PLX4032 – THE B-RAF ANTAGONIST

Introduction: The Effect of B-Raf mutation

Cellular pathways are sequences of chemical reactions that take place in the cell membrane, cytosole and the nucleus to initiate, control or regulate processes necessary for normal function. A protein called RAS is anchored in the cell membrane and upon activation with GTP can bind to another protein called RAF [1]. With transfer of a phosphate group from GTP the RAF protein becomes detached and activated. Moving through the cytosole start the signal of RAF is passed on to MEK and then to ERK by means of phosphorylation. Eventually the signal reaches the cell nucleus and growth processes are started depending on the original signal [1;2]. Therefore, it can be seen that Raf and MEK are two proteins whose major function is to carry a signal from membrane to nucleus to ensure cell survival. Any form of deviation from the normal situation in regards to these two proteins would cause significant problems. With regards to Raf three isoforms have been isolated and characterized known as A,B or C-Raf [3;4]. Genetic profiling has determined that B-Raf is prone to aberrations in its structure leading to conditions of permanent activity. The major mutation of B-Raf is recorded as BRaf^V600E and has been located in significant proportions in numerous cancer types, for example prostate, ovarian, colorectal, papillary and melanoma, to name but a few [5-9]. The B-Raf^v600E mutation has demonstrated sensitivity towards small molecule inhibitors; hence the targeting of this pathway is considered a viable therapeutic route. PLX4032 is a B-Raf inhibitor that was developed to take advantage of this pathway; it has shown activity in many tumor cell lines and xenografts prompting research into its clinical applicability.

PLX4032: Properties and Availability

PLX4032 was originally developed as part of the Daiichi Sankyo portfolio in co-operation with Hoffman-La Roche under the codes RG7204 or RO5185426. It is currently called vemurafenib and is marketed under the trade name Zelboraf. Typical for this type of compound the PLX4032 structure has an anilinoquinazoline core substituted with both a benzyl chloride and a thiosulphoxide side chain. PLX4032 is a pan-inhibitor against EGFR but is only clinically effect against the B-Raf^V600E protein. The PLX4032 IC50 with respect to the B-Raf^V600E is 44 nM. While the PLX4032 solubility in DMSO and ethanol saturable at 200 and 5 mg/ml respectively. PLX4032 stability is no different to other inhibitors of its class with an expiration date of 2 years for solid kept at -20°C. If Researchers wish to buy PLX4032 the a reasonable of PLX4032 suppliers exist. Please note though that the PLX4032 price can range from $42 up to $895 for a 50 mg vial. With the PLX4032 cost dependent on the supplier it is recommended researchers shop carefully for this product.

PLX4032: Preclinical investigation

Initial research demonstrated the PLX4032 was a highly effective inhibitor of the B-Raf^V600E mutation, in light of this evidence work was pursued in regard to metastatic melanoma where 50 - 70% of patients have this specific mutation [10]. Phase 1 trials established that PLX4032 was effective in wild type B-Raf patients. The resistance mechanism is suspected to be due to the activation of a survival pathway rather than the apoptotic pathway found in the mutated gene patients [11-14]. However, sufficient evidence in melanoma studies was gathered to indicate that this molecule could benefit metastatic melanoma patients with mutated B-Raf gene. At phase I a total of 87 patients with the BRaf^v600E mutation were assessed with 37/87 demonstrating either a partial response or complete remission while the progression free survival was over 7 months for the remainder [15].

PLX4032: Clinical Status and FDA Approval

PLX4032 clinical trials conducted in melanoma patients highlighted the need for patient screening by pharamcogenomic with BRaf^V600E patients responding to treatment and wild type not [16]. In December of 2011 PLX4032 was approved for first line treatment of individuals with unresectable or metastatic melanoma carrying the BRaf^V600E mutation. Since the BRaf mutation is common in a variety of other tumor types several clinical trials have been initiated with patient screening by an approved RBaf^V600E assay, these trials are currently recruiting as of 2012.

References

 

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  10.    Bollag G, Hirth P et al. Clinical efficacy of a RAF inhibitor needs broad target blockade in BRAF-mutant melanoma. Nature 2010; 467(7315):596-599.

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  15.    Flaherty KT, Puzanov I et al. Inhibition of mutated, activated BRAF in metastatic melanoma. N Engl J Med 2010; 363(9):809-819.

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