Introduction
Nucleoside analogue inhibitors of the reverse transcriptase (RT) enzyme of HIV-1 were the first class of compounds to be discovered and used for anti-HIV-1 therapy and currently represent an essential component of highly active antiretroviral therapy (HAART). Despite the effectiveness of these compounds when used in combination regimens with a protease inhibitor (PI) or a non-nucleoside reverse transcriptase inhibitor (NNRTI), long term or suboptimal use of these drugs can result in the selection of viral resistance [1].
Additionally, in recent years the proportion of individuals who have become infected with resistant strains of HIV-1 has increased [2-4]. Resistance to antiretroviral agents is often associated with a variable level of cross-resistance to drugs from the same class [5-8]. Thus, the identification of new anti-HIV-1 agents with activity against drug-resistant strains of HIV-1 is important for the future treatment of patients.
Amdoxovir (DAPD; (-)-β-D-2, 6-diaminopurine dioxolane), a nucleoside reverse transcriptase inhibitor (NRTI) is deaminated in vivo by adenosine deaminase to (-)-(-D-dioxolane guanine (DXG), which is then bioconverted to DXG-TP which is the active metabolite. DXG has potent antiviral activity in vitro against clinical isolates of HIV-1 resistant to currently available NRTI, NNRTI and PI [less than four-fold change in 50% effective concentration (EC50)]. Median EC50 values determined for DAPD and DXG against a HIV-1 laboratory isolate in peripheral blood mononuclear cells are 4.0 ± 2.2 and 0.25 ± 0.17 μmol/l, respectively. In particular, DAPD has activity in vitro against clinical isolates containing mutations at positions 41, 67, 70, 184, 215 and 219 of the reverse transcriptase that are resistant to zidovudine/lamivudine and/or stavudine/lamivudine [9].
In preclinical studies, resistance to DXG was only observed in recombinant variants containing the 65R/151M double mutations [10,11]. Although the K65R mutation can be selected by abacavir, didanosine, tenofovir and zalcitabine in vitro or in vivo, its prevalence in treatment-experienced patients is very low (< 2%) [12,13].
The aim of this phase 1 study in HIV-1 infected individuals was to establish the antiviral activity of amdoxovir and preliminarily assess a dose and a schedule of administration for amdoxovir.
It has been difficult to establish the pharmacodynamic characteristics of antiretroviral agents without studying their effects as single agents in HIV-infected patients. Depending on the drug, the duration of administration and the population, selection of viral populations with reduced sensitivity is a risk of monotherapy. Understanding viral kinetics through clinical experience with other antiretroviral agents allowed us to design a trial that would limit the risk of selecting resistant virus as the dominant quasispecies, while retaining the ability to define preliminary pharmacodynamics, safety and tolerability [14,15]. A 2-week treatment period with amdoxovir monotherapy was designed, based on the following rationale. First, the kinetics of HIV decline are rapid upon initiation of antiviral exposure, with a sharp reduction in viral load and peak effects within 1 to 2 weeks [14-17]. HIV replication is rapid, allowing time for multiple cycles of replication within 14 days. Second, in vitro selection of HIV with mutations associated with resistance to DXG occurs slowly and only with prolonged exposure to increasing concentrations of drug during passage of HIV in tissue culture. Bazmi et al [18] required 13 to 14 passages of HIV in culture (2 months) before either a L74V or K65R mutation was observed. Data from Borroto-Esoda et al [10] have shown a similar pattern with the appearance of a L74V mutation appearing between 45 and 70 days of repeated passage of HIV-1 in the presence of DXG. The L74V mutation confers low level resistance to DXG [approximately four-fold increase in 50% inhibitory concentration (IC50)] [10]. Based upon these considerations, it was determined that this 2-week monotherapy period to evaluate multiple doses of amdoxovir would be a safe and rigorous design for pharmacodynamic evaluation of amdoxovir in HIV-infected patients.
Methods
Study design
DAPD-101 investigated the antiviral activity of multiple escalating doses of amdoxovir in treatment-naive and treatment-experienced HIV-1-infected patients with viremia. Six doses of amdoxovir were evaluated (25, 100, 200, 300 and 500 mg twice daily and 600 mg once daily) as monotherapy and two doses, 300 and 500 mg twice daily, as add-on therapy to patients' existing antiretroviral regimens. The study drug was administered in an open-label fashion for 14 days, after which individuals were followed for 7 days.
The study was conducted in three parts. The first part was a dose escalation evaluation in HIV-infected naive patients at the following doses: 25, 100, 200, 300, 500 mg twice daily and 600 mg once daily. In the second part, the doses of amdoxovir that had produced an antiviral effect and acceptable safety and tolerance in treatment-naive individuals (200, 300 and 500 mg twice daily) were administered to six to eight treatment-experienced patients who had discontinued all antiretroviral treatment for a minimum of 7 days prior to study entry. In part three of the study, treatment-experienced patients added DAPD to their existing antiretroviral regimen at one of two doses: 300 or 500 mg twice daily amdoxovir.
A central drug allocation scheme was utilized to assign patients to treatment cohorts.
Study participants
An Institutional Review Board reviewed the study protocol and consent form for each study center and all patients provided written informed consent. Ninety HIV-1-infected adults were enrolled into the study at 13 centers in the USA (n = 11) and France (n = 2). Eligible patients had plasma HIV-1 RNA levels between 5000 and 250 000 copies/ml and were either naive to antiretroviral therapy or on stable antiretroviral therapy for at least 6 months prior to randomization. Treatment-experienced patients had previously received a combination treatment regimen including zidovudine/lamivudine or stavudine/lamivudine for at least 6 months and had been considered virologic failures while on therapy as defined as viral load of > 5000 copies/ml (Roche Amplicor assay; Roche Diagnostics, Branchburg, New Jersey, USA) or > 2000 copies/ml (Chiron branched DNA assay; Chiron, Emeryville, California, USA) on two consecutive measurements. Treatment-experienced patients in part two of the study had to discontinue antiretroviral therapy at least 7 days prior to study entry. For the add-on cohorts, treatment-experienced patients were required to be on their current regimens, excluding any experimental agents, for at least 30 days before entry into the study without having to be considered virologic failures on zidovudine/lamivudine or stavudine/lamivudine at the time of entry into the study. Treatment-experienced patients who harbored the K65R, L74V and/or T69S-SS insert mutations were excluded. Treatment regimens could include PIs and NNRTIs in addition to one or more nucleoside analogues.
Amdoxovir was supplied by Gilead Sciences Inc. (Durham, North Carolina, USA), as capsules containing 25 or 100 mg of the active ingredient.
Assessments
Unless otherwise specified, all referenced laboratory values were obtained from samples sent to a central laboratory.
Plasma HIV-1 RNA levels were assessed at all time points using the standard Amplicor HIV-1 Monitor Test [Roche Amplicor assay; limit of quantification (LOQ) of 400 copies/ml]. Plasma HIV-1 RNA levels were measured at baseline (prior to the first dose of the study medication), at days 1, 2, 3, 4, 5, 8, 10 and 12, and pre-dose and 12, 24, 48 h after the last dose on day 15 or premature termination. Plasma HIV-1 RNA measurement was performed by Gilead Sciences, Inc.
Safety and tolerability were evaluated by assessing treatment-emergent adverse events and clinical laboratory values at baseline, days 8, 15 and 21.
Plasma for genotypic analysis of the HIV-1 reverse transcriptase and protease genes was obtained at each visit up to completion of the study. Genotypic analysis was performed at baseline and at day 15 with Affymetrix Genechip (Affymetrix, Inc., Santa Clara, California, USA) or with ABI 377 (Applied Biosystems, Inc., Foster City, California, USA) by Gilead Sciences, Inc.
Plasma concentrations of DAPD/DXG were determined using a validated liquid chromatography (LC)/mass spectrometry (MS)/MS method and urine concentrations of DAPD/DXG were determined using a validated high-performance liquid chromatography (HPLC)/ultraviolet (UV) method developed by Gilead Sciences, Inc.
Statistical considerations
Antiviral activity was compared between treatment arms using log10 HIV-1 RNA based on average area under the curve minus baseline (AAUCMB) to day 15. The AAUCMB was calculated as the area of the trapezoid under the HIV-1 RNA versus time curve, divided by the time to the last available evaluation minus the baseline HIV-1 RNA value, as follows:
Equation (Uncited)Image Tools
where Cj,i is the ith patient measurement at time tj, and tj is the actual time of the jth visit.
Patients with at least one post-baseline evaluation were included in this analysis. For patients whose plasma HIV-1 RNA levels decreased below the limit of quantification, 400 copies/ml was used as the plasma HIV-1 RNA value.
No formal sample size calculations were made for this study.
Data were analyzed for the intent-to-treat (ITT) population. ITT was the primary population for all analyses of efficacy and baseline characteristics, which included data from all patients who were randomized into the study and received at least one dose of study medication with no data exclusion.
Results
Patient population
The study period began in January 1999 (first individual enrolled) and ended in October 2001 (final individual observation). Ninety HIV-1 infected adults were enrolled into the study. A total of six patients prematurely discontinued the study due to adverse events (n = 2), investigator and/or patient decision to withdraw (n = 3) and lost to follow up (n = 1). Of the 90 patients, 41 were treatment-naive and 49 were treatment-experienced. All treatment-naive patients as well as twenty treatment-experienced patients received amdoxovir as monotherapy, amdoxovir was added to the patient's current antiviral regimen in 29 treatment-experienced patients.
Overall, the majority of patients enrolled were male (86%). The racial make-up of the population was predominantly Caucasian (42%) and African-American origin (39%). Patient ages ranged from 19 to 57 years, with a mean of 38.6 years.
The demographic/baseline characteristics of the treatment-naive and treatment-experienced patient populations are summarized in Table 1.
Overall, the median CD4+ cell count at screening was 362 × 106 cells/l (range: 49-1578 × 106 cells/l) and the median plasma HIV-1 RNA at baseline was 4.53 log10 copies/ml (range: 3.02-5.79 log10 copies/ml).
Follow up and discontinuation of treatment
Two individuals discontinued due to adverse events (psychosis n = 1, pneumonia n = 1) not thought to be related to treatment. One individual was lost to follow up and three others withdrew consent.
Antiviral activity
All dose cohorts showed a reduction in plasma HIV-1 RNA at the end of dosing (day 15) from baseline levels (Fig. 1).
In treatment-naive patients, the median reductions in plasma HIV-1 RNA levels were greatest for patients who received the highest amdoxovir doses 300 mg (1.5 log10) and 500 mg (1.3 log10) twice daily, compared with the lowest doses 25 mg (0.42 log10), 100 mg (0.69 log10) and 200 mg (1.14 log10) twice daily. The results of the AAUCMB analysis from baseline to day 15 are presented in Fig. 2. There was a trend for a statistically significant difference between the groups with respect to the AAUCMB to day 15 (P = 0.0075). In treatment-experienced patients, a reduction in viral load was observed at all doses tested (Fig. 1).
No dose-related trend in antiviral activity was identified in treatment-experienced patients receiving amdoxovir monotherapy, although the 500 mg twice daily amdoxovir dose tended to have the greatest activity at day 15 (0.78 log10). In the add-on cohorts, the median decrease from baseline at day 15 was 0.31 and 0.65 log10 for the 300 and 500 mg twice daily doses, respectively; this difference was not statistically significant. The analysis of AAUCMB showed comparable activity at both amdoxovir doses in the add-on cohorts (-0.36 and -0.45 log10) at day 15. Conversely, the proportion of individuals who had a 0.5 log10 decrease or greater from baseline was higher in the 500 mg twice daily cohort (67%) than in the 300 mg twice daily (36%); however this difference did not reach statistical significance. Among all treatment-experienced patients combining the monotherapy and add-on cohorts, 38% of individuals in the 500 mg twice daily cohort also demonstrated a 1.0 log10 decrease from baseline as compared with 24% in the 300 mg twice daily cohort. Furthermore, four patients (8%) had plasma HIV-1 RNA levels < 400 copies/ml at the end of the study, of which three were in the 500 mg twice daily cohort.
The 500 mg twice daily amdoxovir dose had the greatest activity (0.7 log10 versus 0.2 log10 for 300 mg twice daily), although this difference was not significant.
Figures 1 and 2 summarize the antiviral activity results in treatment-experienced patients.
Safety and tolerability
During the 15-day treatment phase, 43 individuals (48%) reported at least one adverse event at least possibly related to study drug. The most frequently reported adverse events were headache (11%), nausea (10%) and diarrhea (10%). All of these adverse events were reported as mild or moderate in severity. The profile of adverse events was similar across all treatment groups and no dose related effects were identified. Furthermore, there was no report of visual abnormalities.
Two individuals had serious adverse events; one received amdoxovir 25 mg twice daily and one received amdoxovir 500 mg twice daily. Both of these events were considered by the investigator to be unrelated to study medication. The first individual was hospitalized for chest discomfort and the episode resolved the following day. The second individual was admitted to the hospital for presumptive Mycobacterium avium complex infection at day 22 (after the dosing period), which was confirmed during a second hospitalization at day 40.
Overall, 66% of patients had at least one treatment-emergent laboratory abnormality (grade 1 to 4). The majority of treatment-emergent laboratory abnormalities were grade 1 or 2 in severity and none required modification of dosing with study drug. The incidence of grade 3 and 4 laboratory abnormalities was 17% in treatment-naive patients and included increased creatine kinase (n = 4), decreased neutrophils (n = 1) and increased triglycerides (n = 2), and 28% in treatment-experienced patients and included increased aspartate aminotransferase (AST) (n = 2), increased creatine kinase and amylase (n = 4, each), increased pancreatic amylase (n = 2), increased glucose, lipase, triglyceride and total bilirubin (n = 1, each) and decreased neutrophils (n = 1).
Pharmacokinetic evaluations
Pharmacokinetic analysis of amdoxovir was performed in all patients except the treatment-naive 25 mg twice daily cohort.
Plasma samples were collected before the first dose of amdoxovir on days 5, 8 and 12 and thus were considered to represent trough plasma samples at steady state.
Following oral administration, DAPD was rapidly absorbed and converted to DXG by adenosine deaminase, the active anti-HIV moiety of DAPD. Peak plasma concentrations for both DAPD and DXG occurred between 1 to 2 h post-DAPD dose. Plasma DXG concentrations were much higher than those of DAPD with a median AUC ratio (DXG to DAPD) of 5 (range 3 to 9). DAPD was rapidly eliminated from plasma (half-life (t½) approximately 1-2 h) primarily by conversion to DXG, while DXG had a longer half-life in plasma ranging from 4 to 7 h at steady state. Plasma DAPD/DXG concentrations increased in a dose-related fashion. Steady-state plasma concentrations of DAPD and DXG were predictable based on single-dose pharmacokinetic data and the disposition of DAPD and DXG followed linear kinetics. There were no major differences in DAPD/DXG plasma pharmacokinetics between treatment-naive and -experienced patients. Plasma concentrations of DXG reached levels well above the in vitro EC50 over the entire dosing intervals for doses of 100 mg twice daily and higher consistent with the antiviral activity observed in the clinic.
Dose-response relationship in antiretroviral activity
Based on a simple Emax model using individual AAUCMB at day 15 as response endpoint, it was predicted that a 500 mg dose twice daily of amdoxovir would produce a response that was approximately 94% of the maximum predicted response (0.95 log10 HIV-1 RNA copies/ml) in the treatment-naive patients, while the predicted response in the treatment-experienced patients was only about 62% of the maximum predicted response (1.08 log10 HIV-1 RNA copies/ml). Due to the large inter-patient variability in the anti-viral activity, the dose-response relationship of amdoxovir for the treatment-experienced patient population requires further evaluation.
Genotypic analysis
Baseline HIV-1 genotypic data were obtained for treatment-naive and treatment-experienced patients.
In the treatment-naive patients, nine patients (24%) had at least one mutation at baseline, and two patients developed a new mutation at day 15 and the end of the study.
Consistent with the extensive prior use of nucleoside analogs (zidovudine, lamivudine and stavudine) for treatment-experienced patients, Baseline HIV-1 genotypic data revealed that 43 out of 49 (88%) of these patients had at least one resistance mutation. The most frequent mutations in the RT gene were M184V (55%), T215Y (39%), M41L (33%), D67N (31%), K70R (20%) and L210W (16%). At the end of the study, six (12%) treatment-experienced patients developed a new NRTI mutation; L210W and T215Y (zidovudine-associated mutations) were the most frequent. Two individuals developed specific protease mutations while receiving prior and/or current antiretroviral therapy including protease inhibitors.
No patient developed the K65R mutation in this study. Table 2 summarizes the genotypic findings.
Multiple regression analysis of predictive factors for antiviral response
To identify independent predictive factors for the antiviral response, a multivariate linear regression model was constructed by using a stepwise model selection method to determine which factors alone, and when combined, were associated with the antiretroviral response. Factors tested in the linear regression models included: dose, baseline HIV-RNA, baseline CD4+ cell count, duration of HIV treatment and the presence of the following at baseline: thymidine-analogue mutations (TAMs), NNRTI mutations, all NRTI mutations, PI mutations, M41L, D67N, T69D, K70R, I74V, V75T, M184V, L210W, T215Y, T215F, K219E and K219Q.
In treatment-naive patients, dose was the only factor associated with antiviral response (P = 0.0005).
In treatment-experienced patients, the probability of a 0.5 log10 decrease in HIV-1 RNA was correlated with the baseline CD4+ cell count (P < 0.0001) and inversely correlated with the presence of TAMs at baseline (P = 0.0009). The number of TAMs or specific RT resistance mutations or dose of amdoxovir was not associated with response. The same trend was noted for a 1.0 log10 decrease from baseline.
The results of the univariate and multivariate analysis are summarized in Table 3.
Discussion
Study DAPD-101 demonstrated that amdoxovir produced antiviral activity in all treatment-naive patients and in 24 of 49 (49%) treatment-experienced patients who had at least a 0.5 log10 decline in HIV-RNA at day 15. In treatment-naive patients receiving short-term amdoxovir monotherapy, a reduction of 1.5 log10 at the highest doses was observed.
In treatment-experienced patients, the reduction in viral load observed at each dose was less than that observed in treatment-naive patients in apparent contradiction to the in vitro virology data [18]. During the study, nine treatment-experienced patients developed up to three new nucleoside reverse transcriptase-associated mutations. Three individuals (6%) developed L210W, two developed T215Y, one developed Y188C, one developed M41L, one developed L90M and one developed I54V. The specific amino acid substitutions observed among the patients who developed nucleoside-associated RT mutations suggest that prior and/or current background therapy was at least partially responsible for the development of these mutations. The five patients who developed typical zidovudine-associated mutations were concomitantly receiving or had prior use of zidovudine (n = 3) or stavudine (n = 2). No patient developed K65R which can be selected in MT2 cells by amdoxovir [18]. Five of the nine patients in this study who developed nucleoside-associated RT mutations did not achieve at least a 0.5 log10 decrease in plasma HIV-RNA from baseline over the 15 day treatment period. The median decrease in plasma HIV-RNA in these five patients was 0.31 log10 at day 15.
In the stepwise linear regression analysis, in treatment-naive patients, DAPD dose was the only predictive factor for antiviral response. In treatment-experienced patients, the only predictive factors for antiviral response were the presence of TAMs at baseline and baseline CD4+ cell count. Individuals with HIV not containing TAMs at baseline and with higher baseline CD4+ cell count had a greater reduction in viral load while taking DAPD. Previous results suggested that different patterns of TAMs could influence the virologic response to nucleosides or nucleotide RT inhibitors and this could have an impact in clinical practice [19-23]. The presence of other primary NRTI, NNRTI, or PI mutations did not seem to affect DAPD antiviral activity.
In the treatment-experienced add-on therapy patients the presence of the M184V mutation was a factor associated with better antiviral response (P = 0.058). Patients with HIV mutant at M184V had a higher frequency of virologic response (> 0.5 log10 copies/ml decrease from baseline) as compared to HIV wild type at the 184 position (odds ratio, 4.1; 95% confidence interval, 0.30-55.7). This finding has been previously reported with tenofovir [24]. The dose of amdoxovir did not appear to predict antiviral activity in the treatment-experienced patients.
In summary, results of antiviral activity, safety and pharmacokinetic analyses obtained in this study indicate that amdoxovir has potent antiviral activity in treatment-naive patients. Amdoxovir also demonstrated antiviral activity in heavily treatment-experienced patients, with significant antiviral activity in patients with no TAMs and higher baseline CD4+ cell counts.
References
1. Turner D, Brenner B, Wainberg MA. Relationships among various nucleoside resistance-conferring mutations in the reverse transcriptase of HIV-1. J Antimicrob Chemother 2004; 53:53-57.
2. Mocroft A, Phillips AN, Friis-Moller N, Colebunders R, Johnson AM, Hirschel B, et al, the EuroSIDA study group. Response to antiretroviral therapy among patients exposed to three classes of antiretrovirals: results from the EuroSIDA study. Antivir Ther 2002; 7:21-30.
3. Tamalet C, Fantini J, Tourres C, Yahi N. Resistance of HIV-1 to multiple antiretroviral drugs in France: a 6-year survey (1997-2002) based on an analysis of over 7000 genotypes. AIDS 2003; 17:2383-2388.
4. Little SJ, Holte S, Routy JP, Daar ES, Markowitz M, Collier AC, et al. Antiretroviral-drug resistance among patients recently infected with HIV. N Engl J Med 2002; 347:385-394.
5. Walmsley S. Individualized therapy for the treatment-experienced patient. AIDS Read 2003; 13(6 Suppl):S11-S15.
6. Marcelin AG, Delaugerre C, Wirden M, Viegas P, Simon A, Katlama C, et al. Thymidine analogue reverse transcriptase inhibitors resistance mutations profiles and association to other nucleoside reverse transcriptase inhibitors resistance mutations observed in the context of virological failure. J Med Virol 2004; 72:162-165.
7. Gallant JE, Gerondelis PZ, Wainberg MA, Shulman NS, Haubrich RH, St Clair M, et al. Nucleoside and nucleotide analogue reverse transcriptase inhibitors: a clinical review of antiretroviral resistance. Antivir Ther 2003; 8:489-506.
8. Deeks SG. Treatment of antiretroviral-drug-resistant HIV-1 infection. Lancet 2003; 362(9400):2002-2011.
9. Jeffrey JL, Feng JY, Qi CC, Anderson KS, Furman PA. Dioxolane guanosine 5'-triphosphate, an alternative substrate inhibitor of wild-type and mutant HIV-1 reverse transcriptase. Steady state and pre-steady state kinetic analyses. J Biol Chem 2003; 278:18971-18979.
10. Mewshaw JP, Myrick FT, Wakefield DA, Hooper BJ, Harris JL, McCreedy B, et al. Dioxolane guanosine, the active form of the prodrug diaminopurine dioxolane, is a potent inhibitor of drug-resistant HIV-1 isolates from patients for whom standard nucleoside therapy fails. J Acquir Immune Defic Syndr 2002; 29:11-20.
11. Gu Z, Wainberg MA, Nguyen-Ba N, L'Heureux L, de Muys JM, Bowlin TL, et al. Mechanism of action and in vitro activity of 1′, 3′-dioxolanylpurine nucleoside analogues against sensitive and drug-resistant human immunodeficiency virus type 1 variants. Antimicrob Agents Chemother 1999; 43:2376-2382.
12. Bloor S, Kemp SD, Hertogs K, Alcorn T, Larder BA. Patterns of HIV drug resistance in routine clinical practice: a survey of almost 12,000 samples from the USA in 1999 [abstract 169]. Antivir Ther 2000; 5:132.
13. Lanier ER, Scott J, Ait-Khaled M, et al. Prevalence of mutations associated with resistance to antiretroviral therapy from 1999-2002 [poster 635]. In: Programs and Abstracts of the 10th Conference on Retrovirus and Opportunistic Infections (Boston). Arlington, VA: Foundation for Retrovirology and Human Health, 2003. p. 285.
14. Rousseau FS, Wakeford C, Mommeja-Marin H, Sanne I, Moxham C, Harris J, et al, FTC-102 Clinical Trial Group. Prospective randomized trial of emtricitabine versus lamivudine short-term monotherapy in human immunodeficiency virus-infected patients. J Infect Dis 2003; 188:1652-1658.
15. Rousseau FS, Kahn JO, Thompson M, Mildvan D, Shepp D, Sommadossi JP, et al. Prototype trial design for rapid dose selection of antiretroviral drugs: an example using emtricitabine (Coviracil). J Antimicrob Chemother 2001; 48:507-513.
16. Ho DD, Neumann AU, Perelson AS, Chen W, Leonard JM, Markowitz M. Rapid turnover of plasma virions and CD4 lymphocytes in HIV-1 infection. Nature 1995; 373(6510):123-126.
17. Wei X, Ghosh SK, Taylor ME, Johnson VA, Emini EA, Deutsch P, et al. Viral dynamics in human immunodeficiency virus type 1 infection. Nature 1995; 373(6510):117-122.
18. Bazmi HZ, Hammond JL, Cavalcanti SC, Chu CK, Schinazi RF, Mellors JW. In vitro selection of mutations in the human immunodeficiency virus type 1 reverse transcriptase that decrease susceptibility to (-)-beta-D-dioxolane-guanosine and suppress resistance to 3'-azido-3'-deoxythymidine. Antimicrob Agents Chemother 2000; 44:1783-1788.
19. Miller V, Larder BA. Mutational patterns in the HIV genome and cross-resistance following nucleoside and nucleotide analogue drug exposure. Antivir Ther 2001; 6(Suppl 3):25-44.
20. Gonzales MJ, Wu TD, Taylor J, Belitskaya I, Kantor R, Israelski D, et al. Extended spectrum of HIV-1 reverse transcriptase mutations in patients receiving multiple nucleoside analog inhibitors. AIDS 2003; 17:791-799.
21. Bocket L, Yazdanpanah Y, Ajana F, Gerard Y, Viget N, Goffard A, et al. Thymidine analogue mutations in antiretroviral-naive HIV-1 patients on triple therapy including either zidovudine or stavudine. J Antimicrob Chemother 2004; 53:89-94.
22. Barrios A, de Mendoza C, Martin-Carbonero L, Ribera E, Domingo P, Galindo MJ, et al. Role of baseline human immunodeficiency virus genotype as a predictor of viral response to tenofovir in heavily pretreated patients. J Clin Microbiol 2003; 41:4421-4423.
23. McColl DJ, Miller MD. The use of tenofovir disoproxil fumarate for the treatment of nucleoside-resistant HIV-1. J Antimicrob Chemother 2003; 51:219-223.
24. Miller MD, Margot N, Lu B, Zhong L, Chen SS, Cheng A, et al. Genotypic and phenotypic predictors of the magnitude of response to tenofovir disoproxil fumarate treatment in antiretroviral-experienced patients. J Infect Dis 2004; 189:837-846.
Appendix
A.1 DAPD-101 Study Group
Roberto Arduino, Houston, Texas; Constance A. Benson, Denver, Colorado; Steven Grant Deeks, San Francisco, California; Joseph J. Eron Jr., Chapel Hill, North Carolina; Charles Frank Farthing, West Hollywood, California; Jeffrey Martin Jacobson, New York, New York; Harold Kessler, Chicago, Illinois; Jeffrey P. Nadler, Tampa, Florida; Gary Richmond, Fort Lauderdale, Florida; François Raffi, Nantes, France; Daniel Sereni, Paris, France; Melanie Thompson, Atlanta, Georgia; Francesca J. Torriani, San Diego, California.
© 2005 Lippincott Williams & Wilkins, Inc.