Koen KA Van Rompay: Primate Models for Anti-HIV Drug Studies
Several nonhuman primate models are used in HIV and AIDS research. In contrast to HIV-1 infection
of chimpanzees, infection of macaque species with simian immunodeficiency virus (SIV) isolates results
in a disease (simian AIDS) that shares many similarities with HIV infection and AIDS in humans. Al-
though each animal model has its limitations and can never completely mimic HIV infection of humans,
a carefully designed study allows experimental approaches, such as the control of certain variables,
that are not feasible in humans, but that are often the most direct way to gain better insights in disease
pathogenesis and provide proof-of-concept for novel intervention strategies. In the early days of the
HIV pandemic, nonhuman primate models played a relatively minor role in the anti-HIV drug develop-
ment process. During the past decade, however, the development of better virologic and immunologic
assays, a better understanding of disease pathogenesis, and the availability of better drugs have made
these animal models more practical for drug studies. In particular, nonhuman primate models have
played an important role in demonstrating: (i) preclinical efficacy of novel drugs such as tenofovir; (ii)
the benefits of chemoprophylaxis, early treatment and immunotherapeutic strategies; (iii) the virulence
and clinical significance of drug-resistant viral mutants; and (iv) the role of antiviral immune respons-
es during drug therapy. Comparison of results obtained in primate models with those observed in
human studies will lead to further validation and improvement of these animal models. Accordingly,
well-designed drug studies in nonhuman primates can continue to provide a solid scientific basis to
advance our scientific knowledge and to guide future clinical trials. (AIDS Reviews 2005;7:67-83)
Macaque. Monkey. Prophylaxis. Chemotherapy. Resistance.
AIDS Reviews 2005;7:67-83
Koen KA Van Rompay
California National Primate Research Center
University of California
Davis, CA 95616, USA
Introduction: the need for
an appropriate animal model
An increasing arsenal of anti-HIV drugs is currently
being used, and many novel candidates are continu-
ously being developed1. The main anti-HIV drugs that
have been approved or are being developed target
several key steps or enzymes in the viral replication
cycle: attachment, fusion, reverse transcriptase (RT),
integrase or protease. During recent years, combina-
tion therapy of these compounds, so-called highly ac-
tive antiretroviral therapy (HAART), has led to major
improvements in the clinical management of HIV-in-
fected people2. Despite this considerable success,
there is no reason for complacency as long-term ad-
ministration of these drugs is associated with prob-
lems of cost, toxicity, compliance, and drug resis-
tance. Accordingly, the quest for better antiviral drug
regimens continues. The ideal antiviral drug regimen
would be one that induces strong and persistent sup-
pression of virus replication, gives prolonged immuno-
logic and clinical benefits without toxicity, can be ad-
ministered at infrequent dosage intervals, is affordable
and easy to store, and can thus benefit the greatest
number of HIV-infected people, including those in de-
Antiretroviral Drug Studies in Nonhuman Primates:
a Valid Animal Model for Innovative Drug Efficacy
and Pathogenesis Experiments
Koen K. A. Van Rompay
California National Primate Research Center, University of California, Davis, CA, USA
AIDS Reviews 2005;7
The pipeline that new drug candidates need to cross
between the first demonstration of in vitro antiviral ef-
fects and approval for clinical use is tedious, time-
consuming, and very expensive. Most compounds that
inhibit virus replication in vitro are never further devel-
oped (due to lack of resources), or they fail in pre-
clinical testing or clinical trials due to unfavorable phar-
macokinetics, toxicity, or insufficient antiviral efficacy.
A confounding obstacle in the drug development pro-
cess is that many drugs have already been approved
for HIV-infected patients. It is considered unethical to
treat “control” groups with anything less than the cur-
rently available “gold standard” of combination therapy.
Therefore, the efficacy of new drugs is now often eval-
uated by including the compound as part of a combina-
tion regimen, often in patients failing currently available
HAART regimens, who may have existing drug-resis-
tance mutations, low CD4+ cell counts, or poor adher-
ence. Thus, the response in such “worst-case scenario”
patients may underestimate the potency of the drug for
treatment-naive patients. These dilemmas underscore
the need for an evaluation of the role of animal models
in the drug development process. Appropriate animal
models that allow rapid evaluation of the efficacy and
toxicity of antiviral compounds can assist in sorting out
those drugs which are promising and deserve to enter
human clinical trials first, from those drugs that should
probably be discarded3.
While murine and feline models are appropriate for
initial screening, further testing is best done in nonhuman
primate models that better resemble HIV infection of hu-
mans. Nonhuman primates are phylogenetically the clos-
est to humans. The similarities in physiology (including
drug metabolism, placentation, fetal and infant develop-
ment, etc.) and immunology allow a more reliable ex-
trapolation of results obtained in primate models to clini-
cal applications for humans. While chimpanzees can be
infected with HIV-1, this animal model is not practical due
to the low availability, high price, low viral virulence, and
ethical issues4,5. Many nonhuman primate species in Af-
rica are naturally infected with simian immunodeficiency
virus (SIV) strains; despite persistent high-level virus rep-
lication, these natural hosts do not develop disease, pos-
sibly because infection is associated with little immune
activation6,7. In contrast however, infection of non-natural
hosts, such as macaques, with virulent SIV isolates re-
sults in a disease which resembles human AIDS (includ-
ing generalized immune activation, CD4+ T-cell deple-
tion, opportunistic infections, weight loss and wasting),
and the same laboratory markers can be used to monitor
disease progression8. Compared to HIV infection of hu-
mans, infection of macaques with virulent SIV or simian-
human immunodeficiency virus (SHIV) isolates results in
an accelerated course, as most animals develop clinical
disease within one to three years. Similar to observations
in HIV-infected human infants, the disease course in new-
born macaques following inoculation with virulent SIV
strains is usually accelerated9,10. It is important, however,
to remember that SIV or SHIV infection of macaques is
not necessarily fatal, as there are many attenuated or
nonpathogenic virus isolates which give transient or low-
level viremia, and slow or no disease. This wide spectrum
of infection outcomes makes this model suitable to assess
how genetic changes in the virus (e.g. drug-resistance
mutations) affect viral virulence.
Primate models are powerful tools in many areas of HIV
research. In addition to allowing investigators to unravel
virus-host interactions during disease pathogenesis and
to test vaccines8, macaques allow us to model the differ-
ent aspects of antiviral drug treatment, including pharma-
cokinetics, toxicity, and antiviral efficacy. The balance
among all these in vivo interactions (which is impossible
to model accurately in vitro) determines the long-term
clinical usefulness of the antiviral drug (Fig. 1).
Besides being a test system for preclinical screening
of novel drug regimens, an animal model can also be
used to test hypotheses that are difficult or impossible
to explore in humans. By manipulating certain variables
(e.g. the initiation of drug treatment relative to virus in-
oculation, duration of treatment, the age of the animals,
the virulence and drug susceptibility of the virus inocu-
lum, the status of the immune system), investigators can
design studies to address very specific questions. As
discussed further in this review, examples of this are
studies focused on evaluating chemoprophylaxis, the in
vivo virulence and clinical implications of drug-resistant
viral mutants, and the role of antiviral immune respons-
es on antiviral drug efficacy.
Macaque species and virus isolates used
in antiviral drug studies
Anti-HIV drug studies in macaques generally used
rhesus macaques (Macaca mulatta) or cynomolgus
macaques (M. Fascicularis)11. The SIV isolates usually
belonged to a few groups, in particular SIVmac,
SIVsmm and SIVmne. Because the polymerase region
of these SIV isolates has about 60% and 85% amino
acid homology to HIV-1 and HIV-2, respectively, SIV is
susceptible to many of the same nucleoside RT in-
hibitors (NRTI; e.g. zidovudine), nucleotide RT inhibi-
tors (tenofovir, adefovir), integrase and protease in-
Koen KA Van Rompay: Primate Models for Anti-HIV Drug Studies
hibitors12-16. Due to their CCR5 chemokine coreceptor
usage, SIV isolates are also susceptible to CCR5-tar-
geting entry inhibitors17. Some compounds, however,
including nonnucleoside RT inhibitors (NNRTI) such as
nevirapine and efavirenz, are active only against HIV-1
and not against HIV-2 or SIV18. The construction of
infectious SIV/HIV-1 chimeric viruses, in which the RT
gene of SIV was replaced by its counterpart of HIV-1
(so called RT-SHIV), has been proven useful to evalu-
ate NNRTI in primate models19-23. Other SHIV have
been constructed and contain the envelope region (so
called env-SHIV) or other genes of HIV-1. Many env-
SHIV are attenuated. Most pathogenic env-SHIV such
as SHIV-89.6P, while useful to address specific ques-
tions, have the limitation that their disease pathogen-
esis (including CXCR4 coreceptor usage and very
rapid CD4+ cell depletion) is different from the typical
course seen with HIV and SIV infection24. Currently
available CCR5-using env-SHIV (such as SHIV-
SF162P)25 have the limitation that, after the initial peak
of viremia, many untreated animals are able to sup-
press viremia to undetectable levels; while these iso-
lates are useful to test prophylactic or early post-infec-
tion interventions, this large variability in chronic viremia
set-point and disease outcome makes them less prac-
tical for testing antiviral drug efficacy during chronic
infection, especially with limited animal availability. Ac-
cordingly, SIV is in general a more appropriate and
practical model to test anti-HIV strategies26,27.
Development of primate models:
from initial obstacles to validation
During the first decade of the HIV pandemic, the role
of nonhuman primate models in testing anti-HIV drugs
was rather limited. Although SIV is susceptible to many
anti-HIV drugs in vitro, many initial drug studies in ma-
caques were not very successful in demonstrating in
vivo efficacy3,28. Several factors are responsible for
these observations. Most drugs that were available at
that time had complicated dosage regimens (e.g. a
short half-life necessitating frequent administration) or
problems of toxicity and were thus not suitable for long-
term administration. The time course of SIV disease
progression in juvenile and adult macaques is highly
variable as the asymptomatic period can range from
months to years; it was therefore hard to determine
whether a small difference in clinical outcome was due
to host factors or to the drug treatment, especially with
only relatively small numbers of animals and short-term
treatment regimens29. In retrospect, another important
reason for the poor efficacy results of the initial drug
studies was that at that time the role of antiviral immune
responses in determining antiviral drug efficacy was not
Figure 1. Overall outcome of antiviral drug treatment. The ultimate goal of drug treatment is to improve the overall health of the host and in-
definitely delay disease progression. This outcome is determined by many interactions between the virus, the host, and the antiviral drugs, most
of which cannot be mimicked appropriately by in vitro studies. Animal models allow us to control and manipulate certain variables through ex-
perimental approaches that are not feasible in humans (such as experimental inoculation of animals with drug-resistant mutants, or in vivo
depletion of certain immune cells), but that are often the most direct way to address certain questions regarding antiviral drug treatment.
AIDS Reviews 2005;7
recognized. Untreated macaques infected with virulent
isolates such as SIVmac251 have higher viremia, lower
cell-mediated antiviral immune responses, and a more
rapid disease course than HIV-infected humans30. As
discussed further in this review, an antiretroviral drug
becomes less effective in suppressing viremia without
the assistance of effective antiviral immune responses.
As the drugs available at that time were not very potent
in suppressing viremia in HIV-infected humans, it is now
no surprise that they were even less effective in sup-
pressing viremia in immunodeficient SIV-infected ma-
caques. Finally, sensitive assays to accurately quanti-
tate viremia were not available at that time.
Many of these problems have been solved in the past
decade. Sensitive assays, similar to those used to mon-
itor HIV infection of humans, have been developed to
monitor virus replication in SIV-infected macaques, in-
cluding quantitative viral RNA assays31-33. The develop-
ment of a pediatric SIV model has also been very useful,
as the more uniformly rapid disease course (~ 3 to 4
months) observed in infant macaques infected with
virulent SIV isolates permits evaluation of drug efficacy,
including viremia and disease-free survival, in a rela-
tively short time29,34,35. Infant macaques are also easier
to handle for drug administration and require less drug,
which is useful especially for compounds that are ini-
tially very expensive to produce in test quantities. The
first report on the RT inhibitor tenofovir (9-[2-(R)-(phosp
honomethoxy)propyl]adenine; PMPA) in 1995 was a
milestone in validating this animal model because it was
the first compound found to be highly effective against
SIV infection34,36. The strong therapeutic benefits ob-
served with tenofovir in the monkey studies have been
predictive of tenofovir’s efficacy in HIV-infected humans,
and have contributed to its clinical development37-39.
Altogether, these developments over the past decade
have sparked further interest in using nonhuman pri-
mate models for antiretroviral drug studies.
Drug studies in nonhuman primates:
overview and lessons learned
Pharmacokinetics and toxicity
Macaques, which are similar in physiology and me-
tabolism to humans, have been very useful for studying
the toxicity and pharmacokinetics of antiviral drugs,
including the effects of pregnancy and drug transfer
across the placenta and into breast milk40-46. While
most studies used short-term drug administration (in
the order of days to weeks), studies with tenofovir have
also assessed the safety of prolonged treatment (> 1
to 10 years), starting at birth and continuing throughout
adulthood, including pregnancy47. These studies found
that prolonged daily treatment with a high dose of te-
nofovir resulted in a Fanconi-like syndrome (proximal
renal tubular disorder) with bone pathology, while
short-term administration of relatively high doses and
prolonged low-dose regimens were safe47. Such long-
term studies in primates are very relevant as they
mimic life-long treatment of HIV-infected humans.
Prophylaxis: prevention of infection
Many studies in nonhuman primates have focused on
investigating whether drug administration starting near
the time of virus inoculation could prevent infection.
Prevention of infection is traditionally considered as the
complete absence of any viral or immunologic evidence
of infection; however, the development of more sensitive
techniques (including DNA PCR, viral RNA quantitation)
has sometimes resulted in transient detection of low-
level signs of infection, usually within the first months
after virus inoculation48,49. Accordingly, for the purposes
of this review, prophylaxis is defined as “protection
against persistent infection”, with persistent infection be-
ing defined as “persistent viremia or persistently detect-
able virus-specific immune responses”.
A few studies in macaque models have evaluated
the efficacy of antiviral compounds as topical microbi-
cides against mucosal infection; topical high-dose ad-
ministration of a number of compounds protected adult
macaques against intravaginal or intrarectal SIV or
SHIV infection at varying rates of efficacy50-56.
Most studies have used systemic drug administration
to try to prevent infection. Early studies, which mostly
used zidovudine (AZT), were not very effective in pre-
venting infection, but a likely reason for this was the high
dose of virus used in these experiments57-61. In subse-
quent studies, when a lower dose of virus was used to
inoculate animals, administration of several drugs (includ-
ing zidovudine, adefovir (PMEA), tenofovir (PMPA) and
3’-fluorothymidine) starting prior to or at the time of virus
inoculation was able to prevent virus infection48,49,62-69.
Very few compounds have been shown to prevent infec-
tion when treatment was started after virus inoculation:
i.e. tenofovir, BEA-005, and GW420867. A combination
of the timing and duration of drug administration was
found to determine their success rate21,36,63,70-72. Of these
three compounds, tenofovir was effective following virus
inoculation by different routes (intravenous, oral, intra-
vaginal, intrarectal), and is currently the only one ap-
Koen KA Van Rompay: Primate Models for Anti-HIV Drug Studies
proved for therapeutic use in humans; BEA-005 and
GW420867 are no longer in clinical development.
The demonstration that antiviral drugs can prevent
infection in macaques has provided a solid scientific
rationale to administer anti-HIV drugs to humans follow-
ing exposure to HIV in several clinical settings. Antiviral
drugs are now recommended, usually as combination
regimens, to prevent HIV infection following occupa-
tional exposure (e.g. needlestick accidents of health
care workers) and non-occupational exposure (e.g. sex
or injection-drug use)73,74. Similarly to the animal studies,
transient viremia has been described in some humans
receiving postexposure prophylaxis75.
Because an efficacious HIV vaccine has so far not
been identified, tenofovir’s prophylactic success in the
macaque models has sparked clinical trials to investi-
gate whether uninfected adult persons who engage in
high-risk behavior will have a lower infection rate by
taking tenofovir once daily. The ethical controversies
surrounding these trials, which are being held at sev-
eral international sites and target different high-risk
populations, are reviewed elsewhere76.
Antiviral drugs, especially zidovudine and nevirap-
ine, have played a very important role in the prevention
of mother-to-infant transmission of HIV, including in
developing countries77-79. To counteract potential prob-
lems of drug-resistance mutations that are induced by
the nevirapine regimen in women in developing coun-
tries80, the promising data of a two-dose tenofovir
regimen in the newborn macaque model49,64 have
spurred interest to test the feasibility of a two-dose
tenofovir regimen to reduce perinatal HIV transmission
(PACTG-394 and HPTN-057).
Therapy: treatment of infection
Many studies in the macaque model have demon-
strated that, even when infection was not prevented,
early drug treatment delayed or reduced the peak of
acute viremia that occurs during the first weeks of infec-
tion, enhanced antiviral immune responses, and delayed
disease progression16,19,21,29,57,59,60,66,81-94. These same
benefits of early treatment have now been confirmed in
When macaques were started on short-term drug
regimens during the stage of acute viremia, the outcome
once treatment was withdrawn depended on the virus
isolate. With pathogenic env-SHIV isolates, short-term
suppression of acute viremia was usually effective to
induce strong antiviral immune responses that controlled
virus replication and delayed disease for an extended
time in the absence of drug treatment16,90,101. In contrast,
with highly virulent SIV isolates (such as SIVmac251),
viremia usually increased again once short-term drug
treatment was stopped, similarly to what is observed in
most HIV-infected humans26,27,94,102-105.
Macaque studies have also investigated the effects of
antiviral therapy on established, chronic SIV infection
(i.e. after the acute viremia stage), and the often disap-
pointing results have puzzled researchers for a long
time. Initial studies with zidovudine were not very suc-
cessful in reducing viremia once SIV infection was es-
tablished29,62,106. As selection for zidovudine-resistant
viral mutants was slow107, these data are consistent with
the relative weakness of zidovudine monotherapy com-
pared to newer compounds. Lamivudine (3TC) and em-
tricitabine ((-)-FTC) treatment of SIVmac251-infected
infant macaques also had little effect on viremia and
disease progression. However, there was rapid emer-
gence of drug-resistant mutants with the M184V muta-
tion in RT, suggesting that drug levels were sufficient to
inhibit replication of wild-type virus108. The CCR5 in-
hibitor CMPD 167 reduced viremia fourfold to 200-fold
in chronically SIV-infected macaques, but in some ani-
mals this effect was transient17. Similarly, efavirenz treat-
ment led to reduced viremia in RT-SHIV infected ani-
mals, and selection for drug-resistant mutants led in
some animals to viral rebound23. The integrase inhibitor
L-870812 reduced viremia in SHIV-89.6P-infected ma-
caques if initiated during early infection (before CD4+
cell depletion)16. In most studies, tenofovir has been
highly effective to reduce established viremia34,109-112.
During prolonged tenofovir therapy, the emergence of
viral mutants with reduced in vitro susceptibility did not
always lead to a rebound in viremia as some animals
maintained low viremia34,113. However, there have been
reports where tenofovir therapy was not effective in sup-
pressing viremia despite the presence of drug-suscep-
tible virus at the onset of treatment35,101,109,112,114, sug-
gesting that antiviral drug therapy is more complex than
just a matter of having sufficient drug levels and sus-
ceptible virus. As discussed below, a growing body of
evidence obtained from monkey studies creates a pic-
ture of drug therapy in which the efficacy of a drug
regimen to reduce viremia is the combined result of
several factors: (i) direct inhibitory activity of the drug(s)
against the virus, determined by pharmacokinetic and
pharmacodynamic factors; (ii) drug resistance (includ-
ing likelihood of emergence, level of reduced suscepti-
bility, effect of mutations on viral replication fitness and
virulence); and (iii) the status of the host immune system
(including antiviral immune responses). Primate studies
AIDS Reviews 2005;7
have provided valuable insights into these interactions.
The demonstration of tenofovir’s antiviral efficacy in
SIV-infected macaques has sparked many other drug
studies in this animal model. Tenofovir-containing regi-
mens have been used to gain a better understanding of
disease pathogenesis and drug therapy, and to test
additional intervention strategies. While SIV infection
leads to rapid depletion of CD4+ T-cells from gut-as-
sociated lymphoid tissue (GALT) and gastrointestinal
dysfunction115-117, early tenofovir therapy was found to
decrease mucosal virus levels and restore the CD4+
T-cell population in GALT; this was associated with up-
regulation of growth factors and genes involved in repair
and regeneration of the mucosal epithelium118,119. Com-
bination treatment of SIV-infected macaques with teno-
fovir and two protease inhibitors (indinavir and nelfinavir)
was found to improve immune responses against other
organisms such as mycobacterium120. The macaque
model has also been used to investigate the viral reser-
voirs during drug treatment: SIV-infected pigtailed ma-
caques treated with tenofovir and emtricitabine were
found to have viral reservoirs in resting CD4+ T-lympho-
cytes121. Similar to observations in humans, a combina-
tion of tenofovir, lamivudine, and Efavirenz was also
found to be very effective to suppress viremia in RT-
SHIV infected macaques, with no detectable emergence
of drug-resistant mutants during treatment122.
A number of studies have combined antiviral drug
treatment with other strategies aimed at enhancing anti-
viral immune responses, so that when drug treatment
was stopped, viremia was controlled better. These im-
munotherapeutic strategies include structured treatment
interruption, the combination of antiviral therapy with ac-
tive immunization with or without cytokine administration,
and immune reconstitution via administration of autolo-
gous CD4+ T-cells collected prior to SIV infection123-130.
A caveat in interpreting the data of several of these stud-
ies, however, is that the combination of a high dose of
tenofovir, didanosine, and hydroxyurea in macaques is
plagued by problems of pancreatic toxicity (probably
due to didanosine), which sometimes results in life-
threatening diabetes (including after drug withdrawal);
the published reports do not discuss whether drug-re-
lated toxicity may have contributed to the mortality ob-
served in some of these studies.
The value of primate models in studying
Many individuals do not show the desired strong and
persistent suppression of viral replication during HAART.
Although other factors, such as compliance and indi-
vidual variability in pharmacokinetics, also contribute to
reduced efficacy of HAART, a major limiting factor is the
emergence of viral mutants with reduced in vitro suscep-
tibility to antiviral drugs (so called “drug-resistant mu-
tants”)131. Due to the high mutation rate of the virus, in-
complete suppression of replication selects for viral
variants with mutations that allow better replication in the
presence of drugs. The relationship between drug ad-
herence and the emergence of drug-resistant mutants is
complex and seems to depend on the drug class132.
While the correlation between specific mutations in
the viral genome and in vitro reduced susceptibility has
been well documented for most antiviral compounds,
many unanswered questions remain regarding the ex-
act clinical implications of these drug-resistant variants
in vivo, and how to use this information to make treat-
ment decisions. If drug resistance means that the drug
is no longer effective, then it can just as well be with-
drawn; but if there is still a partial response, then it will
be counterproductive to discontinue drug administra-
tion unless better alternatives can be offered133-135.
Many studies, including those utilizing drug interrup-
tions, have demonstrated that HAART can still have
therapeutic virologic and/or immunologic benefits even
in the presence of drug-resistant virus, and this may
be due to some residual drug activity and/or the al-
tered pathogenesis of drug-resistant variants136-146.
Thus, it is important to note that the terms “drug resis-
tance” and “reduced susceptibility” are in vitro mea-
sures, and “drug resistance” does not necessarily im-
ply that drug efficacy is completely abolished in vivo.
An important question about mutants with reduced
in vitro susceptibility to drugs concerns the replicative
fitness and virulence of such mutants in comparison to
wild-type virus. Because the mutations that reduce
susceptibility are at very low or undetectable frequen-
cy in the absence of drug treatment, these mutations
are expected to reduce the ability of the virus to repli-
cate. However, primary drug-resistance mutations are
often followed by compensatory mutations to improve
replicative fitness. So what is the final result? Are drug-
resistant mutants attenuated in virulence (i.e. their abil-
ity to cause disease) to such extent that the purpose
of continuing drug therapy could be to prevent rever-
sion to the more virulent wild-type form?
Studies measuring in vitro replication kinetics of
drug-resistant HIV mutants can never completely pre-
dict their in vivo virulence. In vivo virulence is deter-
mined by complex pharmacologic, viral and host fac-
tors (including many tissue- and cell-specific factors)
Koen KA Van Rompay: Primate Models for Anti-HIV Drug Studies
that are difficult to mimic in vitro, such as drug phar-
macokinetics, primary and compensatory mutations
(and their impact on replication fitness, but also on
immunogenicity), cell tropism, and the complex role of
the immune system (which supports virus replication,
but at the same time also tries to contain it). Studies in
the SIV-macaque model have demonstrated repeat-
edly that the correlation between in vitro markers (viral
replication fitness, cell tropism, and cytopathogenicity)
and in vivo measures (replication fitness, cell tropism,
and virulence) is often weak as virus isolates that rep-
licate well and are very cytopathogenic in vitro can be
severely attenuated or have a different cell tropism
following inoculation in macaques147-149. Thus, the ex-
trapolation of results from in vitro growth kinetic studies
to decisions affecting clinical management of HIV-in-
fected patients should be performed with caution.
Similarly, it has been difficult to correlate data of in
vitro drug susceptibility assays (which can demon-
strate small to large changes in susceptibility) with
changes in antiviral efficacy in vivo150.
Some information regarding the relative replication fit-
ness and stability of drug-resistant HIV mutants in vivo
can be gathered from case reports, such as those doc-
umenting primary infection with drug-resistant HIV-1, as
well as those monitoring the reversion of drug-resistant
virus to wild-type following discontinuation of drug treat-
ment144,151,152. An animal model, however, allows ap-
proaches which are impossible in humans, but which are
the most direct ways to study the clinical implications of
drug-resistant virus: animals can be inoculated with
drug-resistant viral mutants or their wild-type counter-
parts, and their replication fitness and virulence can be
compared in drug-treated versus untreated animals.
Drug-resistance studies in the macaque
Several methods have been used to generate drug-
resistant SIV variants in vitro, including selection through
serial passage as well as site-directed mutagenesis of
molecular clones23,153,154. Only a few studies have evalu-
ated the emergence of drug-resistant viral mutants in
treated macaques. Treatment of RT-SHIV infected ma-
caques with nevirapine or efavirenz gave rise to the emer-
gence of mutations at codon 103 and 181 in RT, similar
to observations in treated HIV-1 infected patients22,23.
A zidovudine-treated SIVmac251-infected macaque
developed a glutamine-to-methionine substitution at
codon 151 of RT (Q151M), associated with high-level
(> 100-fold) in vitro resistance to zidovudine29,107. In-
oculation of the Q151M SIVmac isolate into naive new-
born macaques demonstrated that this mutation did
not significantly reduce viral replication and viral viru-
lence; the Q151M mutation (which is the result of two
base changes) was also very stable in the absence of
zidovudine treatment107. This Q151M mutation has not
been found in HIV-1 infected patients receiving zidovu-
dine monotherapy, but has been found in HIV-1 in-
fected patients receiving sequential or combination
therapy with dideoxynucleoside analogues155,156. How-
ever, the Q151M mutation is found frequently in HIV-2
infected patients receiving NRTI therapy157,158. This lat-
ter observation indicates that, due to much sequence
homology, HIV-2 and SIV use similar mutational path-
ways that are sometimes distinct from those of HIV-1.
Treatment of SIV-infected infant macaques with lami-
vudine (3TC) or emtricitabine ((-)-FTC) gave rise to the
emergence of viral mutants with the expected M184V
mutation in RT within five weeks of treatment108. The
clinical implication of the M184V mutation was subse-
quently investigated by inoculating juvenile macaques
with SIVmac239 clones having either wild-type se-
quence or the M184V mutation in RT (SIVmac239-
184V). In comparison to wild-type virus, SIVmac239-
184V was replication-impaired, based on virus levels
one week after inoculation, and on the reversion of
SIVmac239-184V to wild-type sequence in untreated
animals. However, this reduced replication fitness was
not sufficient to affect viral virulence, as animals inocu-
lated with SIVmac239-184V and treated with emtric-
itabine (to prevent reversion) had similar viremia from
two weeks after infection onwards, and the disease
course and survival was indistinguishable from that of
animals infected with wild-type virus108. In a different
study, the M184V mutation did not revert in macaques
inoculated with SIVmac239 containing both the M184V
and E89G mutations; however, the M184V mutation in
that study was engineered with two base changes in
codon 184 (instead of the single base change that is
normally seen during in vitro or in vivo selections)159.
Long-term treatment of SIVmac251-infected macaques
with tenofovir resulted in the emergence of virus with
fivefold reduced in vitro susceptibility to tenofovir, as-
sociated with a lysine-to-arginine substitution at codon
65 (K65R) of RT34,114. Tenofovir also selects for the K65R
mutation in HIV-1 RT160-162. The emergence of K65R in
SIV was followed by additional RT mutations, which were
likely to be compensatory mutations34. The emergence
and distribution of K65R mutants is a complex process,
with considerable variability among animals and among
tissues114. The SIV macaque model has provided impor-
AIDS Reviews 2005;7
tant information regarding the clinical implications of
K65R viral mutants during tenofovir treatment. Although
some SIVmac251-infected animals show an increase in
viremia following the emergence of K65R viral mutants,
other animals continue to suppress viremia to low or
undetectable levels for years (> 3 to 9 years)34,113,163.
This success in persistently suppressing replication of
the highly virulent SIVmac251 isolate with tenofovir
monotherapy is unprecedented in this animal model26,27.
To investigate whether this observation of suppressed
viremia in some animals despite K65R virus was caused
by an attenuating effect of the K65R mutation on viral
replication fitness and virulence, two K65R SIV isolates
were inoculated into new animals. In the absence of
tenofovir treatment, the K65R SIV isolates were as fit and
virulent as wild-type SIVmac251, based on their ability
to induce high viremia and rapid disease (≤ 4 months)
in newborn macaques163. However, in the presence of
prolonged tenofovir treatment, the disease course was
changed and two scenarios were possible: (i) K65R vi-
remia was reduced and could become undetectable
with prolonged disease-free survival (> 9 years)113,163; (ii)
viremia remained high (> 106 to 107 RNA copies/mL
plasma), but with continued tenofovir treatment, survival
was increased significantly more than predicted based
on viral RNA levels and CD4+ T-cell counts35,113,163. Such
findings have not been observed with other antiviral
drugs in the SIV-macaque model, which suggests that
tenofovir treatment may have rather unusual interactions
with the immune system. These observations instigated
further in vivo experiments that identified a major role of
the immune system in determining the efficacy of antivi-
ral drug therapy to reduce viremia.
The role of the immune system
on the efficacy of drug therapy
Viral kinetics during drug therapy depend on viral
replication fitness, drug susceptibility of the virus, and
drug potency164-166. When virus levels in plasma are
reduced rapidly following the onset of drug therapy, the
antiviral drugs are lauded for their potency, while the
role of antiviral immune responses during drug therapy
is less clear166. In this context, one is inclined to con-
sider antiviral immune responses mostly as a backup
plan to try to contain viremia whenever drug treatment
is withdrawn or if drug-resistant virus would emerge103.
Recently, however, a growing body of evidence from
human and primate studies suggests that antiviral im-
mune responses play a previously unrecognized role
during drug therapy, which merits proper cred-
it16,35,113,143,167. Drug studies in macaques have demon-
strated the concept that the efficacy of antiviral drug
therapy in reducing viremia is not only determined by
the intrinsic potency of the drug in directly inhibiting
virus replication, but is also strongly dependent on the
status of the immune system16,35,113. In other words,
antiviral drugs require the assistance of immune re-
sponses to reach full effectiveness in reducing viremia,
both at the onset of treatment when the virus has wild-
type susceptibility, as well as during prolonged treat-
ment in the presence of drug-resistant mutants113.
Several key studies using experimental depletion of
CD8+ cells in vivo (through administration of anti-CD8
monoclonal antibody) are summarized in figure 2, and
support the model shown in figure 3. When tenofovir
treatment was started during acute viremia with wild-
type SIVmac251, the efficacy of tenofovir to suppress
acute viremia with wild-type SIVmac251 was signifi-
cantly reduced in the absence of CD8+ cells113. These
observations indicate that the otherwise rapid decline
of productively infected cells (with half-life of ~ 1 to 2
days) after the onset of drug therapy is due to CD8+
cell-mediated killing or inhibition, rather than the natu-
ral death rate (as determined by the cytopathogenicity
of the virus)113. In this model of drug therapy (Fig. 3),
CD8+ cell-mediated antiviral immune responses con-
tribute significantly to the antiviral effects of anti-HIV
drugs, presumably by reducing the burst of virus rep-
lication in productively infected cells via cytolytic or
noncytolytic pathways. In the absence of CD8+ cells,
productively infected cells had a long half-life, sug-
gesting that virulent SIV, during concomitant tenofovir
treatment, is not as cytopathic as expected113.
Even after the emergence of K65R SIV mutants, some
tenofovir-treated animals were able to reach undetect-
able viremia34,113. A tempting explanation for this surpris-
ing observation, especially if seen in tenofovir-treated
humans, would be to ascribe it to (i) a severe reduction
in replication fitness caused by the K65R mutation
(which, as discussed earlier, is not the case for K65R
SIV isolates)163, and/or (ii) sufficient residual inhibitory
effect of tenofovir against these viral mutants (with ~ 5-
fold reduced in vitro susceptibility). However, CD8+ cell-
depletion experiments, which are not feasible in humans,
revealed that the suppressed viremia of K65R SIV mu-
tants during prolonged tenofovir treatment of macaques
was largely due to strong CD8+ cell-mediated antiviral
immune responses because, in the absence of CD8+
cells, (i) K65R viral mutants were very replication-com-
petent, and (ii) tenofovir treatment alone was not suffi-
cient to inhibit K65R SIV replication in vivo (Fig. 2)113.
Koen KA Van Rompay: Primate Models for Anti-HIV Drug Studies
Further experiments demonstrated that continued teno-
fovir treatment was required to maintain suppression of
K65R SIV replication because tenofovir withdrawal led to
a slow increase in viremia (Fig. 2)113. Thus, both tenofo-
vir and effective CD8+ cells were required to maximally
suppress replication of virulent virus in this animal mod-
el. Because the anti-CD8 antibody depletes both
CD8+CD3+ T-lymphocytes and CD8+CD3- natural killer
(NK) cells, the relative contribution of these two cell
populations and their antiviral effector mechanisms could
not be identified in these experiments113. These observa-
tions of reduced viremia of K65R SIV mutants associated
with improved antiviral immune responses in tenofovir-
treated macaques are consistent with clinical observa-
tions of strong antiviral immune responses in HAART-
treated HIV-1-infected people who have low-level viremia
with drug-resistant virus143,168. Temporal variability in the
strength of such immune responses may also be the
direct cause of transient blips of viremia that are ob-
served in many HAART-treated individuals169,170. Antiviral
immune responses may thus also play a role in determin-
ing viral reservoirs in HAART-treated patients171.
As mentioned previously, tenofovir treatment initiated
during early stages of SIV infection was usually very
effective in reducing viremia. In contrast, several stud-
ies documented that tenofovir therapy was not very
effective in rapidly suppressing viremia, despite the
presence of drug-susceptible virus at the onset of
treatment, especially when tenofovir therapy was start-
ed later in infection, with more virulent isolates, and in
animals with high viremia and immunodeficien-
cy35,101,109,112,114. However, the rapid emergence of
K65R virus that has been described in some of these
studies is a reflection of strong selection pressure, and
Figure 2. Importance of CD8+ cells for the efficacy of tenofovir treatment: summary of CD8+ cell-depletion experiments. A schematic simplifica-
tion of previously published data is presented113. In Experiment 1, animals were inoculated with wild-type virulent SIVmac251 and started on te-
nofovir therapy two weeks later. While untreated animals had persistently high viremia (not shown), animals started on tenofovir treatment (closed
square) showed a rapid reduction of viremia (A), with estimated half-life of productively infected cells of 1 to 2 days in the presence of CD8+ cells.
At the onset of tenofovir treatment, one group (open square and circle) was also depleted of CD8+ cells via administration of the anti-CD8 mono-
clonal antibody (cM-T807); in the absence of CD8+ cells, tenofovir-treated animals had little reduction in viremia (B), suggesting a half-life of
productively infected cells of 4 to 6 days. When CD8+ cells became detectable, viremia was reduced rapidly with a half-life of 1 to 2 days (C).
Despite the emergence of K65R mutants (with fivefold reduced in vitro susceptibility to tenofovir), some animals were able to reach undetectable
viremia after prolonged tenofovir treatment113. In Experiment 2, when such chronically treated animals were depleted of CD8+ cells, viremia of
K65R virus increased transiently and returned to baseline values upon return of CD8+ cells. Thus, tenofovir treatment alone was not sufficient to
control viremia of K65R mutants in the absence of CD8+ cells. In Experiment 3, when prolonged tenofovir treatment was withdrawn, viremia of
K65R virus increased slowly, demonstrating that CD8+ cell-mediated immune responses alone were not sufficient to maintain maximal suppres-
sion of viremia. Thus, both tenofovir and CD8+ cells were required for optimal suppression of viremia, both at the onset of therapy (when virus
was still wild-type) as well as during prolonged therapy (when virus had reduced in vitro susceptibility and the K65R mutation in RT)113.
AIDS Reviews 2005;7
indicates efficient inhibition of wild-type virus replica-
tion by the tenofovir regimen35. An integrase inhibitor
was also found to be less effective in reducing viremia
when initiated during late infection16. These data pro-
vide further support for this model in which antiviral
immune responses assist anti-HIV drugs in reducing
viremia. In the absence of effective antiviral immune
responses, antiviral drugs face a more daunting task
to control viremia as already infected cells survive lon-
ger and produce more viral progeny (Fig. 3D)35,113.
Because virulent SIV isolates induce immune dysfunc-
tion at many stages of the immune response (including
antigen presentation and CD4+ T-helper cell func-
tion172,173), CD8+ cell-mediated immune responses
become inactive at lower levels of antigen, and thus it
is less likely that viremia can be suppressed to low or
undetectable levels, especially once drug-resistant
mutants emerge174-176. This model in which both drugs
and antiviral immune responses play a role in reducing
viremia helps to explain the different patterns of viremia
that are seen in drug-treated SIV-infected macaques
and HIV-infected infants and adults177,178. Several main
scenarios of models of viremia during drug therapy are
presented in figure 4. Note, however, that an individu-
al’s pattern may shift to another one based on chang-
es in drug regimen, the potential of immune restoration
(including increased potency of antiviral immune re-
sponses), and the acquisition of additional drug resis-
tance mutations (which can affect virulence and repli-
cation fitness). Even in an individual host, patterns of
viral kinetics and turnover may vary among different
tissues, based on tissue-specific differences in target
cells, drug levels, and antiviral immune-effector mech-
anisms; this could explain observations of highly un-
even distribution of SIV mutants in drug-treated ma-
caques114. Such mechanisms of immune-mediated
clearance of virus during drug therapy are probably
not unique to lentiviruses, as a similar correlation has
been described between the status of the immune
system and clearance of hepatitis B virus following
lamivudine treatment in patients with dual HIV and
hepatitis B infection179. Despite this recent progress in
better appreciating the role of antiviral immune re-
sponses during drug therapy, we need to acknowl-
edge the big gaps that still remain in our knowledge
of these antiviral immune responses. Direct in vivo ma-
Figure 3. Proposed model of drug and immune-mediated effects on virus replication. A: Without drug treatment, virulent virus can replicate
to high titers because of high infection rates of CD4+ T-helper cells and antigen-presenting cells which are unable to provide sufficient as-
sistance to CD8+ cell-mediated immune responses to contain virus replication. B: A potent drug regimen reduces the number of CD4+ T-
helper cells and antigen-presenting cells that become newly infected. Potent CD8+ cell-mediated immune responses reduce the half-life, and
thus the burst size of viral progeny, for those cells that already became infected. The combined antiviral activities of drug and antiviral CD8+
cells are efficient to induce and maintain low viremia, even after the emergence of drug-resistant viral mutants (as shown for tenofovir in the
macaque model113). C: During artificial CD8+ cell depletion, productively infected cells survive longer and produce more progeny virus, result-
ing in higher viremia (see also Fig. 2)113. D: During immunodeficiency, the reduced function of antigen presenting cells and CD4+ T-helper
cells results in insufficient assistance to antiviral CD8+ cells to remain active, especially at lower levels of viremia. Even when infection of
new cells is reduced by an efficient drug regimen, the half-life of the productively infected cells is long, resulting in a slower decrease of
viremia. Without sufficient immune restoration, the emergence of drug-resistant mutants is likely to lead to a rebound in viremia16,35.
Modified from reference 113.
Koen KA Van Rompay: Primate Models for Anti-HIV Drug Studies
nipulations of the immune system (such as experimen-
tal depletions), which are often the best way to get a
better understanding of in vivo antiviral immune mech-
anisms, can be performed in animal models, but are
usually not feasible in humans. Instead, the need to
rely on in vitro and ex vivo immune assays has the
limitation that the currently available assays, especial-
ly when performed on peripheral blood, are not able to
accurately grasp the variety, breadth, and strength of
antiviral immune-effector mechanisms that control virus
replication in vivo, especially in the lymphoid tissues
and at mucosal sites143,180-184.
Figure 4. Models of viremia during antiviral drug therapy: interaction of drugs and antiviral immune responses. Several scenarios are presented
using different combinations of variables, including the strength of antiviral immune responses, the potency of the antiviral drug regimen against
the virus, and the virulence and replication fitness of the virus. Tx indicates the start of drug treatment; R indicates the emergence of drug-resis-
tant mutants with sufficient replication fitness, while S indicates viremia of wild-type virus (and/or drug-resistant mutants with severely reduced
replication fitness). Intermediate levels of viral fitness are possible (not shown). “Potent drug” indicates a highly effective (single or combination)
drug regimen that would completely prevent infection of new cells. A: Without effective antiviral immune responses and antiviral drugs (or in the
presence of totally ineffective therapy due to complete drug resistance), viremia remains persistently high and leads to rapid disease. B: In the
absence of anti-HIV drug therapy, some individuals are able to mount strong antiviral immune responses that initially control viremia, but usually
are lost (due to progressive immune dysfunction and/or the emergence of immune escape mutants). C: Starting a potent drug regimen at a time
of strong antiviral immune responses (e.g. during acute viremia) leads to rapid reduction of viremia; viremia can become and remain undetectable,
even after the emergence of replication-fit drug-resistant virus (as observed in tenofovir-treated SIV-infected macaques113; see Fig. 2). D: Starting
drug treatment at a moment of partial immunity (e.g. most HIV-infected patients with chronic infection) leads to a first phase of rapid decline in
viremia, followed by phases of slower decline. These phases, generally believed to reflect distinct populations of infected cells164, may alterna-
tively also reflect antiviral immune responses that, without sufficient assistance of antigen-presenting cells or T-helper cells, become less active
at lower levels of antigen196. In the absence of sufficient immune restoration, the emergence of drug-resistant virus or withdrawal of drug treatment
is likely to lead to increased viremia. E: Without effective antiviral immune responses (e.g. SIV- or SHIV-infected macaques with severe immu-
nodeficiency)16,35,108, treatment with an otherwise highly potent drug does not result in rapid reduction in viremia, despite the presence of wild-type
virus. Viremia can only continue to decrease if the drug is 100% effective in preventing infection of new cells and there is no emergence of drug-
resistant mutants. F: With a partially effective drug regimen (or suboptimal levels of a potent drug), the reduction in viremia is limited because the
relative increase in CD4+ cells provides more target cells for virus replication; as a result, viremia can stabilize at a lower level. Because wild-type
virus can still replicate (albeit at reduced levels), the detection of drug-resistant mutants is delayed (e.g. zidovudine29,107).
AIDS Reviews 2005;7
It is important to note that the effects of antiviral im-
mune responses during drug therapy are not mutually
exclusive of the effects of reduced replication fitness
of mutant virus and/or residual drug activity. In par-
ticular, even a relatively minor decrease in replication
fitness, or a partial inhibition of virus replication by the
drug regimen, can have a major impact on viremia if it
provides more opportunity for effective antiviral im-
mune responses to kill productively infected cells prior
to the major viral burst. In contrast, in the absence of
effective antiviral immune responses (such as during
late-stage disease), a small difference in replication
fitness may not translate into any significant difference
in viremia and clinical outcome108,113,185.
As mentioned previously, a surprising observation
was that tenofovir-treated animals that maintained high
viremia of K65R virus had prolonged disease-free sur-
vival, significantly more than predicted based on viral
RNA levels and CD4+ T-cell counts35,163. This improved
survival despite high viremia was only observed in the
presence of tenofovir treatment, and has so far not
been described for any other drugs in this animal
model107,108. This prolonged survival despite high vire-
mia in tenofovir-treated macaques is reminiscent of
“discordant” or “paradoxical” results that have been
described in HAART-treated HIV-infected adults and
children, especially with regimens containing protease
inhibitors. In such discordant patients, there is immu-
nologic benefit (as measured by improved CD4+ T-
lymphocyte counts and/or antigen-specific immune
responses) and clinical benefits despite virologic fail-
ure140-142,144,177,186-188. The available data suggest that
a combination of factors plays a role in such discordant
results, including a decreased replicative fitness and
T-cell activating ability of the drug-resistant mu-
tants136,138,144,146, an anti-apoptotic effect of protease
inhibitors that preserves CD4+ T-cells189, improved vi-
rus-specific cellular immunity190, and direct antimicro-
bial properties of protease inhibitors191,192. Our study
with tenofovir-treated SIV-infected macaques had the
surprising finding that improved survival despite high
viremia was even observed in animals in the absence
of a significant immunologic response (based on stan-
dard immunologic parameters such as CD4+ T-cell
counts and antibody responses to SIV and test anti-
gens)35,163. Such clinical benefits would be difficult to
detect in human studies as it requires years of follow-
up, and without a good virologic and immunologic re-
sponse, drug regimens would probably be changed in
the meantime. As discussed elsewhere, it is unclear
whether this phenomenon of prolonged disease-free
survival in tenofovir-treated macaques with high vire-
mia is due to residual antiviral activity of tenofovir
against K65R virus in particular cell types (for example,
antigen-presenting cells), potentially leading to relative
preservation of innate immunity, or due to immuno-
modulatory effects that are independent of its antiviral
effects, but that may partially protect the immune sys-
tem against the deleterious effects of persistent virus
replication and/or immune activation35. Tenofovir, which
has many immunomodulatory effects in murine mod-
els193, primed rhesus macaque cells for increased in-
terleukin-12 secretion in vitro194.
Such observations further highlight our relatively
poor understanding of disease pathogenesis, and the
need for further research to unravel the complex inter-
actions between viral, host, and pharmacologic factors
that determine (i) control of virus replication, and (ii)
overall clinical outcome. The data of these macaque
studies also suggest that the criteria for changing treat-
ment regimens that were established with older drug
regimens (based on correlations between viral RNA
levels, CD4+ cell counts and disease progression)
may have to be modified for regimens that include
newer drugs (such as tenofovir). Please note, however,
that tenofovir-treated animals with high viremia, despite
having improved survival, eventually still develop dis-
ease. Thus, the ultimate goal of antiviral therapy re-
mains to inhibit virus replication maximally and restore
the immune system, using regimens that are feasible
with regard to safety, cost, and adherence.
Studies in SIV-infected macaques have shown that
improvement of immunologic control of viremia is pos-
sible with adoptive transfer of autologous antigen-pre-
senting cells, CD4+ T-helper cells, or other immunization
strategies124-130,195. The studies with tenofovir in ma-
caques have proven the concept that the combination
of a potent drug regimen and good antiviral immune re-
sponses is able to induce long-term suppression of vire-
mia and prolonged disease-free survival (> 3 to 9 years),
even in the presence of mutants with reduced drug sus-
ceptibility113. Accordingly, these primate studies provide
a strong scientific rationale to explore other strategies to
boost or restore antiviral immune responses during anti-
viral therapy. The demonstration in SIV-infected ma-
caques that antiviral immune responses already contrib-
ute significantly to rapidly reducing viremia immediately
after the onset of drug therapy (Fig. 2) provides the
scientific impetus to also explore the feasibility of starting
immunotherapeutic strategies near to or simultaneously
with the onset of antiviral drug therapy, instead of waiting
until viremia has reached lower levels.
Koen KA Van Rompay: Primate Models for Anti-HIV Drug Studies
The development of better reagents and more sensi-
tive virologic and immunologic assays, the discovery of
more potent drugs, and a better understanding of dis-
ease pathogenesis have made nonhuman primate mod-
els a more practical and adaptable system (i) to rapidly
evaluate novel prophylactic and therapeutic drug strate-
gies, and (ii) to test hypotheses that cannot be mimicked
appropriately by in vitro experiments and are difficult to
explore in humans. The comparison and correlation of
results obtained in monkey and human studies is leading
to a growing validation and recognition of the relevance
of this animal model. Although each animal model has
its limitations, carefully designed drug studies in nonhu-
man primates can continue to advance our scientific
knowledge and guide future clinical trials.
The author wishes to thank Dr. M. Marthas and E.
Blackwood for critical reading of, and helpful sugges-
tions to the manuscript.
1. De Clercq E. HIV-chemotherapy and -prophylaxis: new drugs, leads
and approaches. Int J Biochem Cell Biol 2004;36:1800-22.
2. Sabin C, Hill T, Lampe F, et al. Treatment exhaustion of highly active
antiretroviral therapy (HAART) among individuals infected with HIV in
the United Kingdom: multicentre cohort study. Br Med J 2005;
doi:10.1136/bmj.38369.669850.8F (published 4 March 2005).
3. Van Rompay K, Marthas M. Non-human primate models for testing
anti-HIV drugs; In: J. Kreuter, R. Unger and Rüebsamen-Waigmann,
editors. Antivirals against AIDS. New York: Marcel Dekker, Inc.; 2000.
4. Nath B, Schumann K, Boyer J. The chimpanzee and other non-hu-
man-primate models in HIV-1 vaccine research. Trends Microbiol
5. Novembre F, Saucier M, Anderson D, et al. Development of AIDS
in a chimpanzee infected with HIV. J Virol 1997;71:4086-91.
6. Silvestri G, Sodora D, Koup R, et al. Nonpathogenic SIV infection of
Sooty Mangabeys is characterized by limited bystander immunopa-
thology despite chronic high-level viremia. Immunity 2003;18:1-20.
7. Silvestri G, Fedanov A, Germon S, et al. Divergent host responses
during primary simian immunodeficiency virus SIVsm infection of
natural sooty mangabey and nonnatural rhesus macaque hosts. J
8. Haigwood N. Predictive value of primate models for AIDS. AIDS
9. Bohm R, Martin L, Davison-Fairburn B, et al. Neonatal disease in-
duced by SIV infection of the rhesus monkey (Macaca mulatta).
AIDS Res Hum Retroviruses 1993;9:1131-7.
10. Marthas M, Van Rompay K, Otsyula M, et al. Viral factors determine
progression to AIDS in simian immunodeficiency virus-infected new-
born rhesus macaques. J Virol 1995;69:4198-205.
11. Carlsson H, Schapiro S, Farah I, et al. Use of primates in research:
a global overview. Am J Primatol 2004;63:225-37.
12. Desrosiers R. The simian immunodeficiency viruses. Ann Rev Im-
munol 1990; 8:557-78.
13. Black P, Downs M, Lewis M, et al. Antiretroviral activities of protease
inhibitors against murine leukemia virus and simian immunodeficiency
virus in culture. Antimicrob Agents Chemother 1993;37:71-7.
14. Sager P, Cradock J, Litterst C, et al. In vitro testing of therapeutics
against SIV and HIV. Ann NY Acad Sci 1990;616:599-605.
15. Giuffre A, Higgins J, Buckheit R, Jr., et al. Susceptibilities of simian
immunodeficiency virus to protease inhibitors. Antimicrob Agents
16. Hazuda D, Young S, Guare J, et al. Integrase inhibitors and cellular
immunity suppress retroviral replication in rhesus macaques. Sci-
17. Veazey R, Klasse P, Ketas T, et al. Use of a small molecule CCR5
inhibitor in macaques to treat simian immunodeficiency virus infec-
tion or prevent simian-human immunodeficiency virus infection. J
Exp Med 2003;198:1551-62.
18. De Clercq E. HIV inhibitors targeted at the reverse transcriptase.
AIDS Res Hum Retroviruses 1992;8:119-34.
19. Überla K, Stahl-Hennig C, Böttiger D, et al. Animal model for the
therapy of acquired immunodeficiency syndrome with reverse tran-
scriptase inhibitors. Proc Natl Acad Sci USA 1995;92:8210-4.
20. Balzarini J, De Clercq E, Überla K. SIV/HIV-1 hybrid virus expressing
the reverse transcriptase gene of HIV-1 remains sensitive to HIV-1-
specific reverse transcriptase inhibitors after passage in rhesus ma-
caques. J Acquir Immune Defic Syndr Human Retrovirol 1997;15:1-4.
21. Mori K, Yasumoti Y, Sawada S, et al. Suppression of acute viremia
by short-term postexposure prophylaxis of simian/human immuno-
deficiency virus SHIV-RT-infected monkeys with a novel reverse
transcriptase inhibitor (GW420867) allows for development of po-
tent antiviral immune responses resulting in efficient containment of
infection. J Virol 2000;74:5747-53.
22. Zuber B, Böttiger D, Benthin R, et al. An in vivo model for HIV resis-
tance development. AIDS Res Hum Retroviruses 2001;17:631-5.
23. Hofman M, Higgins J, Matthews T, et al. Efavirenz therapy in rhesus
macaques infected with a chimera of simian immunodeficiency virus
containing reverse transcriptase from HIV type 1. Antimicrob Agents
24. Nishimura Y, Igarashi T, Donau O, et al. Highly pathogenic SHIVs and SIVs
target different CD4+ T cell subsets in rhesus monkeys, explaining their
divergent clinical courses. Proc Natl Acad Sci USA 2004;101:12324-9.
25. Harouse J, Gettie A, Tan R, et al. Distinct pathogenic sequela in
rhesus macaques infected with CCR5 or CXCR4 utilizing SHIVs.
26. Feinberg M, Moore J. AIDS vaccine models: challenging challenge
viruses. Nature Med 2002;8:207-10.
27. Lifson J, Martin M. One step forwards, one step back. Nature
28. Van Rompay K, Singh R, Marthas M. SIV as a model for AIDS drug
studies; In: M. Bendinelli, S. C. Specter and H. Friedman, editors.
Animal models of HIV disease and control. New York: Kluwer Aca-
demic/Plenum Publishers; 2005; in press.
29. Van Rompay K, Otsyula M, Marthas M, et al. Immediate zidovudine
treatment protects simian immunodeficiency virus-infected newborn
macaques against rapid onset of AIDS. Antimicrob Agents Che-
30. McChesney M, Sawai E, Miller C. Simian immunodeficiency virus;
In: R. Ahmed and I. Chen, editors. Persistent viral infections. New
York: John Wiley & Sons, Ltd; 1998. p. 322-45.
31. Wingfield C, Booth J, Sheridan P, et al. SIV 4.0, performance charac-
teristics of an exceptionally sensitive, quantitative assay for SIV RNA
using branched DNA technology. 20th Annual Symposium on Nonhu-
man Primate Models for AIDS. 2002, Monterey, CA. Abstract 135.
32. Lifson J, Nowak M, Goldstein S, et al. The extent of early viral
replication is a critical determinant of the natural history of simian
immunodeficiency virus infection. J Virol 1997;71:9508-14.
33. Leutenegger C, Higgins J, Matthews T, et al. Real-time Taqman
PCR as a specific and more sensitive alternative to the branched-
chain DNA assay for quantitation of simian immunodeficiency virus
RNA. AIDS Res Hum Retroviruses 2001;17:243-51.
34. Van Rompay K, Cherrington J, Marthas M, et al. 9-[2-(Phosphono
methoxy)propyl]adenine therapy of established simian immunode-
AIDS Reviews 2005;7
ficiency virus infection in infant rhesus macaques. Antimicrob
Agents Chemother 1996;40:2586-91.
35. Van Rompay K, Singh R, Brignolo L, et al. The clinical benefits of te-
nofovir for simian immunodeficiency virus-infected macaques are
larger than predicted by its effects on standard viral and immunologi-
cal parameters. J Acquired Immune Defic Syndr 2004;36:900-14.
36. Tsai C, Follis K, Beck T, et al. Prevention of simian immunodefi-
ciency virus infection in macaques by 9-(2-phosphonylmethoxypro
pyl)adenine (PMPA). Science 1995;270:1197-9.
37. Deeks S, Barditch-Crovo P, Lietman P, et al. Safety, pharmacokinet-
ics and antiretroviral activity of intravenous 9-[2-(R)-(Phosphonome
thoxy)propyl]adenine, a novel anti-HIV therapy, in HIV-infected
adults. Antimicrob Agents Chemother 1998;42:2380-4.
38. Barditch-Crovo P, Deeks S, Collier A, et al. Phase I/II trial of the
pharmacokinetics, safety, and antiretroviral activity of tenofovir diso-
proxil fumarate in HIV-1 infected adults. Antimicrob Agents Che-
39. Schooley R, Ruane P, Myers R, et al. Tenofovir DF in antiretroviral-
experienced patients: results from a 48-week, randomized, double-
blind study. AIDS 2002;16:1257-63.
40. Ha J, Nosbisch C, Conrad S, et al. Fetal toxicity of zidovudine
(azidothymidine) in macaca nemestrina: preliminary observations.
J Acquired Immune Defic Syndr 1994;7:154-7.
41. Lopez-Anaya A, Unadkat J, Schumann L, et al. Pharmacokinetics
of zidovudine (Azidothymidine). III. Effect of pregnancy. J Acquir
Immune Defic Syndr 1991;4:64-8.
42. Odineces A, Pereira C, Nosbisch C, et al. Prenatal and postpartum
pharmacokinetics of stavudine (2’,3’-didehydro-3’-deoxythymidine)
and didanosine (dideoxyinosine) in pigtailed macaques (Macaca
nemestrina). Antimicrob Agents Chemother 1996;40:2423-5.
43. Pereira C, Nosbisch C, Baughman W, et al. Effect of zidovudine
on transplacental pharmacokinetics of ddI in the pigtailed ma-
caque (Macaca nemestrina). Antimicrob Agents Chemother 1995;
44. Ravasco R, Unadkat J, Tsai C, et al. Pharmacokinetics of dideoxy-
inosine in pigtailed macaques (Macaca nemestrina) after intrave-
nous and subcutaneous administration. J Acquired Immune Defic
45. Tuntland T, Nosbisch C, Baughman W, et al. Mechanism and rate
of placental transfer of zalcitabine (2’,3’ - dideoxycytidine) in Ma-
caca nemestrina. Am J Obstet Gynecol 1996; 174:856-63.
46. Van Rompay K, Hamilton M, Kearney B, et al. Pharmacokinetics of
tenofovir in breast milk of lactating rhesus macaques. Antimicrob
Agents Chemother 2005;49:2093-4.
47. Van Rompay K, Brignolo L, Meyer D, et al. Biological effects of
short-term and prolonged administration of 9-[2-(phosphonometho
xy)propyl]adenine (PMPA; tenofovir) to newborn and infant rhesus
macaques. Antimicrob Agents Chemother 2004;48:1469-87.
48. Grob P, Cao Y, Muchmore E, et al. Prophylaxis against HIV-1 infec-
tion in chimpanzees by nevirapine, a nonnucleoside inhibitor of
reverse transcriptase. Nature Med 1997;3:665-70.
49. Van Rompay K, McChesney M, Aguirre N, et al. Two low doses of
tenofovir protect newborn macaques against oral simian immuno-
deficiency virus infection. J Infect Dis 2001;184:429-38.
50. Miller C, Rosenberg Z, Bischofberger N. Use of topical PMPA to prevent
vaginal transmission of SIV. Ninth International Conference on Antiviral
Research. 1996, Fukushima, May 19-24.
51. Manson K, Wyand M, Miller C, et al. Effect of a cellulose acetate
phtalate topical cream on vaginal transmission of simian immuno-
deficiency virus in rhesus monkeys. Antimicrob Agents Chemother
52. Weber J, Nunn A, O’Connor T, et al. ‘Chemical condoms’ for the
prevention of HIV infection: evaluation of novel agents against
SHIV98.6PD in vitro and in vivo. AIDS 2001;15:1563-8.
53. Lederman M, Veazey R, Offord R, et al. Prevention of vaginal SHIV
transmission in rhesus macaques through inhibition of CCR5. Sci-
54. Tsai C, Emau P, Jiang Y, et al. Cyanovirin-N inhibits AIDS virus
infections in vaginal transmission models. AIDS Res Hum Retrovi-
55. Tsai C, Emau P, Jiang Y, et al. Cyanovirin-N gel as a topical micro-
bicide prevents rectal transmission of SHIV89.6P in macaques.
AIDS Res Hum Retroviruses 2003;19:535-41.
56. Wyand M, Manson K, Miller C, et al. Effect of 3-hydroxyphthaloyl-beta-
lactoglobulin on vaginal transmission of simian immunodeficiency virus
in rhesus monkeys. Antimicrob Agents Chemother 1999;43:978-80.
57. Wyand M. The use of SIV-infected rhesus monkeys for the pre-
clinical evaluation of AIDS drugs and vaccines. AIDS Res Hum
58. Fazely F, Haseltine W, Rodger R, et al. Postexposure chemopro-
phylaxis with ZDV or ZDV combined with interferon-α: failure after
inoculating rhesus monkeys with a high dose of SIV. J Acquir Im-
mune Defic Syndr 1991;4:1093-7.
59. Lundgren B, Bottiger D, Ljungdahl-Ståhle E, et al. Antiviral effects
of 3’-fluorothymidine and 3’-azidothymidine in cynomolgus monkeys
infected with simian immunodeficiency virus. J Acquir Immune De-
fic Syndr 1991;4:489-98.
60. McClure H, Anderson D, Ansari A, et al. Nonhuman primate mod-
els for evaluation of AIDS therapy. Ann N Y Acad Sci 1990;
61. Black R. Animal studies of prophylaxis. Am J Med 1997;102
62. Van Rompay K, Marthas M, Ramos R, et al. Simian immunodefi-
ciency virus (SIV) infection of infant rhesus macaques as a model
to test antiretroviral drug prophylaxis and therapy: oral 3’-azido-3’-
deoxythymidine prevents SIV infection. Antimicrob Agents Che-
63. Van Rompay K, Marthas M, Lifson J, et al. Administration of 9-[2-(
phosphonomethoxy)propyl]adenine (PMPA) for prevention of peri-
natal simian immunodeficiency virus infection in rhesus macaques.
AIDS Res Hum Retroviruses 1998;14:761-73.
64. Van Rompay K, Berardi C, Aguirre N, et al. Two doses of PMPA
protect newborn macaques against oral simian immunodeficiency
virus infection. AIDS 1998;12:F79-F83.
65. Böttiger D, Putkonen P, Öberg B. Prevention of HIV-2 and SIV infec-
tions in cynomolgus macaques by prophylactic treatment with 3’-
fluorothymidine. AIDS Res Hum Retrovir 1992;8:1235-8.
66. Böttiger D, Vrang L, Öberg B. Influence of the infectious dose of
simian immunodeficiency virus on the acute infection in cynomolgus
monkeys and on the effect of treatment with 3’-fluorothymidine.
Antivir Chem Chemother 1992;3:267-71.
67. Tsai C, Follis K, Sabo A, et al. Preexposure prophylaxis with 9-(-2-
phosphonylmethoxyethyl)adenine against simian immunodeficiency
virus infection in macaques. J Infect Dis 1994;169:260-6.
68. Subbarao S, Otten R, Ramos A, et al. Chemoprophylaxis with oral
tenofovir disoproxil fumarate (TDF) delays but does not prevent
infection in rhesus macaques given repeated rectal challenges of
SHIV. 12th Conference on Retroviruses and Opportunistic Infec-
tions. 2005, Boston, Massachusetts. Abstract 136LB.
69. Van Rompay K, Lawson J, Colón R, et al. Oral tenofovir DF protects
infant macaques against infection following repeated low-dose
oral exposure to virulent simian immunodeficiency virus. XV Inter-
national AIDS Conference. 2004, Bangkok. (LbOrB10).
70. Böttiger D, Johansson N, Samuelsson B, et al. Prevention of simian
immunodeficiency virus, SIVsm, or HIV-2 infection in cynomolgus
monkeys by pre- and postexposure administration of BEA-005.
71. Otten R, Smith D, Adams D, et al. Efficacy of postexposure prophy-
laxis after intravaginal exposure of pig-tailed macaques to a human-
derived retrovirus (HIV type 2). J Virol 2000;74:9771-5.
72. Tsai C, Emau P, Follis K, et al. Effectiveness of postinoculation (R)-
9-(2-phosphonylmethoxypropyl)adenine treatment for prevention of
persistent simian immunodeficiency virus SIVmne infection depends
critically on timing of initiation and duration of treatment. J Virol
73. Centers for Disease Control and Prevention. Update: provisional
Public Health Service recommendations for chemoprophylaxis after
occupational exposure to HIV. MMWR 1996;45:468-72.
74. Centers for Disease Control and Prevention. Antiretroviral postex-
posure prophylaxis after sexual, injection-drug use, or other nonoc-
Koen KA Van Rompay: Primate Models for Anti-HIV Drug Studies
cupational exposure to HIV in the United States: recommendations
from the U.S. Department of Health and Human Services. MMWR
75. Puro V, Calcagno G, Anselmo M, et al. Transient detection of plas-
ma HIV-1 RNA during postexposure prophylaxis. Infect Control
Hosp Epidemiol 2000;21:529-31.
76. AIDS Vaccine Advocacy Coalition. Will a pill a day prevent HIV?
Anticipating the results of the tenofovir “PREP” trials. (www.avac.
org). March 2005
77. Gaillard P, Fowler M, Dabis F, et al. Use of antiretroviral drugs to
prevent HIV-1 transmission through breastfeeding: from animal
studies to randomized clinical trials. J Acquired Immune Defic
78. Connor E, Sperling R, Gelber R, et al. Reduction of maternal-infant
transmission of HIV type 1 with zidovudine treatment. N Engl J Med
79. Guay L, Musoke P, Fleming T, et al. Intrapartum and neonatal sin-
gle-dose nevirapine compared with zidovudine for prevention of
mother-to-child transmission of HIV-1 in Kampala, Uganda: HIVNET
012 randomized trial. Lancet 1999;354:795-802.
80. Eshleman S, Mracna M, Guay L, et al. Selection and fading of resis-
tance mutations in women and infants receiving nevirapine to prevent
HIV-1 vertical transmission (HIVNET012). AIDS 2001; 15:1951-7.
81. Martin L, Murphey-Corb M, Soike K, et al. Effects of initiation of
3’-azido-3’-deoxythymidine treatment at different times after infec-
tion of rhesus monkeys with simian immunodeficiency virus. J Infect
82. Tsai C, Follis K, Grant R, et al. Effect of dosing frequency on ZDV
prophylaxis in macaques infected with simian immunodeficiency
virus. J Acquired Immune Defic Syndr 1993;6:1086-92.
83. Joag S, Li Z, Foresman L, et al. Early treatment with 9-(2-phospho-
nylmethoxyethyl) adenine reduces virus burdens for a prolonged
period in SIV-infected rhesus macaques. AIDS Res Hum Retroviru-
84. Le Grand R, Clayette P, Noack O, et al. An animal model for anti-
lentiviral therapy: effect of zidovudine on viral load during acute
infection after exposure of macaques to simian immunodeficiency
virus. AIDS Res Hum Retroviruses 1994;10:1279-87.
85. Le Grand R, Vaslin B, Larghero J, et al. Post-exposure prophylaxis
with highly active antiretroviral therapy could not protect macaques
from infection with SIV/HIV chimera. AIDS 2000;14:1864-6.
86. Van Rompay K, Dailey P, Tarara R, et al. Early short-term 9-[2-(phos-
phonomethoxy) propyl] adenine (PMPA) treatment favorably alters
subsequent disease course in simian immunodeficiency virus-in-
fected newborn rhesus macaques. J Virol 1999;73:2947-55.
87. Watson A, McClure J, Ranchalis J, et al. Early postinfection antiviral
treatment reduces viral load and prevents CD4+ cell decline in HIV
type 2-infected macaques. AIDS Res Hum Retroviruses 1997;
88. Rausch D, Heyes M, Murray E, et al. Zidovudine treatment prolongs
survival and decreases virus load in the central nervous system of
rhesus macaques infected perinatally with simian immunodeficien-
cy virus. J Infect Dis 1995;172:59-69.
89. Rosenwirth B, ten Haaft P, Bogers W, et al. Antiretroviral therapy
during primary immunodeficiency virus infection can induce persis-
tent suppression of virus load and protection from heterologous
challenge in rhesus macaques. J Virol 2000;74:1704-11.
90. Smith M, Foresman L, Lopez G, et al. Lasting effects of transient
postinoculation tenofovir [9-R-(2-phosphonomethoxypropyl)adenine
] treatment of SHIVKU2 infection of rhesus macaques. Virology
91. Hodge S, De Rosayro J, Glenn A, et al. Postinoculation PMPA
treatment, but not preinoculation immunomodulatory therapy,
protects against development of acute disease induced by the
unique simian immunodeficiency virus SIVsmmPBj. J Virol 1999;
92. Lifson J, Rossio J, Arnaout R, et al. Containment of simian immu-
nodeficiency virus infection: cellular immune responses and protec-
tion from rechallenge following transient postinoculation antiretrovi-
ral treatment. J Virol 2000;74:2584-93.
93. Lori F, Gallo R, Malykh A, et al. Didanosine but not high doses of
hydroxyurea rescue pigtail macaque from a lethal dose of SIVsmmp-
bj14. AIDS Res Hum Retroviruses 1997;13:1083-8.
94. Spring M, Stahl-Hennig C, Stolte N, et al. Enhanced cellular immune
responses and reduced CD8+ lymphocytes apoptosis in acutely
SIV-infected rhesus macaques after short-term antiretroviral treat-
ment. Virology 2001;279:221-31.
95. Hecht F, Wang L, Collier A, et al. Outcomes of HAART for acute/
early HIV-1 infection after treatment discontinuation. Program and
abstracts of the 12th Conference on Retroviruses and Opportunis-
tic Infections, February 22-25. 2005, Boston, Massachusetts. Ab-
96. Berrey M, Schacker T, Collier A, et al. Treatment of primary HIV
type 1 infection with potent antiretroviral therapy reduces frequency
of rapid progression to AIDS. J Infect Dis 2001;183:1466-75.
97. Kinloch-de Loës S, Hirschel B, Hoen B, et al. A controlled trial of zi-
dovudine in primary HIV infection. N Engl J Med 1995;333:408-13.
98. Lafeuillade A, Poggi C, Tamalet C, et al. Effects of a combination of
zidovudine, didanosine, and lamivudine on primary HIV type 1 infec-
tion. J Infect Dis 1997;175:1051-5.
99. Rosenberg E, Altfeld M, Poon S, et al. Immune control of HIV-1
after early treatment of acute infection. Nature 2000;407:523-6.
100. Kassutto S, Rosenberg ES. Primary HIV type 1 infection. Clin Infect
101. Igarashi T, Brown C, Endo Y, et al. Macrophage are the principal
reservoir and sustain high virus loads in rhesus macaques after the
depletion of CD4+ T cells by a highly pathogenic simian immuno-
deficiency virus/HIV type 1 chimera (SHIV): implications for HIV-1
infections of humans. Proc Natl Acad Sci USA 2001;98:658-63.
102. Hel Z, Venzon D, Poudyal M, et al. Viremia control following antiret-
roviral treatment and therapeutic immunization during primary
SIV251 infection of macaques. Nature Med 2000;6:1140-6.
103. Lori F, Lisziewicz J. Structured treatment interruptions for the man-
agement of HIV infection. JAMA 2001;286:2981-7.
104. Markowitz M, Jin X, Hurley A, et al. Discontinuation of antiretroviral
therapy commenced early during the course of human HIV type 1
infection, with or without adjunctive vaccination. J Infect Dis
105. Daar E, Bai J, Hausner M, et al. Acute HIV syndrome after discon-
tinuation of antiretroviral therapy in a patient treated before sero-
conversion. Ann Int Med 1998;128:827-9.
106. Böttiger D, Ståhle L, Li S, et al. Long-term tolerance and efficacy
of 3’-azido-thymidine and 3’-fluorothymidine treatment of asymp-
tomatic monkeys infected with simian immunodeficiency virus. An-
timicrob Agents Chemother 1992;36:1770-2.
107. Van Rompay K, Greenier J, Marthas M, et al. A zidovudine-resistant
simian immunodeficiency virus mutant with a Q151M mutation in
reverse transcriptase causes AIDS in newborn macaques. Antimi-
crob Agents Chemother 1997;41:278-83.
108. Van Rompay K, Matthews T, Higgins J, et al. Virulence and reduced
fitness of simian immunodeficiency virus with the M184V mutation
in reverse transcriptase. J Virol 2002;76:6083-92.
109. Tsai C, Follis K, Beck T, et al. Effects of (R)-9-(2-phosphonylme-
thoxypropyl) adenine monotherapy on chronic SIV infection. AIDS
Res Hum Retroviruses 1997;13:707-12.
110. Tsai C, Follis K, Sabo A, et al. Efficacy of 9-(2-phosphonylmethoxy
ethyl)adenine treatment against chronic simian immunodeficiency
virus infection in macaques. J Infect Dis 1995;171:1338-43.
111. Nowak M, Lloyd A, Vasquez G, et al. Viral dynamics of primary
viremia and antiretroviral therapy in simian immunodeficiency virus
infection. J Virol 1997;71:7518-25.
112. Silvera P, Racz P, Racz K, et al. Effect of PMPA and PMEA on the
kinetics of viral load in simian immunodeficiency virus-infected ma-
caques. AIDS Res Hum Retroviruses 2000;16:791-800.
113. Van Rompay K, Singh R, Pahar B, et al. CD8+ cell-mediated sup-
pression of virulent simian immunodeficiency virus during tenofovir
treatment. J Virol 2004;78:5324-37.
114. Magierowska M, Bernardin F, Garg S, et al. Highly uneven distribu-
tion of PMPA selected SIV drug resistance genotypes in different
anatomical sites of rhesus macaques. J Virol 2004;78:2434-44.
AIDS Reviews 2005;7
115. Smit-McBride Z, Mattapallil J, Villinger F, et al. Intracellular cytokine
expression in the CD4+ and CD8+ T cells from intestinal mucosa
of simian immunodeficiency virus infected macaques. J Med Prima-
116. Veazy R, DeMaria M, Chalifoux L, et al. Gastrointestinal tract as a
major site of CD4+ T cell depletion and viral replication in SIV infec-
tion. Science 1998; 280:427-31.
117. Heise C, Miller C, Lackner A, et al. Primary acute simian immuno-
deficiency virus infection of intestinal lymphoid tissue is associated
with gastrointestinal dysfunction. J Infect Dis 1994;169:1116-20.
118. Mattapallil J, Smit-McBride Z, Dailey P, et al. Activated memory
CD4+ T helper cells repopulate the intestine early following antiret-
roviral therapy of simian immunodeficiency virus-infected rhesus
macaques but exhibit a decreased potential to produce interleukin-
2. J Virol 1999;73:6661-9.
119. George M, Reay E, Sankaran S, et al. Early antiretroviral therapy for
simian immunodeficiency virus infection leads to mucosal CD4+
T-cell restoration and enhanced gene expression regulating muco-
sal repair and regeneration. J Virol 2005;79:2709-19.
120. Shen Y, Shen L, Sehgal P, et al. Antiretroviral agents restore myco-
bacterium-specific T-cell immune responses and facilitate control-
ling a fatal tuberculosis-like disease in macaques coinfected with
simian immunodeficiency virus and Mycobacterium bovis BCG. J
121. Shen A, Zink M, Mankowski J, et al. Resting CD4+ T lymphocytes
but not thymocytes provide a latent viral reservoir in a simian im-
munodeficiency virus-Macaca nemestrina model of HIV type 1 in-
fected patients on highly active antiretroviral therapy. J Virol
122. North T, Van Rompay K, Higgins J, et al. Suppression of virus load
by highly active antiretroviral therapy in rhesus macaques infected
with a recombinant simian immunodeficiency virus containing re-
verse transcriptase from HIV type 1. J Virol 2005;79:7349-54.
123. Lori F, Lewis M, Xu J, et al. Control of SIV rebound through struc-
tured treatment interruptions during early infection. Science
124. Villinger F, Brice G, Mayne A, et al. Adoptive transfer of simian
immunodeficiency virus (SIV) naive autologous CD4+ T cells to
macaques chronically infected with SIV is sufficient to induce long-
term nonprogressor status. Blood 2002;99:590-9.
125. Hel Z, Nacsa J, Kelsall B, et al. Impairment of gag-specific CD8+
T-cell function in mucosal and systemic compartments of simian
immunodeficiency virus mac251-and simian-human immunodefi-
ciency virus KU2-infected macaques. J Virol 2001;75:11483-95.
126. Tryniszewska E, Nacsa J, Lewis M, et al. Vaccination of macaques with
long-standing SIVmac251 infection lowers the viral set point after ces-
sation of antiretroviral therapy. J Immunol 2002;169:5347-57.
127. Hel Z, Nacsa J, Tsai W, et al. Equivalent immunogenicity of the
highly attenuated poxvirus-based ALVAC-SIV and NYVAC-SIV vac-
cine candidates in SIVmac251-infected macaques. Virology
128. Nacsa J, Stanton J, Kunstman K, et al. Emergence of cytotoxic T
lymphocyte escape mutants following antiretroviral treatment sus-
pension in rhesus macaques infected with SIVmac251. Virology
129. Boyer J, Nath B, Schumann K, et al. IL-4 increases simian immuno-
deficiency virus replication despite enhanced SIV immune responses
in infected rhesus macaques. Int J Parasitol 2002;32:543-50.
130. Lisziewicz J, Trocio J, Xu J, et al. Control of viral rebound through
therapeutic immunization with DermaVir. AIDS 2005;19:35-43.
131. Richman D. HIV chemotherapy. Nature 2001;410:995-1001.
132. Bangsberg D, Moss A, Deeks S. Paradoxes of adherence and drug
resistance to HIV antiretroviral therapy. J Antimicrob Chemother
133. Darby G, Larder B. The clinical significance of antiviral drug resis-
tance. Res Virol 1992;143:116-20.
134. Richman D. The clinical significance of drug-resistant mutants of
HIV. Res Virol 1992;143:130-1.
135. Richman D. Resistance, drug failure, and disease progression.
AIDS Res Hum Retroviruses 1994;10:901-5.
136. Deeks S, Hoh R, Grant R, et al. CD4+ T cell kinetics and activation
in HIV-infected patients who remain viremic despite long-term treat-
ment with protease inhibitor-based therapy. J Infect Dis 2002;
137. Deeks S, Barbour J, Martin J, et al. Sustained CD4+ T cell response
after virologic failure of protease inhibitor-based regimens in pa-
tients with HIV infection. J Infect Dis 2000;181:946-53.
138. Barbour J, Wrin T, Grant R, et al. Evolution of phenotypic drug
susceptibility and viral replication capacity during long-term viro-
logic failure of protease inhibitor therapy in HIV-infected adults. J
139. Monpoux F, Tricoire J, Lalande M, et al. Treatment interruption for
virologic failure or as sparing regimen in children with chronic HIV-
1 infection. AIDS 2004;18:2401-9.
140. Bélec L, Piketty C, Si-Mohamed A, et al. High levels of drug-resis-
tant HIV variants in patients exhibiting increasing CD4+ T counts
despite virologic failure of protease inhibitor-containing antiretroviral
combination therapy. J Infect Dis 2000;181:1808-12.
141. Miller V, Phillips A, Clotet B, et al. Association of virus load, CD4
cell count, and treatment with clinical progression in HIV-in-
fected patients with very low CD4 cell counts. J Infect Dis 2002;
142. Mezzaroma I, Carlesimo M, Pinter E, et al. Clinical and immuno-
logic responses without decrease in virus load in patients after 24
months of highly active antiretroviral therapy. Clin Infect Dis 1999;
143. Deeks S, Martin J, Sinclair E, et al. Strong cell-mediated immune
responses are associated with the maintenance of low-level viremia
in antiretroviral-treated individuals with drug-resistant HIV type 1.
J Infect Dis 2004;189:312-21.
144. Deeks S, Wrin T, Liegler T, et al. Virologic and immunologic conse-
quences of discontinuing combination antiretroviral-drug therapy in
HIV-infected patients with detectable viremia. N Engl J Med 2001;
145. Deeks S. Durable HIV treatment benefit despite low-level viremia.
Reassessing definitions of success or failure. JAMA 2001;
146. Hunt P, Martin J, Sinclear E, et al. Drug-resistant phenotype is as-
sociated with decreased in vivo T-cell activation independent of
changes in viral replication among patients discontinuing antiretro-
viral therapy. Antiviral Therapy 2003;8:S82.
147. Borda J, Alvarez X, Kondova I, et al. Cell tropism of simian immu-
nodeficiency virus in culture is not predictive of in vivo tropism or
pathogenesis. Amer J Pathol 2004;165:2111-22.
148. Kestler H, III, Ringler D, Mori K, et al. Importance of the nef gene
for maintenance of high virus loads and for development of AIDS.
149. Lohman B, McChesney M, Miller C, et al. A partially attenuated
simian immunodeficiency virus induces host immunity that corre-
lates with resistance to pathogenic virus challenge. J Virol
150. Hirsch M, Brun-Vezinet F, Clotet B, et al. Antiretroviral drug resis-
tance testing in adults infected with HIV type 1: 2003 recommenda-
tions of an International AIDS Society-USA Panel. Clin Infect Dis
151. Gandhi R, Wurcel A, Rosenberg E, et al. Progressive reversion of
HIV type 1 resistance mutations in vivo after transmission of a
multiply drug-resistant virus. Clin Infect Dis 2003;37:1693-8.
152. Izopet J, Souyris C, Hance A, et al. Evolution of HIV type 1
populations after resumption of therapy following treatment inter-
ruption and shift in resistance genotype. J Infect Dis 2002;
153. Cherry E, Slater M, Salomon H, et al. Mutations at codon 184 in
simian immunodeficiency virus reverse transcriptase confer resis-
tance to the (-) enantiomer of 2’,3’-dideoxy-thiacytidine. Antimicrob
Agents Chemother 1997;41:2763-5.
154. Murry J, Higgins J, Matthews T, et al. Reversion of the M184V
mutation in simian immunodeficiency virus reverse transcriptase is
selected by tenofovir, even in the presence of lamivudine. J Virol
Koen KA Van Rompay: Primate Models for Anti-HIV Drug Studies
155. Schmit J, Cogniaux J, Hermans P, et al. Multiple drug Resistance
to nucleoside analogues and non-nucleoside reverse transcriptase
inhibitors in an efficiently replicating HIV-1 patient strain. J Infect
156. Shafer R, Winters M, Iversen A, et al. Genotypic and phenotypic
changes during culture of a multinucleoside-resistant HIV type 1
strain in the presence and absence of additional reverse transcrip-
tase inhibitors. Antimicrob Agents Chemother 1996;40:2887-90.
157. Colson P, Henry M, Tivoli N, et al. Polymorphism and drug-selected
mutations in the reverse transcriptase gene of HIV-2 from patients
living in southern France. Antiviral Therapy 2003;8:S161.
158. Descamps D, Damond F, Matheron S, et al. High frequency of
selection of the Q151M mutation in HIV-2-infected patients receiving
nucleoside reverse transcriptase inhibitor-containing regimen. An-
tiviral Therapy 2003;8:S162.
159. Newstein M, Desrosiers R. Effects of reverse-transcriptase muta-
tions M184V and E89G on simian immunodeficiency virus in rhesus
monkeys. J Infect Dis 2001;184:1262-7.
160. Ruane P, Luber A. K65R-Associated Virologic Failure in HIV-Infected
Patients Receiving Tenofovir-Containing Triple Nucleoside/Nucleotide
Reverse Transcriptase Inhibitor Regimens. MedGenMed 2004;6:31.
161. Valer L, Martín-Carbonero L, de Mendoza C, et al. Predictors of
selection of K65R: tenofovir use and lack of thymidine analogue
mutations. AIDS 2004;18:2094-6.
162. Margot N, Isaacson E, McGowan I, et al. Extended treatment with
tenofovir disoproxil fumarate in treatment-experienced HIV-1-in-
fected patients: genotypic, phenotypic, and rebound analyses. J
Acquired Immune Defic Syndr 2003;33:15-21.
163. Van Rompay K, Cherrington J, Marthas M, et al. 9-[2-(Phosphonometh
oxy)propyl]adenine (PMPA) therapy prolongs survival of infant ma-
caques inoculated with simian immunodeficiency virus with reduced
susceptibility to PMPA. Antimicrob Agents Chemother 1999;43:802-12.
164. Perelson A. Modelling viral and immune system dynamics. Nature
Immunology Reviews 2002;2:28-36.
165. Wodarz D, Nowak M. Mathematical models of HIV pathogenesis
and treatment. Bioessays 2002;24:1178-87.
166. Bonhoeffer S, May R, Shaw G, et al. Virus dynamics and drug
therapy. Proc Natl Acad Sci USA 1997;94:6971-6.
167. Buseyne F, Lechenadec J, Burgard M, et al. Gag-specific memory
CTL responses and antiretroviral drugs act in synergy to control HIV-
1 replication in infected children. 11th Conference on Retroviruses
and Opportunistic Infections. 2004, San Francisco. Abstract 217.
168. Alatrakchi N, Duvivier C, Costagliola D, et al. Persistent low viral
load on antiretroviral therapy is associated with T cell-mediated
control of HIV replication. AIDS 2005;19:25-33.
169. Cohen Stuart J, Wensing A, Kovacs C, et al. Transient relapses
(“blips”) of plasma HIV RNA levels during HAART are associated with
drug resistance. J Acquir Immune Defic Syndr 2001;28:105-13.
170. Di Mascio M, Markowitz M, Louie M, et al. Viral blip dynamics dur-
ing highly active antiretroviral therapy. J Virol 2003;77:12165-72.
171. Blankson J, Persaud D, Siliciano R. The challenge of viral reservoirs
in HIV-1 infection. Annu Rev Med 2002;53:557-93.
172. McKay P, Barouch D, Schmitz J, et al. Global dysfunction of CD4
T-lymphocyte cytokine expression in simian-human immunodefi-
ciency virus/SIV-infected monkeys is prevented by vaccination. J
173. Zimmer M, Larregina A, Castillo C, et al. Disrupted homeostasis of
Langerhans cells and interdigitating dendritic cells in monkeys with
AIDS. Blood 2002; 99:2859-68.
174. Kaech S and Ahmed R. CD8 T cells remember with a little help.
175. Lieberman J, Shankar P, Manjunath N, et al. Dressed to kill? A re-
view of why antiviral CD8 T lymphocytes fail to prevent progressive
immunodeficiency in HIV-1 infection. Blood 2001;98:1667-77.
176. McMichael A, Rowland-Jones S. Cellular immune responses to HIV.
177. Ghaffari G, Passalacqua D, Caicedo J, et al. Two-year clinical and
immune outcomes in HIV-infected children who reconstitute CD4 T
cells without control of viral replication after combination antiretroviral
therapy. Pediatrics 2004;114:e604-11.
178. Huang W, De Gruttola V, Fischl M, et al. Patterns of plasma HIV
type 1 RNA response to antiretroviral therapy. J Infect Dis 2001;
179. Haverkamp M, Smit M, Weersink A, et al. The effect of lamivudine on
the replication of hepatitis B virus in HIV-infected patients depends
on the host immune status (CD4 cell count). AIDS 2003;17:1572-4.
180. Abel K, Alegria-Hartman M, Zanotto K, et al. Anatomic site and
immune function correlate with relative cytokine mRNA expression
levels in lymphoid tissues of normal rhesus macaques. Cytokine
181. Altfeld M, van Lunzen J, Frahm N, et al. Expansion of pre-existing,
lymph node-localized CD8+ T cells during supervised treatment inter-
ruptions in chronic HIV-1 infection. J Clin Invest 2002;109:837-43.
182. Pantaleo G, Koup R. Correlates of immune protection in HIV-1 infec-
tion: what we know, what we don’t know, what we should know. Nat
183. Betts M, Ambrozak D, Douek D, et al. Analysis of total HIV-specific
CD4+ and CD8+ T-cell responses: relationship to viral load in un-
treated HIV infection. J Virol 2001;75:11983-91.
184. Van Rompay K, Abel K, Lawson J, et al. Attenuated poxvirus-based
SIV vaccines given in infancy partially protect infant and juvenile
macaques against repeated oral challenge with virulent SIV. J Ac-
quired Immune Defic Syndr 2005;38:124-34.
185. Frost S, Nijhuis M, Schuurman R, et al. Evolution of lamivudine resis-
tance in HIV type 1-infected individuals: the relative roles of drift and
selection. J Virol 2000;74:6262-8.
186. Deeks S, Barbour J, Grant R, et al. Duration and predictors of CD4
T-cell gains in patients who continue combination therapy despite
detectable plasma viremia. AIDS 2002;16:201-7.
187. Renaud M, Katlama C, Mallet A, et al. Determinants of paradoxical
CD4 cell reconstitution after protease inhibitor-containing antiretro-
viral regimen. AIDS 1999;13:669-76.
188. Ledergerber B, Egger M, Opravil M, et al. Clinical progression and
virological failure on highly active antiretroviral therapy in HIV-1
patients: a prospective cohort study. Lancet 1999;353:863-8.
189. Sloand E, Kumar P, Kim S, et al. HIV type 1 protease inhibitor
modulates activation of peripheral blood CD4+ T cells and de-
creases their susceptibility to apoptosis in vitro and in vivo. Blood
190. Price D, Scullard G, Oxenius A, et al. Discordant outcomes following
failure of antiretroviral therapy are associated with substantial differ-
ences in HIV-specific cellular immunity. J Virol 2003;77:6041-9.
191. Atzori C, Angeli E, Mainini A, et al. In vitro activity of HIV prote-
ase inhibitors against pneumocystis carinii. J Infect Dis 2000;
192. Cassone A, Tacconelli E, De Bernardis F, et al. Antiretroviral ther-
apy with protease inhibitors has an early, immune reconstitution-
independent beneficial effect on Candida virulence and oral can-
didiasis in HIV-infected subjects. J Infect Dis 2002;185:188-95.
193. Zídek Z, Frankova D, Holy A. Activation by 9-(R)-[2-(phosphonome-
thoxy) propyl] adenine of chemokine (RANTES, macrophage inflam-
matory protein 1α) and cytokine (tumor necrosis factor alpha, inter-
leukin-10 [IL-10], IL-1β) production. Antimicrob Agents Chemother
194. Van Rompay K, Marthas M, Bischofberger N. Tenofovir primes
rhesus macaque cells in vitro for enhanced interleukin-12 secretion.
Antiviral Res 2004;63:133-8.
195. Lu W, Wu X, Lu Y, et al. Therapeutic dendritic-cell vaccine for sim-
ian AIDS. Nature Med 2003;9:27-32.
196. Arnaout R, Nowak M, Wodarz D. HIV-1 dynamics revisited: biphasic
decay by cytotoxic T lymphocyte killing? Proc R Soc Lond