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JAIDS Journal of Acquired Immune Deficiency Syndromes:
Clinical Science

Intracellular Pharmacokinetics of Tenofovir Diphosphate, Carbovir Triphosphate, and Lamivudine Triphosphate in Patients Receiving Triple-Nucleoside Regimens

Hawkins, Trevor MD*; Veikley, Wenoah AC, RN*; St. Claire, Robert L III PhD†; Guyer, Bill PharmD‡; Clark, Nicole PhD†; Kearney, Brian P PharmD‡

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From the *Southwest CARE Center, Santa Fe, NM; †Gilead Sciences, Inc., Durham, NC; and ‡Gilead Sciences, Inc., Foster City, CA.

Received for publication December 20, 2004; accepted March 29, 2005.

Technical and financial support for this study was provided by Gilead Sciences, Inc.

Reprints: Trevor Hawkins, Southwest CARE Center, 649, Harkle Rd, Ste E, Santa Fe, NM 87505 (e-mail: thawkins@southwestcare.org).

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Abstract

Objective: To evaluate the potential for a pharmacologic mechanism to explain suboptimal virologic responses observed in a triple-nucleoside only regimen containing tenofovir disoproxil fumarate (TDF), abacavir (ABC), and lamivudine (3TC).

Methods: This was a prospective evaluation of intracellular concentrations and pharmacokinetics of tenofovir diphosphate (TFV-DP), carbovir triphosphate (CBV-TP), and lamivudine triphosphate (3TC-TP) in patients on triple-nucleoside regimens. Fifteen patients on a stable TDF plus ABC plus a third nucleoside reverse transcriptase (RT) inhibitor (3TC [n = 13], stavudine [n = 2]) regimen discontinued TDF or ABC, replacing it with a nonnucleoside RT inhibitor or protease inhibitor. Peripheral blood mononuclear cells were collected after the last dose of TDF or ABC at baseline and over 12 to 96 hours as well as at days 14 and 28 after discontinuation. Nucleotide concentrations were measured directly using liquid chromatography with tandem mass spectrometry; changes after ABC or TDF discontinuation would provide evidence of an intracellular drug interaction.

Results: Intracellular nucleotide concentrations of the continued drugs were unaffected when TDF or ABC was discontinued. Intracellular levels of TFV-DP exhibited less inter- and intrapatient variability than CBV-TP or 3TC-TP. TFV-DP also had persistent intracellular levels on TDF discontinuation (median half-life of 150 hours, range: 60 to >175 hours). CBV-TP concentrations fell to below the limit of detection in all patients by 72 hours after the last ABC dose in accordance with a median half-life of 18 hours (range: 12-19 hours).

Conclusions: An intracellular drug interaction does not explain the suboptimal viral response in patients treated with the nucleoside-only regimen of TDF, ABC, and 3TC.

The approval of a number of new once-daily nucleoside analogue reverse transcriptase inhibitors (NRTIs) and data supporting once-daily use of older agents in this class, including didanosine (ddI), abacavir (ABC), and lamivudine (3TC), prompted study of a number of triple-nucleoside only multiclass-sparing antiretroviral regimens. These regimens were thought to offer improved adherence by avoiding high pill burdens, short-term side effects, long-term toxicities, and drug-drug and drug-food interactions. Despite these theoretical advantages and the demonstrated potency of the individual components in short-term monotherapy and/or as part of multiclass highly active antiretroviral (HAART) regimens, a growing body of data indicates that triple-nucleoside antiretroviral regimens are associated with poor virologic response when used in treatment-naive patients.

Data from large government-sponsored (AIDS Clinical Trials Group [ACTG] A5095),1 industry-sponsored (ESS30009, Glaxo-SmithKline),2 and small, investigator-initiated clinical studies3-5 have demonstrated the inferiority of a number of triple-NRTI regimens relative to standard of care treatments that include 2 classes of agents. Of particular interest, the ESS30009 study investigated a once-daily regimen containing 300 mg of tenofovir disoproxil fumarate (TDF), 600 mg of ABC, and 300 mg of 3TC in comparison to once-daily efavirenz (EFV) administered at a dose of 600 mg with 600 mg of ABC plus 300 mg of 3TC. In both study arms, 3TC and ABC were dosed using recently approved once-daily dosing schedules. In an unplanned interim analysis, 49% of patients in the TDF/ABC/3TC arm experienced what was considered virologic failure versus only 5% in the EFV-containing arm, resulting in termination of the triple-nucleoside regimen of the study. Although the findings of ACTG A5095 had elucidated the limitations of a triple-nucleoside regimen composed of twice-daily zidovudine/3TC/ABC versus a regimen containing EFV, the high rate of virologic nonresponse and failure of the TDF/ABC/3TC regimen in the ESSS30009 study was unexpected, considering the potency of the individual agents. This finding led to questions regarding the possibility of antiviral antagonism and/or pharmacologic interaction between these agents.

No interaction has been observed between TDF, 3TC, or ABC in plasma.6-8 ABC or TDF in combination with 3TC and EFV has demonstrated high rates of virologic success in short- and long-term clinical safety and efficacy studies of 48 and 144 weeks in duration, respectively.9,10 No data are available regarding the cell-associated (intracellular) clinical pharmacokinetics of the active anabolites of nucleotides at their site of action during coadministration. We evaluated the intracellular pharmacokinetics of tenofovir diphosphate (TFV-DP), carbovir triphosphate (CBV-TP), and lamivudine triphosphate (3TC-TP) to ascertain the possibility of an intracellular drug-drug interaction that would explain the failure of triple-nucleoside regimens containing TDF, ABC, and 3TC.

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METHODS

Patient Population

Twenty-one HIV-infected patients at our clinic were receiving triple-nucleoside regimens that contained TDF and ABC with 3TC or stavudine (d4T). No patients had evidence of virologic failure on these regimens; however, because of the failure of triple-nucleoside regimens, all patients were encouraged to modify their treatment by replacing TDF or ABC with a non-NRTI or protease inhibitor. At the end of the last common dosing interval for all 3 medications and immediately before the regimen change, (baseline) blood samples were collected for HIV-1 RNA, CD4+ cell count, and isolation of peripheral blood mononuclear cells (PBMCs) for measurement of intracellular nucleotide analogue metabolites of TFV (TFV-DP), ABC (CBV-TP), and 3TC (3TC-TP). PBMC sampling was also conducted at 4 visits up to 96 hours and then 14 and 28 days after discontinuation of TDF or ABC. At each visit, adverse events were monitored and laboratory tests as well as HIV-1 RNA and CD4 measurements were obtained, including weeks 4 and 12 after the regimen change.

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Methods
Sample Collection and Processing

For each pharmacokinetic sample collection time point, PBMCs were immediately isolated from 16-mL of whole blood using cell preparation tubes (CPT) tubes (Becton-Dickinson) and processed per the manufacturer's instructions. The resultant PBMC-containing plasma was transferred to a 45-mL conical tube to which 30-mL of normal saline was added. A 0.5-mL aliquot of cells was then immediately sent to a reference laboratory (Tricore Laboratories, University of New Mexico) for automated cell counting within 60 minutes of collection. The conical tube containing PBMCs was then centrifuged for 15 minutes at 400× relative centrifugal force (RCF) to yield a cell pellet to which 1.0-mL of 70:30 methanol/H2O solution was added. Samples were stored at or below −20°C until shipment to Gilead Sciences (Durham, NC) on dry ice for analysis.

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Bioanalytic and Pharmacokinetic Analyses

Intracellular concentrations of the active nucleotides TFV-DP, CBV-TP, and 3TC-TP were directly measured in 1 to 10 × 106 PBMCs using liquid chromatography with tandem mass spectrometry (LC/MS/MS). These analytic procedures are based on the original nucleotide analysis technology previously reported.11 Intracellular measurements of d4T-TP were not performed because of method unavailability. On each day of analysis, blank PBMC extracts and study samples were thawed, spiked with appropriate calibration and/or internal standard solutions (adefovir diphosphate or emtricitabine triphosphate), and subjected to ion-paring solid phase extraction. After elution, the samples were evaporated to dryness and reconstituted in a 5-mM ammonium phosphate/2-mM tetrabutylammonium hydroxide solution. Samples were filtered and transferred to injection vials for analysis. Chromatographic separations were performed by an ion-pairing step gradient method using a MS C18 analytic column (Waters Xterra MS C18 analytical; Waters Corporation, Millford, MA) and tandem mass spectrometry (MS/MS) detection (Micromass Quattro-LC; Waters Corporation) using positive ion electrospray ionization. A standard curve was generated at the beginning of each sample run, and a series of low- and high-concentration quality control samples were placed at the midpoint and end of the run. The lower limits of quantitation (LLOQ) for TFV-DP, 3TC-TP, and CBV-TP were 15, 24, and 76 fmol on-column, respectively. Calibration curves were obtained using a quadratic regression algorithm of the peak area ratio of the active nucleotide of interest to internal standard. A 1/concentration2 weighting factor was used to fit the curve to the standard points. All correlation coefficients (r) exceeded 0.999. Within-run accuracy (% error) values ranged from 1.3% to 14.5%, and precision (% RSD) values ranged from 0.01% to 10.9% for the 3 compounds. Between-run accuracy (% error) values ranged from −0.7% to 13.9%, and precision (% RSD) values ranged from 2.45% to 7.85% for the 3 compounds. All calculated values for quality control samples fell within industry guidelines of ±20% for the LLOQ and ±15% for the upper limit of quantitation.

Changes in the intracellular levels of TFV-DP, CBV-TP, and/or 3TC-TP after discontinuation of ABC or TDF were evaluated to provide evidence for diminution of cellular phosphorylation and/or efflux of the continuing nucleotides, thereby indicating a drug-drug interaction. Intracellular concentrations of active nucleotides were summarized using descriptive statistics and, where possible, exploratory parametric and nonparametric statistical tests. Additionally, the amounts of inter- and intrapatient variability in intracellular concentrations of active nucleotide triphosphates observed were compared using descriptive statistics. Finally, intracellular decay rates of the withdrawn drug were calculated (WinNonlin; Pharsight Corporation, Mountain View, CA; GraphPad Software, San Diego, CA) to assess the degree of pharmacokinetic persistence and intracellular half-life of each nucleotide analogue anabolite.

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RESULTS

Patient Demographics and Clinical Follow-Up

Written informed consent was obtained from all patients after the study was approved by the Institutional Review Board at St. Vincent's Hospital in Santa Fe, New Mexico. Fifteen of 21 potential patients were enrolled; the other 6 patients switched regimens before enrollment in this study began. Nine male and 6 female patients were enrolled. Eight patients were Hispanic, 6 were white, and 1 was Native American; patient age ranged from 31 to 65 years (mean = 45 years); and patient weight ranged from 44 to 114 kg (mean = 69 kg) at baseline. All patients were receiving a triple-nucleoside regimen that contained TDF and ABC and 3TC (n = 13) or d4T (n = 2). One treatment-naive patient receiving this regimen as initial therapy was experiencing favorable viral load suppression and was approaching an undetectable HIV-1 viral load (270 copies/mL) at the time of study, a reduction of >2.3 log10 over 32 weeks. The other patients had received this regimen for an average period of 15 months with no viral load rebound or reductions in CD4+ cell counts, which was suggestive of treatment failure. Fourteen of 15 patients' viral load was <400 copies/mL (<50 copies/mL in 12 of 14 patients), and 1 patient with a baseline HIV-1 RNA level of 1856 copies/mL had an HIV genotype of reverse transcriptase (RT) and protease that showed no evidence of resistance mutations.

Overall, 7 patients discontinued ABC and 8 patients stopped TDF. The choice of drug discontinuation was decided by the patient and the investigator based on treatment history and future therapeutic options. Of the patients discontinuing ABC, 4, 2, and 1 began atazanavir/ritonavir (ATV/r) at a dose of 300/100 mg, nevirapine (NVP), and d4T, respectively. Of the patients discontinuing TDF, 3 started EFV, 3 began ATV (2 ATV/r), and 2 initiated NVP.

At 4 weeks after the regimen change, all patients achieved and/or maintained an HIV-1 RNA load of <400 copies/mL (<50 copies/mL in 14 of 15 patients) and the mean CD4 count (n = 15) was 522 cells/mm3. One patient developed hepatotoxicity on an NVP-based regimen, requiring discontinuation; he subsequently started an EFV-based regimen that was well tolerated. One patient became jaundiced, with a direct bilirubin level of 6.8 mg/dL, while continuing TDF and beginning an ATV/r regimen; she elected to stay on the regimen, and the bilirubin level fell to 3.4 mg/dL by week 12. There were no other significant clinical adverse events or laboratory abnormalities through 12 weeks after the regimen change.

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Intracellular Pharmacokinetics

After the final oral dose of the triple-nucleoside/nucleotide regimen, baseline trough concentrations of TFV-DP obtained in patients at 12 hours (n = 8) or 24 hours (n = 7) after dosing were similar (84.0 and 87.2 fmol/106 cells, median values; P = 0.69, Mann-Whitney U test). Baseline 12- and 24-hour trough levels for CBV-TP were 42.0 to 691 (n = 12) and 57.4 to 158 (n = 2) fmol/106 cells, respectively. Baseline 3TC-TP 12- and 24-hour trough concentrations were 12,600 (n = 7) and 8680 (n = 6) fmol/106 cells, respectively (P = 0.181, Mann-Whitney U test). Baseline concentrations of CBV-TP and 3TC-TP were similar or higher than those reported in the literature, indicating no significant deficiency in intracellular levels of these agents during their coadministration.12,13

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Intracellular Nucleotide Concentrations of Continued Drugs

In patients continuing TDF or ABC, median TFV-DP and CBV-TP concentrations at baseline were 89.9 (n = 7) and 157 (n = 8) fmol/106 cells, respectively. Intracellular concentrations of TFV-DP and CBV-TP did not demonstrate evidence of substantial changes over time after discontinuation of the other drug (Figs. 1, 2). Median TFV-DP and CBV-TP concentrations at the final study visit 1 month after the regimen change were 89.6 (n = 7) and 120 (n = 8) fmol/106 cells, respectively. The range of variability observed in absolute TFV-DP and CBV-TP concentrations between patients was 15.5-fold and 43.5-fold respectively, whereas median concentrations across visits exhibited 1.29-fold and 2.27-fold interpatient variability, respectively (Table 1). TFV-DP and CBV-TP exhibited median 1.80-fold and 4.87-fold intrapatient fluctuation across visits, respectively (see Table 1).

Table 1
Table 1
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Figure 1
Figure 1
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Figure 2
Figure 2
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3TC-TP concentrations were not significantly different based on the drug discontinued (P = 0.62) or pharmacokinetic visit (P = 0.14), with similar values at baseline or 1 month after the regimen change (9380 and 8760 fmol/106 cells [n = 13]) by 2-way ANOVA. Significant interpatient variability was observed acrossvisits (P < 0.0001), with absolute and median 3TC-TP concentration variabilities of 19.8-fold and 1.64-fold, respectively. Within an individual patient, 3TC-TP levels exhibited a median 2.15-fold (1.25-fold to 11.2-fold, minimum-maximum values) intrapatient fluctuation across visits (see Table 1).

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Intracellular Decay After Drug Discontinuation

Figures 3 and 4 present the concentration-time profiles of TFV-DP (n = 8) and CBV-TP (n = 7) on discontinuation of TDF and ABC, respectively. TFV-DP concentrations exhibited a virtually flat concentration-time profile over 60 to 72 hours after TDF discontinuation with slow intracellular elimination in all patients with quantifiable concentrations in PBMCs in 6 of 8 and 2 of 8 patients 14 and 28 days after the last dose of TDF, respectively (see Fig. 3). Estimates of intracellular decay rates suggest a median intracellular half-life of TFV-DP of approximately 150 hours (range: 60 to >175 hours).

Figure 3
Figure 3
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Figure 4
Figure 4
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In patients discontinuing ABC, median CBV-TP concentrations decreased approximately 50% over the first 12 hours after dosing, falling to below detectable concentrations in 1 patient by 48 hours and in all 7 patients by 72 hours after dosing (see Fig. 4). Analyses of individual patient and the aggregate CBV-TP decay curves yielded a median intracellular half-life of 18 hours (range: 12-19 hours).

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DISCUSSION

Despite the availability of a number of potent agents within this class, clinical studies have demonstrated poor virologic outcomes with triple-nucleoside regimens in treatment-naive patients, particularly compared with those demonstrated with regimens combining a 2-drug nucleoside backbone with a nonnucleoside reverse transcriptase inhibitor (NNRTI) or protease inhibitor. The reason for the poor performance of triple-nucleoside regimens containing once-daily TDF/3TC/ABC may stem from limitations in antiviral potency and/or common pathways to resistance. Little is understood regarding the intracellular pharmacokinetics of and interplay between nucleoside/nucleotide analogues at their site of action. The primary goal of this study was to evaluate in HIV-infected patients the possibility of a substantial intracellular pharmacologic interaction that may have led to insufficient levels of active nucleotides when TDF/3TC/ABC was coadministered.

The patients in our study were successfully switched from a triple-nucleoside regimen with few and easily addressable issues of tolerability and mild to moderate adverse event/laboratory abnormalities. During coadministration of this triple-nucleoside/nucleotide analogue regimen, concentrations of CBV-TP and 3TC-TP were marginally higher than those reported in the literature.12,13 Intracellular concentrations of TFV-DP in this study are the first reported in HIV-infected patients. Higher levels in this study likely reflect better recovery of intracellular drug because of automated cell counting and direct LC/MS/MS quantitation of active nucleotides without extensive multiple or processing steps used in earlier methods (ie, separation of 3TC-monophosphate, -DP, or -TP and/or dephosphorylation before analysis).

Furthermore, no substantial change in the intracellular level of any active nucleotide analogue was observed after the withdrawal of TDF or ABC. These findings concur with data from experiments demonstrating no interaction between these agents with respect to the degree of intracellular phosphorylation at suprapharmacologic concentrations and their demonstrated additive to synergistic antiviral activity in vitro.14,15 Together with in vitro and clinical pharmacokinetic data from a previous study that demonstrated no systemic pharmacokinetic drug-drug interaction,7 the data from this study strongly indicate the absence of an antagonistic pharmacokinetic/pharmacodynamic interaction between these agents. Although this study was conducted in patients who were virologically well controlled, similar conclusions have been reached by the investigators from the TONUS study, who, on exploring intracellular data from patients failing this regimen, concluded that a drug-drug interaction did not seem to explain virologic failure which is most likely due to a combination of host factors and drug PK/PD.16

Kakuda et al17 have speculated that in the absence of a direct drug-drug interaction between these agents, TDF, alone or possibly in combination with ABC, may lead to increases in endogenous nucleoside/nucleotide concentrations that would attenuate antiviral potency and/or result in cell toxicity. This hypothesis is not supported by in vitro studies, which show that TDF and ABC at concentrations many times higher than those observed in vivo do not alter the intracellular levels of the natural purines deoxyadenosine triphosphate (dATP) and deoxyguanosine triphosphate (dGTP)15 and that TDF is not cytotoxic to T cells at concentrations up to 1 mM.18,19 It therefore seems that the failure of this regimen likely stems from the high degree of selective pressure exerted by all 3 drugs on the nucleotide binding and/or incorporation sites in HIV RT, causing the early selection of the M184V mutant, followed by the K65R mutation in some patients. Alternatively, the overall potency of this once-daily triple NRTI regimen may simply be inadequate to control HIV-1 replication enough to prevent resistance development. Preliminary data regarding the use of ABC/3TC/TDF with zidovudine suggest that there may be a role for a thymidine-containing analogue quadruple-nucleoside only regimen.20,21 The basis of this hypothesis is that the use of agents with nonoverlapping resistance profiles forces the virus to develop mutations that allow for excision and binding/incorporation discrimination, which are pathways that do not seem to favorably coexist.

This clinical study confirmed that in addition to differences in plasma pharmacokinetics, there are significant differences in the intracellular kinetics of nucleoside/nucleotide analogues in HIV-infected patients that can be studied in a clinical setting. We observed that intracellular TFV-DP concentrations exhibit a low degree of fluctuation over the dosing interval and less inter- and intrapatient variability relative to CBV-TP and 3TC-TP.12,13 TFV-DP and CBV-TP had a long intracellular half-life relative to their parent drugs in plasma (17.0 and 1.0-2.0 hours, respectively)22,23; however, TFV-DP exhibited sustained levels within PBMCs, whereas CBV-TP concentrations sharply declined over its dosing interval. These findings for CBV-TP are similar to those observed in patients by Harris et al24 (≥12 hours) but somewhat shorter than those reported by Piliero et al12 (20.6 hours), where sampling was limited to 24 hours after dosing. Because of its long intracellular half-life, questions have arisen regarding the risk for the development of resistance during periods of TFV-DP monotherapy when patients stop regimens of medications with shorter half-lives (eg, TDF plus 3TC plus a boosted protease inhibitor). Although efforts to avoid monotherapy are advisable, data from 2 viral dynamic/efficacy studies in which TDF monotherapy was studied over 21 and 28 days, respectively, showed that no patients developed the hallmark K65R mutation in RT.25,26

The differences in the plasma pharmacokinetics of nucleoside/nucleotide analogues are well established, and the availability of better research tools and instrumentation now allows for in-depth study of the interplay of plasma and intracellular pharmacokinetics on tissue distribution and suppression of viral replication. For example, examinations should be conducted to learn if there are benefits to using regimens with sustained intracellular drug exposure to maintain viral suppression during inevitable times of suboptimal adherence and/or treatment interruptions. This question may be of particular importance in treatment-naive patients, who are frequently treated with NNRTIs that have a long systemic half-life and can rapidly select for resistance in settings of monotherapy.

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ACKNOWLEDGMENTS

The authors thank Jaymin Shah, Michael Miller, Adrian Ray, and Arnold Fridland for their input and collaboration on this study and manuscript.

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REFERENCES

1. Gulick RM, Ribaudo HJ, Shikuma CM, et al. Triple-nucleoside regimens versus efavirenz-containing regimens for the initial treatment of HIV-1 infection. N Engl J Med. 2004;350:1850-1861.

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13. Yuen GJ, Lou Y, Bumgarner NF, et al. 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. Antimicrob Agents Chemother. 2004;48:176-182.

14. Mulato AS, Cherrington JM. Anti-HIV activity of adefovir (PMEA) and PMPA in combination with antiretroviral compounds: in vitro analyses. Antiviral Res. 1997;36:91-97.

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16. Landman R, Peytavin G, Descamps D, et al. Low genetic barrier to resistance is a possible cause of early virologic failures in once daily regimen of abacavir, lamivudine and tenofovir-Tonus study [abstract 52]. Presented at: 11th Conference on Retroviruses and Opportunistic Infections; 2004; San Francisco.

17. Kakuda TN, Anderson PL, Becker SL. CD4 cell decline with didanosine and tenofovir and failure of triple nucleoside/nucleotide regimens may be related. AIDS. 2004;18:2442-2444.

18. Fridland A, Ray A, Olson L, et al. Intracellular activation of tenofovir and abacavir in human PBMC and CEM cells [abstract 2.3]. Presented at: Fifth International Workshop on Clinical Pharmacology of HIV Therapy; 2004; Rome.

19. Robbins BL, Srinivas RV, Kim C, et al. Anti-human immunodeficiency virus activity and cellular metabolism of a potential prodrug of the acyclic nucleoside phosphonate 9-R-(2-phosphonomethoxypropyl)adenine (PMPA), Bis(isopropyloxymethylcarbonyl)PMPA. Antimicrob Agents Chemother. 1998;42:612-617.

20. Moyle G, Nelson M, Higgs C, et al. A randomised open label comparative study of Combivir + efavirenz (2 class triple therapy) versus Trizivir + tenofovir (single class quadruple therapy) in initial therapy for HIV-1 infection [abstract H-1131]. Presented at: 44th Annual Interscience Conference on Antimicrobial Agents and Chemotherapy; 2004; Washington, DC.

21. DeJesus E, Elion R, Cohen C, et al. Week-24 analysis of once-daily (QD) Trizivir (TZV) and tenofovir DF (TDF) in antiretroviral-naive subjects (COL40263) [abstract H-564]. Presented at: 44th Annual Interscience Conference on Antimicrobial Agents and Chemotherapy; 2004; Washington, DC.

22. Kearney BP, Flaherty JF, Shah J. Tenofovir disoproxil fumarate: clinical pharmacology and pharmacokinetics. Clin Pharmacokinet. 2004;43:595-612.

23. Wang LH, Chittick GE, McDowell JA. Single-dose pharmacokinetics and safety of abacavir (1592U89), zidovudine, and lamivudine administered alone and in combination in adults with human immunodeficiency virus infection. Antimicrob Agents Chemother. 1999;43:1708-1715.

24. Harris M, Back D, Kewn S, et al. Intracellular carbovir triphosphate levels in patients taking abacavir once a day. AIDS. 2002;16:1196-1197.

25. Barditch-Crovo P, Deeks SG, Collier A, et al. Phase I/II trial of the pharmacokinetics, safety, and antiretroviral activity of tenofovir disoproxil fumarate in human immunodeficiency virus-infected adults. Antimicrob Agents Chemother. 2001;45:2733-2739.

26. Louie M, Hogan C, Hurley A, et al. Determining the antiviral activity of tenofovir disoproxil fumarate in chronically HIV-1 infected individuals. AIDS. 2003;17:1151-1156.

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Keywords:

intracellular; pharmacokinetics; tenofovir/abacavir

© 2005 Lippincott Williams & Wilkins, Inc.

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