Background: MEM-288 is a conditionally-replicative oncolytic adenovirus expressing human IFNβ and a recombinant membrane-stable form of CD40L (MEM40). Preclinical studies show MEM-288 induces robust dendritic cell-mediated systemic T cell responses capable of inhibiting abscopal tumor growth as monotherapy and synergizes with immune checkpoint inhibitors (ICI). Methods: MEM-288 is being evaluated in this Phase 1 open-label trial (NCT05076760) in pts with select solid tumors including NSCLC (a) refractory to standard therapy and (b) with a tumor lesion deemed feasible for biopsy and MEM-288 intratumoral (i.t.) injection. The primary objective is to determine using a BOIN design a recommended phase II dose of MEM-288 across 3 dose levels (DL1-3) spanning 1e10 to 1e11 viral particles by i.t. injection once every 3 weeks. Secondary objectives include efficacy assessment, including response rate of injected and non-injected tumors assessed separately. Tumor biopsies obtained immediately prior to the 1st and 2nd injections are used to explore biomarkers and anti-tumor immune responses. Results: As of February 2023, 12 pts (11 NSCLC and 1 pancreatic cancer; n = 3 DL1, n = 5 DL2, n = 4 DL3) have enrolled. The median number of MEM-288 i.t. injections was 2 (1-3), median age was 63 (39-76), and median prior lines of therapy was 3 (1-5). Radiology-guided injections were administered in superficial/palpable tumors and visceral lesions. No dose limiting toxicities occurred. Treatment-related adverse events observed in > 1 pt include grade 1 injection site reaction (42%) and chills (17%). Of 10 response-evaluable pts, 4 (40%) pts had shrinkage of injected tumor (range -26 to -54%): 3 (30%) PR and 1 (10%) SD. Multiple pts also had stabilization or shrinkage of distal non-injected lesions. Best overall response was 3 (30%) stable disease and 7 (70%) progressive disease. After a single MEM-288 injection, biopsies show decreased tumor cell percentage concomitant with substantial increases in overall CD8⁺ T cells, increased T clonotype diversity in both tumor and peripheral blood, and increased TCF1⁺ stem-like CD8⁺ T cells that are strongly associated with response to ICIs. Plasma cytokine analysis showed increases in IFNg and of IFN-inducible cytokines and chemokines, supportive of stimulation of systemic response after MEM-288. Of note, a pt with strong stimulation of tumor and systemic T cell immunity after MEM-288 had subsequent CR (ongoing > 7 months) to docetaxel + ramucirumab following prior treatment failure with platinum doublet + ICI received before MEM-288. Conclusions: Preliminary safety, antitumor and immune response data are encouraging. Updated results and immune data from this study will be presented. An expansion arm is planned with combination MEM-288 and anti-PD1 antibody in pts with advanced NSCLC refractory to ICI. Clinical trial information: NCT05076760.