May 26 (Reuters) - An experimental Mirati Therapeutics Inc drug that targets a specific genetic mutation shrank tumors in 44% of advanced lung cancer patients in clinical trials, but also caused serious side effects in 43% of them, the company said on Thursday.

Details from studies of the drug, adagrasib, were released ahead of the early June annual meeting of the American Society of Clinical Oncology (ASCO), sending shares of Mirati down more than 21% in after hours trading.

The oral drug is 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 type of lung cancer.

The first drug in this class, Amgen Inc's Lumakras, was approved by the U.S. Food and Drug Administration last year. Amgen in April reported data showing that its drug shrank tumors in 41% of advanced NSCLC patients.

The drugs are part of a growing trend of medicines that target gene mutations driving cancer regardless of which organ the disease originated.

The FDA is slated to make an approval decision on Mirati's drug by mid-December.

Mirati on Thursday said updated data from the Phase II study under review at the FDA shows that 43% of trial participants experienced serious treatment-related side effects, including increased levels of liver enzymes and anemia.

Two treatment-related deaths were reported and 7% of trial participants discontinued the treatment.

Mirati at the ASCO meeting is scheduled to present additional data on how effective adagrasib is at treating NSCLC that has spread to the brain.

The California-based company is also testing the drug as an initial treatment for NSCLC, both alone and in combination with Merck & Co's immunotherapy Keytruda.

Shares of Mirati, which closed at $58.76 in regular trading, were down $12.46, or 21.3%, at $46 after hours. (Reporting By Deena Beasley; Editing by Bill Berkrot and Richard Pullin)