UPDATE 1-Rash most common side effect in Vectibix trials

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LOS ANGELES, May 15 (Reuters) - The most common side effect observed in ongoing trials of Amgen Inc's (AMGN.O: Quote, Profile, Research, Stock Buzz) cancer drug Vectibix is skin rash, according to interim analyses of pivotal trials of the drug in colorectal cancer released on Thursday.

Vectibix is already approved in the United States for treating colon cancer patients who have stopped responding to chemotherapy, and Amgen is studying the drug in earlier stages of the disease as well as in other types of cancer.

One Phase III study of Vectibix, also known as panitumumab, is looking at its effectiveness as a first-line treatment in combination with a chemotherapy regimen known as Folfox, compared with chemo alone.

An interim safety analysis of the first 601 patients in the trial shows that 54 percent of Vectibix patients reported a skin rash, compared with 11 percent of chemo-only patients.

Skin rash and acne have come to be associated with a response to biologic drugs designed to block a protein called epidermal growth factor -- a class that includes Vectibix as well as ImClone Systems Inc's (IMCL.O: Quote, Profile, Research, Stock Buzz) Erbitux.

A mutation in a gene known as KRAS has been linked to a poor response to the drugs.

Amgen said the interim safety results also show Vectibix was associated with diarrhea in 43 percent of patients, nausea in 39 percent and low levels of infection-fighting white blood cells in 38 percent of patients.

The company said updated pooled safety data from around 900 patients in the trial will be presented later this month at a meeting of the American Society of Clinical Oncology (ASCO).