The session on Mechanisms of Antiretroviral Resistance provided new insights into the crystal structures of both the HIV reverse transcriptase (RT) and protease (PR) enzymes, in regard to diminished sensitivity to antiviral drugs. In abstract 20, experiments conducted by David Stammer and colleagues showed that two different second generation NNRTI, efavirenz and the investigational thiocarboxanilide compound UC-781, can structurally redistribute and rearrange themselves within the pockets of several mutated RT enzymes, thus rendering them more resilient to the effects of resistance mutation.

Further mechanistic insight into AZT resistance was provided in abstract 21 by Peter Meyer et al, who reported that the RT enzyme from AZT-resistant viruses can remove AZT monophosphate (AZT-MP) through nucleotide-dependent primer unblocking, and thereby continue to synthesize proviral DNA . Ordinarily, in the case of wild-type virus, transcription of proviral DNA is terminated when the active, triphosphorylated nucleoside (in this case AZT) is incorporated into the elongating DNA chain. Meyer et al discovered that in the presence of ATP, mutant RT was able to remove AZT-MP by essentially "donating" the chain-terminating nucleoside analog to an ATP molecule (a nucleotide "acceptor"), resulting in (1) a shortened, extendable primer and (2) an inactive dinucleoside polyphosphate. Since foscarnet is an analog of pyrophosphate, the authors conjectured and showed that foscarnet could both block the synthesis of dinucleoside polyphosphate and suppress the ZDV-resistant phenotype in biochemical assays. This work suggests that foscarnet might also suppress the ZDV-resistance phenotype in vivo and synergize with ZDV.

Abstract 22 evaluated mechanistic aspects of RT containing the 184V mutation and showed that this enzyme was severely compromised in regard to pyrophosphorylysis or the cleavage of ZDV-terminated primers. These data help to explain both the fact that 184V can resensitize ZDV-resistant viruses to ZDV, at least on a temporary basis, and reinforced the notion that 3TC and ZDV can act jointly against viruses that are resistant to either drug alone. This analysis was extended in abstract 23, whose authors demonstrated the chain-terminating effects of several nucleoside-analog triphosphates during the synthesis of (+) strand DNA. This work opens up the possibility of using anti-viral drugs to specifically antagonize individual steps during the complex process of reverse transcription. This work also showed that NNRTIs have the effect of converting natural pause sites during synthesis of (+) strand DNA into the equivalent of chain termination sites. Finally, data were presented to suggest that the 184V mutation in tissue culture can be effectively maintained by drug pressure with Ziagen, 3TC, and a novel compound termed dOTC, and less well with ddI. This finding may have practical significance for design of clinical trials that seek to preserve the 184V mutation; several papers suggest that this substitution may have clinical benefit.

Abstract 24 also presented data on RT enzymes resistant to NNRTIs and showed that certain mutations in the genes coding for these enzymes cause a deficiency in the RNAse H cleavage reaction. Abstract 25 presented work on a novel mechanism that would account for 3TC resistance, which involved the appearance of mutations at positions 44D and 118I. The latter substitutions can also exist as polymorphisms in patients prior to initiation of 3TC therapy, and their full significance is not yet established.

Abstract 26 described complex patterns of cross-resistance among different NNRTIs in 5000 clinical samples. The results indicated that the most common double combination was 103N/18IC, while the most common triple combination was 103N/181C/190A. Of 71 NNRTI-resistant samples, 56% displayed resistance to all three drugs, while 34% were resistant to two and 10% to one. While all efavirenz-resistant samples also displayed diminished sensitivity to nevirapine, many nevirapine-resistant viruses remained sensitive to both delavirdine and efavirenz.

Abstract 27 showed that resistance to PIs could also be due, in part, to high-level expression of P Glycoprotein (PgP) and that levels of expression of the latter were elevated in patients with PI experience. The authors argued that levels of PgP expression should be examined more routinely in patients on anti-retroviral therapy.

Other interesting presentations were Abstract 34, that showed that a deletion in RT at codon 67 could account for resistance to ZDV. Abstracts 35 and 36 reiterated that a variable pattern of insertions in the _3-_4 fingers subdomain of RT could give rise to multiple drug resistance. Abstract 38 suggested that mutations associated with ZDV resistance might be selected in patients on long-term stavudine-containing regimens.