Background: IMM-101 is a suspension of heat-killed whole cell Mycobacterium obuense (NCTC 13365), which enhances the innate immune response and dendritic cell maturation. In animal models, it increases antigen specific responses and number of CD8+ CTL and CD4 + Th1 cells. The clinical studies with IMM-101 have shown promising efficacy signals in pancreatic cancer when combined with gemcitabine and in melanoma as adjunctive or single agent. Methods: IMM-101-015 is an open-label Phase 2a study to investigate the safety and efficacy of IMM-101 in combination with checkpoint inhibitors (CPIs) in patients (pts) with advanced melanoma who were either treatment-naive (cohort A), or whose disease had progressed during PD-1 blockade (cohort B). Pts with evaluable lesions, adequate performance status and organ function were eligible. Pts received 1.0 mg of IMM-101 every 2 weeks for the first 3 doses followed by a rest period of 4 weeks, then every 2 weeks for the next 3 doses, and thereafter every 4 weeks. Nivolumab was given every 2 or 4 weeks (dependent on Investigator choice). Pts in cohort B had the option to change to ipilimumab and IMM-101 if their disease continued to progress. Biopsies and blood samples were obtained at baseline and during treatment for assessment of tumor biomarkers and immune correlatives. The primary objectives of the study were to evaluate the overall response rate (ORR) after a maximum of 18 months of treatment by RECIST 1.1 and to assess the safety and tolerability of the combination of IMM-101 + CPIs. Results: Sixteen pts (11 Cohort A and 5 Cohort B) were treated between October 2018 and May 2021. The median age was 68.5 yrs (range 36-92) and 11 (69%) were male. The ECOG ps was 0 in 9 (56%) and 1 in 7 (44%) pts. Four (25%) had unresectable stage III melanoma and 12 (75%) stage IV. In Cohort A (3 stage III, 5 stage IV M1b and 3 M1c) 3 pts (27%) had an elevated baseline LDH, 6 (55%) a positive PD-L1 status and 3 (27%) a BRAF mutation. Pts in cohort A were on study for a median time of 8.5 months (range 1.5 - 19.1) and those in cohort B for 3.0 months (range 1.5 - 7.4). All pts were evaluated for response. The ORR was 73% (95% CI 39.03, 93.98) in cohort A whereas all pts in cohort B reported progressive disease. With respect to cohort A, 2 (18%) pts had CR, 6 (55%) PR and 1 (9%) SD. The median progression-free survival time was 10.2 months (95% CI 2.50, NE) with 41% of the pts progression-free at 18 months. The most frequent treatment emergent adverse events (TEAEs) were injection site reaction (63%), pruritus (44%), fatigue (38%), skin rash (25%), hypothyroidism (25%) and diarrhoea (19%). There were no grade 4 TEAEs. Grade 3 TEAEs occured in 10 patients (63%), mostly skin toxicity (19%) and lab abnormalities (13%). Conclusions: IMM-101 in combination with nivolumab is safe and shows encouraging antitumor activity in treatment-naive patients with advanced melanoma. Clinical trial information: NCT03711188.