Vemurafenib

Vemurafenib (INN), sold under the brand name Zelboraf, is a medication used for the treatment of late-stage melanoma. It is an inhibitor of the B-Raf enzyme and was developed by Plexxikon.

Vemurafenib causes programmed cell death in melanoma cell lines. Vemurafenib interrupts the B-Raf/MEK step on the B-Raf/MEK/ERK pathway − if the B-Raf has the common V600E mutation.

Vemurafenib only works in melanoma patients whose cancer has a V600E BRAF mutation (that is, at amino acid position number 600 on the B-Raf protein, the normal valine is replaced by glutamic acid). About 60% of melanomas have this mutation. It also has efficacy against the rarer V600K BRAF (the normal valine is replaced by lysine) mutation. Melanoma cells without these mutations are not inhibited by vemurafenib; the drug paradoxically stimulates normal BRAF and may promote tumor growth in such cases.

Three mechanisms of resistance to vemurafenib (covering 40% of cases) have been discovered:

At the maximum tolerated dose (MTD) of 960 mg twice a day 31% of patients get skin lesions that may need surgical removal. The BRIM-2 trial investigated 132 patients; the most common adverse events were arthralgia in 58% of patients, skin rash in 52%, and photosensitivity in 52%. In order to better manage side effects, some form of dose modification was necessary in 45% of patients. The median daily dose was 1750 mg, 91% of the total daily MTD.

In a phase I clinical study, vemurafenib (then known as PLX4032) was able to reduce numbers of cancer cells in over half of a group of 16 patients with advanced melanoma. The treated group had a median increased survival time of 6 months over the control group.

A second phase I study, in patients with a V600E mutation in B-Raf, ~80% showed partial to complete regression. The regression lasted from 2 to 18 months.

In early 2010 a Phase I trial for solid tumors (including colorectal cancer), and a phase II study (for metastatic melanoma) were ongoing.