Treatment and Resistance to Paradox-Breaker PLX8394 in cutaneous melanoma

FDA-approved BRAF inhibitors produce high response rates and improve overall survival in patients with BRAF V600E/K mutant melanoma, but are linked to pathologies associated with paradoxical ERK1/2 activation in wild-type BRAF cells. To overcome this limitation, a next-generation paradox breaking RAF inhibitor (PLX8394) has been designed. Here we show that by using a quantitative reporter assay, PLX8394 rapidly suppressed ERK1/2 reporter activity and growth of mutant BRAF melanoma xenografts. Ex vivo treatment of xenografts and use of a patient-derived explant system (PDeX) revealed that PLX8394 suppressed ERK1/2 signaling and elicited apoptosis more effectively than the FDA-approved BRAF inhibitor, vemurafenib. Furthermore, PLX8394 was efficacious against vemurafenib-resistant BRAF splice-variant expressing tumors and reduced splice-variant homodimerization. Importantly, PLX8394 did not induce paradoxical activation of ERK1/2 in wild-type BRAF cell lines or PDeX. Continued in vivo dosing of xenografts with PLX8394 led to the development of acquired resistance via ERK1/2 reactivation through heterogeneous mechanisms; however, resistant cells were found to have differential sensitivity to ERK1/2 inhibitor. These findings highlight the efficacy of a paradox-breaking selective BRAF inhibitor and the use of PDeX system to test efficacy of therapeutic agents. Overall design: RNAs were obtained from parental and PBRT resistant cell lines established from in vivo tumors PBRT15 and PBRT16 were treated with 500 nM PLX8394