Fibrogen Inc. became the Street's new biotech sweetheart Friday, pricing its upsized initial public offering (IPO) of 8.1 million shares at $18 apiece, the high end of its range, to raise $145.8 million. The company could add another $21.9 million and become the largest U.S.-based biotech IPO this year, if underwriters fill a 30-day overallotment option to purchase an additional 1.2 million shares, eclipsing the $146.6 million raised by Santa Monica, Calif.-based Kite Pharma Inc. in its IPO.