Genentech Inc. announced that Activase, its new blood-clot dissolving drug, rang up hefty sales of $58 million after only 45 days on the market--making it one of the most successful drugs ever introduced, according to analysts. The company also predicted earnings for its 1987 fiscal year would leap by 250% when a charge for a buyout of a partnership is excluded. Sales are expected to rise more than 60% over revenue of $134 million in 1986, the company said. Activase sales figures and the earnings prediction caused Genentech shares to rise $1.875 to $44.625. Activase, one of the most expensive drugs on the market at a price of $2,200 per injection, is used to treat heart attack victims.