Nektar Therapeutics Inc., of San Francisco, commenced an underwritten public offering of $175 million of shares of its common stock. In connection with that offering, Nektar will also grant to the underwriters a 30-day option to purchase up to an additional $26.25 million shares. J.P. Morgan is acting as the sole book-running manager. Nektar said it intends to use the net proceeds for general corporate purposes, including research and development funding and working capital.