At Chiron Corporation, business is business for a landmark interleukin-2 (IL-2) study. In October, the company announced it would pull the plug on its trial of the immune therapy after three and a half years. Dubbed SILCAAT, the safety-and-efficacy trial charted 1,975 HIVers in 11 countries. All had low (50 to 299) CD4s; one half got HAART alone, the other half got HAART plus periodic shots of IL-2 to stimulate CD4-cell production.

"It was primarily a business decision," says Chiron VP Bruce Scharschmidt, MD. "The size and duration of the trial increased, and our thinking regarding the overall probability of success changed. To continue an investment for five years or more would have jeopardized our ability to initiate or advance some of our more promising programs in our pipeline." Scharschmidt estimates that it would cost some $100 million to complete SILCAAT in five years.

But critics claim that Chiron could cover the tab: On October 20, three days before lowering the boom, the company reportedly made a takeover offer of $705 million to Brit-based vaccine-maker PowderJect.

So are SILCAATs up a tree? No, says Scharschmidt. Chiron promises to make IL-2 available to the subjects for "some extended period of time." It also asked the FDA to approve IL-2 for HAART-takers with low CD4s. Recent small studies suggest that IL-2 does in fact produce significant and sustained CD4 gains. Still, long-term effects remain to be seen. But with the Chiron trial abandoned, we may never know if IL-2 actually works -- by decreasing AIDS-defining illness and deaths. Therefore, Chiron may regret letting SILCAAT out of the bag.