Shares in US group Genentech slipped 1.5% in after-hours trading on the New York Stock Exchange last night, as investors seemed to concentrate on a disappointing first-quarter performance of one of its key drugs instead of the 48% growth in profit it booked for the period.

The world’s second-largest biotechnology company, which is majority-owned by Swiss drug giant Roche, said it had generated profit of $421 million, or $0.39 per share, for the first quarter of 2006, compared to $284 million, or $0.27 a share, for the year-ago period.

Growth was driven by strong sales, which jumped 39% to just under $1.2 billion, led by solid US sales of its cancer drugs Avastin (bevacizumab), rocketing 96% to $390 million, and Herceptin (trastuzumab), which shot up 123% to $290 million, as well as higher than expected royalty revenues for the period. Other drugs turning in solid results were: Tarceva (erlotinib), up 94% to $93 million; Xolair (omalizumab), climbing 46% to $95 million; and Raptiva (efalizumab), which grew 24% to $21 million.

But sales of Rituxan (rituximab) came in under par, climbing just 8% from the year-ago period to $477 million, but slipping from the prior quarter’s $484.4 million, disappointing both investors and analysts, who had been expecting the drug to perform better.

All-in-all, however, the first quarter was pretty positive for Genentech, which said it is raising its forecast for full-year earnings per share growth to 45%-55%, excluding special items, from its original 40%-50% target, on higher expectations for product sales and royalty revenues.