Correlation of pharmacokinetics of CPT-11 in FOLFIRI with phenotyping of CYP3A4 and UGT1A1 activity.

Abstract

e13002 Background: CPT-11 has complex PK and PD; genotyping of UGT1A1 is recommended for higher doses in *28 carriers (Caucasian) and *6 carriers (Asian) because of a higher risk of toxicity. Raltegravir is mainly metabolized to raltegravir glucuronide by UGT1A1. CPT-11 is mainly eliminated by cytochrome P450 isoform 3A (CYP3A)-mediated oxidation and by esterase cleavage to form SN-38, which is further conjugated by uridine-diphosphate glucuronosyltransferase (UGT) isozymes to the inactive SN-38 glucuronide. We prospectively explored the correlation of midazolam and raltegravir PK with CPT-11 clearance (CL) and SN-38 levels respectively and toxicity in patients undergoing chemotherapy with CPT-11. Methods: Twenty-five Asian patients with advanced cancer received CPT-11 as a 2-weekly 90-minute infusion (180 mg/m2) as part of the FOLFIRI regimen. Subjects were administered raltegravir 400 mg orally (as a UGT1A1 probe) and intravenous midazolam 1 mg (as a CYP3A4 probe) one day before the first dose of their chemotherapy. Blood sampling for drug concentrations was performed for 2 days after probe drug administration. Non-compartmental PK analyses were performed using WinNonLin. Genomic DNA was isolated and screened for the known genetic variants in UGT1A1*6. Results: One patient developed grade 3 diarrhea and 16 developed grade 3-4 neutropenia. The mean CPT-11 CL was 21.9 + 7.8L/h. SN-38G/SN-38 AUC ratio was highly correlated with Raltegravir glucuronide/ Raltegravir AUC ratio (r=0.784 p<0.01). Midazolam CL was highly correlated with CPT-11 CL (r=0.563 p<0.01). SN38 AUC correlated well with neutropenia (Log10Nadir Absolute Neutrophil Count (ANC) (r=-0.397 p<0.05). Neither CPT-11 nor midazolam CL correlated with Log10 Nadir ANC. Of the 24 patients with genotyping of UGT1A1*6 done, 17 were wild type (GG), 7 heterozygotes (AG) and 0 were mutant (AA). No differences in toxicity or SN38 pharmacokinetics were observed in the UGT1A1*6 variants. Conclusions: Raltegravir glucuronide ratio is a good predictor of SN-38 glucuronidation, and has potential to predict neutropenia from FOLFIRI treatment. Prospective studies with dose adjustments should be done to develop raltegravir as a probe to optimize CPT-11 therapy.