Background: Dipeptidyl peptidase IV (DPP4), also known as Cluster of differentiation 26 (CD26) is a commonly expressed cell surface protein found in many cell types that can function as a tumor suppressor or activator, depending on the cancer type its interactions with the tumor microenvironment. DPP4 has been studied as a tumor biomarker due to its expression in various primary tumors and metastases. Recent literature established benefits of DPP4 inhibitors on progression free survival (PFS) of diabetic patients with advanced colorectal and airway cancers. In this study, we examine the effect of DPP4 inhibitors on PFS in diabetic patients (pts) with advanced-stage prostate cancer. Methods: We performed a multi-center retrospective analysis at Moffitt Cancer Center and the University of Florida of diabetic pts with advanced stage (III or IV) prostate cancer. All pts were on anti-hyperglycemic therapy and received either surgery, radiation therapy, or both for their prostate cancer. The control group included pts on metformin and a sulfonylurea, and the study group included pts on a DPP4 inhibitor and metformin. PFS was calculated for both groups, and Cox proportional hazards regression was used to determine statistical significance. Kaplan-Meier survival analysis was done to predict the effect of DPP4 inhibition and radiation therapy on PFS. Results: Our study population consisted of 161 pts, with 120 pts in the control group and 41 in the study group. No significant difference in progression of prostate cancer was found between those on DPP4 inhibitor vs. Metformin. Specifically, 24 of 41 (59%) of patients on DPP4 inhibitors progressed, while 73 of 120 (61%) of patients on Metformin had progression of prostate cancer (HR 1.01, 95% CI 0.64 to 1.61, p= 0.955). Median time to progression was 3.3 years in the DPP4 group versus 3.5 years in the Metformin group. However, radiation therapy was associated with improved PFS (HR 0.56, 95% CI 0.37 to 0.84, p= 0.0006). Conclusions: Contrary to what has been shown in pts with diabetes and advanced colorectal or airway cancers, exposure to DPP4 inhibitors did not lead to statistically significant improvement in PFS in pts with advanced stage prostate cancer. Recent data suggests that DPP4 may act as a tumor suppressor gene to the androgen receptor (AR) pathway, and that DPP4 inhibition can result in emergence of resistance to androgen deprivation therapy (ADT). Further analysis with larger sample sizes is necessary to elucidate if this is applicable to prostate cancer as a whole, or if this effect is specific to castrate-sensitive prostate cancer (CSPC) vs. castrate-resistant prostate cancer (CRPC).