Shares of South San Francisco, Calif.-based

Exelixis

(EXEL) - Get Report

soared Tuesday after

GlaxoSmithKline

(GSK) - Get Report

agreed to help fund its research over the next six years and buy a small stake in the firm.

Exelixis shares shot up almost 50% to $5.30 after the companies said they'd collaborate on new treatments for cancer, inflammatory diseases and vascular biology.

Glaxo agreed to pay $30 million for 2 million newly issued shares of Exelixis at $7 a pop, nearly double its current stock price, giving it a roughly 3% stake.

Glaxo will provide Exelixis with at least $90 million in research and development funding over the next six years, and Exelixis will also receive milestone payments based on the success of its small-molecule compounds that meet specific criteria in an early phase II clinical trial. If successful, Glaxo will have the right to further develop those compounds, although Exelixis retains copromotional rights in North America.

"For GlaxoSmithKline this alliance promises to complement our own accelerating drug-discovery activities by gaining access to the expanding array of capabilities which Exelixis has assembled rapidly in recent years," said Tadataka Yamada, Glaxo's chairman of research and development.

Based on successful development of the compounds, payments to Exelixis could range from $220 million to $350 million, the companies said. In addition, Glaxo will make available a loan facility to Exelixis of up to $85 million.