Test shows when pills help lung cancer patients

*Test shows 71 pct of patients with mutation benefit

*Could return Iressa to US market

By Maggie Fox, Health and Science Editor

WASHINGTON, May 14 (Reuters) - A test that looks for a certain mutation in tumors can predict which lung cancer patients will be helped by targeted pills such as Iressa and Tarceva, researchers reported on Thursday.

The study, to be presented at a meeting of the American Society of Clinical Oncology, should help doctors better choose patients who can take the pills, outside experts said.

AstraZeneca Inc. (AZN.L) stopped most U.S. sales of Iressa, known generically as gefitinib, in 2004 after it was found to help only about 10 percent to 15 percent of cancer patients.

But it has followed up on tantalizing evidence that some people -- notably non-smokers, Asians and women -- did better on Iressa.

Dr. Masahiro Fukuoka of Kinki University in Osaka, Japan and Dr. Tony Mok of the Chinese University of Hong Kong and colleagues tested tissue samples from 683 patients with non-small-cell lung cancer, the most common type.

Iressa and Tarceva, known generically as erlotinib and made by Genentech, now owned by Roche AG (ROG.VX), are monoclonal antibodies -- genetically engineered immune system molecules that target a molecule called epidermal growth factor receptor or EGFR. Tumors use EGFR to grow themselves blood supplies.