Merck & Co. (MRK  ), one of the largest pharmaceutical companies on the planet, has announced that it plans on purchasing Acceleron Pharma Inc. (XLRN  ) for $11.5 billion. With its rare disease drug candidate, sotatercept, Acceleron represents a new source of sales for Merck as one of its biggest moneymakers approaches the end of its market exclusivity.

Keytruda, a lung cancer medication, brought in $4.18 billion in the second quarter of 2021 alone, making up more than a third of Merck's total sales for that quarter. However, in 2028, Keytruda will lose its market exclusivity, something that pharmaceutical insiders say "could mean the end of a product's value". At that point, generic versions of the drug can be put on the market.

Acceleron's sotatercept, used to treat a certain high blood pressure disease called pulmonary arterial hypertension (PAH), could bring in billions of dollars in revenue once it goes on the market, according to Merck.

PAH is a fatal disease that currently has no cure. Potential treatment sotatercept is currently still undergoing late-stage testing, but the NASDAQ reported that peak sales of the drug are estimated at $2 billion. Merck says it foresees the total PAH treatment market being worth $7.5 billion by 2026.

Sotatercept isn't the only promising drug in Acceleron's portfolio. Another drug that the NASDAQ estimates will bring in $2 billion at its peak is Acceleron's Rebozyl, marketed in partnership with Bristol Myers Squibb (BMY  ). Rebozyl is used to treat anemia in patients with myelodysplastic syndromes (MDS).

Some Acceleron shareholders say that Merck's offer of $180 in cash per share, 2.6% above Acceleron's closing price the day before the offer was made, is too low given the potential money to be made off of this potentially life-changing drug.

"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," Avaro Capital, which owns 7% of Acceleron, wrote in a statement.

Cantor Fitzgerald analyst Louise Chen wrote in a note to the investment firm's clients that, while sotatercept isn't likely to make up for the loss of revenue Merck will suffer when generic versions of Keytruda hit the market, it will help soften the blow.

Merck is planning for a 2024-2025 launch date for the PAH treatment, meaning the company's exclusivity on the drug would likely last through 2036 or 2037.

The NASDAQ reports that Merck's shares have been likely undervalued over the past year, given that they've fallen in value despite increases in the company's revenue. Share prices hit their 52-week high in September of last year.