Enzymatic assay for measurement of zidovudine triphosphate in peripheral blood mononuclear cells

Enzymatic Assay for Rapid Measurement of Antiretroviral Drug Levels

Poor adherence to pre-exposure prophylaxis (PrEP) and antiretroviral therapy (ART) can lead to human immunodeficiency virus (HIV) acquisition and emergence of drug-resistant infections, respectively. Measurement of antiviral drug levels provides objective adherence information that may help prevent adverse health outcomes. Gold-standard drug-level measurement by liquid chromatography/mass spectrometry is centralized, heavily instrumented, and expensive and is thus unsuitable and unavailable for routine use in clinical settings. We developed the REverSe TRanscrIptase Chain Termination (RESTRICT) assay as a rapid and accessible measurement of drug levels indicative of long-term adherence to PrEP and ART. The assay uses designer single-stranded DNA templates and intercalating fluorescent dyes to measure complementary DNA (cDNA) formation by reverse transcriptase in the presence of nucleotide reverse transcriptase inhibitor drugs. We optimized the RESTRICT assay using aqueous solutions of tenofovir diphosphate (TFV-DP), a metabolite that indicates long-term adherence to ART and PrEP, at concentrations over 2 orders of magnitude above and below the clinically relevant range. We used dilution in water as a simple sample preparation strategy to detect TFV-DP spiked into whole blood and accurately distinguished TFV-DP drug levels corresponding to low and high PrEP adherences. The RESTRICT assay is a fast and accessible test that could be useful for patients and clinicians to measure and improve ART and PrEP adherence.

The search for nucleoside/nucleotide analog inhibitors of dengue virus

Nucleoside analogs represent the largest class of antiviral agents and have been actively pursued for potential therapy of dengue virus (DENV) infection. Early success in the treatment of human immunodeficiency virus (HIV) infection and the recent approval of sofosbuvir for chronic hepatitis C have provided proof of concept for this class of compounds in clinics. Here we review (i) nucleoside analogs with known anti-DENV activity; (ii) challenges of the nucleoside antiviral approach for dengue; and (iii) potential strategies to overcome these challenges. This article forms part of a symposium in Antiviral Research on flavivirus drug discovery.

Mass spectrometry in the quantitative analysis of therapeutic intracellular nucleotide analogs

Nucleoside analogs are widely used in anti-cancer, anti-(retro)viral, and immunosuppressive therapy. Nucleosides are prodrugs that require intracellular activation to mono-, di-, and finally triphosphates. Monitoring of these intracellular nucleotides is important to understand their pharmacology. The relatively involatile salts and ion-pairing agents traditionally used for the separation of these ionic analytes limit the applicability of mass spectrometry (MS) for detection. Both indirect and direct methods have been developed to circumvent this apparent incompatibility. Indirect methods consist of de-phosphorylation of the nucleotides into nucleosides before the actual analysis. Various direct approaches have been developed, ranging from the use of relatively volatile or very low levels of regular ion-pairing agents, hydrophilic interaction chromatography (HILIC), weak anion-exchange, or porous graphitic carbon columns to capillary electrophoresis and matrix-assisted light desorption--time of flight (MALDI-TOF) MS. In this review we present an overview of the publications describing the quantitative analysis of therapeutic intracellular nucleotide analogs using MS. The focus is on the different approaches for their direct analysis. We conclude that despite the technical hurdles, several useful MS-compatible chromatographic approaches have been developed, enabling the use of the excellent selectivity and sensitivity of MS for the quantitative analysis of intracellular nucleotides.

Plasma and cellular markers of 3'-azido-3'-dideoxythymidine (AZT) metabolism as indicators of DNA damage in cord blood mononuclear cells from infants receiving prepartum NRTIs

Several systemic and cellular markers of 3'-azido-3'-dideoxythymidine (AZT) metabolism and AZT incorporation into nuclear DNA were measured in cord blood from uninfected infants born to HIV-1-infected mothers receiving prepartum therapies based on AZT or AZT in combination with 2',3'-dideoxy-3'-thiacytidine (3TC). In addition, the relationships among these pharmacological end points, levels of AZT-DNA incorporation, and the previously reported mutagenic responses in these infants were evaluated. AZT- and 3TC-specific radioimmunoassays (RIAs), or HPLC coupled with AZT-RIA, were used to measure plasma levels of AZT and the AZT-glucuronide, and cellular levels of AZT, phosphorylated AZT, and DNA incorporation of AZT or 3TC in cord blood mononuclear cells from treated infants compared with unexposed controls born to HIV-uninfected mothers. Fewer infants had detectable AZT-DNA incorporation levels in the group exposed to AZT (71%; n = 7) compared with those receiving AZT-3TC (100%; n = 21), and the mean AZT-DNA incorporation for AZT-exposed infants (14.6 +/- 6.3 AZT/10(6) nucleotides) was significantly lower than that in AZT-3TC exposed infants (51.6 +/- 10.2 AZT/10(6) nucleotides; P = 0.028). Low levels of 3TC-DNA incorporation found in a few AZT-3TC-exposed newborns correlated with AZT-DNA incorporation values in the same samples. Among the metabolites studied, there were positive correlations between levels of AZT-diphosphate and AZT-triphosphate, and AZT-triphosphate and AZT-DNA incorporation, in nucleoside analog-exposed infants. Levels of AZT-DNA incorporation, however, did not correlate well with the reported frequencies of somatic mutations in the same population of nucleoside analog-treated children. While these data support the continued use of AZT-based therapies during pregnancy, infants receiving prepartum AZT should be monitored long-term for adverse health effects.

Simultaneous measurement of intracellular triphosphate metabolites of zidovudine, lamivudine and abacavir (carbovir) in human peripheral blood mononuclear cells by combined anion exchange solid phase extraction and LC-MS/MS

All nucleoside reverse transcriptase inhibitors (NRTI) must first be metabolized to their triphosphate forms in order to be active against HIV. Zidovudine (ZDV), abacavir (ABC) and lamivudine (3TC) have proven to be an efficacious combination. In order simultaneously to measure intracellular levels of the triphosphates (-TP) of ZDV, ABC (carbovir, CBV) and 3TC, either together or individually, we have developed a cartridge-LC-MS/MS method. The quantitation range was 2.5-250 pg/microl for 3TC-TP, 0.1-10.0 pg/microl for ZDV-TP and 0.05-5.00 pg/microl for CBV-TP. This corresponds to 0.1-11.0 pmol 3TC-TP per million cells, 4-375 fmol ZDV-TP per million cells and 2-200 fmol CBV-TP per million cells, extracted from 10 million cells. Patient samples demonstrated measured levels in the middle regions of our standard curves both at pre-dose and 4h post-dose times.

Simultaneous quantitation of the nucleotide analog adefovir, its phosphorylated anabolites and 2'-deoxyadenosine triphosphate by ion-pairing LC/MS/MS

The nucleotide analog adefovir is an important therapy for hepatitis B viral infection. The study of nucleoside/tide pharmacology has been hampered by difficulties encountered when trying to develop LC/MS/MS methods for these polar analytes. In an attempt to identify a more convenient, selective and sensitive alternative to the analysis of the metabolism of radiolabeled parent nucleotide traditionally used for in vitro cell culture studies, an LC/MS/MS method was developed for the quantitative detection of adefovir and its phosphorylated metabolites in cellular samples. Ion-pairing reversed phase LC using tetrabutylammonium (TBA) and ammonium phosphate had the best compromise between chromatographic separation and positive mode MS/MS detection. Using microbore reverse phase columns and a low flow acetonitrile gradient it was possible to quantitate adefovir, its metabolites and 2'-deoxyadenosine triphosphate. A cross-validation showed comparable levels of adefovir and its metabolites were determined using either LC/MS/MS or radioactivity detection. However, initial methods were conducted at high pH and utilized an acetonitrile step gradient causing unacceptable column life and unpredictable equilibration. Further method optimization lowered the concentration of TBA and phosphate, decreased pH and applied a linear gradient of acetonitrile. This work resulted in a method that was found to have sensitivity, accuracy and precision sufficient to be a useful tool in the study of the intracellular pharmacology of adefovir in vitro and may be more broadly applicable.

The effects of prolonged treatment with zidovudine, lamivudine, and abacavir on a T-lymphoblastoid cell line

A human T-lymphoblastoid cell line that is resistant to the antiviral activity of zidovudine (ZDV) and moderately resistant to lamivudine (3TC) has been obtained as a result of prolonged treatment with a combination of three nucleoside analogues (NA), ZDV, 3TC, and abacavir (ABV). These cells, called CEM(ZLA), are fully sensitive to ABV. The cellular resistance of the CEM(ZLA) cells to ZDV correlates with significant reductions in thymidine kinase (TK) activity and in the amount of intracellular TK protein. Interestingly, the reduction in TK activity led to impairment of the ability of CEM(ZLA) to accumulate the triphosphate metabolite of ZDV. However, the moderately 3TC-resistant phenotype of CEM(ZLA) cannot be ascribed to a similar reduction in deoxycytidine kinase activity. Compared to the parental CEM cells, CEM(ZLA) cells express a high level of multidrug resistance protein 4 (MRP4), which could reduce the intracellular concentration of 3TC. This study shows that the exposure of cells to a combination of NAs is capable of simultaneously affecting more than one target site to confer resistance and that NAs display differing abilities to select cellular resistance mechanisms.

Effect of ribavirin on intracellular and plasma pharmacokinetics of nucleoside reverse transcriptase inhibitors in patients with human immunodeficiency virus-hepatitis C virus coinfection: results of a randomized clinical study

The intracellular triphosphorylation and plasma pharmacokinetics of lamivudine (3TC), stavudine (d4T), and zidovudine (ZDV) were assessed in a pharmacokinetic substudy, in 56 human immunodeficiency virus-hepatitis C virus (HIV-HCV) coinfected patients receiving peginterferon alfa-2a (40KD) 180 microg/week plus either placebo or ribavirin (RBV) 800 mg/day in the AIDS PEGASYS Ribavirin International Coinfection Trial. There were no significant differences between patients treated with RBV and placebo in plasma pharmacokinetics parameters for the nucleoside reverse transcriptase inhibitors (NRTIs) at steady state (weeks 8 to 12): ratios of least squares mean of area under the plasma concentration-time curve (AUC(0-12 h)) were 1.17 (95% confidence interval, 0.91 to 1.51) for 3TC, 1.44 (95% confidence interval, 0.58 to 3.60) for d4T and 0.85 (95% confidence interval, 0.50 to 1.45) for ZDV, and ratios of least squares mean plasma C(max) were 1.33 (95% confidence interval, 0.99 to 1.78), 1.06 (95% confidence interval, 0.68 to 1.65), and 0.84 (95% confidence interval, 0.46 to 1.53), respectively. Concentrations of NRTI triphosphate (TP) metabolites in relation to those of the triphosphates of endogenous deoxythymidine-triphosphate (dTTP) and deoxcytidine-triphosphate (dCTP) were similar in the RBV and placebo groups. Differences (RBV to placebo) in least squares mean ratios of AUC(0-12 h) at steady state were 0.274 (95% confidence interval, -0.37 to 0.91) for 3TC-TP:dCTP, 0.009 (95% confidence interval, -0.06 to 0.08) for d4T-TP:dTTP, and -0.081 (95% confidence interval, -0.40 to 0.24) for ZDV-TP:dTTP. RBV did not adversely affect HIV-1 replication. In summary, RBV 800 mg/day administered in combination with peginterferon alfa-2a (40KD) does not significantly affect the intracellular phosphorylation or plasma pharmacokinetics of 3TC, d4T, and ZDV in HIV-HCV-coinfected patients.

Quantitation of zidovudine triphosphate concentrations from human peripheral blood mononuclear cells by anion exchange solid phase extraction and liquid chromatography-tandem mass spectroscopy; an indirect quantitation methodology

To facilitate the assessment of drug safety and determination of phamacokinetics, an anion exchange isolation of zidovudine triphosphate (ZDV-TP) from human peripheral blood mononuclear cells (hPBMC), coupled with dephosphorylation, desaltation, and detection by liquid chromatography-tandem mass spectroscopy (LC-MS-MS) was validated. hPBMCs were harvested from whole blood, lysed, and a suspension of intracellular ZDV-TP was produced. ZDV-TP was isolated from ZDV, ZDV-monophosphate (ZDV-MP), and ZDV-diphosphate (ZDV-DP), which were all present in the cell lysate, by performing a salt gradient anion exchange SPE. Isolated ZDV-TP was dephosphorylated with acid phosphatase to its parent drug form, ZDV. ZDV was then desalted and concentrated for tandem mass spectral detection. An LC-MS-MS methodology was developed and validated for the determination of molar ZDV directly corresponding to the intra-hPBMC molar ZDV-TP concentration. ZDV-TP concentrations were determined in femtomoles per million hPBMCs (fmol/10(6)cells). The assay was able to determine ZDV-TP concentrations accurately and precisely within the range of 5-640 fmol/10(6)cells with 10 million cells per sample analyzed. Inter- and intra-day accuracy and precision data for back calculated standards and quality controls fell within 15% of nominal. The assay correlated well with a previous ELISA method developed and validated in our laboratory, and has been successfully used to quantitate ZDV-TP concentrations in patients being routinely monitored and treated with ZDV.

Development and validation of a liquid chromatographic/tandem mass spectrometric assay for the quantitation of nucleoside HIV reverse transcriptase inhibitors in biological matrices

Besides liquid chromatographic (LC)/UV methods adapted to therapeutic drug monitoring, there is still a need for more powerful techniques that can be used for pharmacological research and clinical purposes. We developed an LC method coupled with tandem mass spectrometry (MS/MS) to separate, detect and quantify with high sensitivity the nucleoside analogues used in multitherapies (zidovudine, stavudine, zalcitabine, didanosine, lamivudine and abacavir) in plasma and in the intracellular medium. We worked on two essential issues: (i) the need to use two ionization modes in order to achieve the best sensitivity, which leads to the optimization of the chromatographic separation of drugs detected in the positive ionization mode and drugs detected in the negative ionization mode, and (ii) the need to optimize the extraction step in order to enhance sample recovery. The peripheral blood mononuclear cells were lysed in Tris buffer-MeOH. A clean-up procedure was performed by solid-phase extraction only for plasma samples. The LC separation was carried out on a Zorbax Stable Bond C(18) column followed by MS/MS analysis after electrospray ionization in either the negative or positive mode. The positive ionization mode was applied at the beginning of the run to detect zalcitabine and lamivudine, then the ionization mode was changed to negative for the detection of didanosine, stavudine, internal standard and zidovudine. The calibration range for all the analytes was 0.5-200 ng ml(-1). The recoveries were between 64 and 90%, with coefficients of variation (CVs) lower than 15%. The inaccuracy (bias) was +/-15% with CVs always lower than 12%. The analytes were stable at room temperature and in the extraction solvent for at least 24 h, after storage at -80 degrees C for 3 months, after three freeze-thaw cycles and in the injection solvent after 48 h at 4 degrees C. Together with the measurement of intracellular triphosphorylated metabolites thanks to the powerful plasma and intracellular assay method for intact drugs, it is possible to describe the behaviour of nucleoside analogues against HIV through plasma pharmacokinetics, cell membrane diffusion including drug transport involvement, and also the intracellular metabolism.

Pharmacodynamic Effects of Zidovudine 600 mg Once/Day versus 300 mg Twice/Day in Therapy-Naïve Patients Infected with Human Immunodeficiency Virus

STUDY OBJECTIVE

To compare the virologic activity of zidovudine monotherapy administered as 600 mg once/day versus 300 mg twice/day.

DESIGN

Phase II, randomized (1:1), open-label study.

SETTING

Thirteen medical centers in the United States.

PATIENTS

Thirty-two antiretroviral-naive patients infected with human immunodeficiency virus (HIV).

INTERVENTION

Patients were administered either zidovudine 600 mg every 24 hours (16 patients) or 300 mg every 12 hours (16 patients) for 13 days.

MEASUREMENTS AND MAIN RESULTS

Plasma HIV-1 RNA concentration was measured daily. Study end points were between-group differences in change from baseline of log10-transformed HIV-1 RNA and in rates of viral load decline measured by the slope of HIV-1 RNA over time. At baseline, mean HIV-1 RNA was similar in the once/day and twice/day groups (4.33 and 4.40 log10 copies/ml, respectively). At day 14, a trend toward lower mean reduction in HIV-1 RNA from baseline was observed in the once/day group (-0.585, 95% confidence interval [CI] -0.728 to -0.442 log10 copies/ml) compared with the twice/day group (-0.849, 95% CI -1.067 to -0.630 log10 copies/ml, p=0.056). Viral load reduction also tended to be slower in the once/day group, as indicated by the smaller slope of viral load decline in the once/day group than in the twice/day group during days 1-14 (-0.045 vs -0.065 logic copies/ml/day, p=0.065). Both zidovudine regimens were similarly well tolerated.

CONCLUSION

Zidovudine 600 mg once/day has antiviral activity, although less pronounced and more slowly achieved than that seen with zidovudine 300 mg twice/day. No differences were observed between the two treatment groups with respect to safety profile or tolerability

Equivalent steady-state pharmacokinetics of lamivudine in plasma and lamivudine triphosphate within cells following administration of lamivudine at 300 milligrams once daily and 150 milligrams twice daily

Once-daily administration of 300 mg of lamivudine in combination with other antiretroviral agents has been proposed as a possible way to optimize anti-human immunodeficiency virus (HIV) treatment and to facilitate adherence. A single-center, randomized, two-way, crossover study was conducted in 60 healthy subjects to compare the steady-state pharmacokinetics of lamivudine in plasma and its putative active anabolite, lamivudine 5'-triphosphate (lamivudine-TP), in peripheral blood mononuclear cells (PBMCs) following 7 days of treatment with lamivudine at 300 mg once daily and 7 days of the standard regimen of 150 mg twice daily. Serial blood samples were collected over 24 h for determination of plasma lamivudine concentrations by liquid chromatography-mass spectrometry and intracellular lamivudine-TP concentrations in peripheral blood mononuclear cells by high-performance liquid chromatography/radioimmunoassay methods. Pharmacokinetic parameters were calculated based on lamivudine and lamivudine-TP concentration-time data. Regimens were considered bioequivalent if 90% confidence intervals (CI) for the ratio (once daily/twice daily) of geometric least-squares (GLS) means for lamivudine and lamivudine-TP pharmacokinetic values fell within the acceptance range of 0.8 to 1.25. Steady-state plasma lamivudine pharmacokinetics following the once- and twice-daily regimens were bioequivalent with respect to the area under the drug concentration-time curve from 0 to 24 h at steady state (AUC(24,ss)) (GLS mean ratio, 0.94; 90% CI, 0.92, 0.97) and average plasma lamivudine concentration over the dosing interval (C(ave,ss)) (GLS mean ratio, 0.94; 90% CI, 0.92, 0.97). Steady-state intracellular lamivudine-TP pharmacokinetics after the once- and twice-daily regimens were bioequivalent with respect to AUC(24,ss) (GLS mean ratio, 0.99; 90% CI, 0.88, 1.11), C(ave,ss) (GLS mean ratio, 0.99; 90% CI, 0.88, 1.11), and maximum lamivudine concentration (C(max,ss)) (GLS mean ratio, 0.93; 90% CI, 0.81, 1.07). Lamivudine-TP trough concentrations were modestly lower (by 18 to 24%) during the once-daily regimen; the clinical importance of this is unclear, given the large intersubject variability in values that was observed (coefficient of variation, 48 to 124%). Once-daily lamivudine was as well tolerated as the twice-daily regimen. Overall, the results of this study suggest that for key AUC-related parameters, lamivudine at 300 mg once daily is pharmacokinetically equivalent to lamivudine at 150 mg twice daily.

Metabolism of tenofovir and didanosine in quiescent or stimulated human peripheral blood mononuclear cells

OBJECTIVE

As tenofovir disoproxil fumarate substantially increases plasma concentrations of didanosine in patients with human immunodeficiency virus-1 infection, we sought to determine whether tenofovir and didanosine showed a similar intracellular interaction in human peripheral blood mononuclear cells (PBMCs).

DESIGN

Comparative in vitro incubation of two antiretrovirals in lymphocytes.

SETTING

Clinical research laboratory.

MATERIAL

Radiolabeled tenofovir and didanosine in human PBMCs.

MEASUREMENTS AND MAIN RESULTS

Phosphorylation of 2 and 20 microM didanosine to dideoxyadenosine triphosphate (ddATP) was determined in quiescent and stimulated PBMCs in the presence or absence of 5 microM tenofovir. Similarly, phosphorylation of 5 microM tenofovir to tenofovir diphosphate (TFVpp) was examined in the presence or absence of 2 and 20 microM didanosine. Intracellular amounts of ddATP and TFVpp were determined by incubating PBMCs with radiolabeled tenofovir or didanosine alone and together for up to 16 hours and then separating the anabolites by high-performance liquid chromatography for quantitation. The presence of tenofovir did not affect the amount of ddATP in quiescent or stimulated PBMCs with 2 or 20 microM didanosine. In addition, didanosine did not alter the amount of TFVpp that formed. The amount of ddATP was modestly (1.5-3-fold) but consistently higher in stimulated than in quiescent PBMCs, but the amount of TFVpp did not differ.

CONCLUSION

There is no significant interaction between tenofovir and didanosine in human PBMCs as determined by the extent of formation of the phosphorylated anabolites. This suggests that adjusting didanosine dosage, when given with tenofovir, to achieve similar didanosine plasma concentrations, may be sufficient to accommodate the systemic drug interaction.

Time-dependent changes in HIV nucleoside analogue phosphorylation and the effect of hydroxyurea

BACKGROUND

Nucleoside analogues are activated to their triphosphates, which compete with endogenous deoxynucleoside triphosphate (dNTP) pools to inhibit HIV reverse transcriptase. Hydroxyurea has been administered with nucleoside analogues to modulate intracellular dNTP pools and thus the ratio of drug triphosphate:endogenous triphosphate.

OBJECTIVES

To examine changes in drug activation over time and investigate the effects of hydroxyurea on intracellular phosphorylation of antiretroviral nucleoside analogues.

PATIENTS

A total of 229 HIV-infected individuals receiving abacavir, lamivudine and zidovudine were randomly assigned to receive or not nevirapine and hydroxyurea. Twenty-four patients were recruited to an observational substudy measuring intracellular drug triphosphate and dNTP concentrations at 0, 2, 6, 12, 24 and 48 weeks.

METHODS

Drugs were extracted from isolated peripheral blood mononuclear cells before analysis of endogenous dNTP and drug triphosphates by primer extension assays.

RESULTS

Twenty-two out of 24 patients were followed to completion of the substudy. Hydroxyurea had no demonstrable effect on endogenous dNTP or drug triphosphate levels at any timepoint. However, the ratio of zidovudine triphosphate to endogenous deoxythymidine triphosphate was significantly increased with hydroxyurea. A significant decrease in lamivudine triphosphate (3TCTP) and the 3TCTP:endogenous deoxycytidine triphosphate ratio was seen over 48 weeks. In five patients who failed therapy in the first 24 weeks, significantly reduced 3TCTP was seen.

CONCLUSION

Hydroxyurea does not affect measurable pools of endogenous nucleosides in vivo. Decreased lamivudine phosphorylation over time may provide a novel pharmacological explanation for the mechanism of resistance to this drug.

Development of a direct assay for measuring intracellular AZT triphosphate in humans peripheral blood mononuclear cells

Direct LC/MS/MS methods have recently been developed for measuring triphosphate anabolites of several nucleosidic reverse transcriptase inhibitor (NRTI) in peripheral blood mononuclear cells (PBMCs) from HIV-positive patients. Whereas AZT is one of the most-used NRTIs, no such method has been developed for AZT-TP, its active anabolite, mainly because of the presence of endogenous nucleotides that interfere with such an assay. In this paper, we first describe the development of two enzyme immunoassays (EIA) of AZT-TP in PBMCs: one directly measuring AZT-TP content; the other, measuring the nucleoside AZT after selective extraction of AZT-TP and dephosphorylation. The precision of these two assays was too low to achieve precise determination of AZT-TP in PBMC samples. Direct LC/MS/MS is not specific enough for AZT-TP, since at least two interfering endogenous nucleotides (same m/z ratio and fragment as well as retention time close to that of AZT-TP) are found in the intracellular medium of PBMCs. The off-line combination of immunoaffinity extraction (IAE) and LC/MS/MS proved to be a successful strategy allowing without dephosphorylation appropriate specificity and sensitivity (limit of quantification established as 9.3 fmol/10(6) cells) to determine AZT-TP in PBMCs from 7 mL of blood of HIV-infected patients. Validation of this IAE-LC/MS/MS method demonstrated CV percent for repeatability and intermediate precision lower than 15%. More than 150 samples/week can be analyzed by one analyst, making this method suitable for routine analysis during clinical studies.

Development of enzymatic assays for quantification of intracellular lamivudine and carbovir triphosphate levels in peripheral blood mononuclear cells from human immunodeficiency virus-infected patients

In this paper, we describe the development and use of enzymatic assays to determine intracellular lamivudine triphosphate (3TCTP) and carbovir triphosphate (CBVTP) concentrations in peripheral blood mononuclear cells (PBMCs) from human immunodeficiency virus (HIV)-infected patients. The assays involve inhibition of HIV reverse transcriptase (RT), which normally incorporates radiolabeled deoxynucleoside triphosphates into a synthetic template primer. For the 3TCTP assay, a preincubation procedure was added whereby 3TCTP becomes incorporated before [(3)H]dCTP. At a 1:400 template primer dilution, control product formation was reduced by 88.0% with 0.8 pmol of 3TCTP. Standard 3TCTP inhibition curves were performed using this procedure. For the CBVTP assay, 0.1 pmol of CBVTP inhibited control product formation with and without the use of a preincubation step, so inhibition curves were constructed using both procedures. However, reduced template primer stability with assays using preincubation steps led to a single-incubation procedure being adopted for future studies. The presence of PBMC extracts interfered with the 3TCTP assay. However, this was overcome by the addition of CuSO(4). PBMC extracts did not interfere with the CBVTP assay. Intracellular 3TCTP and CBVTP concentrations were determined in PBMCs from HIV-infected patients over 24 h or greater. Peak concentrations were obtained 6 to 8 h after dosing, and the half-lives of the anabolites suggested the possibility of once-daily dosing. These assays are currently being used for determination of 3TCTP and CBVTP concentrations in clinical studies.

Improved method for the simultaneous determination of d4T, 3TC and ddl intracellular phosphorylated anabolites in human peripheral-blood mononuclear cells using high-performance liquid chromatography/tandem mass spectrometry

There is still a need for direct determination (i.e. without dephosphorylation) of nucleoside reverse transcriptase inhibitor (NRTI) triphosphorylated nucleotides in the peripheral-blood mononuclear cells (PBMCs) of HIV-positive patients. The objective of this paper was first to improve our previously described direct liquid chromatography/tandem mass spectrometry (LC/MS/MS) assay for stavudine triphosphate (d4T-TP). Preparation of PBMCs was modified to reduce degradation of d4T-TP during cell preparation and to simplify this step for routine use in clinical units. The performance of several HPLC columns was compared in order to improve the stability of peak shape over time. The SMT C(18) column was replaced by a Supelcogel ODP-50, thereby reducing two-fold the concentration of the first standard. Various internal standards were compared to optimize peak shape and remove an interfering peak in LC. 2-Chloroadenosine 5prime prime or minute-triphosphate was chosen as the most appropriate internal standard. Substitution of the narrowbore column by a microbore column (150 x 0.32 mm) is also presented and discussed. Secondly, this improved method was successfully applied to the simultaneous determination of d4T-TP, dideoxyadenosine triphosphate (ddA-TP) and lamivudine triphosphate (3TC-TP) in PBMCs, which is useful in view of the common use of NRTI combinations. The method was subsequently applied to clinical samples from HIV-positive patients receiving antiretroviral therapy containing d4T, ddl and/or 3TC. This method can be used simply and routinely on approximately 200 samples per week, using commercially available instruments and with a simple cell lysis as sample treatment.

The production and evaluation of antibodies for enzyme immunoassay of AZTTP

We describe the development of the first enzyme immunoassay for quantifying AZTTP that does not use of radioactive labeling. Anti-AZTTP antibodies were raised in rabbits by immunizing with an AZTTP-kelhoyle limpet hemocyanin (KLH) conjugate. Competitive immunoassays indicated a nanomolar sensitivity to AZTTP. One of the antisera produced was specific for AZTTP.

Influence of prior exposure to zidovudine on stavudine phosphorylation in vivo and ex vivo

Intracellular phosphorylation of stavudine (d4T) and zidovudine (ZDV) was investigated in peripheral blood mononuclear cells (PBMCs) isolated from ZDV-naive and ZDV-experienced human immunodeficiency virus (HIV)-positive patients. An in vivo study measured the amount of d4T triphosphate (d4TTP), while an ex vivo study assessed the capacity of cells to phosphorylate added d4T. Endogenous dTTP was also measured. d4TTP and dTTP were determined in vivo using a reverse transcriptase chain termination assay. In ex vivo studies, d4T (1 microM) was incubated in resting and phytohemagglutinin-stimulated (10 microg ml(-1); 72 h) PBMCs for 24 h. After washing and methanol extraction, radiolabeled anabolites were detected by high-performance liquid chromatography. d4TTP reached its highest level 2 to 4 h after dosing (0.21 +/- 0.14 pmol/10(6) cells; n = 27 [mean +/- standard deviation]). Comparison of ZDV-naive and ZDV-experienced individuals showed no significant difference in levels of d4TTP (ZDV naive, 0.23 +/- 0.17 pmol/10(6) cells [n = 7] versus ZDV experienced, 0.20 +/- 0.14 pmol/10(6) cells [n = 20]; P = 0.473) or the d4TTP/dTTP ratio (0.14 +/- 0.12 [n = 7] and 0.10 +/- 0.08 [n = 20], respectively; p = 0.391). Ex vivo data demonstrated no significant difference in the formation of d4TTP or total d4T phosphates in naive and experienced patients (0.086 +/- 0.055 pmol/10(6) cells in ZDV-naive patients [n = 17] versus 0.081 +/- 0.038 pmol/10(6) cells in ZDV-experienced patients [n = 22]; P = 0.767). The ability of HIV-infected patients to phosphorylate d4T in vivo and ex vivo was unchanged with increasing exposure to ZDV.

Amino acid phosphoramidate monoesters of 3'-azido-3'-deoxythymidine: relationship between antiviral potency and intracellular metabolism

A series of phosphoramidate monoesters of 3'-azido-3'-deoxythymidine (AZT) bearing aliphatic amino acid methyl esters (3a, 3c, 4a, 4c, 5-7) and methyl amides (3b, 3d, 4b, 4d) was prepared and evaluated for anti-HIV-1 activity in peripheral blood mononuclear cells (PBMCs). These compounds, which showed no cytotoxicity at concentrations of 100 microM, were effective at inhibiting HIV-1 replication at concentrations of 0.08-30 microM. Since the D-phenylalanine and D-tryptophan derivatives exhibited equivalent or enhanced antiviral activity compared to their L-counterparts, there appears to be no specific stereochemical requirement for the amino acid side chain. In addition, except for the D-phenylalanine derivatives, the methyl amides had greater antiviral activity than the corresponding methyl esters. On the basis of the observed antiviral activity of AZT phosphoramidate monoesters 3a and 4a in PBMCs and CEM cells, the mechanism of action of these two compounds was investigated. AZT-MP and substantial amounts of either phosphoramidate were detected in PBMCs and CEM cells treated with either 3a or 4a. Biological mechanistic studies demonstrated that 3a and 4a affect viral replication at a stage after virus entry and preceding viral DNA integration. Quantitation of the intracellular levels of AZT-TP in PBMCs and CEM cells treated with 3a and 4a in the presence and absence of exogenous thymidine correlated the intracellular levels of AZT-TP to the antiviral activity and suggested that AZT-TP was responsible for the activity observed. In addition, the reduced toxicity of 3a and 4a toward CEM cells relative to AZT correlated with reduced levels of total phosphorylated AZT and not AZT-TP. Stable carbamate analogues of 3a and 4a were prepared and shown to inhibit the production of AZT-MP from cell-free extracts of CEM cells, further suggesting that a phosphoramidate hydrolase may be responsible for intracellular P-N bond cleavage. Taken together, these results suggest that the biological activity and intracellular metabolism of nucleoside phosphoramidate monoesters are distinct from that of phosphoramidate diesters.

Phosphorylation of nucleoside analog antiretrovirals: a review for clinicians

Nucleoside analogs (zidovudine, didanosine, zalcitabine, stavudine, abacavir, lamivudine) have been administered as antiretroviral agents for more than a decade. They undergo anabolic phosphorylation by intracellular kinases to form triphosphates, which inhibit human immunodeficiency virus replication by competitively inhibiting viral reverse transcriptase. Numerous methods are used to elucidate the intracellular metabolic pathways of these agents. Intracellular and extracellular factors affect intracellular phosphorylation. Lack of standardization and complexity of methods used to study phosphorylation in patients limit interpretation of study results and comparability of findings across studies. However, in vitro and in vivo studies give important insights into mechanisms of action, metabolic feedback mechanisms, antiviral effects, and mechanisms of toxicity, and have influenced dosing regimens of nucleoside analogs.

Direct determination of phosphorylated intracellular anabolites of stavudine (d4T) by liquid chromatography/tandem mass spectrometry

The objective was to develop and validate a routine assay for active intracellular anabolites of stavudine (d4T), a nucleoside reverse transcriptase inhibitor in human PBMC, applicable to pharmacokinetic studies and treatment monitoring. This was achieved using liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS), which theoretically allies optimum sensitivity, specificity and high sample throughput. After cellular lysis in a Tris/methanol buffer, the extract spiked with 2[H(8)]-ATP (internal standard) is directly injected into the LC/MS/MS system. Phosphorylated metabolites of d4T as well as deoxythymidine-triphosphate, the competitor on the reverse transcriptase, are separated from d4T on a reverse-phase microbore column with ion pairing. The detection is performed in the multiple reaction monitoring (MRM) mode after drug ionisation in negative mode electrospray. The limit of quantitation for d4T-TP was 138 fmol per 7 mL blood (9.8 fmol per 10(6) cells) and CV% for repeatability and intermediate precision were lower than 15%. Stability of compounds was checked before and during the process of isolation of PBMC. Cellular samples from several d4T-treated patients were successfully analysed using this method and d4T-triphosphate and deoxythymidine triphosphate were recovered. In conclusion, we have developed and validated a routine LC/MS/MS method that allows the simultaneous determination of mono-, di- and triphosphorylated anabolites of d4T in PBMC as well as the natural corresponding triphosphate in one analysis. For the first time, the chain terminator ratio (d4T-TP/dT-TP) could be directly measured. This method can be used simply and routinely on more than 35 samples per day. Extension to other nucleoside analogues is under development.

Fluorometric determination of 2'-beta-fluoro-2',3'-dideoxyadenosine 5'-triphosphate, the active metabolite of a new anti-human immunodeficiency virus drug, in human lymphocytes

A sensitive precolumn derivatization method has been developed to measure the 5'-triphosphate of 2'-beta-fluoro-2',3'-dideoxyadenosine (F-ddA, lodenosine), a new anti-HIV drug, in human lymphocytes by HPLC using fluorescence detection. Reaction of chloroacetaldehyde with F-ddA triphosphate in extracts from human lymphocytes produces a highly fluorescent etheno adduct. This derivative is then separated and quantitated by reverse-phase paired-ion chromatography. Degradation of natural nucleic acid ribosides, such as ATP, using periodate oxidation simplifies the chromatogram and minimizes interference with detection of the target analyte. This method, modeled using cultured MOLT-4 T-lymphocytes, achieves a linear detector response for peak area measurements over the range 2.5 to 22.5 pmol (50-450 nM using 50 microl sample). Analyte recovery is greater than 90%, and the method achieves a limit of detection and limit of quantitation of 1.4 and 2.5 pmol per HPLC injection (50 microl sample containing cellular extract from 2.5 x 10(6) cells), respectively. Application of this method to measure F-ddATP in peripheral blood mononuclear cells from HIV-infected patients treated with F-ddA at 3.2 mg/kg twice daily for 22 days shows F-ddATP levels which range from 1.5 to 3.5 pmol/10(6) cells.

Simultaneous quantitation of the 5'-triphosphate metabolites of zidovudine, lamivudine, and stavudine in peripheral mononuclear blood cells of HIV infected patients by high-performance liquid chromatography tandem mass spectrometry

A high-performance liquid chromatography (HPLC) method utilizing triple quadrupole mass spectrometry (MS) detection was developed and validated for the simultaneous measurement of the intracellular nucleoside 5'-triphosphate anabolites of zidovudine (ZDV-TP), lamivudine (3TC-TP), and stavudine (d4T-TP). These compounds were extracted from patient peripheral blood mononuclear cells (PBMCs) which are the sites of HIV replication and drug action. Ion-exchange solid phase extraction (SPE) followed by enzymatic digestion with alkaline phosphatase was utilized to yield the measurable nucleoside forms of the nucleotides. Reversed phase C-18 SPE with addition of a nucleoside internal standard, 3'-azido-2',3'-dideoxyuridine (AzdU) allowed for the indirect measurement of the original 5'-triphosphate concentration by HPLC/MS/MS. Quantitation was performed from calibration curves generated from authentic 5'-triphosphate standards spiked in PBMCs from healthy volunteers. Analytical range for the three 5'-triphosphates was equivalent to 50-45,000 pg. Mean interassay accuracies for 3TC-TP, d4T-TP, and ZDV-TP (n > 90) were 99.4%, 100.1%, and 108.0%, respectively. Mean interassay precisions (%C.V.) for 3TC-TP, d4T-TP, and ZDV-TP (n > 90) were 8.8%, 10.4%, and 8.2%, respectively. Recovery of the extraction method was 79.2%, 83.1%, and 98.3% for 3TC-TP, d4T-TP, and ZDV-TP, respectively. This method can be utilized to measure the intracellular 5'-triphosphate levels in HIV infected patients receiving antiretroviral therapy containing the nucleoside reverse transcriptase inhibitors 3TC, d4T, or ZDV.

Simultaneous quantitation of intracellular zidovudine and lamivudine triphosphates in human immunodeficiency virus-infected individuals

Highly active antiretroviral therapy (HAART) is the standard treatment for infection with human immunodeficiency virus (HIV). The most common HAART regimen consists of the combination of at least one protease inhibitor (PI) with two nucleoside reverse transcriptase inhibitors (NRTIs). Contrary to PIs, NRTIs require intracellular activation from the parent compound of their triphosphate moiety to suppress HIV replication. Simultaneous intracellular determination of two NRTI triphosphates is difficult to accomplish due to their relatively small concentrations in peripheral blood mononuclear cells (PBMCs), requiring large amounts of blood from HIV-positive patients. Recently, we described a method to determine intracellular zidovudine triphosphate (ZDV-TP) concentrations in HIV-infected patients by using solid-phase extraction and tandem mass spectrometry. The limit of quantitation (LOQ) for ZDV-TP was 0.10 pmol, and the method was successfully used for the determination of ZDV-TP in HIV-positive patients. In this study, we enhanced the aforementioned method by the simultaneous quantitation of ZDV-TP and lamivudine triphosphate (3TC-TP) in PBMCs from HIV-infected patients. The LOQ for 3TC-TP was 4.0 pmol, with an interassay coefficient of variation and an accuracy of 7 and 12%, respectively. This method was successfully applied to the simultaneous in vivo determination of the ZDV-TP and 3TC-TP pharmacokinetic profiles from HIV-infected patients receiving HAART.

Efavirenz is a potent nonnucleoside reverse transcriptase inhibitor of HIV type 1 replication in microglia in vitro

The objective of this study was to determine whether reverse transcriptase inhibitors (RTIs) could decrease viral replication in microglia. Human microglia obtained from individuals undergoing temporal lobectomy were cultured and infected with HIV-1 isolates from the central nervous system (CNS) as previously described (Strizki JM, et al. J Virol 1996;70:7654-7662). These microglial cultures were treated with one of three nucleoside RTIs (NRTIs) or with efavirenz, a nonnucleoside RTI (NNRTI), at various time points before and during HIV-1 infection. The drug levels sufficient to provide > 90% inhibition of microglial HIV replication (IC90) were determined by comparison of p24(gag) release in the cultures among treated and untreated microglia. Infectious virus released from the infected cultures was also measured with U373-MAGI-CCR5 cells. Efavirenz, an NNRTI, blocked HIV-1(DS-br) infection of microglia with an IC(90) of 0.7-7 nM. This value is similar to the efavirenz IC(90) values for inhibition of laboratory and clinical isolates in lymphocytes, is 2-3 logs lower than the IC90 values of AZT and d4T, and is 1-2 logs lower than that of ddC in microglia. Efavirenz also inhibited infection with other neurotropic isolates, and with viruses isolated from other compartments that also replicated well in microglia. Thus, efavirenz is a potent inhibitor of HIV-1 infection in microglia. Furthermore, efavirenz IC(90) drug levels are present in the cerebrospinal fluid (CSF) of patients taking this once daily NNRTI.

Zidovudine triphosphate and lamivudine triphosphate concentration-response relationships in HIV-infected persons

OBJECTIVE

To quantitate intracellular concentrations of zidovudine and lamivudine triphosphate and explore relationships with virologic and immunologic responses to antiretroviral therapy.

DESIGN

Eight antiretroviral-naive, HIV-infected persons with CD4 T cell counts > 100 x 10(6) cells/l, and HIV RNA in plasma > 5000 copies/ml participating in a prospective, randomized, open-label study of standard dose versus concentration-controlled therapy with zidovudine, lamivudine, and indinavir.

METHODS

Peripheral blood mononuclear cells and plasma were collected frequently throughout the study for quantitation of intracellular zidovudine triphosphate and lamivudine triphosphate concentrations, and zidovudine and lamivudine concentrations in plasma. CD4 T cells and HIV RNA in plasma (Roche Amplicor Ultrasensitive Assay) were measured at baseline and every 4 weeks throughout the study. Relationships among intracellular and plasma concentrations, and CD4 T cells and HIV RNA in plasma were investigated with regression analyses.

RESULTS

Significant relationships were observed between the intracellular concentrations of zidovudine triphosphate and lamivudine triphosphate and the baseline level of CD4 cells. Lamivudine triphosphate concentrations were related in a linear manner to the apparent oral clearance of lamivudine from plasma. A direct linear relationship was found between the intracellular concentrations of zidovudine triphosphate and lamivudine triphosphate. The percent change in CD4 cells during therapy and the rate of decline in HIV RNA in plasma were related to the intracellular concentrations of zidovudine triphosphate and lamivudine triphosphate.

CONCLUSION

These studies into the intracellular clinical pharmacology of nucleoside reverse transcriptase inhibitors illustrate potential clinical implications as determinants of therapeutic success. Moreover, these findings provide several leads and a strong impetus for future investigations with nucleoside reverse transcriptase inhibitors particularly when given in combination and sequentially.

Synthesis and antiviral activity of amino acid carbamate derivatives of AZT

Lipophilic amino acid methyl ester and methyl amide carbamates of 3'-azido-3'-deoxythymidine (AZT) were synthesized and their anti-HIV-1 activity in PBMCs was determined. The methyl amides were more potent (EC50s = 1.8-4.0 microM) than the methyl esters (EC50s = 2.0-20 microM). Carbamate hydrolysis by cell lysates and liberation of AZT was not observed for representative methyl ester or methyl amide AZT carbamates. No evidence of direct inhibition of HIV reverse transcriptase or integrase was observed.

The pharmacokinetics of lamivudine phosphorylation in peripheral blood mononuclear cells from patients infected with HIV-1

OBJECTIVE

To assess the pharmacokinetics of lamivudine phosphorylation in peripheral blood mononuclear cells (PBMC) from patients infected with HIV-1.

DESIGN

Single-center, open-label, randomized, two-period, cross-over study in 10 asymptomatic, antiretroviral-experienced, HIV-1-infected patients who had a CD4+ lymphocyte count of 200-500 x 10(6)/l and had received combination treatment with lamivudine 150 mg twice a day plus zidovudine 600 mg a day (divided into two or three doses) for > or = 16 weeks prior to study entry.

METHODS

Patients were randomly assigned to receive lamivudine 150 mg twice a day or lamivudine 300 mg twice a day for 14 days, with at least a 48-h washout period between treatments. Serial blood samples were collected over 36 h for determination of lamivudine serum concentrations using liquid chromatography/mass spectrometry and intracellular phosphate PBMC concentrations using high performance liquid chromatography/radioimmunoassay methods. Pharmacokinetic parameters were calculated based on lamivudine and lamivudine anabolite concentration-time data.

RESULTS

Intracellular pharmacokinetic parameters were highly variable between patients (coefficient of variations approximately 50%). The two regimens produced lamivudine-total phosphate (totP) values of a similar magnitude. Although the 300-mg regimen tended to produce higher lamivudine-monophosphate (MP) and -triphosphate (TP) values, differences from values produced by the 150-mg regimen were not statistically significant. As lamivudine diphosphate (DP) was the predominant anabolite, accounting for 50-55% of lamivudine-totP (compared with 30-35% for lamivudine-MP and 15-20% for lamivudine-TP), the conversion of lamivudine-DP to lamivudine-TP can be regarded as the rate-limiting step. The median lamivudine-TP intracellular half-life (t1/2) for the 150-mg and 300-mg regimens did not differ significantly (15.3 and 16.1 h, respectively). Serum lamivudine pharmacokinetic parameters were consistent with those observed in previous studies in HIV-1-infected patients. No apparent linear relationships were observed between lamivudine intracellular anabolite and serum data.

CONCLUSIONS

The intracellular pharmacokinetics of lamivudine phosphorylation in PBMC from asymptomatic HIV-1-infected patients are highly variable and do not differ statistically between the 150- and 300-mg twice a day regimens. The variations in intracellular lamivudine-TP concentrations following these two lamivudine dosage regimens are unlikely to result in differences in clinical effect. This was confirmed by the results of a large phase III study in HIV-1-infected patients which showed no differences in HIV-1 RNA or CD4+ lymphocyte counts between the 150- and 300-mg lamivudine regimens in combination with zidovudine.

Quantitation of intracellular triphosphate of emtricitabine in peripheral blood mononuclear cells from human immunodeficiency virus-infected patients

An analytical methodology combining solid-phase extraction (SPE) and high-performance liquid chromatography (HPLC) was developed to quantitate the intracellular active 5'-triphosphate (TP) of beta-L-2',3'-dideoxy-5-fluoro-3'-thiacytidine (emtricitabine) (FTC) in human peripheral blood mononuclear cells (PBMCs). The FTC nucleotides, including 5'-mono-, di-, and triphosphates, were successively resolved on an anion-exchange SPE cartridge by applying a gradient of potassium chloride. The FTC-TP was subsequently digested to release the parent nucleoside that was finally analyzed by HPLC with UV detection (HPLC-UV). Validation of the methodology was performed by using PBMCs from healthy donors exposed to an isotopic solution of [(3)H]FTC with known specific activity, leading to the formation of intracellular FTC-TP that was quantitated by an anion-exchange HPLC method with radioactive detection. These levels of FTC-TP served as reference values and were used to validate the data obtained by HPLC-UV. The assay had a limit of quantitation of 4. 0 pmol of FTC-TP (amount on column from approximately 10(7) cells). Intra-assay precision (coefficient of variation percentage of repeated measurement) and accuracy (percentage deviation of the nominal reference value), estimated by using quality control samples at 16.2, 60.7, and 121.5 pmol, ranged from 1.3 to 3.3% and -1.0 to 4. 8%, respectively. Interassay precision and accuracy varied from 3.0 to 10.2% and from 2.5 to 6.7%, respectively. This methodology was successfully applied to the determination of FTC-TP in PBMCs of patients infected with human immunodeficiency virus after oral administration of various dosing regimens of FTC monotherapy.

Pharmacologic considerations for therapeutic success with antiretroviral agents

OBJECTIVE

To explore pharmacologic elements that contribute to therapeutic success with agents used for the treatment of HIV infection.

DATA SOURCES

Primary articles were identified through MEDLINE (January 1990-January 1999) and secondary sources. Conference abstracts were selected by review of published proceedings.

STUDY SELECTION AND DATA EXTRACTION

The author evaluated all articles, and relevant information was selected for incorporation into this review.

DATA SYNTHESIS

Heterogeneity in the response to antiretroviral therapy has been attributed to virologic, immunologic, behavioral, and pharmacologic differences among patients. Patients receiving the same dose of an antiretroviral agent will not have the same systemic or intracellular concentration because of interpatient variability in drug absorption, distribution, metabolism, and excretion. Pharmacologic theory and available clinical data support relationships between drug concentration and anti-HIV effect. Thus, variability in drug concentrations contributes to variability in antiviral effect. Desirable pharmacologic characteristics for an antiretroviral drug include high oral bioavailability, a long elimination half-life, low intra- and interpatient pharmacokinetic variability, and a predictable dose-concentration-effect relationship.

CONCLUSIONS

Treatment of the individual who is infected with HIV is a challenging long-term undertaking. Optimal management requires synthesis of a rapidly evolving base of basic and clinical knowledge. The selection of an antiretroviral regimen based on an understanding of the degree to which each individual agent possesses the desired pharmacologic characteristics should help the healthcare provider to translate the therapeutic principles of treatment of HIV infection into clinical reality for all patients.

Intracellular phosphorylation of zidovudine in an in vitro hollow fiber model

Combination antiretroviral therapy that includes zidovudine has proved much better in treating human immunodeficiency virus infection than monotherapy Diminished responses to zidovudine, especially when it was given alone, was likely due to factors including interpatient variability in pharmacokinetics, nonadherence, emergence of resistant mutants, and reduced cellular enzymatic processes to phosphorylate the drug. This study evaluated the intracellular metabolism of zidovudine up to 6 weeks using a hollow fiber cellular model system that simulated exposure of cells to steady-state concentrations achieved in humans. The CEM-T4 lymphocytes were exposed to simulated 200-, 600-, and 1200-mg daily doses of zidovudine. Samples were analyzed for monophosphate, diphosphate, and triphosphate metabolites of zidovudine by high-performance liquid chromatography separation and radiochemical detection. The monophosphate metabolite increased as simulated doses increased, but no corresponding increases in the active triphosphate metabolite occurred. In addition, intracellular metabolism of zidovudine did not change after exposure for 6 weeks. These results suggest that the active triphosphate metabolite of zidovudine does not change much when doses are increased or when exposed for at least 6 weeks. Hollow fiber models may be used effectively to investigate intracellular metabolism of antiviral agents and for some duration of time.

Heterodimer-loaded erythrocytes as bioreactors for slow delivery of the antiviral drug azidothymidine and the antimycobacterial drug ethambutol

Disseminated infection with Mycobacterium avium complex (MAC) remains the most common serious bacterial infection in patients with advanced AIDS. The organisms that make up this complex are found ubiquitously in the environment, yet rarely cause disseminated disease in nonimmunocompromised human patients; on the contrary, up to 50% of patients with AIDS may ultimately develop the pathology. Hence, therapeutic strategies able to inhibit HIV and Mycobacterium replication are needed. Because of the rapid plasma elimination and toxicity of the most commonly used drugs, daily multiple-drug therapies must often be continued throughout life, frequently causing major side effects and, as a consequence, poor patient compliance. Therefore, alternative strategies that reduce the toxicity of the drugs and allow prolonged application intervals are sorely needed. Since erythrocytes (RBCs) can behave as bioreactors able to convert impermeant prodrugs to membrane-releasable active drugs, new compounds (AZTpEMB, AZTpEMBpAZT, and AZTp2EMB) consisting of both an antiretroviral and an antimicrobial drug were designed and synthesized. Among these, only AZTp2EMB was hydrolyzed by erythrocyte enzymes and could be encapsulated inside RBCs. AZTp2EMB-loaded RBCs slowly released AZT and EMB in culture medium, reducing its concentration by one-half about every 48 hr of incubation at 37 degrees C. Moreover, when AZTp2EMB-loaded erythrocytes were incubated for 6 days in the presence of human macrophages infected with Mycobacterium avium (M. avium) a marked bactericidal effect (>1 log) was observed. Thus, AZTp2EMB-loaded erythrocytes could be used as endogenous bioreactors for AZT and EMB delivery in the treatment of HIV and M. avium infection.

A critical analysis of the pharmacology of AZT and its use in AIDS

The triphosphorylated form of the nucleoside analogue 3'-azido-3'-deoxythymidine (Zidovudine, AZT) is claimed to interrupt the HIV replication cycle by a selective inhibition of viral reverse transcriptase, thereby preventing the formation of new proviral DNA in permissive, uninfected cells. Given that initial HIV infection of an individual instigates abundant HIV replication from inception until death, and that the life of infected T-cells is only several days, the administration of AZT should lead both in vitro and in vivo (i) to decreased formation of proviral DNA; and thus (ii) to decreased frequencies of 'HIV isolation' (detection of p24 or reverse transcription or both) in stimulated cultures/cocultures of T-cells from seropositive individuals; (iii) to decreased synthesis of HIV p24 and RNA ('antigenaemia', 'plasma viraemia', 'viral load') ultimately resulting in low or absent levels of all three parameters; and (iv) to a perfect and direct correlation between all these parameters. A critical analysis of the presently available data shows that no such evidence exists, an outcome not unexpected given the pharmacological data on AZT. HIV experts all agree that only the triphosphorylated form of AZT (AZTTP) and not the unphosphorylated form administered to patients, nor its mono- or diphosphate, is the active agent. Furthermore, the mechanism of action is the ability of AZTTP to halt the formation of HIV-DNA (chain termination). However, although this claim was posited from the outset, AZT underwent clinical trials and was introduced as a specific anti-HIV drug many years before there were any data proving that the cells of patients are able to triphosphorylate the parent compound to a level considered sufficient for its putative pharmacological action. Notwithstanding, from the evidence published since 1991 it has become apparent that no such phosphorylation takes place and thus AZT cannot possess an anti-HIV effect. However, the scientific literature does elucidate: (i) a number of biochemical mechanisms which predicate the likelihood of widespread, serious toxicity from use of this drug; (ii) in vitro data proving that AZT has significant antibacterial and antiviral properties which confound interpretation of its effects when administered to patients. Based on all these data it is difficult if not impossible to explain why AZT was introduced and still remains the most widely recommended and used anti-HIV drug.

Intracellular nucleotides of (-)-2',3'-deoxy-3'-thiacytidine in peripheral blood mononuclear cells of a patient infected with human immunodeficiency virus

An analytical methodology was developed to quantitate the intracellular nucleotides including mono-, di-, and triphosphates and the diphosphocholine derivative of (-)-2', 3'-deoxy-3'-thiacytidine (3TC) in human peripheral blood mononuclear cells (PBMCs). The procedure includes the resolution of 3TC nucleotides by solid-phase extraction (SPE) on an anion-exchange cartridge, with subsequent enzyme digestion of the resulting phosphates to the parent drug that is ultimately quantitated by high-performance liquid chromatography with UV detection (HPLC-UV). Validation was performed with PBMCs from healthy donors exposed to [3H]3TC, leading to the formation of intracellular nucleotides that were quantitated by anion-exchange HPLC with radioactive detection (HPLC-RA). These nucleotide levels served as reference values and were used for cross-validation with data obtained by HPLC-UV. An excellent correlation was established between the results obtained by HPLC-RA and those obtained by HPLC-UV, with a slope of the regression lines close to unity and intercepts near nullity as well as a correlation coefficient close to unity for all 3TC phosphates. The assay was characterized by a limit of quantitation below 1 ng (amount on column) with a precision (percentage of coefficient of variation of repeated measurement) ranging from 0.8 to 18.1% and an accuracy (deviation of the amount determined by HPLC-UV from the nominal reference value) varying from -14.8 to 19.4%. This methodology was successfully applied to determine the quantity of 3TC nucleotides in PBMCs of a patient infected with human immunodeficiency virus after oral administration of 3TC and stavudine.

Concentration-controlled zidovudine therapy

BACKGROUND

Heterogeneity in the response to antiretroviral therapy has been attributed to pharmacologic, immunologic, and virologic differences between patients. Currently available antiretroviral agents used for the treatment of human immunodeficiency virus (HIV) infection in adults are administered in standard fixed doses. The active moiety of nucleoside anti-HIV drugs is the intracellular anabolite. Therefore the heterogeneity in response to nucleoside agents may arise as a result of pharmacologic variability at both the systemic and cellular level.

OBJECTIVES

To determine whether a novel concentration-controlled zidovudine regimen could improve anti-HIV response compared with the standard fixed-dose approach.

DESIGN

At the Outpatient Clinic of the General Clinical Research Center at the University of Minnesota, 20 persons with HIV infection received an oral regimen of zidovudine designed to achieve a target concentration in plasma of 0.7 mumol/L and the 500 mg/day standard dose in a randomized, crossover 24-week study.

RESULTS

The concentration-controlled regimen achieved overall higher systemic concentrations with reduced interpatient variability: steady-state average zidovudine plasma concentrations were 0.76 mumol/L (coefficient of variation, 12%) versus 0.62 mumol/L (coefficient of variation, 32%) for the standard regimen. There was no difference in safety and tolerance between regimens. Intracellular zidovudine triphosphate concentrations averaged 160 fmol/10(6) peripheral blood mononuclear cells (PBMCs) with concentration-controlled versus 92 fmol/10(6) PBMCs for standard therapy. The percentage change from baseline in CD4 cells was a 22% increase for the concentration-controlled regimen versus a 7% decrease with standard therapy.

CONCLUSIONS

These data indicate that pharmacologic variability affects antiretroviral response. Furthermore, these findings provide a framework to characterize the pharmacologic determinants of effect and quantitate their contribution to the heterogeneity in clinical response to optimize therapeutic benefit.

Intracellular phosphorylation of zidovudine (ZDV) and other nucleoside reverse transcriptase inhibitors (RTI) used for human immunodeficiency virus (HIV) infection

Dramatic reductions of viral load and increased survival have been achieved in patients infected with the Human Immunodeficiency Virus (HIV) with the introduction of combination antiretroviral therapy. Currently 11 agents including nucleoside reverse transcriptase inhibitors (RTI), non-nucleoside RTI and protease inhibitors are available for the use for treatment of HIV infection. Recent studies have demonstrated that certain combinations of these drugs are advantageous over their individual use as monotherapy with an even more sustained viral suppression. Much emphasis has therefore been put on studies evaluating the interactions of these different compounds. Especially the intracellular metabolism of nucleoside RTI has been evaluated to some extent, by both in vitro and in vivo studies. These compounds need to undergo phosphorylation to their active 5'-triphoshates involving several enzymatic steps and the nucleoside concentration in the plasma may not correlate with intracellular concentrations of active drug. It is therefore of great importance to study these drugs at an intracellular level in order to evaluate their efficacy. This review summarizes the intracellular phosphorylation of Zidovudine and other nucleoside analogs investigated by in vitro experiments and the efforts of measuring the active anabolites in vivo in cells isolated from HIV infected patients on nucleoside therapy.

Analysis of antiretroviral nucleosides by electrospray ionization mass spectrometry and collision induced dissociation

Antiretroviral nucleoside drugs used against the human immunodeficiency virus (HIV) infection have been analyzed using negative ion electrospray ionization (ESI) mass spectrometry and collision-induced dissociation (CID-MS/MS). Mass fragmentation of azidothymidine (AZT), didanosine (ddI), dideoxycytidine (ddC) and dideoxythiacytidine (3TC) were obtained at different cone voltages and collision energies. Fragmentation of purines and pyrimidines occurred by different pathways. For purines (ddI), the fragmentation was similar to those found in endogenous nucleosides; mainly the pseudo molecular ion is present (M-H)- and a cleavage through the glycosidic bond forming (B)- was observed. For pyrimidines (AZT, ddC, 3TC), the fragmentation pathways were different from endogenous nucleosides; for AZT, the fragmentation occurred primarily through the elimination of the azido group in the 3'-position (M-H2-N3)-, whereas ddC and 3TC presented more complex fragmentation patterns. For ddC, fragmentation appeared to be dominated by a retro Diels-Alder mechanism (M-CONH)-. For 3TC, the sulfur atom in the sugar moiety provided greater stability to the charge, producing fragments where the charge resided initially in the dideoxyribose (M-C2O2H6)-.

Probing the mechanism of action and decomposition of amino acid phosphomonoester amidates of antiviral nucleoside prodrugs

The decomposition pathways in peripheral blood mononuclear cells (PBMCs) and the in vitro anti-HIV-1 activity of the structurally similar 3'-azido-3'-deoxythymidine (AZT) phosphoramidates 1-6 and 3'-fluoro-3'-deoxythymidine (FLT) phosphoramidates 7-10 are reported. The AZT phosphoramidates exhibited no cytotoxicity toward CEM cells at concentrations as high as 100 microM, whereas the FLT phosphoramidates 9 and 10 had CC50 values of 95.6 and 35.1 microM, respectively. All 10 compounds exhibited no cytotoxicity toward PBMCs at concentrations as high as 100 microM and were effective at inhibiting viral replication. In particular, the AZT phosphomonoester amidate 4 displayed comparable antiviral activity to the parent nucleoside analog AZT. Mechanistic studies on the amino acid carbomethoxy ester phosphomonoester amidates revealed that their decomposition pathway differs from that of amino acid carbomethoxy ester aryl phosphodiester amidates of nucleotide prodrugs. AZT phosphomonoester amidates are internalized by lymphocytes to the same extent as AZT by a nonsaturable process. In lymphocytes, the amino acid carbomethoxy ester phosphomonoester amidates of AZT are not significantly metabolized to either AZT or the mono-, di-, or triphosphate of AZT. The amount of active anabolite, AZT-5'-triphosphate, formed in PBMCs incubated with the AZT phosphomonoester amidates 3 and 4 was 2- and 3-fold less than that observed after treatment with AZT, respectively. In contrast, FLT phosphomonoester amidates are rapidly converted to FLT-5'-monophosphate by a process that is antagonized by the corresponding AZT derivative 4. These results suggest that the metabolism of aromatic amino acid carbomethoxy ester phosphomonoester amidate nucleotide prodrugs by PBMCs does not require prior conversion to the corresponding carboxylic acid before proceeding to P-N bond cleavage.

Quantitation of intracellular zidovudine phosphates by use of combined cartridge-radioimmunoassay methodology

This report describes the development of a potentially clinical method to measure the cellular metabolites of zidovudine (ZDV) in patients receiving the drug. This new method combines the use of Sep-Pak cartridges to separate ZDV phosphates with radioimmunoassaying to quantitate ZDV. The detection limit is 0.02 pmol/10(6) cells, and this assay can measure a wide range of intracellular drug concentrations. The use of the cartridge-radioimmunoassay methodology should prove very useful for in vivo cellular pharmacokinetic studies of ZDV.

Pharmacologic considerations for antiviral drug development

OBJECTIVE

To discuss pharmacologic considerations for the development of antiviral agents.

DATA SOURCES

English-language literature pertaining to the development and clinical evaluation of antiviral compounds, primarily agents targeted against herpes group viruses and HIV.

STUDY SELECTION AND DATA EXTRACTION

Pertinent information, as judged by the author, was selected for discussion.

DATA SYNTHESIS

Drug development of antiviral agents presents unique problems compared with that of antimicrobial and other agents. Understanding the mechanism of action and both pharmacokinetic and pharmacodynamic considerations is critical to developing a rational dosing strategy and safe, effective use. The lack of standardized methods for antiviral susceptibility testing and the influence of factors such as strain of virus, host cell type, culture medium, inoculum size, end point, and method of measurement on the results obtained illustrate factors that complicate preclinical pharmacologic analysis of antiviral agents. Acyclovir offers a model for clinical drug development. Its mechanism of action, pharmacokinetics, and pharmacodynamics have been studied extensively. Rational guidelines for usage are available, including guidelines in special patient populations such as kidney transplant recipients and neonates. A pregnancy registry has allowed evaluation of the incidence of birth defects in fetuses exposed to systemic acyclovir. Several pitfalls in antiviral drug development are associated with inadequate pharmacologic information. The development of dextran sulfate and fialuridine provides two examples. Integration of pharmacokinetic and pharmacodynamic analyses using modern sampling and analysis techniques may facilitate more rapid development of antiviral agents and more informed dosage regimens to achieve the highest probability of therapeutic success.

Zidovudine phosphorylation after short-term and long-term therapy with zidovudine in patients infected with the human immunodeficiency virus

BACKGROUND

Resistance of human immunodeficiency virus (HIV) to zidovudine (AZT) has been associated with mutations in the viral reverse transcriptase gene. However, recent studies suggest that host cellular factors such as a decreased thymidine kinase activity or an increased cellular P-glycoprotein expression may be important. This study compared concentrations of zidovudine monophosphate, zidovudine diphosphate, and zidovudine triphosphate with P-glycoprotein expression in peripheral blood mononuclear cells from patients receiving long-term (> 18 months) and short-term (< 2 months) zidovudine treatment.

METHODS

Ten subjects in the short-term group and 11 subjects in the long-term group with CD4 counts between 300 and 500 received a single oral dose of zidovudine (200 mg) after a 24-hour washout period. Blood samples were collected at 0, 1, 2, 4, and 6 hours. Intracellular nucleotide concentrations were measured by a combined HPLC-radioimmunoassay method, and P-glycoprotein expression was determined by fluorescence activated cell sorting (FACS) analysis with use of the monoclonal mouse antibody MRK-16.

RESULTS

Zidovudine monophosphate was the predominant compound, accounting for 73.4% +/- 7.1% (SD) of the total phosphates in the long-term treatment group and 74.2% +/- 15.0% (SD) in the short-term group. Zidovudine diphosphate accounted for 13.3% +/- 3.3% (SD) in the long-term group and 12.5% +/- 6.6% (SD) in the short-term group. Zidovudine triphosphate accounted for 13.4% +/- 4.1% (SD) in the long-term group and 13.5% +/- 8.3% (SD) in the short-term group. Mean peak concentrations for the active zidovudine triphosphate were 0.04 +/- 0.02 (SD) pmol/10(6) cells in both groups. Comparison of the individual zidovudine phosphate concentrations and P-glycoprotein expression revealed no significant difference in the two patient populations.

CONCLUSIONS

These data suggest that intracellular phosphorylation does not change over time and that zidovudine does not select for P-glycoprotein expressing cells.

Clinical pharmacokinetics of zidovudine. An update

The battle against the acquired immune deficiency syndrome (AIDS) is now into its second decade, and substantial advancements have been made in our understanding of the complex life cycle of, and the immunopathology associated with, human immunodeficiency virus (HIV) infection, as well as of the drugs used to modify the course of disease. Zidovudine was the first agent approved for treatment of HIV disease, and since its widespread availability in 1987 the pharmacokinetic disposition and clinical effects of zidovudine have been extensively evaluated. This article reviews the absorption, distribution, metabolism and elimination characteristics of zidovudine, focusing on more recent information. In addition, factors that may or may not affect zidovudine disposition are discussed. These include selected drug interactions and concomitant disease states such as renal and hepatic insufficiency. Issues such as bodyweight normalisation, maternal-fetal transfer, pregnancy and intracellular phosphorylation are discussed in relation to the pharmacokinetics and clinical efficacy of zidovudine. Finally, information regarding the clinical pharmacodynamics of zidovudine is presented. This includes possible relationships between zidovudine pharmacokinetics and markers of efficacy and toxicity, and the significance of linking pharmacokinetic and pharmacodynamic information.

Pharmacokinetic individualisation of zidovudine therapy. Current state of pharmacokinetic-pharmacodynamic relationships

Zidovudine is the cornerstone of current antiretroviral treatment of human immunodeficiency virus (HIV) infection. Its use, however, frequently leads to adverse reactions, including myelosuppression. Zidovudine pharmacokinetics show large interindividual variation with indications of pharmacokinetic-pharmacodynamic relationships, but a clear therapeutic window has not yet been defined. Individualisation of zidovudine therapy with monitoring of drug concentrations might be desirable. This review considers (intracellular) monitoring of zidovudine and anabolites for individualisation of zidovudine therapy and the achievements in describing pharmacokinetic-pharmacodynamic relationships so far.

Quantification of zidovudine and individual zidovudine phosphates in peripheral blood mononuclear cells by a combined isocratic high performance liquid chromatography radioimmunoassay method

Zidovudine (ZDV, AZT) is the first clinically effective drug licensed for use in the treatment of human immunodeficiency virus (HIV) infection. Activation of ZDV requires phosphorylation to ZDV triphosphate by cellular kinases. It is important, therefore, to determine the intracellular levels of the active form because measurement of ZDV concentrations in plasma have not reflected any direct relationship with activity or toxicity. In this paper a validated assay for the measurement of both ZDV and its three phosphorylated anabolites, ZDV mono-, di- and triphosphate, in peripheral blood mononuclear cells (PBMCs) is described. The method consisted of a combination of isocratic high performance liquid chromatography (HPLC) separation and radioimmunoassay (RIA). The PBMCs were separated from whole blood and ZDV and ZDV nucleotides were extracted and separated by isocratic elution with an ion-pairing mobile phase on a reversed-phase HPLC column. The collected ZDV and individual ZDV nucleotide fractions were dephosphorylated to ZDV, cleaned by solid phase extraction and assayed by a commercially available RIA kit. The assay developed was successfully used to determine intracellular ZDV and anabolite concentrations of 10 PBMC samples taken from HIV positive patients on ZDV treatment.

Metabolism of Zidovudine

1. The anti-HIV drug zidovudine (3'-azido-2',3'-dideoxythymidine; ZDV) has three important pathways of metabolism. ZDV is a prodrug and must be phosphorylated in lymphocytes in order to exert its antiviral action. However, in quantitative terms this is a minor pathway probably accounting for less than 1% of the overall metabolic profile. The predominant pathway of metabolism is glucuronidation to GZDV and the metabolite is renally excreted. A further metabolite, derived by reduction of the azido moiety is 3'-amino-3'-deoxythymidine (AMT). 2. Zidovudine glucuronidation has been characterised in human liver microsomes. A number of drugs (e.g., naproxen, indomethacin and probenecid) have been shown to inhibit the in vitro conjugation of ZDV. Some of these drugs have also been co-administered with ZDV in HIV-positive patients. Significant pharmacokinetic interactions have been demonstrated with probenecid, naproxen and fluconazole. 3. 3'-amino-3'-deoxythymidine formation is probably mediated by both cytochrome P450 isozymes and NADPH-cytochrome P450 reductase. Peak plasma concentrations of AMT are approximately 10-15% of ZDV in patients. This is a potentially important metabolite because of its alleged cytotoxicity. 4. Measurement of intracellular ZDV phosphates in patients provides the key to our understanding of both the efficacy and toxicity of ZDV. Important recent work has demonstrated that as patients deteriorate (i.e., CD4 counts decrease below 100 x 10(6)/L), there is a corresponding increase in intracellular ZDV-monophosphate. This could have toxicological implications.

Oral and intravenous zidovudine pharmacokinetics: the effect of granulocyte-macrophage colony stimulating factor

Combination therapy with zidovudine and recombinant human granulocyte-macrophage colony stimulating factor (rHu GM-CSF) may be warranted, owing to the bone marrow suppressive effects of zidovudine. A study of 16 patients, 8 of whom had acquired immune deficiency syndrome (AIDS) and 8 of whom were infected with human immunodeficiency virus (HIV) but were asymptomatic, was conducted. The effect of 4 days of rHU GM-CSF versus placebo on intermittent zidovudine therapy (200 mg every 8 hours) was evaluated using a randomized, cross-over study design. Pharmacokinetics of oral and intravenous zidovudine were determined on days 1 (oral), 3 (oral), and 4 (intravenous) of rHu-GM-CSF (placebo) administration. After intravenous dosing, zidovudine plasma clearance for placebo and rHu GM-CSF averaged 1.4 +/- 0.2 and 1.3 +/- 0.2 L/hr/kg, respectively (P = 0.017), mean residence time averaged 1.5 +/- 0.5 and 1.9 +/- 0.6 hours, respectively (P = 0.012), and the steady-state volume of distribution was 2.0 +/- 0.7 and 2.3 +/- 0.7 L/kg, respectively (P = 0.027) for the two treatment arms. Stratified data for patients with AIDS and those with asymptomatic HIV infection revealed no significant difference in plasma clearance or mean residence time between the two patient groups. These pharmacokinetic results indicate that dosage adjustments for zidovudine are not warranted when administered with rHu GM-CSF owing to the small changes observed. However, the statistically significant increase in Vss suggests the possibility of enhanced zidovudine cellular uptake in the presence of rHu GM-CSF.

Pharmacokinetic optimisation of antiretroviral therapy in patients with HIV infection

More than 7 years after the introduction of zidovudine for treatment of HIV infection, little use has been made of the pharmacokinetic properties of this or any of the subsequently approved antiretroviral agents to optimise therapy. This is partly because of the limits of technologies developed to measure clinically relevant forms and concentrations of these drugs, and partly because the clinical community has been slow to recognise the potential benefits of pharmacokinetic optimisation of nucleoside analogue therapy in any disease. Nonetheless, for some of these agents, progress in understanding the relationship between pharmacokinetics and pharmacodynamics has been made. With zidovudine, for example, even though plasma concentrations have little clinical utility, evidence suggests that concentrations of active phosphorylated forms of zidovudine inside target cells are related to disease progression and toxicity. Furthermore, a decreased ability to phosphorylate zidovudine might be a prerequisite for the emergence of zidovudine-resistant HIV strains. Measurements of phosphorylated zidovudine inside cells similarly suggest that 100 mg of oral zidovudine every 8 hours approximates the optimal initial dosage regimen in asymptomatic patients. Increased plasma didanosine concentrations have been associated with several measures of clinical improvement in patients, and may be associated with an increased risk of toxicity as well. For zalcitabine and stavudine, however, the picture is much less clear. Their pharmacokinetic and pharmacodynamic relationships have not been studied in patients. Furthermore, there is insufficient data on the effects of age, gender, race and concurrent underlying conditions on the pharmacokinetics of all of these agents. Mounting evidence suggests that monitoring of these compounds could lead to individually optimised intervention strategies. Given the marginal benefits of therapy with these agents, their proven toxic effects and the lack of proven alternatives, it is critical that the clinical community strive to make the most effective use of these agents in the treatment of their patients.

Foscarnet and ganciclovir pharmacokinetics during concomitant or alternating maintenance therapy for AIDS-related cytomegalovirus retinitis

INTRODUCTION

The use of foscarnet and ganciclovir as a combination treatment for cytomegalovirus retinitis is increasing because of limitations associated with single agent therapy.

METHODS

The pharmacokinetics of foscarnet and ganciclovir were determined in 13 patients receiving either concomitant therapy (regimen A) or daily alternating therapy (regimen B) for maintenance of cytomegalovirus disease. For regimen A, 60 mg/kg intravenous foscarnet and 3.75 mg/kg ganciclovir were sequentially administered daily; for regimen B, 120 mg/kg foscarnet and 6 mg/kg ganciclovir were administered on alternating days. For both regimens, serial blood sampling for pharmacokinetic analysis was performed for each drug alone (day 1 or 2) and after 2 weeks of combination therapy. Plasma samples for foscarnet and ganciclovir analysis were performed by means of high-performance liquid chromatography. Pharmacokinetic analysis was performed with noncompartmental methods.

RESULTS

For regimen A, the plasma clearance (CL) of foscarnet did not change in the presence of ganciclovir, averaging 0.12 +/- 0.08 and 0.11 +/- 0.02 L/hr/kg on study days 2 and 14, respectively (p = 0.34). The volume of distribution (VSS) and mean residence time (MRT) also did not change significantly. CL and MRT of foscarnet did not change for regimen B, although a slight increase in VSS was observed before (0.38 +/- 0.05 L/kg) and after (0.46 +/- 0.07 L/kg) alternating therapy (p = 0.03). Ganciclovir CL did not change for either regimen, with mean values of 0.21 +/- 0.10 and 0.25 +/- 0.10 L/hr/kg (regimen A, p = 0.17) and 0.32 +/- 0.10 and 0.34 +/- 0.11 L/hr/kg (regimen B, p = 0.24). MRT and VSS were also not significantly different.

CONCLUSION

These plasma data suggest that further dosage adjustments are unnecessary for or alternating maintenance therapy.