Shares of Five Prime Therapeutics Inc. blasted 353% higher on heavy volume toward a three-year high in premarket trading Wednesday, after the company said its treatment for gastric cancer met all three of its efficacy endpoints from Phase 2 trial. Trading volume ballooned to 2.9 million shares, compared with the full-day average of about 659,000 shares. "These results bring us one step closer to the first potential targeted therapy for advanced gastric cancer in over a decade," said Five Prime Chief Medical Officer Helen Collins in a statement late Tuesday. She said she "looks forward to discussing next steps with health authorities worldwide." J.P. Morgan analyst Eric Joseph followed on Wednesday by upgrading Five Prime to neutral from underweight, saying the results were an "upside surprise to our low held expectations." The stock has climbed 16.3% year to date through Tuesday, while the iShares Nasdaq Biotechnology ETF /zigman2/quotes/206189322/composite IBB +1.13% has advanced 14.3% and the S&P 500 /zigman2/quotes/210599714/realtime SPX +0.31% has gained 9.7%.