Highly active antiretroviral therapy (HAART) has reduced the morbidity and mortality of HIV infection in the U.S. and Europe. However, long-term success stories, in which patients with complete virologic suppression demonstrate good adherence to well tolerated drug regimens for years and years, constitute only a fraction of clinical courses. In many cases, increasing time on treatment is associated with increasing intolerance to drugs, drug interruptions, the development (and transmission) of drug resistance and, ultimately, treatment failure.

The evolution of drug resistance is recognized as a major obstacle in achieving and maintaining viral suppression. However, despite increasing numbers of patients with drug-resistant HIV, the marked decrease in mortality first noted with the introduction of protease inhibitor-containing combination therapy in the mid 1990's is still evident. This suggests that drug resistance may, through a decrease in the fitness or replicative capacity of HIV, provide continuing clinical benefit in the presence of rebounding viral loads.

In the coming years, reduced-cost generic drugs and access programs will allow large numbers of HIV-infected individuals throughout the world to gain access to ARV therapies. The prospect of implementing effective ARV therapy in large populations of treatment-naive individuals evokes an important question: Will the lessons learned during six years of experience with HAART in the U.S. and Europe be applied to the development of strategies for prolonging the efficacy of ARV treatment globally?

Increasingly, phenotypic and genotypic drug resistance testing is being used to improve virologic outcomes of patients receiving successive HAART regimens. However, most studies of drug resistance, as well as interpretations of genotypic changes in HIV-1 reverse transcriptase (RT) and protease (PR), are based on work with HIV-1 subtype B viruses. Worldwide, however, non-B subtypes are more common: subtype C virus now accounts for the majority of infections globally and subtype B infection is increasingly less common. There is evidence that the prevalence of HIV-1 non-B subtypes are increasing in North America and there are clinics in Europe where the majority of patients are infected with non-subtype B viruses. Data on resistance in non-subtype B (NSB) HIV-1 are limited, as is the correlation of genotypic and phenotypic data with antiretroviral drugs.

Studies of patients with NSB viruses from Europe, Israel, Brazil and Africa have demonstrated clinical responses to HAART regimens (in drug-naive patients) that are consistent with the 50% to 70% rates of virologic response seen in subtype B [1]. There are, however, differences among HIV subtypes with respect to genotyping methods, both in terms of obtaining sequences and interpreting sequence data to discern drug susceptibility.

One example is the high prevalence of K20R, M36I, M46I and I93L substitutions in wild-type, subtype C isolates from patients who have not been exposed to ARV drugs [2]. In subtype B viruses, these are considered secondary PR mutations which may increase fitness or add to PI resistance, while in NSB viruses these polymorphisms occur with high frequency in the absence of drug pressure. Limited phenotypic studies have shown that at least M36I-containing viruses are susceptible to PI. But a recent, large analysis of patients in Italy suggests that M36I (when combined with other resistance-associated polymorphisms at 71 or 77) may be associated with an increased rate of virologic failure with initial PI-containing regimens [3]. There are a number of polymorphic sites in PR adjacent to sites of known resistance mutations, where substitutions in different subtypes may alter their susceptibility to PI. Thus far, we have no data to suggest that NSB viruses are less susceptible to PI, but there are few long-term clinical studies and little information on the susceptibility of NSB viruses and clinical response to salvage therapies after failure of an initial regimen.

Due to their low manufacturing costs, patent expirations and other factors, the first generation of NRTI will become increasingly available in resource-limited settings. NNRTI are also relatively inexpensive to produce and are likely to be included in the first multi-class ARV regimens used in such settings on a large scale. Triple-drug regimens including NRTI and NNRTI raise many of the same questions about polymorphisms in the RT gene and the susceptibility of NSB viruses. The most important example of NNRTI resistance is the absence of the binding pocket in HIV-2, and the consequent ineffectiveness of these drugs for treatment of HIV-2. Among diverse HIV-1 subtypes, there are a few reports of NNRTI resistance-associated mutations and variability in the susceptibility of some isolates of wild-type viruses. These include relative NNRTI resistance in subtype F viruses from Latin America, as well as the presence of NNRTI resistance mutations at K103N in several subtype D viruses isolated from untreated patients in Uganda. Whether polymorphisms and differences in NSB viruses hasten the development of NNRTI resistance is not known.

Dr. Mark Wainberg and colleagues recently presented evidence that serial passage of subtype C HIV cultured in subinhibitory concentrations of efavirenz selected NNRTI resistance-associated mutations far more rapidly than in subtype B viruses [4]. Whether they can be ascribed to the enhanced replicative capacity of subtype C viruses, increased fitness of viruses with NNRTI mutations or the background polymorphisms in subtype C virus, these findings may be harbingers of an increased rate of resistance in NSB viruses. Studies in Uganda, Zimbabwe and South Africa suggest a high rate of selection of nevirapine resistance, particularly in subtypes C and D, following the administration of single-dose nevirapine to prevent mother-to-child transmission.

Since its emergence, HIV-1 has shown a remarkable capacity to diversify in response to selective pressure through mutations and recombination, such that even within highly conserved functional genes, there is as much as a 20% genetic difference between subtypes. In North America and Europe, examples of the rapidly increasing rate of transmission of drug-resistant viruses, as well as the persistence of drug resistance among patients receiving long-term ARV therapy, provide evidence of the ability of subtype B virus to adapt in the presence of currently available drugs. As treatment options increase around the world, particularly in hyper-endemic settings, a new selective pressure will be exerted on NSB viruses which may impact the long-term effectiveness of ARV drugs. Optimizing the interpretation of genotypic mutation data to determine salvage regimens in NSB viruses may require analysis of emerging data and the development of NSB resistance algorithms in practice, as well as the performance of clinical trials in settings where NSB viruses predominate.

Today, most genotypic test reports provide subtype information. The internationalization of AIDS treatment, and the increased use of genotyping in salvage (and in some cases initial) regimens will provide new information about the range and variation of resistance across subtypes.

References

Frater AJ, Beardall A, Ariyoshi K, Churchill D, Galpin S, Clarke JR, Weber JN, McClure MO. Impact of baseline polymorphisms in RT and protease on outcome of highly active antiretroviral therapy in HIV-1-infected African patients. AIDS. 2001;15(12):1493-502.
Grossman Z, Vardinon N, Chemtob D, Alkan ML, Bentwich Z, Burke M, Gottesman G, Istomin V, Levi I, Maayan S, Shahar E, Schapiro JM. Genotypic variation of HIV-1 reverse transcriptase and protease: comparative analysis of clade C and clade B. AIDS. 2001;15(12):1453-60.
Perno CF, Cozzi-Lepri A, Balotta C, Forbici F, Violin M, Bertoli A, Facchi G, Pezzotti P, Cadeo G, Tositti G, Pasquinucci S, Pauluzzi S, Scalzini A, Salassa B, Vincenti A, Phillips AN, Dianzani F, Appice A, Angarano G, Monno L, Ippolito G, Moroni M, Monforte Ad. Secondary mutations in the protease region of human immunodeficiency virus and virologic failure in drug-naive patients treated with protease inhibitor-based therapy. J Infect Dis. 2001;184(8):983-91.
Loemba H, Brenner B, Spira B, Petrella, Maayan S, Wainberg M. Genotypic and phenotypic characteristics of HIV-1 subtype C originating from Ethiopia. 1st IAS Conference on HIV Pathogenesis and Treatment, 8-11 July 2001, Buenos Aires.Abstract 395