Viral resistance and the selection of antiretroviral combinations.

Mounting scientific evidence suggests that viral replication and evolution of diversification, with the resulting emergence of variants including drug-resistant strains, is responsible for the gradual destruction of the immune system and is the mechanism of disease progression in HIV-infected patients. Monotherapy does not give long-term suppression of viral replication and evolution, and combination therapy is viewed as a potentially more effective long-term approach based on increased and more durable suppression of HIV replication. Because of the large number of drugs that could be used in combination therapy regimens, it is important to investigate, in clinical studies, only those combinations likely to be most effective. In vitro studies are therefore critical in the selection of such combinations. Resistance mutations to many antiretroviral agents have been documented and their patterns of emergence elucidated. A number of studies have since demonstrated the phenomenon of cross-resistance, in which resistant virus emerging under the selective pressure of one therapy is also cross-resistant to a second antiviral agent. This information can be used to avoid combining agents to which cross-resistance can occur. Suppression or "phenotypic reversal" of zidovudine resistance by the lamivudine-resistance mutation at codon 184 has been demonstrated in in vitro studies. It is encouraging that this finding has led to the clinical evaluation of these two agents with promising results. Moreover, in vitro scientific data are critical in facilitating the identification of potentially effective combination therapies.