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. Methods: Vemurafenib (960 mg PO BID) combined with escalating doses of XL888 (30, 45, 90 or 135 mg PO twice weekly) was investigated in 21 patients with advanced BRAF V600 -mutant melanoma. Primary endpoints 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 15/20 evaluable patients (75%; 95% CI: 51-91%), with 3 complete and 12 partial responses. Median progression-free 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 patients9 PBMCs showed increased day 8 HSP70 expression compared to 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. Conclusion. 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.