Abstract 1168: Anti-CSF1R antibody monotherapy inhibits minimal residual disease in a novel immunocompetent murine colorectal cancer metastasis model

Abstract Background: The high mortality rate of colorectal cancer (CRC) is primarily due to metastatic disease. Microscopic minimal residual disease (MRD) detected in patients by circulating tumor DNA (ctDNA) may remain radiographically undetectable and persist following therapeutic intervention, resulting in recurrence. Preclinical models are urgently needed to elucidate the mechanisms whereby MRD escapes anti-tumor immune surveillance. We have developed and characterized a preclinical liver MRD model that recapitulates immune response, with the goal to discover novel strategies to improve outcomes for patients with ctDNA+ MRD. Methods: We generated genetically engineered mouse colonic organoids CDX2 CRE; Rosa26 LSL-CAS9-GFP with APC and TP53 mutation and performed intrasplenic injection in syngeneic C57BL/6 mice to recapitulate microsatellite stable CRC metastasis to the liver. Mice were monitored for radiographically detectable disease using MRI. Tumors were resected at various timepoints, with comparison between metastases at various sizes of development (dichotomized as micro- and macro-metastases). We performed multiplex immunofluorescence staining, single cell RNA seq, and spatial transcriptomic analysis to delineate the immune compartments of micro- and macro-metastasis in untreated mice. After identifying colony stimulating factor -1 receptor (CSF1R) as a target, we performed a vehicle controlled anti-CSF1R monotherapy study for 4 weeks. Treatment began at either 7 days before metastasis is radiographically detectable to mimic treatment of MRD, or 21 days post tumor engraftment. Results: Immune exclusion was confirmed through intratumoral CD45+ leukocyte densities, with transition to an immune excluded phenotype occurring above 0. 5mm in size (defined as macro-metastases). In our preclinical model, immunosuppressive populations of macrophages (IBA1+CD163+) dominated in the micro-metastases. This observation was confirmed in 13 patients with paired micro- and macro-metastases to the liver (p=0. 0065). Strikingly, anti-CSF1R treatment in the murine model eradicated IBA1+CD163+ macrophages (p0. 0001) and induced complete remission of liver micro-metastasis (0/7 mice with residual disease), in contrast to limited efficacy for macro-metastases (5/5 mice with progressed radiographically disease). Additionally, a long-term study confirmed 100% survival (5/5 mice) and no evidence of disease via MRI at 6 months upon anti-CSF1R treatment compared to vehicle (0/5 mice). Conclusions: We established a reproducible preclinical model of MRD with similar immunopathological features of MRD in patients. Our data further identified infiltration of immunosuppressive macrophage populations as an early determinant of MRD progression and indicated CSF1R as a potential new clinical therapeutic target for ctDNA+-defined MRD patients. Citation Format: Alaa M. Mohamed, Kyung Serk Cho, Melinda Soeung, Amanda Anderson, Jumanah Alshenaifi, Ganiraju Manyam, Oscar Villareal, Jennifer Davis, Will Norton, Sisi Gao, Christopher Bristow, Federica Carbone, Stefania Napolitano, Oluwadara Coker, Isha Khanduri, Justin Huang, Dipen M. Maru, Mauro Di Pilato, Ryan Sun, Linghua Wang, David G. Menter, Giannicola Genovese, Giulio Draetta, Natalie Fowlkes, Scott Kopetz. Anti-CSF1R antibody monotherapy inhibits minimal residual disease in a novel immunocompetent murine colorectal cancer metastasis model abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 1168.