Patient-derived Colorectal Microtumors Predict Response to anti-PD1 Therapy

Duy T. Nguyen^1*Matthew A. Schaller¹Krista P. Terracina¹Xia Xu²Diego I. Pedro¹Alfonso Pepe¹Juan M. Urueña¹Zadia Dupee¹Nickolas Diodati¹Ryan Smolchek¹Jack E. Famiglietti¹Nhi Tran Yen Nguyen¹Gerik Tushoski¹Kuoyuan Cheng²Lan Chen²Doug Linn²Vania Vidimar²Aquila Fatima²Soon Woo Kwon²Dongyu Sun²Hongmin Chen²Haiyan Xu²Brian Long²Lily Y. Moy²Bonnie J Howell²George H. Addona²W. Gregory Sawyer¹

• ¹University of Florida, Gainesville, United States
• ²Merck & Co Inc, Rahway, United States

Immune checkpoint inhibitors have made remarkable impacts in treating various cancers, including colorectal cancer (CRC). However, CRC still remains a leading cause of cancer-related deaths. While microsatellite instability (MSI) colorectal cancer (CRC) has shown positive responses to anti-PD1-therapy, this subgroup represents a minority of all CRC patients. Extensive research has focused on identifying predictive biomarkers to understand treatment response in CRC. Interestingly, a growing number of clinical cases have reported favorable outcomes from a subtype of supposedly non-responder microsatellite stable (MSS) CRC, characterized by DNA polymerase ε (POLE) proofreading domain mutations with high tumor mutational burden (TMB). This subtype has shown a notable response, either partial or complete, to pembrolizumab as salvage treatment, often following significant disease progression. To improve efficiency, cost-effectiveness, and clinical outcomes, there is an essential need for a testing platform capable of promptly identifying evidence of anti-PD-1 response to inform treatment strategies. Here, we established a novel 3D ex vivo immunotherapy model using patient-derived tumor microexplants (or microtumors <1 mm) co-cultured with autologous peripheral blood mononuclear cells (PBMCs) from treatment-naïve CRC patients. We demonstrate here that long-term ex vivo treatment with pembrolizumab induced a heterogeneous but appreciable interferon-gamma (IFN-gamma) secretion, accompanied by infiltrating PBMCs. Intriguingly, a case study involving an MSS CRC phenotype harboring POLE mutation and associated ultrahigh TMB demonstrated a response to PD-1 blockade, potentially from the intratumoral immune cell population. Ultimately, this novel model could serve as a valuable tool in complementing clinical diagnostics and guiding personalized treatment plans for CRC patients, particularly those with specific phenotypes and mutational profiles.