Modulation of glucuronidation of SN-38, the active metabolite of irinotecan, by valproic acid and phenobarbital

Cancer Chemother Pharmacol. 1997;39(5):440-4. doi: 10.1007/s002800050595.


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 pretreated 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.

MeSH terms

  • Animals
  • Antineoplastic Agents, Phytogenic / pharmacokinetics
  • Biotransformation / drug effects
  • Camptothecin / analogs & derivatives*
  • Camptothecin / pharmacokinetics
  • Female
  • Glucuronates / metabolism*
  • Glucuronosyltransferase / metabolism
  • Irinotecan
  • Metabolic Clearance Rate
  • Phenobarbital / pharmacology*
  • Rats
  • Rats, Wistar
  • Valproic Acid / pharmacology*


  • Antineoplastic Agents, Phytogenic
  • Glucuronates
  • Valproic Acid
  • Irinotecan
  • Glucuronosyltransferase
  • Camptothecin
  • Phenobarbital