Background: SD-101 is a synthetic CpG-ODN agonist of TLR 9 that stimulates dendritic cells to release IFN-alpha and mature into antigen presenting cells to activate T cell anti-tumor responses. Pembro is a PD-1 inhibitor approved for the treatment of metastatic melanoma. This study, MEL-01 (NCT02521870), assesses the safety and preliminary efficacy of SD-101 in combination with pembro in stage IIIC-IV melanoma. Methods: A modified 3+3 design was used for SD-101 dose escalation of 1, 2, 4, and 8 mg injected in a single tumor lesion Q1W x 4 then Q3W x 7 in combination with pembro (200 mg IV Q3W). Tumor responses were assessed per investigator using RECIST v1.1. Results: In phase Ib, 22 pts were enrolled: median age 64 y/o, male 68%, white 82%, Stage IV/IIIc 86%/14%, LDH > 1 ULN 27%, ≥ 3 prior lines therapy 36%, anti-PD-1 naïve (n = 9) and experienced (n = 13). There has been no dose limiting toxicity (DLT) to date. The most common (≥20%) treatment-related AEs (TRAEs) were transient low-grade fatigue, myalgia, headache, chills and injection site reactions. Grade ≥ 3 TRAEs were observed in 59.1% pts (most common: myalgia 13.6% and injection site pain 13.6%). Immune-related AEs occurred in 2 pts. One had a G2 pneumonitis on Day 23 resulting in drug withdrawal and the other G3 hypophysitis (85 days after last treatment). No deaths occurred. Responses were observed at all doses in PD-1 inhibitor naïve pts, both at the injected and non-injected lesions. A response was seen at the 8 mg dose in PD-1 inhibitor experienced pts. With median f/u of 97 days (max 382), the ORR was 66.7% in the PD-1 inhibitor naïve patients with best overall response of CR 22.2% (n = 2), PR 44.4% (n = 4), SD 11.1% (n = 1), PD 11.1% (n = 1), and NE 11.1% (n = 1). In the PD-1 inhibitor experienced pts: PR 7.7% (n = 1) and SD 38.5% (n = 5). Conclusions: The combination of SD-101 and pembro was well tolerated and demonstrates no worsening of the expected toxicities of each of the individual monotherapies. These interim data support enhanced activity of adding SD-101 to pembro in anti-PD-1 naive metastatic melanoma as well as potential activity in anti-PD-1 experienced pts. Additional follow up data through May 15, 2017 will be presented. Clinical trial information: NCT02521870