Estimate of the Frequency of Human Immunodeficiency Virus Type 1 Protease Inhibitor Resistance within Unselected Virus Populations In Vitro

The frequency of drug-resistant human immunodeficiency virus type 1 (HIV-1) variants in virus populations not previously exposed to drug was determined in vitro by using HIV-1(RF) and the protease inhibitor SC-55389A. Two variants with single mutations responsible for drug resistance (V82A and N88S) were quantifiably isolated after only one round of replication, yielding a crude frequency estimate of at least 1 SC-55389A-resistant variant per 3.5 × 10(5) wild-type infectious units.