SOUTH SAN FRANCISCO, Calif. (AP) — Rigel Pharmaceuticals Inc. said Monday that a licensing agreement announced last month with AstraZeneca has gone into effect after the expiration of a waiting period under the Hart-Scott-Rodino Antitrust Improvements Act.

Rigel said it is eligible to receive an upfront payment of $100 million from AstraZeneca tied to the effectiveness of the agreement, and additional milestone payments.

The companies signed a worldwide licensing deal for a potential rheumatoid arthritis treatment that has completed midstage clinical trials.

Under the deal, AstraZeneca gains development and sales rights to Rigel's fostamatinib disodium, or R788. The deal includes potential for up to $345 million in additional milestone payments and up to $800 million in payments based on future sales.

AstraZeneca, based in London, expects to begin late-stage studies on the drug during the second half of 2010, with the goal of asking for regulatory approval in 2013.

AstraZeneca also will receive exclusive rights to Rigel's portfolio of oral Syk inhibitors, as well as for additional indications for fostamatinib disodium beyond rheumatoid arthritis.

Rigel shares added 8 cents to $7.96 in morning trading. AstraZeneca shares rose 15 cents to $44.76.