Sept 30 (Reuters) - Merck & Co (MRK.N) said on Thursday it would buy Acceleron Pharma Inc (XLRN.O) for about $11.5 billion, broadening its portfolio beyond aging cancer drug Keytruda with potential treatments that could bring in fresh revenue. The deal gives Merck access to Acceleron"s rare disease drug candidate, sotatercept, which the company expects could bring in billions of dollars in sales, and comes as Keytruda moves toward the loss of market exclusivity in 2028. The lung cancer treatment accounted for 36.7%, or $4.18 billion, of Merck"s total sales in the second quarter. Sotatercept is currently in a late-stage study, testing it as a treatment for a rare cardiovascular disease called pulmonary arterial hypertension (PAH), a type of high blood pressure that affects blood vessels in the lungs. Merck sees PAH as a roughly $7.5 billion market by 2026. The drugmaker will pay $180 per Acceleron share in cash, representing a premium of about 2.6% to the stock"s closing price on Wednesday, according to Refinitiv data. Avoro Capital, which owns 7% of Acceleron stock along with its managed funds, said the deal undervalues the drugmaker. "We believe there should be no urgency to sell at a low price now since the value of the company will only increase as additional clinical trial data is released," it said in a statement. Although the Acceleron deal will not fully remove the overhang of generic competition for Keytruda, it does help diversify sales, which addresses a large shareholder concern, Cantor Fitzgerald analyst Louise Chen wrote in a client note. Merck is targeting a U.S. launch for sotatercept in 2024-2025 and its market exclusivity for PAH is expected to extend through 2036-2037. The deal also adds FDA-approved blood-related disorder drug Reblozyl, which Acceleron markets with partner Bristol Myers Squibb (BMY.N), to Merck"s portfolio.