(Reuters) - Pfizer Inc. said on Tuesday it plans to raise $31billion from a debt offering to finance its proposed acquisition of Seagen Inc.

Pfizer struck a $43 billion deal in March to acquire Seagen and its targeted cancer therapies as it prepares for a steep fall in COVID-19 sales and generic competition for some top-selling drugs.

The company has been pumping billions of dollars into research and acquisitions to mitigate an anticipated $17 billion hit to revenue by 2030 from patent expirations for top drugs, and a decline in demand for COVID products.

The debt offering, which would be in eight tranches, is expected to close on May 19, the company said, adding that BofA Securities, Citigroup, Goldman Sachs and J.P. Morgan are acting as the joint lead managers and joint book-running managers.