US pharmaceutical giant Gilead Sciences confirmed Sunday that it is buying American biotech firm Immunomedics, which makes a breast cancer drug, for $21 billion.

Under the transaction, to be finalized in the fourth quarter of 2020, Gilead will buy Immunomedics at a price of $88 per share, more than double the current value of $42.25.

The California drug giant plans to fund the transaction with $15 billion in cash and $6 billion in new bonds.

"This acquisition represents significant progress in Gilead's work to build a strong and diverse oncology portfolio," Gilead Chairman and CEO, Daniel O'Day said in a statement.

New Jersey-based Immunomedics makes Trodelvy, an antibody drug approved in April by the FDA, which is used to treat , a particularly aggressive form of the disease.

Triple-negative represents about 10 to 15 percent of all breast cancers, according to the American Cancer Society.

Studies are underway to assess the efficacy of Trodelvy in treating lung and bladder cancer.

Immunomedics stock has jumped more than 90 percent on Wall Street since Trodelvy's approval.

Gilead took a giant step forward in the rapidly developing oncology sector with its $4.9 billion April acquisition of Forty Seven, which is developing a promising treatment for blood cancers.