Emergence of zidovudine resistance in HIV-infected patients receiving stavudine

A novel genetic pathway of human immunodeficiency virus type 1 resistance to stavudine mediated by the K65R mutation

Stavudine (d4T) and zidovudine (AZT) are thymidine analogs widely used in the treatment of human immunodeficiency virus type 1 (HIV-1)-infected persons. Resistance to d4T is not fully understood, although the selection of AZT resistance mutations in patients treated with d4T suggests that both drugs have similar pathways of resistance. Through the analysis of genotypic changes in nine recombinant viruses cultured with d4T, we identified a new pathway for d4T resistance mediated by K65R, a mutation not selected by AZT. Passaged viruses were derived from treatment-naïve persons or HIV-1(HXB2) and had wild-type reverse transcriptase (RT) or T215C/D mutations. K65R was selected in seven viruses and was associated with a high level of enzymatic resistance to d4T-triphosphate (median, 16-fold; range, 5- to 48-fold). The role of K65R in d4T resistance was confirmed in site-directed mutants generated in three different RT backgrounds. Phenotypic assays based on recombinant single-cycle replication or a whole-virus multiple replication cycle were unable to detect d4T resistance in d4T-selected mutants with K65R but detected cross-resistance to other nucleoside RT inhibitors. Four of the six viruses that had 215C/D mutations at baseline acquired the 215Y mutation alone or in association with K65R. Mutants having K65R and T215Y replicated less efficiently than viruses that had T215Y only, suggesting that selection of T215Y in patients treated with d4T may be favored. Our results demonstrate that K65R plays a role in d4T resistance and indicate that resistance pathways for d4T and AZT may not be identical. Biochemical analysis and improved replication assays are both required for a full phenotypic characterization of resistance to d4T. These findings highlight the complexity of the genetic pathways of d4T resistance and its phenotypic expression.

Changes in the human immunodeficiency virus p7-p1-p6 gag gene in drug-naive and pretreated patients

Resistance to antiretroviral agents often results from mutations within the human immunodeficiency virus (HIV) pol gene. Moreover, insertions within the p6 gag-pol region have recently been found to be involved with resistance to nucleoside analogs. Overall, we found that 21% of 156 specimens collected from HIV-infected individuals (17.6% from 74 drug-naive patients and 24.4% from 82 pretreated patients) harbored these insertions. Insertions around the KQE (Lys-Gln-Glu) motif were found in 12.2% of the pretreated patients but in none of the drug-naive subjects (P = 0.002). In contrast, insertions around the PTAP (Prol-Thre-Ala-Prol) motif were seen at similar rates ( approximately 15%) among drug-naive and pretreated patients, which supports the idea that they may be natural polymorphisms.

Subtle decreases in stavudine phenotypic susceptibility predict poor virologic response to stavudine monotherapy in zidovudine-experienced patients

To identify the level of phenotypic susceptibility for stavudine (d4T) that is associated with a diminished virologic response to d4T therapy, phenotyping was performed on archived baseline HIV isolates from 26 subjects who received d4T monotherapy in AIDS Clinical Trials Group (ACTG) 302 who had received >3 years of prior zidovudine (ZDV) monotherapy. Seven of 26 subjects achieved a virologic response of >0.3-log10 copies/mL reduction in plasma HIV RNA after 8 weeks of d4T. Responders had lower fold changes in susceptibility to d4T (1.0 vs. 1.6, p=.003), lower baseline viral loads (4.26 vs. 4.74 log10 copies/mL, p=.004), and fewer thymidine analog mutations (TAMS) (1 vs. 2, p=.059). Lower baseline d4T fold change in susceptibility predicted greater reductions in HIV RNA from baseline to week 8 after adjusting for baseline HIV RNA, ZDV fold change in susceptibility, and number of TAMS. Using the same phenotypic assay, drug susceptibility among 240 antiretroviral-naive patients found all HIV isolates to have d4T susceptibility <or=1.4-fold change. Using <or=1.4 as the d4T cutoff, the positive predictive value for a virologic response in this study was 44%, and the negative predictive value was 100%. d4T susceptibility greater than 1.4-fold change was associated with failure to achieve significant viral load reduction after 8 weeks of d4T monotherapy.

Persistence of wild-type virus and lack of temporal structure in the latent reservoir for human immunodeficiency virus type 1 in pediatric patients with extensive antiretroviral exposure

Although highly active antiretroviral therapy (HAART) for human immunodeficiency virus type 1 (HIV-1) infection can reduce levels of HIV-1 RNA in plasma to below the limit of detection, replication-competent forms of the virus persist in all infected individuals. One form of persistence involves a stable reservoir of latent but potentially infectious virus that resides in resting memory CD4(+) T cells. The mechanisms involved in maintaining this latent reservoir are incompletely understood. In the present study, we examined the dynamic characteristics of this reservoir in a cohort of children who developed drug-resistant HIV-1 as a result of extensive exposure to inadequately suppressive one- or two-drug regimens prior to the advent of HAART. We have previously shown that drug-resistant viruses selected by nonsuppressive pre-HAART regimens can enter and persist in this reservoir. We have extended these findings here by demonstrating that archival wild-type HIV-1 persists in this reservoir despite the fact that in these patients drug-resistant mutants have been favored by the selective conditions for many years. Phylogenetic analysis of replication-competent viruses persisting in resting CD4(+) T cells revealed a striking lack of temporal structure in the sense that isolates obtained at later time points did not show greater sequence divergence than isolates from earlier time points. The persistence of drug-sensitive virus and the lack of temporal structure in the latent reservoir provide genetic evidence for the idea that HIV-1 can persist in a latent form free of selective pressure from antiretroviral drugs in long-lived resting memory CD4(+) T cells. Although there may be other mechanisms for viral persistence, this stable pool of latently infected cells is of significant concern because of its potential to serve as a lasting source of replication-competent viruses, including the infecting wild-type form and all drug-resistant variants that have arisen subsequently.

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.

Efficacy and safety of stavudine plus didanosine in asymptomatic HIV-infected children with plasma HIV RNA below 50,000 copies per milliliter

BACKGROUND

Simple antiretroviral drug combinations might provide a comparable benefit to standard triple regimens in patients with mild HIV disease, because poor adherence and toxicities often compromise the sustained benefit of the latest triple regimens, especially when protease inhibitors are used. Bad adherence is the main cause of virological failure in HIV-positive children. The activity and safety of a double combination of nucleosides with high genetic barrier for resistance (stavudine plus didanosine) was assessed in children with nonadvanced HIV disease.

METHOD

From February 1998 to March 1999, 16 children were enrolled in six Spanish hospitals in a trial in which didanosine (180 mg/m2/day) and stavudine (2 mg/Kg/day) were administered for 48 weeks to asymptomatic naive children with plasma HIV RNA below 50,000 copies/mL and CD4 counts above 15%. Genotypic resistance to nucleoside analogues was examined at baseline and at the end of the study.

RESULTS

At baseline, median age was 6.5 years (range, 2-14). The absolute and percentage mean CD4 counts were 864 and 32%, respectively (z score: -0.48 and -1.1). Mean plasma viral load was 4.05 log. No clinical events occurred during the 1-year study period. Minor side effects were recorded in two thirds of children, although none led to drug discontinuation. Lipoatrophy was not recognized in any of the participants. Plasma HIV RNA below 400 copies/mL was reached by 43% and 44% of patients at 24 and 48 weeks, respectively. The z score for absolute and percentage CD4 count increased significantly at 48 weeks (+0.63 and +0.97, respectively) in respect to baseline (p <.05). Resistance mutations linked to didanosine (L74V or M184V) or stavudine (V75T) were not recognized and neither were multinucleoside resistant genotypes (151 complex or 69 inserts). However, four children developed AZT-like mutations T215Y and/or M41L.

CONCLUSION

Treatment with a dual combination of didanosine plus stavudine in naive children with nonadvanced HIV disease is safe and provides a satisfactory virological outcome at 1 year. Toxicity and drug resistance seem to occur rarely when this combination is used, which allows good adherence and spares other future treatment options.

Laboratory Guidelines for the Practical Use of HIV Drug Resistance Tests in Patient Follow-Up

HIV drug resistance is one of the major limitations in the successful treatment of HIV-infected patients using currently available antiretroviral combination therapies. When appropriate, drug susceptibility profiles should be taken into consideration in the choice of a specific combination therapy. Guidelines recommending resistance testing in certain circumstances have been issued. Many clinicians have access to resistance testing and will increasingly use these results in their treatment decisions. In this document, we comment on the different methods available, and the relevant issues relating to the clinical application of these tests. Specifically, the following recommendations can be made: (i) genotypic and phenotypic HIV-1 drug resistance analyses can yield complementary information for the clinician. However, insufficient information currently exists as to which approach is preferable in any particular clinical setting; (ii) when HIV-1 drug resistance testing is required, it is recommended that testing be performed on plasma samples obtained before starting, stopping or changing therapy, on samples that have a viral load above the detection limit of the resistance test; (iii) the panel recommends that genotypic and phenotypic HIV-1 drug resistance testing for clinical purposes be performed in a certified laboratory under strict quality control and quality assurance standards; and (iv) the panel recommends that resistance testing laboratories provide clinicians with resistance reports that include a list of drug-related resistance mutations (genotype) and/or a list of drug-related fold resistance values (phenotype), with interpretations of each by an experienced virologist. The interpretation of genotypic and phenotypic analysis is a complex and developing science, and in order to understand HIV-1 drug resistance reports, communication between the requesting clinician and the expert that interpreted the resistance report is recommended.

Differential removal of thymidine nucleotide analogues from blocked DNA chains by human immunodeficiency virus reverse transcriptase in the presence of physiological concentrations of 2'-deoxynucleoside triphosphates

Removal of 2',3'-didehydro-3'-deoxythymidine-5'-monophosphate (d4TMP) from a blocked DNA chain can occur through transfer of the chain-terminating residue to a nucleotide acceptor by human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT). ATP-dependent removal of either d4TMP or 3'-azido-3'-deoxythymidine-5'-monophosphate (AZTMP) is increased in AZT resistant HIV-1 RT (containing D67N/K70R/T215F/K219Q mutations). Removal of d4TMP is strongly inhibited by the next complementary deoxynucleoside triphosphate (50% inhibitory concentration [IC(50)] of approximately 0.5 microM), whereas removal of AZTMP is much less sensitive to this inhibition (IC(50) of >100 microM). This could explain the lack of cross-resistance by AZT-resistant HIV-1 to d4T in phenotypic drug susceptibility assays.