PURPOSE: This phase I study assessed the safety, pharmacokinetics (PKs), and efficacy of MIW815 (ADU-S100), a novel synthetic cyclic dinucleotide that activates the stimulator of IFN genes (STING) pathway, in patients with advanced/metastatic cancers. PATIENTS AND METHODS: Patients (n = 47) received weekly i.t. injections of MIW815, 50 to 6,400 μg, on a 3-weeks-on/1-week-off schedule. RESULTS: A maximum tolerated dose was not reached. Most common treatment-related adverse events were pyrexia (17%), chills, and injection-site pain (each 15%). MIW815 was rapidly absorbed from the injection site with dose-proportional PK, a rapid terminal plasma half-life (approximately 24 minutes), and high interindividual variability. One patient had a partial response (PR; Merkel cell carcinoma); two patients had unconfirmed PR (parotid cancer, myxofibrosarcoma). Lesion size was stable or decreased in 94% of evaluable, injected lesions. RNA expression and immune infiltration assessments in paired tumor biopsies did not reveal significant on-treatment changes. However, increases in inflammatory cytokines and peripheral blood T-cell clonal expansion suggested systemic immune activation. CONCLUSIONS: MIW815 was well tolerated in patients with advanced/metastatic cancers. Clinical activity of single-agent MIW815 was limited in this first-in-human study; however, evidence of systemic immune activation was seen.