Shares of the South San Francisco company at last check quadrupled to $20.88.
"These results bring us one step closer to the first potential targeted therapy for advanced gastric cancer in over a decade," Helen Collins, a physician who is executive vice president and chief medical officer, said in a statement.
The trial enrolled 155 patients in 15 countries across Asia, the European Union, and the U.S. Several analysts responded positively to the test results.
J.P. Morgan analyst Eric Joseph upgraded the stock to neutral from underweight. He said in a note to investors that "in an upside surprise to our low held expectations, Five Prime announced positive top-line data from the Phase 2 Fight trial of bemarituzumab."
Wedbush analyst Robert Driscoll, who rates the stock outperform and tripled his price target to $28 from $9, said, "Given the consistency of the data with other [first-line] gastric cancer studies, [the] high quality nature of this double-blind, placebo-controlled study, as well as the significant unmet need, we see potential for accelerated approval for bema in FGFR2b-overexpressing gastric cancers."
SVB Leerink analyst Jonathan Chang raised his target to $33 a share from $9.
"While there are both positive and negative takeaways in the top-line data disclosure, as well as unanswered questions, the update is clearly a net positive and a big win for the stock amidst persistently low investor expectations," said Chang, who has an outperform rating on Five Prime.
Jefferies analyst Eun Yang reiterated his buy rating on Five Prime and raised his price target to $23 from $8, saying the test is a “key inflection point” for the company.