April 11, 2006

Plexxikon has initiated IND-enabling studies with PLX4032, the company's oncogenic B-Raf inhibitor, a novel, small molecule drug candidate for the treatment of cancer. This compound selectively inhibits the cancer causing form of the B-Raf gene, a mutation found in 70 percent of skin cancers and a significant number of colorectal, ovarian and thyroid cancers. At the 97th Annual Meeting of the American Association for Cancer Research, Plexxikon scientists presented preclinical data demonstrating that a low dose of this oral V600E B-Raf inhibitor preferentially stopped tumor growth in malignant melanoma and colorectal cancer cells.

The V600E B-Raf inhibitor showed strong selectivity and potency against specific melanoma and colorectal cell lines. In addition, this compound demonstrated robust efficacy in xenograft models using a once-daily low oral dose, with evidence of tumor shrinkage and prolonged delay of tumor growth for a significant post-dosing period.