WASHINGTON (Reuters) - An experimental new immunosuppressant drug could help prevent rejection of transplanted organs with fewer side effects, U.S.-based researchers said on Thursday.

Monkeys and mice treated with the drug after getting heart or kidney transplants survived much longer than untreated animals, the researchers report in Friday's issue of the journal Science.

None of the treated animals showed evidence of common side effects of immunosuppression such as higher cholesterol and blood glucose levels, increased blood pressure or lymphoproliferation, the researchers found.

The drug, known by its experimental name CP-690,550, is being tested jointly by Pfizer Global Research and Development, a division of Pfizer Inc., the National Institute of Arthritis and Musculoskeletal and Skin Diseases and Stanford University in California.

The drug is a specific inhibitor of Janus kinase 3, which is required for signaling by multiple cytokines. The enzyme is found only in immune system cells, so suppression of JAK3 seems not to affect other cells.

Science 2003;302:875-878.

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