Irish drug company Elan Corp. (ELN) has an arrangement whereby its U.S. strategic partner, Johnson & Johnson (JNJ), will help Elan finance the purchase of another 50% stake in multiple-sclerosis drug Tysabri - but only if that opportunity arises, Chief Financial Officer Shane Cooke said Tuesday.

Elan and U.S. company Biogen Idec Inc. (BIIB) are joint-venture partners on Tysabri, whose annual revenue is fast reaching the $1 billion mark, according to Elan. If there is a change of ownership in Biogen Idec, then Elan has the option to buy the other 50% of Tysabri, or to sell its 50% stake in the MS drug.

A Biogen spokeswoman declined to comment on any agreement between Elan and Johnson & Johnson, but noted that the change-of-control provision with Elan allows "several different methodologies" of valuing the Tysabri stake, including a third-party valuation.

In a separate $1.5 billion deal recently announced between Elan and Johnson & Johnson, J&J will acquire the assets of and rights to Elan's Alzheimer's program co-partnered with U.S. firm Wyeth (WYE) and invest $1 billion in Elan in exchange for new American Depositary Receipts of the Irish company, representing 18.4% of its outstanding ordinary shares. Elan will retain a 49.9% interest in the new Johnson & Johnson subsidiary holding those assets.

Johnson & Johnson will assume and continue Elan's 50-50 joint-venture Alzheimer's program with Wyeth, and will initially commit up to $500 million toward the development and launch of Alzheimer's treatment bapineuzumab, or AAB-001.

Company Web site: www.elan.com

-By Quentin Fottrell, Dow Jones Newswires; +353-1-6762189; quentin.fottrell@dowjones.com

(Thomas Gryta contributed to this report.)