Background: Early virological response (VR) to enfuvirtide-based salvage regimens at week 12 has been described as a predictor of long-term therapeutic success. The relationship between enfuvirtide plasma exposure and VR has not yet been investigated in the clinical setting. Our aim was to investigate the role of enfuvirtide plasma exposure as a determinant of early VR in the clinical setting. Methods: Forty-two multidrug-experienced patients starting a salvage enfuvirtide-based regimen were prospectively evaluated over a 12 week period. HIV-RNA levels and enfuvirtide Ctrough were regularly measured. VR was considered as achievement of viral load (VL) undetectability and/or a decrease of more than 1 log at week 12. Results: Optimized background score (OBS) and enfuvirtide Ctrough concentrations were associated with VL decrease at week 12. An OBS ≥2 and enfuvirtide Ctrough >2100 ng/mL were associated with VR. The pharmacokinetic/pharmacodynamic (PK/PD) analysis confirmed this exposure-response relationship both in the total population and in different groups according to OBS 50 were calculated for the OBS 0 (0.31 versus 0.21 log) and Imax (-64 versus -3.33 log). Conclusions: Enfuvirtide plasma exposure and OBS were found to significantly influence the magnitude and rate of early VR. The PK/PD modelling of enfuvirtide concentrations was different in our clinical setting, compared with previous data obtained under trial conditions. Therefore, optimization of enfuvirtide plasma exposure could deserve further evaluation as a determinant of therapeutic response in HIV-positive patients.
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