Modulation of glucuronidation of SN-38, the active metabolite of irinotecan, by valproic acid and phenobarbital |
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Authors: | E Gupta Xiaolin Wang Jacqueline Ramirez M J Ratain |
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Institution: | (1) Section of Hematology/Oncology, Department of Medicine, The University of Chicago Medical Center, 5841 S. Maryland Ave., MC 2115, Chicago, IL 60637, USA Tel. (312) 702-4400; Fax (312) 702-0693, US;(2) Committee on Clinical Pharmacology and Cancer Research Center, The University of Chicago Medical Center, 5841 S. Maryland Ave., MC 2115, Chicago, IL 60637, USA, US |
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Abstract: | Purpose: Irinotecan (CPT-11) is hydrolyzed to its active metabolite SN-38 which is subsequently conjugated by uridine diphosphate
glucuronosyl transferase (UDP-GT) to the glucuronide (SN-38G). Both preclinical and clinical data indicate that conjugation
is a primary clearance mechanism for SN-38 with the plasma glucuronide levels being substantially higher than those of SN-38.
This investigation was designed to determine the possibility of modulation of glucuronidation of SN-38 and its effect on the
disposition of the parent drug and metabolites. Methods: Female Wistar rats were pretreated with 200 mg/kg valproic acid (VPA), an inhibitor of glucuronidation, 5 min prior to the
administration of 20 mg/kg irinotecan. The control rats were given 20 mg/kg irinotecan only. To study the effect of inducers
of UDP-GT activity, rats were pre- treated with phenobarbital (PB) before irinotecan administration. Results: Pretreatment with VPA caused a 99% inhibition in the formation of SN-38G leading to a 270% increase in the area under plasma
concentration-time curve (AUC) of SN-38 compared with the control rats. The irinotecan estimations were unchanged in the two
groups. PB pretreatment caused a 1.7-fold increase in the AUC of SN-38G and a concomitant 31% and 59% reduction in the AUCs
of SN-38 and irinotecan, respectively. Conclusions: The most plausible explanation for the alterations in SN-38G disposition is inhibition of SN-38 conjugation by VPA and induction
of the conjugation by PB.
Received: 5 February 1996 / Accepted: 30 July 1996 |
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Keywords: | SN-38 Glucuronidation Inhibition Induction Pharmacokinetics |
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