Background: MIW815 (ADU-S100) is a novel synthetic cyclic dinucleotide that activates the STimulator of INterferon Genes (STING) pathway impacting tumor cells, tumor microenvironment, vasculature, tumor-associated fibroblasts, and priming APC and CD8+ T cells. Spartalizumab is a humanized IgG4 mAb that blocks the binding of PD-1 to PD-L1/2. Preclinical data support synergistic antitumor effects when MIW815 (ADU-S100) is combined with checkpoint inhibitors. Methods: In this Phase Ib dose escalation study, pts with advanced/metastatic solid tumors or lymphoma received MIW815 (ADU-S100) (intratumoral injections [50–800 µg] either weekly [3 weeks on/1 week off] or Q4W) and spartalizumab (400 mg IV Q4W). Injected and non-injected tumor biopsies were obtained at baseline and on treatment. Primary objectives are to determine safety and identify a dose/schedule for future studies. Preliminary activity, pharmacokinetics (PK), and pharmacodynamics (PD) are also being explored. Results: As of Jan 11, 2019, 66 pts (median age: 61 y) with various solid tumors or lymphomas have been treated. Treatment was discontinued in 49 pts (74%) due to disease progression (n = 28), pt/physician decision (n = 18), AE (n = 2), or death (n = 1). No DLTs were reported during the first cycle at any dose level. Most common (≥5 pts) treatment-related AEs (TRAEs) were injection site pain (12%), pyrexia (11%), and diarrhea (9%). Grade 3/4 TRAEs (in ≥2 pts) were increased AST and ALT (3% each). Serious TRAEs were pyrexia (3%), increased amylase, increased lipase, diarrhea, fatigue, hyperthyroidism, partial seizures, dyspnea, and pneumonitis (all 2%). Partial responses in pts with PD-1–naive TNBC and PD-1–relapsed/refractory melanoma have been observed. MIW815 (ADU-S100) plasma exposure generally increased in a dose-dependent manner with a rapid terminal half-life. Response data, PK and PD analyses will be presented. Conclusions: Thus far, MIW815 (ADU-S100) + spartalizumab has demonstrated antitumor activity in PD-1–naive TNBC and PD-1–relapsed/refractory melanoma. The combination is well tolerated, with no DLTs reported to date. The MTD has not been reached and dose escalation is ongoing. Clinical trial information: NCT03172936.