Dideoxynucleoside resistance emerges with prolonged zidovudine monotherapy. The RV43 Study Group

2′-Fluoro-2′,3′-Dideoxyarabinosyladenine (F-ddA): Activity against Drug-Resistant Human Immunodeficiency Virus Strains and Clades A-E

2′-Fluoro-2′,3′-dideoxyarabinosyladenine (F-ddA), an anti-human immunodeficiency virus (HIV) drug currently in clinical trial, was compared with zidovudine (AZT), ddl and ddC for anti-HIV activity and potency in HIV-1 strains both sensitive and resistant to zidovudine, ddl and non-nucleoside reverse transcriptase inhibitors. A variety of host cell systems [MT-2, MT-4, phytohaemagglutinin (PHA)-stimulated peripheral blood mononuclear cells (PBMC)] was used. F-ddA was effective against each of the drug-resistant isolates, including the strain resistant to ddl, the other purine dideoxynucleoside evaluated in this study. The anti-HIV-1 activities of F-ddA and zidovudine were also determined against clades A-E in PHA-PBMCs. Although activities were similar, zidovudine was significantly more potent than F-ddA in the PHA-PBMC system.

Low-frequency nevirapine (NVP)-resistant HIV-1 variants are not associated with failure of antiretroviral therapy in women without prior exposure to single-dose NVP

BACKGROUND

Low-frequency nevirapine (NVP)-resistant variants have been associated with virologic failure (VF) of initial NVP-based combination antiretroviral therapy (cART) in women with prior exposure to single-dose NVP (sdNVP). We investigated whether a similar association exists in women without prior sdNVP exposure.

METHODS

Pre-cART plasma was analyzed by allele-specific polymerase chain reaction to quantify NVP-resistant mutants in human immunodeficiency virus-infected African women without prior sdNVP who were starting first-line NVP-based cART in the OCTANE/A5208 trial 2. Associations between NVP-resistant mutants and VF or death were determined and compared with published results from women participating in the OCTANE/A5208 trial 1 who had taken sdNVP and initiated NVP-based cART.

RESULTS

Pre-cART NVP-resistant variants were detected in 18% (39/219) of women without prior sdNVP exposure, compared to 45% (51/114) with prior sdNVP exposure (P < .001). Among women without prior sdNVP exposure, 8 of 39 (21%) with NVP-resistant variants experienced VF or death vs 31 of 180 (17%) without such variants (P = .65); this compares with 21 of 51 (41%) vs 9 of 63 (14%) among women with prior exposure (P = .001).

CONCLUSIONS

The risk of VF on NVP-based cART from NVP-resistant variants differs between sdNVP-exposed and -unexposed women. This difference may be driven by drug-resistance mutations emerging after sdNVP exposure that are linked on the same viral genome.

CLINICAL TRIALS REGISTRATION

NCT00089505.

Genetic diversity of simian immunodeficiency virus encoding HIV-1 reverse transcriptase persists in macaques despite antiretroviral therapy

The impact of antiretroviral therapy (ART) on the genetics of simian immunodeficiency virus (SIV) or human immunodeficiency virus (HIV) populations has been incompletely characterized. We analyzed SIV genetic variation before, during, and after ART in a macaque model. Six pigtail macaques were infected with an SIV/HIV chimeric virus, RT-SHIV(mne), in which SIV reverse transcriptase (RT) was replaced by HIV-1 RT. Three animals received a short course of efavirenz (EFV) monotherapy before combination ART was started. All macaques received 20 weeks of tenofovir, emtricitabine, and EFV. Plasma virus populations were analyzed by single-genome sequencing. Population diversity was measured by average pairwise difference, and changes in viral genetics were assessed by phylogenetic and panmixia analyses. After 20 weeks of ART, viral diversity was not different from pretherapy viral diversity despite more than 10,000-fold declines in viremia, indicating that, within this range, there is no relationship between diversity and plasma viremia. In two animals with consistent SIV RNA suppression to <15 copies/ml during ART, there was no evidence of viral evolution. In contrast, in the four macaques with viremias >15 copies/ml during therapy, there was divergence between pre- and during-ART virus populations. Drug resistance mutations emerged in two of these four animals, resulting in virologic failure in the animal with the highest level of pretherapy viremia. Taken together, these findings indicate that viral diversity does not decrease with suppressive ART, that ongoing replication occurs with viremias >15 copies/ml, and that in this macaque model of ART drug resistance likely emerges as a result of incomplete suppression and preexisting drug resistance mutations.

A combination of decreased NRTI incorporation and decreased excision determines the resistance profile of HIV-1 K65R RT

OBJECTIVE

To determine the mechanisms of resistance of K65R mutant reverse transcriptase (RT) to the currently approved nucleoside and nucleotide RT inhibitors (NRTI).

METHODS

Susceptibilities of K65R mutant HIV-1 to NRTI were determined in cell culture. The Ki/Km values were measured to determine the relative binding or incorporation of the NRTI, and ATP-mediated excision of incorporated NRTI was measured to determine NRTI stability as chain terminators.

RESULTS

K65R HIV-1 had decreased susceptibility to most NRTI, but increased susceptibility to zidovudine (ZDV). Ki/Km values were increased 2- to 13-fold for K65R compared to wild-type RT for all NRTI, indicating decreased binding or incorporation. However, K65R also showed decreased excision of all NRTI compared to wild-type, indicating greater stability once incorporated. At physiological nucleotide concentrations, excision of ZDV, carbovir (the active metabolite of abacavir; ABC), stavudine (d4T), and tenofovir was further decreased, while excision of didanosine (ddI), zalcitabine (ddC), lamivudine (3TC), and emtricitabine (FTC) was unchanged. The decreased binding or incorporation of ZDV by K65R appeared counteracted by decreased excision resulting in overall increased susceptibility to ZDV in cell culture. For ABC, tenofovir, and d4T, despite having decreased excision, decreased binding or incorporation resulted in reduced susceptibilities to K65R. For ddI, ddC, 3TC, and FTC, decreased binding or incorporation by K65R appeared responsible for the decreased susceptibilities in cell culture.

CONCLUSIONS

NRTI resistance in cells can consist of both altered binding or incorporation and altered excision of the NRTI. For K65R, the combination of these opposing mechanisms results in decreased susceptibility to most NRTI but increased susceptibility to ZDV.

The 3'-azido group is not the primary determinant of 3'-azido-3'-deoxythymidine (AZT) responsible for the excision phenotype of AZT-resistant HIV-1

The mechanism of human immunodeficiency virus (HIV) 1 resistance to 3'-azido-3'-deoxythymidine (AZT) involves reverse transcriptase (RT)-catalyzed phosphorolytic excision of the chain-terminating AZT-5'-monophosphate (AZTMP). Primers terminated with AZTMP are generally better substrates for this reaction than those terminated with 2',3'-dideoxynucleoside-5'-monophosphate (2',3'-ddNMP) analogs that lack a 3'-azido moiety. This led to the hypothesis that the 3'-azido group is a major structural determinant for maintaining the primer terminus in the appropriate site for phosphorolytic excision of AZTMP by AZT-resistant (AZT(R)) RT. To test this hypothesis, we evaluated the incorporation, phosphorolytic excision, and antiviral activity of a panel of 3'-azido-2',3'-ddN including 3'-azido-2',3'-ddA (AZddA), 3'-azido-2',3'-ddC (AZddC), 3'-azido-2',3'-ddG (AZddG), AZT, and 3'-azido-2',3'-ddU (AZddU). The results indicate that mutations correlated with resistance to AZT (D67N/K70R/T215F/K219Q) confer resistance to the 3'-azidopyrimidine nucleosides (AZddC, AZT, and AZddU) but not to the 3'-azidopurine nucleosides (AZddA and AZddG). The data suggest that the presence of a 3'-azido group on the 3'-terminal nucleotide of the primer does not confer increased phosphorolytic excision by AZT(R) RT for all 3'-azido-ddNMP analogs. Thus, the 3'-azido group cannot be the only structural determinant important for the enhanced phosphorolytic excision of AZTMP associated with HIV resistance to AZT. Other structural components, such as the base, must play a role in defining the specificity of the excision phenotype arising from AZT resistance mutations.

Viral resistance patterns selected by antiretroviral drugs and their potential to guide treatment choice

Massive viral turnover and reverse transcriptase's high error rate create the potential for drug-resistant viral variants to appear rapidly under the selective pressure of antiretroviral therapy. Loss of antiviral effect in treatment-adherent persons is most commonly coincident with the appearance of viral mutants with reduced drug sensitivity. Thus, detection of viral resistance may represent an early marker of therapy failure. Similarly, control of viral replication in the plasma compartment, as defined by plasma viral load below the levels of assay quantification, is associated with a sustained therapeutic response and delayed development of viral resistance. Information on patterns of resistance to and cross-resistance between antiretroviral agents is increasingly well characterised and represents an important consideration when deciding how to combine and/or sequence antiretrovirals to achieve optimal antiviral effects. Given the limited number of antiretrovirals presently available or in advanced development, it is important not to limit future therapeutic options by using therapies early in the treatment sequence which may select for cross-resistant viral variants and hence potentially reduce the magnitude of therapeutic response when treatment is changed to another member of that drug class. However, no studies using resistance to guide clinical decision making have been reported to date and available sequencing studies have focused largely on switching or adding therapies to patients experienced with zidovudine monotherapy. Thus, no resistance driven treatment algorithm is currently available.

HIV-1 genotypic and phenotypic resistance

Antiretroviral failure caused by the development of drug resistance in HIV-1 is an increasingly common clinical problem. Two types of resistance assays are available to clinicians. Genotypic assays determine the presence of mutations associated with drug resistance. The interpretation of mutations is often complicated, however, and may require expert opinion. Phenotypic assays provide a direct measure of the drug susceptibility of the virus. The magnitude of increase, however, in viral drug inhibitory concentration that is predictive of clinical drug failure remains unknown for several antiretroviral drugs. The mutational patterns underlying resistance to each antiretroviral drug are often diverse, and cross-resistance patterns within each of the currently available classes are complex. Currently, resistance testing is recommended for patients who have virologic failure on an antiretroviral regimen. Furthermore, testing should also be considered in treatment-native patients when the prevalence of transmitted drug-resistant virus is expected to be high.

Sustained high proportion of zidovudine-resistant HIV variants despite prolonged substitution of zidovudine by other nucleoside reverse transcriptase inhibitors

The consequences of zidovudine (ZDV) replacement by other nucleoside reverse transcriptase inhibitors on the expression of resistance mutations at codons 215 and 41 of the reverse transcriptase (RT) gene was investigated prospectively in 66 patients harboring mutant genotypes who were changed to an effective two- or three-drug combination antiretroviral regimen. Quantitation of mutant (MUT) viral populations at codon 215 by means of RT-PCR with differential hybridization of amplicons specific for MUT and wild (WT) variants revealed no difference in the proportion of 215 MUT variants prior to (93.5 +/- 2.4%) and 12 to 20 months after (96.9 +/- 1.9%) ZDV replacement, independently of a therapeutic change for stavudine. The fitness of the variants harboring the ZDV-resistant MUT 215 genotype following drug withdrawal was calculated to be 96 to 99% of that of the variants harboring the WT 215 genotype. The apparent stability of ZDV-resistant variants in the study population may have two main complementary explanations: persistent selective pressure secondary to partial cross-resistance due to the new regimens given after the therapeutic alteration and suppression of viral replication after the therapeutic alteration that could have hampered the replacement of less fit variants by fitter variants. These findings indicate that, at least within 15 months following discontinuation of ZDV, an effective antiretroviral therapy is insufficient to allow for ZDV-resistant strains to disappear, and thus to allow for the safe re-introduction of the drug.

Genotypic testing for human immunodeficiency virus type 1 drug resistance

There are 16 approved human immunodeficiency virus type 1 (HIV-1) drugs belonging to three mechanistic classes: protease inhibitors, nucleoside and nucleotide reverse transcriptase (RT) inhibitors, and nonnucleoside RT inhibitors. HIV-1 resistance to these drugs is caused by mutations in the protease and RT enzymes, the molecular targets of these drugs. Drug resistance mutations arise most often in treated individuals, resulting from selective drug pressure in the presence of incompletely suppressed virus replication. HIV-1 isolates with drug resistance mutations, however, may also be transmitted to newly infected individuals. Three expert panels have recommended that HIV-1 protease and RT susceptibility testing should be used to help select HIV drug therapy. Although genotypic testing is more complex than typical antimicrobial susceptibility tests, there is a rich literature supporting the prognostic value of HIV-1 protease and RT mutations. This review describes the genetic mechanisms of HIV-1 drug resistance and summarizes published data linking individual RT and protease mutations to in vitro and in vivo resistance to the currently available HIV drugs.

Clinical use of genotypic and phenotypic drug resistance testing to monitor antiretroviral chemotherapy

Assays that detect antiretroviral drug resistance in human immunodeficiency virus have recently become available to clinicians. Phenotypic assays measure the drug susceptibility of the virus by determining the concentration of drug that inhibits viral replication in tissue culture. Genotypic assays determine the presence of mutations that are known to confer decreased drug susceptibility. Although each type of assay has specific advantages, limitations associated with these tests often complicate the interpretation of results. Several retrospective clinical trials have suggested that resistance testing may be useful in the assessment of the success of salvage antiretroviral therapy. Prospective, controlled trials have demonstrated that resistance testing improves short-term virological response. Resistance testing is currently recommended to help guide the choice of new drugs for patients after treatment has failed and for pregnant women. Resistance testing should also be considered for treatment-naïve patients, to detect transmission of resistant virus.

Phosphorylation of nucleoside analog antiretrovirals: a review for clinicians

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

Mutational Patterns in the HIV Genome and Cross-Resistance following Nucleoside and Nucleotide Analogue Drug Exposure

A variety of key mutations in HIV reverse transcriptase (RT) have been associated with nucleoside reverse transcriptase inhibitor (NRTI) exposure, which give rise to a diverse range of effects in terms of altered drug susceptibilities, viral replicative capacity and RT biochemistry. There are three basic mechanisms of resistance conferred by specific mutations in the coding region of RT. The first is drug discrimination, whereby a particular drug or drugs are either selectively excluded from uptake or from the RT–primer–template catalytic complex. Drug discrimination is, for the most part, relatively specific for individual drugs. Repositioning of the template–primer to prevent a catalytically competent complex in the presence of a bound drug molecule has also been observed in some instances, and forms a second mechanism. The third, and potentially most significant for long-term efficacy of the NRTIs, is pyrophosphorolysis, the primary mode of resistance to zidovudine. Mutations selected by this drug or stavudine serve to elevate the natural rate of the reverse reaction for RT. Pyrophosphorolysis uncouples the last nucleoside monophosphate added to the proviral transcript, and attaches it to either a free pyrophosphate (regenerating a deoxynucleoside triphosphate) or to a nucleoside di- or triphosphate (usually ATP). Uncoupling a chain-terminating NRTI residue therefore rescues reverse transcription and reduces drug susceptibility across the class, since the process is not specific for the selecting drug. Of all the nucleoside-associated mutations, the best known and most studied are the six associated with thymidine analogue exposure. These six mutations (M41L, D67N, K70R, L210W, T215Y/F, K219Q) enhance RT pyrophosphorolysis to confer high-level viral resistance to zidovudine, and clinically significant loss of response to stavudine and didanosine. They have also been found to confer reduced susceptibility to lamivudine and abacavir, particularly when present alongside other NRTI-induced changes. Other key mutations generally confer more limited resistance to specific agents, although the primary lamivudine- and abacavir-associated M184V substitution generates a broad spectrum of drug-dependent phenotypes, and uncommon mutational complexes conferring resistance across the entire class are well known. In addition to ‘classical’ multi-nucleoside-resistant genotypes, database-driven ‘virtual phenotyping’ for accumulations of NRTI-associated mutations around a core of thymidine analogue-induced changes predicts drug susceptibilities below wild-type across the entire NRTI class, even in the absence of key mutations associated with individual agents. When the natural range of drug susceptibilities for treatment-naive isolates is used as the basis for defining resistance, retrospective analysis of clinical isolates in the Virco database shows a significantly increased incidence of reduced susceptibility for the dideoxy NRTIs (didanosine, stavudine and zalcitabine) that was undetected in previous assays. These data imply a cumulative degradation of response to NRTI drugs incident on the failure of thymidine analogue-based combinations, consistent with observations of treatment-experienced versus treatment-naive individuals. Among the investigational agents, response to tenofovir disproxil fumarate (TDF) appears to be essentially independent of baseline genotype in NRTI-experienced individuals, and its sole selected resistance mutation, K65R, has been observed to emerge only rarely (2%) and without loss of clinical response. In vitro results also show very little effect on TDF susceptibility for the most common of the multi-nucleoside resistance patterns. This drug has also been shown to display a substantially reduced sensitivity to pyrophosphorolytic uncoupling in vitro, which may, in part, explain the surprisingly sustained response observed over 48 weeks for TDF intensification of an existing regimen.

Phenotypic and genotypic resistance patterns of HIV-1 isolates derived from individuals treated with didanosine and stavudine

OBJECTIVE

To evaluate the phenotypic susceptibilities and genotypic resistance patterns to both didanosine and stavudine of baseline and follow-up HIV-1 isolate pairs, derived from antiretroviral naive subjects treated with this dual nucleoside combination.

DESIGN AND METHODS

Phenotypic drug susceptibility testing was performed in peripheral blood mononuclear cells on 34 viral isolate pairs derived from patients participating in the BMS AI-460 trial. Sequencing of the complete reverse transcriptase of 36 study isolate pairs, baseline and follow-up, was performed using standard dideoxy techniques.

RESULTS

The mean fold change in susceptibilities to didanosine was 1.6 (P= 0.278) and to stavudine 1.9 (P= 0.002, Wilcoxon's signed rank test). Mutations classically associated with zidovudine resistance were observed to emerge in 7 out of 36 isolates, T215Y/F (four), M41L +T215Y/F (two) and D67N (one). Other mutations observed included the A62V, V751, F77L, F116Y, Q151 M multinucleoside resistance complex (one), the Q151M mutation (two) and the rare V75T mutation (two). No mutations classically associated with didanosine exposure and resistance were observed. No relationship was evident between the emergence of zidovudine associated mutations and the level of phenotypic resistance to either stavudine or didanosine or between the emergence of zidovudine associated mutations and changes in plasma HIV RNA levels.

CONCLUSION

These comprehensive data demonstrate modest (< twofold) mean reductions in didanosine and stavudine susceptibilities at follow-up. The emergence of zidovudine associated mutations in this retroviral-naive population treated with combination didanosine and stavudine therapy is notable. Furthermore, the emergence of these mutations and of the Q151 M multinucleoside resistance complex raise concerns for potential nucleoside analog cross-resistance. The potential mechanisms driving the selection of the zidovudine associated mutations in the setting of didanosine and stavudine therapy and the relevance of these findings to current three and four drug regimens merit further evaluation.

Improved uptake and retention of lipophilic prodrug to improve treatment of HIV

Dideoxynucleosides currently in use for anti-HIV therapy have been found to be inefficient in passing through the blood-brain barrier to enter and maintain therapeutic drug levels in brain, a very significant reservoir of HIV. The low bioavailability of these drugs combined with the bone marrow toxicity of AZT (3'-azido, 3'-deoxythymidine, Zidovudine), resulting in anemia and leukopenia, pancreatitis with ddI (2',3'-dideoxyinosine, Didanosine) and painful peripheral neuropathy in case of ddC (2',3-dideoxycytosine, Zalcitabine) are the limiting factors in their use. In addition, the emergence of strains of HIV resistant to AZT, the most commonly used drug, further restricts its use. Thus the control of AIDS and its complications, needs special therapeutic approaches to combat the disease. In order to overcome these limitations, AZT and ddI have been synthesized as ester-linked ceramide- and phosphatidylcholine-linked prodrugs possessing therapeutic attributes lacking in the parent compounds. There is greater uptake and longer retention of these prodrugs in NIH/3T3 cells in vitro. Pretreatment with our prodrugs blocked infection of these cells by Moloney murine leukemia virus (M-MuLV) for an extended period, which the parent drugs failed to do. When human CD4+ HeLa cells were continuously exposed to the AZT prodrug, subsequent infection of these cells by HIV was blocked. Similar results were obtained with NIH/3T3 cells exposed to M-MuLV. AE(6)C, a prodrug of AZT linked to ceramide via a cleavable ester bond and a six carbon linker, was less toxic to both mouse and human bone marrow progenitor cells than free AZT. Most significantly, the prodrugs concentration was greater and the retention longer, in well known sanctuaries for HIV, such as the brain, testes and thymus.

Antiretroviral Drug Resistance in HIV-1

Progress in understanding antiretroviral resistance has evolved rapidly in recent years. Specific resistance mutations have been associated with virologic failure of different nucleoside reverse transcriptase inhibitors (NRTIs). These mutations vary in the extent of cross resistance they confer to other drugs in the same class. In addition, two novel mutational patterns conferring resistance to multiple NRTIs have been recognized. Considerable class-specific cross resistance also exists among viruses with reduced susceptibility to nonnucleoside reverse transcriptase inhibitors (NNRTIs). Among protease inhibitors, low level resistance that arises early during virologic failure may be drug specific in some situations, but high level resistance seen later during suboptimal therapy is likely to confer cross resistance to the entire class. Prevalence of drug resistance in infected patients appears to be considerable, and transmission of multidrug-resistant virus has been documented. Current methods of testing for resistance are promising, but they have significant limitations and require further clinical validation. The best approach to prevent resistance is to start treatment early during infection with a regimen that engenders good compliance and is potent enough to decrease viral load to below detection limits of the most sensitive assay available. Once resistance arises, salvage regimens in general have compromised efficacy and should be planned with attention to the patient's prior drug treatment history and the viruses' suspected or demonstrated resistance patterns.

Managing resistance to anti-HIV drugs: an important consideration for effective disease management

Current recommendations for the treatment of HIV-infected patients advise highly active antiretroviral therapy (HAART) consisting of combinations of 3 or more drugs to provide long-term clinical benefit. This is because only a complete suppression of virus replication will be able to prevent virus drug resistance, the main cause of drug failure. Virus drug resistance may remain a cause of concern in patients who have already received suboptimal mono- or bitherapy, or for patients who do not experience complete shut-down of virus replication under HAART. For these patients, replacement of one combination therapy regimen by another at drug failure, taking into account the existing resistance profile, will be needed. The development of new drugs will remain necessary for those patients who have failed to respond to all currently available drugs, as will be the institution of more effective and less toxic HAART regimens.

Incidence of discontinuation of highly active antiretroviral combination therapy (HAART) and its determinants

OBJECTIVE

To determine the incidence and determinants for discontinuation of initial highly active antiretroviral therapy (HAART).

DESIGN

In this retrospective follow-up study from hospital files and pharmacy dispensing data, a standard dataset was collected including patient characteristics, therapy characteristics, and HIV-monitoring parameters (e.g., CD4+ lymphocyte counts, viral load determinations). Kaplan-Meier estimates of the cumulative probability of discontinuation of initial HAART were calculated. Cox proportional hazard analysis was used to identify determinants for discontinuation of initial HAART.

PATIENTS

All patients starting HAART (n = 99) during June 1996 to February 1997 at our regional AIDS center.

MAIN OUTCOME MEASURES

Incidence and determinants for discontinuation of HAART.

RESULTS

During the mean follow-up of 450+/-10 days, 27 patients switched initial HAART, 3 patients stopped any antiretroviral therapy. Reasons for switching were increasing viral load (18x), insufficient decrease of viral load (3x), and adverse events (6x). Nonnaivete for antiretroviral therapy and a lower CD4+ lymphocyte count at start were identified as determinants for discontinuation of initial HAART.

CONCLUSIONS

The overall incidence density for discontinuation of initial HAART was 25 per 100 patients/year. The main reason for switching was an increasing viral load. CD4+ lymphocyte counts at start and nonnaivete for antiretroviral therapy were identified as determinants for discontinuation.

A combinatorial ledge: reverse transcriptase fidelity, total body viral burden, and the implications of multiple-drug HIV therapy for the evolution of antiviral resistance

The chronicity, high mutation rates, and high circulating titers of HIV during the 'stable' phase of infection make rapid evolution of resistance mutations a key predictor of antiretroviral efficacy. Recent advances in measurement of viral RNA titers, turnover dynamics and the in vivo spectrum of resistance mutations allow realistic in vivo estimates of important kinetic parameters of within-patient evolution of viral resistance. First-order estimates of the frequency of viral genotypes necessary for resistance to many antiretroviral combination regimens indicate that many such genotypes pre-exist in patients prior to initiation of therapy. The combinatorial nature of observed multiply-resistant genotypes, however, along with current estimates of total-body viral load and viral turnover dynamics, imply a strikingly sharp transition associated with the change from two-drug to three-drug antiretroviral regimens: pre-existing resistance being near-certain in the first instance but highly unlikely in the second. This abrupt change, a 'combinatorial ledge', carries with it a number of important implications for the understanding and control of HIV infection and other potential targets of antiviral therapy.

Selective vertical transmission of HIV-1 antiretroviral resistance mutations

OBJECTIVE

To examine the patterns of vertical transmission of zidovudine (ZDV) resistance mutations.

DESIGN

HIV-1 reverse transcriptase codons 10-250 were sequenced from 24 pairs of ZDV-exposed women and their HIV-infected infants as part of the Women and Infants Transmission Study.

METHODS

Viral RNA was extracted from tissue culture supernatants and sequenced using fluorescent dye-primer chemistry and an automated sequencer.

RESULTS

For 17 of these pairs, maternal and infant sequences were identical to one another and lacking known ZDV resistance mutations. The remaining seven maternal sequences contained known mutations associated with ZDV resistance at reverse transcriptase codons 70, 210, 215 and 219. In each case where the maternal HIV isolate showed a pure mutant species, the infant sequence was identical. When the maternal sequence showed the presence of a sequence mixture at codon 70 or 219, the infant's virus showed only wild-type sequence even when the ZDV-resistant mutant was quantitatively dominant in the mother. The single maternal HIV isolate showing mixed sequence at codon positions 210 and 215 transmitted an unmixed mutant to the infant at both positions. When maternal mixtures were present at sites not associated with ZDV resistance, only the dominant species appeared in the infant.

CONCLUSIONS

When maternal HIV isolates contained mixed wild-type and ZDV-resistant subpopulations, only a single component of the mixture could be detected in the infected infants. Resistance mutants without the codon 215 mutation were not transmitted from mixtures, even when the mutants formed the majority of circulating maternal virus. In perinatal HIV transmission, specific ZDV-resistant HIV genotypes circulating in the maternal virus pool may influence whether infection in the infant will be established by a wild-type or ZDV-resistant HIV strain.

Efficacy and safety of stavudine and didanosine combination therapy in antiretroviral-experienced patients

OBJECTIVES

To assess the efficacy, tolerance, and safety of combination antiretroviral therapy with didanosine and stavudine in HIV-infected patients with CD4+ cell counts > 100 x 10(6)/l and HIV plasma RNA > 10(4) copies/ml previously treated with other antiretroviral agents for at least 3 months.

DESIGN

In this open, multicentre, non-randomized, Phase II pilot study, adult patients were administered didanosine (200 mg twice daily) plus stavudine (40 mg twice daily) for 6 months. Patients for whom the first regimen had led to undetectable HIV RNA levels were offered a second 6-month course of treatment; those who had achieved insufficient immunological and virological gains in the first 6 months were given a new combination.

METHODS

Primary evaluation of efficacy was based on viral load measured by branched DNA second-generation testing (lower limit of detection, 500 copies/ml) and CD4+ cell counts; secondary evaluations included AIDS-defining events and clinical side-effects.

RESULTS

Sixty-five patients with median prior antiretroviral therapy of 24 months (65 with zidovudine, 29 with zalcitabine) were included in the study. At baseline, median CD4+ cell count was 198 x 10(6)/l and median plasma HIV RNA was 80000 copies/ml (4.9 log10 copies/ml). In this heavily pretreated population, an increase in the mean CD4+ cell count was observed (+70 x 10(6)/l at 24 weeks). In addition, rapid and prolonged antiviral activity was seen, with a mean maximal decrease of 1.1 log10 copies/ml at week 4, a mean decrease of 0.89 log10 copies/ml at week 24, and a plasma RNA viraemia < 500 copies/ml achieved in 14% of patients at week 24.

CONCLUSIONS

Combination therapy with stavudine and didanosine is safe and leads to a sustained antiviral effect, even in patients with prolonged prior antiretroviral exposure and low CD4+ cell counts.

Pharmaceutical properties of related calanolide compounds with activity against human immunodeficiency virus

The present studies were undertaken to compare the relative pharmacokinetic parameters and bioavailability of two chemically related natural products which are nonnucleoside inhibitors of reverse transcriptase. Both (+)-calanolide A (Cal A; NSC 675451) and (+)-dihydrocalanolide A (DHCal A; NSC 678323) are currently under development for the treatment of HIV infections. HPLC-based analytical assays were developed for both compounds using modifications of a previously published procedure. The assays were used to compare the intravenous pharmacokinetics of the dihydro analogue relative to the parent compound, Cal A, and to determine the relative oral bioavailability of each drug in CD2F1 mice. Although the pharmacokinetic parameters of each drug were similar (Cal A, 25 mg/kg: AUC: 9.4 [microg/mL]. hr, t1/2beta: 0.25 h,, t1/2gamma: 1.8 h, clearance: 2.7 L/h/kg versus DHCal A, 25 mg/kg: AUC: 6.9 [microg/mL].hr, t1/2beta: 0.22 h,, t1/2gamma: 2.3 h, clearance: 3.6 L/h/kg), the oral bioavailability of DHCal A (F = 46. 8%) was markedly better than that obtained for Cal A (F = 13.2%). The relative ability of Cal A and DHCal A to change to their inactive epimer forms, (+)-calanolide B and (+)-dihydrocalanolide B, respectively, was also determined. While conversion of active to inactive forms of the drugs was noted to occur in vitro especially under acidic conditions, no epimer forms of either compound were noted in plasma of mice after administration of either CalA or DHCal A. Considered together with preliminary toxicology findings, the pharmacokinetic data obtained in the present series of experiments suggest that selection of the dihydro derivative of (+)-calanolide A may be a reasonable choice for further preclinical development and possible Phase I clinical evaluation.

The M184V mutation in HIV-1 reverse transcriptase (RT) conferring lamivudine resistance does not result in broad cross-resistance to nucleoside analogue RT inhibitors

OBJECTIVE

To investigate the prevalence and magnitude of M184V-mediated changes in susceptibility to zalcitabine, didanosine, stavudine and abacavir (1592U89 succinate) in a cohort of lamivudine-treated patients.

DESIGN AND METHODS

A total of 255 samples from patients treated with lamivudine and zidovudine with or without other nucleoside reverse transcriptase inhibitors (NRTI) were analysed for susceptibility to zidovudine, lamivudine, zalcitabine, didanosine and stavudine using a recombinant virus assay. Seventy-three samples originated from patients exposed to zidovudine and lamivudine only. A subset of 27 samples was investigated for cross-resistance to abacavir. Resistance was defined as a change in median inhibitory concentration more than fivefold compared with wild-type (high-level resistance, > 10-fold). A genotypic analysis of plasma-derived reverse transcriptase coding regions was carried out in samples with cross-resistance.

RESULTS

The majority of samples displayed wild-type or greater than wild-type sensitivity to zalcitabine, didanosine and stavudine: resistance was seen in 17.2, 9 and 6.3% of the total sample population, respectively. Of these, 1.2, 2.7 and 2.4%, respectively, showed high-level resistance. The prevalence of resistance to a particular NRTI was lower in samples from patients not pretreated with that NRTI and in samples from patients exposed to zidovudine-lamivudine only. Cross-resistance was more prevalent in samples with high ZDV resistance. There was no obvious correlation between cross-resistance and genotype; all but two samples were mutant at codon 184. There were no consistent changes at positions associated with zidovudine resistance. The majority of samples from a subset (n=27) were four- to eightfold less sensitive to abacavir. There were no other genotypic changes in addition to M184V known to be associated with abacavir resistance.

CONCLUSIONS

Cross-resistance was not commonly observed in this lamivudine-treated cohort. M184V per se is not expected to compromise subsequent treatment with NRTI such as didanosine-stavudine or combinations containing abacavir.

Anti-human immunodeficiency virus drug combination strategies

It is now generally accepted that mono- and bitherapy for human immunodeficiency virus type 1 (HIV-1) infection are only transiently efficient mainly due to virus drug resistance. To obtain a sustained benefit from antiviral therapy, current guidelines recommend at least triple-drug combinations, or the so-called highly active antiretroviral therapy (HAART). In some patients, HAART can be problematic, either because it is difficult to remain compliant or because previous suboptimum therapies have limited the choice of drugs. For compliant drug-naive patients, HAART should be able to offer long-term virus suppression, when changing from first- to second- to third-line HAART at drug failure. Long-term treatment might ultimately result in multi-drug resistant virus leaving few options for salvage therapy. HIV drug resistance testing to guide this salvage therapy and the development of new drugs to allow new options will therefore remain priorities in anti-HIV drug research.

Antiretroviral therapy for HIV infection. A knowledge-based approach to drug selection and use

In the absence of evidence that eradication of HIV from an infected individual is feasible, the established goal of antiretroviral therapy is to reduce viral load to as low as possible for as long as possible. Achieving this with the currently available antiretroviral agents involves appropriate selection of components of combination regimens to obtain an optimal antiviral response. In addition, consideration of a plan for a salvage or second-line regimen is required if initial therapy fails to achieve an optimal response or should loss of virological control occur despite effective initial therapy. Such a planned approach, based on consideration of the likely modes of therapeutic failure (viral resistance, cellular resistance, toxicity) could be called rational sequencing. Choice of therapy should never involve compromise in terms of activity. However, the choice of drug should also be guided by tolerability profiles and considerations of coverage of the widest range of infected cells, compartmental penetration, pharmacokinetic interactions and, importantly, the ability of an agent or combination to limit future therapeutic options through selection of cross-resistant virus. Available clinical end-point data clearly indicate that combination therapy is superior to monotherapy, with clinical and surrogate marker data supporting the use of triple drug (or double protease inhibitor) combinations over double nucleoside analogue combinations. Thus, 3-drug therapy should represent current standard practice in a nontrials setting. Treatment should be considered as early as practical, and may be best guided by measurement of viral load, with a range of other markers having potential utility in individualising treatment decisions. Therapeutic failure may be defined clinically, immunologically or, ideally, virologically, and should prompt substitution of at least 2, and preferably all, components of the treatment regimen. Drug intolerance may also be best managed by rational substitution.

(S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC) inhibits HIV-1 replication in epithelial cells, but not T-lymphocytes

PMEA [9-(2-phosphonylmethoxyethyl)adenine] inhibited both HSV-1 and HIV-1 replication in MT-2 and HeLa-CD4 cells. (S)-1-[3-hydroxy-2-(phosphonylmethoxy)propyl]cytosine (HPMPC) inhibited both these viruses in the epithelioid HeLa-CD4 cells, but did not inhibit either virus in the T-lymphocytic MT-2 cells. PMEA and HPMPC are metabolized to their diphosphorylated forms within cells, which then inhibit viral polymerases. We therefore compared the metabolism of PMEA and HPMPC in MT-2 and HeLa CD4 cells. PMEApp formation was efficient in both the cell types, whereas HPMPCpp levels were approximately 3-10 fold lower in MT-2 cells, compared to HeLa-CD4 cells. These results indicate that HPMPC can inhibt HIV replication in the appropriate cell types, and show that differences in their metabolism cannot account entirely for the lack of antiviral efficacy of HPMPC in MT-2 cells.

Prevalence and incidence of resistance to zidovudine and other antiretroviral drugs

Drug-resistant human immunodeficiency virus (HIV)-1 has been detected in patients on all of the currently available antiretroviral drug regimens. Continuous, high-level virus replication with an error-prone reverse transcriptase enzyme and potential viral recombination events lead to each patient having numerous viral quasispecies and promote the emergence of drug-resistant strains. Drug resistance is associated with one or more point mutations in the HIV gene of the protein that is targeted by the drug. Factors associated with rapid emergence of drug resistance include host factors, such as advanced HIV disease and low CD4 cell counts; viral factors, such as high plasma HIV RNA, pre-existing drug-resistant virus, and possibly syncytium-inducing (SI) phenotype; and drug-related factors, such as suboptimal drug levels or poor compliance. High-level drug resistance has emerged after weeks to months of therapy for lamivudine (3TC) and nevirapine where drug-resistant quasispecies pre-exist in essentially all patients. Resistance has emerged more slowly for zidovudine (ZDV) and HIV protease inhibitors, which require the sequential accumulation of multiple mutations to develop high-level resistance. Certain drugs, such as didanosine (DDI), dideoxycytidine (DDC), and stavudine (D4T) have only produced viruses with low-level resistance, despite prolonged therapy. Development of drug-resistant HIV-1 has been associated with declining CD4 cell counts and rising plasma viral load. Zidovudine-resistant HIV-1 has been associated with adverse clinical outcomes independent of baseline CD4 cell counts and plasma HIV-1 RNA levels. Combination therapy offers the possibility of delaying or preventing the development of HIV drug resistance via interacting drug resistance mutations or potent antiviral activity. Widespread use of ZDV has been associated with transmission of ZDV-resistant HIV-1 in approximately 10% of adult seroconverters and a significant percentage of infants who fail the AIDS Clinical Trial Group (ACTG) 076 prophylactic regimen.

Analysis of HIV-2 RT mutants provides evidence that resistance of HIV-1 RT and HIV-2 RT to nucleoside analogs involves a repositioning of the template-primer

Mutations that confer resistance to nucleoside analogs do not cluster around the deoxynucleotide triphosphate (dNTP) binding site. Instead, these mutations appear to lie along the groove in the enzyme where the template-primer binds. Based on such structural data and on complementary biochemical analyses, it has been suggested that resistance to nucleoside analogs involves repositioning of the template-primer. We have prepared mutations in HIV-2 RT that are the homologs of mutations that confer resistance to nucleoside analogs in HIV-1 RT. Analysis of the behavior of HIV-2 RT mutants (Leu74Val, Glu89Gly, Ser215Tyr, Leu74Val/Ser215Tyr and Glu89Gly/Ser215Tyr) in vitro confirms the results obtained with HIV-1 RT: resistance is a function of the length of the template overhang. These analyses also suggest that the homolog in HIV-2 RT of one of the mutations that confers resistance to AZT in HIV-1 RT (Thr215Tyr) confers resistance by repositioning of the template-primer.

Human immunodeficiency virus type 1 reverse transcriptase genotype and drug susceptibility changes in infected individuals receiving dideoxyinosine monotherapy for 1 to 2 years

The genetic mechanisms of human immunodeficiency virus type 1 (HIV-1) resistance to dideoxyinosine (ddI) in vivo have been described based on data from primary HIV-1 isolates. To better define the spectrum of HIV-1 reverse transcriptase (RT) changes occurring during ddI therapy, we determined the genotype and ddI susceptibility of the RT gene of HIV RNA isolated from the plasma of 23 patients who had received 1 to 2 years (mean, 87 +/- 16 weeks) of ddI monotherapy. Population-based sequencing of plasma virus showed that 12 of 23 (52%) patients developed known ddI resistance mutations: L74V (7 patients), K65R (2 patients), L74V with M184V (3 patients), and L74V with K65R (1 patient). Five patients developed one or more known zidovudine resistance mutations (at codons 41, 67, 70, 215, and/or 219) during the study. Other amino acid substitutions were found, but only S68G and L210W occurred in more than one patient. Studies of sensitivity to ddI were performed on population-based recombinant-virus stocks generated by homologous recombination between a plasmid containing an HXB2 clone with the RT gene deleted and RT-PCR products of the RT genes from patients' plasma RNA. The sequences of the virus stocks produced by this procedure were typically identical to the sequence of the input PCR product from plasma RNA. Both an MT-2 cell-based culture assay and a cell-free virion-associated RT inhibition assay showed that viruses possessing an L74V and/or M184V mutation or a K65R mutation had reduced sensitivity to ddI. Viruses without these specific mutations had no change in sensitivity to ddI. The results presented here show that the spectrum of RT mutations in a population of patients on ddI monotherapy is more complex than previously described. The development of multiple mutational patterns, including those that confer resistance to other nucleoside analogs, highlights the complexity of using the currently available nucleoside analogs for antiretroviral therapy.

Evolution of human immunodeficiency virus type 1 env sequence variation in patients with diverse rates of disease progression and T-cell function

We examined the relationship between env sequence variation and disease progression in 10 human immunodeficiency virus type 1 (HIV-1)-seropositive subjects selected from a longitudinal cohort receiving zidovudine therapy. Five subjects were chosen for stable clinical status and CD4 counts (slow progressors), and five were selected for rapid clinical deterioration and CD4 count decline (rapid progressors). The slow progressors had significantly lower plasma viral RNA loads and greater lymphoproliferative responses to mitogens than the rapid progressors. DNA sequences representing the C1 through C3 regions of env were amplified from two peripheral blood mononuclear cell DNA samples from each subject separated by an average of 2.5 years. Molecular clones of these amplicons were then sequenced, and DNA sequence and deduced amino acid sequence distances were compared. Inter-time point sequence comparison showed a higher rate of sequence evolution for the rapid progressors in three of five matched pairs of rapid progressors and slow progressors and for the slow progressors in the remaining two subject pairs. However, intra-time point sequence comparisons showed that four of five slow progressors developed a more diverse quasispecies over time and one showed no change. In contrast, four of five rapid progressors showed no change in quasispecies diversity over time and one showed a significant decrease in diversity. The overall C1 through C3 region quasispecies diversity in the slow progressors at baseline was lower than that for the rapid progressors, but this difference was not significant at the follow-up time points. These diversity relationships were obscured if sequence analyses were limited to the 300-bp C2 to V3 region. Thus, HIV-1 quasispecies diversity increased over time in subjects with more functional immune systems.

Zidovudine alone or in combination with didanosine or zalcitabine in HIV-infected patients with the acquired immunodeficiency syndrome or fewer than 200 CD4 cells per cubic millimeter. Investigators for the Terry Beirn Community Programs for Clinical Research on AIDS

BACKGROUND

We compared two combinations of nucleosides with zidovudine alone in patients with advanced human immunodeficiency virus (HIV) infection.

METHODS

A total of 1102 patients with the acquired immunodeficiency syndrome or fewer than 200 CD4 cells per cubic millimeter were randomly assigned to receive zidovudine alone or zidovudine combined with either didanosine or zalcitabine. Disease progression, survival, toxic effects, and the CD4 cell response were assessed.

RESULTS

After a median follow-up of 35 months, disease progression or death occurred in 62 percent of the 363 patients assigned to zidovudine plus didanosine, 63 percent of the 367 assigned to zidovudine plus zalcitabine, and 66 percent of the 372 assigned to zidovudine only (P=0.24). As compared with zidovudine therapy, treatment with zidovudine plus didanosine was associated with a relative risk of disease progression or death of 0.86 (95 percent confidence interval, 0.71 to 1.03), and treatment with zidovudine plus zalcitabine was associated with a relative risk of 0.92 (95 percent confidence interval, 0.76 to 1.10). Survival was similar in the three groups. In a subgroup analysis, combination therapy delayed disease progression or death in patients who had previously received zidovudine for 12 months or less. Therapy with zidovudine plus didanosine resulted in more gastrointestinal adverse effects, and treatment with zidovudine plus zalcitabine, more neuropathy. The mean increases in CD4 cell counts at two months were higher with combination therapy than with zidovudine alone.

CONCLUSIONS

In patients with advanced HIV infection, combination therapy with zidovudine and either didanosine or zalcitabine is not superior to zidovudine therapy alone. However, these combinations may be more effective than zidovudine monotherapy in patients with little or no previous zidovudine treatment.

Reverse transcriptase genotype and antiretroviral susceptibility of human immunodeficiency virus isolates from patients with advanced disease treated with didanosine: correlation with virologic response and survival

To identify correlates of virologic response and survival, the reverse transcriptase (RT) genotype and in vitro antiviral susceptibility of human immunodeficiency virus (HIV) isolates from 20 patients treated with didanosine were studied. Patients had advanced HIV disease and were intolerant to or had failed zidovudine. Neither RT genotype nor antiviral susceptibility testing, as determined by a peripheral blood mononuclear cell-based assay, correlated with a virologic response to didanosine, as determined previously by quantitative serum culture. Only one (8%) of 12 isolates obtained after 6-12 months of treatment showed mutation at codon 74 conferring didanosine resistance. Reversions were seen in three of five patients with pre-treatment zidovudine resistance mutations at codons 70, but in none of eight with mutations at codon 215. Pretreatment isolates encoding mutations at RT codon 215 or encoding codon 123 asp were associated with both significantly greater CD4 lymphocyte depletion and shorter survival. In this cohort of patients with advanced HIV disease, neither rapid emergence of didanosine resistance nor rapid reversion of zidovudine resistance was observed. To better understand the relationship between virologic response and in vitro susceptibility to didanosine, more precise tools may be needed.

Use of viral resistance patterns to antiretroviral drugs in optimising selection of drug combinations and sequences

High rates of viral replication throughout HIV infection, and the frequency of mutation occurring during each replication cycle due to the inaccuracy of reverse transcriptase, drive the potential for drug-resistant viral variants to appear under the selective pressure of antiretroviral therapy. Loss of antiviral effect with a variety of antiretroviral agents has been reported to coincide with the appearance of viral mutants with reduced drug sensitivity. Additionally, the presence of both phenotypic and genotypic zidovudine resistance is associated with an increased risk of clinical disease progression and death, independent of a change of therapy to didanosine. The patterns of resistance to and cross-resistance between antiretroviral agents are increasingly well characterised, and represent an important consideration when deciding how to combine and/or sequence antiretrovirals to achieve optimal antiviral effects. Given the limited number of antiretrovirals currently available or in advanced development, it is important not to potentially limit future therapeutic options by using, early in the treatment sequence, therapies which may select for cross-resistant viral variants and hence potentially reduce the additional therapeutic response when treatment is changed to another member of that drug class.

Modeling combinations of antiretroviral agents in vitro with integration of pharmacokinetics: guidance in regimen choice for clinical trial evaluation

We propose a method for the selection of doses and dosing schedule for drugs to be used in combination. This approach uses the simulation of steady-state concentrations of the drugs in the combination and overlays these concentrations onto a three-dimensional effect surface. The MacSynergy II program is used to construct the three-dimensional drug interaction surface from the direct evaluation of drug combination effect in vitro. The study examined the combination of an inhibitor of the human immunodeficiency virus protease, A-77003, and the nucleoside analog zidovudine. Zidovudine concentrations from a steady-state interval were simulated on the basis of the administration of 100 mg every 12 h by mouth, while for A-77003 simulation profiles were for intravenous administration of 800 mg every 4 h as well as a continuous infusion of 200 mg/h. The average percentage of the maximal effect was taken as a measure of regimen effectiveness. Three different schedules of administration were examined. If both drugs were to be administered simultaneously, the model predicts a mean maximal effect of a steady-state interval (12 h) of 67%. If the drug doses were offset by 2 h, the mean maximal effect predicted was 71%. If A-77003 was to be given by continuous infusion, the mean maximal effect predicted was 90%. This method holds promise as a way of quickly evaluating potential combinations of agents that takes into account the drug interaction in a mathematically robust way and that allows the evaluation of the effect of each drug's pharmacokinetic profile.

Synthesis, chromatographic resolution, and anti-human immunodeficiency virus activity of (+/-)-calanolide A and its enantiomers

The anti-HIV agent (+/-)-calanolide A (1) has been synthesized in a five-step approach starting with phloroglucinol [-->5-->6-->11-->18-->(+/-)-1], which includes Pechmann reaction, Friedel-Crafts acylation, chromenylation with 4,4-dimethoxy-2-methylbutan-2-ol, cyclization, and Luche reduction. Cyclization of chromene 11 to chromanone 18 was achieved by employing either acetaldehyde diethyl acetal or paraldehyde in the presence of trifluoroacetic acid and pyridine or PPTS. Luche reduction of chromanone 18 at lower temperature preferably yielded (+/-)-1. Reduction of chromone 12, synthesized by Kostanecki-Robinson reaction from chromene 11, failed to afford (+/-)-1. The synthetic (+/-)-1 has been chromatographically resolved into its optically active forms, (+)- and (-)-1. The anti-HIV activities for synthetic (+/-)-1, as well as resultant (+)- and (-)-1, have been determined. Only (+)-1 accounted for anti-HIV activity, which was similar to the data reported for the natural product, and (-)-1 was inactive.

Determination of human immunodeficiency virus RNA in plasma and cellular viral DNA genotypic zidovudine resistance and viral load during zidovudine-didanosine combination therapy

Eleven human immunodeficiency virus (HIV)-infected subjects on long-term zidovudine (ZDV) therapy had didanosine (ddI) added to their antiretroviral regimen. HIV RNA in plasma was quantitated by branched-DNA signal amplification assay. Peripheral blood mononuclear cell (PBMC) HIV viral DNA was quantitated by PCR. The relative amounts of wild-type (WT) sequence, ddI resistance-associated codon changes (reverse transcriptase [RT] gene codon 65 K-->R [RT K65R], RT 174V, RT I135K/T/V, and RT M184I/V), and ZDV resistance-associated codon change (RT T215Y/F) from HIV RNA in plasma and RT T215Y/F from PBMC viral DNA were determined by differential hybridization of PCR products from 10 of 11 subjects. All subjects had evidence of RT T215Y/F mutation in both RNA in plasma and PBMC DNA at baseline. Subjects with a mixture of WT and RT T215Y/F HIV RNA in plasma at baseline demonstrated a decline in RNA levels in plasma after the addition of ddI. However, after 6 months of ZDV-ddI therapy, WT HIV RNA in plasma was undetectable in all subjects who had demonstrated a mixture at baseline. Subjects with only RT T215Y/F RNA present in plasma at baseline remained so and demonstrated no decline in RNA levels in plasma. In all subjects, no significant changes in PBMC DNA viral load and RT T215Y/F or WT levels were seen. HIV RNA in plasma demonstrated a significantly higher RT T215Y/F mutant/WT ratio than that of PBMC viral DNA, both at baseline and after ZDV-ddI combination therapy in all subjects. No subjects developed mutations associated with ddI resistance at codons 65, 74, 135, and 184 during this study. This study suggests that determination of relative amounts of RT T215Y/F and WT species from HIV RNA in plasma at baseline may be predictive of virologic response during ZDV-ddI combination therapy.

Antiviral therapy for human immunodeficiency virus infections

Depending on the stage of their intervention with the viral replicative cycle, human immunodeficiency virus inhibitors could be divided into the following groups: (i) adsorption inhibitors (i.e., CD4 constructs, polysulfates, polysulfonates, polycarboxylates, and polyoxometalates), (ii) fusion inhibitors (i.e., plant lectins, succinylated or aconitylated albumins, and betulinic acid derivatives), (iii) uncoating inhibitors (i.e., bicyclams), (iv) reverse transcription inhibitors acting either competitively with the substrate binding site (i.e., dideoxynucleoside analogs and acyclic nucleoside phosphonates) or allosterically with a nonsubstrate binding site (i.e., non-nucleoside reverse transcriptase inhibitors), (v) integration inhibitors, (vi) DNA replication inhibitors, (vii) transcription inhibitors (i.e., antisense oligodeoxynucleotides and Tat antagonists), (viii) translation inhibitors (i.e., antisense oligodeoxynucleotides and ribozymes), (ix) maturation inhibitors (i.e., protease inhibitors, myristoylation inhibitors, and glycosylation inhibitors), and finally, (x) budding (assembly/release) inhibitors. Current knowledge, including the therapeutic potential, of these various inhibitors is discussed. In view of their potential clinical the utility, the problem of virus-drug resistance and possible strategies to circumvent this problem are also addressed.

Limited sequence diversity of the HIV type 1 protease gene from clinical isolates and in vitro susceptibility to HIV protease inhibitors

Proviral DNAs from 3 laboratory strains and 21 clinical isolates of HIV-1 were extracted from infected cells after proteinase K digestion and the protease gene was PCR amplified and sequenced directly by the Sanger method. In vitro susceptibilities of the virus isolates to protease inhibitors were determined by the ACTG/DoD consensus assay. Four different HIV protease inhibitors were tested including P9941, a C2 symmetrical diol (Du Pont-Merck); A80987, an asymmetric mono-ol (Abbott); XM323, a cyclic urea (Du Pont-Merck); and Ro31-8959, an asymmetric hydroxyethylene isostere (Roche). Maximum sequence variation was 10% at both the nucleic and amino acid levels. Purine-purine substitutions were most common. Five noncontiguous regions were conserved across all isolates and corresponded to amino acids 1-9 (amino terminal), 21-32 (catalytic site), 47-56 ("flap" region), 78-88 (substrate-binding region), and 94-99 (carboxy terminal). All clinical isolates demonstrated in vitro susceptibility to the protease inhibitors. There was no significant difference between the susceptibility of the reference strains and the clinical isolates. These data suggest that the variable regions of protease do not contain sites that are important for interactions with the inhibitors tested.