The frequency of resistant mutant virus before antiviral therapy.

OBJECTIVE To calculate the expected prevalence of resistant HIV mutants before antiviral therapy. DESIGN HIV replication generates virus mutants. The prevalence of these mutants is determined by mutation and selection/fitness. Some mutations will confer drug resistance and it is crucial for the success of antiviral drug therapy to determine whether these resistant viruses are present before the initiation of therapy. METHODS A quasispecies equation was used to calculate the expected frequency of drug-resistant virus prior to therapy. RESULTS AND CONCLUSIONS We show how the pretreatment frequency of resistant virus depends on the number of point mutations between wild-type and mutant virus, the selective disadvantage of the resistant mutant and the intermediate mutants, and the mutation rate.