Background: Treatment of locally advanced rectal cancer currently involves tri-modal therapy with pre-operative chemo-radiation (CRT), surgery, and combination chemotherapy. Total neoadjuvant therapy (TNT) is emerging as an important treatment pathway that can increase rates of complete response (CR) leading to better overall outcomes. Novel radio-sensitizing agents may improve rates of CR. Trifluridine/tipiracil (TFD/TPI) is a novel fluoropyrimidine that has demonstrated clinical efficacy in patients with treatment-refractory metastatic colorectal cancer. Importantly, TFD/TPI has superior radio-sensitization in vitro compared to 5-fluorouracil, and we hypothesize that this can improve the tumor response of patients. This phase 1 study assesses the safety and preliminary efficacy of using TFD/TPI for long-course CRT for rectal cancer followed by FOLFOX chemotherapy in the TNT setting. Methods: NCT04104139 is a prospective, dose-finding phase I study using Bayesian Optimal Interval (BOIN) design to determine the maximum tolerated dose (MTD) of TFD/TPI with concurrent standard dose/fractionation radiation for resectable rectal cancer. Adults with previously untreated rectal adenocarcinoma, with clinical stage II (T3-4aN0) and stage III (T1-4aN1+) by MRI and endoscopy are eligible. Patients must first receive CRT with standard radiation (45-50 Gy over 25-28 fractions) followed by up to 8 cycles of standard FOLFOX, or alternatively 5 cycles of CAPOX chemotherapy. During CRT, the first dose level of TFD/TPI is PO 25mg/m2 twice a day from Mon to Fri (5 weekdays) alternating on weeks 1, 3, and 5 concurrent with 5-6 weeks of radiation. The second dose level is 30mg/m2, and third dose level is 35mg/m2. The MTD of TFD/TPI in combination with radiation is the dose corresponding to a dose limiting toxicity (DLT) probability of 30% using the BOIN design with escalation/de-escalation of each dose level occurring in cohorts of 3 participants until a maximum total of 18 participants, sub-total of 12 patients within one dose level, or another stopping rule based on serious toxicities. The primary endpoint is proportion of DLTs for TFD/TPI at MTD. Secondary endpoints include overall tolerability profile, delayed toxicities during FOLFOX, rates of clinical CR, rates of pathologic CR, and correlative biomarkers. The study was activated December 2019 and has accrued 8 of intended 18 patients. Clinical trial information: NCT04104139.