Tesaro's stock drops, as stock offering comes a day after stock doubles

Shares of Tesaro Inc. tumbled 5.2% in premarket trade Thursday, after the cancer-focused biopharmaceutical company said it launched a public offering of $300 million worth of its common stock. The offering comes a day after the stock more than doubled--up 108%--after the company announced positive results from a late-stage trial of its treatment for ovarian cancer. The company granted the underwriters of the offering options to buy an additional $45 million worth of its stock. Based on Wednesday's closing price of $77.40, if the underwriters exercise the options, the company could be selling 4.45 million new shares, which could increase the shares outstanding by 9.7%, according to FactSet data. Tesaro said the offering was subject to market conditions. The stock has surged 48% year to date through Wednesday, while the S&P 500 has gained 1.3%.

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