Rewrites throughout to include details on share reversal

By Christy Santhosh

June 1 (Reuters) - Shares of Summit Therapeutics SMMT.O fell 11%, reversing premarket gains as concerns over weaker overall survival benefits in older patients outweighed strong late-stage trial results for its experimental lung cancer drug.

In a head-to-head study conducted in China, patients with advanced squamous non-small cell lung cancer who received ivonescimab and chemotherapy lived an average of 27.9 months, compared with 23.7 months for those who received Tevimbra and chemotherapy.

At first glance, the trial data was a hit with the "overall survival data clearing even the highest investor expectations," said Evercore analyst Cory Kasimov.

At least three brokerages, however, flagged concerns around ivonescimab's efficacy across ages after the trial data — presented at the American Society of Clinical Oncology meeting in Chicago by Summit's China-based partner Akeso 9926.HK — showed a weaker survival signal in patients aged 65 and above.

The median overall survival improvement of 4.2 months seemed smaller than what the hazard ratio suggested, said Jefferies analyst Faisal Khurshid.

A hazard ratio compares the risk of an event like disease progression or death occurring in one group versus another over a specific period of time.

Experts also questioned the China trial data's translatability to a global population, patient selection and efficacy in elderly patients.

Summit holds the rights for the drug in the U.S., Canada, Europe and Japan through a deal worth up to $5 billion, while Akeso retains the rights for China and the rest of the world.

Khurshid said investors were likely to focus on whether similar benefits can be replicated in global trials, particularly in the U.S. and Europe.

Summit shares fell 7.2% to $16.29 in afternoon trading.

Separately, shares of Revolution Medicines RVMD.O, which also presented late-stage data at the meeting, rose 3.7% to $163.71.

In a trial testing patients with advanced pancreatic cancer who had failed one round of chemotherapy, Revolution's once-daily pill, daraxonrasib, doubled survival compared to standard chemotherapy.

Raymond James analyst Sean McCutcheon called daraxonrasib's overall survival results a "home run" and said he expected a rapid and broad uptake in patients with advanced pancreatic cancer.