Identification of Genotypic Changes in Human Immunodeficiency Virus Protease That Correlate with Reduced Susceptibility to the Protease Inhibitor Lopinavir among Viral Isolates from Protease Inhibitor-Experienced Patients

The association of genotypic changes in human immunodeficiency virus (HIV) protease with reduced in vitro susceptibility to the new protease inhibitor lopinavir (previously ABT-378) was explored using a panel of viral isolates from subjects failing therapy with other protease inhibitors. Two statistical tests showed that specific mutations at 11 amino acid positions in protease (L10F/I/R/V, K20M/R, L24I, M46I/L, F53L, I54L/T/V, L63P, A71I/L/T/V, V82A/F/T, I84V, and L90M) were associated with reduced susceptibility. Mutations at positions 82, 54, 10, 63, 71, and 84 were most closely associated with relatively modest (4- and 10-fold) changes in phenotype, while the K20M/R and F53L mutations, in conjunction with multiple other mutations, were associated with >20- and >40-fold-reduced susceptibility, respectively. The median 50% inhibitory concentrations (IC(50)) of lopinavir against isolates with 0 to 3, 4 or 5, 6 or 7, and 8 to 10 of the above 11 mutations were 0.8-, 2.7-, 13.5-, and 44.0-fold higher, respectively, than the IC(50) against wild-type HIV. On average, the IC(50) of lopinavir increased by 1.74-fold per mutation in isolates containing three or more mutations. Each of the 16 viruses that displayed a >20-fold change in susceptibility contained mutations at residues 10, 54, 63, and 82 and/or 84, along with a median of three mutations at residues 20, 24, 46, 53, 71, and 90. The number of protease mutations from the 11 identified in these analyses (the lopinavir mutation score) may be useful for the interpretation of HIV genotypic resistance testing with respect to lopinavir-ritonavir (Kaletra) regimens and may provide insight into the genetic barrier to resistance to lopinavir-ritonavir in both antiretroviral therapy-naive and protease inhibitor-experienced patients.