Seaport Therapeutics Inc., shares jumped 17% in the firm’s trading debut, after the company raised nearly $255 million in an upsized initial public offering.

The Boston-based company’s shares opened at $21 each, above the IPO price of $18 apiece. The biotech firm sold 14.16 million shares in the IPO, after having offered 11.8 million shares for $16 to $18 each.