ANTIMICROBIAL AGENTS AND CHEMOTHERAPY, Aug. 2005, p. 3558–3561
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Vol. 49, No. 8
Pharmacokinetics of Nelfinavir and Efavirenz in Antiretroviral-Naı ¨ve,
Human Immunodeficiency Virus-Infected Subjects when
Administered Alone or in Combination with
Nucleoside Analog Reverse
Patrick F. Smith,1,6* Gregory K. Robbins,2,6Robert W. Shafer,3,6Hulin Wu,4,6Song Yu,2,6
Martin S. Hirsch,2,6Thomas C. Merigan,3,6Jeong-Gun Park,2,6Alan Forrest,1,6
Margaret A. Fischl,5,6Gene D. Morse,1,6and the ACTG 384-5006 Team6
University at Buffalo, School of Pharmacy and Pharmaceutical Sciences, Buffalo, New York1; Harvard University, School of
Medicine, Boston, Massachusetts2; Stanford University, School of Medicine, Palo Alto, California3; University of
Rochester School of Medicine and Dentistry, Rochester, New York4; University of Miami, School of
Medicine, Miami, Florida5; and Adult AIDS Clinical Trials Group, National Institute
of Allergy and Infectious Diseases, Bethesda, Maryland6
Received 2 December 2004/Returned for modification 16 January 2005/Accepted 5 May 2005
Pharmacokinetic studies were conducted with human immunodeficiency virus-infected patients receiving
efavirenz, nelfinavir, or both agents at weeks 4 and 32. Reductions of 25% and 45% were observed in the mean
nelfinavir area under the concentration-time curve and minimum concentration of the drug in serum, and
there was a 31% more rapid half-life for patients receiving both drugs compared to patients receiving nelfinavir
alone. There were no significant differences in efavirenz pharmacokinetics.
Therapy for antiviral-naı ¨ve human immunodeficiency virus
(HIV)-infected individuals usually includes dual nucleoside
analogue reverse transcriptase inhibitors with a nonnucleoside
reverse transcriptase inhibitor and/or a protease inhibitor (10).
Adult AIDS Clinical Trials Group (AACTG) protocol 384 was
initiated to evaluate if efavirenz or a protease inhibitor would
be more effective with a dual nucleoside analogue reverse
transcriptase inhibitor backbone and whether two sequential
three-drug regimens were superior to a single four-drug regi-
men. The primary study results reported that a regimen of
zidovudine-lamivudine-efavirenz was as effective as zidovu-
dine-lamivudine-nelfinavir (NFV)-efavirenz, but the four-drug
regimen exhibited a longer time to treatment failure (7, 8).
A pharmacokinetic substudy was conducted to examine
nelfinavir, M8, and efavirenz pharmacokinetics after 4 weeks
and 32 weeks of therapy. A previous healthy-volunteer study
similarly evaluated this interaction, utilizing a thrice-daily reg-
imen of nelfinavir, and reported no effect of efavirenz on nelfi-
navir pharmacokinetics (W. D. Fiske, I. H. Benedek, S. J.
White, K. A. Pepperess, J. L. Joseph, and D. M. Kornhauser,
Abstr. Conf. Retrovir. Oppor. Infect., abstr. 349, 1998).
Adult HIV-infected patients were randomized to receive
zidovudine-lamivudine or didanosine-stavudine plus efavirenz
(600 mg daily), nelfinavir (1,250 mg twice daily), or efavirenz-
nelfinavir in a double-blind fashion with matching placebos.
Steady-state pharmacokinetics of efavirenz, nelfinavir, and its
M8 metabolite were determined at weeks 4 and 32. Blood
samples were collected at 0, 1, 2, 3, 4, 6, 8, 10, and 12 h
following oral dosing, and the exact times of the prior three
doses were recorded. Efavirenz, nelfinavir, and M8 concentra-
tions were measured by use of a validated assay method which
has been previously described (6). Measurements performed
on blinded samples demonstrated a variation for efavirenz
ranging from 4.6 to 7.0% and 6 to 15% for M8 and nelfinavir.
The limits of quantitation were 0.050 ?g/ml for efavirenz and
M8 and 0.100 ?g/ml for nelfinavir. Pharmacokinetic parame-
ters were determined by standard noncompartmental methods
(WinNonlin Professional 4.1; Pharsight Corporation, Cary,
NC). Statistical comparisons between treatment groups were
by repeated-measures mixed-effects modeling.
For nelfinavir, 73 intensive pharmacokinetic studies were
conducted: 36 studies were with patients receiving nelfinavir
alone, and 37 were with patients receiving the combination of
nelfinavir and efavirenz. Forty patients were studied at week 4,
and 26 were studied at week 32. For M8, assay results were
available for 34 subjects receiving nelfinavir alone and 27 sub-
jects receiving nelfinavir and efavirenz concurrently. Pharma-
cokinetic parameters for nelfinavir and M8 are summarized in
Table 1. For efavirenz, 77 pharmacokinetic studies were con-
ducted with 46 patients. Totals of seven and eight patients were
studied only at weeks 4 and 32, respectively.
As illustrated in Fig. 1, nelfinavir pharmacokinetic parame-
ters differed significantly for subjects receiving both nelfinavir
and efavirenz and subjects receiving nelfinavir alone, with a
25% reduction in the mean (standard deviation [SD]) nelfina-
vir area under the concentration-time curve from 0 to 12 h
(AUC0-12) (22.8 [11.2] versus 30.5 [13.6] ?g · h/ml, P ? 0.01),
* Corresponding author. Mailing address: Adult ACTG Pharmacol-
ogy Support Laboratory, Laboratory for Antiviral Research, Depart-
ment of Pharmacy Practice, School of Pharmacy and Pharmaceutical
Sciences, 219 Cooke Hall, University at Buffalo, Buffalo, NY 12460.
Phone: (716) 645-2828, ext. 242. Fax: (716) 645-2886. E-mail: pfsmith
45% reduction in the mean (SD) minimum concentration of
drug in serum (Cmin) (1.1 [0.9] versus 0.6 [0.5] ?g/ml, P ?
0.01), and a 31% more rapid half-life (4.2 [2.2] versus 2.9 [1.5]
h, P ? 0.01). Although the M8 metabolite AUC tended to be
smaller, this difference was not statistically significant. The
AUC ratios of M8 to nelfinavir with and without efavirenz
were similar (0.26 versus 0.25). Pharmacokinetic parameters
are summarized in Table 1.
The mean efavirenz plasma concentration-time profiles are
shown in Fig. 2. The overall efavirenz mean (SD) AUC0-24,
maximum concentration of drug in serum (Cmax), and Cmin
were 48.8 (37.0) ?g · h/ml, 3.7 (1.9) ?g/ml, and 1.8 (1.4) ?g/ml,
respectively. By repeated-measures analysis of variance, there
were no statistically significant differences in efavirenz phar-
macokinetic parameters with regard to either concomitant
nelfinavir or duration of therapy. At week 4, the least-squares
mean (SD) AUC0-24was 45.1 (16.6), and that at week 32 was
48.2 (19.6) (P ? 0.05). When efavirenz was combined with
nelfinavir, the least-squares efavirenz mean (SD) AUC0-24was
43.8 (17.1), compared to 52.6 (18.9) for efavirenz without nelfi-
navir (P ? 0.05).
Efavirenz and nelfinavir both display complex pharmacoki-
netic characteristics during multiple dosing. Efavirenz is highly
bound to plasma proteins, displays a prolonged plasma half-
life, is metabolized via cytochrome P450 2B6 and 3A4, and
induces CYP450 activity during chronic administration (9).
Numerous drug interactions have been reported between efa-
virenz and commonly prescribed medications (3). Nelfinavir is
also highly bound to plasma proteins, is metabolized via 3A4
and 2C19 to an active metabolite (M8), which competitively
inhibits 3A4 activity, and induces cytochrome P450 enzymes
with chronic dosing. The M8 metabolite undergoes subsequent
metabolism via 3A4 (2, 4, 5).
The pharmacokinetic results of efavirenz, both with and
without a protease inhibitor, are consistent with previously
published reports (9, 11). Prior studies have reported the half-
life of efavirenz after repeated dosing to be approximately 40
to 50 h with no major influence noted when a protease inhib-
itor is combined (1, 5, 9). These data also provide new infor-
mation, in that efavirenz pharmacokinetics were stable over 32
The nelfinavir and M8 pharmacokinetic data are consistent
with previous reports (2, 5), including the extent of M8 me-
tabolism (M8-to-NFV ratio, ?20 to 30%). It is difficult to draw
conclusions about the influence of efavirenz induction on M8
formation from our study, due to the noncrossover study de-
sign. However, these results strongly suggest that chronic ad-
ministration may be associated with a smaller amount of total
nelfinavir plus M8. This is consistent with efavirenz induction
of both 2C9 and 3A4. The influence of enzyme inducers on
nelfinavir pharmacokinetics has been previously described dur-
ing phenytoin administration with a lowering of the nelfinavir
AUC (M. J. Shelton, D. Cloen, M. Becker, P. H. Hsyu, J. H.
Wilton, and R. G. Hewitt, Abstr. 40th Intersci. Conf. Antimi-
crob. Agents Chemother., abstr. 426, 2000). Induction effects
of efavirenz on other HIV-1 protease inhibitors have also been
described (1, 5). In addition, a stable AUC ratio of metabolite
to parent was observed in both arms; thus, a possible reduction
in oral bioavailability could also contribute to the observed
results. Possible influences of patients with low nelfinavir con-
centrations dropping out of the nelfinavir-alone arms due to
virological failure also cannot be ruled out.
Patients in this substudy were not evaluated for potential
genetic polymorphisms in the primary CYP450 enzymes re-
sponsible for efavirenz (2B6 and 3A4) and nelfinavir (2C19
and 3A4) metabolism. Efavirenz exposure has been shown to
be correlated with a single-nucleotide polymorphism of
CYP2B6 (516G3T) (D. W. Haas, H. Ribaudo, G. R. Wilkin-
son, R. Gulick, D. Clifford, T. Hulgan, et al., Abstr. 11th Conf.
Retrovir. Oppor. Infect., abstr. 133, 2004). Genetic polymor-
phisms that result in a premature stop codon or altered splice
FIG. 1. Mean nelfinavir and M8 plasma concentrations (? stan-
dard errors of the means) when the drug was administered with or
without efavirenz (EFV).
TABLE 1. Nelfinavir and M8 pharmacokinetic parameters (combined weeks 4 and 32)
Least-squares mean (SD) pharmacokinetic parameter
AUC0–12(?g · h/ml)
AUC0–12(?g · h/ml)
VOL. 49, 2005NOTES3559
site in CYP2C19 confer a poor metabolizer phenotype, result-
ing in reduced nelfinavir metabolism. The potential contribu-
tions of these polymorphisms to the observed results are un-
known and should be evaluated in future studies. Knowledge
of patient-specific genotypes would be expected to account for
a portion of the observed pharmacokinetic variability and may
improve our understanding of drug-drug interactions whereby
the presence and/or magnitude of the interaction may depend
upon the specific genotype and phenotype of a given patient.
A majority of pharmacokinetic studies of antiretrovirals con-
sider acute therapy to be within one to two weeks and long-
term therapy to be over the first month. In ACTG 384-5006,
the design we employed allowed for the investigation of within-
subject patterns over 32 weeks. The observation that nelfinavir
exposure levels were similar between the three-drug nelfinavir
arms but were lower in the four-drug arms suggests that efa-
virenz may have lowered nelfinavir and M8 concentrations
through sustained induction.
With regard to interactions between nelfinavir and efa-
virenz, the only prior pharmacokinetic data that examined the
combined administration were obtained from a seven-day
study with healthy volunteers which utilized thrice-daily nelfi-
navir dosing. This study reported that nelfinavir plasma con-
centrations were increased after seven days by ?20%. These
data are markedly different from those obtained in ACTG
384-5006. Possible reasons for these different findings may be
related to studying healthy volunteers compared to HIV-in-
fected individuals. The duration of drug exposure was much
longer in the present study, reflecting chronic drug adminis-
tration. In addition, potential contributions resulting from the
use of twice-daily nelfinavir dosing in the current study, reflect-
ing current clinical practices, cannot be ruled out.
This study was funded by the following grants from the following
organizations: National Institute of Allergy and Infectious Diseases,
UO1-AI38858; University at Buffalo ACTG Pharmacology Support
Laboratory Harvard University, ACTU AI-27659; Harvard (Massa-
chusetts General Hospital) (A0101), AACTG grant no. AI27659;
NYU/Bellevue (A0401), AACTG grant no. AI27665, GCRC grant no.
M01-R00096; Mount Sinai Medical Center (N.Y.) (A0404), AACTG
grant no. U01-AI-27667 and GCRC grant no. M01-RR-00071; Stan-
ford University (A0501), AACTG grant no. AI27666 and GCRC grant
no. M01-RR00070; University of California, San Diego (A0701),
AACTG grant no. AI27670 and GCRC grant; University of Rochester
Medical Center and SUNY—Buffalo (Rochester) (A1101 and A1102),
AACTG grant no. AI27658 and GCRC grant no. RR00044; University
of Southern California (A1201), AACTG grant no. AI27673 and
GCRC grant; University of Washington (A1401), AACTG grant no.
AI27664 and GCRC grant no. M01-RR-00037; University of Minne-
sota (A1501), AACTG grant no. AI27661 and GCRC grant no.
M01RR00400; University of Cincinnati (A2401), AACTG grant no.
AI25897 and GCRC grant no. M01RR0884; Indiana University Hos-
pital (A2601), AACTG grant no. AI25859 and GCRC grant no.
MO1RR00750; University of North Carolina (A3201), AACTG grant
no. AI25868, GCRC grant no. RR00046, and CFAR no. AI50410;
University of Puerto Rico (A5401), AACTG grant no. AI34832 and
GCRC grant no. 1P20RR11126; Tulane University (A9426), AACTG
grant no. AI35162; Harbor-UCLA (A0601), AACTG grant no.
AI27660 and GCRC grant no. M01-RR00425; University of Pittsburgh
(A1001), AACTG grant no. 5U01-AI46383 and GCRC grant no. M01-
RR00056; and The Cornell Clinical Trials Unit and Chelsea Clinic
(A7803 and A7804), AACTG grant no. AI46386 and GCRC grant no.
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