Dosing concerns aside, some disease areas have more combo-pill potential than others. "Anytime you have multiple mechanisms of action for a single disease, [the success of a combination pill] will be better," says Million.HIV treatment is an ideal area for fixed-dose combination pills.Gilead already has two three-in-one pills on the market-Atripla and Complera-both of which combine the company′s two-drug combo Truvada together with a single antiviral from either BMS or Tibotec Pharmaceuticals.Now, with the Quad, Gilead is adding two of its own investigational drugs to Truvada, creating a four-pronged attack in which each ingredient blocks viral replication in a different way. "It′s a neater package with antiviral agents than it is with other diseases simply because of the nature of the target," says William Valenti, founding director of the AIDS Care clinic in Rochester, New York.Combination pill development is also prevalent in the cardiovascular arena, with pills that strike multiple heart-related conditions in one go.In 2010, the FDA approved Japan′s Daiichi Sankyo′s Tribenzor for high blood pressure; the three-in-one medication contains a diuretic, an angiotensin receptor blocker and a calcium channel blocker.But some drugmakers are aiming higher.Bolstered by pos. phase 2 trial results, India′s Cadila Pharmaceuticals has sold four- and five-drug heart pills composed of generic drugs in its home country since 2009.To prove the pill′s efficacy, Cadila plans to launch a 5,000-person, phase 3 trial for the four-drug version in early 2012, and phase 3 trials for a similar heart pill are already underway in Europe, New Zealand, and Australia for the Indian pharmaceutical company Dr.Reddy′s.But combining too many medicines together can compromise chem. stability, not to mention create comically large tablets. "It′s a bioengineering achievement to be able to get all that in a pill that you can get into your mouth," Valenti says.After the Quad, "I′m not sure we need any more agents in there."