Background: SD-101 is a synthetic CpG TLR9 agonist. Pembrolizumab is a PD-1 inhibitor. SYNERGY-001/KEYNOTE-184 assesses the safety and preliminary efficacy of the combination of SD-101 and pembrolizumab in patients naïve to anti-PD-1/L1 therapy with unresectable stage IIIC-IV melanoma. Methods: SD-101 was evaluated as 2 mg/lesion injected into 1 to 4 lesions and 8 mg/lesion in 1 lesion as 4 weekly doses followed by 7 doses Q3W. Pembrolizumab was administered as 200 mg IV Q3W. CT scans were performed every 9 weeks. Responses were assessed per investigator using RECIST v1.1. Responses and post-hoc Kaplan-Meier analyses of PFS in the ITT population were compared for patients who received 2 mg/lesion with 8 mg/lesion. Results: 86 patients (2 mg: N = 45; 8 mg: N = 41) have been enrolled with similar baseline characteristics: median age = 66 years; male = 67%; ECOG stage 0 = 67%; disease stage: IIIC = 21%; IVM1a/b = 50%; IVM1c = 28%; LDH ≤ ULN = 71%; treatment naïve = 73%. Median follow up to date is 8.1 months in 2 mg group and 8.3 months in 8 mg group. SD-101 safety profile comprises flu-like symptoms with most frequent grade ≥3 SD-101-related AEs of headache (7%), fatigue (7%), malaise (5%), myalgia (4%), and chills (4%). Immune-related AEs were reported in 19%. ORR in 2 mg group = 71% (95% CI: 57, 82) (CR: 13%) and in 8 mg group = 49% (95% CI: 33, 65) (CR: 7%) with responses in both injected and non-injected lesions, including visceral. DOR = not reached (NR) in either group. ORR by baseline PD-L1 expression in 62 patients, 53% of whom were PD-L1 positive: 2 mg = 80%/79% (PD-L1 positive/negative); 8 mg = 62%/40% (PD-L1 positive/negative). PFS was higher in 2 mg group with median PFS in 2 mg = NR (95% CI: Not estimable [NE], NE) and in 8 mg = 10.4 months (95% CI: 4.2, NE), HR = 0.45 (95% CI: 0.21, 0.98), p = 0.036. 6 month PFS rate in 2 mg = 81% and in 8 mg = 60%. 6 month OS rate in 2 mg = 98% and in 8 mg = 92%. Conclusions: The TLR9 innate immune stimulant, SD-101, in combination with pembrolizumab has been well-tolerated, and is showing promising high response rates and PFS, regardless of PD-L1 expression, particularly in patients who received 2 mg SD-101. Clinical trial information: NCT02521870