A mechanistic study on altered pharmacokinetics of irinotecan by St. Johns wort

Irinotecan (CPT-11) is an important anticancer drug in management of advanced colon cancer. A marked protective effect on CPT-11-induced blood and gastrointestinal toxicity is obtained by combination of St. John's wort (SJW) in recent clinical and rat studies. However, the mechanism is unclear. This study aimed to explore the effects of SJW on the pharmacokinetics of CPT-11 and its major metabolites (SN-38 and SN-38 glucuronide) in rats and the underlying mechanisms using several in vitro models. Short-term (3 days) and long-term (14 days) pretreatment with SJW were conducted in rats to examine the effects of co-administered SJW on the plasma pharmacokinetics of CPT-11, SN-38 and SN- 38 glucuronide. Rat liver microsomes and a rat hepatoma cell line, H4-II-E cells, were utilized to study the effects of aqueous and ethanolic extracts (AE and EE) and major active components (hyperforin, hypericin and quercetin) of SJW on CPT-11 and SN-38 metabolism and intracellular accumulation. Co-administered SJW for consecutive 14 days significantly decreased the initial plasma concentration (C0) of CPT-11, the area under the concentration-time curve (AUC0-10hr) and maximum plasma concentration (Cmax) of SN-38. The ethanolic extracts (EE) of SJW at 5 μ g/ml significantly decreased SN-38 glucuronidation by 45% (P < 0.05) in rat hepatic microsomes. Pre-incubation of aqueous SJW extracts (AE) at 10 g/ml, SJW EE at 5 μg/ml, and quercetin at 10μ M significantly increased the glucuronidation of SN-38 in H4- II-E cells. A 2-hr pre-incubation of quercetin (100μ M) significantly increased the intracellular accumulation of CPT-11 (P < 0.05). However, pre-incubation of hypericin (20 nM and 200 nM) and hyperforin (1μ M) significantly decreased the intracellular accumulation of CPT-11. In addition, pre-incubation of hypericin, SJW EE and quercetin significantly increased the intracellular accumulation of SN-38. Aqueous and ethanolic SJW extracts and its major active components did not alter the plasma protein binding of CPT-11 and SN-38. These results indicated that the aqueous and ethanolic extracts of SJW and its major active components could markedly alter glucuronidation of SN-38 and intracellular accumulation of CPT-11 and SN-38, which probably provides partial explanation for the altered plasma pharmacokinetics of CPT-11 and SN-38 and the antagonizing effects on the toxicities of CPT-11. Further studies are needed to explore the role of both pharmacokinetic and pharmacodynamic components in the protective effect of SJW against the toxicities of CPT-11.

Pharmacokinetic mechanisms for reduced toxicity of irinotecan by coadministered Thalidomide

The clinical use of irinotecan (CPT-11) is hindered by dose-limiting diarrhea and myelosuppression. Recent clinical studies indicate that thalidomide, a known tumor necrosis factor-alpha inhibitor, ameliorated the toxicities induced by CPT-11. However, the mechanisms for this are unknown. This study aimed to investigate whether combination of thalidomide modulated the toxicities of CPT-11 using a rat model and the possible role of the altered pharmacokinetic component in the toxicity modulation using in vitro models. The toxicity model was constructed by treatment of healthy rats with CPT-11 at 60 mg/kg per day by intravenous (i.v.) injection. Body weight, acute and delayed-onset diarrhea, blood cell counts, and macroscopic and microscopic intestinal damages were monitored in rats treated with CPT-11 alone or combined therapy with thalidomide at 100 mg/kg administered by intraperitoneal (i.p.) injection. Single dose and 5-day multiple-dose studies were conducted in rats to examine the effects of concomitant thalidomide on the plasma pharmacokinetics of CPT-11 and its major metabolites SN-38 and SN-38 glucuronide (SN-38G). The effect of CPT-11 on thalidomide's pharmacokinetics was also checked. Rat liver microsomes and a rat hepatoma cell line, H4-II-E cells, were used to study the in vitro metabolic interactions between these two drugs. H4-II-E cells were also used to investigate the effect of thalidomide and its hydrolytic products on the transport of CPT-11 and SN-38. In addition, the effect of thalidomide and its hydrolytic products on rat plasma protein binding of CPT-11 and SN-38 was examined. Administration of CPT-11 by i.v. for 4 consecutive days to rats induced significant body weight loss, decrease in neutrophil and lymphocyte counts, severe acute- and delayed-onset diarrhea, and intestinal damages. These toxicities were alleviated when CPT-11 was combined with thalidomide. In both single-dose and 5-day multiple-dose pharmacokinetic study, coadministered thalidomide significantly increased the area under the plasma concentration-time curve (AUC) of CPT-11, but the AUC and elimination half-life (t(1/2)) of SN-38 were significantly decreased. However, CPT-11 did not significantly alter the pharmacokinetics of thalidomide. Thalidomide at 25 and 250 microM and its hydrolytic products at a total concentration of 10 microM had no significant effect on the plasma protein binding of CPT-11 and SN-38, except for that thalidomide at 250 microM caused a significant increase in the unbound fraction (f(u)) of CPT-11 by 6.7% (P < 0.05). The hydrolytic products of thalidomide (total concentration of 10 microM), but not thalidomide, significantly decreased CPT-11 hydrolysis by 16% in rat liver microsomes (P < 0.01). The formation of both SN-38 and SN-38G from CPT-11, SN-38 glucuronidation, or intracellular accumulation of both CPT-11 and SN-38 in H4-II-E cells followed Michaelis-Menten kinetics with the one-binding site model being the best fit for the kinetic data. Coincubation or 2-hr preincubation of thalidomide at 25 microM and 250 microM and its hydrolytic products at 10 microM did not show any significant effects on CPT-11 hydrolysis and SN-38 glucuronidation. However, preincubation of H4-II-E cells with thalidomide (250 microM), its hydrolytic products (total concentration of 10 microM), or phthaloyl glutamic acid (one major thalidomide hydrolytic product, 10 microM) significantly increased the intracellular accumulation of SN-38, but not CPT-11 (P < 0.01). The dose-limiting toxicities of CPT-11 were alleviated by combination with thalidomide in rats and the pharmacokinetic modulation by thalidomide may partially explain its antagonizing effects on the toxicities of CPT-11. The hydrolytic products of thalidomide, instead of the parental drug, modulated the hepatic hydrolysis of CPT-11 and intracellular accumulation of SN-38, probably contributing to the altered plasma pharmacokinetics of CPT-11 and SN-38. Further studies are needed to explore the role of both pharmacokinetics and pharmacodynamic components in the protective effect of thalidomide against the toxicities of CPT-11.

Novel agents that potentially inhibit irinotecan-induced diarrhea

Irinotecan (CPT-11, 7-ethyl-10-[4-(1-piperidino)-1-piperidino] carbonyloxycamptothecin) has exhibited clinical activities against a broad spectrum of carcinomas by inhibiting DNA topoisomerase I (Topo I). However, severe and unpredictable dosing-limiting toxicities (mainly myelosuppression and severe diarrhea) hinder its clinical use. The latter consists of early and late-onset diarrhea, occurring within 24 hr or ≥ 24 hr after CPT-11 administration, respectively. This review highlights novel agents potentially inhibiting CPT-11-induced diarrhea, which are designed and tested under guidance of disposition pathways and potential toxicity mechanisms. Early-onset diarrhea is observed immediately after CPT-11 infusion and probably due to the inhibition of acetylcholinesterase activity, which can be eliminated by administration of atropine. Lateonset diarrhea appears to be associated with intestinal exposure to SN-38 (7-ethyl-10-hydroxycamptothecin), the major active metabolite of CPT-11, which may bind to Topo I and induce apoptosis of intestinal epithelia, leading to the disturbance in the absorptive and secretory functions of mucosa. CPT-11 and SN-38 may also stimulate the production of pro-inflammatory cytokines and prostaglandins (PGs), thus inducing the secretion of Na⁺ and Cl^-. Early treatment of severe late-onset diarrhea with oral high-dose loperamide has decreased patient morbidity. Extensive studies have been conducted to identify other potential agents to ameliorate diarrhea in preclinical and clinical models. These include intestinal alkalizing agents, oral antibiotics, enzyme inducers, P-glycoprotein (PgP) inhibitors, cyclooxygenase-2 (COX-2) inhibitors, tumor necrosis factor-agr (TNF-α) inhibitors, or blockers of biliary excretion of SN-38. Further studies are needed to identify the molecular targets associated with CPT-11 toxicity and safe and effective agents for alleviating CPT-11-induced diarrhea.