We found generic stavudine, lamivudine and nevirapine in the form of Triomune fails to meet strict bioequivalence criteria in patients with objectively confirmed adherence on stable therapy. While Triomune failed to meet strict bioequivalence, the differences were relatively minor and are unlikely to be clinically significant. Our results are similar to a recently reported bioequivalence study carried out in Malawian HIV-infected patients 
. One exception is that in the former study, the generic Triomune formulation resulted in a significant increase in stavudine Cmax
compared to the brand name. In both the present study and in the Malawian report, plasma levels of nevirapine following administration of either the generic or brand formulations were higher than those previously reported in Caucasian HIV patients 
. A large fraction of nevirapine is metabolized by the polymorphic CYP2B6 enzyme 
. Whether the elevated levels of nevirapine in these African populations is related to the higher allele frequency of the reduced function CYP2B6
516G>T polymorphism in these populations should be investigated in larger samples.
There are several reasons why the differences in drug exposure between brand and generic medication are unlikely to be clinically significant. Over 70% achieved undetectable viral load levels (<400 copies/ml) and had significantly improved their CD4 count at 12 and 24 weeks 
. Even in the case of lamivudine, for which Cmax
and AUC were decreased 20–30% with the generic formulation compared to brand name, the plasma concentrations were similar to those reported following a single dose of Triomune or a second combination drug containing abacavir, lamivudine and zidovudine to healthy subjects 
. Furthermore, the mixed random effects model found no statistically significant difference in Cmax
or AUC between the formulation types for any of the three drugs (results not shown).
We found a high degree of variability between study subjects. The random effects model produced correlation coefficients of about 50% for both log transformed Cmax
and AUC values (). This implies that about 50% of all variability in these parameters was due to differences between study subjects. The interindividual variability in the pharmacokinetics of stavudine, lamivudine, and nevirapine may be attributable to various sources, such as environmental or genetic factors, that were not addressed in this study. In particular, genetic polymorphisms in transporters or drug-metabolizing enzymes for which these drugs are substrates may affect the pharmacokinetics of these drugs. For example, the cytochrome P450 2B6 (CYP2B6) enzyme, which is involved in nevirapine metabolism, contains a genetic polymorphism (516G>T) that has been shown to substantially decrease hepatic protein expression and function 
. In patients with HIV, this polymorphism has been significantly associated with increased nevirapine plasma levels 
. While the nucleoside reverse transcriptase inhibitors lamivudine and stavudine are not extensively metabolized in the liver, it is possible that polymorphisms in membrane transporters could influence the bioavailability of lamivudine and stavudine, thereby modulating plasma drug levels. Recently, a polymorphism in the MRP4 transporter was associated with intralymphocytic lamivudine levels in HIV patients 
; conceivably, this polymorphism may similarly affect MRP4 activity in enterocytes, where drug absorption occurs. Due to the relatively small minor allele frequencies of these polymorphisms, a larger study population is needed to address these pharmacogenetic questions.
We observed a significant difference in the mean nevirapine trough concentrations between the brand and the generic formulation. However, these steady state plasma nevirapine concentrations are far above the concentration required to inhibit 50% viral replication in vitro
; the IC50 for nevirapine is 10.6 ng/ml 
so this difference may not have clinical relevance. The first round of pharmacokinetic sampling does not show a significant difference in the mean nevirapine trough concentrations between the brand and generic formulation. This implies that the observed difference is not a systematic difference in the trough concentrations of the two formulations and may be due to the observed sequence effects. A limitation of our study was that we did not conduct drug content assays and in vitro dissolution tests. A drug content assay for each formulation would rule out formulation problems. In vitro dissolution testing would reveal variations in drug degradation between brand and generic formulations that could lead to in vivo
differences in the rate of nevirapine absorption.
Our analysis model identified significant sequence effects for nevirapine Cmax
and AUC. Although the exact causes of the sequence effects are not known, ‘real’ differences between the groups could contribute to the observed effects. While our study groups were similar in most baseline characteristics, the brand to generic group was generally healthier than the generic to brand group. Ninety one percent (10/11) of the brand to generic group was virologically suppressed (<400 copies/ml) compared to 71% (5/7) in the generic to brand group (). In addition, mean CD4 cell count in the brand to generic group was about 80 units more than the mean CD4 count in the generic to barnd group (402 cells/µl vs. 319 cells/µl). To control for this empirical confounding, baseline CD4 cell count and viral load were added to the model. We found a significant association between baseline CD4 cell count and nevirapine Cmax
0.007) and AUC (p
0.02). Higher CD4 cell count was associated with lower drug levels. This difference in health status may explain the observed sequence effects.
Triomune-40® contains a higher dose of stavudine than is currently recommended in treatment guidelines. Current WHO guidelines now recommend a 12 hourly dose of 30 mg for all patients, irrespective of weight 
. This limits applicability of our results to patients currently on antiretroviral therapy. Studies exploring bioequivalence of fixed dose combination formulations containing 30 mg of stavudine are needed.
In summary, the steady-state pharmacokinetics of the generic formulation (Triomune-40®) in HIV-infected patients did not meet the strict bioequivalence requirement set by the FDA when compared to the brand name formulations of lamivudine, stavudine, and nevirapine. However, based on the measured plasma levels, the generic formulation is expected to produce a similar therapeutic response as the brand name formulations. There was a large degree of interindividual variability in antiretroviral exposure. Variability could have been due to individual disease state and progression, or the genetic variation within the subjects. Understanding the exact sources of this variability will be important for optimization of therapy. These results suggest that bioequivalence and pharmacokinetic studies are needed in specific populations in which these medications are used, to account for unique characteristics that may influence drug disposition. Drug regulatory bodies in countries in which generic antiretroviral medications are used should endeavor to test all antiretrovirals imported into the country to ensure drug quality.