UPDATE 1-Mirati to focus adagrasib/Keytruda study in low PDL1 lung cancer patients

(Adds analyst comment, share price; clarifies details of Amgen studies)

By Deena Beasley

Dec 5 (Reuters) - Early trial data shows that a combination of Mirati Therapeutics experimental drug adagrasib and Merck & Co's immunotherapy Keytruda helped about half of previously untreated metastatic lung cancer patients, with manageable side effects, Mirati said on Monday.

The company expects the largest advantage in patients with lower levels of PDL1, the protein targeted by Keytruda, and plans to launch a phase-three trial enrolling only those patients soon, Mirati chief executive officer David Meek told Reuters.

Wall Street analysts called the early results encouraging, but shares of Mirati were down more than 8% at $84.50 after hours on Monday. More trial details will be presented on Wednesday.

"We believe the data continue to support the regimen's differentiated positioning relative to standard of care and are far more favorable than that of Lumakras plus (immunotherapy)," JP Morgan analyst Eric Joseph said in a research note on Monday, referring to a rival drug sold by Amgen Inc.

Both drugs target a mutated form of a gene known as KRAS that occurs in about 13% of non-small cell lung cancers (NSCLC), the most common form of the disease.

Mirati's phase-three trial will compare adagrasib and Keytruda with chemotherapy plus Keytruda. The initial treatment for patients with higher levels of PDL1 is Keytruda alone, and Mirati will likely begin planning next year for a combination study in those patients, Meek said.

Recent data shows that 19% of NSCLC patients treated with Keytruda plus chemotherapy were alive after five years, compared with 11% of patients treated with a placebo and chemotherapy.

Out of seven phase-two trial patients given the adagrasib combination, four (57%) had tumor shrinkage, and none had tumor growth, Mirati said.

A preliminary analysis of 53 patients showed that 49% of them had responded to the combination treatment.

Serious side effects occurred in 44% of patients, including increased levels of pancreatic enzymes in 11% and higher liver enzymes in 8% to 9%.

Amgen earlier this year said more study was needed after a trial of Lumakras in combination with immunotherapies found that high numbers of patients experienced liver toxicity.

Mirati expects a U.S. Food and Drug Administration decision by Dec. 14 on its application to sell adagrasib for advanced NSCLC patients who have stopped responding to other drugs.

Meek declined to comment on takeover speculation that had pushed shares of Mirati up nearly 40% since mid-November.

"We're laser-focused on crossing the finish line with the approval and then we have to launch the drug .... We're so busy right now," he said. (Reporting by Deena Beasley in Los Angeles; Editing by Matthew Lewis and Bradley Perrett)