Genetic Variants in the UDP-glucuronosyltransferase 1A1 Gene Predict the Risk of Severe Neutropenia of Irinotecan

Purpose Severe toxicity is commonly observed in cancer patients receiving irinotecan. UDP-glucuronosyltransferase 1A1 (UGT1A1) catalyzes the glucuronidation of the active metabolite SN-38. This study prospectively evaluated the association between the prevalence of severe toxicity and UGT1A1 genetic variation. Patients and Methods Sixty-six cancer patients with advanced disease refractory to other treatments received irinotecan 350 mg/m2 every 3 weeks. Toxicity and pharmacokinetic data were measured during cycle 1. UGT1A1 variants (−3279G>T, −3156G>A, promoter TA indel, 211G>A, 686C>A) were genotyped. Results The prevalence of grade 4 neutropenia was 9.5%. Grade 4 neutropenia was much more common in patients with the TA indel 7/7 genotype (3 of 6 patients; 50%) compared with 6/7 (3 of 24 patients; 12.5%) and 6/6 (0 of 29 patients; 0%) (P = .001). The TA indel genotype was significantly associated with the absolute neutrophil count nadir (7/7 < 6/7 < 6/6, P = .02). The relative risk of grade 4 neutropenia ...