734 VOLUME 91 NUMBER 4 | APRIL 2012 | www.nature.com/cpt
nature publishing group
Human leukocyte antigen B (HLA-B) is responsible for
presenting peptides to immune cells and plays a critical role in
normal immune recognition of pathogens. A variant allele, HLA-
B*57:01, is associated with increased risk of a hypersensitivity
reaction to the anti-HIV drug abacavir. In the absence of genetic
prescreening, hypersensitivity affects ~6% of patients and can be
life-threatening with repeated dosing. We provide recommenda-
tions (updated periodically at http://www. pharmkgb.org) for the
use of abacavir based on HLA-B genotype.
The purpose of this guideline is to provide information that
will allow the interpretation of clinical HLA-B genotype tests
so that the results can be used to guide the use of abacavir for
the treatment of HIV. Detailed guidelines regarding selection of
appropriate antiretroviral therapy based on patient demograph-
ics and clinical measurements, viral resistance testing, and cost-
effectiveness analyses, are beyond the scope of this article but are
available at http://aidsinfo.nih.gov. Clinical Pharmacogenetics
Implementation Consortium (CPIC) guidelines are published
and updated periodically on http://www.pharmgkb.org to reflect
new developments in the field.
Focused Literature review
A systematic search of the literature focused on HLA-B genotype
and abacavir use (see Supplementary Data online); reviews1–4
were relied on to summarize much of the earlier literature.
HLA-B is a member of the major histocompatibility complex
(MHC) gene family located on chromosome 6, which consists
of class I, II, and III subgroups. The HLA-B gene product is
a class I HLA molecule that must heterodimerize with β-2
microglobulin to form a functional complex at the cell surface.5
HLA class I molecules are expressed on almost all cells and
are responsible for presenting peptides to immune cells. Cells
in the body are constantly producing new proteins, breaking
down old proteins, and recycling the breakdown products into
new proteins. However, some of these peptides are attached to
MHC molecules instead, and are trafficked to the cell surface.
In a typical cell, the peptides presented are the breakdown
products of normal proteins and are recognized by immune
cells as such (i.e., “self”). However, if a cell becomes infected by
a pathogen, some of the peptides presented will have resulted
from the breakdown of foreign proteins and will be recognized
as “non-self,” triggering an immune response against the anti-
gen. MHC molecules are also critical in the field of transplant
immunology, where careful HLA matching between donor
and recipient minimizes transplant rejection.6 In addition,
in rare cases, some pharmaceuticals are capable of producing
immune-mediated hypersensitivity reactions through interac-
tions with MHC molecules, although the exact mechanism of
these interactions remains unclear. Some suggest that these
drugs may function as haptens that irreversibly bind to the
peptides presented to immune cells, causing them to attack
the peptide-hapten conjugate.7 Another theory suggests that
these compounds might interact directly with MHC molecules
or T-cell receptors, leading to T-cell activation.8
Because of the need to present a wide variety of peptides for
immune recognition, HLA genes are both numerous and highly
polymorphic.9 Other than in identical twins, the probability is
extremely small that two individuals will be an exact HLA match
across all loci. More than 1,500 HLA-B alleles have been identi-
fied, but the guidelines we present here specifically discuss only
the HLA-B*57:01 allele as it relates to abacavir hypersensitivity
Clinical Pharmacogenetics Implementation
Consortium Guidelines for HLA-B Genotype and
MA Martin1, TE Klein2, BJ Dong3, M Pirmohamed4, DW Haas5–7 and DL Kroetz1
1Department of Bioengineering and Therapeutic Sciences, University of California, San Francisco, San Francisco, California, USA; 2Department of Genetics, Stanford
University, Stanford, California, USA; 3Department of Clinical Pharmacy, University of California, San Francisco, San Francisco, California, USA; 4Department of
Pharmacology, University of Liverpool, Liverpool, UK; 5Department of Medicine, Vanderbilt University, Nashville, Tennessee, USA; 6Department of Pharmacology,
Vanderbilt University, Nashville, Tennessee, USA; 7Department of Pathology, Microbiology, and Immunology, Vanderbilt University, Nashville, Tennessee, USA.
Correspondence: DL Kroetz (firstname.lastname@example.org)
Received 7 October 2011; accepted 15 December 2011; advance online publication 29 February 2012. doi:10.1038/clpt.2011.355
CliniCal pharmaCology & TherapeuTiCs | VOLUME 91 NUMBER 4 | APRIL 2012
Genetic test interpretation
Clinical genotyping tests are available to identify HLA-B alleles.
It is preferable to perform only specific tests for HLA-B*57:01
because more extensive HLA genotyping does not add clinically
useful information with regard to abacavir treatment. Unlike
many other pharmacogenetic associations, HLA-B allele status
has no effect on abacavir pharmacodynamics or pharmacoki-
netics; it only influences the likelihood that an HSR will occur.
Furthermore, given the codominant expression of HLA-B, geno-
typing results are either “positive” (HLA-B*57:01 being present
in one or both copies of the HLA-B gene) or “negative” (no
copies of HLA-B*57:01 are present), with no intermediate phe-
notype. The assignment of the likely HLA-B phenotype, based
on allele diplotypes, is summarized in Table 1. The prevalence
pattern of HLA-B alleles varies significantly by population, and
it has been extensively studied in geographically, racially, and
ethnically diverse groups (see Supplementary Tables S1 and S2
online). The frequency of the HLA-B*57:01 allele is lowest in
African and Asian populations and is totally absent in some
African populations as well as in the Japanese. In European
populations, this allele is relatively common, with a frequency
of 6–7%. The highest frequency of HLA-B*57:01 is reported in
Southwest Asian populations, where up to 20% of the popula-
tion are carriers.
available genetic test options
Several methods of HLA-B genotyping are commercially avail-
able. The Supplementary Data online and the Pharmacogenetic
Tests section of PharmGKB (http://pharmgkb.org/resources/
forScientificUsers/pharmacogenomic_tests.jsp) contain more
information on available clinical testing options.
Variations in HLA-B have been associated with several autoim-
mune conditions. For example, the presence of the HLA-B27
type is associated with development of ankylosing spondylitis,10
which commonly occurs alongside other inflammatory condi-
tions, including uveitis, psoriasis, and inflammatory bowel dis-
ease. Despite decades of research, the causative mechanism for
this is still unclear.
Several variants in HLA-B have been associated with other
adverse drug reaction phenotypes. Patients with the HLA-B*15:02
genotype are at increased risk of developing Stevens–Johnson
syndrome from treatment with carbamazepine,11 whereas HLA-
B*58:01 is associated with an increased risk of severe cutaneous
adverse reactions in response to allopurinol.12
In addition to abacavir HSR, HLA-B*57:01 has previously been
linked to flucloxacillin-induced liver injury.13 Although the rela-
tive risk of liver injury was more than 40 times greater in HLA-
B*57:01-positive patients than in HLA-B*57:01-negative ones,
the incidence of flucloxacillin hepatotoxicity is rare (<1 in 5,000),
significantly less than that of abacavir HSR; therefore routine
screening for HLA-B*57:01 is not done to assess susceptibility
to flucloxacillin-induced liver injury.
HLA-B*57:01 has also been shown to be overrepresented in
HIV long-term nonprogressors,14,15 the small group of HIV-
positive patients in whom, despite the absence of antiretroviral
therapy, the condition does not progress to AIDS. This suggests
that HLA-B*57:01 in some way confers a host immune response
that is better able to control the virus. In addition, HLA-B*57:01
has been associated with a lower viral load set point (i.e., the
amount of viral RNA detectable in blood during the asympto-
matic period of HIV) in Caucasians;16 similar associations, with
lower viral loads, have been observed in African Americans with
the closely related allele HLA-B*57:03.17
Abacavir is a nucleoside reverse transcriptase inhibitor indi-
cated for the treatment of HIV infection, in combination with
other medications, as part of highly active antiretroviral therapy.
Abacavir competitively inhibits viral reverse transcriptase, sup-
pressing HIV’s ability to convert its RNA genome into DNA
before insertion into a host cell’s genome. It is commercially
available as a single agent (Ziagen) or coformulated as a fixed-
dose combination with other nucleoside reverse transcriptase
inhibitors, lamivudine (Epzicom/Kivexa) and lamivudine/zido-
vudine (Trizivir). As compared with a tenofovir-based highly
active antiretroviral therapy regimen, an abacavir-based one
showed a significantly shorter time to virologic failure and also
a shorter time to first adverse event in patients with baseline
viral loads >100,000 copies/ml18 but showed no differences in
virologic failure rates in patients with lower baseline viral loads.
Abacavir received significant attention after the report of an asso-
ciation of the drug with an increased risk of myocardial infarc-
tion19 as compared with other nucleoside reverse transcriptase
inhibitors; however, subsequent analyses,20 including a meta-
analysis conducted by the US Food and Drug Administration
(FDA), have failed to show any such association.
Although abacavir is generally well tolerated, ~5–8% of
patients experience HSR during the first 6 weeks of treatment
if genetic prescreening is not performed. Symptoms of HSR
increase in severity over time if the drug is continued despite
the progressive symptoms. Symptoms of an HSR include at
least two of the following: fever, rash, gastrointestinal symp-
toms (e.g., nausea, vomiting, abdominal pain), fatigue, cough,
and dyspnea. Suspicion of an HSR warrants immediate discon-
tinuation of abacavir. If the symptoms of clinically diagnosed
table 1 assignment of likely HLa-B phenotypes based on
Very low risk of
~94%a of patients)
Absence of *57:01 alleles
(reported as “negative”
on a genotyping test)
High risk of hypersensitivity
(~6% of patients)
Presence of at least one
*57:01 allele (reported as
“positive” on a genotyping
HLA-B, human leukocyte antigen B.
aSee supplementary data online for estimates of genotype frequencies among
different ethnic/geographic groups. b*X = any HLA-B genotype other than *57:01.
736 VOLUME 91 NUMBER 4 | APRIL 2012 | www.nature.com/cpt
HSR resolve after discontinuation of abacavir, drug rechallenge
is contraindicated because immediate and life-threatening reac-
tions, including anaphylaxis and even fatalities, can occur.21 In
addition, an allergy to abacavir should be noted in the patient’s
medical record. Previous data have shown that peripheral blood
mononuclear cells from hypersensitive patients have a detect-
able immune response when cultured with abacavir in vitro,22,23
including increased expression of interferon-γ, tumor necrosis
factor-α, and other inflammatory cytokines, showing a clear role
of the immune system in mediating abacavir HSR.
Linking genetic variability to variability in drug-related
There is substantial evidence linking the presence of the
HLA-B*57:01 genotype with phenotypic variability (see
Supplementary Table S3 online). The application of a
grading system to the evidence linking genotypic variability to
phenotypic variability indicates a high quality of evidence in the
majority of cases (see Supplementary Table S3). The evidence
described below and in Supplementary Table S3 provides the
basis for the recommendations in Figure 1 and Table 2.
In 2002, two independent research groups reported the initial
association between HLA-B*57:01 and abacavir HSR24,25 using
cohort and case–control designs. The association was replicated
in a UK population in 2004.26 However, the results were not
broadly generalizable because the populations studied were pre-
dominantly white males. Nevertheless, given the strength of the
observed association, some centers began implementing pro-
spective screening of HLA-B*57:01 in all HIV-positive patients
to exclude HLA-B*57:01 positivity before starting abacavir.
This approach led to significant reductions in the incidence of
HSR.27–29 These studies, along with the retrospective SHAPE
study,30 found that HLA-B*57:01 was also predictive of HSR in
females and in African Americans.
Moreover, the results of PREDICT-1, the first double-blind,
prospective, randomized trial of a genetic test to reduce adverse
drug events, showed that genetic prescreening for HLA-B*57:01
resulted in no immunologically confirmed HSR events among
HLA-B*57:01-negative patients in the genetic testing arm,31 vs.
a 2.7% incidence of immunologically confirmed HSR among
842 unscreened patients in the standard-of-care control arm.
The results of PREDICT-1 and the existing body of evidence
prompted the FDA to implement a black box warning in 2008
about the high risk of HLA-B*57:01-associated HSR. The FDA
recommended that all patients be screened before being treated
with abacavir (including those who had previously tolerated the
drug and were being restarted on the therapy) and that abacavir
not be initiated in carriers of HLA-B*57:01. Abacavir is one of a
limited number of drugs for which the FDA has recommended
genetic testing prior to use, and it remains one of the best exam-
ples to date of pharmacogenetics being integrated into routine
We agree with others32–36 that HLA-B*57:01 screening should
be performed in all abacavir-naive individuals before initiation
of abacavir-containing therapy (see Table 2); this is consistent
with the recommendations of the FDA, the US Department
of Health and Human Services, and the European Medicines
Agency. In abacavir-naive individuals who are HLA-B*57:01-
positive, abacavir is not recommended and should be consid-
ered only under exceptional circumstances when the potential
benefit, based on resistance patterns and treatment history, out-
weighs the risk. HLA-B*57:01 genotyping is widely available in
the developed world and is considered the standard of care prior
to initiating abacavir. Where HLA-B*57:01 genotyping is not
Do not prescribe
abacavir. Select an
Does the patient
have signs and
symptoms of HSR?
abacavir and switch
to alternative agent.
monitor for HSR.
Figure 1 Treatment algorithm for clinical use of abacavir based on HLA-
B*57:01 genotype. HLA-B, human leukocyte antigen B; HSR, abacavir
table 2 recommended therapeutic use of abacavir in relation to
Low or reduced
risk of abacavir
Use abacavir per
Abacavir is not
HLA-B, human leukocyte antigen B.
aRating scheme described in supplementary data online.
CliniCal pharmaCology & TherapeuTiCs | VOLUME 91 NUMBER 4 | APRIL 2012
clinically available (such as in resource-limited settings), some
have advocated initiating abacavir, provided there is appropri-
ate clinical monitoring and patient counseling about the signs
and symptoms of HSR, although this remains at the clinician’s
There is some debate among clinicians regarding whether
HLA-B*57:01 testing is necessary in patients who had previously
tolerated abacavir chronically, discontinued its use for reasons
other than HSR, and are now planning to resume abacavir. The
presence of HLA-B*57:01 has a positive predictive value of ~50%
for immunologically confirmed hypersensitivity,31 indicating
that some HLA-B*57:01-positive individuals can be, and have
been, safely treated with abacavir. However, we were unable to
find any data to show that HLA-B*57:01-positive individuals
with previous, safe exposure to abacavir had zero risk of HSR
upon re-exposure. Although there are isolated case reports
of previously asymptomatic patients developing a hypersen-
sitivity-like reaction after restarting abacavir,37–39 there were
confounding circumstances. Many of the patients had compli-
cating concomitant illnesses that could have masked an HSR
during initial abacavir therapy, and none were immunologi-
cally confirmed, making the case reports difficult to interpret.
Furthermore, most of these case reports precede the availability
of HLA-B*57:01 genetic testing, making it impossible to deter-
mine from the published data whether there could be a risk of
HSR upon re-exposure to abacavir in previously asymptomatic
In addition, there may also exist a small group of patients
who have been on chronic abacavir therapy since before the
introduction of HLA-B*57:01 genotyping. Given that virtually
all abacavir HSR events occur within the first several weeks
of therapy, and that ~50% of HLA-B*57:01 carriers can safely
take abacavir, we were unable to find any evidence to suggest
that HLA-B*57:01-positive individuals on current, long-term,
uninterrupted abacavir therapy are at risk of developing HSR.
Existing clinical guidelines32–36 have a blanket recommendation
that all HLA-B*57:01-positive individuals should avoid abacavir,
regardless of patient history. Although HLA-B*57:01 genotyp-
ing has proven utility in significantly reducing the incidence
of both clinically diagnosed and immunologically confirmed
hypersensitivity7,27,28,31,40 in patients being newly considered for
abacavir therapy, the connection between HLA-B*57:01 geno-
type and risk of HSR in patients with previous asymptomatic
abacavir use is less clear.
recommendations for incidental findings
Although other variants in HLA-B are associated with autoim-
mune diseases and drug response phenotypes, they have not
been associated with abacavir HSR.
Abacavir skin patch testing may be performed after a case
of clinically diagnosed HSR to determine whether it can be
immunologically confirmed. At this time, skin patch testing is
an investigational procedure, and the results should be inter-
preted only by an experienced immunologist. More details on
skin patch testing can be found in the Supplementary Materials
and Methods online.
Potential benefits and risks for the patient
A clear benefit of HLA-B*57:01 testing is that it leads to a reduc-
tion in the incidence of abacavir HSR by identifying patients at
significant risk so that alternative antiretroviral therapy can be
prescribed for them. Importantly, a number of effective and safe
antiretrovirals are available that can be substituted for abacavir
in patients carrying this risk-related allele. HLA-B*57:01’s high
negative predictive value (>99%)31 shows that it is extremely
effective in identifying those at risk of immunologically
confirmed hypersensitivity to abacavir. A potential problem
would be an error in genotyping or in reporting a genotype.
This could result in high-risk patients mistakenly being given
abacavir and potentially having an HSR. However, given that
patients testing negative for HLA-B*57:01 also have a 3% risk
of developing a clinically diagnosed HSR, standard practice
would include patient counseling and careful monitoring for
signs and symptoms of an HSR. Given the lifelong nature of
genotype results, an error in genotyping may also have a broader
adverse impact on a patient’s health care if other associations
with HLA-B*57:01 are found in the future.
caveats: appropriate use and/or potential misuse of
The positive predictive value of HLA-B*57:01 genotyping is
~50%, which means that a significant number of patients will
be denied abacavir on the basis of their genotyping results even
though they would have been able to take abacavir without
experiencing an HSR. There is currently no way to know a
priori which HLA-B*57:01 carriers are and which are not likely
to experience HSRs, although new genetic risk factors may be
found in the future. Given the potential seriousness of HSRs,
the moderate positive predictive value is greatly outweighed
by the very high negative predictive value of HLA-B*57:01
HLA-B*57:01 is not predictive of any other adverse reactions
a patient may experience while on abacavir treatment, nor does
it predict whether abacavir will be effective in treating a patient’s
HIV. In addition, genotyping is not a replacement for appropri-
ate patient education and clinical monitoring for the signs and
symptoms of hypersensitivity. The development of signs and
symptoms of an HSR warrants that serious consideration be
given to discontinuing abacavir, regardless of the HLA-B geno-
CPIC guidelines reflect expert consensus based on clinical evi-
dence and peer-reviewed literature available at the time they
are written and are intended only to assist clinicians in decision
making and to identify questions for further research. New evi-
dence may have emerged since the time a guideline was submit-
ted for publication. Guidelines are limited in scope and are not
applicable to interventions or diseases not specifically identified.
Guidelines do not account for all variations among individual
738 VOLUME 91 NUMBER 4 | APRIL 2012 | www.nature.com/cpt Download full-text
patients and cannot be considered inclusive of all proper meth-
ods of care or exclusive of other treatments. It remains the
responsibility of the health-care provider to determine the best
course of treatment for the patient. Adherence to any guideline
is voluntary, with the ultimate determination regarding its appli-
cation to be made solely by the clinician and the patient. CPIC
assumes no responsibility for any injury to persons or damage
to property related to any use of CPIC’s guidelines, or for any
errors or omissions.
suPPLeMentarY MateriaL is linked to the online version of the paper at
We acknowledge the critical input of members of the Clinical
Pharmacogenetics Implementation Consortium of the Pharmacogenomics
Research Network, funded by the National Institutes of Health (NIH). This
work was funded by NIH grants GM61390 and GM61374.
conFLict oF interest
The authors declared no conflict of interest.
© 2012 American Society for Clinical Pharmacology and Therapeutics
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