Ra Pharmaceuticals Inc. priced its offering of 8.4 million shares at $6 apiece to raise about $50.4 million in gross proceeds.

The Cambridge, Mass.-based biopharmaceutical company granted the underwriters an option to buy up to an additional 1,260,000 common shares.

The company plans to use the net proceeds to fund its phase 2 trial of RA101495 to treat generalized myasthenia gravis — a neuromuscular disease causing skeletal muscle weakness — for other pipeline programs, and for working capital and general corporate purposes.

The offering is expected to close Feb. 16, subject to customary closing conditions.