Pfizer reported positive results from a phase III trial of maraviroc for the treatment of HIV. This 48-week trial was dubbed MERIT (maraviroc versus Efavirenz Regimens as Initial Therapy) and enrolled CCR5-tropic HIV-1 infected subjects who had never received antiretroviral therapy and had no evidence of resistance to the drugs used in the study. MERIT was designed to compare maraviroc (300 mg twice daily) to efavirenz, standard of care, (600 mg once daily), both dosed in combination with zidovudine/lamivudine.

In the full set analysis, the rates of virologic suppression were 70.6% and 73.1% for the maraviroc and efavirenz groups, respectively, for less than 400 copies/ml and 65.3% vs. 69.3%, respectively, at less than 50 copies/ml. Increases in CD4+ cell counts from baseline were also greater with maraviroc (+170 cells/mm3) than with efavirenz (+144 cells/mm3). In addition, fewer subjects in the maraviroc arm experienced Grade 3 or 4 adverse events compared to the efavirenz group.

The FDA issued an approvable letter for maraviroc in June of 2007.