Higher efavirenz concentrations determine the response to viruses carrying non-nucleoside reverse transcriptase resistance mutations.

We examined the influence of both efavirenz plasma concentrations and non-nucleoside reverse transcriptase (NNRTI) resistance mutations on the antiviral activity of efavirenz in patients experiencing early virological failure under nevirapine-containing regimens. Up to 41% of patients reach less than 50 copies/ml at 48 weeks. No association was found between the presence of NNRTI resistance mutations and virological outcome. Nevertheless, patients responding virologically and carrying NNRTI-resistant viruses had higher efavirenz levels than those who did not respond.