The influence of protease inhibitor resistance profiles on selection of HIV therapy in treatment-naive patients.

Although protease inhibitors (PIs) have dramatically improved outcomes in HIV-infected patients, half still fail treatment with PI-based combination therapy. Genetic pressure from incomplete viral suppression rapidly selects for HIV variants with protease gene mutations that confer reduced susceptibility to PI drugs. A number of specific amino acid substitutions have been associated with PI resistance. However, high-level resistance to individual PIs requires the accumulation of several primary and secondary mutations, developing along drug-specific, step-wise pathways. HIV variants resistant to saquinavir and ritonavir usually contain L90M and V82A substitutions, respectively. Indinavir resistance may be linked to substitutions at positions 46 or 82. Resistance to nelfinavir is primarily associated with D30N but may alternatively be found with L90M. Resistance during exposure to amprenavir can follow development of I50V, which also may confer resistance to lopinavir. Failure during treatment with atazanavir is closely linked to 150L. The overlapping of these pathways can lead to multiple-PI resistance, limiting therapeutic options in antiretroviral-experienced patients. Reduced susceptibility to more than one PI is most likely to be associated with amino acid substitutions at six positions: 10, 46, 54, 82, 84 and 90. Other mutations (D30N, G48V, I50V or I50L) are relatively specific for particular PIs and are less likely to produce cross resistance. Certain resistance mutations selected by exposure to one PI may actually increase susceptibility to others. Patients newly diagnosed with HIV infection are increasingly found to harbour virus that is resistant to the more commonly used drugs. Newer PIs may select for mutations that result in less cross resistance with older agents.