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Background Calmangafodipir (CaM, PledOx) demonstrated efficacy in preventing patient-reported chemotherapy-induced peripheral neuropathy (CIPN) in a randomized phase 2 study in patients with metastatic colorectal cancer. The Preventive Treatment of OxaLiplatin Induced peripherAl neuRopathy (POLAR) program aimed to assess efficacy and safety of CaM in the prevention of CIPN in patients treated with oxaliplatin in adjuvant (POLAR-A, ClinicalTrials.gov.NCT04034355) or metastatic (POLAR-M, ClinicalTrials.gov.NCT03654729) settings. Methods Two randomized, placebo-controlled phase 3 trials investigated patient-reported, moderate-to-severe CIPN 9 months after beginning folinic acid, 5-fluorouracil, and oxaliplatin therapy with or without CaM. In POLAR-A, patients with stage III or high-risk stage II colorectal cancer were randomly assigned 1:1 to receive CaM 5 mu mol/kg or placebo. In POLAR-M, patients with metastatic colorectal cancer were randomly assigned 1:1:1 to receive CaM 5 mu mol/kg, CaM 2 mu mol/kg, or placebo. Results POLAR-A (n = 301) and POLAR-M (n = 291) were terminated early following unexpected hypersensitivity reactions in CaM-treated patients. In a combined analysis of month 9 CIPN (primary endpoint) data from both trials (CaM 5 mu mol/kg, n = 175; placebo, n = 176), 54.3% of patients in the CaM group had moderate-to-severe CIPN compared with 40.3% in the placebo group. The estimated relative risk for moderate-to-severe CIPN at month 9 was 1.37 (95% confidence interval = 1.01 to 1.86; P = .045). A higher proportion of patients experienced serious hypersensitivity reactions across both trials with CaM treatment (3.6%) than with placebo (0.8%). Conclusion The POLAR clinical studies failed to meet their primary endpoint. These results highlight the challenges of targeting oxidative stress for preventing CIPN in both the adjuvant and metastatic settings.