Background: Immune checkpoint inhibitors (ICIs) face hurdles in overcoming the immunosuppressive tumor microenvironment. BMS-986299 is first-in-class, NOD-, LRR-, and pyrin domain containing 3 (NLRP3) inflammasome agonist enhancing adaptive immune and T cell memory responses. In-vivo studies demonstrated BMS-986299 synergy with ICIs and induction of durable immunological memory. Methods: This multicenter, open-label, phase I [NCT03444753] study assessed safety and tolerability of intra-tumoral (IT) BMS-986299 monotherapy (Part 1A) and in combination (Part 1B) with the programmed death 1 receptor antibody nivolumab (N), and the anti-cytotoxic T lymphocyte associated protein 4 antibody ipilimumab (I) in advanced solid tumors. Eligible patients (pts) regardless of prior ICI with accessible solid tumors (≥ 2 lesions) received IT BMS-986299 administered on day 1, 8, 15, 22, 29 for Cycle 1 (C1) and on days 1, 29 until C2, 8 weeks apart across 7 dose escalation cohorts in Part 1A (75μg-4000μg; Bayesian Logistic Regression Model (BLRM). In Part 1B, BMS-986299 was given across 3 dose escalation cohorts (75μg-2000μg; BLRM Copula) with nivolumab 480 mg q4weeks and ipilimumab 1 mg/kg q8weeks (fixed dosing) from day 1 up to 24 months (m). Cross-over (CS) was allowed from Part 1A to Part 1B. Results: The study was terminated early, unrelated to safety, due to the COVID-19 pandemic and did not complete enrollment. From April 2018 to February 2022, 50 pts were treated (33; Part 1A and 17; Part 1B). Eleven pts crossed over to Part 1B. Median age of pts enrolled was 59 years and with preponderance in female gender (58.0%) and white (80.0%) pt. participation. Forty-two pts (70%) had treatment related adverse events (TRAEs). Grade (G)3/G4 TRAEs included tubulointerstitial nephritis (1; Part 1A), hepatitis (1; CS), colitis, diabetic ketoacidosis, elevated liver enzymes (1 each; Part 1B). No G5 TRAEs or dose limiting toxicities were seen. In Part 1A, 3 pts (melanoma; bladder; anal) achieved stable disease (SD). In CS cohort, 1 triple negative breast cancer (BC) pt had partial response (PR; -60%) for 27.4 m and 3 had SD (breast; cSCC, squamous oropharynx). In Part 1B, 3 pts had PR including 2 pts with hormone positive BC (1 pt with -86%; 3.2m and 1 pt with -39%; 2m) and 1 pt with cutaneous squamous cell cancer (cSCC) (-43%; 1m). Four pts in part 1B had SD (2 breast, 1 lung, 1 chondrosarcoma). All 4 responders received the combination of BMS-986299 with N + I with 3 responders receiving BMS-986299 at 2000μg and one at 300μg. 3 responders (2 breast; 1 cSCC) had prior ICI therapy. Conclusions: BMS-986299 in combination with ICI resulted in modest clinical activity in select tumor types and carried a manageable toxicity profile. However, definitive conclusions could not be drawn as the study was not powered for inference testing. Further exploration is warranted to define the role of this novel agent in the ICI refractory population. Clinical trial information: NCT03444753.