Combined BRAF and HSP90 Inhibition in Patients with Unresectable BRAFV600E-Mutant Melanoma

Purpose: BRAF inhibitors are clinically active in patients with advanced BRAF(V600)-mutant melanoma, although acquired resistance remains common. Preclinical studies demonstrated that resistance could be overcome using concurrent treatment with the HSP90 inhibitor XL888. Patients and Methods: Vemurafenib (960 mg p.o. b.i.d.) combined with escalating doses of XL888 (30, 45, 90, or 135 mg p.o. twice weekly) was investigated in 21 patients with advanced BRAF(V600)-mutant melanoma. Primary end-points were safety and determination of a maximum tolerated dose. Correlative proteomic studies were performed to confirm HSP inhibitor activity. Results: Objective responses were observed in 15 of 20 evaluable patients [75%; 95% confidence interval (CI), 51%-91%], with 3 complete and 12 partial responses. Median progression-free survival and overall survival were 9.2 months (95% CI, 3.8-not reached) and 34.6 months (6.2-not reached), respectively. The most common grade 3/4 toxicities were skin toxicities, such as rash (n = 4, 19%) and cutaneous squamous cell carcinomas (n = 3, 14%), along with diarrhea (n = 3, 14%). Pharmacodynamic analysis of patients' peripheral blood mononuclear cells (PBMC) showed increased day 8 HSP70 expression compared with baseline in the three cohorts with XL888 doses >= 45 mg. Diverse effects of vemurafenib-XL888 upon intratumoral HSP client protein expression were noted, with the expression of multiple proteins (including ERBB3 and BAD) modulated on therapy. Conclusions: XL888 in combination with vemurafenib has clinical activity in patients with advanced BRAF(V600)-mutant melanoma, with a tolerable side-effect profile. HSP90 inhibitors warrant further evaluation in combination with current standard-of-care BRAF plus MEK inhibitors in BRAF(V600)-mutant melanoma. (C) 2018 AACR. See related commentary by Sullivan, p. 5496