Shares of the drug developer Summit Therapeutics were up more than 12% Thursday morning on news that its drug is performing well in a Chinese lung cancer study.

Summit stock jumped to $16.46 after the Miami-based start-up said that its drug ivonescimab was extending the survival of lung cancer patients better than the world's best-selling drug-Merck's Keytruda-in a head-to-head trial in China run by Summit partner Akeso. The companies didn't disclose the extent of ivonescimab's survival advantage, but they said they will present more details at a Sept. 15 medical conference.

Last week, Akeso reported that ivonescimab delivered a survival advantage in rare, but aggressive, cancers of the bile duct.

Ivonescimab is the furthest-advanced in clinical trials of drugs seen as the successors to Keytruda, which brought in $32 billion in sales to Merck last year. A so-called bispecific antibody, ivonescimab combines Keytruda's immune-enhancing features with the power of drugs like Roche Holding's Avastin to block tumors from building new blood supply. The bispecifics seem to work even better than simultaneous use of the separate drugs.

Summit, Bristol Myers Squibb, and Pfizer are all testing bispecifics in the hope that they can replace Keytruda and other immuno-oncology drugs as part of the standard treatment, with chemotherapy, against many cancers. The first readout in a global trial comparing that standard regime against a bispecific plus chemo will be Summit's Harmoni-3 study in lung cancer-which is expected to start reporting data before the end of this year.

Patents begin to roll off Keytruda in the next few years, but Merck has bet on replacing the chemo side of the standard treatment with drugs known as antibody-drug conjugates, which narrowly target the chemo toxins to cancer cells. But Merck has a bispecific ready for Phase 3 testing if Summit's Harmoni-3 results prove compelling.

Summit is basically a one-drug bet on ivonescimab, whose international rights it licensed several years ago from the Hong Kong-listed Akeso. Shares of Summit have climbed by as much as 400% in the last five years, as Akeso has reported successful trials and approvals for the drug in China. Thursday's price action values the pre-revenue Summit at nearly $13 billion.

UBS analyst David Dai said that he expected Thursday's good news from Akeso's China study, thanks to his statistical conclusion that ivonescimab's advantage increases as time goes by. But he thinks that skepticism about the drug left many traders surprised.

The Akeso study is the fourth Phase 3 study demonstrating ivonescimab's superiority, Dai notes. He rates Summit a Buy and bets the stock can climb to $33.

At H.C. Wainwright, Mitchell Kapoor is less enthusiastic about Summit and its drug. Until ivonescimab's developers report the size of its survival advantage, Kapoor remains unsure it will be large enough to offset the bispecific's non-negligible side effects. He has a Neutral rating on Summit.