Antiviral dynamics and sex differences of zidovudine and lamivudine triphosphate concentrations in HIV-infected individuals

Organic cation transporters and their roles in antiretroviral drug disposition

BACKGROUND

Understanding metabolising processes and drug-transporter interactions is particularly crucial to the management of patients with HIV infection, given the several antiretroviral drugs that must be taken lifelong and the use of other medications for HIV-related and non-HIV-related conditions. Several interactions of antiretroviral drugs with metabolising enzymes, especially cytochrome P450 and ATP-dependent transporter P-glycoprotein, have been described but the role of the organic cation transporters (OCTs) is less clearly defined.

OBJECTIVE

To review the relevance of the OCTs for antiretroviral drug disposition.

METHODS

Interactions of OCTs with antiretroviral drugs and evidence for clinical relevance are discussed.

RESULTS/CONCLUSION

Several antiretroviral drugs show relevant interactions with the OCTs in cell-based experiments and the OCTs are highly upregulated in HIV-infected patients. For evaluating the clinical significance, interaction studies in HIV patients and reliable in vitro models for delineation of in vivo effects are needed.

Recent developments in the clinical pharmacology of anti-HIV nucleoside analogs

PURPOSE OF REVIEW

This review summarizes recent developments regarding the unique clinical pharmacologic profile of nucleoside analog reverse transcriptase inhibitors for management of HIV.

RECENT FINDINGS

First, intracellular data in patients suggest that nucleoside reverse transcriptase inhibitor-triphosphates are compartmentalized in different cell types. Additionally, intracellular drug-drug interactions were identified, which were undetectable in plasma. Second, extracellular data illustrate multiple bidirectional plasma drug-drug interactions between renally eliminated tenofovir and liver-metabolized drugs. Definitive mechanistic details for these interactions are lacking but they appear to involve renal and/or enteric drug transporters. Furthermore, the plasma versus female genital tract disposition of these agents was recently elucidated, which is important for currently investigated indications for pre-exposure and post-exposure prophylaxis. Finally, tenofovir/emtricitabine and abacavir (using a promising human leukocyte antigen-B*5701 genetic test for hypersensitivity)/lamivudine have emerged as common first-line nucleoside analog reverse transcriptase inhibitors because of co-formulations, once-daily dosing, and favorable tolerability and adverse effect profiles. Nevertheless, elucidating the long-term safety profile for all nucleoside analog reverse transcriptase inhibitors remains a priority.

SUMMARY

Knowledge of nucleoside analog reverse transcriptase inhibitor disposition intracellularly and extracellularly has expanded. This provides a basis for rational use of these agents clinically and adds new perspectives for future research.

Implications of gender and pregnancy for antiretroviral drug dosing

PURPOSE OF REVIEW

This review briefly outlines the influences of gender and pregnancy on drug disposition, and describes the available antiretroviral pharmacokinetic data and dosing recommendations in these groups.

RECENT FINDINGS

Recent studies in pregnant women continue to document altered exposure of different classes of drugs during pregnancy. While new information shows that tenofovir exposure is significantly decreased during pregnancy, the magnitude of the decrease will not likely necessitate dose changes, similar to other nucleoside reverse transcriptase inhibitors. In contrast, standard doses of lopinavir/ritonavir in the third trimester showed markedly decreased exposure, and higher doses of this co-formulated agent should be given to women during the third trimester. Likewise, nelfinavir exposure using the new 625-mg tablets is also decreased during pregnancy, and higher doses should be considered in the third trimester.

SUMMARY

The majority of antiretrovirals studied have altered pharmacokinetics during pregnancy. Understanding the extent of these changes is necessary to recommend dose changes during pregnancy when appropriate. The correct dose is critical to maintain efficacy and safety of these agents for both the mother and the fetus. Innovative study designs are needed to facilitate the study of antiretrovirals during pregnancy.

Nucleoside and nucleotide HIV reverse transcriptase inhibitors: 25 years after zidovudine

Twenty-five years ago, nucleoside analog 3'-azidothymidine (AZT) was shown to efficiently block the replication of HIV in cell culture. Subsequent studies demonstrated that AZT acts via the selective inhibition of HIV reverse transcriptase (RT) by its triphosphate metabolite. These discoveries have established the first class of antiretroviral agents: nucleoside and nucleotide reverse transcriptase inhibitors (NRTIs). Over the years that followed, NRTIs evolved into the main component of antiretroviral drug combinations that are now used for the treatment of all populations of HIV infected patients. A total of thirteen NRTI drug products are now available for clinical application: eight individual NRTIs, four fixed-dose combinations of two or three NRTIs, and one complete fixed-dose regimen containing two NRTIs and one non-nucleoside RT inhibitor. Multiple NRTIs or their prodrugs are in various stages of clinical development and new potent NRTIs are still being identified through drug discovery efforts. This article will review basic principles of the in vitro and in vivo pharmacology of NRTIs, discuss their clinical use including limitations associated with long-term NRTI therapy, and describe newly identified NRTIs with promising pharmacological profiles highlighting those in the development pipeline. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, volume 85, issue 1, 2010.

Interindividual variability in pharmacokinetics of generic nucleoside reverse transcriptase inhibitors in TB/HIV-coinfected Ghanaian patients: UGT2B7*1c is associated with faster zidovudine clearance and glucuronidation

There are limited data on the pharmacokinetics of generic nucleoside reverse transcriptase inhibitors (NRTIs) in native African populations, in whom they are commonly used. The authors characterized the pharmacokinetics of lamivudine (n = 27), zidovudine (n = 16), and stavudine (n = 11) in human immunodeficiency virus (HIV)/tuberculosis (TB)-coinfected Ghanaians and evaluated associations between zidovudine metabolism and UDP-glucuronosyltransferase (UGT) 2B7 polymorphisms. Lamivudine, zidovudine, and stavudine apparent oral clearance (CL/F) values (mean +/- SD [% coefficient of variation [CV]) were 7.3 +/- 2.8 (39%), 31.9 +/- 33.6 (106%), and 16.4 +/- 5.8 (35%) mL/min/kg, respectively, whereas half-life values were 4.2 +/- 1.9 (46%), 8.1 +/- 7.9 (98%), and 1.5 +/- 1.0 (65%) hours, respectively. Zidovudine CL/F was 196% higher (P = .004) in UGT2B7*1c (c.735A>G) carriers versus noncarriers. This was confirmed using human liver bank samples (n = 52), which showed 48% higher (P = .020) zidovudine glucuronidation and 33% higher (P = .015) UGT2B7 protein in UGT2B7*1c carriers versus noncarriers. In conclusion, generic NRTI pharmacokinetics in HIV/TB-coinfected Ghanaians are similar to other populations, whereas the UGT2B7*1c polymorphism may explain in part relatively high interindividual variability in zidovudine clearance.

Pharmacologic consideration for the use of antiretroviral agents in the elderly

The prevalence of human immunodeficiency virus (HIV) infection among people older than 50 years is increasing. HIV-infected patients require lifelong treatment with antiretroviral agents to suppress viral replication and maintain immune function. The use of antiretroviral agents in the elderly can be complicated by multiple chronic comorbidities and coadministered non-HIV medications. The pharmacokinetics of antiretroviral agents may be altered due to age-related decrements in hepatic and renal function. The elderly may be more sensitive than younger people to antiretroviral drug toxicity. A better understanding of the pharmacokinetics of antiretroviral agents in the elderly is of importance for the successful management of complex antiretroviral regimens in this population.

Comparative study of the persistence of anti-HIV activity of deoxynucleoside HIV reverse transcriptase inhibitors after removal from culture

Background

Most in vitro assays of drug potency may not adequately predict the performance in vivo. Methods to assess the persistence of antiviral activity of deoxynucleoside analogs, which require intracellular activation to the active metabolites that can persist in cells, will be important for designing dosages, combination regimens, and assessing treatment compliance. Using an HIV-IIIB/TZM-bl indicator cell culture system, we assessed the ability of an inhibitor to protect cells from infection and to delay viral rebound after removal of inhibitor from culture.

Results

The order of protection of cells from HIV-infection was 4'-Ed4T > LFD4C > DDI > D4T > 3TC > AZT > FTC > NVP. The fold-increase in EC₅₀ to delay viral rebound was DDI < 4'-Ed4T < LFD4C < FTC < D4T < 3TC < NVP < AZT. The ranking of persistence of anti-HIV activity of the inhibitors based on the two-component assay was DDI > 4'-Ed4T > LFD4C > FTC = D4T > 3TC > NVP > AZT.

Conclusion

The persistence ranking was derived from assays based on measures of single viral replication-cycle and cumulative inhibition at multiple time-points. Therefore, a better indicator of the pharmacodynamic property of an inhibitor. The persistence of anti-HIV activity assay may complement in vitro potency assays to better predict in vivo performance of nucleoside analogs.

Women facing HIV. Key question on women with HIV infection: Italian consensus workshop

A panel of leading Italian specialists in infectious diseases, obstetrics and gynaecology met in a national consensus workshop on women facing HIV to review critical aspects and discuss recommendations for selected key questions on four issues: (1) women and highly active antiretroviral therapy (HAART): access to care and adherence to therapy, side effects and drug-drug interaction; (2) HIV-infected pregnant women: prevention of mother to child transmission; (3) desire for children among women living with HIV: assisted reproduction; (4) sexually transmitted diseases and genital disturbances. The method of a nominal group meeting was used, and recommendations were graded for their strength and quality of evidence using a system based on the one adopted by the Infectious Diseases Society of America. Main conclusions are summarized and critically discussed, and some of the most recent data supporting recommendations are provided.

Pilot pharmacokinetic study of human immunodeficiency virus-infected patients receiving tenofovir disoproxil fumarate (TDF): investigation of systemic and intracellular interactions between TDF and abacavir, lamivudine, or lopinavir-ritonavir

Previous work has demonstrated the existence of systemic interaction between tenofovir (TFV) disoproxil fumarate (TDF) and didanosine as well as between TDF and lopinavir-ritonavir (LPV/r). Here we investigated TDF interactions with the nucleoside reverse transcriptase inhibitors (NRTIs) lamivudine (3TC) and abacavir (ABC), comparing both the concentrations of nucleoside/nucleotide reverse transcriptase inhibitors in plasma and the intracellular concentrations of their triphosphate metabolites (NRTI-TP) for human immunodeficiency virus-infected patients receiving these NRTIs with TDF and after 4 weeks of TDF interruption. We also looked at interactions between TDF-ABC and LPV/r, comparing patients receiving or not receiving LPV/r. Blood samples were taken at baseline and at 1, 2, and 4 h after dosing. Liquid chromatography-tandem mass spectrometry was used to measure NRTIs and NRTI-TPs. Statistical analyses were performed on pharmacokinetic parameters: the area under the concentration-time curve from 0 to 4 h (AUC(0-4)), the maximum concentration of the drug (C(max)), and the residual concentration of the drug at the end of the dosing interval (C(trough)) for plasma and the AUC(0-4) and C(trough) for intracellular data. Among the groups of patient discontinuing TDF, the very long intracellular half-life of elimination (150 h) of TFV-DP (the diphosphorylated metabolite of TFV, corresponding to a triphosphorylated species) was confirmed. Comparison between groups as well as the longitudinal study showed no significant systemic or intracellular interaction between TDF and ABC or 3TC. Significant differences were observed between patients receiving LVP/r and those receiving nevirapine. For ABC, plasma exposure was decreased (40%) under LVP/r, while, in contrast, plasma exposure to TFV was increased by 50% and the intracellular TFV-DP AUC(0-4) was increased by 59%. A trend for a gender effect was observed for TFV-DP at the intracellular level, with higher and C(trough) values for women.

Considerations for the antiretroviral management of women in 2008

Clinicians should be familiar with sex-specific considerations when managing antiretroviral (ARV) treatment among women. Pregnancy is a critical influence on when to start treatment and what ARVs should be included in a regimen. Sex, pregnancy and hormonal contraceptive therapies can each influence ARV pharmacokinetic profiles. Women may be prone to have higher serum levels with selected ARV treatments, which may improve potency but also increase the risk for toxicities. Several studies have demonstrated that women do have higher frequencies of selected ARV-associated adverse events when compared with men. Although HIV treatment guidelines for nonpregnant women do not differ from men, clinicians should be aware of the high potential for certain ARV-related toxicities and follow suggestions in order to decrease the risk of side effects.

Steady-state pharmacokinetics of abacavir in plasma and intracellular carbovir triphosphate following administration of abacavir at 600 milligrams once daily and 300 milligrams twice daily in human immunodeficiency virus-infected subjects

Abacavir (ABC) is administered either at 600 mg once daily (ABC 600 mg QD) or 300 mg twice daily (ABC 300 mg BID) in anti-human immunodeficiency virus (anti-HIV) combination therapy. Although ABC plasma pharmacokinetics following each regimen has been well defined, no study has directly compared the regimens with respect to pharmacokinetics of ABC's active intracellular anabolite, carbovir-triphosphate (CBV-TP). In an open-label, two-period, crossover study, 34 HIV-infected male and female subjects stabilized on antiretroviral regimens containing either ABC 600 mg QD or ABC 300 mg BID received their usual doses on days -1 and 1 and then switched regimens for days 2 to 11. Serial blood samples collected on days 1 and 11 were assayed for plasma ABC and intracellular CBV-TP concentrations using validated high-performance liquid chromatography-tandem mass spectrometry methods. Pharmacokinetic parameters were calculated using noncompartmental methods. Analysis of variance with a mixed-effect model was performed for treatment and gender comparisons. In 27 evaluable subjects, the regimens provided bioequivalent ABC daily areas under the concentration-time curve from 0 to 24 h (AUC(0-24)) and comparable CBV-TP concentrations at the end of the dosing interval (C(tau)). As expected, ABC QD resulted in 109% higher ABC maximum concentrations of drug in plasma (C(max)) than did ABC BID. ABC QD also resulted in 32% higher CBV-TP AUC(0-24) and 99% higher CBV-TP C(max) than did ABC BID. Females had a 38% higher weight-adjusted ABC AUC(0-24) and 81% higher weight-adjusted CBV-TP AUC(0-24) than did males. Virologic suppression was maintained during regimen switch, and no tolerability differences between regimens were observed. In conclusion, this study showed that ABC 600 mg QD and ABC 300 mg BID regimens led to similar intracellular CBV-TP C(tau) values, thus providing pharmacokinetic support for the interchangeability of these two regimens. Women had higher intracellular CBV-TP exposure than did men.

Abacavir pharmacokinetics during chronic therapy in HIV-1-infected adolescents and young adults

The pharmacokinetics of abacavir and its metabolites were investigated in 30 human immunodeficiency virus (HIV)-infected adolescents and young adults 13-25 years of age, equally divided into two groups: <18 years of age and >or=18 years of age. All the subjects received the recommended adult dose of 300 mg twice daily. The area under the plasma concentration-time curve (AUC) and half-life of abacavir did not differ significantly between the age groups or by gender or race, and there were only modest associations of age with apparent abacavir clearance and with volume of distribution. There were no significant correlations of carboxylate or glucuronide metabolite levels with age or gender, although glucuronide AUC was higher in Hispanic subjects than in African-American subjects. Zidovudine and lamivudine concentration profiles were also similar in the two age groups. A novel aspect of the study included an assessment of intracellular carbovir, zidovudine, and lamivudine triphosphate levels, and these were found to be similar in the two age-based groups. Overall, these findings suggest that current recommendations relating to adult dosages are appropriate for adolescents and young adults.

Pharmacokinetic-pharmacodynamic relationship of NRTIs and its connection to viral escape: an example based on zidovudine

In HIV disease, the mechanisms of drug resistance are only poorly understood. Incomplete suppression of HIV by antiretroviral agents is suspected to be a main reason. The objective of this in silico study is to elucidate the pharmacokinetic origins of incomplete viral suppression, exemplified for zidovudine (AZT) as a representative of the key class of nucleoside reverse transcriptase inhibitors (NRTIs). AZT, like other NRTIs, exerts its main action through its intra-cellular triphoshate (AZT-TP) by competition with natural thymidine triphosphate. We developed a physiologically based pharmacokinetic (PBPK) model describing the intra-cellular pharmacokinetics of AZT anabolites and subsequently established the pharmacokinetic-pharmacodynamic relationship. The PBPK model has been validated against clinical data of different dosing schemes. We reduced the PBPK model to derive a simple three-compartment model for AZT and AZT-TP that can readily be used in population analysis of clinical trials. A novel machanistic, and for NRTIs generic effect model has been developed that incorporates the primary effect of AZT-TP and potential secondary effect of zidovudine monophosphate. The proposed models were used to analyze the efficacy and potential toxicity of different dosing schemes for AZT. Based on the mechanism of action of NRTIs, we found that drug heterogeneities due to temporal fluctuations can create a major window of unsuppressed viral replication. For AZT, this window was most pronounced for a 600 mg/once daily dosing scheme, in which insufficient viral suppression was observed for almost half the dosing period.

Development of an optimized dose for coformulation of zidovudine with drugs that select for the K65R mutation using a population pharmacokinetic and enzyme kinetic simulation model

In vitro selection studies and data from large genotype databases from clinical studies have demonstrated that tenofovir disoproxil fumarate and abacavir sulfate select for the K65R mutation in the human immunodeficiency virus type 1 polymerase region. Furthermore, other novel non-thymine nucleoside reverse transcriptase (RT) inhibitors also select for this mutation in vitro. Studies performed in vitro and in humans suggest that viruses containing the K65R mutation remained susceptible to zidovudine (ZDV) and other thymine nucleoside antiretroviral agents. Therefore, ZDV could be coformulated with these agents as a "resistance repellent" agent for the K65R mutation. The approved ZDV oral dose is 300 mg twice a day (b.i.d.) and is commonly associated with bone marrow toxicity thought to be secondary to ZDV-5'-monophosphate (ZDV-MP) accumulation. A simulation study was performed in silico to optimize the ZDV dose for b.i.d. administration with K65R-selecting antiretroviral agents in virtual subjects using the population pharmacokinetic and cellular enzyme kinetic parameters of ZDV. These simulations predicted that a reduction in the ZDV dose from 300 to 200 mg b.i.d. should produce similar amounts of ZDV-5'-triphosphate (ZDV-TP) associated with antiviral efficacy (>97% overlap) and reduced plasma ZDV and cellular amounts of ZDV-MP associated with toxicity. The simulations also predicted reduced peak and trough amounts of cellular ZDV-TP after treatment with 600 mg ZDV once a day (q.d.) rather than 300 or 200 mg ZDV b.i.d., indicating that q.d. dosing with ZDV should be avoided. These in silico predictions suggest that 200 mg ZDV b.i.d. is an efficacious and safe dose that could delay the emergence of the K65R mutation.

Simultaneous determination of endogenous deoxynucleotides and phosphorylated nucleoside reverse transcriptase inhibitors in peripheral blood mononuclear cells using ion-pair liquid chromatography coupled to mass spectrometry

Nucleoside reverse transcriptase inhibitors (NRTIs) are activated intracellularly to their triphosphate (TP) form, which compete with endogenous deoxynucleotide-triphosphates (dNTP) as substrate for HIV reverse transcriptase. The activity of NRTIs is thus described by the NRTI-TP-to-dNTP ratio in relevant cell types. Therefore, we developed an ion-pair (IP) LC-MS method for the simultaneous analysis of the mono-, di-, and TP forms of NRTIs and endogenous deoxynucleosides in peripheral blood mononuclear cells (PBMC). The IP-LC method was applied on an IT mass spectrometer using the MS-mode as well as on a triple quadrupole mass spectrometer using the MS/MS mode. The MS/MS approach on the triple quadrupole mass spectrometer demonstrated the best clinical applicability due to its higher sensitivity. The LOD (minimum amount on column) were 25 fmol for the TP forms of zidovudine, lamivudine, and stavudine, as well as for their endogenous dNTP counterparts. The linearity (R(2) ) of the calibration curves were>0.99. The obtained LOD readily allow for clinical applications using just one million PBMC obtained from HIV-infected patients under therapy.

Differential extracellular and intracellular concentrations of zidovudine and lamivudine in semen and plasma of HIV-1-infected men

OBJECTIVES

To quantitate extracellular and intracellular zidovudine (ZDV) and lamivudine (3TC) concentrations in blood and semen of HIV-1-infected men.

DESIGN

: Nonblind, single-center, open-label pharmacokinetic (PK) study in 14 subjects receiving ZDV plus 3TC.

METHODS

Paired blood and semen samples were obtained during 1 intensive visit and 3 single time point visits over 2 weeks. Extracellular ZDV and 3TC concentrations were measured in blood plasma (BP) and seminal plasma (SP), and intracellular ZDV and 3TC triphosphate (TP) concentrations were measured in isolated mononuclear cells using validated methods. HIV-1 RNA was measured in blood and semen. PK parameters were estimated using noncompartmental analysis.

RESULTS

Median (interquartile range [IQR]) SP/BP area under the time-concentration curve over the 12-hour dosing interval (AUC0-12h) ratios for ZDV and 3TC were 2.28 (1.48 to 2.97) and 6.67 (4.10 to 9.14), respectively, whereas individual SP/BP concentration ratios ranged from 1.9 to 91.4. Intracellular median (IQR) SP/BP AUC0-12h ratios for ZDV-TP and 3TC-TP were 0.36 (0.30 to 0.37) and 1.0 (0.62 to 1.30), respectively, whereas individual SP/BP concentration ratios ranged from 0.11 to 2.9. HIV-1 RNA was undetectable in both compartments.

CONCLUSIONS

ZDV and 3TC SP exposures are 2- to 6-fold greater than BP exposures. Seminal ZDV-TP exposures are approximately 40% of those found in peripheral blood mononuclear cells (PBMCs), whereas 3TC-TP exposures are similar to PBMC exposures. PK variability makes individual SP/BP ratios a suboptimal surrogate for genital tract exposure.

Cytokine and sex hormone effects on zidovudine- and lamivudine-triphosphate concentrations in vitro

INTRODUCTION

Elevated zidovudine- and lamivudine-triphosphates have been observed in peripheral blood mononuclear cells (PBMCs) from females versus males and in patients with high inflammatory states versus lower inflammatory states. Consistent with high triphosphate exposures, these same patient groups also experience elevated rates of toxicity, including lipoatrophy. The purpose of this study was to evaluate the effects of oestradiol, progesterone and testosterone as well as tumour necrosis factor (TNF)-alpha and interferon (IFN)-alpha on zidovudine- and lamivudine-triphosphates in PBMCs and, for the cytokines, in 3T3-L1 adipocytes.

METHODS

Multiple replicates of adipocytes and human PBMCs were incubated with experimental versus control conditions using several cytokine and sex hormone doses. Zidovudine- and lamivudine-triphosphate concentrations were determined with validated LC-MS-MS assays. A mixed effects, cell means model that accounted for experiment number was used to evaluate the effects of experimental conditions relative to control.

RESULTS

In adipocytes, TNF-alpha doses below 2 ng/mL increased zidovudine-triphosphate by 18% (5-31%). Lamivudine-triphosphate was not detectable in adipocytes. In PBMCs, pooled IFN-alpha doses (0.1-10 U/mL) decreased zidovudine-triphosphate 26% (10-42%); 100 and 1000 ng/mL of progesterone decreased lamivudine-triphosphate by 22% (1-43%) and 47% (25-68%), respectively. Pooled testosterone doses (10-1000 ng/mL) decreased lamivudine-triphosphate by 24% (7-41%). No other statistically significant effects were observed.

CONCLUSIONS

We found evidence that sex hormones and cytokines influence zidovudine-triphosphate and lamivudine-triphosphate slightly in PBMCs and adipocytes in vitro. These findings provide insight and scientific direction to address inflammation-dependent and sex-dependent phosphorylation and responses in patients.

High levels of zidovudine (AZT) and its intracellular phosphate metabolites in AZT- and AZT-lamivudine-treated newborns of human immunodeficiency virus-infected mothers

Newborns from human immunodeficiency virus-infected mothers are given antiretroviral prophylaxis against mother-to-child transmission, including predominantly nucleoside reverse transcriptase inhibitors. Pharmacological monitoring of these drugs in newborns has so far been limited to plasma and cord blood. In this study, samples from newborns (up to 45 days old) treated with zidovudine (AZT) alone (n = 29) or in combination with lamivudine (3TC) (n = 20) were analyzed for both intracellular concentrations of phosphate metabolites in peripheral blood mononuclear cells and levels of parent drugs in plasma. Plasma AZT and intracellular AZT-monophosphate and AZT-triphosphate (TP) concentrations were significantly higher during the first 15 days of life (199 versus 52.7 ng/ml [P < 0.0001], 732 versus 282 fmol/10(6) cells [P < 0.0001], and 170 versus 65.1 fmol/10(6) cells [P < 0.0001], respectively) and then became comparable to those of adults. No difference in intracellular AZT metabolite concentrations was found when AZT- and AZT-3TC-treated groups were compared. Plasma 3TC levels (lower limit of quantification [LLOQ], 1,157 ng/ml; median, 412.5 ng/ml) were not associated with the newborn's age, gender, or weight. Intracellular 3TC-TP concentrations (LLOQ, 40.4 pmol/10(6) cells; median, 18.9 pmol/10(6) cells) determined for newborns receiving the AZT-3TC combination were associated with neither the age nor weight of the newborns. Concentrations in females were significantly higher (1.8-fold [P = 0.0415]) than those in males. Unexpectedly, newborns on AZT monotherapy whose mothers' treatment included 3TC displayed residual plasma 3TC and intracellular 3TC-TP levels up to 1 week after birth.

Initiation of antiretroviral therapy at CD4 cell counts >/=350 cells/mm3 does not increase incidence or risk of peripheral neuropathy, anemia, or renal insufficiency

BACKGROUND

US guidelines recommend deferring initiation of highly active antiretroviral therapy (HAART) for most patients with CD4 counts >350 cells/mm in part because of concerns about antiretroviral toxicity.

METHODS

Incidence rates of peripheral neuropathy, anemia, and renal insufficiency in a cohort of 2165 patients followed more than 3 years (mean) were analyzed in multivariate Cox proportional hazards models by CD4 cell counts at initiation of HAART. A nested cohort of 895 patients restricted to study participants who did or did not start HAART within a CD4 cell count stratum were also compared.

RESULTS

Incidence and risks of all 3 comorbidities decreased with initiation of HAART at CD4 counts >200 cells/mm versus <200 cells/mm. Incidence and risks of renal insufficiency were similar with HAART initiation at CD4 counts >/=350 cells/mm versus 200 to 349 cells/mm, but risk of peripheral neuropathy and anemia were further decreased in persons starting HAART at a CD4 count >/=350 cells/mm. The incidence of these conditions was highest during the first 6 months of treatment at any CD4 cell count and declined up to 19-fold with further therapy.

DISCUSSION

Initiating HAART at CD4 cell counts >/=200 cells/mm reduced the incidence and risk of the 3 comorbid conditions and for anemia and peripheral neuropathy as well by starting at CD4 counts >/=350 cells/mm. The incidence of each condition decreased rapidly and remained low with increasing time on HAART.

Evaluation of sex differences of fosamprenavir (with and without ritonavir) in HIV-infected men and women

PURPOSE

Recent studies focusing on HIV-1-infected women have suggested the existence of sex-related differences in natural history, antiretroviral pharmacokinetics, efficacy, and tolerability. This article analyzes three pivotal trials of the protease inhibitor (PI) fosamprenavir (FPV) with a view to providing a better understanding of potential sex differences in efficacy and safety.

METHOD

A post hoc, descriptive analysis was performed on data from 700 subjects (26% women) in three trials of FPV to evaluate sex differences with regard to efficacy, rates of discontinuation, and treatment-related adverse events.

RESULTS

No major sex differences were found. Men and women had similarly good antiviral responses, with greater than 60% of treatment-naïve subjects achieving virologic suppression (<400 copies/mL) at 48 weeks. PI-experienced women in CONTEXT receiving once-daily FPV/r experienced the highest rates of discontinuations due to virologic failure (29% in women vs. 8% in men). Women generally had slightly lower rates of liver enzyme elevations and fewer abnormalities of total cholesterol and triglycerides.

CONCLUSION

The absence of major sex differences provides reassurance, but the small number of women in these trials limited the ability to draw conclusions. Future trials should be specifically powered to detect sex differences in safety and efficacy.

Antiretroviral pharmacokinetic profile: a review of sex differences

BACKGROUND

Emerging evidence suggests that female sex may be associated with increased risk of developing antiretroviral toxicities. Although the mechanisms of sex-related antiretroviral pharmacodynamic differences remain poorly understood and may be multifactorial, they appear to be mediated through a common pathway of pharmacokinetic variability between the sexes.

OBJECTIVE

This article reviews sex differences in the pharmacokinetics of the major classes of antiretroviral drugs currently approved for HIV treatment by the US Food and Drug Administration, identifies knowledge gaps, and provides recommendations for future research directions.

METHODS

To identify pertinent articles for this review, the MEDLINE database was searched from 1990 to June 2006 using the terms sex, gender, antiretroviral therapy, ART, HAART, pharmacokinetics, pharmacodynamics, NRTI, NNRTI, and protease inhibitors. Search results were restricted to English language and human studies. The reference lists of identified articles were also used, as well as abstracts from relevant conferences. In addition, individual antiretroviral drugs were searched by sex/gender or by pharmacokinetics.

RESULTS

Current evidence, though limited, does suggest the existence of a sex disparity in antiretroviral pharmacokinetics, and such disparity has been shown to have pharmacodynamic implications for some drugs. Sex-mediated intracellular pharmaco-enhancement was associated with superior antiviral activities for the zidovudine and lamivudine members of the nucleoside reverse transcriptase inhibitor class. There appears to be divergent opinions about whether sex is a significant determinant of either nevirapine or efavirenz plasma concentrations. For certain protease inhibitors (PIs) (eg, saquinavir [SQV] and indinavir [IDV]), clinically significant relationships between sex differences in plasma drug concentrations and clinical outcomes have been observed. There appears to be a trend toward higher drug exposure in women than in men when PIs are boosted with ritonavir (RTV). Nelfinavir, the only PI that is currently administered unboosted with RTV, does not exhibit a sex difference in its plasma concentrations. Unboosted amprenavir exposure was lower in women compared with men. Sex differences in the pharmacokinetics of SQV and IDV were observed only in the setting of RTV boosting.

CONCLUSIONS

A common weakness in many studies addressing sex-based differences in the pharmacokinetics of antiretroviral drugs is the relatively small number of women participating. Many of these studies were retrospective in design, and some had limited pharmacokinetic parameters for comparison. Antiretroviral treatment trials should be designed with sufficient power (adequate female participation) to detect sex-based differences both in pharmacokinetics and in clinical response. Future studies should explore the molecular basis for sex-based differences in plasma drug concentrations and antiretroviral drug response. The roles of drug transporter proteins and cellular kinases, and the activities of metabolizing enzymes in mediating differential plasma and intracellular antiretroviral concentrations, should be further assessed.

Prevalence and risk factors associated with antiretroviral resistance in HIV-1-infected children

In the USA and West Europe, nearly 80% of HIV-1-infected adults, experiencing virologic failure, harbored virus strain resistant to at least one antiretroviral drug. Limited data are available on antiretroviral drug resistance in pediatric HIV infection. The aims of this study were to analyze prevalence of HIV-1 drug resistance and to identify risk factors associated with resistance in this population. Prevalence of genotypic resistance was estimated retrospectively in treated children who experienced virologic failure (with HIV-1-RNA > 500 copies/ml) followed in Necker hospital between 2001 and 2003. Among 119 children with resistance testing, prevalence of resistance to any drug was 82.4%. Resistance ranged from 76.5% to nucleoside reverse transcriptase inhibitor (NRTI), to 48.7% to non-nucleoside reverse transcriptase inhibitor (NNRTI) and 42.9% to protease inhibitor (PI). Resistance to at least one drug of two classes and three classes (triple resistance) was 31.9 and 26.9%, respectively. Resistance was not associated with geographic origin, HIV-1 subtype, and CDC status. In multivariate analysis, resistance to any drug remained associated independently with current low viral load and high lifetime number of past PI. Triple resistance was independently associated with the high lifetime number of past PI and with gender, particularly among children aged 11 years old or more with a prevalence seven times higher in boys than in girls. In conclusion, antiretroviral resistance is common among treated HIV-1-infected children and prevalence was similar with those observed in adult population in the same year period. However, adolescent boys seem to be at greater risk.

Concentrations of zidovudine- and lamivudine-triphosphate according to cell type in HIV-seronegative adults

INTRODUCTION

Concentrations of zidovudine (ZDV)- and lamivudine (3TC)-triphosphates (TP) have been quantified in unfractionated peripheral blood mononuclear cells (PBMC) from HIV+ patients. The objective of this study was to determine whether concentrations of ZDV-TP and 3TC-TP in PBMC reflect the concentrations within CD4 T cells in HIV-seronegative adults.

METHODS

Volunteers had taken 300 mg of ZDV plus 150 mg of 3TC twice daily for > or = 7 days. Blood (60 mL) was collected 2 or 5 h post observed dose. PBMC were processed into three cell fractions using CD4 magnetic immunobeads: CD4-purified cells; unfractionated PBMC; and CD4-depleted PBMC. TP were determined in each cell fraction with liquid chromatography-mass spectrometry and compared across cell types by non-parametric analyses.

RESULTS

Six males and two females participated. The median (range) percentage of CD4 T cells (CD4%) in each fraction were: CD4-purified, 99%; unfractionated, 63% (range, 53-70); and CD4-depleted, 14% (range, 4-29). Corresponding median (range) ZDV-TP concentrations were 8.0 (5.3-10.3), 26.5 (12.9-42.2), and 34.2 (16.4-52.2) fmol/1 x 10 cells (Friedman P = 0.0008). The 3TC-TP values were 4.6 (2.3-6.7), 4.8 (3.5-8.8), and 6.8 (4.0-13.1) pmol//1 x 10 cells (Friedman P = 0.01). In mixed model analyses: ZDV-TP (fmol/1 x 10 cells) = 42-0.32 (CD4%); P < 0.001 and 3TC-TP (pmol/1 x 10 cells) = 7.3-0.03(CD4%); P = 0.003.

CONCLUSIONS

In HIV-seronegative volunteers, 3TC-TP concentrations in PBMC reflected the concentrations within CD4 T cells, but ZDV-TP concentrations were more than 70% lower in CD4 T cells than in PBMC. Thus, TP concentrations differ according to cell type in vivo with corresponding efficacy and toxicity implications for cells with low or high triphosphates.

Effects of age and sex on immunological and virological responses to initial highly active antiretroviral therapy

BACKGROUND

Highly active antiretroviral therapy (HAART) has increased longevity. Currently, women comprise >50% of HIV-infected individuals worldwide. It is not known if there are differences between the sexes in the immunological and virological responses to HAART across the age strata.

METHODS

Immunological reconstitution and virological response in the first 6 months of a first HAART regimen in two observational clinical HIV-infected cohorts were compared by both sex and age (>or=50 vs. <50 years old).

RESULTS

A total of 246 individuals (28% women) were included in the study; 63 cases (>or=50 years old) and 183 controls (<50 years old). Over two-thirds of patients had HIV RNA levels <400 HIV-1 RNA copies/mL and CD4 count increases >or=50 cells/microL at 6 months from therapy initiation. There were no differences in immunological reconstitution across age and sex strata (P=0.81) and no differences in virological suppression, even after adjusting for type of HAART (P=0.68) or restricting the analysis to women only (P=0.81). These results suggest that younger and older women and men may have similar short-term initial HAART outcomes.

CONCLUSIONS

Further evaluation of longer term clinical response to initial HAART regimen based on sex and age is indicated, especially with more efficacious and simplified antiretroviral regimens and the associated decrease in mortality.

Intracellular Nucleoside Triphosphate Concentrations in HIV-Infected Patients on Dual Nucleoside Reverse Transcriptase Inhibitor Therapy

Background

Intracellular nucleoside reverse transcriptase inhibitor triphosphate (NRTI-TP) concentrations are crucial in suppressing HIV replication. Little is known about how commonly used dual-NRTI regimens affect the intracellular levels of NRTI-TPs, the active form of these drugs. This study investigates the effect of dual-NRTI therapy in intracellular NRTI-TP levels.

Methods

NRTI and NRTI-TP concentrations were evaluated in HIV-infected patients receiving either lamivudine (3TC) and stavudine (d4T) or lamivudine with zidovudine (ZDV); NRTI and NRTI-TP concentrations were determined using a validated HPLC/MS/MS method. Plasma HIV-1 RNA levels were determined at baseline and monthly to examine the relationship between NRTI-TP concentrations and plasma HIV-1 RNA.

Results

Forty-one subjects completed the study. 3TC-TP significantly increased between day 1 and week 28 from 1.48 to 5.00 pmol/106 peripheral blood mononuclear cells (PBMC; P<0.0001). NRTI-TP concentrations for d4T and ZDV did not significantly increase, with values at week 28 of 0.011 and 0.02 pmol/106 PBMC, respectively. Mean NRTI-TP/plasma ratios were 3%, 0.007% and 0.05% for 3TC, d4T and ZDV, respectively. Linear relationships were observed between ZDV- and 3TC-TP and changes in plasma HIV-1 RNA.

Conclusion

Of the three drugs studied, only 3TC-TP levels increased significantly between day 1 and week 28. ZDV-TP and 3TC-TP levels were unaffected by dual-NRTI therapy relative to monotherapy, regardless of the combination (3TC-ZDV or 3TC-d4T). Intracellular levels of d4T-TP were similar to previous reports for dual-NRTI therapy; however, in the case of d4T, these values appear lower than those achieved with d4T monotherapy.

Lopinavir/Ritonavir pharmacokinetic profile: impact of sex and other covariates following a change from twice-daily to once-daily therapy

The aim of this study was to determine the impact of sex on the pharmacokinetics of lopinavir/ritonavir. Interaction between lopinavir/ritonavir and tenofovir was also evaluated. Steady-state plasma samples were obtained from virologically suppressed HIV-infected patients on lopinavir/ritonavir 800/200-mg soft gel capsule taken once daily. Drug assays were performed by high-performance liquid chromatography. Pharmacokinetic parameters estimated by noncompartmental method were reported as 90% confidence intervals (CIs) about the geometric mean ratio (GMR). There were 9 males and 11 females. No sex differences were observed in lopinavir/ritonavir pharmacokinetics profile. The GMR(sex) (women compared with men) for lopinavir area under the concentration-time curve (AUC(24)), maximum concentration (C(max)), and minimum concentration (C(min)) was 0.95 (90% CI, 0.70-1.29), 0.88 (90% CI, 0.67-1.15), and 1.27 (90% CI, 0.60-2.66), respectively. Similarly, the GMR(sex) for ritonavir AUC(24), C(max), and C(min) was 0.84 (90% CI, 0.57-1.24), 0.79 (90% CI, 0.50-1.22), and 1.02 (90% CI, 0.58-1.80), respectively. Tenofovir coadministration led to a reduction in lopinavir/ritonavir plasma exposure, giving a lopinavir GMR(tenofovir) for C(max) of 0.72 (90% CI, 0.57-0.93) and AUC(24) of 0.74 (90% CI, 0.56-0.98), respectively. No difference in lopinavir/ritonavir plasma concentrations between sexes was demonstrated in this study. However, tenofovir coadministration lowered lopinavir/ritonavir plasma exposure.

Intracellular pharmacokinetics of once versus twice daily zidovudine and lamivudine in adolescents

Zidovudine (ZDV) and lamivudine (3TC) metabolism to triphosphates (TP) is necessary for antiviral activity. The aims of this study were to compare ZDV-TP and 3TC-TP concentrations in adolescents receiving twice daily (BID) and once daily (QD) regimens and to determine the metabolite concentrations of ZDV and 3TC during chronic therapy on a QD regimen. Human immunodeficiency virus-infected patients (12 to 24 years) taking ZDV (600 mg/day) and 3TC (300 mg/day) as part of a highly active antiretroviral therapy regimen received QD and BID regimens of ZDV and 3TC for 7 to 14 days in a crossover design. Serial blood samples were obtained over 24 h on the QD regimen. Intracellular mono-, di-, and triphosphates for ZDV and 3TC were measured. The median ratio of BID/QD for ZDV-TP predose concentrations was 1.28 (95% confidence interval [CI] = 1.00 to 2.45) and for 3TC-TP was 1.12 (95% CI = 0.81 to 1.96). The typical population estimated half-lives (+/- the standard error of the mean) were 9.1 +/- 0.859 h for ZDV-TP and 17.7 +/- 2.8 h for 3TC-TP. Most patients had detectable levels of the TP of ZDV (24 of 27) and 3TC (24 of 25) 24 h after dosing, and half-lives on a QD regimen were similar to previously reported values when the drugs were given BID. Lower, but not significantly different, concentrations of ZDV-TP were demonstrated in the QD regimen compared to the BID regimen (P = 0.056). Although findings were similar between the BID and QD groups, the lower concentrations of ZDV and the number of patients below the level of detection after 24 h suggests that ZDV should continue to be administered BID.

Drug-Sparing Regimens for HIV Combination Therapy: Benefits Predicted for “Drug Coasting”

Structured Treatment Interruptions (STI) during HIV drug therapy were thought to potentially reduce side effects and toxicity, boost immune involvement, and possibly lower the viral set-point. Clinical trials of STI regimens, however, have had mixed results. We use an established mathematical model of HAART to estimate possible therapeutic outcomes for STI and for other, similar patterns in HIV combination therapy. We perform an exhaustive search of patterns of up to 60 days, for triple-drug combinations involving accurate pharmacokinetics for 12 specific antiviral drugs. The results of this analysis are consistent with recent clinical trials which have demonstrated that STI-type patterns, involving therapy interruption of weeks or months, are rarely optimal. Our analysis predicts, however, that the benefit of treatment can often be improved by including very short drug-free periods, during which the patient effectively "coasts" for a day or two on adequate drug concentrations due to the long half-life of some pharmaceuticals. Our analysis predicts many cases in which this may be achieved without increasing the risk of drug-resistance. This suggests that "drug coasting" patterns, significantly shorter than STI patterns, may merit further clinical investigation in efforts to find drug-sparing regimens for HIV.

Intensification of a triple-nucleoside regimen with tenofovir or efavirenz in HIV-1-infected patients with virological suppression

OBJECTIVE

To compare a quadruple-nucleoside with an efavirenz-containing regimen for treatment of HIV-1 infection.

DESIGN

A randomized, open-label study of the AIDS Clinical Trials Group (ACTG).

METHODS

Subjects receiving zidovudine/lamivudine/abacavir on ACTG 5095 with HIV-1 RNA less than 200 copies/ml were randomly assigned to intensify either with tenofovir or efavirenz. Subjects were followed for time to treatment failure, defined as either virological failure or treatment discontinuation. Analyses were intent-to-treat.

RESULTS

One hundred and seventy subjects (21% women; 56% non-white) entered the study. At baseline, 95 and 73% had HIV-1-RNA levels less than 200 and 50 copies/ml, respectively; the median CD4 cell count was 453 cells/microl. Over a median 79 weeks follow-up, 165 (97%) completed the study, three (2%) discontinued, and two (1%) died. Treatment failure occurred in 31 subjects: 18 (21%) (quadruple nucleosides) and 13 (15%) (efavirenz-containing regimen); however the failure-time curves crossed and demonstrated a non-constant treatment effect over time, characterized by more early treatment failures on the efavirenz-containing regimen and more late treatment failures on the four-nucleoside regimen. HIV-1 RNA remained suppressed in more than 88% of subjects to less than 200 copies/ml and in more than 78% to less than 50 copies/ml at weeks 24, 48, and 72, without differences by treatment arm. There were no significant differences between the regimens in CD4 cell increases, time to new grade 3/4 adverse events, or adherence.

CONCLUSION

The safety, tolerability, and efficacy of the four-nucleoside regimen were not significantly different from the efavirenz-containing regimen. These pilot data support further investigation of the quadruple-nucleoside regimen.

La résistance du VIH-1 aux antirétroviraux chez les enfants infectés : du nouveau-né à l'adolescent

Limit of antiretroviral treatment success is the emergence of drug-resistant virus. As reported in adult population, prevalence of resistance was high in treated HIV-infected children with detectable HIV viral load. Resistance increased with number of prior antiretroviral treatments, particularly with protease inhibitors. Adolescent boys seem at greater risk to harbor multi-classes resistant virus. In HIV-infected newborns, prevalence of resistance was 20%. Most of resistance mutations detected were in accord to perinatal antiretroviral exposition. Principal mechanism of resistance acquisition in newborns was transmission of resistant viruses from mother to child with early archive in cellular reservoir and long term persistence with or without treatment. Consequences of long term therapeutic strategies in children are major.

Sex differences in pharmacokinetics and toxicity of antiretroviral therapy

Worldwide, the rate of new HIV infections continues to increase among women. Over the past 5 years there has been a growing focus on the clinical aspects of HIV infection among women. Historically, the majority of subjects enrolled in HIV clinical trials were male patients. Consequently, most knowledge about antiretroviral efficacy and toxicity has been derived from studies of predominately male subjects. More recently, results from clinical trials, especially those that have focused on HIV-infected female subjects, suggest that there may be clinically important gender-related differences in several aspects of HIV disease. These include, but are not limited to, differences in natural history, efficacy and safety of drug treatment. This chapter reviews current data on gender differences in antiretroviral pharmacokinetics and toxicity.

Genetic, ethnic, and gender differences in the pharmacokinetics of antiretroviral agents

Variable antiretroviral therapy (ART) drug response likely reflects the combined influence of environment, underlying disease, concurrent drugs, and genetics. Gender exerts modest or negligible effects on ART disposition, and it is expected to have limited clinical implication, although it should be accounted for in large population studies. Ethnic denominations have, with the notable exception of efavirenz, no clear influence on ART disposition. Exploration of genetic factors might offer a better comprehension to the largely unpredictable and unresolved variability in ART concentrations and related toxicity or treatment outcome. Despite the negative perception of genetic research among the general public, this type of investigation is now widely accepted by concerned parties: patients, relatives, and study volunteers.

Model for intracellular Lamivudine metabolism in peripheral blood mononuclear cells ex vivo and in human immunodeficiency virus type 1-infected adolescents

The pharmacologic variability of nucleoside reverse transcriptase inhibitors such as lamivudine (3TC) includes not only systemic pharmacokinetic variability but also interindividual differences in cellular transport and metabolism. A modeling strategy linking laboratory studies of intracellular 3TC disposition with clinical studies in adolescent patients is described. Data from ex vivo laboratory experiments using peripheral blood mononuclear cells (PBMCs) from uninfected human subjects were first used to determine a model and population parameter estimates for 3TC cellular metabolism. Clinical study data from human immunodeficiency virus type 1-infected adolescents were then used in a Bayesian population analysis, together with the prior information from the ex vivo analysis, to develop a population model for 3TC systemic kinetics and cellular kinetics in PBMCs from patients during chronic therapy. The laboratory results demonstrate that the phosphorylation of 3TC is saturable under clinically relevant concentrations, that there is a rapid equilibrium between 3TC monophosphate and diphosphate and between 3TC diphosphate and triphosphate, and that 3TC triphosphate is recycled to 3TC monophosphate through a 3TC metabolite that remains to be definitively characterized. The resulting population model shows substantial interindividual variability in the cellular kinetics of 3TC with population coefficients of variation for model parameters ranging from 47 to 87%. This two-step ex vivo/clinical modeling approach using Bayesian population modeling of 3TC that links laboratory and clinical data has potential application for other drugs whose intracellular pharmacology is a major determinant of activity and/or toxicity.

The impact of sex and contraceptive therapy on the plasma and intracellular pharmacokinetics of zidovudine

OBJECTIVES

Zidovudine remains part of combination antiretroviral therapy. Pharmacological studies rely on quantitation of active triphosphates in peripheral blood mononuclear cells. This study evaluated the impact of female sex and contraceptive therapy on zidovudine plasma and intracellular pharmacokinetics and the impact of contraceptive therapy on HIV viral load.

METHODS

Serial plasma and intracellular zidovudine pharmacokinetics following oral and intravenous dosing were determined in 18 men and 20 women treated with zidovudine. Women could repeat pharmacokinetics assessment following 2 months oral or injectable contraceptive therapy. Zidovudine plasma and intracellular mono-, di- and triphosphate concentrations were determined by liquid chromatography tandem mass spectrometry. Plasma and cervical viral loads were determined preceding and following 2 months of contraceptive therapy in women.

RESULTS

Men exhibited higher area under the concentration versus time curve for intracellular zidovudine and zidovudine-monophosphate following oral and intravenous dosing and higher zidovudine triphosphate following oral dosing. There was no difference between men and women in plasma zidovudine parameters. Furthermore, contraceptive therapy had no effect on zidovudine plasma or intracellular pharmacokinetics or on plasma or cervical HIV-1 RNA levels.

CONCLUSIONS

Using an optimized pharmacokinetic design, this study indicated men exhibit significantly higher zidovudine-monophosphate and zidovudine-triphosphate exposure following zidovudine oral administration, having implications for drug toxicity and overall tolerance of zidovudine therapy. The lack of an effect of contraceptive therapy on zidovudine pharmacokinetics is surprising in light of previous pharmacokinetic studies for drugs eliminated primarily through glucuronidation. Contraceptive therapy had no effect on plasma or cervical viral load, results consistent with previous findings.

The clinical implications of antiretroviral pharmacogenomics

Heterogeneity exists in the effectiveness and toxic effects of antiretroviral agents between individuals and populations. Although patient-related clinical variables such as age, sex and ethnic origin have been associated with drug response, inherited predispositions may have a significant effect on treatment outcome. The role of host and pathogen pharmacogenomics is gaining increasing interest in the field of both antiretrovirals in development, such as the chemokine (C-C motif) receptor 5 (CCR5) inhibitors, and in established therapies where toxicity and efficacy may be predicted. Despite numerous studies available in the literature, the interpretation of the relationship between genetic polymorphisms and clinical outcomes is often posed with many confounding variables, making clinical interpretations of these results difficult. This review summarizes the key findings in the growing knowledge between human genetics and response to antiretroviral drugs and how these findings may be effectively applied in a clinical context.

Pharmacogenetic characteristics of indinavir, zidovudine, and lamivudine therapy in HIV-infected adults: a pilot study

OBJECTIVE

The aim of the study was to investigate relationships among indinavir, lamivudine-triphosphate, and zidovudine-triphosphate pharmacokinetics and pharmacodynamics with polymorphisms in CYP3A5, MDR1, MRP2, MRP4, BCRP, and UGT1A1 genes.

STUDY DESIGN

Retrospective pilot investigation among 33 subjects who participated in a randomized pharmacological study of indinavir, lamivudine, and zidovudine. Subjects were defined as genetic variant carriers or not. Relationships were investigated with multivariable regression. Indinavir clearance was adjusted for African American race; triphosphates for sex; and HIV-response for study arm, drug exposure, and baseline HIV-RNA.

RESULTS

Genetically determined CYP3A5 expressors had 44% faster indinavir oral clearance versus nonexpressors (P = 0.002). MRP2-24C/T variant carriers had 24% faster indinavir oral clearance (P = 0.05). Lamivudine-triphosphate concentrations were elevated 20% in MRP4 T4131G variant carriers (P = 0.004). A trend for elevated zidovudine-triphosphates was observed in MRP4 G3724A variant carriers (P = 0.06). The log10 changes in HIV-RNA from baseline to week 52 were -3.7 for MDR1 2677 TT, -3.2 for GT, and -2.2 for GG (P < 0.05). Bilirubin increases were 2-fold higher in UGT1A1 [TA]7/[TA]7 genotypes. No relationships were identified with BCRP.

DISCUSSION

Novel relationships were identified among genetic variants in drug transporters and indinavir, lamivudine-triphosphate, and zidovudine-triphosphate concentrations. CYP3A5 expression was associated with faster indinavir oral clearance. These pilot data provide a scientific basis for more rational utilization of antiretroviral drugs.

Host determinants of antiretroviral drug activity

PURPOSE OF REVIEW

As physicians are confronted with a diversity of adverse events and variability in response to medication among patients, this review will focus on the available evidence regarding the impact of genetic variation on drug response. Specifically, variation in drug metabolizing enzymes such as the cytochrome P450s, drug transporters and human leucocyte antigen (for idiosyncratic drug reactions). The potential role of pharmacogenetics in the management of HIV-1 infected individuals and drug discovery will be discussed.

RECENT FINDINGS

Wide inter-individual variability of antiretroviral therapy efficacy and toxicity in HIV-infected patients has been extensively reported in the literature. Since treatment involves multiple drugs and drug classes with the potential for significant variability in drug-host as well as drug-drug interactions, antiretroviral drug therapy represents a significant challenge. Despite this inherent complexity, considerable recent advances have led to a greater understanding of interactions between genetic and host factors that influence the efficacy and toxicity of these agents.

SUMMARY

The goal of pharmacogenetics in antiretroviral therapy is to outline differences within a given population that influence drug efficacy and toxicity. Although to date, despite the numerous studies available in the literature, conclusions on how the analysis of the relationship between single nucleotide polymorphisms and drug efficacy may not always be clinically useful. Further studies are warranted to clarify the role of pharmacogenetics and antiretroviral drug management.

Pharmacogenetics of plasma efavirenz exposure after treatment discontinuation: an Adult AIDS Clinical Trials Group Study

BACKGROUND

Efavirenz has a long plasma half-life and a low genetic barrier to resistance. Simultaneously stopping treatment with all agents in efavirenz-containing regimens may result in functional efavirenz monotherapy that selects for drug-resistant human immunodeficiency virus type 1. Lower plasma efavirenz clearance is associated with a cytochrome P450 2B6 gene (CYP2B6) polymorphism (516G-->T) that is more frequent among African American individuals than among European American individuals.

METHODS

We characterized relationships between this polymorphism and predicted plasma efavirenz concentration-time profiles after discontinuation of therapy with use of data obtained from subjects receiving therapy. Pharmacokinetic parameters were estimated using population-based methods. Concentrations after discontinuation of therapy were predicted from subject-specific estimates. RESULTS. Median estimated efavirenz half-lives were 23, 27, and 48 h for patients with CYP2B6 position 516 GG (78 patients), GT (60), and TT (14) genotypes, respectively (P<.001). After therapy was stopped, plasma efavirenz concentrations in patients with GG, GT, and TT genotypes were predicted to exceed 46.7 ng/mL (the estimated protein-adjusted 95% inhibitory concentration for wild-type virus) for a median of 5.8 days (interquartile range [IQR], 4.4-8.3 days), 7.0 days (IQR, 5.0-8.0 days), and 14 days (IQR, 11.1-21.2 days), respectively (P<.001). Plasma efavirenz levels were predicted to exceed 46.7 ng/mL for >21 days in 5% of subjects with GG genotype, 5% of subjects with GT genotype, and 29% of subjects with TT genotype.

CONCLUSIONS

The CYP2B6 position 516 TT genotype or a prolonged measured elimination half-life may predict increased risk of developing drug resistance among patients who discontinue efavirenz-containing regimens. This has implications for strategies to safely discontinue antiretroviral regimens while avoiding the emergence of drug resistance.

Role of the Inhibitory Quotient in HIV Therapy

A systemic review is presented of all studies that have evaluated the inhibitory quotient (IQ). The IQ is defined as the ratio between (trough) drug concentration and level of drug resistance of the HIV isolate. From the studies presented, it can be concluded that for protease inhibitors (PIs) and efavirenz, the phenotypic IQ is associated with virological response. The genotypic IQ (GIQ) for PIs was also demonstrated to be associated with virological response. An intrinsic limitation of the GIQ is that it is only applicable for PIs, of which resistance is based on the cumulative effect of mutations. As the IQ can be modified by adjustment of the drug dosage, it may be of clinical value. Its application in patient care should therefore be further investigated.

The pharmacology of antiretroviral nucleoside and nucleotide reverse transcriptase inhibitors: implications for once-daily dosing

The trend toward once-daily dosing in HIV antiretroviral therapy is based on the association between adherence, treatment outcome, and patient preferences. Patients prefer simpler treatments, fewer pills, less frequent dosing, and no food restrictions. When a regimen meets a patient's preferences, the patient is more likely to be adherent, and with good adherence, the regimen is more likely to be effective. Nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) have been a prime focus for developing once-daily therapies primarily because they form the backbone of most current regimens. Within the NRTI class, however, drugs differ in their pharmacokinetic properties, such as plasma and intracellular half-lives, and thus in their suitability for once-daily dosing. For example, newer NRTIs, such as tenofovir and emtricitabine, combine longer plasma half-lives with longer intracellular half-lives, prolonging exposure and the period of pharmacologic activity. Of equal importance, the clinical impact of systemic and intracellular interactions between concomitant drugs defines which once-daily drugs may be combined in once-daily regimens. To construct simplified and effective therapies for individual patients, clinicians require an understanding of the plasma and intracellular pharmacokinetic properties of NRTIs and how these properties determine a drug's appropriateness for once-daily dosing and placement within a once-daily regimen.

Current Status and Future Prospects of Therapeutic Drug Monitoring and Applied Clinical Pharmacology in Antiretroviral Therapy

The consensus of current international guidelines for the treatment of HIV infection is that data on therapeutic drug monitoring (TDM) of non-nucleoside reverse transcriptase inhibitors and protease inhibitors provide a framework for the implementation of TDM in certain defined scenarios in clinical practice. However, the utility of TDM is considered to be on an individual basis until more data are obtained from large clinical trials showing the benefit of TDM.

In April 2004, a panel of experts met in Rome, Italy. This followed an inaugural meeting in Perugia, Italy, in October 2000, which resulted in the article published in AIDS 2002, 16(Suppl 1):S5–S37. The objectives of this second meeting were to review the questions surrounding TDM of antiretroviral drugs and discuss the clinical utility, current concerns and future prospects of drug concentration monitoring in the care of HIV-1-infected individuals.

This report, which has been updated to include material published or presented at international conferences up to the end of September 2004, reviews pharmacokinetic and pharmacodynamic data and reports the issues discussed by the panel, offering advice to clinical care providers who may be currently, or are considering incorporating TDM into the routine care of their patients. In addition, the panel formulated a series of position statements that are relevant to the interpretation of current data and can aid the design of future clinical trials.

Part 2 of this Special article, Therapeutic drug monitoring and drug–drug interactions involving antiretroviral drugs, will be published in Antiviral Therapy 10(4).