RAPID REVIEW
Novel Therapies for Malignant Melanoma: A New Generation of High-affinity, Selective BRAF Inhibitors
Robert G. Fenton, MD, PhD, Clinical Associate Professor, Clinical Research Committee Member, University of Maryland, Marlene and Stewart Greenebaum Cancer Center. Dr. Fenton reports no financial relationships relevant to this field of study.
Approximately 60% of human melanomas express a mutant form of the serine/threonine kinase BRAF, in which substitution of glutamic acid for valine at amino acid 600 (V600E) leads to the RAS-independent activation of the BRAF kinase. This constitutive activation of BRAF, with subsequent activation of the MEK1/2 and ERK1/2 kinases, promotes neoplastic features, including enhanced cell-cycle progression, resistance to apoptosis, changes in cell motility, and invasiveness, all of which, in combination with other genetic changes (e.g. those inhibiting cell senescence), define the pathogenesis of malignant melanoma. Recently, high-affinity, small-molecule inhibitors that specifically target BRAF have entered clinical trials and have shown exciting activity as single agents against mutant BRAF melanoma. These early-stage trials will be reviewed here, and new concepts of how BRAF inhibitors work in tumors with different genetic backgrounds will be discussed.
Early-Phase Clinical Trials of the New Generation of BRAF Inhibitors
A phase I study of PLX4032 examining MTD, toxicity, and pharmacokinetic and pharmacodynamic endpoints enrolled 49 patients with metastatic melanoma, three with thyroid cancer, one rectal cancer, and one ovarian cancer, tumor types known to express V600E mutant BRAF.1 PLX4032 had previously been shown to regress V600E-BRAF mutant tumors in human melanoma xenograft models with minimal toxicity to mice.2 The clinical trial utilized two different preparations of PLX4032 with different bioavailabilities, which complicated the dose escalation. Of the 12 metastatic melanoma patients treated with 240 mg BID or higher, seven had BRAF mutations; five of these patients experienced tumor regression, with one RESIST PR and one unconfirmed PR. Two of 5 pts with unknown BRAF status had regression (one confirmed PR). All seven patients with tumor regression were still in remission 4-14 months after treatment. Three thyroid patients had minimal regression (9%-16%) and remained stable for 4-7 months at the time of the report. The MTD was 720 mg PO BID, although the intermediate dose of 960 mg remains to be tested. Dose-limiting toxicities consisted of rash and fatigue. The preliminary conclusion of this ongoing trial was that PLX4032 was active against BRAF-mutant melanoma and was well tolerated.
GSK2118436 is a potent, selective ATP competitive inhibitor with 100-fold greater activity against mutant vs. wild-type BRAF, which had demonstrated tumor regression in a melanoma xenograft model. Sixty-one patients (52 with mutant BRAF) were treated in a phase I study. At the time of abstract presentation, the MTD had not been reached.3 In patients with mutant BRAF melanoma, 18 of 30 had > 20% tumor response by RESIST criteria at the first evaluation, 8-9 weeks after beginning treatment. The preliminary conclusion was that this agent has clinically significant anti-tumor activity with minimal toxicity.
Plexxikon, Inc., in partnership with Roche, is sponsoring the BRIM3 (BRAF Inhibitor in Melanoma) phase III, randomized study in which 700 previously untreated melanoma patients will receive either PLX4032 at a dose of 960 mg BID or dacarbazine (DTIC). The primary endpoint is overall survival, and secondary endpoints include duration of response, progression-free survival, and best overall response rate. BRIM3 is being conducted at 100 sites in the United States, Canada, Australia, and Europe. A second study, BRIM2, will study the efficacy of PLX4042 in 100 previously treated melanoma patients whose tumors express mutated BRAF.
Mechanisms of BRAF-Inhibitor Activity: Current Hypotheses and Future Directions
In vitro, BRAF inhibitors effectively inhibit proliferation of BRAF-mutant cell lines, but have no activity against melanoma cells expressing a mutant NRAS oncogene.4,5 Importantly, recent studies indicate that, under some circumstances, BRAF inhibitors can activate the RAF-MEK-ERK pathway.6 How can one explain the paradoxical activation of BRAF by a BRAF inhibitor, and how does this alter the way clinicians utilize these novel compounds to treat patients with tumors that may encode mutant BRAF?
An important facet in understanding Raf biology was the discovery that Raf molecules exist as side-to-side dimers (either as homodimers or heterodimers of CRAF and BRAF).7 Additionally, it was shown that paradoxical activation of RAF dimers by BRAF inhibitors requires an activated RAS. Thus, paradoxical MAPK activation only occurs when BRAF is inhibited in the presence of oncogenic RAS. The molecular mechanisms by which BRAF inhibitors activate the MAPK pathway remain to be definitively explained. One provocative experiment suggests that inhibitor binding to the ATP-binding site of one partner of a dimer can induce the transactivation of kinase activity of the other partner.8 In this study, a CRAF with an inactivated kinase domain (generated by in vitro mutagenesis) was dimerized to a different CRAF mutant that could not bind the BRAF inhibitor. When introduced into cells, the addition of a BRAF inhibitor led to activation of the kinase of the second mutant CRAF, indicating that inhibitor binding to the kinase-dead CRAF had activated the kinase of its partner.
Realizing that mutant BRAF and activated, oncogenic RAS do not coexist in the same melanoma, it becomes clear that paradoxical BRAF activation by BRAF-specific inhibitors can only occur in those subsets of melanoma patients whose tumor harbors activated NRAS (but not mutant BRAF). Furthermore, there are experimental data in a murine transgenic mouse model that kinase-dead BRAF (mimicking the situation in patients taking BRAF inhibitors) can enhance the in vivo tumorigenicity of mutant NRAS.5 This suggests that BRAF inhibitors may actually enhance the growth of human melanomas harboring NRAS mutations, and that they should not be used in this situation. Hence, the BRAF and NRAS genotypes of a patient's tumor should be known before they are treated with BRAF-selective inhibitors. One way to avoid this would be to combine BRAF inhibitors with MEK inhibitors, with the latter precluding a paradoxical activation of the MAPK pathway. In fact, a new phase I/II study will combine GSK2118436 with a MEK1/2 inhibitor GSK1120212 in patients with BRAF-mutant malignant melanoma. However, it is unclear why the MEK inhibitor is needed in this situation where the tumor cells are known to express mutant BRAF, since the model indicates that paradoxical activation of RAF by BRAF inhibitors requires mutant RAS, which will not be present in BRAF-mutant melanomas; this combination of drugs would seemingly eliminate the specificity of targeting mutant BRAF and would introduce the toxicities associated with MEK inhibitors that are not targeted to the tumor.
While much remains to be learned mechanistically, it is clear that the new generation of potent BRAF inhibitors have important clinical activity against BRAF-mutant melanoma, and it will be of great interest in the years to come to follow their clinical development as they are combined with standard chemotherapeutic agents as well as small molecule inhibitors of other signal transduction pathways.
References
1. Flaherty K, et al. Phase I study of PLX4032: Proof of concept for V600E BRAF mutation as a therapeutic target in human cancer. J Clin Oncol. 2009;27:15s
2. Yang H, et al. RG7204 (PLX4032), a selective BRAF V600E inhibitor, displays potent antitumor activity in preclinical melanoma models. Cancer Res. 2010;70:5518-5527.
3. Kefford R, et al. A phase I/II study of GSK2118436, a selective inhibitor of oncogenic mutant BRAF kinase, in patients with metastatic melanoma and other solid tumors. J Clin Oncol. 2010;28:15s
4. Hatzivassiliou G, et al. RAF inhibitors prime wild-type RAF to activate the MAPK pathway and enhance growth. Nature. 2010;464:431-435.
5. Heidorn SJ, et al. Kinase-dead BRAF and oncogenic RAS cooperate to drive tumor progression through CRAF. Cell. 2010;140:209-221.
6. Cox AD, Der CJ. The Raf inhibitor paradox: Unexpected consequences of targeted drugs. Cancer Cell. 2010;17:221-223.
7. Tsai J, et al. Discovery of a selective inhibitor of oncogenic B-RAF kinase with potent anti-melanoma activity. Proc Natl Acad Sci USA. 2008;105:3041-3046.
8. Poulikakos PI, et al. RAF inhibitors transactivate RAF dimers and ERK signaling in cells with wild-type BRAF. Nature. 2010;464:427-430.
Approximately 60% of human melanomas express a mutant form of the serine/threonine kinase BRAF, in which substitution of glutamic acid for valine at amino acid 600 (V600E) leads to the RAS-independent activation of the BRAF kinase.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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