Irish drug company Elan Corp. PLC (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. firm Biogen Idec (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.

In a separate $1.5 billion deal recently announced between Elan and Johnson & Johnson, J&J will acquire the assets and rights of 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 firm, 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