Background: Regorafenib demonstrated survival benefits as salvage therapy for patients with metastatic colorectal cancer. However, severe toxicities frequently occurred early in the treatment with the standard dose (160 mg/day), resulting in a dose reduction or interruption. To improve the tolerability and maintain sufficient efficacy, we conducted a phase II study of regorafenib with a lower starting dose (120 mg/day).
Patients and methods: Regorafenib was initiated at 120 mg/day, and the dosage was increased to 160 mg/day on day 15 of the first cycle for patients who had met the dose escalation criteria. The primary endpoint was the disease control rate (DCR). The pharmacokinetics of the total and unbound regorafenib and its active metabolites (M2, M5) were assessed.
Results: A total of 70 patients were enrolled from September 2016 to December 2017. Only 6 patients achieved dose escalation to 160 mg on day 15 as planned. For the 68 evaluable patients, the DCR was 32.4% (95% confidence interval, 21.5%-44.8%), which was less than the threshold (30%) of our statistical hypothesis. The serum concentrations of total regorafenib for patients whose dose was escalated to 160 mg/day were significantly lower than those of the patients whose dose was not escalated (median, 3978 vs. 7244 nM; P = .027). The serum unbound concentrations of the sum of regorafenib and the active metabolites correlated significantly with the maximum grade of regorafenib-related symptomatic adverse events in the first cycle (11,138 vs. 19,096 pM; P = .035).
Conclusion: Regorafenib with a low starting dose of 120 mg/day did not achieve the expected DCR. A relationship of unbound exposure with toxicity was found.
Keywords: Advanced colorectal cancer; Dose escalation; Multikinase inhibitor; Pharmacokinetics; Protein binding.
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