Drug Interactions between amobarbital and irinotecan

MONITOR: Coadministration with inducers of the CYP450 3A4 isoenzyme may decrease the plasma concentrations of irinotecan and its pharmacologically active metabolite, SN-38. Irinotecan is partially metabolized by CYP450 3A4, and induction of this process results in less of the drug available in the plasma for conversion to SN-38 via carboxylesterases. The interaction has been reported with St. John's wort and the enzyme-inducing anticonvulsants carbamazepine, phenobarbital, and phenytoin. An approximately 40% reduction in SN-38 systemic exposure (AUC) has been reported in the presence of St. John's wort and greater than 60% reductions have been reported in the presence of enzyme-inducing anticonvulsants. However, all of these agents are known to be potent inducers of CYP450 3A4 as well as other enzymatic pathways (e.g., UDP-glucuronosyl transferase, or UGT; carboxylesterases) and drug transporters (e.g., multispecific organic anion transporter, or MRP2; P-glycoprotein) that may be involved in the clearance of irinotecan and/or SN-38. The extent, if any, to which irinotecan may interact with less potent CYP450 3A4 inducers is unknown.

MANAGEMENT: The antitumour activity of irinotecan may be reduced in patients treated with CYP450 3A4 inducers. Pharmacologic response to irinotecan should be monitored more closely whenever a CYP450 3A4 inducer is added to or withdrawn from therapy, and the irinotecan dosage adjusted as necessary.