A recent study shows that while about one-third of AIDS patients on Pfizer's Viracept medication developed resistance to protease inhibitors over 48 weeks, users of Abbott Laboratories' Kaletra drug did not become resistant. The results may indicate that Kaletra can be utilized as a first-line therapy for AIDS, says the University of Pittsburgh Medical Center's John W Mellors, and could be used more than once in treating HIV, both in early and later stages of the disease. Abbott's director of antiviral-drug development, Eugene Sun, does caution that virus mutations could eventually cause Kaletra users to develop protease inhibitor resistance.