SOUTH SAN FRANCISCO, Calif. (AP) — Onyx Pharmaceuticals Inc. said Wednesday that it expects its stock sale will generate gross proceeds of $358.6 million, which it will use for cancer drug research and development and to help market its drug Kyprolis.

The company is offering 4.4 million shares of its common stock, and has granted underwriters the option to buy another 660,000 shares if demand exceeds initial supply. The projection would estimate a sale price of about $81.50 each.

Kyprolis is an injectable drug approved as a treatment for multiple myeloma, a type of cancer that causes tumors to grow in the bone marrow, preventing the production of normal blood cells. The FDA approved Kyprolis on July 20, allowing Onyx to market it for use in patients who have already been treated with at least two other multiple myeloma drugs without success.

Onyx also has developed a colorectal cancer pill with Bayer, called Stivarga, and developed Nexavar, which is used to treat liver and kidney cancer.

The company said the offering is expected to close on or about January 22. BofA Merrill Lynch and Barclays are acting as joint book-running managers for this offering.

Shares of the South San Francisco-based company fell nearly 5 percent to $79.29 in afternoon trading on three times average daily trading volume.