Nucleoside and nucleotide analogue reverse transcriptase inhibitors: a clinical review of antiretroviral resistance.

Although advances in highly active antiretroviral therapy (HAART) have made long-term suppression of HIV an achievable goal of therapy, a substantial proportion of first-line regimens will eventually fail. Successful longterm treatment requires consideration of downstream treatment options at the time of initiating or changing regimens. An understanding of the patterns and interactions of resistance mutations, and the appropriate use of genotypic and phenotypic testing is an important component of successful drug sequencing. Resistance to multiple nucleoside reverse transcriptase inhibitors (NRTIs) may result from several genotypically distinct pathways, including the Q151M (151 complex), the 69 insertion complex, two distinct thymidine analogue mutational pathways and the K65R mutation. Knowledge of the clinical implications of these and other resistance pathways, as well as the antagonism or synergy between mutations, helps guide individualized treatment choices from initial therapy in the treatment-naive patient to salvage therapy in the highly treatment-experienced individual. The development of effective sequencing strategies will depend upon the continued understanding of drug resistance mutation patterns and their associations with specific HAART combinations. This review summarizes research advances that further the understanding of nucleoside and nucleotide analogue resistance mutations, and their interplay.