12121 Background: Prospective data regarding the prevalence of DPYD variant alleles and the impact of genotype guided dosing on safety and efficacy of fluoropyrimidines (FP) remains limited. We assessed the feasibility and safety impact of routine, prospective, DPYD genotype guided FP dosing. Methods: Adult patients (>18 years), commencing FP for the first time were enrolled. Sanger PCR sequencing was designed to target DPYD variants NM_000110.4:c.1905+1G>A (DPYD*2A), NM_000110.4:c.1679T>G (DPYD*13), NM_000110.4:c.1236G>A and NM_000110.4:c.2846A>T. Dose reductions were implemented as per current international pharmacogenomic guidelines. Gr3/4 toxicity, dose reductions/delays and hospital admissions were reviewed for the first 60 days after commencement. For comparison purposes, a retrospective review of patients receiving FP without testing in a prior year was completed. Results: Between July 2021 – May 2023, 334 patients were enrolled for testing. 95% ECOG 0-1. 57% curative treatment intent. Mean time from phlebotomy to result 6.9 days (range 2-15). Clinician/patient acceptance was high (>95% of eligible patients starting FP were enrolled). 20 (6.0%) variant DPYD allele carriers were detected. 11 heterozygous for c.1236G>A, 5 heterozygous for C.1905+1G>A, 3 heterozygous for c.2846A>T and 1 homozygous for c.1236G>A. 295 of patients tested proceeded to receive FP. 62% received Infusional-FP, 38% Capecitabine. 39 patients commenced treatment prior to DPYD result being available (treating clinician decision). Dose modification recommendations were followed in all but two patients. 24% experienced FP related Grade 3/4 toxicity within 60 days. (23% DPYD wild type, 37% DPYD variant) versus 35% in the retrospective cohort. 16% had FP related dose delays (16% DPYD wild type, 12% DPYD variant) versus 25% retrospective cohort. 14% had FP related dose reductions (13% DPYD wild type, 25% DPYD variant) versus 12% retrospective cohort. 13% had FP related hospital admissions (13% DPYD wild type, 19% DPYD variant) versus 17% retrospective cohort. 1 death in the DPYD wild type cohort was deemed potentially related to FP. Conclusions: Prospective DPYD genotype guided dosing was feasible in clinical practice, with prevalence of variant alleles comparable to published literature. There was a modest reduction in risk of FP related severe toxicity, dose delays, and hospital admissions with DPYD guided dosing compared to a historical, untested cohort. Clinical trial information: ACTRN12621001117808.