Genotype-driven phase I study of irinotecan administered in combination with fluorouracil/leucovorin in patients with metastatic colorectal cancer

Purpose: We aimed to identify the maximum-tolerated dose (MTD) of irinotecan in patients with cancer with UGT1A1*1/*1 and *1/*28 genotypes. We hypothesize that the patients without the *28/*28 genotype tolerate higher doses of irinotecan. Patients and Methods: Patients undergoing first-line treatment for metastatic colorectal cancer (CRC) eligible for treatment with irinotecan plus infusional fluorouracil/leucovorin (FOLFIRI) were screened for the UGT1A1*28/ *28 genotype and excluded from the study. Fifty-nine white patients with either the *1/*1 or the *1/*28 genotype were eligible for dose escalation of irinotecan. The starting dose of biweekly irinotecan was 215 mg/m2 for both genotype groups, whereas the dose of infusional fluorouracil was fixed. Pharmacokinetic data of irinotecan and metabolites were also obtained. Results: The dose of irinotecan was escalated to 370 mg/m2 in patients with the *1/*28 genotype and to 420 mg/m2 in those with the *1/*1 genotype. Dose-limiting toxicities (DLTs) were observed in two of four of *1/*28 patients at 370 mg/m2 and in two of three of *1/*1 patients at 420 mg/m2. No DLTs were observed in 10 *1/*28 patients at 310 mg/m2 and in 10 *1/*1 patients at 370 mg/m2; hence these dose levels were the MTD for each genotype group. The most common grade 3 to 4 toxicities were neutropenia and diarrhea. The pharmacokinetics of irinotecan and SN-38 exhibit linear kinetics. Conclusion: The recommended dose of 180 mg/m2 for irinotecan in FOLFIRI is considerably lower than the dose that can be tolerated when patients with the UGT1A1*28/*28 genotype are excluded. Prospective genotype-driven studies should test the efficacy of higher irinotecan doses in the FOLFIRI schedule.