The implications of P-glycoprotein in HIV: friend or foe?

Effect of ABCB1 most frequent polymorphisms on the accumulation of bictegravir in recombinant HEK293 cell lines

Bictegravir, a key second-generation integrase strand transfer inhibitor in the treatment of HIV, is subject to active efflux transport mediated by ABCB1 (P-glycoprotein). Several coding variants of ABCB1 have been described and associated with variable effects on substrate drugs pharmacokinetics. Here, we investigated the effect of the four most common coding ABCB1 single nucleotide polymorphisms (i.e., c.1199G > A, c.1236C > T, c.2677G > T and c.3435C > T) on the intracellular accumulation of bictegravir. Using a previously validated HEK293 recombinant cell line model, we found decreased bictegravir intracellular concentrations in cell lines overexpressing ABCB1 as compared to control cell lines, in line with the known role of ABCB1 in bictegravir transport. However, we were unable to demonstrate any significant difference in bictegravir intracellular accumulation when comparing HEK293 cells overexpressing the wild type (1236C-2677G-3435C, 1199G) or the variant (1236C-2677G-3435T, 1236T-2677T-3435T or 1199A) proteins. These findings suggest that the ABCB1 c.1199G > A and c.1236C > T-c.2677G > T-c.3435C > T variants have no or at least limited impact on the active transport of bictegravir by ABCB1.

Analysis of drug-drug interactions among patients receiving antiretroviral regimens using data from a large open-source prescription database

PURPOSE

Results of a study of contraindicated concomitant medication use among recipients of preferred antiretroviral therapy (ART) regimens are reported.

METHODS

A retrospective study was conducted to evaluate concomitant medication use in a cohort of previously treatment-naive, human immunodeficiency virus (HIV)-infected U.S. patients prescribed preferred ART regimens during the period April 2014-March 2015. Data were obtained from a proprietary longitudinal prescription database; elements retrieved included age, sex, and prescription data. The outcome of interest was the frequency of drug-drug interactions (DDIs) associated with concomitant use of contraindicated medications.

RESULTS

Data on 25,919 unique treatment-naive patients who used a preferred ART regimen were collected. Overall, there were 384 instances in which a contraindicated medication was dispensed for concurrent use with a recommended ART regimen. Rates of contraindicated concomitant medication use differed significantly by ART regimen; the highest rate (3.2%) was for darunavir plus ritonavir plus emtricitabine-tenofovir disoproxil fumarate (DRV plus RTV plus FTC/TDF), followed by elvitegravir-cobicistat-emtricitabine-tenofovir disoproxil fumarate (EVG/c/FTC/TDF)(2.8%). The highest frequencies of DDIs were associated with ART regimens that included a pharmacoenhancing agent: DRV plus RTV plus FTC/TDF (3.2%) and EVG/c/FTC/TDF (2.8%).

CONCLUSION

In a large population of treatment-naive HIV-infected patients, ART regimens that contained a pharmacoenhancing agent were involved most frequently in contraindicated medication-related DDIs. All of the DDIs could have been avoided by using therapeutic alternatives within the same class not associated with a DDI.

Common ABCB1 polymorphisms in Greek patients with chronic hepatitis C infection: A comparison with hyperlipidemic patients and the general population

BACKGROUND

Hepatitis C virus infectivity and replication efficiency appears to be dependent on the lipid content and organization of the plasma membrane of the host cell, as well as of the intracellular membranous web. As there is increasing awareness of a role played by the efflux pump ABCB1 (p-glycoprotein, P-gp) in lipid homeostasis, its function could be a determinant of chronic HCV infection. The aim of the present study was to examine and compare the distribution of common ABCB1 genotypes in patients with chronic HCV infection (n=168), hyperlipidemic patients (n=168) and a control group (n=173), all from Greece.

METHODS

Participants were genotyped for the ABCB12677G>T/A and 3435C>T polymorphisms with previously reported PCR-RFLP methods. Genotype and allele frequency distributions were compared between the three groups with the χ(2) test of independence.

RESULTS

The ABCB1 2677GG (ancestral) genotypes were significantly over-represented in patients with chronic hepatitis C compared to controls (39.3% vs. 26.6%, p=0.015 according to the dominant model). A similar result was obtained when hyperlipidemic patients were compared to controls (45.2% vs. 26.6%, p<0.001 according to the dominant model). Comparison of ABCB1 3435C>T genotype and allele distributions provided similar but not as significant differences. Genotype and allele distributions for both ABCB12677G>T/A and 3435C>T were very similar between HCV patients and hyperlipidemic patients.

CONCLUSION

Our findings imply an influence of ABCB1 polymorphisms on HCV infectivity, possibly through an effect on lipid homeostasis.

The pharmacogenetics of antiretroviral therapy

PURPOSE OF REVIEW

Although the past 20 years have seen many advances in HIV pharmacotherapy, host variability has only been more recently recognized as an important driver of both treatment response and toxicity. This review will focus on the importance of variability in drug metabolizing enzymes and the potential research and clinical applications of these findings in HIV care.

RECENT FINDINGS

The treatment of HIV is complex and involves chronic treatment with multiple drugs and drug classes. Recent research has been important in uncovering genetic differences in drug metabolizing and drug transporter genes, which underpin dose-dependent toxicity and efficacy, but also in genes defining differences in immune response and human leukocyte antigen genes, which restrict processes that are less dependent on the drug dose.

SUMMARY

The advancement of pharmacogenetics will continue to further our knowledge of disease-drug interactions and pathogenesis. Ultimately, the goals of pharmacogenetics and pharmacogenomics in HIV medicine will be the development of the right drugs for the right patient or population. The strong association between HLA-B5701 and abacavir hypersensitivity reaction shows promise for a genetic screening test to be feasibly incorporated into clinical practice. The clinical applicability of many findings demonstrating associations between single nucleotide polymorphisms, particularly in drug metabolism and transporter genes and drug efficacy and toxicity, are currently uncertain.

Chemokine (C-C motif) receptor 5 -2459 genotype in patients receiving highly active antiretroviral therapy: race-specific influence on virologic success

BACKGROUND

In patients receiving highly active antiretroviral therapy (HAART), antiretroviral drug-metabolizing enzyme and transporter gene polymorphisms, as well as chemokine receptor gene polymorphisms, may influence response to treatment.

METHODS

In a North American, treated, adherent human immunodeficiency virus (HIV)-positive cohort (self-identified whites, n = 175; blacks, n = 218), we investigated whether CYP2B6 (516G>T, 983T>C), UGT2B7 (IVS1+985A>G, 802C>T), MDR1 3435C>T, chemokine (C-C motif) receptor 2 (CCR2) 190G>A, and CCR5 (-2459G>A, Δ32) polymorphisms influenced the time to achieve virologic success (TVLS).

RESULTS

No difference in TVLS was observed between races. In Kaplan-Meier analyses, only 516G>T (log-rank P = .045 for comparison of GG, GT, and TT and P = .02 GG + GT vs TT) and -2459G>A (log-rank P = .04 for GG, GA, and AA and P = .02 for GG + GA vs AA) genotypes were significantly associated with TVLS in black patients but not in white patients. However, in the Cox proportional hazards model that included age, sex, baseline CD4(+) T cell count, and baseline viral load, no significant association was observed between 516G>T and TVLS, whereas the association between -2459G>A and TVLS remained significant even after including CCR2 190G>A as well as all the drug-metabolizing enzyme and transporter genotypes.

CONCLUSIONS

These findings suggest that CCR5 -2459G>A genotype had a strong, race-specific influence on TVLS in this cohort. Understanding the possible mechanisms underlying this influence requires further studies.

Switch from enfuvirtide to raltegravir lowers plasma concentrations of darunavir and tipranavir: a pharmacokinetic substudy of the EASIER-ANRS 138 trial

We compared tipranavir and darunavir concentrations measured at steady state in 20 human immunodeficiency virus (HIV)-infected patients enrolled in the EASIER-ANRS 138 clinical trial who switched from enfuvirtide to raltegravir while maintaining the same background regimen. The geometric mean ratios of the observed predose concentration (C(trough)), maximum concentration of drug observed in plasma (C(max)), and area under the plasma concentration-time curve (AUC) before (day 0) and after (week 24) the switch were 0.49, 0.76, and 0.67 and 0.82, 0.68, and 0.64 for tipranavir and darunavir, respectively. The virologic consequences of these drug interactions have yet to be determined.

Inhibition of P-glycoprotein-mediated Multidrug Resistance (MDR) by N,N-bis(cyclohexanol)amine aryl esters: further restriction of molecular flexibility maintains high potency and efficacy

Conformational modulation of the aryl portion of a set of N,N-bis(cyclohexanol)amine aryl esters (1a-d) that are potent Pgp-dependent MDR inhibitors has been performed. Toward this end the trans-3-(3,4,5-trimethoxyphenyl)acrylic acid present in set 1 was substituted with 3-(3,4,5-trimethoxyphenyl)propanoic and 3-(3,4,5-trimethoxyphenyl)propiolic moieties to give sets 2 and 3, respectively. While the introduction of 3-(3,4,5-trimethoxyphenyl)propanoic moiety resulted in a definite drop in potency and efficacy, esterification with 3-(3,4,5-trimethoxyphenyl)propiolic acid gave four isomers (3a-d) that maintain high potency and possess optimal efficacy. These results are discussed in terms of conformational flexibility of the different sets of compounds.

P-glycoprotein related drug interactions: clinical importance and a consideration of disease states

IMPORTANCE OF THE FIELD

P-glycoprotein (P-gp) is the most characterized drug transporter in terms of its clinical relevance for pharmacokinetic disposition and interaction with other medicines. Clinically significant P-gp related drug interactions appear restricted to digoxin. P-gp may act as a major barrier to current and effective drug treatment in a number of diseases including cancer, AIDS, Alzheimer's and epilepsy due to its expression in tumors, lymphocytes, cell membranes of brain capillaries and the choroid plexus.

AREAS COVERED IN THIS REVIEW

This review summarizes the current understanding of P-gp structure/function, clinical importance of P-gp related drug interactions and the modulatory role this transporter may contribute towards drug efficacy in disease states such as cancer, AIDS, Alzheimer's and epilepsy.

WHAT THE READER WILL GAIN

The reader will gain an understanding that the clinical relevance of P-gp in drug interactions is limited. In certain disease states, P-gp in barrier tissues can modulate changes in regional distribution.

TAKE HOME MESSAGE

P-gp inhibition in isolation will not result in clinically important alterations in systemic exposure; however, P-gp transport may be of significance in barrier tissues (tumors, lymphocytes, brain) resulting in attenuated efficacy.

Effects of experimental setup on the apparent concentration dependency of active efflux transport in in vitro cell permeation experiments

P-Glycoprotein mediated efflux is one of the barriers limiting drug absorption from the intestine. Predictions of the intestinal P-glycoprotein function need to take into account the concentration dependency because high intestinal drug concentrations may saturate P-glycoprotein. However, the substrate binding site of P-glycoprotein lies inside the cells and the drug concentration at the binding site cannot be measured directly. Therefore, rigorous determination of concentration dependent P-glycoprotein kinetics is challenging. In this study, the effects of the aqueous boundary layers, extracellular pH and cellular retention on the apparent saturation kinetics of P-glycoprotein mediated transport of quinidine in an in vitro cell permeation setting were explored. The changes in the experimental conditions caused 1 order of magnitude variation in the apparent affinity to P-glycoprotein (K(m,app)) and a 5-fold difference in the maximum effective P-glycoprotein mediated transport rate of quinidine (V(max,app)). However, fitting the concentration data into a compartmental model which accounted for the aqueous boundary layers, cell membranes and cellular retention suggested that the P-glycoprotein function per se was not altered, it was the differences in the passive transfer of quinidine which changed the apparent transport kinetics. These results provide further insight into the dynamics of the P-glycoprotein mediated transport and on the roles of several confounding factors involved in in vitro experimental setting. Further, the results confirm the applicability of compartmental model based data analysis approach in the determination of active transporter kinetics.

Protease inhibitors atazanavir, lopinavir and ritonavir are potent blockers, but poor substrates, of ABC transporters in a broad panel of ABC transporter-overexpressing cell lines

OBJECTIVES

A possible mechanism for HIV therapy failure is the efflux of HIV drugs from viral target cells or certain body compartments by ATP-binding cassette (ABC) transporters, allowing ongoing viral replication. Here, we investigated the interaction between protease inhibitors (PIs) and ABC transporters.

METHODS

To explore the potential blocking capacity of PIs, we exposed cells overexpressing multidrug resistance 1 P-glycoprotein (MDR1 P-gp), multidrug resistance protein 1 (MRP1) and breast cancer resistance protein (BCRP) to established cytotoxic substrates with or without one of the PIs atazanavir, lopinavir or ritonavir. Furthermore, to assess whether PIs serve as substrates, cell growth-inhibitory effects of these PIs were evaluated on cells overexpressing 1 of 11 ABC transporters and their parental counterparts.

RESULTS

Atazanavir, lopinavir and ritonavir were highly effective in reversing resistance against established substrates in cells overexpressing MDR1 P-gp and MRP1, and, to a lesser extent, BCRP. Concurrently, however, PIs appeared to be relatively poor substrates for ABC transporters. Only a moderate level of resistance to atazanavir was observed in cells overexpressing MRP6 and MRP9 [resistance factor (RF): 2.0-2.6]. Cells overexpressing MDR1 P-gp, MRP3, MRP4 and MRP5 displayed low levels of resistance to atazanavir (RF: 1.3-1.7); MRP7- and MRP9-overexpressing cells to lopinavir (RF: 1.4-1.5); and MRP9-overexpressing cells to ritonavir (RF: 1.4).

CONCLUSIONS

PIs can act as potent blockers of MDR1 P-gp, MRP1 and BCRP, but they are poor substrates for 11 ABC transporters. Consequently, ABC transporters are unlikely to play a major role in PI failure, but still may contribute to drug-specific adverse events and drug-drug interactions.

Role of macrophages in HIV infection and persistence

Current antiretroviral therapy regimens can effectively suppress HIV in patients for prolonged periods of time, but do not constitute a cure, since they are incapable of eradicating viral reservoirs. It is, therefore, necessary for us to refocus on the partially understood pathogenesis of HIV, on the issue of viral persistence, and on the development of strategies for a temporally contained therapy capable of purging HIV from the body. Macrophages play a pivotal role in all three of these scenarios. This review summarizes important aspects of macrophage biology as they relate to HIV and discusses conceptual challenges for virus suppression and eradication in this cell type. We highlight a number of significant recent advances in understanding differences in HIV replication and pharmacotherapy between macrophages and CD4 T cells, as well as the role of macrophages in various aspects of the disease process and in different anatomical compartments. Finally, the importance of infected macrophages in the persistence of HIV, regarding both pathogenesis and advancement of eradication strategies, is discussed.

MDR1 3435T and 1236T alleles delay disease progression to pediatric AIDS but have no effect on HIV-1 vertical transmission

BACKGROUND

Single nucleotide polymorphisms (SNPs) in the MDR1 gene, coding for the drug transporter P-glycoprotein, may modulate the response to antiretroviral therapy and susceptibility to HIV-1 infection. We investigated whether the MDR1 SNPs C1236T (exon 12) and C3435T (exon 26) affect HIV-1 vertical transmission and progression to pediatric AIDS.

METHODS

The MDR1 genotypes were identified by PCR-restriction fragment length polymorphism (RFLP) assays in 219 HIV-infected, 128 exposed uninfected children and 231 HIV-seronegative blood donors. Genotype and haplotype frequencies were estimated in the different groups. The median follow-up time of the infected cohort was 108 months and AIDS-free time was evaluated for the different MDR1 genotypes in 171 HIV-infected children.

RESULTS

We found that both C1236T and C3435T polymorphisms were highly frequent in the studied groups (approximately 0.44) and showed strong linkage disequilibrium. There was no association between MDR1 genotypes and HIV-1 vertical transmission. However, a protective effect against progression to AIDS was associated with MDR1 3435CT, 1236CT and 1236TT genotypes (P = 0.005, P = 0.024 and P = 0.026, respectively). Moreover, haplotype pairs' analysis showed that the 3435CT/1236CT and 3435CT/1236TT exerted a significant protection against progression to pediatric AIDS (P = 0.0025 and P = 0.006, respectively).

CONCLUSION

We conclude that in Argentinean children, MDR1 genotypes are associated with progression to AIDS, but they do not affect HIV-1 susceptibility by vertical transmission. These results support the notion that P-glycoprotein plays a role in HIV-1 infection independently from its role in drug transport.

Impact of ATP-binding cassette transporters on human immunodeficiency virus therapy

Even though potent antiretrovirals are available against human immunodeficiency virus (HIV)-1 infection, therapy fails in a significant fraction of patients. Among the most relevant reasons for treatment failure are drug toxicity and side effects, but also the development of viral resistance towards the drugs applied. Efflux by ATP-binding cassette (ABC-) transporters represents one major mechanism influencing the pharmacokinetics of antiretroviral drugs and particularly their distribution, thus modifiying the concentration within the infected cells, that is, at the site of action. Moreover, drug-drug interactions may occur at the level of these transporters and modulate their activity or expression thus influencing the efficacy and toxicity of the substrate drugs. This review summarizes current knowledge on the interaction of antiretrovirals used for HIV-1 therapy with ABC-transporters and highlights the impact of ABC-transporters for cellular resistance and therapeutic success. Moreover, the suitability of different cell models for studying the interaction of antiretrovirals with ABC-transporters is discussed.

Pharmacogenetics of antiretroviral agents

PURPOSE OF REVIEW

The past year has seen the first whole genome association study for determinants of host control of HIV, as well as a number of hypothesis-driven candidate gene studies defining determinants of pharmacokinetics and toxicity of antiretroviral drugs. This review summarizes some of these findings, but it must be noted that the field is moving with unprecedented speed.

RECENT FINDINGS

A number of novel polymorphisms have been reported in the CYP2B6 locus that influence pharmacokinetics of non-nucleoside reverse transcriptase inhibitors. Among these are some novel nonsynonomous single nucleotide polymorphisms such as 983T > C (CYP2B6 18) and 499C > G (CYP2B6 26), as well as a partial deletion (CYP2B6 29). In addition, the concept of dose reduction according to CYP2B6 genotype has now been tested with some promising but preliminary results. Some other important advances in our knowledge have also been made, such as the association of TA repeats in the UGT1A1 regulatory region (UGT1A1 28) with atazanavir-related hyperbilirubinaemia and the association of ABCC2 and ABCC4 single nucleotide polymorphisms with tenofovir-associated renal toxicity.

SUMMARY

Treatment response to antiretrovirals is governed by genetic and environmental factors as well as adherence to therapy. Variability exists within pharmacological, immune and viral genes, and future studies must co-ordinate these issues.

A dual-fluorescence high-throughput cell line system for probing multidrug resistance

The efflux pump P-glycoprotein (ATP-binding cassette B1, multidrug resistance [MDR] 1, P-gp) has long been known to contribute to MDR against cancer chemotherapeutics. We describe the development of a dual-fluorescent cell line system to allow multiplexing of drug-sensitive and P-gp-mediated MDR cell lines. The parental OVCAR-8 human ovarian carcinoma cell line and the isogenic MDR NCI/ADR-RES subline, which stably expresses high levels of endogenous P-gp, were transfected to express the fluorescent proteins Discosoma sp. red fluorescent protein DsRed2 and enhanced green fluorescent protein, respectively. Co-culture conditions were defined, and fluorescent barcoding of each cell line allowed for the direct, simultaneous comparison of resistance to cytotoxic compounds in sensitive and MDR cell lines. We show that this assay system retains the phenotypes of the original lines and is suitable for multiplexing using confocal microscopy, flow cytometry, or laser scanning microplate cytometry in 1,536-well plates, enabling the high-throughput screening of large chemical libraries.

Comparative description of haplotype structure and genetic diversity of MDR1 (ABCB1) in HIV-positive and HIV-negative populations

Human P-glycoprotein (P-gp), encoded by MDR1 (ABCB1), is an efflux transporter with a wide specificity for substrates/drugs, including HIV protease inhibitors which are commonly used in HIV/AIDS treatment. Three single nucleotide polymorphisms (SNPs) in MDR1 have been shown to affect P-gp expression and function, and may affect HIV/AIDS treatment outcome: 1236C>T [G412G, exon-12], 2677G>T/A [A893S/T, exon-21] and 3435C>T [I1145I, exon-26]. In the present study, our aims were (i) to compare the 3-SNP MDR1 haplotype structure and genetic diversity between North American HIV-positive and HIV-negative individuals belonging to four major ethnic groups and (ii) to determine whether the haplotype structure and genetic diversity observed in these ethnically admixed populations differ from that in ethnically non-admixed populations. For these aims, we analyzed a cohort of 447 HIV/AIDS patients (White [n=193], Black [n=235], Hispanic [n=17], and Asian [n=2]). Results obtained for these patients were compared with the results for (i) HIV-negative individuals (n=356) and (ii) various HapMap and Environmental Genome Project populations. We observed that the genetic characteristics of MDR1 were largely consistent between HIV-positive and HIV-negative populations, but there were striking interethnic differences in the genetic characteristics of MDR1 in both populations. Although it appeared that the genetic characteristics of MDR1 were largely consistent between ethnically admixed and non-admixed populations, genetic characterization of the admixed populations remains to be done. Thus, our results provide useful comparative insights about the genetic characteristics of MDR1 that could be extrapolated across population groups worldwide. For a meaningful interpretation of these results regarding HIV/AIDS treatment outcome, MDR1 haplotype/diplotype structure data, genetic characterization of population admixture, and polymorphisms in other relevant drug transporter and/or metabolizing enzyme genes should be considered in future clinical studies.

NanoART, neuroAIDS and CNS drug delivery

A broad range of nanomedicines is being developed to improve drug delivery for CNS disorders. The structure of the blood-brain barrier (BBB), the presence of efflux pumps and the expression of metabolic enzymes pose hurdles for drug-brain entry. Nanoformulations can circumvent the BBB to improve CNS-directed drug delivery by affecting such pumps and enzymes. Alternatively, they can be optimized to affect their size, shape, and protein and lipid coatings to facilitate drug uptake, release and ingress across the barrier. This is important as the brain is a sanctuary for a broad range of pathogens including HIV-1. Improved drug delivery to the CNS would affect pharmacokinetic and drug biodistribution properties. This article focuses on how nanotechnology can serve to improve the delivery of antiretroviral medicines, termed nanoART, across the BBB and affect the biodistribution and clinical benefit for HIV-1 disease.

Pharmacovirological impact of an integrase inhibitor on human immunodeficiency virus type 1 cDNA species in vivo

Clinical trials of the first approved integrase inhibitor (INI), raltegravir, have demonstrated a drop in the human immunodeficiency virus type 1 (HIV-1) RNA loads of infected patients that was unexpectedly more rapid than that with a potent reverse transcriptase inhibitor, and apparently dose independent. These clinical outcomes are not understood. In tissue culture, although their inhibition of integration is well documented, the effects of INIs on levels of unintegrated HIV-1 cDNAs have been variable. Furthermore, there has been no report to date on an INI's effect on these episomal species in vivo. Here, we show that prophylactic treatment of transgenic rats with the strand transfer INI GSK501015 reduced levels of viral integrants in the spleen by up to 99.7%. Episomal two-long-terminal-repeat (LTR) circles accumulated up to sevenfold in this secondary lymphoid organ, and this inversely correlated with the impact on the proviral burden. Contrasting raltegravir's dose-ranging study with HIV patients, titration of GSK501015 in HIV-infected animals demonstrated dependence of the INI's antiviral effect on its serum concentration. Furthermore, the in vivo 50% effective concentration calculated from these data best matched GSK501015's in vitro potency when serum protein binding was accounted for. Collectively, this study demonstrates a titratable, antipodal impact of an INI on integrated and episomal HIV-1 cDNAs in vivo. Based on these findings and known biological characteristics of viral episomes, we discuss how integrase inhibition may result in additional indirect antiviral effects that contribute to more rapid HIV-1 decay in HIV/AIDS patients.

HIV monotherapy with ritonavir-boosted protease inhibitors: a systematic review

OBJECTIVE

To assess the efficacy of ritonavir-boosted protease inhibitor monotherapy.

DESIGN AND METHODS

Systematic review of all protease inhibitor-monotherapy studies published in peer-reviewed journals or presented at conferences to date. Data of randomized controlled trials were pooled to yield common odds ratios.

RESULTS

Twenty-two protease inhibitor-monotherapy studies were identified. In the intent-to-treat analysis, 395 out of 582 (67.9%) patients had undetectable HIV-RNA at the end of follow-up. In the six randomized controlled trials (all lopinavir/ritonavir monotherapy), the risk of therapy failure was greater on monotherapy: 121 out of 364 (33.2%) patients on monotherapy against 64 out of 280 (22.9%) patients on HAART [pooled odds ratio 1.48 (95% confidence interval 1.02-2.13, P = 0.037)]. Regarding patients with successfully resuppressed HIV-RNA upon (re-)introducing nucleoside reverse transcriptase inhibitors (NRTIs) as nonfailures, the risk of therapy failure was comparable: 98 out of 364 (26.9%) against 64 out of 280 (22.9%) patients [odds ratio 1.05 (95% confidence interval 0.72-1.53, P = 0.81)].

CONCLUSION

The overall efficacy of ritonavir-boosted protease inhibitor monotherapy is inferior to HAART. The efficacy improves in patients started on monotherapy after suppressed HIV-RNA for at least 6 months. Ten percent of patients have viral rebound with HIV-RNA levels between 50 and 500 copies/ml. Possible explanations are lack of HIV suppression in particular cells or compartments, alternative resistance mechanisms, and nonadherence. Once proven that reintroduction of NRTIs, in patients with loss of viral suppression, is safe and effective, a broader use of simplification of HAART to protease inhibitor monotherapy might be justified. This review supports that the majority of patients with prolonged viral suppression on HAART can successfully be treated with protease inhibitor monotherapy. Arguments for this strategy are NRTI/NNRTI side effects, NRTI/NNRTI resistance, and costs.

Modulation of P-glycoprotein at the blood-brain barrier: opportunities to improve central nervous system pharmacotherapy

Pharmacotherapy of central nervous system (CNS) disorders (e.g., neurodegenerative diseases, epilepsy, brain cancer, and neuro-AIDS) is limited by the blood-brain barrier. P-glycoprotein, an ATP-driven, drug efflux transporter, is a critical element of that barrier. High level of expression, luminal membrane location, multispecificity, and high transport potency make P-glycoprotein a selective gatekeeper of the blood-brain barrier and thus a primary obstacle to drug delivery into the brain. As such, P-glycoprotein limits entry into the CNS for a large number of prescribed drugs, contributes to the poor success rate of CNS drug candidates, and probably contributes to patient-to-patient variability in response to CNS pharmacotherapy. Modulating P-glycoprotein could therefore improve drug delivery into the brain. Here we review the current understanding of signaling mechanisms responsible for the modulation of P-glycoprotein activity/expression at the blood-brain barrier with an emphasis on recent studies from our laboratories. Using intact brain capillaries from rats and mice, we have identified multiple extracellular and intracellular signals that regulate this transporter; several signaling pathways have been mapped. Three pathways are triggered by elements of the brain's innate immune response, one by glutamate, one by xenobiotic-nuclear receptor (pregnane X receptor) interactions, and one by elevated beta-amyloid levels. Signaling is complex, with several pathways sharing common signaling elements [tumor necrosis factor (TNF) receptor 1, endothelin (ET) B receptor, protein kinase C, and nitric-oxide synthase), suggesting a regulatory network. Several pathways include autocrine/paracrine elements, involving release of the proinflammatory cytokine, TNF-alpha, and the polypeptide hormone, ET-1. Finally, several steps in signaling are potential therapeutic targets that could be used to modulate P-glycoprotein activity in the clinic.

Effect of an antiretroviral regimen containing ritonavir boosted lopinavir on intestinal and hepatic CYP3A, CYP2D6 and P-glycoprotein in HIV-infected patients

This study aimed to quantify the inhibition of cytochrome P450 (CYP3A), CYP2D6, and P-glycoprotein in human immunodeficiency virus (HIV)-infected patients receiving an antiretroviral therapy (ART) containing ritonavir boosted lopinavir, and to identify factors influencing ritonavir and lopinavir pharmacokinetics. We measured activities of CYP3A, CYP2D6, and P-glycoprotein in 28 patients before and during ART using a cocktail phenotyping approach. Activities, demographics, and genetic polymorphisms in CYP3A, CYP2D6, and P-glycoprotein were tested as covariates. Oral midazolam clearance (overall CYP3A activity) decreased to 0.19-fold (90% confidence interval (CI), 0.15-0.23), hepatic midazolam clearance and intestinal midazolam availability changed to 0.24-fold (0.20-0.29) and 1.12-fold (1.00-1.26), respectively. In CYP2D6 extensive metabolizers, the plasma ratio AUC(dextromethorphan)/AUC(dextrorphan) increased to 2.92-fold (2.31-3.69). Digoxin area under the curve (AUC)(0-12) (P-glycoprotein activity) increased to 1.81-fold (1.56-2.09). Covariates had no major influence on lopinavir and ritonavir pharmacokinetics. In conclusion, CYP3A, CYP2D6, and P-glycoprotein are profoundly inhibited in patients receiving ritonavir boosted lopinavir. The covariates investigated are not useful for a priori dose selection.

The impact of host pharmacogenetics on antiretroviral drug disposition

The ability to predict efficacy and toxicity during antiretroviral therapy for HIV would be of obvious advantage. The substantial variability between patients in terms of bioavailability and distribution of current regimens is likely driven by genetic and environmental factors. Protease inhibitors and nucleoside/nucleotide reverse transcriptase inhibitors are metabolized by cytochrome P450 enzymes. Their bioavailability and excretion may also be affected by variability in drug transporters of the ABC and SLC families. In pharmacokinetics and efficacy studies, issues are complicated by multiple loci effects (driven by the large number of proteins contributing to disposition) and heterogeneity in both study populations and the virus (ie, the target). Some of these issues are now being tackled, but studies need to be sufficiently powered and the phenotype carefully characterized. This review aims to summarize the current understanding of pharmacogenetic determinants of antiretroviral disposition.

Pharmacokinetics and safety of saquinavir/ritonavir and omeprazole in HIV-infected subjects

We investigated the pharmacokinetics and safety of saquinavir/ritonavir when administered with omeprazole simultaneously and 2 h apart to human immunodeficiency virus (HIV) subjects. Saquinavir/ritonavir 12-h pharmacokinetics was assessed with and without omeprazole 40 mg. Subjects were randomized to group A (saquinavir/ritonavir and omeprazole simultaneously/2 h apart) or group B (saquinavir/ritonavir and omeprazole 2 h apart/simultaneously). Saquinavir/ritonavir pharmacokinetics was assessed on days 1, 8, and 22. Within-subject changes were evaluated by geometric mean ratios and 90% confidence interval (CI). Twelve subjects completed the study. GM (90% CI) for saquinavir area under the curve (AUC)(0-12) (ng h/ml), trough concentration (C(trough)) (ng/ml), and maximum concentration (C(max)) (ng/ml) were 14,698 (13,242-20,636), 433 (368-758), 2,513 (2,243-3,329) without omeprazole; 22,646 (18,536-131,861), 750 (619-1,280), 3,890 (3,223-5,133) with omeprazole simultaneously; and 24,549 (20,884-38,894), 851 (720-1,782), 4,141 (3,554-5,992) with omeprazole 2 h earlier. Simultaneous administration of omeprazole significantly increased saquinavir AUC(0-12), C(trough), and C(max) by 54, 73, and 55%, whereas staggered administration by 67, 97, and 65%. No grade 3/4 toxicity or lab abnormalities were observed. In the presence of omeprazole, saquinavir plasma exposure is significantly increased in HIV-infected subjects whether administered simultaneously or 2 h apart.

MDR1 and CYP3A4 polymorphisms are associated with HIV seropositivity in Israeli patients but do not influence the course of HIV disease

Recent studies have examined the relationship between polymorphic alleles of the MDR1 gene and the course of HIV. Such polymorphisms may alter the metabolism of antiretroviral medications or influence susceptibility to HIV infectivity. We therefore studied a polymorphism in MDR1 (C3435T), and the CYP3A4*1B variant allele, the latter of which has not been previously studied in HIV. Ninety-six patients of either Ethiopian (57) or Caucasian (39) ethnicity and 276 controls were studied including serial CD4 counts, clinical course, and AIDS-defining illnesses. For both ethnic groups, the C allele of MDR1 C3435T was highly associated with being infected with HIV (p < 0.0001) compared to controls, but genotype did not influence change in CD4 counts over time in the patients, whether or not they were treated with antiretrovirals. CYP3A4*1B was also significantly associated with being infected with HIV (p < 0.0001) both in heterozygotes and in homozygotes for the polymorphism, but only for Ethiopians (p < 0.023 compared to Caucasians, p = 0.44). CYP3A4*1B did not influence CD4 count or AIDS defining illnesses. We conclude that in Israeli patients, polymorphisms in drug metabolism and disposition genes may influence infectivity of HIV but do not influence the course of the disease. Larger studies are needed to confirm these findings.

Factors impacting the expression of membrane-bound proteins in lymphocytes from HIV-positive subjects

BACKGROUND

The cellular transport proteins ABCB1, ABCC1 and ABCG2 have been implicated in the efflux of some antiretroviral drugs, thus decreasing their intracellular concentrations. Decreased drug accumulation in lymphocytes may allow viral replication and the subsequent emergence of viral resistance leading to treatment failure. Expression of HIV co-receptors on the surface of lymphocytes may influence viral tropism and therefore viral pathogenicity and disease progression. Here, we describe the relationship between expression of transport proteins and chemokine receptors in lymphocytes isolated from HIV-infected individuals and also investigate their relationship with demographic, therapeutic and virological factors.

METHODS

Peripheral blood mononuclear cells (PBMC) isolated from HIV-positive individuals were co-stained for expression of CD4 and ABCB1, ABCC1, ABCG2, CXCR4 and CCR5. The influence of gender, ethnicity, treatment status, viral load and CD4 count was assessed on expression of each protein as well as correlations between expression of the proteins by univariate and multivariate analyses.

RESULTS

Expression of ABCB1 was independently associated with gender (n = 98) and expression of ABCG2 and CXCR4. Gender also correlated with expression of ABCC1 and CXCR4 in univariate analysis with lower expression being detected in females compared with males.

CONCLUSIONS

Here we confirm that the previously reported correlation between ABCB1 and CXCR4 observed in PBMC isolated from healthy volunteers is also found in HIV-positive individuals. The influence of gender on the expression of drug efflux proteins could be a determinant of intracellular drug concentrations in vivo.

The Effect of ABCB1 Polymorphism on the Pharmacokinetics of Saquinavir Alone and in Combination with Ritonavir

This genotype panel study investigated the effect of ABCB1 polymorphism in exon 26 (C3435T), exon 21 (G2677T/A), and exon 12 (C1236T) on saquinavir pharmacokinetics and on the expression and activity of P-glycoprotein (P-gp) in peripheral blood monocytic cells (PBMCs). One hundred and fifty healthy volunteers were genotyped to identify 15 TT3435 and 15 CC3435 individuals. In these individuals, saquinavir pharmacokinetics were assessed after administration of a single oral dose of saquinavir 1,000 mg and saquinavir/ritonavir 1,000/100 mg. PBMC P-gp expression and activity were assessed in 15 and 19 subjects. The co-administration of ritonavir on study day 2 caused a significant increase in saquinavir exposure, in both TT3435 and CC3435 individuals. No correlation was observed between the ABCB1 C3435T, G2677T/A, and C1236T polymorphisms, separately and in haplotypes, with saquinavir pharmacokinetics, administered with or without ritonavir and with PBMC P-gp expression and activity. In conclusion, ABCB1 polymorphism has no pronounced effect on saquinavir exposure.

Pharmacokinetic Interactions between Buprenorphine and Antiretroviral Medications

Buprenorphine is used for the treatment of opioid dependence. As the number of persons receiving buprenorphine treatment and antiretroviral therapy continues to grow, so too does the existence and clinical impact of drug interactions between buprenorphine and medications for treating human immunodeficiency virus (HIV) infection. Awareness that such interactions exist may deter some patients and physicians from initiating potentially lifesaving therapy or lead to complications among patients whose treatment is already under way. Complications include nonadherence to antiretroviral therapy and the development of viral resistance. Illicit drug use is a frequent consequence of adverse drug effects experienced by injection drug users. The occurrence of unrecognized drug interactions can lead to unsuccessful therapy for HIV infection and the treatment of substance dependence. The present review is organized to provide a working background of buprenorphine pharmacology. Review of the current state of knowledge regarding specific interactions between buprenorphine and antiretrovirals is followed by a review of the clinical applicability of these interactions.

A cationic chalcogenoxanthylium photosensitizer effective in vitro in chemosensitive and multidrug-resistant cells

Pentacyclic thio- (1) and seleno- (2) analogues of tetramethylrosamine (TMR) were prepared with a julolidyl fragment replacing the 3-dimethylamino substituent in the xanthylium core. The pentacylic structure increases the lipophilicity of 1 and 2 relative to TMR-S and TMR-Se and locks the lone-pair of electrons on the julolidyl N atom into conjugation with the xanthylium core. This conformational rigidization leads to longer wavelengths of absorption, but has little impact on other photophysical properties such as quantum yields for fluorescence and singlet-oxygen generation and fluorescence lifetimes in 1 and 2 relative to TMR-S and TMR-Se. Both 1 and 2 are effective photosensitizers against chemosensitive AUXB1 cells in vitro at 1x10(-7)M and compound 2 is an effective photosensitizer against multidrug-resistant CR1R12 cells in vitro at 1x10(-7)M. While the uptake TMR-S into CR1R12 cells as measured by fluorescence is significantly lower than uptake into chemosensitive AUXB1 cells, there is no significant difference in the uptake of 1 into either AUXB1 or CR1R12 cells. The addition of 2x10(-4)M verapamil to the cells prior to treatment with 1 had no significant effect on the uptake of 1 into either AUXB1 or CR1R12 cells. Treating lipid-activated, purified Pgp with 2 and light gave complete inhibition of Pgp ATPase activity.

Pharmacogenetics of HIV therapy

Drug treatment in HIV disease is characterized by variable responses, in terms of both efficacy and toxicity. Both genetic and environmental factors are important determinants of this variability, although the relative contributions are unclear and likely to vary with different drugs. Many of the antiretrovirals are metabolized by polymorphically expressed enzymes (cytochrome P450, CYP450; glucuronyl transferase, GT) and/or transported by drug transporters (ABC and SLC families). Initial studies of antiretroviral efficacy have therefore focused on these genes. For example, it has recently been shown that a CYP2B6 genetic variant predicts higher plasma efavirenz exposure and possibly increased central nervous system toxicity. A large number of studies on ABCB1 genetics with antiretrovirals have also been undertaken; however, as in other therapeutic areas, the data have been contradictory, and currently, no firm conclusions can be reached on the effect of ABCB1 variability as a determinant of efficacy. Indeed, this highlights the need for validation of initial association studies in pharmacogenetic research. By contrast, the clearest association between genetic variants and response relates to the hypersensitivity reaction that occurs with abacavir. The identification that the major histocompatibility complex haplotype 57.1 acts as a strong genetic predisposing factor can be regarded as a prime example of how fundamental research can be translated into a pharmacogenetic test. Nevirapine hypersensitivity has also been related to an HLA gene (HLA-DRB1*0101) but the predictive value does not appear to be sufficient to implement in clinical practice. Much more work needs to be done to define the genetic factors determining response to antiretroviral agents. These studies need to be sufficiently powered and utilize a modern genotyping strategy. Most importantly, the phenotype needs to be carefully characterized. We also need to disseminate this information: a pivotal resource for this can be found at www.HIV-pharmacogenomics.org.

Effect of omeprazole on the pharmacokinetics of saquinavir-500 mg formulation with ritonavir in healthy male and female volunteers

INTRODUCTION

Recent studies have described reduced absorption of certain protease inhibitors when administered with agents known to increase gastric pH. No clinically significant interactions between saquinavir absorption and gastric pH have previously been shown. We evaluated the effect of omeprazole, a proton-pump-inhibitor, on the pharmacokinetics of the recently developed saquinavir-500 mg formulation co-administered with ritonavir.

METHODS

Eighteen healthy subjects (n = 6 women and 12 men) received 1000/100 mg saquinavir/ritonavir twice daily in an open-label study for 15 days. On days 11-15, subjects were administered omeprazole 40 mg daily with the morning dose. Serial plasma samples were collected for 12-h pharmacokinetic profiles of saquinavir and ritonavir on days 10 and 15 and safety analysis on days 1, 4, 10, 15 and 29.

RESULTS

The geometric mean and 95% confidence interval (CI), for the area under time-concentration curve (AUC; ng h/ml), trough plasma concentration (C trough; ng/ml) and maximum observed plasma concentration (Cmax; ng/ml) of saquinavir were 20599 (14396-29360) and 37511 (28733-48970); 737 (482-1127) and 1521 (1039-2227); 3227 (2370-4393) and 5611 (4507-7710) on days 10 and 15, respectively, with geometric mean ratios of 1.82, 2.06 and 1.75. No significant changes were observed in saquinavir elimination half life, ritonavir pharmacokinetic parameters or in safety laboratory tests. No unexpected adverse events attributed to study medication were noted.

CONCLUSIONS

In the presence of omeprazole, total saquinavir plasma exposure is significantly increased (82% increase in AUC). The mechanism of this interaction requires elucidation. Despite the significant increase in saquinavir exposure, no short term toxicities were observed.

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.

Mitochondria are sensors for HIV drugs

Highly active anti-retroviral therapy (HAART) has drastically altered the course of HIV-1 infection, resulting in a major decrease in morbidity and mortality. However, adverse drug reactions and long-term toxicities associated with HAART are now a concern. A major toxicity that has been highlighted by the increased use of HAART is related to mitochondrial side-effects. At the same time, analysis of the biochemical pathways involved in programmed cell death has revealed that mitochondria are main sensors in this process. In this article, the regulation of mitochondrial damage following the use of nucleoside analogue reverse transcriptase inhibitors (NRTIs) and protease inhibitors is discussed, with a particular focus on the putative molecular mechanisms involved.