RP Management LLC, an investor in royalty streams from pharmaceuticals, offered to buy Elan Corp. (ELN) for about $6.5 billion, threatening the Irish drugmaker’s plan to embark on its own acquisitions. A sale of Elan would allow shareholders to avoid the substantial risks of the company’s plan to make purchases with the $3.25 billion it will receive from selling its stake in the Tysabri multiple-sclerosis drug to Biogen Idec Inc. (BIIB), the New York-based firm, known as Royalty Pharma, said in a statement today. The informal offer of $11 per American depositary receipt is 3.8 percent above the closing price Feb. 22, and represents “the full value of Elan today,” RP said. The offer is a challenge to Dublin-based Elan’s strategy of reinvesting the proceeds from the Tysabri divestiture, which will leave the company with virtually no operations. Chief Executive Officer Kelly Martin’s plan to buy drugs that are on the market, late-stage experimental products or some early-stage clinical research projects drew skepticism from investors when announced Feb. 6. The company said Feb. 22 it also would buy back $1 billion of stock. “The conservative investors will say let’s tender the shares because of the uncertainty about what management will do with the cash,” Olav Zilian, an analyst at Helvea SA in Geneva who has a reduce rating on the stock, said in a telephone interview. “It’s still open as to where the cash will be reinvested and shareholders possibly have no say in that.” No Response: Royalty Pharma said it went public with its offer after Elan failed to respond an approach made last week.