THE SELECTION AND CHARACTERIZATION OF HIV-1 VIRUS ISOLATES RESISTANT TO NONNUCLEOSIDE REVERSE TRANSCIPTASE INHIBITORS: AN EVALUATION OF FACTORS INVOLVED IN THE SELECTION OF RESISTANT VIRUS ISOLATES

Abstract

Millions of people worldwide are infected with the human immunodeficiency virus (HIV). Reverse transcriptase (RT) inhibitors, the only compounds approved for clinical use, result in the rapid selection of resistant viruses both in vivo and in vitro. Drug-resistance has led to the investigation of using combinations of RT inhibitors to suppress the selection of resistant virus isolates. Drug-resistance is a critical issue which must be fully investigated in the search to identify novel RT inhibitor candidates or combinations of inhibitors with therapeutic utility. These issues in HIV research will require additional knowledge of the factors which affect the in vivo selection of resistant virus isolates. This study was designed to evaluate the role of several factors in the resistance selection process. Comparative analysis of two resistance selection methodologies indicate that emergence of resistant viruses is not significantly influenced by the use of gradual increases in antiviral concentration or constant concentrations of the compound. Factors other than the selection process are critical in the outgrowth of resistant virus populations. Specifically, the genetic composition of the virus strain used in the selection process influences the selection of resistant virus isolates Genetic heterogeneity exists in laboratory established virus pools and in clinical virus strains, thus the virus strain chosen for resistance studies may determine the resistant virus populations which emerge during treatment. The HIV-1 subpopulation that appeared to be presentin greatest frequency in our virus pool, Y181C. emerged rapidly in the presence of UC84 and various UC analogs, followed by the selection of secondary mutations. While the resistant virus population most prevalent in the wild type virus pool may influence results of the resistance selection process, the nature of the interaction of various nonnucleoside reverse transcriptase inhibitors (NNRTI) with the RT is also an important factor in the selection of resistant viruses. The structure of the antiviral inhibitor determines the specific interactions between the compound and RT, thus influencing the selection of the resistant virus isolates and the reproducibility of the selection process. Some NNRTIs reproducibly select virus isolates possessing specific mutations, while others continually select various secondary mutations. UC analogs select resistant virus isolates at different rates which exhibit different levels of resistance to the selecting compound. Structural features of the compound may be responsible for selection of resistant viruses more slowly than others. These data suggest that the NNRTIs, while binding at the nonnucleoside binding pocket, may specifically interact with different amino acids in the binding pocket. Data from this study also suggests that different selection pressures exist among the human cell lines available for in vitro resistance studies and these factors determine the rate of HIV replication and the resistant virus population selected. These studies indicate that the virus strain, the cell line, and the selecting agent all influence the resistant virus isolates that emerge during the resistanceselection process. Standardized protocols among laboratories to evaluate potential antiviral compounds used alone or in combination studies may prove useful. The use of cell lines and virus strains most closely related to those encountered in the clinical setting may also provide more accurate profiles of potential antivirals being considered for clinical use. These studies further define the role of these factors in antiviral resistance selection and may be useful in the process of identifying and evaluating potential antiviral compounds for therapeutic utility.