By Deena Beasley

June 2 (Reuters) - Amgen's AMGN.O Imdelltra reduced the risk of death by 40% compared to chemotherapy for small cell lung cancer patients whose disease had worsened after an initial round of chemo, according to interim data from a late-stage trial presented at a major medical meeting on Monday.

The Phase 3 trial of 509 patients showed that Imdelltra extended median overall survival by more than five months to 13.6 months, compared with 8.3 months for standard-of-care chemotherapy, the company said.

The median length of time patients lived without their disease worsening, a measure known as progression-free survival, was 4.2 months for the Imdelltra group and 3.7 months for patients who received chemotherapy.

Amgen said the trial results are intended to serve as confirmatory evidence to support last year's accelerated approval by the U.S. Food and Drug Administration of Imdelltra for patients with extensive stage small cell lung cancer whose disease worsened on or after platinum-based chemotherapy.

The findings were presented at the American Society of Clinical Oncology's annual meeting in Chicago and published in The New England Journal of Medicine.

Most lung cancer cases are non-small cell, while up to 15% are the more aggressive small cell variety targeted by Imdelltra, according to the American Cancer Society.

The Amgen drug belongs to a class of treatments called bispecific antibodies designed to attach to a cancer cell and an immune cell, bringing them together so that the body's immune system can kill the cancer.

Amgen said 27% of trial patients treated with Imdelltra experienced serious side effects, including low white blood cell counts, compared with 62% of chemotherapy patients. Patients on Imdelltra reported improvements in cough and shortness of breath compared to the chemotherapy group.

Cytokine release syndrome, a potentially dangerous condition that occurs when the body's immune system responds over aggressively to infection or immunotherapy drugs, was primarily low-grade and manageable, Amgen said.

(Reporting By Deena Beasley in Los Angeles; Editing by Bill Berkrot)

((deena.beasley@thomsonreuters.com; 213 955 6746; Reuters Messaging: deena.beasley.thomsonreuters.com@reuters.net))