Monday, September 19, 2005—GlaxoSmithKline (GSK) has stumbled upon “severe” liver-toxicity problems with the experimental HIV drug aplaviroc (also known as GSK-873,140) and abruptly canceled a clinical trial studying its effect on HIVers who’ve never taken meds before. Aplaviroc is a CCR5 antagonist, a new type of entry inhibitor that researchers hoped would avoid the kinds of serious side effects seen with other classes of HIV drugs. GSK’s announcement included a notice that it would be closely monitoring liver function in another aplaviroc trial as well, this one enrolling treatment-experienced HIVers. The other two experimental CCR5 antagonists in the race for FDA approval are Pfizer’s maraviroc and Schering-Plough’s vicriviroc.