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HIV-1 Population Dynamics in Patients Treated with Ritonavir W. RESCH*, T. SMITH, J. NELSON, and R. SWANSTROM.
Univ. of North Carolina at Chapel Hill The development of resistance still limits the value of combination therapy for HIV-1 infection. Substitutions at specific amino acid positions in the protease gene (pro) have been shown to result in viral populations with substantial resistance to one or more protease inhibitors.
Thus far the investigation of competing viral populations in the course of resistance evolution has been challenging. We have developed a modified Heteroduplex Tracking Assay (HTA) which is selectively sensitive to six resistance relevant sites in pro (46, 48, 54, 82, 84, 90).
We investigated the population dynamics of resistance evolution in patients failing ritonavir monotherapy by HTA. We examined pro sequences present in plasma-derived viral RNA of 14 patients who failed therapy following a good initial response to ritonavir. Control groups consisted of either patients given placebo or those without measurable virologic response to ritonavir. Overall, the patterns of resistance evolution were highly variable between patients. In each patient, there were between one and seven HTA bands corresponding to distinct populations. Three patients maintained populations of low inferred resistance while simultaneously developing more resistant populations. Approximately half of non-responding patients and none of the placebo patients showed significant changes in viral populations. The evolution of resistance on the level of population dynamics appears to be a two-step process. First, in greater than 90% of initial responders we found the evolution of new populations with resistance markers at the time of rebound. Virtually all viral populations underwent further evolution in a second step after the rebound during a period of stable viral load. We measured the relative fitness of viral populations in vivo when possible, and found a significant increase in fitness for both steps. This increase in fitness may be attributed to increased resistance, higher replication rates, or both.
In summary, response of the virus to strong selection by a protease inhibitor led to dynamic changes in the virus populations as defined by genotypic variants in pro. Most patients develop resistance in a two step process with succesive gains of fitness.
Key Words: Evolution, Population Dynamics, Protease Resistance
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