SAN FRANCISCO - Biotechnology pioneer Genentech mulled Roche's $43.7 billion offer for its remaining shares Monday as investors made clear their feelings that the Swiss pharmaceutical giant should pay considerably more.

But some analysts said Roche, already Genentech's majority shareholder, has leverage to keep the price for the maker of the blockbuster cancer therapy Avastin and other cancer treatments from spiraling much higher.

Genentech's stock price climbed well above Roche's offer of $89 per share in Monday trading -- a bet that the independent directors weighing the Swiss company's bid will demand more money.

Shares jumped $12.06, or nearly 15 percent, to close at $93.88.

Many Wall Street analysts said that Roche's offer did not reflect the profit potential of Avastin or the San Francisco company's other popular drugs. Roche owns about 55.9 percent of Genentech.

"We believe Roche is attempting to capture Genentech's significant future growth on the cheap," Oppenheimer & Co. analyst Bret Holley wrote to investors. Some analysts predicted Roche's offer would have to climb to more than $100 per share to be acceptable to Genentech.

Still, an agreement outlined in Securities and Exchange Commission filings for Genentech's 1999 public offering of stock indicates Roche has clout that could help it negotiate a final bid closer to its initial offer.

Under the agreement, a majority of non-Roche shareholders must approve any sale or merger of Genentech with Roche. If shareholders reject the bid, according to the agreement, Roche can hire investment banks to set the price for Genentech's remaining shares.

Rather than gamble on banks setting an even lower price than Roche has already offered, Genentech's board members would likely negotiate an agreement within a few dollars of the $89-per-share bid, said Eric Schmidt, an analyst for Cowen and Company.

Genentech declined to comment on any aspect of the offer or how quickly the company would respond except to say in a statement that a special committee of the independent directors "will be convened promptly to determine what action to take."

Among the directors considering the offer is Herbert Boyer, 71, who helped create the biotech industry when he co-founded Genentech in 1976.

Since then, the company has scored repeated successes with some of the most popular biopharmaceuticals on the market. Its top-selling cancer drug Avastin brought in $2.3 billion in sales in 2007, a number analysts expect to see go higher following Avastin's surprise approval in April as a treatment for breast cancer.

Avastin is already approved to treat lung and colorectal cancer.

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