Semen and serum pharmacokinetics of zidovudine and zidovudine-glucuronide in men with HIV-1 infection

Functional characteristics of 3'-azido-3'-deoxythymidine transport at the blood-testis barrier

3'-Azido-3'-deoxythymidine (AZT), an antiretroviral drug, is often adopted in the therapy for human immunodeficiency virus (HIV) infection, and the characteristics of AZT transport at the blood-testis barrier (BTB) were investigated in this study. In the integration plot analysis that evaluates the transport activity in vivo, the apparent influx clearance of [³H]AZT was significantly greater than that of [¹⁴C]D-mannitol, a non-permeable paracellular transport marker. In the uptake study in vitro with TM4 cells derived from mouse Sertoli cells, [³H]AZT uptake exhibited a time- and concentration-dependent manner, of which K_m and V_max values being 20.3 µM and 102 pmol/(min·mg protein), respectively. In the inhibition analysis, [³H]AZT uptake was not affected by extracellular inorganics and some substrates of transporters putatively involved in AZT transport. In the further inhibition analyses to elucidate the characteristics of AZT transport, [³H]AZT uptake was strongly reduced in the presence of several nucleosides, that are categorized as 2'-deoxynucleosides with pyrimidine, whereas little effect on [³H]AZT uptake was exhibited in the presence of other nucleosides, nucleobases, and antiretrovirals. These results suggest the influx transport of AZT from the circulating blood to the testis, and the involvement of carrier-mediated process at the BTB, which selectively recognizes 2'-deoxynucleosides with a pyrimidine base.

Semen Extracellular Vesicles From HIV-1-Infected Individuals Inhibit HIV-1 Replication In Vitro, and Extracellular Vesicles Carry Antiretroviral Drugs In Vivo

BACKGROUND

Extracellular vesicles (EVs) are cell-derived vesicles with diverse functions in intercellular communication including disease and infection, and EVs seem to influence HIV-1 pathogenesis. EVs isolated from HIV-1-uninfected semen (SE), but not blood (BE), contain factors that interfere with HIV-1 infection and replication in target cells. The reason for this dichotomy is unknown. Furthermore, the effect of HIV-1 infection and antiretroviral (ARV) drugs on the anti-HIV-1 effects of SE and BE is unknown. Here, we characterize EVs and EV-free plasma isolated from HIV-infected donor semen and blood and their effects on HIV infection.

METHODS

EVs and EV-free plasma were purified from autologous blood and semen of HIV-negative, HIV-infected antiretroviral therapy (ART)-naïve, and HIV-infected ART-treated participants. HIV infection was assessed in a TZM-bl cell reporter system. ARV concentrations were analyzed using liquid chromatography-mass spectrometry.

RESULTS

SE isolated from both HIV-negative and HIV-infected, ART-naïve donors inhibited HIV-1 infection, but BE and semen and blood EV-free plasma did not. By contrast, BE, SE, and EV-free plasma from HIV-infected, ART-treated donors inhibited HIV-1. Importantly, exosomes isolated from ART-treated donors contained concentrations of ARV drugs (ART-EVs) at biologically relevant inhibitory levels.

CONCLUSIONS

The HIV-1-inhibitory phenotype of SE is independent of donor HIV-1 or ART status, and ARV drugs and their metabolites are SE- and BE-associated in vivo.

Development of a Novel Maternal-Fetal Physiologically Based Pharmacokinetic Model I: Insights into Factors that Determine Fetal Drug Exposure through Simulations and Sensitivity Analyses

Determining fetal drug exposure (except at the time of birth) is not possible for both logistical and ethical reasons. Therefore, we developed a novel maternal-fetal physiologically based pharmacokinetic (m-f-PBPK) model to predict fetal exposure to drugs and populated this model with gestational age-dependent changes in maternal-fetal physiology. Then, we used this m-f-PBPK to: 1) perform a series of sensitivity analyses to quantitatively demonstrate the impact of fetoplacental metabolism and placental transport on fetal drug exposure for various drug-dosing regimens administered to the mother; 2) predict the impact of gestational age on fetal drug exposure; and 3) demonstrate that a single umbilical venous (UV)/maternal plasma (MP) ratio (even after multiple-dose oral administration to steady state) does not necessarily reflect fetal drug exposure. In addition, we verified the implementation of this m-f-PBPK model by comparing the predicted UV/MP and fetal/MP AUC ratios with those predicted at steady state after an intravenous infusion. Our simulations yielded novel insights into the quantitative contribution of fetoplacental metabolism and/or placental transport on gestational age-dependent fetal drug exposure. Through sensitivity analyses, we demonstrated that the UV/MP ratio does not measure the extent of fetal drug exposure unless obtained at steady state after an intravenous infusion or when there is little or no fluctuation in MP drug concentrations after multiple-dose oral administration. The proposed m-f-PBPK model can be used to predict fetal exposure to drugs across gestational ages and therefore provide the necessary information to assess the risk of drug toxicity to the fetus.

Development of a Novel Maternal-Fetal Physiologically Based Pharmacokinetic Model II: Verification of the model for passive placental permeability drugs

Fetal exposure to drugs cannot be readily estimated from single time point cord blood sampling at the time of delivery. Therefore, we developed a physiologically based pharmacokinetic (PBPK) model to estimate fetal drug exposure throughout pregnancy. In this study, we report verification of this novel maternal-fetal PBPK (m-f-PBPK) model for drugs that passively diffuse across the placenta and are not metabolized/transported there. Our recently built m-f-PBPK model was populated with gestational age-dependent changes in maternal drug disposition and maternal-fetal physiology. Using midazolam as an in vivo calibrator, the transplacental passive diffusion clearance of theophylline and zidovudine was first estimated. Then, for verification, the predicted maternal plasma (MP) and umbilical venous (UV) plasma drug concentrations by our m-f-PBPK were compared against those observed at term. Overall, our m-f-PBPK model well predicted the maternal and fetal exposure to the two verification drugs, theophylline and zidovudine, at term, across a range of dosing regimens, with nearly all observed MP and UV plasma drug concentrations falling within the 90% prediction interval [i.e., 5th-95th percentile range of a virtual pregnant population (n = 100)]. Prediction precision and bias of theophylline MP and UV were 14.5% and 12.4%, and 9.4% and 7.5%, respectively. Furthermore, for zidovudine, after the exclusion of one unexpectedly low MP concentration, prediction precision and bias for MP and UV were 50.3% and 30.2, and 28.3% and 15.0%, respectively. This m-f-PBPK should be useful to predict fetal exposure to drugs, throughout pregnancy, for drugs that passively diffuse across the placenta.

Pharmacokinetic Modeling of Lamivudine and Zidovudine Triphosphates Predicts Differential Pharmacokinetics in Seminal Mononuclear Cells and Peripheral Blood Mononuclear Cells

The male genital tract is a potential site of viral persistence. Therefore, adequate concentrations of antiretrovirals are required to eliminate HIV replication in the genital tract. Despite higher zidovudine (ZDV) and lamivudine (3TC) concentrations in seminal plasma (SP) than in blood plasma (BP) (SP/BP drug concentration ratios of 2.3 and 6.7, respectively), we have previously reported lower relative intracellular concentrations of their active metabolites, zidovudine triphosphate (ZDV-TP) and lamivudine triphosphate (3TC-TP), in seminal mononuclear cells (SMCs) than in peripheral blood mononuclear cells (PBMCs) (SMC/PBMC drug concentration ratios of 0.36 and 1.0, respectively). Here, we use population pharmacokinetic (PK) modeling-based methods to simultaneously describe parent and intracellular metabolite PK in blood, semen, and PBMCs and SMCs. From this model, the time to steady state in each matrix was estimated, and the results indicate that the PK of 3TC-TP and ZDV-TP in PBMCs are different from the PK of the two in SMCs and different for the two triphosphates. We found that steady-state conditions in PBMCs were achieved within 2 days for ZDV-TP and 3 days for 3TC-TP. However, steady-state conditions in SMCs were achieved within 2 days for ZDV-TP and 2 weeks for 3TC-TP. Despite this, or perhaps because of it, ZDV-TP in SMCs does not achieve the surrogate 50% inhibitory concentration (IC50) (as established for PBMCs, assuming SMC IC50 = PBMC IC50) at the standard 300-mg twice-daily dosing. Mechanistic studies are needed to understand these differences and to explore intracellular metabolite behavior in SMCs for other nucleoside analogues used in HIV prevention, treatment, and cure.

Pharmacokinetics of antiretrovirals in genital secretions and anatomic sites of HIV transmission: implications for HIV prevention

The incidence of HIV remains alarmingly high in many parts of the world. Prophylactic use of antiretrovirals, capable of concentrating in the anatomical sites of transmission, may reduce the risk of infection after an unprotected sexual exposure. To date, orally and topically administered antiretrovirals have exhibited variable success in preventing HIV transmission in large-scale clinical trials. Antiretroviral mucosal pharmacokinetics may help explain the outcomes of these investigations. Penetration and accumulation of antiretrovirals into sites of transmission can influence dosing strategies and pre-exposure prophylaxis clinical trial design. Antiretroviral tissue distribution varies widely within and between drug classes, attributed in part to their physicochemical properties and tissue-specific drug transporter expression. Nucleoside(-tide) reverse transcriptase inhibitors, the CCR5 antagonist maraviroc, and the integrase inhibitor raltegravir demonstrate the highest penetration into the male and female reproductive tracts and colorectal tissue relative to blood. This review describes antiretroviral exposure in anatomic sites of transmission, and places these findings in context with the prevention of HIV and the efficacy of pre-exposure prophylactic strategies.

Antiretroviral bioanalysis methods of tissues and body biofluids

Research in the many areas of HIV treatment, eradication and prevention has necessitated measurement of antiretroviral (ARV) concentrations in nontraditional specimen types. To determine the knowledgebase of critical details for accurate bioanalysis, a review of the literature was performed and summarized. Bioanalytical assays for 31 ARVs, including metabolites, were identified in 205 publications measuring various tissues and biofluids. 18 and 30% of tissue or biofluid methods, respectively, analyzed more than one specimen type; 35-37% of the tissue or biofluid methods quantitated more than one ARV. 20 and 76% of tissue or biofluid methods, respectively, were used for the analysis of human specimens. HPLC methods with UV detection predominated, but chronologically MS detection began to surpass. 40% of the assays provided complete intra- and inter-assay validation data, but only 9% of publications provided any stability data with even less for the prevalent ARV in treatments.

Localization of multidrug resistance-associated proteins along the blood-testis barrier in rat, macaque, and human testis

The blood-testis barrier (BTB) prevents the entry of many drugs into seminiferous tubules, which can be beneficial for therapy not intended for the testis but may decrease drug efficacy for medications requiring entry to the testis. Previous data have shown that some of the transporters in the multidrug resistance-associated protein (MRP) family (ABCC) are expressed in the testis. By determining the subcellular localization of these transporters, their physiologic function and effect on drug disposition may be better predicted. Using immunohistochemistry (IHC), we determined the site of expression of the MRP transporters expressed in the testis, namely, MRP1, MRP4, MRP5, and MRP8, from immature and mature rats, rhesus macaques, and adult humans. We determined that in all species MRP1 was restricted to the basolateral membrane of Sertoli cells, MRP5 is located in Leydig cells, and MRP8 is located in round spermatids, whereas MRP4 showed species-specific localization. MRP4 is expressed on the basolateral membrane of Sertoli cells in human and nonhuman primates, but on the apical membrane of Sertoli cells in immature and mature rats, representing a potential caution when using rat models as a means for studying drug disposition across the BTB. These data suggest that MRP1 may limit drug disposition into seminiferous tubules, as may MRP4 in human and nonhuman primates but not in rats. These data also suggest that MRP5 and MRP8 may not have a major impact on the penetration of drugs across the BTB.

Basolateral uptake of nucleosides by Sertoli cells is mediated primarily by equilibrative nucleoside transporter 1

The blood-testis barrier (BTB) prevents the entry of many xenobiotic compounds into seminiferous tubules thereby protecting developing germ cells. Understanding drug transport across the BTB may improve drug delivery into the testis. Members of one class of drug, nucleoside reverse transcriptase inhibitors (NRTIs), do penetrate the BTB, presumably through interaction with physiologic nucleoside transporters. By investigating the mechanism of nucleoside transport, it may be possible to design other drugs to bypass the BTB in a similar manner. We present a novel ex vivo technique to study transport at the BTB that employs isolated, intact seminiferous tubules. Using this system, we found that over 80% of total uptake by seminiferous tubules of the model nucleoside uridine could be inhibited by 100 nM nitrobenzylmercaptopurine riboside (NBMPR, 6-S-[(4-nitrophenyl)methyl]-6-thioinosine), a concentration that selectively inhibits equilibrative nucleoside transporter 1 (ENT1) activity. In primary cultured rat Sertoli cells, 100 nM NBMPR inhibited all transepithelial transport and basolateral uptake of uridine. Immunohistochemical staining showed ENT1 to be located on the basolateral membrane of human and rat Sertoli cells, whereas ENT2 was located on the apical membrane of Sertoli cells. Transepithelial transport of uridine by rat Sertoli cells was partially inhibited by the NRTIs zidovudine, didanosine, and tenofovir disoproxil fumarate, consistent with an interaction between these drugs and ENT transporters. These data indicate that ENT1 is the primary route for basolateral nucleoside uptake into Sertoli cells and a possible mechanism for nucleosides and nucleoside-based drugs to undergo transepithelial transport.

Pharmacokinetics of antiretroviral drugs in anatomical sanctuary sites: the male and female genital tract

HIV resides within anatomical 'sanctuary sites', where local drug exposure and viral dynamics may differ significantly from the systemic compartment. Suboptimal antiretroviral concentrations in the genital tract may result in compartmentalized viral replication, selection of resistant mutations and possible re-entry of wild-type/resistant virus into the systemic circulation. Therefore, achieving adequate antiretroviral exposure in the genital tract has implications for the prevention of sexual and vertical transmission of HIV. Penetration of antiretrovirals in the genital tract is expressed by accumulation ratios derived from the measurement of drug concentrations in time-matched seminal plasma/cervicovaginal fluid and plasma samples. Penetration varies by gender and may be drug (as opposed to class) specific with high interindividual variability. Concentrations in seminal plasma are highest for nucleoside analogues and lowest for protease inhibitors and efavirenz. Seminal accumulation of newer agents, raltegravir and maraviroc, is moderate (rank order of accumulation is nucleoside/nucleotide reverse transcriptase inhibitors [lamivudine/zidovudine/tenofovir/didanosine > stavudine/abacavir] > raltegravir > indinavir/maraviroc/nevirapine >> efavirenz/protease inhibitors [amprenavir/atazanavir/darunavir > lopinavir/ritonavir > saquinavir] > enfuvirtide). In the female genital tract, the nucleoside analogues exhibit high accumulation ratios, whereas protease inhibitors have limited penetration; however, substantial variability exists between individuals and study centres. Second generation non-nucleoside reverse transcriptase inhibitor etravirine, and maraviroc and raltegravir, demonstrate effective accumulation in cervicovaginal secretions (rank order of accumulation is nucleoside/nucleotide reverse transcriptase inhibitor [zidovudine/lamivudine/didanosine > emtricitabine/tenofovir] > indinavir > maraviroc/raltegravir/darunavir/etravirine > nevirapine/abacavir > protease inhibitors [amprenavir/atazanavir/ritonavir] > lopinavir/stavudine/efavirenz > saquinavir).

Antiretroviral drug concentrations in the male and female genital tract: implications for the sexual transmission of HIV

PURPOSE OF REVIEW

To summarize the recent literature (2008-2010) on antiretroviral (ARV) drug disposition into the male and female genital tract.

RECENT FINDINGS

Recent studies have confirmed that penetration of antiretroviral agents into the male and female genital tracts are both drug and sex specific. Concentrations achieved vary considerably depending on the class of drug studied, the sampling techniques used and the times samples are obtained.

SUMMARY

There appear to be several patterns of drug penetration into the male and female genital tract. In addition there appear to be different patterns of genital shedding under the influence of antiretroviral therapy. What effect these factors will have on the sexual transmission of HIV or the evolution and transmission of resistant HIV remains to be seen.

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.

Quantification of antiretroviral drugs for HIV-1 in the male genital tract: current data, limitations and implications for laboratory analysis

Antiretroviral therapy has reduced the morbidity and mortality associated with HIV-1/AIDS in developed countries. Viral replication in blood plasma is suppressed by antiretroviral drugs, whereas virus in the male genital tract is genetically and phenotypically unique and may not be suppressed. This viral compartmentalization affects antiretroviral drug penetration of the male genital tract and capacity for antiretroviral therapy to reduce sexual transmission. The problem of having two distinct viral populations within any given individual is compounded by the fact that antiretroviral drugs penetrate semen to varying degrees. Incomplete suppression of genital tract virus may yield drug-resistant virus and increase the risk of sexual transmission. This review critically appraises current studies of antiretroviral drug quantification in semen and suggests recommendations to address observed limitations.

Male genital tract pharmacology: developments in quantitative methods to better understand a complex peripheral compartment

A substantial health burden results from medical problems affecting the male genital tract, including chronic morbidity conditions affecting a large proportion of men, such as benign prostatic hypertrophy and prostatitis, and potentially lethal conditions, such as prostate cancer and human immunodeficiency virus transmission. Rational approaches to therapeutics in these conditions should benefit from understanding local pharmacokinetics and pharmacodynamics of active drugs within the male genital tract. However, the description of drug distribution into the male genital tract has been largely limited to total, rather than protein-free drug concentrations in the whole ejaculate at one or two time points within a dosing interval, which may be misleading in understanding local drug action. Recent innovations enable a quantitative understanding of protein-free drug kinetics into semen and drug distribution into individual male genital tract glands. These methods may benefit therapeutics through optimization of targeted drug delivery and facilitate drug development for diseases related to these glands.

Effect of semen sampling frequency on seminal antiretroviral drug concentration

Study of male genital tract (MGT) pharmacology is relevant to the treatment of prostatitis, prostate cancer, infertility, and seminal human immunodeficiency virus transmission. However, the time course of drug concentrations in the MGT is largely unknown. To determine the feasibility of frequent semen sampling in assessing the pharmacokinetics of the MGT, we administered efavirenz, indinavir, and zidovudine to subjects to achieve steady-state levels and then collected semen samples at sequentially decreasing ejaculation intervals. The volume of seminal plasma decreased from 4.0 (1.2-5.1) ml (median with range) at 48 h after the baseline ejaculation to 0.72 (0.45-1.6) ml 1 h after a previous ejaculation, which was still adequate for drug concentration assessment. The seminal fructose concentration also decreased. However, the concentration of prostate-specific antigen and all three drugs did not decrease, even if the ejaculation intervals decreased to 1 h. Thus, semi-intensive semen sampling can be used to assess MGT pharmacokinetics.

Pharmacokinetic profiles of nevirapine and indinavir in various fractions of seminal plasma

Limited data are available on antiretroviral drug concentrations in seminal plasma during a dosing interval. Further, since human ejaculate is composed of fluids originating from the testes, the seminal vesicles, and the prostate, all having different physiological characteristics, drug concentrations in total seminal plasma do not necessarily reflect concentrations in the separate compartments. Five human immunodeficiency virus type 1-infected patients on nevirapine (NVP; 200 mg twice a day [b.i.d.]) and/or indinavir (IDV; 800 mg b.i.d. with ritonavir, 100 mg b.i.d.) regimens used a split ejaculate technique to separate seminal plasma in two fractions, representing fluids from the testes and prostate (first fraction) and fluids from the seminal vesicles (second fraction). Split-ejaculate samples were provided at 0, 2, 5, and 8 h after drug ingestion, on separate days after 3 days of sexual abstinence. NVP and IDV showed time-dependent concentrations in seminal plasma, with peak concentrations in both fractions at 2 and 2 to 5 h, respectively, after drug ingestion. The NVP concentrations were not significantly different between the first and second fractions of the ejaculate at all time points measured and were in the therapeutic range, except for the predose concentration in two patients. The median (range) predose IDV concentrations in the first and second fractions of the ejaculate were 448 (353 to 1,015) ng/ml and 527 (240 to 849) ng/ml, respectively (P = 0.7). In conclusion, NVP and IDV concentrations in seminal plasma are dependent on the time after drug ingestion. Furthermore, our data suggest that NVP and IDV achieve therapeutic concentrations in both the testes and prostate and the seminal vesicles throughout the dosing interval.

Domestic cat model for predicting human nucleoside analogue pharmacokinetics in blood and seminal plasma

To establish whether a feline model can predict nucleoside analogue behavior in human semen, zidovudine (ZDV) and lamivudine (3TC) pharmacokinetic parameters (PKs) were determined in the blood and seminal plasma of healthy cats. Our results show considerable similarity in ZDV and 3TC PKs between cats and humans. As in humans, ZDV and 3TC tend to accumulate in feline seminal plasma. Area under the blood plasma concentration-time curve was predictive of seminal plasma excretion. The felid model offers a unique in vivo experimental alternative for investigating the pharmacokinetics of nucleoside analogues in the male genital tract.

Antiretroviral drug concentrations in semen of HIV-1 infected men

Because semen is a major vehicle for the sexual transmission of HIV-1, control of viral replication within the sanctuary of the male genital tract should be a goal of antiretroviral therapy. Local immune responses, virus specific factors, and the degree of viral and cellular trafficking all appear to be important in controlling viral replication and evolution. However, the most important factor influencing viral replication and evolution within the male genital tract may be the disposition of antiretroviral agents into genital tissues and fluids. This review proposes possible mechanisms of antiretroviral distribution into the male genital tract by using other sanctuary barriers; such as the placenta, renal tubules, and blood-brain barrier; as models. In addition, this review summarises recent clinical studies regarding the disposition of currently available antiretroviral drugs into the seminal plasma and discusses some of the difficulties in interpreting drug concentration in the genital tract.