Abstract CT165: Modifying the tumor microenvironment in gastric and gastroesophageal junction adenocarcinoma using a cGMP-phosphodiesterase inhibitor

Abstract Background: The current standard approach for locoregional gastric and gastroesophageal junction adenocarcinoma (GAC) includes perioperative chemoimmunotherapy with 5-florouracil, oxaliplatin, docetaxel, and durvalumab (FLOT-D). However, FLOT-D shows limited efficacy with suboptimal response rates, high recurrence risk, and unproven long-term outcomes, highlighting an unmet need for new targets. Our murine preclinical studies identified a subset of Schlafen4 (SLFN4) expressing myeloid derived suppressor cells (MDSCs) that correlate with metaplasia. These SLFN4+-MDSCs are programmed death-ligand1 (PD-L1) positive and display a strong type I interferon inducible transcriptional program and potent T cell suppressor activity. Sildenafil suppressed SLFN4+-MDSCs, restored T cell function, and reduced metaplasia, supporting a therapeutic potential for cGMP-dependent phosphodiesterase (PDE) inhibition. Here we present the results of a first in human, proof of principle, window of opportunity, clinical trial, designed to modify GAC tumor microenvironment (TME) marked by inhibiting cGMP-PDEs and SLFN12L+-MDSCs. Methods: Using single-cell RNA sequencing, we examined tissues from stage I-III GAC patients (n=10) enrolled in this phase II, single arm study (NCT05709574). The patients received tadalafil (20mg) for 14 days, followed by combined tadalafil and FLOT for 8 weeks. Gastric tissue and fluids (blood and urine) were collected i) before treatment, ii) after 14 days of tadalafil, and iii) after 8 weeks of combined treatment. The primary objective was demonstrating the safety of combining tadalafil with FLOT. The secondary objectives were the pathologic and radiologic responses. Exploratory objectives included testing for SLFN12L+-MDSCs and other genetic biomarkers. Results: Patient accrual has been completed. Median age at diagnosis was 74years; 60% patients were males, 40% were Hispanic, 60% were distal tumors while intestinal and diffuse histologies were equally represented. Tadalafil alone suppressed >50% Gr-MDSCs marked by SLFN12L in both the tissue and blood with further reductions after combined therapy. There was a corresponding decrease in exhausted T cells and increase in CD8+ lymphocytes and decrease in CD4+ T-helper lymphocytes. Two patients had complete pathologic responses, one near complete response (>90%) and two patients had partial responses as per Becker’s criteria. One patient had a complete radiologic response, two near complete and three partial responses. Data from 3 patients are pending. Only one patient experienced grade 3 side effects from tadalafil requiring a dose reduction. Tissue and blood testing identified MIR130b as a potential diagnostic biomarker for GAC. Conclusion: Results from trial show that tadalafil with FLOT is well tolerated, decreases SLFN12L+-MDSCs, reduces tumor immunosuppression, and may correlate with clinical responses. The study used a pre-immunotherapy standard, with future larger trials to incorporate FLOT-D for validation. Citation Format: Junaid Arshad, Lin Ding, Palash Mallick, Mohammad Khreiss, Aaron J. Scott, Rachna Shroff, Juanita Merchant. Modifying the tumor microenvironment in gastric and gastroesophageal junction adenocarcinoma using a cGMP-phosphodiesterase inhibitor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(8_Suppl):Abstract nr CT165.