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

Lamivudine/Tenofovir Disoproxil Fumarate is an Appropriate PrEP Regimen

Oral pre-exposure prophylaxis (PrEP) containing tenofovir disoproxil fumarate (TDF) co-formulated with emtricitabine (FTC) or lamivudine (3TC) is recommended as an additional prevention option for persons at substantial risk of HIV infection by both the World Health Organization (WHO) and the US President's Emergency Plan for AIDS Relief (PEPFAR). The WHO and PEPFAR consider 3TC clinically interchangeable with FTC for PrEP given comparable pharmacologic equivalence, resistance and toxicity patterns, and indirect clinical trial evidence from TDF-containing studies. Globally, FTC/TDF has been widely used in clinical trials, open-label extension studies and demonstration projects. Thus, most PrEP efficacy and safety data are based on FTC/TDF use in heterosexual women and men, men who have sex with men, and people who inject drugs. However, generic 3TC/TDF is less expensive than FTC/TDF, is already available in supply chains for HIV drugs, and has 60-70% of the global adult market share, making it particularly appealing in settings with limited availability or affordability of FTC/TDF. Compelling indirect evidence suggests that scaling up use of 3TC/TDF is potentially cost saving for HIV programs in settings where restricting drug choice to FTC/TDF would delay PrEP implementation. Guideline committees and public health decision-makers in countries should encourage flexibility in PrEP drug selection, support off-label use of 3TC/TDF, and approve use of generic formulations to decrease the cost of PrEP medications and accelerate PrEP delivery through the public and private sectors.

Direct and indirect quantification of phosphate metabolites of nucleoside analogs in biological samples

Nucleoside reverse transcriptase inhibitors (NRTIs) are prodrugs that require intracellular phosphorylation to active triphosphate nucleotide metabolites (NMs) for their pharmacological activity. However, monitoring these pharmacologically active NMs is challenging due to their instability, high hydrophilicity, and their low concentrations in blood and tissues. Liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) is the gold standard technique for the quantification of NRTIs and their phosphorylated NMs. In this review, an overview of the publications describing the quantitative analysis of intracellular and total tissue concentration of NMs is presented. The focus of this review is the comparison of the different approaches and challenges associated with sample collection, tissue homogenization, cell lysis, cell counting, analyte extraction, sample storage conditions, and LC-MS analysis. Quantification methods of NMs via LC-MS can be categorized into direct and indirect methods. In the direct LC-MS methods, chromatographic retention of the NMs is accomplished by ion-exchange (IEX), ion-pairing (IP), hydrophilic interaction (HILIC), porous graphitic carbon (PGC) chromatography, or capillary electrophoresis (CE). In indirect methods, parent nucleosides are 1st generated from the dephosphorylation of NMs during sample preparation and are then quantified by reverse phase LC-MS as surrogates for their corresponding NMs. Both approaches have advantages and disadvantages associated with them, which are discussed in this review.

A Pharmacokinetic/Pharmacodynamic Model to Predict Effective HIV Prophylaxis Dosing Strategies for People Who Inject Drugs

The goal of this work was to evaluate dosing strategies for tenofovir disoproxil fumarate (TDF), tenofovir alafenamide (TAF), and emtricitabine (FTC) for pre-exposure prophylaxis (PrEP) with injection drug use with a pharmacokinetic/pharmacodynamics analysis of concentration data generated from two single-dose clinical studies conducted in healthy women. Population pharmacokinetic models were developed using measured intracellular metabolite, endogenous nucleotide competitors, and extracellular parent drug concentrations. Intracellular metabolite concentrations were normalized to endogenous competitors and compared with an EC90 target for PrEP efficacy. Monte Carlo simulations were used to select effective dose strategies of single agents (TAF, TDF, and FTC) and combinations (TDF + FTC and TAF + FTC). Daily, intermittent, and event-driven dosing regimens at varying dosage amounts were explored. When combined, TDF + FTC and TAF + FTC both provided quick (0.5 hours) and durable (up to 84 and 108 hours, respectively) protection of ≥99% after a single dose. When dosed twice per week, protection remained at 100%. Single-agent regimens provided lower estimates of protection than either combination tested. Here, the application of pharmacokinetic modeling to in vitro target concentrations demonstrates the added utility of including FTC in a successful PrEP regimen. While no TAF-based PrEP data are currently available for comparison, this analysis suggests TAF + FTC could completely protect against percutaneous exposure with as little as two doses per week.

Simultaneous quantification of intracellular lamivudine and abacavir triphosphate metabolites by LC-MS/MS

Nucleoside reverse transcriptase inhibitors (NRTIs) require intracellular phosphorylation to active triphosphate (TP) nucleotide metabolites before they can inhibit the HIV reverse transcriptase. However, monitoring these pharmacologically active TP metabolites is challenging due to their instability and their low concentrations at the pg/ml levels in blood and tissues. The combination of lamivudine (3TC) and abacavir (ABC) is one of the first lines for HIV therapy. Therefore, a sensitive, selective, accurate, and precise LC-MS/MS method was developed and validated for the simultaneous quantification of 3TC- and ABC-TP metabolites in mouse blood and tissues. Calibration curves were linear over the range of 10-100,000 pg/ml for 3TC-TP and 4-40,000 pg/ml for carbovir-TP (CBV-TP; phosphorylated metabolite of ABC). This corresponds to 2.1-21,322 fmol/10⁶ cells for 3TC-TP and 0.8-8000 fmol/10⁶ cells for CBV-TP. Accuracy and precision were less than 15% for all quality control sample (QCs), and absolute extraction recovery of were >65% for 3TC-TP and >90% for CBV-TP. The method was optimized to ensure stability of TP samples and standards during sample collection, preparation, analysis, and storage conditions. This method has enhanced sensitivity and requires smaller amounts of blood and tissue samples compared to previous LC-MS/MS methods for 3TC- and CBV-TP quantification. The developed method was successfully applied to characterize the pharmacokinetic profile of TP metabolites in mouse peripheral blood mononuclear cells (PBMCs), spleen, lymph nodes, and liver cells. In addition, another direct, simple, and high-throughput method for the quantification of TP standards was developed and used for the analysis of stability samples.

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

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

Ultrasensitive method to quantify intracellular zidovudine mono-, di- and triphosphate concentrations in peripheral blood mononuclear cells by liquid chromatography-tandem mass spectrometry

Although zidovudine (AZT) is not the preferred antiretroviral drug for adult HIV-infected patients, it is still widely used in infants for both prevention of mother-to-infant HIV-1 transmission and treatment of HIV-infected children. However, it is difficult to measure intracellular concentrations of AZT metabolites in small blood samples due to their extremely low concentrations in peripheral blood mononuclear cells and interference by endogenous nucleotide triphosphates, residual plasma phosphates and electrolytes. We developed an ultrasensitive assay using liquid chromatography-tandem mass spectrometry (LC-MS/MS) for measurement of intracellular concentrations of zidovudine (AZT)-monophosphate (AZT-MP), -diphosphate (AZT-DP) and -triphosphate (AZT-TP). The high sensitivity was due to the improvement of peripheral blood mononuclear cells extraction for complete removal of plasma and electrolytes, alkalization of LC buffer and use of alkaline-stable high performance liquid chromatography column and tetrabutylammonium hydroxide as the ion pair. Using this method, the lower limits of quantification of AZT, AZT-MP, -DP and -TP were 6, 6, 10 and 10 fmol per sample, respectively. Accuracy ranged 89-115% and precision was lower than 15% in the quantification range of 6-6000 fmol/sample for plasma AZT and intracellular AZT-MP and 10-10 000 fmol/sample for AZT-DP and -TP. The validation parameters met the international requirements. Among nine AZT-treated HIV-infected adult patients, five had low AZT-TP levels (<10 fmol/10(6) cells). Our assay has high sensitivity and is advantageous for evaluation of AZT phosphates in children and infants based on minimum blood sampling requirement.

Drug-induced nanocarrier assembly as a strategy for the cellular delivery of nucleotides and nucleotide analogues

The natural nucleotide adenosine triphosphate (ATP) and nucleotide analogues such as azidothymidine triphosphate (AZT-TP) display important pharmacological activities for the treatment of ischemia and HIV infections, respectively. Their clinical use is, however, limited mostly due to their hydrophilicity, which highly restricts their diffusion into the target cells. Few nanocarriers have been proposed to address the challenge of ATP/AZT-TP cellular delivery, but the loading efficiency, preparation complexity, and efficient cellular delivery remain important barriers to their development. In this study, we propose an original, straightforward and versatile design of nucleotide and nucleotide analogue nanocarriers based on the natural polysaccharide chitosan (CS). We show that the drugs ATP and AZT-TP can induce ionotropic gelation of CS, leading to CS/ATP and CS/AZT-TP nanoparticles with high drug entrapment efficiency and loading rate-up to 44%. Such nanocarriers release ATP and AZT-TP in physiological media and allow an efficient in vitro cellular delivery of these molecules down to the cell cytoplasm.

Effect of HIV-1 infection and sex on the cellular pharmacology of the antiretroviral drugs zidovudine and lamivudine

The cellular pharmacology of zidovudine (ZDV) and lamivudine (3TC) in vivo is not completely understood. This prospective longitudinal study investigated the relationship between HIV-1 serostatus, sex, race, and time on therapy with intracellular and plasma ZDV and 3TC concentrations. Of 20 HIV-seronegative and 23 HIV-seropositive volunteers enrolled, 16 (8 women) and 21 (5 women) completed all 12 study days, respectively. Volunteers began ZDV-3TC therapy (plus a third active drug in HIV-seropositive volunteers), and steady-state concentrations (C(ss)) were determined after days 1, 3, 7, and 12. A repeated-measures mixed model was utilized. HIV-seronegative status was associated with 22% (95% confidence interval [CI], 0%, 50%) and 37% (15%, 67%) higher C(ss) estimates compared to those of HIV-seropositive individuals for intracellular ZDV-TP and 3TC-TP levels, respectively. African-Americans had 36% (8%, 72%) higher ZDV-TP estimates than non-African-Americans. Sex was not associated with ZDV-TP or 3TC-TP (P > 0.19). Women had 36% (4%, 78%) higher plasma ZDV, but the effect was lessened when normalized by lean body weight (5% [-19%, 38%]; P = 0.68). Plasma 3TC was 19% (0%, 41%) higher in HIV-seropositive volunteers and 22% (0%, 48%) higher in African American volunteers, but these effects were not significant when corrected for creatinine clearance (7% [-9%, 20%] and -5% [-26%, 12%] for HIV serostatus and race, respectively; P > 0.35). These results suggest that HIV-seropositive status decreases and African American race elevates the cellular triphosphates of ZDV and 3TC. This information extends knowledge of ZDV and 3TC cellular pharmacology in vivo and provides new leads for future cellular pharmacology studies aimed at optimizing HIV prevention/treatment with these agents.

Cytosine deoxyribonucleoside anti-HIV analogues: a small chemical substitution allows relevant activities

The search for new nucleoside analogue compounds targeting the virally encoded reverse transcriptase was developed by modifying the nucleoside structure to create inhibitor compounds. In this review, the structure-activity relationship of antiviral compounds synthesised from the naturally existing cytosine deoxyribonucleoside (dC) was evaluated. The line of research starting from dC led to the synthesis of 2',3'-dideoxycytidine (ddC; zalcitabine), 2',3'-dideoxy-3'-thiacytidine (3TC; lamivudine) and 2',3'-dideoxy-5-fluoro-3'-thiacytidine (FTC; emtricitabine) and looks very interesting because each product comes from a single small change in the chemical structure of the former compound, resulting in a progressive improvement in terms of activity, pharmacokinetics, tolerability and emergence of resistance.

Determination of nucleoside analog mono-, di-, and tri-phosphates in cellular matrix by solid phase extraction and ultra-sensitive LC-MS/MS detection

An ultra-sensitive liquid chromatography tandem mass spectrometry (LC-MS/MS) assay was developed and validated to facilitate the assessment of clinical pharmacokinetics of nucleotide analogs from lysed intracellular matrix. The method utilized a strong anion exchange isolation of mono-(MP), di-(DP), and tri-phosphates (TP) from intracellular matrix. Each fraction was then dephosphorylated to the parent moiety yielding a molar equivalent to the original nucleotide analog intracellular concentration. The analytical portion of the methodology was optimized in specific nucleoside analog centric modes (i.e. tenofovir (TFV) centric, zidovudine (ZDV) centric), which included desalting/concentration by solid phase extraction and detection by LC-MS/MS. Nucleotide analog MP-, DP-, and TP-determined on the TFV centric mode of analysis include TFV, lamivudine (3TC), and emtricitibine (FTC). The quantifiable linear range for TFV was 2.5-2000 fmol/sample, and that for 3TC/FTC was 0.1 200 pmol/sample. Nucleoside analog MP-, DP-, and TP-determined on the ZDV centric mode of analysis included 3TC and ZDV. The quantifiable linear range for 3TC was 0.1 100 pmol/sample, and 5-2000 fmol/sample for ZDV. Stable labeled isotopic internal standards facilitated accuracy and precision in alternative cell matrices, which supported the intended use of the method for MP, DP, and TP determinations in various cell types. The method was successfully applied to clinical research samples generating novel intracellular information for TFV, FTC, ZDV, and 3TC nucleotides. This document outlines method development, validation, and application to clinical research.

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.

Emerging mutations at virological failure of HAART combinations containing tenofovir and lamivudine or emtricitabine

OBJECTIVE

To compare the emergence of drug-resistant HIV variants at failure of lamivudine (3TC)/tenofovir (TDF)-containing or emtricitabine (FTC)/TDF-containing HAART as a consequence of the different 3TC and FTC intracellular half-lives.

DESIGN

Retrospective evaluation of 859 patients selected from an Italian HIV resistance database (Antiretroviral Resistance Cohort Analysis).

METHODS

Patients were selected for analysis if treated with a HAART whose nucleoside/nucleotide reverse transcriptase inhibitor backbone was either 3TC/TDF or FTC/TDF; if they experienced a virological failure after at least 6 months of plasma HIV-RNA undetectability; and if HIV genotypes before treatment and at failure were available. Univariate and multivariate logistic regression analyses were done to detect predictors of resistance mutations emerging at failure.

RESULTS

Of 714 patients failing with 3TC/TDF and 145 with FTC/TDF, 35.8 and 21.1% were in Centers for Disease Control and Prevention stage C, and 8.8 and 15.2% were on first-line HAART, respectively. At multivariate analysis, the emergence of K70R (P = 0.002), M184V (P = 0.031), T215F (P = 0.020) and Y181C (P = 0.005) was significantly more common in 3TC-treated than in FTC-treated patients, with an odds ratio of 4, 1.56, 1.89 and 3.84, respectively.

CONCLUSION

Despite their close structural similarity, 3TC and FTC are associated with a significantly different rate of drug resistance at treatment failure when combined with TDF in HAART regimens independently of the third drug used.

Zidovudine and Lamivudine for HIV Infection

Zidovudine and lamivudine (ZDV and 3TC) are long-standing nucleoside analog-reverse transcriptase inhibitors (NRTIs) with extensive clinical experience in a wide spectrum of patients from in utero through childhood and adult ages. The safety profiles of both drugs are well-known and side effects for ZDV most commonly include nausea/vomiting, fatigue, anemia/neutopenia, and lipoatrophy; while 3TC is well-tolerated. ZDV-3TC is currently a viable alternative NRTI backbone for initial three-drug therapy of HIV infection when tenofovir disoproxil fumarate-emtricitabine (TDF-FTC) cannot be used because of a relative or absolute contraindication. ZDV-3TC continue to be viable alternatives for children, pregnant women and in resource limited settings where other recommended options are not readily available. ZDV-3TC penetrate the Central Nervous System (CNS) well, which makes ZDV-3TC attractive for use in patients with HIV-associated neurological deficits. Additional benefits of these drugs may include the use of ZDV in combination with certain NRTIs to exert selective pressure to prevent particular drug resistance mutations from developing, and giving a short course of ZDV-3TC to prevent resistance after prophylactic single dose nevirapine.

HPLC-MS/MS method for the intracellular determination of ribavirin monophosphate and ribavirin triphosphate in CEM ss cells

A sensitive and specific method using high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) for the determination of ribavirin monophosphate (RBV-MP) and ribavirin triphosphate (RBV-TP) in cells has been developed and validated. In this method, ribavirin phosphorylated metabolites were extracted and separated by anion exchange solid phase extraction (SPE). The RBV-MP and RBV-TP fractions were dephosphorylated using acid phosphatase and further purified by phenyl boronate SPE prior to HPLC-MS/MS analysis. (13)C(5)-uridine was added as internal standard to obtain better accuracy and precision of the analysis. The MS/MS detector was optimized at multiple reaction monitoring (MRM) using positive electrospray ionization to detect 245-->113 and 250-->133 transitions for ribavirin and internal standard, respectively. The calibration curve was linear over a concentration range of 0.01-10 microg/mL with a limit of quantitation of 0.01 microg/mL. Mean inter-assay accuracy and precision for RBV-MP and RBV-TP quality control samples at 0.03, 0.3 and 8 microg/mL were 5% and 10%, respectively. This method was successfully used for the in vitro determination of RBV-MP and RBV-TP in CEM(ss) cells cultured with RBV.

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

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

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

OBJECTIVES

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

DESIGN

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

Background

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

Methods

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

Results

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

Conclusion

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

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

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

Improved method to quantify intracellular zidovudine mono- and triphosphate in peripheral blood mononuclear cells by liquid chromatography-tandem mass spectrometry

The determination of intracellular triphosphate metabolites of nucleoside analogs used in anti-HIV therapy is very challenging. Despite the well-known sensitivity and selectivity of LC-MS/MS, the measurement of the triphosphate metabolite of zidovudine (AZT-TP) remains difficult because of the interferences induced by endogenous nucleotides triphosphates. We describe a new approach that allows improved determination of AZT-TP simultaneously with AZT-monophosphate (MP). This was obtained, first, by monitoring a transition from the molecular ion of AZT-TP to a minor but very specific product ion. Then, the spiking of samples with a constant amount of AZT-TP allowed the signal to emerge from background, leading to increased sensitivity. Finally, the analytical run time was reduced to less than 10 min. The low limits of quantification were at 150 and 300 fmol per sample for AZT-TP and AZT-MP, respectively. Recoveries were higher than 85%. Inaccuracy and precision were lower than 10% and 15% (17% at the limit of quantification), respectively. The new method offers the possibility of determining simultaneously other nucleotide phosphates, as shown here for d4T-TP (the triphosphate metabolite of another nucleoside analog, stavudine or d4T) and 2'-deoxythymidine-5'-triphosphate or dTTP (the corresponding natural nucleotide triphosphate).

LC-MS/MS method for simultaneous determination of viramidine and ribavirin levels in monkey red blood cells

A high performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) method has been developed for the simultaneous determinations of total viramidine (viramidine, viramidine monophosphate, viramidine diphosphate, and viramidine triphosphate) and total ribavirin (ribavirin, ribavirin monophosphate, ribavirin diphosphate, and ribavirin triphosphate) in monkey red blood cells (RBC). The method involves the addition of internal standards and perchloric acid, conversion of viramidine or ribavirin phosphorylated metabolites to viramidine or ribavirin, purification with an aminopropyl (NH(2)) solid phase extraction (SPE) cartridge, and LC-MS/MS analysis. The MS/MS is selected to monitor m/z 245-->113, 250-->113, 244-->112, and 249-->112 for ribavirin, [(13)C]ribavirin, viramidine, and [(13)C]viramidine, respectively, using positive electrospray ionization. The calibration curves are linear over a concentration range of 100-10,000 ng/mL (0.412-41.2 microM) with a lower limit of quantification (LLOQ) of 100 ng/mL for both compounds. Mean inter-assay recoveries for ribavirin are 101%, 98.9%, and 96.0%, with coefficient of variance (%CV) values between 1.95 and 4.50% for 100, 1000, and 10,000 ng/mL quality control (QC) samples, respectively. Mean inter-assay recoveries for viramidine are 96.3%, 101%, and 102%, with coefficient of variation (%CV) values between 3.61 and 7.22%, for 100, 1000, and 10,000 ng/mL QC samples, respectively. Over-curve dilution QC at 400 microg/mL (1639 microM) for both viramidine and ribavirin are used to ensure the dilution accuracy (25 X dilutions) for monkey samples. The method has been used to simultaneously determine the total concentrations of ribavirin and viramidine in monkey RBC following 5, 15, and 36 weeks dosing of viramidine or ribavirin (60 mg/kg). The concentrations of total ribavirin following ribavirin dosing are 1242 microM at week 5, 1257 microM at week 15, and 1146 microM at week 36. The concentrations of total ribavirin following viramidine dosing are 634 microM at week 5, 716 microM at week 15, and 683 microM at week 36. Only small amounts of viramidine are detected in RBC following viramidine dosing, 7.80 microM at week 5, 6.63 microM at week 15, and 10.4 microM at week 36. The results suggest that ribavirin levels in RBC were at steady state at week 5 of ribavirin or viramidine dosing. At steady state, ribavirin levels in RBC are approximately 2x after ribavirin dosing than viramidine dosing. The relatively small percentage of viramidine in RBC suggests that viramidine either poorly penetrated into RBC or was extensively converted to ribavirin following entry into RBC.

Hybrid polymer nanocapsules enhance in vitro delivery of azidothymidine-triphosphate to macrophages

One of the main limitations in the use of nucleoside reverse transcriptase inhibitors (NRTIs) such as azidothymidine (AZT) lies in their poor intracellular activation by cellular kinases into their active tri-phosphorylated form. Thus, the direct administration of triphosphate NRTIs like azidothymidine-triphosphate (AZT-TP), has been considered for bypassing this metabolic bottleneck, but these molecules do not diffuse intracellularly, due to their too hydrophilic character. Therefore, poly(iso-butylcyanoacrylate) (PIBCA) aqueous-cored nanocapsules have been tested as carriers to overcome the cellular delivery of AZT-TP. However, encapsulation of AZT-TP remained challenging because this molecule, due to its relatively low molecular weight, rapidly leaked out of the nanocapsules. In this study, we show that association of AZT-TP to a cationic polymer such as poly(ethyleneimine) (PEI) allowed to reach high entrapment efficiency of AZT-TP in PIBCA nanocapsules (up to 90%) as well as gradual in vitro release. The resulting hybrid PIBCA/PEI nanocapsules efficiently delivered AZT-TP in vitro to macrophages: the cellular uptake was increased by 30-fold compared to the free molecule, reaching relevant cellular concentrations for therapeutic purposes.

Lack of evidence for in vivo transformation of zidovudine triphosphate to stavudine triphosphate in human immunodeficiency virus-infected patients

The in vivo and in vitro determination of significant intracellular stavudine (d4T) triphosphate (d4TTP) concentrations in human immunodeficiency virus (HIV)-infected subjects and NS-1 cells treated with zidovudine (ZDV) has recently been reported. This study was conducted to corroborate these findings with in vivo samples from HIV-infected subjects taking ZDV and in vitro CEM(SS) cells incubated with different ZDV concentrations. Previously, we have reported on our validated high-performance liquid chromatography coupled with tandem mass spectrometry methodology for the simultaneous determination of d4TTP, lamivudine triphosphate, and ZDV triphosphate (ZDVTP) concentrations. Using this methodology, we monitored the d4TTP concentration in more than 100 samples from HIV-infected subjects treated with d4T. In addition, we simultaneously monitored the concentrations of d4TTP and ZDVTP in more than 500 samples from HIV-infected individuals who were taking ZDV. Finally, we performed in vitro studies by incubating CEM(SS) cells with 10 microM, 50 microM, and 100 microM ZDV and monitored the formation of d4TTP at 24 and 48 h. We could measure d4TTP concentrations from HIV-infected individuals with a limit of quantitation (LOQ) of 2.7 fmol/10(6) cells (total injection, 54 fmol). In the in vivo studies, we measured the d4TTP concentrations among patients receiving d4T treatment, but the samples from patients taking ZDV did not provide d4TTP concentrations above the LOQ. Furthermore, in vitro samples did not produce any signal for d4TTP, despite the detection of substantial ZDVTP concentrations in CEM(SS) cells. Thus, contrary to the previous report, we found no evidence for the in vivo or in vitro transformation of ZDVTP to d4TTP in HIV-infected subjects or CEM(SS) cells.

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.

Pharmacokinetic properties of nucleoside/nucleotide reverse transcriptase inhibitors

Options for antiretroviral therapy in patients infected with HIV continue to expand as new drugs are integrated into treatment regimens. Nonetheless, nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs/NtRTIs) remain the backbone of highly active antiretroviral therapy (HAART). With the approval of emtricitabine in 2003, there are now 8 Food and Drug Administration (FDA)-approved NRTIs/NtRTIs. Several of these agents are effective as once-daily therapy, including didanosine, lamivudine, extended-release stavudine (FDA approved, but not currently available), tenofovir DF, and emtricitabine. Recent results from pharmacokinetic and clinical trials indicate that another NRTI, abacavir, may also be effective as a once-daily therapy, and FDA approval of once-daily dosing is anticipated. NRTIs are inactive as administered, requiring anabolic phosphorylation within target cells to achieve their antiretroviral effects. All NRTIs are converted to nucleoside triphosphates, which serve as the active metabolites (the NtRTI, tenofovir DF, only requires conversion to the diphosphate form). Frequency of drug administration is closely related to the pharmacokinetic properties of a drug. The key parameter is the half-life; however, the plasma elimination half-life of the NRTIs/NtRTIs as administered is of little use in developing a dosing schedule. Rather, the intracellular half-life of the nucleoside triphosphate is the relevant parameter. This article reviews the pharmacokinetic properties, particularly those of the various phosphorylation steps, of the NRTIs/NtRTIs.

Simultaneous determination of HIV-protease inhibitors lamivudine and zidovudine in pharmaceutical formulations by micellar electrokinetic chromatography

A micellar electrokinetic chromatographic (MEKC) method for the simultaneous separation and determination of lamivudine (LMV) and zidovudine (ZDV) in pharmaceutical formulation has been developed. Factors that affect the separation, such as buffer pH, surfactant concentration (sodium dodecyl sulfate, SDS), organic solvents and applied voltage were optimized. Buffer consisting of 12.5 mM sodium tetraborate decahydrate and 15 mM boric acid adjusted at pH 10.8, containing 90 mM SDS and 5% (v/v) acetonitrile (ACN) was found to be suitable for the separation of the drugs. p-Aminobenzoic acid (PABA) was used as internal standard (I.S.). Detection of analytes and I.S. was performed at a wavelength of 210 nm. It was observed that both the drugs and I.S. were migrated within 20 min at the applied voltage of +10 kV. Validation of the method was performed in terms of linearity, accuracy, precision, limit of detection (LOD) and quantification (LOQ). An excellent linearity was obtained in the concentration range 10-80 microg/ml for LMV and 10-100 microg/ml for ZDV. The detection limits for LMV and ZDV were found to be 2.5 and 2.0 microg/ml, respectively. The optimized method was applied to the simultaneous determination of LMV and ZDV in pharmaceutical formulation and human plasma (spiked) samples. Recovery of both the drugs in tablet dosage form and spiked drugs in plasma were > or =99.72% (relative standard deviation (R.S.D.)< or =1.84%) and > or =80.4% (R.S.D.< or =5.4%), respectively. In the electropherogram no interfering peaks were observed in the region of analytes and I.S. due to inactive ingredients in the tablets and matrices in plasma.

Intracellular measurements of anti-HIV drugs indinavir, amprenavir, saquinavir, ritonavir, nelfinavir, lopinavir, atazanavir, efavirenz and nevirapine in peripheral blood mononuclear cells by liquid chromatography coupled to tandem mass spectrometry

A sensitive and accurate liquid chromatography-tandem mass spectrometric (LC-MS/MS) method for the intracellular determination of nine antiretroviral drugs in human peripheral blood mononuclear cells (PBMCs) is proposed. PBMCs are isolated by density gradient centrifugation using Vacutainer CPT tubes and cell count is performed with a Coulter instrument. Single-step extraction of drugs from PBMCs pellets is performed with MeOH 50% (with clozapine added as internal standard, I.S.) and the supernatant is injected onto a 2.1 mm x 30 mm SymmetryShield 3.5 microm-RP18 column equipped with a 2.1 x 10 mm guard column. Chromatographic separations are performed using a gradient program with a mixture of 2 mM ammonium acetate containing 0.1% formic acid and acetonitrile with 0.1% formic acid. Analytes quantification is performed by electro-spray ionisation-triple quadrupole mass spectrometry using the selected reaction monitoring (SRM) detection mode. The positive mode is used for the HIV protease inhibitors (PIs) indinavir, amprenavir, saquinavir, ritonavir, nelfinavir, lopinavir, atazanavir and the non-nucleoside reverse transcriptase inhibitors (NNRTIs) nevirapine, and the negative mode is applied for efavirenz. The calibration curves are prepared using blank PBMCs spiked with antiretroviral drugs at concentrations ranging from 0.5 to 100 ng/ml of cell extracts and fitted to a quadratic regression model weighted by 1/(concentration)(2). The lower limit of quantification is less than 0.5 ng/ml. The mean extraction recovery for all PIs/NNRTIs is always above 88%. The method is precise, with mean inter-day CV% within 0.6-10.2%, and accurate (range of inter-day deviation from nominal values -7.2 to +8.3%). This analytical method can be conveniently used in clinical research for the assessment of intracellular levels of all PIs/NNRTIs commercially available at present using a simple one-step cell extraction of PBMCs followed by liquid chromatography coupled with tandem triple quadripole mass detection.

K65R-associated virologic failure in HIV-infected patients receiving tenofovir-containing triple nucleoside/nucleotide reverse transcriptase inhibitor regimens

High rates of early virologic failure associated with the emergence of the K65R mutation in HIV-1 reverse transcriptase (RT) have been reported among HIV-infected patients who received novel, tenofovir-containing, triple-nucleoside/nucleotide reverse transcriptase inhibitor (NRTI/NtRTI) regimens as their initial therapy. This review surveys the findings of prospective and retrospective studies in this regard, examines the significance of the K65R mutation and other factors associated with reports of early virologic failure among patients receiving tenofovir-containing NRTI/NtRTI regimens, and discusses clinical approaches to preventing and managing HIV drug resistance and treatment failure associated with the K65R mutation.

Determination of ddATP levels in human immunodeficiency virus-infected patients treated with dideoxyinosine

Clinical failures of the highly active antiretroviral therapy could result from inefficient intracellular concentrations of antiviral drugs. The determination of drug contents in target cells of each patient would be useful in clinical investigations and trials. The purpose of this work was to quantify the intracellular concentration of ddATP, the active metabolite of dideoxyinosine (ddI), in peripheral blood mononuclear cells (PBMCs) of human immunodeficiency virus (HIV)-infected patients treated with ddI. We have raised antibodies against ddA-citrate, a stable isostere of ddATP selected on the basis of its structural and electronic analogies with ddATP. The anti-ddA-citrate antibodies recognized ddATP and ddA with nanomolar affinities and cross-reacted neither with any of the nucleotide reverse transcriptase inhibitors used in HIV therapy nor with their phosphorylated metabolites. The three phosphorylated metabolites of ddI (ddAMP, ddADP, and ddATP) were purified by anion exchange chromatography and the amount of each metabolite was determined by radioimmunoassay with or without prior phosphatase treatment. The intracellular levels of the three ddI metabolites were measured both in an in vitro model and in PBMCs of HIV-infected patients under ddI treatment. The possibility to measure intracellular levels of ddATP from small blood samples of HIV-infected patients treated with ddI could be exploited to develop individual therapeutic monitoring.

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

The cellular pharmacology of nucleoside- and nucleotide-analogue reverse-transcriptase inhibitors and its relationship to clinical toxicities

Nucleoside- and nucleotide-analogue reverse-transcriptase inhibitors (NRTIs) require intracellular phosphorylation for anti-human immunodeficiency virus (HIV) activity and toxicity. Long-term toxicities associated with NRTIs may be related to overactivation of this process. In vitro experiments have shown increased rates of NRTI and endogenous nucleoside phosphorylation to be associated with cellular activation. Patients with advanced HIV disease often have overexpression of cytokines, which corresponds to an elevated cellular activation state. These patients also have higher rates of NRTI phosphorylation and NRTI toxicity, suggesting an interaction between a proinflammatory biological state, NRTI phosphorylation, and toxicity. Studies suggest that women may have higher rates of NRTI phosphorylation than do men, as well as increased risk for NRTI-induced toxicity. Future research is needed to understand the NRTI activation process and improve the long-term toxicity profile of NRTIs. Such research should include comparisons of NRTI phosphorylation according to sex and cellular activation state (i.e., elevated vs. low).

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

OBJECTIVES

Nucleoside analog reverse transcriptase inhibitors (NRTI) are used in virtually all anti-HIV regimens. Clinical response depends on the intracellular formation of the pharmacologically active triphosphate moiety. Our objective was to quantify the pharmacological characteristics of zidovudine and lamivudine triphosphate in HIV-infected individuals.

METHODS

Peripheral blood mononuclear cells were obtained at multiple planned intervals from antiretroviral-naive adults participating in a study of zidovudine, lamivudine and indinavir, and triphosphate levels were determined by immunoassay and high-performance liquid chromatography/mass spectrometry. Plasma HIV-RNA, CD4 cell counts, and plasma drug concentrations were collected over 18 months. Data were analysed using non-parametric, regression and time-to-event methods.

RESULTS

Thirty-three subjects were evaluated. The estimated half-lives of zidovudine and lamivudine triphosphate were 7 and 22 h, respectively. Triphosphate concentrations were elevated in individuals with low baseline CD4 cell counts. Triphosphate concentrations in women were higher than in men by 2.3 and 1.6-fold for zidovudine and lamivudine, respectively. Women reached an HIV-RNA level under 50 copies/ml twice as fast as men. Zidovudine triphosphate above 30 fmol/10(6) cells was independently predictive of the time to under 50 copies/ml. Lamivudine triphosphate above 7017 fmol/10(6) cells was independently predictive of a longer virological response. Indinavir concentrations were related to antiviral responses in univariate analyses.

CONCLUSION

Zidovudine and lamivudine triphosphate concentration thresholds were independently associated with the antiviral activity of zidovudine, lamivudine, and indinavir. The significantly elevated triphosphate concentrations in women and individuals with low baseline CD4 cell counts, groups that historically experience high rates of serious NRTI toxicities, provide a hypothesis for the pathogenesis of these events.

Liquid chromatography-tandem mass spectrometric quantitative determination of the HIV protease inhibitor atazanavir (BMS-232632) in human peripheral blood mononuclear cells (PBMC): practical approaches to PBMC preparation and PBMC assay design for high-throughput analysis

A selective, accurate, and reproducible LC/MS/MS assay was developed and validated for the determination of the HIV protease inhibitor atazanavir (BMS-232632) in human peripheral blood mononuclear cells (PBMC) samples. In addition to the details of the validated LC/MS/MS method, a practical procedure is described in great detail for the preparation of large supplies of control (blank) PBMC from units of blood (each unit of blood is about 500 ml) for making the calibration standards and quality control (QC) samples. The PBMC assay design, intended for high-throughput sample analysis, is also described in some detail in regards to the composition and concentration expressions of the calibration standards and QC samples, the lysing procedure of the PBMC samples, and the final analysis/quantitation procedure. The method involved automated solid-phase extraction (SPE) of atazanavir and a stable isotope analog internal standard (I.S.) using 3M Empore C2-SD 96-well plates. A portion of the reconstituted sample residue was injected onto a YMC Basic analytical column which was connected to a triple quad mass spectrometer for analyte determination by positive-ion electrospray in the selected reaction monitoring (SRM) mode. The standard curve, which ranged from 5 to 2500 fmol per one million cells (fmol/10(6) cells), was fitted to a quadratic regression model weighted by 1/concentration. The lower limit of quantitation (LLOQ) was 5 fmol/10(6) cells. The inter- and intra-run coefficients of variation (CV) for the assay were <9% and the accuracy was 94-104%. Atazanavir was stable in PBMC for at least 24h at room temperature and for at least 129 days at -15 degrees C.

A strategy for liquid chromatography/tandem mass spectrometric assays of intracellular drugs: application to the validation of the triphosphorylated anabolite of antiretrovirals in peripheral blood mononuclear cells

The pharmacokinetics of intracellular drugs have recently aroused new interest because monitoring a drug's behaviour near the site of action can enhance knowledge of its efficacy and toxicity. Liquid chromatography coupled with tandem mass spectrometry (LC/MS/MS) is particularly attractive for intracellular analytes. Very few papers deal precisely with special features encountered in intracellular drug assay or with how closely the assay matches the actual recommendations. Particular problems are encountered mainly because the analytes are located intracellularly. This mainly concerns the handling of biological media, including provision of blank samples using Ficoll gradient separation, cell counts, optimisation of cell lysis, sample extraction, plotting standard curves using either fmol/10(6) cells or fmol/ml of extract or fmol/sample, the matrix effect as a function of the number of cells, stability before and during cell separation, as well as in storage conditions using clinical samples, biological matrix replacement and interference by endogenous compounds. This paper describes a strategy for the full validation and routine use of an LC/MS/MS assay applied to the simultaneous intracellular determination of the triphosphorylated anabolites of didanosine (2',3'-dideoxyadenosine triphosphate or ddA-TP) and stavudine (2',3'-didehydro-3'-deoxythymidine triphosphate or d4T-TP), two nucleoside reverse transcriptase inhibitors of HIV, in human peripheral blood mononuclear cells (PBMCs), as a guide for further LC/MS/MS assay of intracellular drugs.

Liquid chromatography-tandem mass spectrometry assays for intracellular deoxyribonucleotide triphosphate competitors of nucleoside antiretrovirals

This study was aimed to apply an LC-MS-MS method previously developed for intracellular nucleoside reverse transcriptase inhibitors-triphosphate (NRTI-TPs) to the determination of natural deoxyribonucleotides (dNTPs) in human peripheral blood mononuclear cells. The LC-MS-MS method was directly used in assay of dATP and dTTP. Interferences by ribonucleotides (rNTPs) prevented direct application to the two other analytes: dGTP and dCTP. A periodate oxidation procedure was therefore optimized to remove rNTPs from the cell medium in order to quantitate dCTP and dGTP. The determination of the intracellular ratio of NRTI-TP/dNTP in HIV-infected patients now involves use of the same chromatographic system for simultaneous assay of several analytes.

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.

A new sensitive cartridge-RIA method for determination of stavudine (D4T) triphosphate in human cells in vivo

We describe a simple and sensitive method to determine stavudine triphosphate, the active intracellular anabolite of stavudine (D4T). Quantification of D4T triphosphate was performed with a combined cartridge-radioimmunoassay (cartridge-RIA) which enabled us to measure concentrations of D4T triphosphate as low as 0.5 ng/ml, or an intracellular concentration which corresponds to 20 fmol/10(6) cells if diluted like our previously published zidovudine (ZDV) assay. The only alternate methodology at present employs liquid chromatography mass spectroscopy (LC-MS/MS). The use of the cartridge-RIA methodology provides a cost-effective alternative for the determination of in vivo cellular pharmacokinetics studies of D4T in human immunodeficiency virus (HIV)-infected persons.

Lamivudine can exert a modest antiviral effect against human immunodeficiency virus type 1 containing the M184V mutation

The M184V mutation in human immunodeficiency virus (HIV) reverse transcriptase is associated with high-level resistance to both (-)2',3'-dideoxy-3'-thiacytidine (3TC) and (-)2',3'-dideoxy-5-fluoro-3'-thiacytidine as well as low-level resistance to 2',3'-dideoxyinosine, 2',3'-dideoxycytidine, and abacavir. This mutation is also associated with diminished HIV replicative fitness as well as several functional changes in enzyme activity, including diminutions in polymerase processivity, pyrophosphorylysis, and nucleotide primer unblocking. Despite the fact that M184V encodes up to 1,000-fold resistance to 3TC, we asked whether this drug might still display some antiviral effect in regard to viruses containing this mutation. Cell-free assays revealed that high concentrations of 3TC triphosphate (i.e., >100 micro M) could affect chain termination and/or inhibit purified reverse transcriptase containing the M184V substitution. This effect became more pronounced with elongation of reverse transcriptase products. In newly infected cells (i.e., peripheral blood mononuclear cells), we found that the amount of full-length reverse transcriptase product was diminished in the presence of 2 to 10 micro M 3TC, although no decrease in the first product of the reverse transcriptase reaction, i.e., minus strong-stop DNA, was observed. In the presence of two other HIV inhibitors, e.g., nevirapine and indinavir, 3TC exerted additive effects in tissue culture at concentrations only marginally higher than the 50% inhibitory concentration (IC(50)). Reverse transcriptases cloned from clinical isolates harboring M184V in the context of multidrug resistance had similar IC(50) values for 3TC triphosphate compared to reverse transcriptase containing only the M184V mutation. These results suggest that viruses containing M184V can retain a higher degree of sensitivity to 3TC than previously assumed.

Cellular pharmacology of D-d4FC, a nucleoside analogue active against drug-resistant HIV

The backbone of effective highly active antiretroviral therapy regimens for the treatment of HIV infections currently contains at least two nucleosides. Among the features that influence the potency of each component of a regimen and the overall efficacy of the combination are the cellular uptake and bioconversion of nucleoside analogues to their active triphosphate form, and the extent of possible interactions in these steps that might occur when more than one nucleoside is used in a regimen. D-d4FC (Reverset), a new cytidine analogue with the ability to inhibit many nucleoside-resistant viral variants, was examined for these parameters. In phytohemaglutinin-stimulated human peripheral blood mononuclear cells, D-d4FC was taken up in a rapid (8 h to 50% maximal value), saturable (plateau above 10 microM parent nucleoside concentration) process, resulting in levels of D-d4FC triphosphate that should provide potent antiviral activity against a variety of virus genotypes. Based on measurement of antiviral effects in cell culture, additive and in some cases, synergistic interactions were observed with protease inhibitors, non-nucleoside reverse transcriptase inhibitors or other nucleosides, including cytidine analogues.

Enzymatic assay for measurement of intracellular DXG triphosphate concentrations in peripheral blood mononuclear cells from human immunodeficiency virus type 1-infected patients

DXG ([2R-cis]-2-amino-1,9-dihydro-9-[2-[hydroxymethyl]-1,3-dioxolan-4-yl]-6H-purin-6-one) and its prodrug DAPD ([2R-cis]-4-[2,6-diamino-9H-purin-9-yl]-1,3-dioxolane-2-methanol; amdoxovir) are novel 2',3'-dideoxynucleosides (ddNs) displaying activity against human immunodeficiency virus type 1 (HIV-1). In this paper, we describe the development of an enzymatic assay for determining the intracellular active metabolite of DXG and DAPD, DXG triphosphate (DXGTP), in peripheral blood mononuclear cells (PBMCs) from HIV-infected patients. The assay involves inhibition of HIV reverse transcriptase (RT), which normally incorporates radiolabeled deoxynucleoside triphosphates (dNTPs) into a synthetic template primer. DXGTP (0.6 pmol) inhibited control product formation with or without a preincubation step. Inhibition was greatest when the template primer was most diluted. DAPDTP inhibited control product formation only at very high levels (50 pmol) and when a preincubation procedure was used. However, reduced template primer stability in assays using preincubation steps, coupled with potential interference by DAPDTP, led to the current assay method for DXGTP being performed without preincubation. Standard DXGTP inhibition curves were constructed. The presence of PBMC extracts or endogenous dGTP did not interfere with the DXGTP assay. Intracellular DXGTP and dGTP concentrations were determined in PBMCs from HIV-infected patients receiving oral DAPD (500 mg b.i.d.). Peak concentrations of DXGTP were obtained 8 h after dosing and were measurable through 48 h postdose. Levels of endogenous dGTP were also determined over 48 h. No direct relationship was observed between concentrations of DXGTP and dGTP. Quantification of DXGTP concentrations in PBMCs from patients receiving a clinically relevant dose of DAPD is possible with this enzymatic assay.

Development of a pharmacodynamic model for HIV treatment with nucleoside reverse transcriptase and protease inhibitors

There is a need for models useful for predicting the efficacy of agents developed for treating human immunodeficiency virus (HIV) based on information obtained during the drug development process. A pharmacodynamic model that superimposes the pharmacokinetics of anti-HIV nucleoside reverse transcription (RT) and protease inhibitors over a previously published predator-prey model of HIV and CD4 dynamics was developed to address this need. This model was applied to in vitro measurements and patient-derived pharmacokinetics of the unbound antiviral drugs to simulate HIV-1 and CD4 counts versus time and dose. The primary mechanism for nucleoside RT inhibitors was assumed to be competitive inhibition of HIV-1-RT by the active nucleoside triphosphates (NTP). Cellular accumulation and breakdown rates of the NTP were estimated from previous in vivo pharmacokinetic studies. Median inhibition concentrations for the HIV-1 RT enzyme were estimated from previously published cell-free binding studies. The concentration of active protease inhibitor available for binding with HIV-1 protease was assumed equal to the unbound fraction in the plasma. The resulting simulations for mono- and dual nucleoside therapy with zidovudine and lamivudine single dose regimen with the protease inhibitor indinavir, produced similar HIV and CD4 response profiles to those reported in large Phase II and III clinical trials. Based on these findings this pharmacodynamic model can be applied to predict starting doses for a new agent based on simulated biological responses as a function of time for dosage regimens comprising one or two agents. However, the model overestimated the efficacy of highly effective drug combinations where all three agents are combined as in highly active anti-retroviral therapy.

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.