Achillion Pharmaceuticals Inc. stock soared nearly 21 percent by close Wednesday following the company's pricing of an underwritten public offering of 9.6 million shares at $5.90 a share, a 1.8 percent discount. The New Haven, Conn.-based company expects net proceeds of about $52.9 million, and could add about $7 million more through an underwriters' overallotment option for as many as 1.44 million shares.