Background Anti-PD1 based therapy has been the mainstay of treatment for advanced melanoma for several years. HBI-8000 is a Class I selective oral HDACi with immunomodulatory effects including enhanced cell-mediated toxicity, enhanced tumor infiltration by cytotoxic T-cells and reduced tumor infiltration by T-regulatory cells. In a phase 1b/2 trial in melanoma, kidney cancer and non-small cell lung cancer, the recommended phase 2 dose of HBI-8000 was determined to be 30mg orally twice weekly (BIW) combined with nivolumab administered at the approved dosing schedule (JITC 2018;P346). This report describes the tolerability of this combination in all enrolled melanoma patients, and efficacy in the expansion cohort of anti-PD1 TN-MEL.

Methods Patients with unresectable or advanced melanoma and measurable disease, of ECOG performance status 0-1, and with adequate hematologic and biochemical parameters were enrolled. Treated brain metastases not requiring steroids were permitted. Tumor response was assessed by RECIST v1.1 and iRECIST with staging every 8 weeks; treatment continued for 24 months, disease progression or unacceptable toxicity. Data cut-off was Jan 31, 2020 for the reported analyses

Results Forty-nine patients (32 anti-PD1 naïve, 17 with prior anti-PD1 therapy) were treated with HBI-8000 (47 patients at 30 mg BIW; 2 patients at 40mg BIW in Phase 1b) in combination with nivolumab. The median age was 63 years (range 28-84); 57% were male. In the anti-PD1 naïve cohort, most (30/32) had normal LDH. The most common all grade treatment related adverse events (AEs) included fatigue (n=25), diarrhea (n=24), abdominal pain (n=14), and lymphopenia (n=13). Although HBI-8000 related thrombocytopenia (n=25) and neutropenia (n=15) were common, clinically significant bleeding or febrile neutropenia were not observed. The most frequent >/= G3 AEs related to HBI-8000 were hypophosphatemia (n=7), neutropenia (n=4), thrombocytopenia (n=3) and lymphopenia (n=2). Twelve patients discontinued treatment for AEs. Among 31 anti-PD1 naïve patients evaluable for response, there were 23 objective responses (4 CR, 19 PR; ORR 74%), 5 stable disease (disease control rate 90%), and 3 progressive disease. Median time to response was 1.9 months. At a median follow-up for this cohort of 8.9 months (range, 0.9-35.5 months), the median duration of response and median progression-free survival have not been reached.

Conclusions The combination of HBI-8000 and nivolumab is well tolerated and demonstrates very encouraging efficacy in patients with anti-PD1-naïve advanced melanoma. Follow-up to assess durability of response is ongoing, and further investigation of this promising combination is planned.

Trial Registration NCT02718066

Ethics Approval The study was approved by participating study sites’ Institutional Review Boards and the Sponsor has conducted the trial in full compliance with all GCP and FDA regulations.