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 3 trial enrolling only those patients by the end of this year, Mirati Chief Executive Officer David Meek told Reuters.

The trial will compare the adagrasib combination with the current standard of care, which is chemotherapy plus Keytruda, also known as pembrolizumab. 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 study data shows that 19.4% of previously untreated, metastatic NSCLC patients treated with Keytruda plus chemotherapy were alive after five years, compared with 11.3% of patients treated with a placebo and chemotherapy.

Adagrasib is an oral drug designed to 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, and less frequently in some other solid tumors.

Out of seven Phase 2 trial patients given the combination treatment, at a median follow-up of 19 months, four (57%) had tumor shrinkage of at least 30%, and none had tumor growth, Mirati said.

A preliminary analysis of 53 patients showed that 49% of them had responded to the combination treatment, and 89% had controlled disease.

Serious side effects occurred in 44% of patients, including increased levels of pancreatic enzymes in 11%, and higher liver enzymes in 8-9%. Mirati said the rates were consistent with levels seen when either drug is used on its own.

Amgen Inc, which sells a KRAS-inhibitor called Lumakras, earlier this year said it had shelved immunotherapy combination studies after finding that high numbers of patients experienced liver toxicity.

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

Meek declined to comment on takeover speculation that has 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 )