Presence of lamivudine or emtricitabine is associated with reduced emergence of nonnucleoside reverse transcriptase inhibitor mutations in an efavirenz-based intermittent antiretroviral treatment regimen.

Efavirenz concentrations were measured in 21 patients during an interruption cycle of the ANRS 106 Window trial. The median efavirenz concentrations in the patients 12 h, 3 days, and 7 days after discontinuation of the drug were 1,962 ng/ml, 416 ng/ml, and 112 ng/ml, respectively. The half-life ranged from 27 to 136 h. No relationship between efavirenz exposure and detection of nonnucleoside reverse transcriptase inhibitor (NNRTI) mutations was demonstrated. Patients who were treated by a lamivudine- or emtricitabine-based regimen had a lower risk of NNRTI mutation selection.