Genentech Inc.'s $50-million-plus investment in the Activase GUSTO trial apparently is showing at least a short-term pay off, as second-quarter sales jumped 25 percent to $55.8 million from $44.6 million in the year-prior quarter and 14 percent from $49.2 million in the pre-GUSTO first quarter of 1993.

The 41,000-patient GUSTO results on April 30 showed that Activase tissue plasminogen activator reduced mortality in heart attack patients by 14 percent compared with another clot buster, streptokinase. Although Activase is priced