601 Role of Resistance and Fitness in Driving the Evolution of the Protease in Patients Failing Treatment with Protease Inhibitors C. Charpentier, D. Lecossier, F. Clavel, A. J. Hance* INSERM U552, Hosp Bichat-Claude Bernard, Paris, France
Background: In patients (pts) failing treatment with protease inhibitors (PI), the evolution of protease genotypes is characterized by the accumulation of resistance mutations. These changes can occur either through the step-wise addition of mutations to the genotype of the pre-existing majority viral population, or result from the emergence of minority populations with distinct genotypes. The relative contribution of improvements in drug resistance or viral fitness in driving this process is not well understood.
Methods: Recombinant viral clones carrying the majority protease sequence observed at successive steps in viral evolution were prepared for 2 pts. The selective advantage of these viruses was evaluated using a modification of a previously reported single-cycle assay measuring viral fitness as a function of drug concentration. For both pts, pairs of viruses representing the successive steps in the evolution of resistance were compared in at least 4 independent experiments.
Results: In both pts, the initial steps in the development of resistance (addition of 4 or 5 resistance mutations) resulted in viruses with improved resistance but some impairment in fitness. Subsequent modifications in resistance genotype occurring in the absence of treatment changes resulted in improved resistance and reduced fitness in two cases (addition of I84V, addition of I93L), but improved fitness without a change in resistance in one case (substitution of L90M for L24I). When 1 pt was switched from saquinavir to nelfinavir, the new majority population expressed L90M instead of I84V. Interestingly, viruses with this genotype had been detected as a minority population prior to the change in therapy, and the strain expressing L90M was demonstrated to have better resistance to nelfinavir than that expressing I84V, but significantly lower resistance to saquinavir.
Conclusions: The improvement of resistance is the major force driving the evolution of the protease in pts failing PI and is often accompanied by a loss of viral fitness. Improvements in fitness without loss of resistance can also occur. Pre-existing minority populations with better resistance to the new agent can emerge following changes in treatment.