Question

Why does the population of HIV in a patient resistant to AZT revert to "wild type" when the drug is removed from the patient?

Answer 1

HIV-1 drug resistance is caused by mutations in the reverse transcriptase (RT) and protease enzymes, the molecular targets of antiretroviral therapy. At the beginning of the year 2000, two expert panels recommended that HIV-1 RT and protease susceptibility testing be used to help select antiretroviral drugs for HIV-1-infected patients. Genotypic assays have been developed to detect HIV-1 mutations known to confer antiretroviral drug resistance. Genotypic assays using dideoxynucleoside sequencing provide extensive insight into the presence of drug-resistant variants in the population of viruses within an individual. However, the interpretation of these assays in clinical settings is formidable because of the large numbers of drug resistance mutations and because these mutations interact with one another and emerge in complex patterns. In addition, cross-resistance between antiretroviral drugs is greater than that anticipated from initial in vitro studies.

Fifteen antiretroviral drugs have been approved for the treatment of HIV-1 infection, including six nucleoside RT inhibitors (NRTI), six protease inhibitors (PI), and three non-nucleoside RT inhibitors (NNRTI). In previously untreated individuals with drug-susceptible HIV-1 strains, combinations of three or more drugs from two drug classes can lead to prolonged virus suppression and immunological reconstitution. However, the margin of success for achieving and maintaining virus suppression is narrow. The extraordinary patient effort is required to adhere to drug regimens that are expensive, inconvenient, and often associated with dose-limiting side effects. In addition to these hurdles, the development of drug resistance looms as both a cause and consequence of incomplete virus suppression that threatens the success of future treatment regimens.

The evolution of HIV-1 drug resistance within an individual depends on the generation of genetic variation and on the selection of drug-resistant variants during drug therapy. HIV-1 genetic variability is caused by the inability of HIV-1 RT to proofread nucleotide sequences during replication1. It is exacerbated by the high rate of HIV-1 replication in vivo, the accumulation of proviral variants during the course of HIV-1 infection, and genetic recombination when viruses with different sequences infect the same cell. As a result, innumerable genetically distinct variants (quasispecies) evolve in individuals in the months following primary infection2.

The risk of developing drug resistance depends on the size and heterogeneity of the HIV-1 population within an individual, the extent to which virus replication continues during drug therapy, the ease of acquisition of a particular mutation (or set of mutations), and the effect of drug-resistance mutations on changes in drug susceptibility and virus fitness. Some mutations selected during drug therapy confer measurable phenotypic resistance by themselves, whereas other mutations arise to compensate for the diminished replicative activity that can be associated with drug resistance, or produces measurable resistance only when present in combination. Resistant virus strains can also be transmitted between individuals. In the United States and Europe, about 10% of new infections are with HIV-1 strains harboring resistance to at least one of three drug classes.

transmission of an AZT-resistant isolate of HIV-1 was first reported in 1992 (8). Recent studies show a prevalence of AZT resistance mutations in viral sequences from newly infected individuals of 5–10% (9, 10), documenting a dramatic increase in the transmission of AZT-resistant HIV-1 over the past decade. In the absence of continued selective pressure from AZT-containing therapy, resistant variants eventually are replaced by AZT-susceptible revertants. Such revertants frequently involve the substitution at codon 215 of unusual amino acids such as aspartate, asparagine, cysteine, or serine in place of the mutant and wild-type residues more commonly encountered at this position. The 215D(GAC), 215N(AAC), 215C(TGC), and 215S(TCC) variants can each arise as a result of single nucleotide changes from 215Y or -F and appear to be more common than wild-type 215T revertants. In contrast to strains carrying the 215Y or -F mutation, variants with these alternative amino acid residues are AZT-sensitive. Moreover, these partial revertants appear to be fitter than the 215Y virus when tested in growth competition experiments.

Treatment with AZT will also select for a mutation (K70R) in this region of RT, but for reasons yet unclear, the K70R HIV-1 is not stable and is eventually outgrown by another AZT-resistant HIV-1 containing a T215Y/F change (12). This mutation and the rarer M41L, K219E/Q, and L210W AZT-resistant mutations (21, 26, 29) are found outside the polymerase-active site in the palm subdomain, a region of unknown function. With the exception of a modest increase in processivity during DNA synthesis by an AZT-resistant RT (9), no differences have been confirmed between the wild-type or AZT-resistant RT in competitive inhibition by AZT-5′-triphosphate (AZT-TP), incorporation of AZT-TP, RNase H activity, and fidelity (28).

In this study, we have employed a tissue culture infection system to study AZT-resistant mechanisms, rather than an in vitro assay, so as not to omit any cellular or viral factors. Amounts of proviral DNA synthesized by various AZT-resistant clinical isolates, AZT-resistant clones, and wild-type HIV-1 were measured in the presence or absence of AZT. An obvious trend was apparent from these analyses, i.e., AZT appears to stimulate reverse transcription in cells infected with AZT-resistant viruses containing the M41L and/or T215Y mutations. No such stimulation was observed in cells exposed to wild-type virus or to HIV-1 carrying only the K70R mutation. Thus, we hypothesize that an AZT anabolite (e.g., AZT-5′-monophosphate) (AZT-MP) may mediate a stimulation of reverse transcription by AZT-resistant, M41L and T215Y HIV-1 to overcome any inhibitory effects of AZT-TP. Considering that all triphosphorylated nucleoside analogs inhibit HIV-1 reverse transcription in a similar fashion, we examined whether AZT could also mediate cross-resistance by AZT-resistant viruses to other nucleoside analogs. We found that M41L and T215Y HIV-1 was resistant to each of 2′,3′-didehydro-2′,3′-deoxythymidine (d4T), ddI, and ddC, but only in the presence of low concentrations of AZT.