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Plexxikon Advances First Oncology Development Candidate

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Plexxikon Inc. 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% 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.

In the oral presentation titled "Development of a Novel Inhibitor of Oncogenic B-Raf," Plexxikon scientists report that the company's 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.

"Plexxikon's V600E B-Raf inhibitor may represent the first opportunity to target a disease-specific gene found only in cancerous cells and not in normal cells. With its clean selectivity profile, and specific targeting of tumors, we anticipate this drug candidate will be very safe for patients," said K. Peter Hirth, Ph.D., chief executive officer of Plexxikon Inc.

"Plexxikon's advancement of this oncology candidate to development, together with our broad program in diabetes and metabolic disease, further demonstrate the depth and breadth of our novel drug discovery platform. We intend to file an IND (Investigational New Drug) application and initiate Phase 1 clinical trials in the second half of 2006 for our oncogenic B-Raf inhibitor."

Plexxikon's Oncology Programs Driven by Advent of Personalized Medicine The development of Plexxikon's novel B-Raf inhibitor will be expedited by the creation of a diagnostic test to detect a biomarker for the B-Raf mutation.

Such a diagnostic test will be utilized to select patients with the B-Raf mutation for clinical trials and treatment. This mutation, known as V600E, occurs consistently in 90% of tumors harboring the oncogenic BRAF gene.

The oncogenic B-Raf inhibitor is Plexxikon's lead oncology program, with a number of other oncology opportunities with similar development profiles at the preclinical and lead optimization stage.

Plexxikon's strategy in oncology focuses on targeting cancers produced by specific genetic mutations or other aberrations which correlate to poorer patient outcome and survival.

By pre-selecting this sub-population of patients most likely to benefit from a selective V600E B-Raf inhibitor, Plexxikon expects to enable more rapid and effective clinical trials.

The company's clinical development will be further facilitated by monitoring biomarkers that aid in dose selection and give an early read on the response of a tumor under therapy.