Gilead Sciences' (NASDAQ:GILD) mission to create a better oncology portfolio took a big step forward on Sunday. The company best known for antiviral treatments made Immunomedics (NASDAQ:IMMU) a juicy buyout offer that values the recently successful cancer drug developer at $21 billion.

Immunomedics shareholders will receive $88 per share in cash from Gilead Sciences when the deal closes, most likely before the end of the year. The price Gilead offered is 108% more than Immunomedics' most recent closing price.

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Acquiring Immunomedics will add the recently approved breast cancer treatment, Trodelvy, to an oncology lineup at Gilead that generated sales of just $334 million in the first half of 2020. Trodelvy is an antibody-drug conjugate that delivers a dose of chemotherapy directly to tumor cells after recognizing a protein called Trop-2 commonly found on their surface.

In April, the FDA approved Trodelvy as the first available treatment for the underserved population of patients with triple-negative breast cancer (TNBC), and Trop-2 is also known to appear on tumors that originate in the pancreas and bladder. As a first available treatment for cancer patients who lack effective options, annual sales of Trodelvy are expected to reach $2.3 billion by 2026.

The FDA granted Trodelvy accelerated approval based on impressive tumor response rates from a single-arm trial with 108 advanced-stage TNBC patients who had already received at least two previous lines of therapy. Immunomedics will present longer-term survival results at the virtual meeting of the European Society for Medical Oncology, which will take place Sept. 19-21.