Antiretroviral Therapy With Efavirenz Accentuates Pregnancy-Associated Reduction of Dihydroartemisinin-Piperaquine Exposure During Malaria Chemoprevention.

Dihydroartemisinin (DHA)-piperaquine is promising for malaria chemoprevention in pregnancy. We assessed the impacts of pregnancy and efavirenz-based antiretroviral therapy on exposure to DHA and piperaquine in pregnant Ugandan women. Intensive sampling was performed at 28 weeks gestation in 31 HIV-uninfected pregnant women, in 27 HIV-infected pregnant women receiving efavirenz, and in 30 HIV-uninfected nonpregnant women. DHA peak concentration and area under the concentration time curve (AUC0-8hr ) were 50% and 47% lower, respectively, and piperaquine AUC0-21d was 40% lower in pregnant women compared to nonpregnant women. DHA AUC0-8hr and piperaquine AUC0-21d were 27% and 38% lower, respectively, in pregnant women receiving efavirenz compared to HIV-uninfected pregnant women. Exposure to DHA and piperaquine were lower among pregnant women and particularly in women on efavirenz, suggesting a need for dose modifications. The study of modified dosing strategies for these populations is urgently needed.

Population Pharmacokinetics of Piperaquine in Young Ugandan Children Treated With Dihydroartemisinin-Piperaquine for Uncomplicated Malaria.

This prospective trial investigated the population pharmacokinetics of piperaquine given with dihydroartemisinin to treat uncomplicated malaria in 107 Ugandan children 6 months to 2 years old, an age group previously unstudied. Current weight-based dosing does not adequately address physiological changes in early childhood. Patients were administered standard 3-day oral doses and provided 1,282 capillary plasma concentrations from 218 malaria episodes. Less than 30% of treatments achieved 57 ng/mL on day 7. A three-compartment model with first-order absorption described the data well. Age had a statistically significant effect (P < 0.005) on clearance/bioavailability in a model that accounts for allometric scaling. Simulations demonstrated that higher doses in all children, but especially in those with lower weight for age, are required for adequate piperaquine exposure, although safety and tolerance will need to be established. These findings support other evidence that both weight- and age-specific guidelines for piperaquine dosing in children are urgently needed.

Alteration in cytochrome P450 3A4 activity as measured by a urine cortisol assay in HIV-1-infected pregnant women and relationship to antiretroviral pharmacokinetics.

OBJECTIVES: Pregnancy results in physiological changes altering the pharmacokinetics of drugs metabolized by cytochrome P450 3A4 (CYP3A4). The urinary ratio of 6-ß hydroxycortisol to cortisol (6ßHF : F) is a marker of CYP3A4 induction. We sought to evaluate its change in antiretroviral (ARV)-treated HIV-1-infected women and to relate this change to ARV pharmacokinetics. METHODS: Women receiving various ARVs had pharmacokinetic evaluations during the third trimester of pregnancy (>30 weeks) and postpartum with determination of 6ßHF : F carried out on the same days. The Wilcoxon signed rank test was used to compare the ratio antepartum to postpartum. The relationship between the change in ratio and the change in pharmacokinetics was analysed using Kendall's tau. RESULTS: 6ßHF : F ratios were available for 107 women antepartum, with 54 having postpartum values. The ratio was higher antepartum (P=0.033) (median comparison 1.35; 95% confidence interval 1.01, 1.81). For 71 women taking a protease inhibitor (PI), the antepartum vs. postpartum 6ßHF : F comparison was marginally significant (P=0.058). When the change in the 6ßHF : F ratio was related to the change in the dose-adjusted ARV area under the plasma concentration vs. time curve (AUC) between antepartum and postpartum, the 35 subjects in the lopinavir/ritonavir (LPV/r) arms demonstrated an inverse relationship (P=0.125), albeit this correlation did not reach statistical significance. CONCLUSIONS: A 35% increase in the urinary 6ßHF : F ratio was measured during late pregnancy compared with postpartum, indicating that CYP3A induction occurs during pregnancy. The trend towards an inverse relationship between the change in the 6ßHF : F ratio and the change in the LPV AUC antepartum vs. postpartum suggests that CYP3A induction may be one mechanism behind altered LPV exposure during pregnancy.

Lopinavir protein binding in HIV-1-infected pregnant women.

BACKGROUND: Pregnancy may alter protein binding (PB) of highly bound protease inhibitors due to changes in plasma concentrations of albumin and alpha-1 acid glycoprotein (AAG). Small changes in PB can greatly impact the fraction of drug unbound (FU) exerting pharmacological effect. We report lopinavir (LPV) PB during third trimester (antepartum, AP) compared to > or =1.7 weeks postpartum (PP) to determine if FU changes compensate for reduced total concentrations reported previously. METHODS: P1026s enrolled women receiving LPV/ritonavir, soft gel capsules 400/100 mg or 533/133 mg twice daily. LPV FU, albumin and AAG were determined AP and PP. RESULTS: AP/PP samples were available from 29/25 women respectively with all but one woman receiving the same dose AP/PP. LPV FU was increased 18% AP vs. PP (mean 0.96+/-0.16% AP vs. 0.82+/-0.21% PP, P=0.001). Mean protein concentrations were reduced AP (AAG=477 mg/L; albumin=3.28 mg/dL) vs. PP (AAG=1007 mg/L; albumin=3.85 mg/dL) (P<0.0001 for each comparison). AAG concentration correlated with LPV binding. Total LPV concentration did not correlate with LPV FU AP or PP. However, higher LPV concentration PP was associated with reduced PB and higher FU after adjustment for AAG. CONCLUSIONS: LPV FU was higher and AAG lower AP vs. PP. The 18% increase in LPV FU AP is smaller than the reduction in total LPV concentration reported previously and is not of sufficient magnitude to eliminate the need for an increased dose during pregnancy.

Pharmacokinetic evaluation of the effects of ribavirin on zidovudine triphosphate formation: ACTG 5092s Study Team.

OBJECTIVES: Ribavirin (RBV) is used for the treatment of hepatitis C virus (HCV) infection in subjects with HIV-1 infection who may require antiretroviral treatment (ART) with nucleoside reverse transcriptase inhibitors including zidovudine (ZDV). We sought to investigate the potential antagonism between RBV and ZDV by evaluating the impact of RBV on the formation of intracellular ZDV triphosphate (TP) in HIV-infected patients receiving ZDV who were treated for HCV infection. METHODS: Serial plasma and intracellular ZDV TP pharmacokinetics (PK) were determined in 14 subjects at entry (within 2 weeks prior to RBV administration) and at 8 weeks following initiation of RBV. Intracellular ZDV TP in peripheral blood mononuclear cells (PBMC) was quantified by a validated cartridge/liquid chromatography/tandem mass spectrometry method. PK exposure was estimated from the steady-state area under the concentration vs time curve (AUC(0-12 h)) in plasma and PBMC. RESULTS: Ribavirin did not have a statistically significant impact on ZDV TP AUC(0-12 h), plasma ZDV AUC(0-12 h) or the ratio of ZDV TP AUC(0-12 h) to plasma ZDV AUC(0-12 h), although there was a trend towards an increase post-RBV ratio compared with pre-RBV. There was extensive variability in the ZDV TP AUC(0-12 h). CONCLUSIONS: Ribavirin did not inhibit formation of ZDV TP in PBMC in 14 patients receiving ZDV as part of ART and RBV-based HCV therapy for 8 weeks. These results are consistent with those of a previously published limited study in seven subjects. These PK findings should be weighed carefully against emerging clinical reports of significant anaemia associated with combination ZDV and high-dose RBV therapy.

Characterization of nelfinavir binding to plasma proteins and the lack of drug displacement interactions.

OBJECTIVES: To determine the characteristics of the binding of nelfinavir and active M8 to alpha1-acid glycoprotein (AAG) and human serum albumin (HSA), and to examine the displacement effects of drugs binding extensively to AAG (ritonavir and saquinavir) or to HSA (salicylic acid and valproic acid). METHODS: Free drugs were separated by equilibrium dialysis after incubation with human plasma or purified plasma proteins and after co-incubation with potential displacers. Association constants were estimated from double-reciprocal plots of the data. RESULTS: Nelfinavir and M8 free fractions [fractions of unbound drug (fus)] were 0.42+/-0.08% (mean+/-standard deviation) and 0.64+/-0.07%, respectively. For the two analytes, respectively, association constants were 7.25 x 10(7)/m and 3.33 x 10(7)/m for AAG and 1.11 x 10(6)/m and 7.92 x 10(5)/m for HSA. Nelfinavir fu in an AAG solution was significantly (P < 0.01) increased by the addition of ritonavir or saquinavir, whereas it was unaltered by addition of these drugs to whole plasma. Similarly, fu in an HSA solution was significantly increased (P < 0.01) by the addition of salicylic acid or valproic acid, whereas there was no difference in the free fraction in plasma. CONCLUSIONS: The affinity of nelfinavir for human plasma proteins was higher than that of M8, and both nelfinavir and M8 showed higher affinity to AAG than to HSA. The free fraction of nelfinavir was not affected by drugs that bind extensively to AAG or albumin when these drugs were added to whole plasma in combination, suggesting a compensatory effect of alternate binding proteins.

A direct determination of thymidine triphosphate concentrations without dephosphorylation in peripheral blood mononuclear cells by LC/MS/MS.

A rapid, sensitive and specific analytical method with minimal sample preparation for the measurement of thymidine triphosphate (TTP) in peripheral blood mononuclear cells (PBMC) by LC/MS/MS has been developed. PBMC were separated from whole blood or buffy coat. The analyte and internal standard were extracted from PBMC with 70% methanol (pH 7.2). These extracts after centrifugation were directly injected onto LC/MS/MS without need for any further sample preparation. The calibration curve was linear over the range 0.8-800 ng/ml. Mean inter- and intra-assay coefficients of variation (CVs) over the range of the standard curve were less than 10%. The overall recovery of TTP was 103.5%.

An LC-MS-MS method for the determination of indinavir, an HIV-1 protease inhibitor, in human plasma.

A method for the determination of indinavir (IDV) (L-735 524) in human plasma by LC-MS-MS is discussed, and the validation data is presented. The analyte and internal standard are isolated from plasma by a simple acetonitrile precipitation of plasma proteins followed by centrifugation. LC-tandem mass spectrometry in positive ion, multiple reaction monitoring mode used pairs of ions at m/z of 614/421 for indinavir and 628/421 for internal standard, respectively. The calibration curve had a linear range from 3.0 to 12320 ng/ml when linear least square regression weighing 1/x was applied to the concentration versus peak area plot. The advantages of this method are the fast sample preparation, wide dynamic assay range and quick analysis taking only 5 min for each sample run. The robust nature of this assay has been further verified during routine use over several months involving multiple analysts.

Liquid chromatographic determination of urinary 6beta-hydroxycortisol to assess cytochrome p-450 3A activity in HIV positive pregnant women.

Assessing the activity of CYP3A4 is important for predicting the pharmacokinetic behavior of protease inhibitors in HIV positive patients, especially in pregnant women. The endogenous hormonal ratio of 6beta-hydroxycortisol (beta-OHF) to cortisol (F) in the urine is an index for metabolic enzyme activity of cytochrome p-450 (CYP) 3A4. Because the ratio is a unique way to assess the enzyme activity without using any exogenous probes for this isozyme, it is practical for use in pregnant women. In this paper, we describe a method using high performance liquid chromatography (HPLC) for 6beta-OHF in urine from pregnant women to estimate the ratio of 6beta-OHF/F. Urinary 6beta-OHF was measured by using C18-cartridge solid phase extraction and isocratic HPLC. Aliquots (1 ml) of urine samples spiked with internal standard, 6beta-hydroxyprednisolone (6beta-OHPSL), were alkalinized with NaOH, then applied to C18-cartridges, which were washed with water and hexane and eluted with ethyl acetate. After the effluents were dried and reconstituted in 10% acetonitrile, the samples were analyzed by HPLC using an isocratic mobile phase (acetic acid/acetonitrile/50 mM potassium dihydrogenphosphate: 0.2/9/90.8; v/v) and ultraviolet detection at 245 nm. The recoveries of 6beta-OHF from C18 cartridges were 93.2 and 93.9% when the authentic 6beta-OHF was added to the urine sample at the concentration of 50 and 300 ng/ml, respectively. Intra- and inter-day variations estimated at concentrations of 113-674 ng/ml were 2.9-5.6 and 4.9-8.1%, respectively. The method was applied to morning urine samples collected from HIV-positive pregnant women managed with protease inhibitor containing anti-retroviral regimens.

Drug concentration monitoring of immunosuppressive agents: focus on tacrolimus, mycophenolate mofetil and sirolimus.

Several new immunosuppressive agents have recently been approved for use in solid organ transplantation. Many of these agents have narrow therapeutic ranges, which necessitates drug concentration monitoring in order to optimise efficacy, minimise toxicity and individualise dosages. Some of the lessons learned with the clinical use of the revolutionary agent cyclosporin have been applied to the newer agents tacrolimus and sirolimus. The agent mycophenolate mofetil has been in clinical use without widespread drug concentration monitoring; however, recent data suggest that therapeutic monitoring may improve clinical outcomes, especially in certain high risk subsets of patients. This review focuses on the literature published to date on drug concentration monitoring of the newer immunosuppressive agents - tacrolimus, mycophenolate mofetil and sirolimus. In addition, pertinent aspects of the clinical pharmacokinetics and metabolism of each agent are reviewed.

The penetration of anti-infectives into the central nervous system.

The blood-brain barrier, blood-cerebrospinal fluid (CSF) barrier, and meninges are a complex and difficult-to-study system charged with protecting the central nervous system (CNS) from toxins, including drugs. Current estimates of CNS drug exposure are limited to CSF to blood ratios, of which area-under-the curve (AUC) estimates provide the most robust measure of drug exposure. Different classes of drugs and individual drugs within classes have different CNS penetration potential that is dependent upon a variety of biologic and pharmacologic factors. Clinical data (AUC and point ratios) regarding the penetration of several anti-infective agents used for the treatment of CNS infections are provided in this article.

Antiviral therapy for HIV patients with renal insufficiency.

Patients with HIV infection and HIV-related opportunistic infections are treated extensively with a spectrum of drugs. Introduction of new antiretroviral drugs, such as protease inhibitors and nonnucleoside reverse transcriptase inhibitors in addition to nucleoside reverse transcriptase inhibitors, has created exciting dimensions in treatment strategies. Renal dysfunction is also common in HIV-infected patients. Because some drugs used in HIV are primarily excreted unchanged by the kidney, dose adjustments are necessary in patients with renal insufficiency. Drugs such as foscarnet, cidofovir and adefovir are directly nephrotoxic, whereas acyclovir can crystallize in the kidneys, and indinavir may cause nephrolithiasis. This paper reviews the impact of renal insufficiency on pharmacokinetics of antiviral drugs used in HIV disease and discusses dosage recommendations needed to avoid toxicity. Finally, we summarize the effects of dialysis on removal of these drugs.

Pharmacokinetics of fosphenytoin in patients with hepatic or renal disease.

PURPOSE: The pharmacokinetic behavior of fosphenytoin (FOS), the water-soluble prodrug of phenytoin (PHT), has been characterized in normal subjects. This is the first study of the effect of hepatic or renal disease on the rate and extent of conversion of FOS to PHT. METHODS: A single dose of fosphenytoin (250 mg over a period of 30 min) was administered to subjects with hepatic cirrhosis (n = 4), renal disease requiring maintenance hemodialysis (n = 4), and healthy controls (n = 4). Serial plasma concentrations were measured, and pharmacokinetic parameters were calculated. RESULTS: The mean time to reach the peak plasma FOS concentration was similar for each of the three groups. However, the mean time to achieve peak plasma concentrations of PHT tended to occur earlier in the hepatic or renal disease groups than in healthy subjects. The half-life of FOS was 4.5, 9.2, and 9.5 min for the three groups, respectively. There was a trend toward increased FOS clearance and earlier peak PHT concentration in subjects with hepatic or renal disease. This finding is consistent with decreased binding of FOS to plasma proteins and increased fraction of unbound FOS resulting from decreased plasma protein concentrations associated with these disease states. The conversion of FOS to PHT was equally efficient in subjects with hepatic or renal disease and healthy subjects. CONCLUSIONS: Although the differences in pharmacokinetic parameters between the three groups were not statistically significant, these data suggest the need for close clinical monitoring during FOS administration to patients with hepatic or renal disease. To minimize the incidence of adverse effects in this patient population, FOS may need to be administered at lower doses or infused more slowly.

Effect of renal disease and hemodialysis on foscarnet pharmacokinetics and dosing recommendations.

BACKGROUND: Foscarnet is an antiviral agent commonly used for managing patients with cytomegalovirus infection. Despite its clinical usefulness, foscarnet is associated with substantial adverse effects including nephrotoxicity. Moreover, foscarnet is primarily eliminated unchanged through the kidneys, thus requiring aggressive dose adjustment during kidney failure. To develop specific dosage guidelines, information on the disposition of this compound in patients with varying degrees of renal function and those requiring dialysis is essential. DESIGN: Twenty-six subjects were enrolled in this study and divided into five groups depending on their degree of renal dysfunction. Group 1 included subjects with normal renal function; group 5 included subjects requiring maintenance hemodialysis. Nondialysis study subjects received a single 60-mg/kg intravenous dose of foscarnet whereas hemodialysis subjects received two intravenous doses, separated by 1 week, to compare the effects of conventional and high-flux dialysis methods. RESULTS: Mean plasma clearance in control subjects averaged 2.1+/-0.7 ml/minute/kg and declined proportionally with changing renal function as indicated by the regression equation: Clp (ml/minute/kg) = 1.48 [CrCl (ml/minute/kg)]-0.08 (r2 = 0.82). Mean half-life averaged 1.9+/-0.1 hours in normal subjects and increased to a mean of 25+/-19 hours in study subjects with severe impairment not on dialysis. Foscarnet dialysis clearance (based on dialysate recovery) averaged 183 ml/minute with conventional dialysis methods and 253 ml/minute during high-flux procedures, which resulted in removal of 37% and 38% of a dose for the two methods, respectively. CONCLUSIONS: These data indicate that substantial dosage adjustments must be made in renal failure patients. Therefore, a patient-specific dosage nomogram has been developed.

Pharmacokinetics of cidofovir in renal insufficiency and in continuous ambulatory peritoneal dialysis or high-flux hemodialysis.

BACKGROUND: Cidofovir is an antiviral agent used for the treatment of cytomegalovirus infection in patients with acquired immunodeficiency syndrome. Because cidofovir is primarily eliminated by the kidneys and because its main adverse effect is nephrotoxicity, an understanding of the pharmacokinetic disposition of cidofovir in patients with renal insufficiency is necessary. METHODS: Twenty-four subjects were enrolled into this study and were divided into 6 groups depending on their degree of renal dysfunction, including subjects receiving maintenance continuous ambulatory peritoneal dialysis and high-flux hemodialysis. The creatinine clearance (CLCR) for subjects not receiving dialysis ranged from 12 to 164 mL/min. Each subject received a single 0.5 mg/kg intravenous dose of cidofovir over 1 hour. Subjects not receiving dialysis were given intravenous hydration with 1 L normal saline solution and concomitant oral probenecid. Serial serum and urine samples were collected to determine pharmacokinetic parameters with use of noncompartmental methods. RESULTS: Mean +/- SD cidofovir clearance (CL) in control subjects (normal renal function; n = 5) was 1.7 +/- 0.1 mL/min/kg, which decreased with declining renal function as indicated by the regression equation: CL (mL/min/kg) = 0.94 x CLCR (mL/min/kg) + 0.064 (r2 = 0.91). Mean volume of distribution at steady state did not change significantly in subjects with kidney disease and cidofovir serum elimination half-life was significantly increased in subjects with severe renal impairment. Cidofovir was not significantly cleared during continuous ambulatory peritoneal dialysis, but high-flux hemodialysis resulted in the removal of 52% +/- 11% of the dose administered. CONCLUSION: The significant (P < .001) correlation observed between CLCR and CL in subjects with varying degrees of renal insufficiency indicates that aggressive dosage reduction of cidofovir would be necessary in subjects with kidney disease to ensure comparable drug exposure based on serum levels.

Simple high-performance liquid chromatographic determination of the protease inhibitor indinavir in human plasma.

Indinavir is a member of a class of protease inhibitors that actively prevent the acquired immunodeficiency syndrome virion from maturing. A high-performance liquid chromatographic (HPLC) assay was developed and validated for the determination of indinavir in human plasma. Indinavir and the internal standard were isolated from the plasma by ether extraction. The residue after evaporation of ether was reconstituted with buffer and injected onto a C4 reversed-phase column eluted isocratically with a mobile phase consisting of 35:65 (v/v) of acetonitrile and buffer. A wavelength of 210 nm was found to be optimum for detection. The calibration range of this assay was from 10 to 5000 ng/ml and coefficients of variation for the assay ranged from 4.6% to 11.0% for three different drug concentrations and the limit of quantitation was 10 ng/ml. During the validation, short-term stability of the drug in plasma, stability during heat deactivation and on repeated freezing and thawing of plasma was evaluated. The overall recovery of indinavir by the ether extraction method was 91.4%. This HPLC assay was found to be a simple and reproducible method for monitoring indinavir levels in human plasma obtained during clinical trials of the drug.

Is there a pharmacokinetic interaction between foscarnet and zalcitabine during concomitant administration?

Foscarnet, an antiviral agent used in the treatment of cytomegalovirus infection, and zalcitabine, an antiretroviral nucleoside analogue used in the treatment of human immunodeficiency virus infection, are commonly used concomitantly. Foscarnet and zalcitabine may interact pharmacokinetically, as both compounds are partially eliminated by renal tubular secretion. Owing to dose-related toxicities associated with these two drugs, it is essential that we have data regarding their pharmacokinetic disposition during concomitant therapy. Twelve patients randomly received either foscarnet (four doses) or zalcitabine (five doses) (Phase 1), followed by concomitant foscarnet (four doses) and zalcitabine (six doses) (Phase 2), followed by dosing with the drug not received in Phase 1 (Phase 3). Following the last dose in each phase of the study, serial plasma samples were collected over 8 hours for zalcitabine and over 12 hours for foscarnet to determine the pharmacokinetics of each drug using noncompartmental analysis. Foscarnet plasma and urine levels were determined using high-performance liquid chromatography, and zalcitabine levels were determined using radioimmunoassay. No clinically significant alterations in the pharmacokinetics of foscarnet or zalcitabine occurred in this study. Thus despite the potential for foscarnet and zalcitabine to compete for renal tubular secretion, no apparent pharmacokinetic interaction exists between these two drugs at the clinically relevant doses studied.

Pharmacokinetics of mycophenolate mofetil and intravenous ganciclovir alone and in combination in renal transplant recipients.

STUDY OBJECTIVES: To evaluate the pharmacokinetics of mycophenolic acid and its glucuronide metabolite alone and in the presence of ganciclovir, and to determine the pharmacokinetics of ganciclovir alone and in combination with mycophenolate mofetil. DESIGN: Randomized, open-label, three-way crossover study. PATIENTS: Twelve kidney transplant recipients. INTERVENTIONS: Mycophenolate mofetil 1500 mg orally and ganciclovir 5 mg/kg intravenously were each given alone and in combination with at least a 1-week washout period between treatment arms. MEASUREMENTS AND MAIN RESULTS: Blood samples were obtained to measure mycophenolic acid and ganciclovir by high-performance liquid chromatography. Mean (+/-SD) oral plasma clearance for mycophenolic acid alone and with ganciclovir was 3.11 +/- 0.72 and 3.19 +/- 0.72 ml/min/kg (p = 0.64). The overall disposition of the major metabolite, MPA-glucuronide, was unchanged, with approximately 70% of the administered dose eliminated as the glucuronide conjugate for both arms of the study. Mean genciclovir serum clearance was 1.80 +/- 0.58 ml/min/kg for ganciclovir and 1.70 +/- 0.55 ml/min/kg for ganciclovir plus mycophenolate mofetil (p = 0.11; 10 patients). Renal clearance of ganciclovir was decreased when the drugs were administered in combination, 1.43 +/- 0.54 (ganciclovir) and 1.26 +/- 0.44 (both drugs) ml/min/kg (p = 0.02; 10 patients). CONCLUSION: The single-dose pharmacokinetics of mycophenolic acid and its glucuronide metabolite were unchanged by the addition of ganciclovir. Total serum clearance of ganciclovir was unchanged by the addition of mycophenolate mofetil, however, renal clearance was slightly decreased.

Pharmacokinetics of intracellular zidovudine and its phosphorylated anabolites in the absence and presence of stavudine using an in vitro human peripheral blood mononuclear cell (PBMC) model.

Both zidovudine (ZDV) and stavudine (D4T) must be intracellularly converted to their respective active triphosphate anabolites (ZDV-TP and D4T-TP). It is hypothesized that the combination of ZDV and D4T may lead to altered formation of phosphorylated anabolites for either drug. The objective of this study was to investigate the effect of D4T on intracellular ZDV phosphorylation. Human PBMCs were incubated with [(3)H]ZDV in the presence and absence of D4T. Cells were harvested at several time points over 12 h to determine area under the intracellular concentration versus time curve (AUC) of ZDV and its phosphorylated anabolites. Radiolabled ZDV and anabolites were quantified using HPLC and LS. The AUC for ZDV-TP was 0.53 and 0.52 pmol x h/10(6) PBMC in the absence and presence of D4T, respectively. The AUC for ZDV monophosphate was 157.45 and 172.44 pmol x h/10(6) PBMC pre and post D4T. D4T does not appear to affect the formation of intracellular ZDV phosphates in human PBMCs under the conditions studied.