Shares of Nastech Pharmaceutical ( NSTK) sank Thursday after the company said it priced a public offering of 3.25 million shares at $13.20 a share. The stock fell 5.7% to $13.26. The $42.9 million offering is being made under a shelf registration that became effective Nov. 29, the company says. All of the shares are being sold by Nastech. UBS Investment Bank acted as sole bookrunner for the offering. The company has granted the underwriter an option to purchase an additional 487,500 shares to cover overallotments, if any.