Background: Preoperative chemo-radiotherapy with or without sequential chemotherapy, followed by surgical intervention, is standard of care for patients with locally advanced rectal cancer (LARC). However, 1/3 of these patients still develop distant metastases, indicating the need for more effective therapies. DNA-dependent protein kinase (DNA-PK) regulates a key DNA damage repair pathway for double-strand break repair. Peposertib (M3814), a potent, selective, orally administered DNA-PK inhibitor, has been shown to potentiate the effect of ionizing radiation in a human colon cancer xenograft model and several colon cancer cell lines. Peposertib is being investigated in several different trials across multiple indications. This Phase Ib/II study (NCT03770689) aims to evaluate the safety, tolerability, pharmacokinetics (PK), and efficacy of the neoadjuvant treatment combination of peposertib, capecitabine, and radiotherapy (RT) in patients with LARC. Methods: Patients aged ≥18 years with histologically confirmed and resectable Stage II/III rectal adenocarcinoma are eligible. Induction chemotherapy is permitted, but residual disease must first be documented by MRI, digital rectal examination and endoscopy. Patients who received other anticancer therapies or those with prior pelvic RT are excluded. During open-label Phase Ib (open), 18–30 patients (n = 3 per cohort) are due to receive peposertib + capecitabine (orally, 825 mg/m² twice daily [BID]) + RT (45–50 Gy), 5 days/week. Peposertib 50 mg once daily (QD) is the starting dose. Additional dose levels will be between 100─800 mg QD. Dose escalation is determined by the safety monitoring committee and guided by a Bayesian 2-parameter logistic regression model. At Phase II (planned), 150 patients will be randomized (1:1) to receive oral capecitabine (825 mg/m² BID) + RT (45–50 Gy), with either oral peposertib (recommended phase II dose [RP2D]) or placebo, QD for 5 days/week. Primary objectives are to define a maximum tolerated dose and RP2D (Phase Ib), and to evaluate the efficacy of peposertib + capecitabine + RT in terms of pathological/clinical complete response (Phase II). Secondary objectives include assessment of antitumor activity (Phase Ib), quality of life outcomes (Phase II), and PK of peposertib, and the safety and tolerability of the combination therapy (both phases). One patient has received peposertib 50 mg QD and six patients have received peposertib 100 mg QD. Patients are currently receiving peposertib 150 mg QD. Clinical trial information: NCT03770689.