Human immunodeficiency virus type 1 expressing the lamivudine-associated M184V mutation in reverse transcriptase shows increased susceptibility to adefovir and decreased replication capability in vitro

Negative effect of the M184V mutation in HIV-1 reverse transcriptase on initiation of viral DNA synthesis

The M184V mutation in HIV reverse transcriptase (RT) is associated with high-level resistance against the nucleoside inhibitor lamivudine as well as diminished viral replication capacity. We have previously demonstrated that HIV variants containing the M184V mutation were relatively unable to successfully undergo compensatory mutagenesis following deletion of an A-rich loop located upstream of the primer binding site (PBS). To understand the mechanisms involved, we synthesized viral RNA templates containing different compensatory mutations that were emergent during the long-term culture of the A-rich loop-deleted viruses. These templates were then used in cell-free reverse transcription initiation assays and in tRNA primer placement assays performed with either recombinant wild-type RT or recombinant RT containing the M184V substitution. The results showed that the RNA template that contained the A-rich loop deletion was impaired in ability to initiate reverse transcription and that the presence of the M184V substitution in RT amplified this effect. Clearance from pausing at position +3 during synthesis of viral DNA was identified as a sensitive step in this reaction that could not be efficiently bypassed with the M184V mutant enzyme. Increased efficiency of initiation was seen with the deleted RNA templates that also contained mutations identified in the revertant viruses, provided that these mutations facilitated formation of a competent binary tRNA/RNA complex. These findings provide biochemical evidence that initiation of tRNA(Lys3)-primed DNA synthesis is an important rate-limiting step in reverse transcription that correlates with viral replication fitness.

A current review of Ebola virus: pathogenesis, clinical presentation, and diagnostic assessment

Ebola hemorrhagic fever (EHF) is an acute viral syndrome that presents with fever and an ensuing bleeding diathesis that is marked by high mortality in human and nonhuman primates. Fatality rates are between 50% and 100%. Due to its lethal nature, this filovirus is classified as a biological class 4 pathogen. The natural reservoir of the virus is unknown. As a result, little is understood about how Ebola virus is transmitted or how it replicates in its host. Although the primary source of infection is unknown, the epidemiologic mode of transmission is well defined. A variety of tests have proven to be specific and useful for Ebola virus identification. There is no FDA-approved antiviral treatment for EHF. Incubation ranges from 2 to 21 days. Patients who are able to mount an immune response to the virus will begin to recover in 7 to 10 days and start a period of prolonged convalescence. Supportive management of infected patients is the primary method of treatment, with particular attention to maintenance of hydration, circulatory volume, blood pressure, and the provision of supplemental oxygen. Since there is no specific treatment outside of supportive management and palliative care, containment of this potentially lethal virus is paramount. In almost all outbreaks of EHF, the fatality rate among health care workers with documented infections was higher than that of non-health care workers.

Ebola virus: immune mechanisms of protection and vaccine development

Vaccination is one of our most powerful antiviral strategies. Despite the emergence of deadly viruses such as Ebola virus, vaccination efforts have focused mainly on childhood communicable diseases. Although Ebola virus was once believed to be limited to isolated outbreaks in distant lands, forces of globalization potentiate outbreaks anywhere in the world through incidental transmission. Moreover, since this virus has already been transformed into weapon-grade material, the potential exists for it to be used as a biological weapon with catastrophic consequences for any population vulnerable to attack. Ebola hemorrhagic fever (EHF) is a syndrome that can rapidly lead to death within days of symptom onset. The disease directly affects the immune system and vascular bed, with correspondingly high mortality rates. Patients with severe disease produce dangerously high levels of inflammatory cytokines, which destroy normal tissue and microcirculation, leading to profound capillary leakage, renal failure, and disseminated intravascular coagulation. Vaccine development has been fraught with obstacles, primarily of a biosafety nature. Case reports of acutely ill patients with EHF showing improvement with the transfusion of convalescent plasma are at odds with animal studies demonstrating further viral replication with the same treatment. Using mRNA extracted from bone marrow of Ebola survivors, human monoclonal antibodies against Ebola virus surface protein have been experimentally produced and now raise the hope for the development of a safe vaccine.

Adefovir Dipivoxil

Each month, subscribers to The Formulary Monograph Service receive five to six well-documented monographs on drugs that are newly released or are in late Phase III trials. The monographs are targeted to your Pharmacy and Therapeutics Committee. Subscribers also receive monthly one-page summary monographs on the agents that are useful for agendas and pharmacy/nursing in-services. A comprehensive target drug utilization evaluation (DUE) is also provided each month. The monographs are published in printed form and on diskettes that allow customization. Subscribers to the The Formulary Monograph Service also receive access to a pharmacy bulletin board, The Formulary Information Exchange (The F.I.X.). All topics pertinent to clinical and hospital pharmacy are discussed on The F.I.X.

Through the cooperation of The Formulary, Hospital Pharmacy publishes selected reviews in this column. If you would like information about The Formulary Monograph Service or The F.I.X., call The Formulary at 800–322–4349. The January 2003 monograph topics are ezetimibe, enfurvitide, aripiprazole, atomoxetine, and amoxicillin/clavulanate potassium extended-release tablets. The DUE is on ezetimibe.

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

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

Reversion of the M184V mutation in simian immunodeficiency virus reverse transcriptase is selected by tenofovir, even in the presence of lamivudine

The methionine-to-valine mutation in codon 184 (M184V) in reverse transcriptase (RT) of human immunodeficiency virus type 1 (HIV-1) or simian immunodeficiency virus (SIV) confers resistance to (-)-2'-deoxy-3'-thiacytidine (3TC; lamivudine) and increased sensitivity to 9-[2-(phosphonomethoxy)propyl]adenine (PMPA; tenofovir). We have used the SIV model to evaluate the effect of the M184V mutation on the emergence of resistance to the combination of 3TC plus PMPA. A site-directed mutant of SIVmac239 containing M184V (SIVmac239-184V) was used to select for resistance to both 3TC and PMPA by serial passage in the presence of increasing concentrations of both drugs. Under these selection conditions, the M184V mutation reverted in the majority of the selections. Variants resistant to both drugs were found to have the lysine-to-arginine mutation at codon 65 (K65R), which has previously been associated with resistance to PMPA in both SIV and HIV. Similarly, in rhesus macaques infected with SIVmac239-184V for 46 weeks and treated daily with (-)-2'-deoxy-5-fluoro-3'-thiacytidine [(-)-FTC], there was no reversion of M184V, but this mutation reverted to 184 M in all three animals within 24 weeks of treatment with (-)-FTC and PMPA. Although the addition of PMPA to the (-)-FTC therapy induced a decrease in virus loads in plasma, these loads eventually returned to pre-PMPA levels in each case. All animals receiving this combination developed the K65R mutation. These results demonstrate that the combination of PMPA with 3TC or (-)-FTC selects for the K65R mutation and against the M184V mutation in SIV RT.

Using a Database of HIV Patients Undergoing Genotypic Resistance Test after Haart Failure to Understand the Dynamics of M184V Mutation

Objective

M184V/I mutation is associated with high-level phenotypic resistance to lamivudine (3TC). The aim of the present analysis was to correlate the time of appearance/disappearance of M184V/I with duration of 3TC treatment.

Methods

Overall, 211 patients were selected from a database of HIV patients undergoing genotypic resistance test after virological failure of HAART regimens in two major reference centres in Rome between 1999 and 2001. At the time of genotyping, 120 of them (56.9%) were failing a 3TC-including HAART, while 91 (43.1%) received 3TC only in previous HAART. Duration of the current 3TC-containing regimen and the time from the end of last 3TC treatment to genotypic resistance test (GRT) were analysed.

Results

Among patients currently undergoing 3TC-containing HAART, the prevalence of M184V/I was 82.5% (78.3/4.2%, respectively) and significantly associated to current 3TC use at GRT. Prevalence of M184V/I was associated to longer history of 3TC (from 47.1% in patients treated with 3TC for <6 months, to 84.0% among those treated for 7–12 months; 100.0% of patients with >42 months of current 3TC carried M184V. At logistic regression analysis, the rate of increase of M184V/I in 3TC-failing patients was statistically significant (OR: 1.066 per month of current 3TC therapy, 95% CI: 1.020–1.114, P<0.01), suggesting a 6.6% monthly increase of probability of M184V/I. Among patients who interrupted 3TC, overall prevalence of M184V/I was 23.1%: proportion of patients carrying the M184V/I dropped from 83.3% among those who interrupted 3TC from ≤3 months, to 56.3, 20, 10.5 and 0% for those interrupting 3TC from 6, 12, 24 and ≥24 months, respectively. At logistic regression, the rate of disappearance of M184V/I was also statistically significant (OR: 0.883 per month, 95% CI: 0.804–0.970, P=0.01), indicating a 11.7% monthly decrease of probability of M184V/I after 3TC interruption.

Conclusions

Dynamics of appearance/disappearance of M184V/I mutation is rapid after 3TC failure/interruption, suggesting ease of development of such a mutation, but also suggesting a remarkable growth disadvantage for HIV. From the clinical perspective, recycling of drugs whose antiviral activity is affected by M184V mutation can be successful after appropriate drug wash-out, also in heavily pretreated patients.

Tenofovir: a nucleotide analog for the management of human immunodeficiency virus infection

Tenofovir disoproxil fumarate, an acyclic nucleotide analog of adenosine monophosphate, is the most recent addition to the antiretroviral arsenal. After conversion to tenofovir by diester hydrolysis, subsequent phosphorylation by cellular enzymes to form the active tenofovir diphosphate is necessary for antiretroviral activity. Preliminary data suggest that tenofovir is as safe and efficacious as stavudine when given in combination with lamivudine and efavirenz for the treatment of antiretroviral-naïve patients. In antiretroviral-experienced patients, the addition of tenofovir to stable background antiretroviral therapy resulted in approximately a 0.6 log10 copies/ml reduction in viral load relative to placebo. Extended follow-up suggests that such virologic gains may be durable. In vitro, recombinant human immunodeficiency virus (HIV) expressing the K65R mutation showed a 3-4-fold increase in the 50% inhibitory concentrations of tenofovir when compared with wild type. In vivo, this mutation thus far appears to occur infrequently and is associated with variable virologic responses. Response rates to tenofovir vary with the number and pattern of thymidine analog mutations present before starting treatment with this agent. Tenofovir appears to be a well-tolerated agent in patients who are heavily pretreated and who have advanced disease. The main adverse effects appear to be gastrointestinal in nature and include nausea, vomiting, and diarrhea. In animals, osteomalacia and nephrotoxicity have occurred with tenofovir at exposures much higher than those observed in humans. Although no patient had to discontinue therapy as a result of elevated creatinine levels or hypophosphatemia through 58 weeks of treatment, the toxicities associated with long-term tenofovir therapy in humans are unknown. Concomitant administration of tenofovir and didanosine increases the area under the concentration-time curve of the latter by 44-60%; monitoring for signs and symptoms of didanosine toxicity is recommended. The approved dosage of tenofovir is 300 mg (one tablet) once/day with meals. Given the ease of administration and relative safety from the perspectives of adverse effects and drug interactions, tenofovir has the potential to assume a large role in the treatment of patients with HIV infection.

Molecular mechanisms of resistance to human immunodeficiency virus type 1 with reverse transcriptase mutations K65R and K65R+M184V and their effects on enzyme function and viral replication capacity

Human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) resistance mutations K65R and M184V result in changes in susceptibility to several nucleoside and nucleotide RT inhibitors. K65R-containing viruses showed decreases in susceptibility to tenofovir, didanosine (ddI), abacavir, and (-)-beta-D-dioxolane guanosine (DXG; the active metabolite of amdoxovir) but appeared to be fully susceptible to zidovudine and stavudine in vitro. Viruses containing the K65R and M184V mutations showed further decreases in susceptibility to ddI and abacavir but increased susceptibility to tenofovir compared to the susceptibilities of viruses with the K65R mutation. Enzymatic and viral replication analyses were undertaken to elucidate the mechanisms of altered drug susceptibilities and potential fitness defects for the K65R and K65R+M184V mutants. The relative inhibitory capacities (K(i)/K(m)) of the active metabolites of tenofovir, ddI, and DXG were increased for the RT containing the K65R mutation compared to that for the wild-type RT, but the relative inhibitory capacity of abacavir was only minimally increased. For the mutant viruses with the K65R and M184V mutations, the increase in tenofovir susceptibility compared to that of the mutants with K65R correlated with a decrease in the tenofovir inhibitory capacity that was mediated primarily by an increased K(m) of dATP. The decrease in susceptibility to ddI by mutants with the K65R and M184V mutations correlated with an increase in the inhibitory capacity mediated by an increased K(i). ATP-mediated removal of carbovir as well as small increases in the inhibitory capacity of carbovir appear to contribute to the resistance of mutants with the K65R mutation and the mutants with the K65R and M184V mutations to abacavir. Finally, both the HIV-1 K65R mutant and, more notably, the HIV-1 K65R+M184V double mutant showed reduced replication capacities and reduced RT processivities in vitro, consistent with a potential fitness defect in vivo and the low prevalence of the K65R mutation among isolates from antiretroviral agent-experienced patients.

HIV-1 drug resistance: can we overcome?

Highly active antiretroviral therapy (HAART) targeting the viral reverse transcriptase and protease enzymes has advanced the treatment of HIV/AIDS. Nucleoside and non-nucleoside reverse transcriptase inhibitors and protease inhibitors used in combination can suppress viral replication thereby delaying disease progression. Emergence of HIV-1 mutated strains, resistant to one or more antiretroviral inhibitors or drug classes, remains one of the leading causes of treatment failure among patients living with HIV/AIDS. While advances in genotypic and phenotypic testing allow for drug resistance guided therapeutic management, the increasing prevalence of multi-drug resistance and an absence of new drug classes forewarn new problems in sustaining the effectiveness of HAART. One promising hope for continued benefit of antiretroviral therapy despite emergent resistance is the observed reduction in replicative ability or 'fitness' of multimutated viruses. This review discusses the development and influence of known drug mutations on drug susceptibility versus viral fitness.

M184V is associated with a low incidence of thymidine analogue mutations and low phenotypic resistance to zidovudine and stavudine

The resistance of HIV clinical isolates with or without M184V was analysed in relation to plasma HIV-1-RNA level and time on therapy. The number of thymidine analogue mutations (TAMs) was lower in isolates with M184V, this was independent of plasma HIV-1-RNA level and time on therapy for T215F/Y, D67N and L210W. This suggests a direct effect of M184V on the reduced selection of TAMs. Lamivudine use was significantly associated with lower median fold resistance to zidovudine and stavudine.

ATP-dependent removal of nucleoside reverse transcriptase inhibitors by human immunodeficiency virus type 1 reverse transcriptase

Removal of nucleoside chain terminator inhibitors mediated by human immunodeficiency virus (HIV) reverse transcriptase (RT) using ATP as an acceptor molecule has been proposed as a novel mechanism of HIV resistance. Recombinant wild-type and mutant HIV type 1 (HIV-1) RT enzymes with thymidine analog resistance mutations D67N, K70R, and T215Y were analyzed for their ability to remove eight nucleoside reverse transcriptase inhibitors in the presence of physiological concentrations of ATP. The order for the rate of removal of the eight inhibitors by the mutant RT enzyme was zidovudine (AZT) > stavudine (d4T) >> zalcitabine (ddC) > abacavir > amdoxovir (DAPD) > lamivudine (3TC) > didanosine (ddI) > tenofovir. Thymidine analogs AZT and d4T were the most significantly removed by the mutant enzyme, suggesting that removal of these inhibitors by the ATP-dependent removal mechanism contributes to the AZT and d4T resistance observed in patients with HIV expressing thymidine analog resistance mutations. ATP-dependent removal of tenofovir was 22- to 35-fold less efficient than removal of d4T and AZT, respectively. The addition of ATP and the next complementary deoxynucleoside triphosphate caused a reduction of ATP-mediated removal of d4T, ddC, and DAPD, while AZT and abacavir removal was unaffected. The reduction of d4T, ddC, and DAPD removal in the presence of the deoxynucleoside triphosphate could explain the minor changes in susceptibility to these drugs observed in conventional in vitro phenotypic assays using cells that have higher deoxynucleoside triphosphate pools. The minimal removal of abacavir, ddC, DAPD, 3TC, ddI, and tenofovir is consistent with the minor changes in susceptibility to these drugs observed for HIV mutants with thymidine analog resistance mutations.

Genotypic and phenotypic analyses of HIV-1 in antiretroviral-experienced patients treated with tenofovir DF

OBJECTIVE

To evaluate the virologic responses and mutational profiles in antiretroviral-experienced patients adding tenofovir DF once-daily to their existing regimens.

DESIGN

Resistance analyses were performed for patients in a phase II placebo-controlled clinical trial of tenofovir DF.

METHODS

HIV-1 reverse transcriptase and protease genes from plasma samples were analyzed genotypically and phenotypically at baseline, week 24, and week 48.

RESULTS

Of 184 patients, 173 (94%) had baseline HIV-1 expressing one or more nucleoside reverse transcriptase inhibitor-associated resistance mutation. Protease inhibitor and non-nucleoside reverse transcriptase inhibitor (NNRTI) resistance mutations were observed in 57% and 32% of patients, respectively. Compared to placebo, significant reductions in HIV-1 RNA were observed for tenofovir DF-treated patients who had thymidine analog- (TAM), lamivudine- (M184V), NNRTI- or protease inhibitor-associated mutations. Patients with phenotypic susceptibility to tenofovir within 4-fold of wild-type responded durably to tenofovir DF 300 mg therapy with a decline in plasma HIV-1 RNA of > or = 0.5 log10 copies/ml; few patients had a more than 4-fold reduced susceptibility to tenofovir at baseline. Four patients (2%) developed the K65R mutation (selected by tenofovir in vitro) and showed 3- to 4-fold reductions in tenofovir susceptibility but no evidence of rebound viremia. Thirty-four percent of patients developed additional TAMs, coincident with concurrent zidovudine or stavudine therapy, but also showed durable HIV-1 reductions. There was no evidence of novel resistance to tenofovir.

CONCLUSIONS

Adding tenofovir DF 300 mg to an existing regimen in patients with ongoing viral replication and a wide range of genotypic resistance patterns resulted in significant and durable HIV-1 RNA reductions. In addition, there was a low incidence of genotypic or phenotypic resistance to tenofovir DF arising during 48 weeks of therapy.

Virulence and reduced fitness of simian immunodeficiency virus with the M184V mutation in reverse transcriptase

Drug-resistant mutants with a methionine-to-valine substitution at position 184 of reverse transcriptase (M184V) emerged within 5 weeks of initiation of therapy in four newborn macaques infected with simian immunodeficiency virus (SIVmac251) and treated with lamivudine (3TC) or emtricitabine [(-)-FTC] (two animals per drug). Thus, this animal model mimics the rapid emergence of M184V mutants of HIV-1 during 3TC therapy of human patients. One animal of each treatment group developed fatal immunodeficiency at 12 weeks of age, which is similar to the rapid disease course seen in most untreated SIVmac251-infected infant macaques. To further evaluate the effect of the M184V mutation on viral fitness and virulence, groups of juvenile macaques were inoculated with the molecular clone SIVmac239 with either the wild-type sequence (group A [n = 5]) or the M184V sequence (SIVmac239-184V; group B [n = 5] and group C [n = 2]). The two SIVmac239-184V-infected animals of group C did not receive any drug treatment, and in both animals the virus population reverted to predominantly wild type (184M) by 8 weeks after inoculation. The other five SIVmac239-184V-infected animals (group B) were treated with (-)-FTC to prevent reversion. Although virus levels 1 week after inoculation were lower in the SIVmac239-184V-infected macaques than in the SIVmac239-infected animals, no significant differences were observed from week 2 onwards. Two animals in each group developed AIDS and were euthanized, while all other animals were clinically stable at 46 weeks of infection. These data demonstrate that the M184V mutation in SIV conferred a slightly reduced fitness but did not affect disease outcome.

The impact of the M184V substitution in HIV-1 reverse transcriptase on treatment response

The M184V mutation in the HIV-1 reverse transcriptase gene is primarily associated with rapid, high-level lamivudine (3TC) resistance. It has also been observed to arise under selective pressure by abacavir, to which it confers low-level resistance. Although the development of viral drug resistance remains a major concern in antiretroviral therapy, it is known that some immunological and clinical benefit can still be derived from highly active antiretroviral therapy (HAART) regimens despite resistance-associated virological failure. This residual benefit on a failing regimen is commonly attributed to the preservation of fitness-reducing protease inhibitor (PI) resistance mutations under continued drug pressure. However, fitness-reducing nucleoside reverse transcriptase inhibitor (NRTI) mutations may also contribute to the effect. M184V is both common in the treated population and fitness-reducing. A number of studies, both of dual nucleoside therapy and HAART, have noted a residual treatment effect for 3TC despite the assumed or observed presence of M184V and high-level phenotypic resistance. The speed and consistency with which this mutation is selected by 3TC under suboptimal viral suppression therefore makes M184V a particularly interesting model for further clinical studies on the association of drug resistance with ongoing treatment benefit. While fitness considerations are likely to be a major contributor to the clinical observations noted, there are a number of other potential mechanisms that may contribute to a continuing response to 3TC in the presence of M184V. These include the delay and reversal of zidovudine (ZDV) resistance, hypersensitization to other NRTIs, reduced reverse transcriptase (RT) processivity and a possible reduction in RT pyrophosphorolysis. The full impact of M184V on therapeutic prospects will require further elucidation; ideally, the risk/benefit of preserving this substitution would be investigated in randomized trials. However, existing data suggest that the presence of this mutation may preserve some benefit in spite of the loss of 3TC susceptibility which, with further study, may prove valuable.

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.

Persistence and fitness of multidrug-resistant human immunodeficiency virus type 1 acquired in primary infection

This study examines the persistence and fitness of multidrug-resistant (MDR) viruses acquired during primary human immunodeficiency virus infection (PHI). In four individuals, MDR infections persisted over the entire study period, ranging from 36 weeks to 5 years, in the absence of antiretroviral therapy. In stark contrast, identified source partners in two cases showed expected outgrowth of wild-type (WT) virus within 12 weeks of treatment interruption. In the first PHI case, triple-class MDR resulted in low plasma viremia (1.6 to 3 log copies/ml) over time compared with mean values obtained for an untreated PHI group harboring WT infections (4.1 to 4.3 log copies/ml). Increasing viremia in PHI patient 1 at week 52 was associated with the de novo emergence of a protease inhibitor-resistant variant through a recombination event involving the original MDR virus. MDR infections in two other untreated PHI patients yielded viremia levels typical of the untreated WT group. A fourth patient's MDR infection yielded low viremia (<50 to 500 copies/ml) for 5 years despite his having phenotypic resistance to all antiretroviral drugs in his treatment regimen. In two of these PHI cases, a rebound to higher levels of plasma viremia only occurred when the M184V mutation in reverse transcriptase could no longer be detected and, in a third case, nondetection of M184V was associated with an inability to isolate virus. To further evaluate the fitness of MDR variants acquired in PHI, MDR and corresponding WT viruses were isolated from index and source partners, respectively. Although MDR viral infectivity (50% tissue culture infective dose) was comparable to that observed for WT viruses, MDR infections in each case demonstrated 2-fold and 13- to 23-fold reductions in p24 antigen and reverse transcriptase enzymatic activity, respectively. In dual-infection competition assays, MDR viruses consistently demonstrated a marked replicative disadvantage compared with WT virus. These results indicate that MDR viruses that are generated following PHI can establish persistent infections as dominant quasispecies despite their impaired replicative competence.

GCV resistance due to the mutation A594P in the cytomegalovirus protein UL97 is partially reconstituted by a second mutation at D605E

A ganciclovir (GCV)-resistant human cytomegalovirus (HCMV) was isolated from an AIDS patient. Molecular analysis of the HCMV UL97 gene revealed two point mutations, A594P and D605E, respectively. In order to evaluate quantitatively the impact of the individual mutations on GCV phosphorylation, recombinant vaccinia viruses (rVVs) were generated carrying either the two mutations (rVV-594/605) or only one mutation (rVV-594 or rVV-605, respectively). In cells infected with the rVV-594/605 double mutant, the GCV phosphorylation was decreased to 50% compared with the phosphorylation in cells infected with the rVV-UL97 wild-type. In cells infected with the rVV-594, however, the GCV phosphorylation was further decreased to 30%. Interestingly, the mutation D605E led to an even better GCV phosphorylation than that measured in cells infected with the rVV-UL97 wild type. These results were confirmed by plaque reduction assays, indicating that rVV-594 was more resistant to GCV than rVV-594/605. In contrast, rVV-605 was more sensitive to GCV than the rVV-UL97 wild type. Therefore, our results demonstrated for the first time that compensatory mutations can also occur in HCMV, as already shown for human immunodeficiency virus type 1.

Virus population dynamics, fitness variations and the control of viral disease: an update

Viral quasispecies dynamics and variations of viral fitness are reviewed in connection with viral disease control. Emphasis is put on resistance of human immunodeficiency virus and some human DNA viruses to antiviral inhibitors. Future trends in multiple target antiviral therapy and new approaches based on virus entry into error catastrophe (extinction mutagenesis) are discussed.

Phase i/ii trial of the pharmacokinetics, safety, and antiretroviral activity of tenofovir disoproxil fumarate in human immunodeficiency virus-infected adults

Tenofovir DF is an antiviral nucleotide with activity against human immunodeficiency virus type 1 (HIV-1). The pharmacokinetics, safety, and activity of oral tenofovir DF in HIV-1-infected adults were evaluated in a randomized, double-blind, placebo-controlled, escalating-dose study of four doses (75, 150, 300, and 600 mg given once daily). Subjects received a single dose of tenofovir DF or a placebo, followed by a 7-day washout period. Thereafter, subjects received their assigned study drug once daily for 28 days. Pharmacokinetic parameters were dose proportional and demonstrated no change with repeated dosing. Reductions in plasma HIV-1 RNA were dose related at tenofovir DF doses of 75 to 300 mg, but there was no increase in virus suppression between the 300- and 600-mg dose cohorts, despite dose-proportional increases in drug exposure. Grade III or IV adverse events were limited to laboratory abnormalities, including elevated creatine phosphokinase and liver function tests, which resolved with or without drug discontinuation and without sequelae. No patients developed detectable sequence changes in the reverse transcriptase gene.

Cross-resistance testing of antihepadnaviral compounds using novel recombinant baculoviruses which encode drug-resistant strains of hepatitis B virus

Long-term nucleoside analog therapy for hepatitis B virus (HBV)-related disease frequently results in the selection of mutant HBV strains that are resistant to therapy. Molecular studies of such drug-resistant variants are clearly warranted but have been difficult to do because of the lack of convenient and reliable in vitro culture systems for HBV. We previously developed a novel in vitro system for studying HBV replication that relies on the use of recombinant baculoviruses to deliver greater than unit length copies of the HBV genome to HepG2 cells. High levels of HBV replication can be achieved in this system, which has recently been used to assess the effects of lamivudine on HBV replication and covalently closed circular DNA accumulation. The further development of this novel system and its application to determine the cross-resistance profiles of drug-resistant HBV strains are described here. For these studies, novel recombinant HBV baculoviruses which encoded the L526M, M550I, and L526M M550V drug resistance mutations were generated and used to examine the effects of these substitutions on viral sensitivity to lamivudine, penciclovir (the active form of famciclovir), and adefovir, three compounds of clinical importance. The following observations were made: (i) the L526M mutation confers resistance to penciclovir and partial resistance to lamivudine, (ii) the YMDD mutations M550I and L526M M550V confer high levels of resistance to lamivudine and penciclovir, and (iii) adefovir is active against each of these mutants. These findings are supported by the limited amount of clinical data currently available and confirm the utility of the HBV-baculovirus system as an in vitro tool for the molecular characterization of clinically significant HBV strains.

In vitro susceptibilities of wild-type or drug-resistant hepatitis B virus to (-)-beta-D-2,6-diaminopurine dioxolane and 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil

Prolonged treatment of chronic hepatitis B virus (HBV) infection with lamivudine ([-]-beta-L-2',3'-dideoxy-3' thiacytidine) or famciclovir may select for viral mutants that are drug resistant due to point mutations in the polymerase gene. Determining whether such HBV mutants are sensitive to new antiviral agents is therefore important. We used a transient transfection system to compare the sensitivities of wild-type HBV and four lamivudine- and/or famciclovir-resistant HBV mutants to adefovir [9-(2-phosphonyl-methoxyethyl)-adenine; PMEA] and the nucleoside analogues (-)-beta-D-2, 6-diaminopurine dioxolane (DAPD) and 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil (L-FMAU). The drug-resistant mutants contained amino acid substitutions in the polymerase protein. We found that the M550I and M550V plus L526M substitutions, which confer lamivudine resistance, did not confer cross-resistance to adefovir or DAPD, but conferred cross-resistance to L-FMAU. The M550V substitution in isolation conferred a similar phenotype to M550I, except that it did not confer significant resistance to L-FMAU. The L526M substitution, which is associated with famciclovir resistance, conferred cross-resistance to L-FMAU but not to adefovir or DAPD. Inhibition of HBV secretion by DAPD, L-FMAU, and adefovir did not always correlate with inhibition of the generation of intracellular HBV replicative intermediates, suggesting that these analogs may preferentially inhibit specific stages of the viral replication cycle.

Adefovir and tenofovir susceptibilities of HIV-1 after 24 to 48 weeks of adefovir dipivoxil therapy: genotypic and phenotypic analyses of study GS-96-408

OBJECTIVE

To determine whether genotypic changes in HIV-1 (HIV) reverse transcriptase (RT) occur during adefovir dipivoxil (ADV) therapy that may alter the susceptibility of HIV to adefovir or the related nucleotide inhibitor, tenofovir.

DESIGN AND METHODS

GS-96-408 was a 1:1 randomized, double-blind, phase III clinical trial assessing the safety and efficacy of 120-mg daily ADV compared with placebo for the treatment of HIV when added to stable background antiretroviral therapy (ART). Of 442 patients enrolled, 142 were prospectively selected for a virology substudy. Baseline and posttreatment (weeks 24-48) plasma samples were genotypically analyzed in HIV RT. HIV from ADV-treated patients who developed RT mutations at week 24 were also phenotypically analyzed.

RESULTS

Nucleoside-associated RT mutations arose with similar frequency among the ADV-and placebo-treated patients, 32% (n = 23) and 28% (n = 20), respectively, during the 24-week blinded treatment phase. RT mutations previously selected by adefovir in vitro (K70E or K65R) did not develop in any patient. Most mutations were typical zidovudine (ZDV)-resistance mutations (e.g., M41L, D67N, K70R, T215Y) in patients taking ZDV or stavudine (d4T) concomitantly, demonstrating directly in the placebo arm that d4T is able to select for these mutations. There appeared to be more patients developing D67N and K70R mutations in the ADV arm versus more T215Y mutations in the placebo arm. Between weeks 24 and 48, 19 of 50 patients (38%) in the ADV arm developed similar RT mutations. The mean HIV RNA responses at weeks 24 and 48 among the ADV-treated patients developing RT mutations were -0.68 log(10) copies/ml (n = 23) and -0.52 log(10) copies/ml (n = 19), respectively, similar to the overall week-24 and week-48 responses (-0.53 and 0.48 log(10) copies/ml, respectively). Patient-derived HIV expressing the observed RT mutations showed insignificant decreases in adefovir susceptibility compared with wild-type in 12 of 16 cases (< threefold). HIV from 1 patient showed significantly reduced susceptibility to tenofovir, which was in association with a double insertion mutation after codon 69 that was also present at baseline.

CONCLUSIONS

HIV RT changes that arose during ADV therapy appear attributable to the patient's background ART. ADV therapy may have influenced the pattern of ZDV-associated resistance mutations that developed, but this did not result in an observed loss of viral load suppression. There was a trend toward decreased phenotypic susceptibility to adefovir in ADV-treated patients, with 4 of 16 analyzed patients showing mild, but significantly decreased susceptibility associated with the additional ZDV-associated mutations. Decreased susceptibility to the related nucleotide analog, tenofovir, was not observed to develop in ADV-treated patients.

Tenofovir, adefovir, and zidovudine susceptibilities of primary human immunodeficiency virus type 1 isolates with non-B subtypes or nucleoside resistance

New antiretroviral drugs with activity against strains of human immunodeficiency virus type 1 (HIV-1) with non-B subtypes and with resistance to current antiretroviral drugs are needed. The activity of two nucleotide analogs, tenofovir and adefovir (PMPA and PMEA, respectively), against non-B subtypes and nucleoside-resistant primary HIV-1 isolates was assessed. Tenofovir and adefovir were fully active against a panel of subtypes A, C, D, E, F, G, and group O primary HIV-1 isolates as compared with their respective activity against subtype B isolates. Moreover, the susceptibility of a panel of 10 primary HIV-1 isolates with >10-fold mean resistance to zidovudine, lamivudine, and abacavir was within 2.2-fold of wild-type tenofovir susceptibility for each isolate. An oral prodrug of tenofovir, tenofovir disoproxil fumarate (DF), is currently in phase III clinical trials for the treatment of HIV-1 infection. These in vitro susceptibility results suggest that tenofovir DF may be active in vivo against HIV-1 with nucleoside resistance as well as against HIV-1 with non-B subtypes.

Antiviral activity of tenofovir (PMPA) against nucleoside-resistant clinical HIV samples

The presence of the lamivudine-associated M184V RT mutation increases tenofovir susceptibility in multiple HIV genotypes. Tenofovir is uniquely active against multinucleoside-resistant HIV expressing the Q151M mutation, but shows reduced susceptibility to the T69S insertion mutations. HIV with common forms of zidovudine and lamivudine resistance are susceptible to tenofovir, corroborating phase II clinical results demonstrating the activity of tenofovir DF in treatment-experienced patients.

Increased Drug Susceptibility of HIV-1 Reverse Transcriptase Mutants Containing M184V and Zidovudine-Associated Mutations: Analysis of Enzyme Processivity, Chain-Terminator Removal and Viral Replication

The presence of the HIV reverse transcriptase (RT) resistance mutation, M184V, induced by lamivudine and abacavir treatment results in increased tenofovir, adefovir and zidovudine susceptibility for HIV-1 with zidovudine-associated RT mutations in vitro. Treatment with oral prodrugs of tenofovir and adefovir has resulted in substantial HIV-1 RNA reductions in antiretroviral-experienced patient populations who have lamivudine-and zidovudine-resistant HIV-1. An enzymatic analysis was undertaken to elucidate the mechanisms of altered drug susceptibilities of HIV-1 containing zidovudine-associated mutations in the presence or absence of M184V. The inhibition constants (Ki) for the active metabolites of tenofovir, adefovir and zidovudine did not vary significantly between recombinant mutant and wild-type RT enzymes. Although increased removal of chain-terminating inhibitors by pyrophosphorolysis and ATP-dependent unblocking correlated with reduced susceptibility of viruses with zidovudine-associated mutations, a reduction in the removal of chain-terminators was not observed, which would explain the increased drug susceptibility of mutants containing M184V plus zidovudine-associated mutations. However, analyses of single-cycle processivity of the mutant RT enzymes on heteropolymeric RNA templates showed that all M184V-containing mutant RT enzymes were less processive than wild-type RT, most notably for mutants expressing both zidovudine-associated mutations and M184V. Similarly, the in vitro replication capacity of a mutant virus expressing a zidovudine-associated mutation and M184V was significantly reduced compared with wild-type virus. The observed decrease in enzymatic processivity of the M184V-expressing RT enzymes might result in decreased viral replication, which then might contribute to the increased drug susceptibility of HIV-1 expressing these RT mutations.

Primary genotypic and phenotypic HIV-1 drug resistance in recent seroconverters in Madrid

OBJECTIVE

Transmission of drug-resistant HIV-1 strains is increasing with widespread use of antiretroviral drugs in developed countries. This study examined the prevalence of resistant viruses in recent seroconverters in Madrid, Spain.

DESIGN

HIV isolates from 30 consecutive participants with positive or indeterminate HIV antibody test results and a negative test result at a mean of 6.6 months earlier were examined for HIV drug resistance. All study subjects admitted to having very recently engaged in high-risk practices. All were therapeutically naive and were recruited between 1997 and 1999 in a referring health care facility for sexually transmitted diseases.

METHODS

Population-based sequencing of the viral reverse transcriptase (RT) and protease (PR) regions derived from plasma viral RNA was performed. Phenotypic resistance was assessed by a recombinant virus assay.

RESULTS

Overall prevalence of genotypes associated with reduced susceptibility was 26.7% (8 of 30 participants). Resistance mutations were seen against nucleoside analogues in 7 (23.3%), nonnucleoside reverse transcriptase inhibitors in 1 (3.3%), and protease inhibitors in 2 (6.7%). Zidovudine-resistance mutations M41L and/or T215Y were the commonest, found in 20% (6 of 30 participants). Resistance mutations to at least two antiretroviral families (multidrug-resistance) were detected in 2 (6.7%) study subjects. A median infectious dose (IC50) increase of fourfold for any drug was found in 7 patients, and in 2 was > tenfold for zidovudine (genotype M41L + T215Y) and lamivudine (genotype M184V), respectively.

CONCLUSIONS

Drug-resistant HIV variants were present in over one quarter of individuals recently diagnosed as infected in Madrid, Spain. Therefore, resistance testing at baseline should be considered for the optimal design of first-line antiretroviral combinations.

Altered viral fitness of HIV-1 following failure of protease inhibitor-based therapy

HIV-1 isolated from patients with improved CD4+ T-cell counts despite virologic failure on a nucleoside reverse transcriptase inhibitor (NRTI) and protease inhibitor (PI)-containing regimen were characterized. Five paired virus isolates from patients before and after zidovudine, lamivudine, and ritonavir treatment were tested. Human peripheral blood leukocyte-reconstituted severe combined immunodeficient (hu-PBL-SCID) mice were infected with pre-or posttreatment isolates and plasma HIV-1 RNA levels and CD4+ T cells were measured. Two of five post-treatment isolates exhibited decreased replication in hu-PBL-SCID mice compared with the paired pretreatment isolate, and both had the V82A mutation in protease associated with resistance to PI. One additional posttreatment isolate with the M184V mutation in reverse transcriptase showed diminished replication. CD4+ T-cell depletion was similar following infection with either the pre-or posttreatment isolates. Subtle losses in the replication capacity of PI-or NRTI-resistant viruses may contribute to relative preservation of CD4+ T-cell counts in persons who experience virologic failure. Cytopathic effects of viral infection for target T cells vary from patient to patient but appear not to be influenced by mutations associated with failure of therapy in this system.

Resistance against reverse transcriptase inhibitors

The response to antiretroviral therapy in human immunodeficiency virus (HIV)-infected patients is limited by the emergence of drug resistance. This resistance is a consequence of the high rate of HIV mutation, the high rate of viral replication (especially when potent multidrug therapies are not used or taken reliably), and the selective effect of these drugs, which favors emergence of mutations that can establish clinical drug resistance. The introduction of highly active antiretroviral therapy (HAART), which typically includes at least 2 nucleoside reverse transcriptase inhibitors (RTIs) and a protease inhibitor or a nonnucleoside RTI, for most treatment-naive patients results in a reduction of viral load below the limit of detection determined by currently available HIV RNA assays. It is this marked reduction that results in durable viral suppression, usually only possible by the simultaneous use of 3 or 4 drugs. The RTI components of HAART are crucial for these benefits of combination therapy. Specific amino acid changes are associated with resistance to several RTIs, but new mutation complexes have been observed that can confer broad cross-resistance within this class. Genotypic and phenotypic resistance assays to measure drug resistance are being developed, but refinements in both methodology and our ability to interpret results of these assays are necessary before they are introduced into widespread clinical use.

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.

In vitro selection and characterization of HIV-1 with reduced susceptibility to PMPA

9-(2-phosphonomethoxypropyl)adenine (PMPA) has demonstrated remarkable anti-simian immunodeficiency virus (SIV) activity in macaque models of SIV infection and transmission prevention. Recently, PMPA and its oral prodrug, bis-POC PMPA, have also shown potent anti-human immunodeficiency virus type 1 (HIV-1) activity in Phase I clinical studies. In vitro experiments were performed to address the resistance properties of PMPA. After eight passages in increasing concentrations of PMPA, HIV-1IIIB was able to grow in the presence of 2 microM PMPA, fivefold above the IC50 of PMPA for wild-type parental virus. Sequence analysis of the reverse transcriptase (RT) genes from four of 15 RT clones demonstrated the presence of a K65R substitution in RT and recombinant HIV expressing the K65R RT mutation showed a threefold to fourfold increase in IC50 value for PMPA as compared to wild-type. Additional experiments demonstrated that viruses expressing other nucleoside-associated RT resistance mutations all showed wild-type or < threefold reduced susceptibility to PMPA in vitro. Interestingly, lamivudine-resistant viruses expressing the M184V RT mutation showed wild-type to slightly increased susceptibility to PMPA in vitro and addition of the M184V mutation to HIV with the K65R mutation resulted in reversion to wild-type susceptibility for PMPA. In agreement with the cell culture findings, Escherichia coli-expressed K65R RT showed fivefold reduced susceptibility to PMPA diphosphate, the active moiety of PMPA. Furthermore, in combination experiments, PMPA with hydroxyurea showed synergistic inhibition of HIV replication in vitro. The potent antiretroviral activity and favourable resistance profile of PMPA and bis-POC PMPA are being further investigated in ongoing clinical trials.

Efavirenz- and adefovir dipivoxil-based salvage therapy in highly treatment-experienced patients: clinical and genotypic predictors of virologic response

OBJECTIVE

To determine the impact of prior nonnucleoside reverse transcriptase inhibitor (NNRTI) therapy, genotypic resistance, and other variables on response to efavirenz (EFV)- and adefovir dipivoxil (ADV)-based salvage therapy.

DESIGN

Retrospective clinical cohort study.

SETTING

One university and one community-based HIV clinic.

STUDY SUBJECTS

All 33 patients who were coenrolled in both the EFV and ADV expanded access programs.

INTERVENTIONS

Patients received EFV 600 mg/day and ADV 120 mg/day in addition to other antiretroviral agents.

OUTCOME MEASURE

HIV viral load (<500 copies/ml) at 12 and 24 weeks.

RESULTS

10 of 33 (30%) patients at 12 weeks and 8 of 33 (24%) patients at 24 weeks had viral loads <500 copies/ml. Prior NNRTI use and a history of any NNRTI-associated mutations predicted failure. Patients with Y181C or G190A single mutations had an initial greater magnitude of viral load suppression than those with K103N, but this advantage was short lived. No one with any NNRTI mutations responded with a viral load <500 copies/ml at 12 or 24 weeks.

CONCLUSIONS

EFV/ADV-based salvage yielded viral load suppression at 24 weeks in 42% (8 of 19) of patients who were highly NRTI and protease inhibitor experienced but NNRTI naive. NNRTI-experienced study subjects had a poor response regardless of the specific NNRTI resistance mutation they harbored.

Antiviral activity of 2'-deoxy-3'-oxa-4'-thiocytidine (BCH-10652) against lamivudine-resistant human immunodeficiency virus type 1 in SCID-hu Thy/Liv mice

Oral administration of 2'-deoxy-3'-oxa-4'-thiocytidine (BCH-10652), a nucleoside analog structurally similar to lamivudine (3TC), caused dose-dependent inhibition of viral replication in SCID-hu Thy/Liv mice infected with human immunodeficiency virus type 1 NL4-3 and with an NL4-3 clone containing the M184V mutation in reverse transcriptase that confers resistance to 3TC. These experiments demonstrate the utility of this mouse model for evaluating drug resistance and for performing direct comparisons between antiviral compounds in vivo.

Adefovir dipivoxil

Adefovir dipivoxil is an ester prodrug of the nucleoside reverse transcriptase inhibitor adefovir (PMEA), the prototype compound of the acyclic nucleoside phosphonates. It has better oral bioavailability than the parent compound. Adefovir dipivoxil 120mg once daily significantly reduced viral load compared with placebo when added to standard antiretroviral therapy in a 6-month, double-blind study in patients with HIV infection. Viral suppression was maintained during an additional 6-month nonblind extension phase. The drug was most effective in patients with baseline isolates containing the M184V lamivudine resistance mutation according to data from a virological substudy of a large placebo-controlled trial. Adefovir dipivoxil 60mg was as effective as 120mg (both once daily) after 20 weeks' treatment in a randomised double-blind study in antiretroviral-experienced (protease inhibitor-naive) patients. Viral suppression was generally maintained in patients who developed new reverse transcriptase mutations during adefovir dipivoxil monotherapy or combination therapy for up to 12 months. No clear pattern of particular clinical resistance mutations has emerged. GI disturbances, hepatic effects and delayed renal abnormalities are the principal adverse events seen with adefovir dipivoxil. Reductions in serum free carnitine levels may occur and coadministration of L-carnitine is recommended.

A new point mutation (P157S) in the reverse transcriptase of human immunodeficiency virus type 1 confers low-level resistance to (-)-beta-2',3'-dideoxy-3'-thiacytidine

A P157S mutation in the reverse transcriptase (RT) of human immunodeficiency virus type 1 conferred fivefold resistance to (-)-beta-2',3'-dideoxy-3'-thiacytidine in cell culture. Interestingly, the P157S mutation resulted in increased sensitivity (two- to threefold) to 3'-azido-3'-deoxythymidine (AZT) and to (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA). A similar increase in susceptibility to AZT and to PMPA was also conferred by the M184V mutation in RT.

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.