Apellis Pharmaceuticals Inc. priced its offering of 9.5 million common shares at $37 per share.

The Waltham, Mass.-based biopharmaceutical company also granted underwriters a 30-day option to buy up to an additional 1,425,000 common shares at the same price.

Gross proceeds are expected to be about $351.5 million.

Citigroup, J.P. Morgan and Evercore ISI are acting as joint book-running managers for the offering, while Cantor Fitzgerald & Co. and Baird are acting as lead managers.

Apellis develops therapies through the inhibition of the complement system for autoimmune and inflammatory diseases. In Jan. 7, the company said a late-stage study showed that its drug APL-2 increased hemoglobin protein in patients with paroxysmal nocturnal hemoglobinuria — a rare life-threatening disease that destroys red blood cells during a late-stage study.