Prevalence and predictive value of intermittent viremia with combination hiv therapy.

CONTEXT: In HIV-infected patients having virologic suppression (plasma HIV RNA <50 copies/mL) with antiretroviral therapy, intermittent episodes of low-level viremia have been correlated with slower decay rates of latently infected cells and increased levels of viral evolution, but the clinical significance of these episodes is unknown. OBJECTIVE: To determine if HIV-infected patients with intermittent viremia have a higher risk of virologic failure (confirmed HIV RNA >200 copies/mL). DESIGN AND SETTING: Retrospective analysis of subjects in well-characterized cohorts, the AIDS Clinical Trials Group (ACTG) 343 trial of induction-maintenance therapy (August 1997 to November 1998) and the Merck 035 trial (ongoing since March 1995). PATIENTS: Two hundred forty-one ACTG 343 patients, of whom 101 received triple-drug therapy throughout the study, and a small group of 13 patients from Merck 035 having virologic suppression after 6 months of indinavir-zidovudine-lamivudine. MAIN OUTCOME MEASURES: Association of intermittent viremia (plasma HIV RNA >50 copies/mL with a subsequent measure <50 copies/mL) with virologic failure (2 consecutive plasma HIV RNA measures >200 copies/mL) in both study groups; evidence of drug resistance in 7 patients from the small (n = 13) study group with long-term follow-up. RESULTS: Intermittent viremia occurred in 96 (40%) of the 241 ACTG 343 patients of whom 32 (13%) had 2 consecutive HIV RNA values >50 copies/mL during the median 84 weeks of observation (median duration of observation after first intermittent viremia episode was 46 weeks). Of the 101 individuals receiving triple-drug therapy throughout, 29% had intermittent viremia; the proportion of episodes occurring during the maintenance period was 64% for the entire cohort and 68% for the group not receiving triple-drug therapy throughout vs 55% for those who did (P =.25). Intermittent viremia did not predict virologic failure: 10 (10.4%) of 96 patients with and 20 (13.8%) of 145 patients without intermittent viremia had virologic failure (relative risk, 0.76; 95% confidence interval [CI], 0.29-1.72). In a Cox proportional hazards model, the risk for virologic failure was not significantly greater in the ACTG 343 patients with intermittent viremia (hazard ratio, 1.28; 95% CI, 0.59-2.79). Median viral load in 10 ACTG 343 patients assessed between 24 and 60 weeks of therapy using an ultrasensitive 2.5-copies/mL detection level assay was 23 copies/mL in those with intermittent viremia vs <2.5 copies/mL in those without (P =.15). Intermittent viremia occurred in 6 of 13 patients from the small study group assessed after 76 to 260 weeks of therapy (using the 2.5-copies/mL detection level assay) and was associated with a higher steady state of viral replication (P =.03), but not virologic failure over 4.5 years of observation. Viral DNA sequences from 7 patients did not show evolution of drug resistance. CONCLUSIONS: Intermittent viremia occurred frequently and was associated with higher levels of replication (Merck 035), but was not associated with virologic failure in patients receiving initial combination therapy of indinavir-zidovudine-lamivudine (ACTG 343 and Merck 035). In this population, treatment changes may not be necessary to maintain long-term virologic suppression with low-level or intermittent viremia.

Baseline human immunodeficiency virus type 1 phenotype, genotype, and RNA response after switching from long-term hard-capsule saquinavir to indinavir or soft-gel-capsule saquinavir in AIDS clinical trials group protocol 333.

AIDS Clinical Trials Group protocol 333 was an open-label trial of a switch from saquinavir (SQV) hard capsules (SQVhc) to indinavir (IDV) or saquinavir soft-gel capsules (SQVsgc) after >48 weeks of prior treatment with SQVhc. Eighty-nine subjects received IDV or SQVsgc or continued to receive SQVhc and continued unchanged treatment with non-protease-inhibitor antivirals for 8 weeks. Subjects receiving SQVhc then switched treatment to IDV. Baseline drug susceptibility and protease gene sequencing were done; 12 codons related to IDV and SQV resistance were analyzed. After 112 weeks (median) of SQVhc, the fall in human immunodeficiency virus (HIV) type 1 RNA level from baseline was significantly greater with IDV and was inversely correlated with the number of protease substitutions. The number of substitutions also correlated with baseline CD4 cell count, HIV-1 RNA level, SQV experience, and drug susceptibility. Substitution at codon 10, which occurred only in isolates with >/=2 substitutions, was associated with blunted RNA response. IDV IC(50) correlated with HIV-1 RNA response after the switch to IDV but added little predictive power once the genotype was considered.

Drug resistance and predicted virologic responses to human immunodeficiency virus type 1 protease inhibitor therapy.

The extent to which human immunodeficiency virus (HIV) type 1 drug resistance compromises therapeutic efficacy is intimately tied to drug potency and exposure. Most HIV-1 protease inhibitors maintain in vivo trough levels above their human serum protein binding-corrected IC(95) values for wild-type HIV-1. However, these troughs are well below corrected IC(95) values for protease inhibitor-resistant viruses from patients experiencing virologic failure of indinavir and/or nelfinavir. This suggests that none of the single protease inhibitors would be effective after many cases of protease inhibitor failure. However, saquinavir, amprenavir, and indinavir blood levels are increased substantially when each is coadministered with ritonavir, with 12-h troughs exceeding corrected wild-type IC(95) by 2-, 7-, and 28-79-fold, respectively. These indinavir and amprenavir troughs exceed IC(95) for most protease inhibitor-resistant viruses tested. This suggests that twice-daily indinavir-ritonavir and, to a lesser extent, amprenavir-ritonavir may be effective for many patients with viruses resistant to protease inhibitors.

3-year suppression of HIV viremia with indinavir, zidovudine, and lamivudine.

BACKGROUND: Antiretroviral regimens containing HIV protease inhibitors suppress viremia in HIV-infected patients, but the durability of this effect is not known. OBJECTIVE: To describe the 3-year follow-up of patients randomly assigned to receive indinavir, zidovudine, and lamivudine in an ongoing clinical trial. DESIGN: Open-label extension of a randomized, double-blind study. SETTING: Four clinical research units. PATIENTS: 33 HIV-infected, zidovudine-experienced patients with serum HIV RNA levels of at least 20,000 copies/mL and CD4 counts ranging from 50 to 400 cells/mm3. INTERVENTION: Indinavir, zidovudine, and lamivudine. MEASUREMENTS: Safety assessments, HIV RNA levels, CD4 cell counts, and genotypic analyses. RESULTS: After 3 years of follow-up, 21 of 31 contributing patients (68% [95% CI, 49% to 83%]) had serum viral load levels less than 500 copies/mL. Twenty of 31 (65% [CI, 45% to 80%]) had levels less than 50 copies/mL. The median increase in CD4 count from baseline was 230 cells/mm3 (interquartile range, 150 to 316 cells/mm3). Nephrolithiasis occurred in 12 of 33 patients (36%). CONCLUSION: A three-drug regimen of indinavir, zidovudine, and lamivudine suppressed viremia in two thirds of patients for at least 3 years.

Associations between amino acids in the evolution of HIV type 1 protease sequences under indinavir therapy.

Significant diversity exists in amino acid sequences encoding HIV-1 protease in individuals naive for protease inhibitors, which could influence the rate of evolution of resistance. High-level resistance to indinavir requires multiple substitutions among at least 11 amino acid sites, and no single substitution was observed in all of 29 resistant isolates obtained from patients on long-term indinavir monotherapy. We have analyzed the evolution of PR in these sequences. The divergence from the baseline amino acid sequence by week 24 was 4%, increasing more than 7% by week 60. The mean difference between sequences from different patients at baseline was 6% (3-9%), rising to 10% after 40 weeks (3-16%), although at all time points nonsynonymous substitutions were less frequent than synonymous nucleotide changes. Analysis of associations between variants at different amino acid sites using a mutual information statistic revealed four pairs of sites to be significantly associated. In three cases these associations included residue 82. Clusters of baseline and week 24 amino acid sequences identified by maximum parsimony did not correlate significantly with the IC95 to indinavir, although a weak correlation of baseline clusters with phenotype at the week 24 time point was suggested.

Genotypic analysis methods for detection of drug resistance mutations in the HIV-1 proteinase and reverse transcriptase genes.

Understanding the basis of human immunodeficiency virus (HIV) drug resistance represents a key requirement for individualized HIV patient care. The genotypic data generated to date have already provided significant insight. However, it is clear that the relationship between genotype, phenotype and clinical outcome is complex and still poorly defined. In this review, we describe methods currently available to obtain genotypic data for the HIV-1 proteinase and reverse transcriptase genes. Different sample preparation strategies and DNA sequencing methods are discussed dividing the latter into two categories, those that give sequence information at specific positions and those that provide continuous sequence data for a particular region. In addition, we also address some of the broad biological and technical issues, which must be considered when interpreting the results of these tests and describe the advantages and disadvantages of individual methods.

Factors influencing the emergence of resistance to indinavir: role of virologic, immunologic, and pharmacologic variables.

A major problem with the use of human immunodeficiency virus type 1 (HIV-1) protease inhibitors as monotherapy has been an unacceptably high rate of emergence of resistance. To examine possible influences on the time to emergence of resistance, 24-week data were examined from five studies in which indinavir had been administered as monotherapy or as a component of combination therapy. Monotherapy data indicated a correlation between the level of HIV-1 RNA achieved and the risk of emergence of resistance: the lower the level, the lower the risk. When combination and monotherapy regimens were compared, the group receiving indinavir + lamivudine + zidovudine had a significantly lower risk of resistance, even after adjusting for the minimum HIV-1 RNA level achieved. The findings indicate that if at all possible, HIV-1-infected patients should receive combination chemotherapy to minimize the emergence of resistance to the protease inhibitor portion of the regimen. The goal of therapy should be to decrease the HIV-1 RNA load to a less-than-detectable level.

Simultaneous vs sequential initiation of therapy with indinavir, zidovudine, and lamivudine for HIV-1 infection: 100-week follow-up.

CONTEXT: Combination antiretroviral therapy can markedly suppress human immunodeficiency virus (HIV) replication but the duration of HIV suppression varies among patients. OBJECTIVE: To compare the antiretroviral effect of a 3-drug regimen started simultaneously or sequentially in patients with HIV infection. DESIGN: A multicenter, randomized, double-blind study, modified after at least 24 weeks of blinded therapy to provide open-label 3-drug therapy with follow-up through 100 weeks. SETTING: Four clinical research units PATIENTS: Ninety-seven patients with HIV infection who had taken zidovudine for at least 6 months with serum HIV RNA level of at least 20000 copies/mL and CD4 cell count of 0.05 to 0.40 x 10(9)/L. INTERVENTIONS: Patients were initially randomized to receive 1 of 3 antiretroviral regimens: indinavir, 800 mg every 8 hours; zidovudine, 200 mg every 8 hours and lamivudine, 150 mg every 12 hours; or all 3 drugs. After at least 24 weeks of blinded therapy, all patients received open-label 3-drug therapy. MAIN OUTCOME MEASURES: Antiretroviral activity was assessed by changes in HIV RNA level and CD4 cell count from baseline. Data through 100 weeks were summarized. RESULTS: Simultaneous initiation of indinavir, zidovudine, and lamivudine suppressed HIV RNA in 78% (25/32) of contributing patients to less than 500 copies/mL and increased CD4 cell count to a median of 0.209 x 10(9)/L above baseline at 100 weeks. When these 3 drugs were initiated sequentially, only 30% to 45% of contributing patients (10 of 33 in the zidovudine-lamivudine group and 13 of 29 in the indinavir group, respectively) had a sustained reduction in HIV RNA to less than 500 copies/mL, and median CD4 cell count increased to 0.101 to 0.163 x 10(9)/L above baseline at 100 weeks. CONCLUSIONS: A 3-drug combination of indinavir, zidovudine, and lamivudine started simultaneously has durable antiretroviral activity for at least 2 years. Sequential initiation of the same 3 drugs is much less effective.

Resistance to HIV protease inhibitors.

Resistance to the HIV-1 protease inhibitor indinavir involves the accumulation of multiple amino acid substitutions in the viral protease. A minimum of 11 amino acid positions have been identified as potential contributors to phenotypic resistance. Three or more amino acid substitutions in the protease are required before resistance becomes measurable (> or = four-fold). Further losses in susceptibility follow the stepwise accumulation of additional amino acid substitutions, indicating that antiviral activity (selective pressure) is maintained despite the appearance of multiple amino acid substitutions in the viral protease. Importantly, the sequential nature of these changes indicates that the effects of these substitutions are additive, and that the evolution of resistance is driven by viral replication. This result has significant implications for therapy. It predicts that viral variants resistant to indinavir are unlikely to pre-exist in protease inhibitor-naive patients, and further, that high-level resistance can only develop if the virus is allowed to replicate in the presence of the drug. The use of indinavir in combination with other antiretroviral agents has been demonstrated to dramatically reduce the incidence of resistance mutations, suggesting that with maximal suppression of viral replication, long-term control of HIV-1 infection may be achievable. Thus, the goal of therapy must be to never to allow the virus to replicate. This can be best accomplished by initiating therapy with a maximally suppressive regimen, to reduce viral replication as much as possible, and by imposing a high genetic barrier to resistance. Previous use of other protease inhibitors or inadequate adherence to therapy may compromise the long-term benefit of indinavir by allowing the virus to gain a foothold through the development of resistance. An understanding of these issues will be critical in realizing the full potential of this potent new drug for the control of HIV-1 infection.

Virological and clinical implications of resistance to HIV-1 protease inhibitors.

Resistance to HIV-1 protease inhibitors is associated with 25 or more amino acid substitutions in the protease and its cleavage sites. These appear in variable combinations and in different orders, and their effects depend on the combinations in which they occur. Substitutions within and away from the enzyme active site may engender resistance by drug-specific or drug-independent mechanisms. The correlates of resistance to the different protease inhibitors overlap substantially, and each can select for viral variants that are cross-resistant to others. An understanding of protease inhibitor resistance is critical for the appropriate use of these potent inhibitors of viral replication.

Genetic correlates of in vivo viral resistance to indinavir, a human immunodeficiency virus type 1 protease inhibitor.

Indinavir (IDV) (also called CRIXIVAN, MK-639, or L-735,524) is a potent and selective inhibitor of the human immunodeficiency virus type 1 (HIV-1) protease. During early clinical trials, in which patients initiated therapy with suboptimal dosages of IDV, we monitored the emergence of viral resistance to the inhibitor by genotypic and phenotypic characterization of primary HIV-1 isolates. Development of resistance coincided with variable patterns of multiple substitutions among at least 11 protease amino acid residues. No single substitution was present in all resistant isolates, indicating that resistance evolves through multiple genetic pathways. Despite this complexity, all of 29 resistant isolates tested exhibited alteration of residues M-46 (to I or L) and/or V-82 (to A, F, or T), suggesting that screening of these residues may be useful in predicting the emergence of resistance. We also extended our previous finding that IDV-resistant viral variants exhibit various patterns of cross-resistance to a diverse panel of HIV-1 protease inhibitors. Finally, we noted an association between the number of protease amino acid substitutions and the observed level of IDV resistance. No single substitution or pair of substitutions tested gave rise to measurable viral resistance to IDV. The evolution of this resistance was found to be cumulative, indicating the need for ongoing viral replication in this process. These observations strongly suggest that therapy should be initiated with the most efficacious regimen available, both to suppress viral spread and to inhibit the replication that is required for the evolution of resistance.

In vivo selection of HIV-1 variants with reduced susceptibility to the protease inhibitor L-735,524 and related compounds.

L-735,524 is a potent inhibitor of the HIV-1 protease. In cell culture, the compound interferes with virus replication by causing production of noninfectious immature viral particles containing an unprocessed gag-pol polyprotein. Initial human clinical studies demonstrated that treatment with the inhibitor caused circulating viral levels to decline and this decline was associated with increases in the CD4 count of varying magnitude. However, in most patients, antiviral activity is lost as viral variants with reduced susceptibility to the inhibitor are selected. The resistant phenotype appears to require an amino acid substitution at protease codon 82. However, this amino acid alteration alone is insufficient for expression of the resistance phenotype. Co-expression of various additional alterations seems to be required, but the nature of these additional substitutions differs among resistant isolates. HIV-1 variants, cross-resistant to a panel of structurally diverse protease inhibitors, were isolated from patients following prolonged L-735,524 therapy.

In vivo emergence of HIV-1 variants resistant to multiple protease inhibitors.

Inhibitors of the human immunodeficiency virus type 1 (HIV-1) protease have entered clinical study as potential therapeutic agents for HIV-1 infection. The clinical efficacy of HIV-1 reverse transcriptase inhibitors has been limited by the emergence of resistant viral variants. Similarly, variants expressing resistance to protease inhibitors have been derived in cell culture. We now report the characterization of resistant variants isolated from patients undergoing therapy with the protease inhibitor MK-639 (formerly designated L-735,524). Five of these variants, isolated from four patients, exhibited cross-resistance to all members of a panel of six structurally diverse protease inhibitors. This suggests that combination therapy with multiple protease inhibitors may not prevent loss of antiviral activity resulting from resistance selection. In addition, previous therapy with one compound may abrogate the benefit of subsequent treatment with a second inhibitor.

Purification and characterization of HIV-1 reverse transcriptase having a 1:1 ratio of p66 and p51 subunits.

Wild-type and several mutant forms of recombinant human immunodeficiency virus type-1 reverse transcriptase were overexpressed as either the p66 or the p51 subunit in a protease-deficient strain of Escherichia coli. Immediately prior to cell lysis, p51 cell paste was mixed with cell paste containing the corresponding overexpressed p66 subunit in a ratio resulting in an excess of the smaller subunit with respect to the larger. During the subsequent chromatography steps stable heterodimer p66/p51 was purified to homogeneity. This protein was characterized by amino acid analysis, denaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis, analytical gel filtration HPLC, laser desorption mass spectroscopy, and isoelectric focusing. In addition, we were able to obtain crystals of the purified enzyme complexed with a quinazolinone class nonnucleoside inhibitor that diffracted to 3.2 A resolution. A potential application of this expression/purification methodology is the ability to alter specific amino acids residues, by site-directed-mutagenesis, of only one subunit of the RT-dimer.

Susceptibilities of human immunodeficiency virus type 1 enzyme and viral variants expressing multiple resistance-engendering amino acid substitutions to reserve transcriptase inhibitors.

To evaluate the potential that multiply resistant human immunodeficiency virus type 1 variants may arise during combination nucleoside and nonnucleoside reverse transcriptase inhibitor therapy, we constructed a series of mutant reverse transcriptase enzymes and viruses that coexpressed various combinations of resistance-associated amino acid substitutions. Substitutions at residues 100 (Leu-->Ile) and 181 (Tyr-->Cys), which mediate resistance to the nonnucleosides, suppressed resistance to 3'-azido-3'-deoxythymidine (AZT) when coexpressed with AZT-specific substitutions. However, a number of viral variants that exhibited significantly reduced susceptibilities to both classes of inhibitors were constructed.

Human immunodeficiency virus type 1 protease inhibitors: evaluation of resistance engendered by amino acid substitutions in the enzyme's substrate binding site.

The human immunodeficiency virus type 1 (HIV-1) protease is a homodimeric aspartyl endopeptidase that is required for virus replication. A number of specific, active-site inhibitors for this enzyme have been described. Many of the inhibitors exhibit significant differences in activity against the HIV-1 and HIV type 2 (HIV-2) enzymes. An initial study was conducted to ascertain the HIV-1 protease's potential to lose sensitivity to several test inhibitors while retaining full enzymatic activity. The substrate binding sites of the HIV-1 and HIV-2 enzymes are almost fully conserved, except for four amino acid residues at positions 32, 47, 76, and 82. Accordingly, recombinant mutant type 1 proteases were constructed that contained the cognate type 2 residue at each of these four positions. The substitution at position 32 resulted in a significant adverse effect on inhibitor potency. However, this substitution also mediated a noted increase in the Km of the substrate. Individual substitutions at the remaining three positions, as well as a combination of all four substitutions, had very little effect on enzyme activity or inhibitor susceptibility. Hence, the four studied active site residues are insufficient to be responsible for differences in inhibitor sensitivity between the HIV-1 and HIV-2 proteases and are unlikely to contribute to the generation of inhibitor-resistant mutant HIV-1 protease.

The genetic and functional basis of HIV-1 resistance to nonnucleoside reverse transcriptase inhibitors.

The nonnucleoside reverse transcriptase (RT) inhibitors are structurally diverse compounds that are specific inhibitors of the human immunodeficiency virus type 1 RT enzyme. The compounds are largely functionally identical and bind to a common site in the enzyme. HIV-1 variants that exhibit reduced susceptibility to these inhibitors have been derived in cell culture and, more recently, from HIV-1-infected patients undergoing experimental therapy. The variants express amino acid substitutions at RT positions that apparently interact directly with the inhibitors. Effects of specific substitutions at these positions vary among the compounds, suggesting subtle differences in how the compounds physically interact with the enzyme.

Comprehensive mutant enzyme and viral variant assessment of human immunodeficiency virus type 1 reverse transcriptase resistance to nonnucleoside inhibitors.

The nonnucleoside reverse transcriptase (RT) inhibitors comprise a class of structurally diverse compounds that are functionally related and specific for the human immunodeficiency virus type 1 RT. Viral variants resistant to these compounds arise readily in cell culture and in treated, infected human. Therefore, the eventual clinical usefulness of the nonnucleoside inhibitors will rely on a thorough understanding of the genetic and biochemical bases for resistance. A study was performed to assess the effects of substitutions at each RT amino acid residue that influences the enzyme's susceptibility to the various nonnucleoside compounds. Single substitutions were introduced into both purified enzyme and virus. The resulting patterns of resistance were markedly distinct for each of the tested inhibitors. For instance, a > 50-fold loss of enzyme susceptibility to BI-RG-587 was engendered by any of four individual substitutions, while the same level of relative resistance to the pyridinone derivatives was mediated only by substitution at residue 181. Similarly, substitution at residue 181. Similarly, substitution at residue 106 had a noted effect on virus resistance to BI-RG-587 but not to the pyridinones. The opposite effect was mediated by a substitution at residue 179. Such knowledge of nonucleoside inhibitor resistance profiles may help in understanding the basis for resistant virus selection during clinical studies of these compounds.