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

 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     

UGT1A1 genotypes and glucuronidation of SN-38, the active metabolite of irinotecan

Background: Irinotecan (CPT-11) is metabolized by esterase to form a SN-38, which is further conjugated by UGT1A1. Genetic polymorphism has been shown in a promoter region of UGT1A1 and is related to its activity. We investigated whether there might be an inter-individual difference in pharmacokinetics of SN-38 and its glucuronide, depending on the genotypes of UGT1A1.Patients and methods: Nine male patients with lung cancer were treated with irinotecan (50 mg/m²) and carboplatin. Pharmacokinetic parameters were calculated with full sampling plasma data. Genotypes were determined by analyzing the sequence of TATA box of UGT1A1 of genomic DNA from the patients.Results: The genotyping analysis revealed one heterozygote (6/7) and one homozygote (7/7) for (TA)₇TAA allele (UGT1A1^*28). The remaining seven patients were homozygote for (TA)₆TAA allele (6/6, wild type). The metabolic ratios (SN-38/SN-38 glucuronide) in the patient with 7/7 genotype were uncharacteristically higher than those in the patients with other genotypes (6/6 and 6/7). Biliary index was 6980 versus 2180 ± 1110 (range 840–3730) in patients with 7/7 versus 6/6 genotypes, respectively.Conclusion: These results support the idea that the patient with 7/7 genotype has an impaired capacity for glucuronidation of SN-38.     

Pharmacokinetic and pharmacogenetic determinants of the activity and toxicity of irinotecan in metastatic colorectal cancer patients

This study aims at establishing relationships between genetic and non-genetic factors of variation of the pharmacokinetics of irinotecan and its metabolites; and also at establishing relationships between the pharmacokinetic or metabolic parameters and the efficacy and toxicity of irinotecan. We included 49 patients treated for metastatic colorectal cancer with a combination of 5-fluorouracil and irinotecan; a polymorphism in the UGT1A1 gene (TA repeat in the TATA box) and one in the CES2 gene promoter (830C>G) were studied as potential markers for SN-38 glucuronidation and irinotecan activation, respectively; and the potential activity of CYP3A4 was estimated from cortisol biotransformation into 6beta-hydroxycortisol. No pharmacokinetic parameter was directly predictive of clinical outcome or toxicity. The AUCs of three important metabolites of irinotecan, SN-38, SN-38 glucuronide and APC, were tentatively correlated with patients' pretreatment biological parameters related to drug metabolism (plasma creatinine, bilirubin and liver enzymes, and blood leukocytes). SN-38 AUC was significantly correlated with blood leukocytes number and SN-38G AUC was significantly correlated with plasma creatinine, whereas APC AUC was significantly correlated with plasma liver enzymes. The relative extent of irinotecan activation was inversely correlated with SN-38 glucuronidation. The TATA box polymorphism of UGT1A1 was significantly associated with plasma bilirubin levels and behaved as a significant predictor for neutropoenia. The level of cortisol 6beta-hydroxylation predicted for the occurrence of diarrhoea. All these observations may improve the routine use of irinotecan in colorectal cancer patients. UGT1A1 genotyping plus cortisol 6beta-hydroxylation determination could help to determine the optimal dose of irinotecan.     

Pharmacogenetic impact of polymorphisms in the coding region of the UGT1A1 gene on SN-38 glucuronidation in Japanese patients with cancer

Pharmacogenetic testing for UDP-glucuronosyltransferase (UGT) 1A1*28, a promoter variant of the UGT1A1 gene, is now carried out clinically to estimate the risk of irinotecan-associated toxicity. We studied the clinical significance of UGT1A1*6 and UGT1A1*27, two variants in exon 1 of the UGT1A1 gene that are found mainly in Asians. The study group comprised 46 Japanese patients who received various regimens of chemotherapy including irinotecan at doses from 50 to 180 mg/m(2). Pharmacogenetic relationships were explored between the UGT1A1 genotype and the ratio of the area under the plasma concentration-time curve (AUC) of the active metabolite of irinotecan (SN-38) to that of SN-38 glucuronide (SN-38G), used as a surrogate for UGT1A1 activity (AUC(SN-38)/AUC(SN-38G)). No patient was homozygous for UGT1A1*28, and none had UGT1A1*27. Two were heterozygous for UGT1A1*28. Two were homozygous and 15 heterozygous for UGT1A1*6, all of whom were wild type with respect to UGT1A1*28. Two patients were simultaneously heterozygous for UGT1A1*28 and UGT1A1*6, present on different chromosomes. The other 25 patients had none of the variants studied. The two patients simultaneously heterozygous for UGT1A1*28 and UGT1A1*6 and the two patients homozygous for UGT1A1*6 had significantly higher AUC(SN-38)/AUC(SN-38G) ratios than the others (P = 0.0039). Concurrence of UGT1A1*28 and UGT1A1*6, even when heterozygous, altered the disposition of irinotecan remarkably, potentially increasing susceptibility to toxicity. Patients homozygous for UGT1A1*6 should also be carefully monitored. UGT1A1 polymorphisms in the coding region of the UGT1A1 gene should be genotyped in addition to testing for UGT1A1*28 to more accurately predict irinotecan-related toxicity, at least in Asian patients.     

The UGT1A1128 polymorphism correlates with erlotinib’s effect on SN-38 glucuronidation

The combination of irinotecan and erlotinib has been evaluated in clinical trials, although toxicity has been significant. We aimed to investigate the effect of erlotinib on SN-38 glucuronidation and the association between UGT1A polymorphisms and SN-38 glucuronidation activity in the presence of erlotinib. The inhibitory effect of erlotinib on SN-38 glucuronidation was determined by measuring the formation rates for SN-38 glucuronide, using recombinant human UGT1A1, pooled human liver microsomes (HLMs) and 52 Caucasian liver microsomes in the absence or presence of erlotinib. Inhibition kinetic studies were conducted. AUC ratios were used to predict the risk of potential drug–drug interactions (DDI) in vivo. Our data showed that erlotinib exhibited potent non-competitive inhibition against SN-38 glucuronidation in pooled HLMs and UGT1A1. Using the physiological and pharmacokinetic parameters obtained from the literature, we estimated the in vivo concentrations of unbound erlotinib available for UGT1A1 active site and thus the AUC ratios of SN-38 were also quantitatively predicted. It is estimated that erlotinib administered at 50 mg/day or higher doses may result in at least a 24% increase in SN-38 AUC. Significant correlations were observed between SN-38 glucuronidation activity in the presence of erlotinib and UGT1A1128 in 52 Caucasian liver microsomes. Our results suggest that erlotinib is a potent inhibitor of SN-38 glucuronidation via UGT1A1 inhibition. The coadministration of erlotinib with irinotecan may result in clinically significant DDI. UGT1A1128 polymorphism correlates with erlotinib’s effect on SN-38 glucuronidation. The present findings shed light on the development and optimisation of combinations involving irinotecan and erlotinib.     

Effects of ketoconazole on glucuronidation by UDP-glucuronosyltransferase enzymes.

PURPOSE: Ketoconazole has been shown to inhibit the glucuronidation of the UGT2B7 substrates zidovudine and lorazepam. Its effect on UGT1A substrates is unclear. A recent study found that coadministration of irinotecan and ketoconazole led to a significant increase in the formation of SN-38 (7-ethyl-10-hydroxycamptothecine), an UGT1A substrate. This study investigates whether ketoconazole contributes to the increase in SN-38 formation by inhibiting SN-38 glucuronidation. EXPERIMENTAL DESIGN: SN-38 glucuronidation activities were determined by measuring the rate of SN-38 glucuronide (SN-38G) formation using pooled human liver microsomes and cDNA-expressed UGT1A isoforms (1A1, 1A7 and 1A9) in the presence of ketoconazole. Indinavir, a known UGT1A1 inhibitor, was used as a positive control. SN-38G formation was measured by high-performance liquid chromatograph. RESULTS: Ketoconazole competitively inhibited SN-38 glucuronidation. Among the UGT1A isoforms screened, ketoconazole showed the highest inhibitory effect on UGT1A1 and UGT1A9. The K(i) values were 3.3 +/- 0.8 micromol/L for UGT1A1 and 31.9 +/- 3.3 micromol/L for UGT1A9. CONCLUSIONS: These results show that ketoconazole is a potent UGT1A1 inhibitor, which seems the basis for increased exposure to SN-38 when coadministered with irinotecan.     

Usefulness of one-point plasma SN-38G/SN-38 concentration ratios as a substitute for UGT1A1 genetic information after irinotecan administration

Background

It was recently reported that genetic polymorphisms of UDP glucuronyltransferase-1 polypeptide A1 (UGT1A1), a glucuronidation enzyme, were associated with irinotecan (CPT-11) metabolism. The active metabolite of CPT-11, 7-ethyl-10-hydroxycamptothecin (SN-38) was glucuronidated (SN-38G) by UGT1A1. Genetic polymorphisms of UGT1A1 were associated with potentially serious adverse events, including neutropenia. Several studies have suggested that the dose of CPT-11 should be decreased in patients homozygous for UGT1A1*6 or UGT1A1*28, or double heterozygotes (*6/*28). However, the reference dose for patients with these genetic polymorphisms is unclear.

Methods

We investigated the relationship between the SN-38G/SN-38 concentration ratio and the dose of CPT-11 in 70 patients with colorectal cancer who received FOLFIRI-based regimens, by measuring the plasma concentrations of CPT-11, SN-38, and SN-38G.

Results

The SN-38G/SN-38 concentration ratio was lower in patients who were homozygous for UGT1A1*6, heterozygous for UGT1A1*6 or UGT1A1*28, or were double heterozygotes compared with patients with wild-type genes. The relative decreases in the SN-38G/SN-38 concentration ratio in patients homozygous for UGT1A1*6 and in double heterozygotes were greater than in patients heterozygous for UGT1A1*6 or UGT1A1*28. Interestingly, decreases in the SN-38G/SN-38 concentration ratio were associated with decreases in the neutrophil count and the final infusion dose of CPT-11.

Conclusion

Our results suggest that the SN-38G/SN-38 concentration ratio is an important factor for guiding dose adjustments, even in patients with wild-type genes. Therefore, the SN-38G/SN-38 concentration ratio, as an index of the patient’s metabolic capacity, is useful for assessing dose adjustments of CPT-11.     

Down-regulation of lipids transporter ABCA1 increases the cytotoxicity of Nitidine

Purpose

This study examined the pharmacokinetics of irinotecan (CPT-11), active metabolite 7-ethyl-10-hydroxycamptothecin (SN-38), SN-38 glucuronide (SN-38G) amrubicin (AMR), and active metabolite amrubicinol (AMR-OH) after intravenous administration of this combination therapy in rats.

Methods

Male Sprague-Dawley rats were treated with 10 mg/kg CPT-11 with 10 mg/kg AMR. AMR, AMR-OH, CPT-11, SN-38 and SN-38G were measured in plasma, bile, and tissues using high-performance liquid chromatography.

Results

Co-administration of CPT-11 resulted in a significant decrease in plasma concentrations and area under the curves (AUC) of AMR-OH compared with treatment with AMR alone. On the other hand, co-administration of AMR resulted in a slight increase in the initial plasma concentration of SN-38; however, there were no differences in AUC values in CPT-11 and SN-38. The cumulative biliary excretion curves of AMR, CPT-11, and their active metabolites were not changed. CPT-11 inhibited the conversion of AMR to AMR-OH in rat cytosolic fractions.

Conclusions

CPT-11 did not affect the pharmacokinetic of AMR but decreased the plasma concentration of AMR-OH and might affect the formation of AMR-OH from AMR in hepatocytes.     

Pharmacokinetic study of CPT-11, SN-38 and SN-38 glucuronide in the ascites,plasma and bile after intraperitoneal administration of CPT-11

We studied the pharmacokinetics of CPT-11 with intraperitoneal administration in a patient with a PTCD tube. The patient had advanced gastric cancer with peritoneal metastasis. CPT-11 was administrated in a dose of 40 mg and the intraperitoneal, plasma and bile levels of CPT-11, SN-38 and SN-38 glucuronide (SN-38 GLU) were measured periodically. The results showed that the periodical concentration pattern of CPT-11, SN-38 and SN-38 GLU in the bile was closely related to that of CPT-11 in the abdominal cavity.     

Metabolism of irinotecan and its active metabolite SN-38 by intestinal microflora in rats

One of the dose-limiting toxicities of irinotecan (CPT-11) is delayed-onset diarrhea, which is the greatest barrier to treatment with CPT-11-containing regimens. CPT-11 is converted to its active metabolite, SN-38, which is conjugated by hepatic uridine diphosphate glucuronosyl transferase to SN-38 glucuronide (SN-38G). SN-38G, once excreted in the intestinal lumen via bile, is extensively deconjugated by bacterial beta-glucuronidase with the regeneration of SN-38 in the intestinal lumen, which may cause diarrhea. However, the metabolism of CPT-11 and its metabolites by intestinal microflora are yet to be reported. This study was carried out to investigate the microbial transformation of CPT-11 and SN-38 using an anaerobic mixed culture of rat cecal microorganisms. No reaction in the mixed cultures was observed when CPT-11 or SN-38 lactone was added to the culture medium. When CPT-11 was added to the culture broth, a significant amount of water-soluble CPT-11 was detected in the spent culture medium. In contrast, only a slight amount of SN-38 was found in the supernatant when SN-38 lactone was added to the broth. A significant quantity of SN-38 was found in the sediment. In conclusion, these results strongly suggest that SN-38 produced from SN-38G by the action of bacterial beta-glucuronidase is rapidly adsorbed by the intestinal bacterial cell walls in the sediment because of the hydrophobic and lipophilic nature of SN-38, and a small amount of SN-38 remains in the intestinal luminal fluid. Thus, we need to reconsider the role of SN-38 in the intestinal lumen in CPT-11-induced late-onset diarrhea.     

Involvement of UDP-glucuronosyltransferase activity in irinotecan-induced delayed-onset diarrhea in rats

We assessed the involvement of UDP-glucuronosyltransferase (UGT) activity in episodes of irinotecan hydrochloride (CPT-11)-induced delayed-onset diarrhea using a mutant rat strain with an inherited deficiency of UGT1A (Gunn rats). Gunn rats exhibited severe diarrhea after the intravenous administration of CPT-11 at a dose of 20 mg/kg, whereas Wistar rats did not. In the epithelium of the small intestine and cecum in Gunn rats, the shortening of villi, degeneration of crypts, and destruction of the nucleus were observed. The AUC, MRT, and t _1/2 of CPT-11, and the AUC of 7-ethyl-10-hydroxycamptothecin (SN-38) in plasma were, respectively, 1.6-fold, 1.5-fold, 1.7-fold, and 6.5-fold higher, and the cumulative biliary excretion rate of SN-38 was 2.3-fold higher, in Gunn rats than Wistar rats. SN-38 glucuronide excreted via bile in Wistar rats was not de-conjugated in the small intestinal lumen. The SN-38 AUC values in small intestinal tissues were also 5.0 to 5.8-fold higher in Gunn rats than Wistar rats. In conclusion, Gunn rats developed severe delayed-onset diarrhea after i.v. administration of CPT-11 at a much lower dose. Severe intestinal impairments would be induced in Gunn rats through exposure to SN-38 at high levels for a long period mainly via the intestinal lumen and partially via the bloodstream. These results clarified that the deficiency of UGT activity contributed greatly to the induction of the CPT-11-induced delayed-onset diarrhea and epithelial impairment in the intestine. In the clinic, great care is needed when using chemotherapy with CPT-11 in patients with poor UGT activity.     

A limited sampling strategy to estimate the pharmacokinetic parameters of irinotecan and its active metabolite, SN-38, in patients with metastatic digestive cancer receiving the FOLFIRI regimen

In this study we propose for the first time a limited sampling strategy to estimate the individual pharmacokinetic parameters of both irinotecan and SN-38 in patients treated with the irinotecan plus 5-fluorouracil (FOLFIRI) regimen. The pharmacokinetics of irinotecan and SN-38 were studied in 74 patients with advanced inoperable digestive cancer. Plasma concentrations were taken during and up to the 42 h following a 90-min infusion of irinotecan (180-225 mg/m(2)). Data splitting was used to create model-building and validation data sets, and data were analysed with the NONMEM program. The disposition of SN-38 was dependent on the disposition of irinotecan. The estimated pharmacokinetic parameters of irinotecan [terminal half-life (t(1/2)), 11.5 h; total clearance (CL), 25.0 l h(-1); area under curve (AUC), 14.9 mg x h l(-1)] and SN-38 (terminal t(1/2), 32.2 h; AUC, 0.42 mg x h l(-1)) were similar to those determined in other studies. The protocol involving two sampling times, at 1 and 24 h following the beginning of the infusion, allowed for a precise and accurate determination of the individual pharmacokinetic parameters of the two drugs. The limited sampling strategy developed in this study ought to facilitate future studies on the pharmacology and toxicity of irinotecan-based therapy.     

Disposition of irinotecan and SN-38 following oral and intravenous irinotecan dosing in mice

The present study was conducted to quantitate the disposition of irinotecan lactone and its active metabolite SN-38 lactone in mice following oral and intravenous administration, and to evaluate the systemic exposure of irinotecan lactone and SN-38 lactone associated with antitumor doses of irinotecan lactone in mice bearing human tumor xenografts. Nontumor-bearing mice were given a single oral or intravenous irinotecan dose (5, 10, 40, or 75 mg/kg), and serial plasma samples were subsequently obtained. Irinotecan and SN-38 lactone plasma concentrations were measured using an isocratic HPLC assay with fluorescence detection. The disposition of intravenous irinotecan lactone was modeled using a two-compartment pharmacokinetic model, and the disposition of oral irinotecan and SN-38 lactone was modeled with noncompartmental methods. Irinotecan lactone showed biphasic plasma disposition following intravenous dosing with a terminal half-life ranging between 1.1 to 3 h. Irinotecan lactone disposition was linear at lower doses (5 and 10 mg/kg), but at 40 mg/kg irinotecan lactone clearance decreased and a nonlinear increase in irinotecan lactone AUC was observed. The steady-state volume of distribution ranged from 19.1 to 48.1 l/m². After oral dosing, peak irinotecan and SN-38 lactone concentrations occurred within 1 h, and the irinotecan lactone bioavailability was 0.12 at 10 mg/kg and 0.21 at 40 mg/kg. The percent unbound SN-38 lactone in murine plasma at 1000 ng/ml was 3.4 ± 0.67%, whereas at 100 ng/ml the percent unbound was 1.18 ± 0.14%. Irinotecan and SN-38 lactone AUCs in micebearing human neuroblastoma xenografts were greater than in nontumor-bearing animals. Systemic exposure to unbound SN-38 lactone in nontumor-bearing animals after a single oral irinotecan dose of 40, 10, and 5 mg/kg was 28.3, 8.6, and 2.9 ng h/ml, respectively. Data from the present study provide important information for the design of phase I studies of oral irinotecan. Received: 30 August 1996 / Accepted: 27 November 1996     

Factors involved in prolongation of the terminal disposition phase of SN-38: clinical and experimental studies.

The active metabolite of irinotecan (CPT-11), 7-ethyl-10-hydroxycamptothecin (SN-38), is either formed through enzymatic cleavage of CPT-11 by carboxyl esterases (CEs) or through cytochrome P-450 3A-mediated oxidation to 7-ethyl-10-[4-(1-piperidino)-1-amino] carbonyloxycamptothecin (NPC) and a subsequent conversion by CE. In the liver, SN-38 is glucuronidated (SN-38G) by UGT1A1, which also conjugates bilirubin. Fourteen patients were treated with 350 mg/m2 CPT-11, and we performed pharmacokinetic analysis during a 500-h collection period. The half-life and area under the plasma concentration-time curve of SN-38 were 47+/-7.9 h and 2.0+/-0.79 microM x h, respectively, both representing a 2-fold increase as compared with earlier reported estimates (A. Sparreboom et al, Clin. Cancer Res., 4: 2747-2754, 1998). As an explanation for this phenomenon, we noted substantial formation of SN-38 from CPT-11 and NPC by plasma CE, consistent with the low circulating levels of NPC observed. In addition, transport studies in Caco-2 monolayers indicated that nonglucuronidated SN-38 could cross the membrane from apical to basolateral, indicating the potential for recirculation processes that can prolong circulation times. Interestingly, individual levels of fecal beta-glucuronidase, which is known to mediate SN-38G hydrolysis, were not related to any of the SN-38 kinetic parameters (r = 0.09; P = 0.26), suggesting that interindividual variation in this enzyme is unimportant in explaining SN-38 pharmacokinetic variability. We have also found, in contrast to earlier data, that SN-38G/SN-38 plasma concentration ratios decrease over time from approximately 7 (up to 50 h) to approximately 1 (at 500 h). This decrease could be explained by the fact that glucuronidation of SN-38 and bilirubin is increasingly competitive at lower drug levels. In addition, no evidence was found for SN-38G transport through the Caco-2 cells. Our findings indicate that until now the circulation time of SN-38 has been underestimated. This is of crucial importance to our understanding of the clinical action of CPT-11 and for future pharmacokinetic/pharmacodynamic relationships.     

Streptomycin alleviates irinotecan-induced delayed-onset diarrhea in rats by a mechanism other than inhibition of β-glucuronidase activity in intestinal lumen

Irinotecan hydrochloride (CPT-11) is a useful drug for cancer chemotherapy but sometimes induces severe diarrhea clinically. CPT-11 is mainly activated to SN-38 by carboxylesterase (CES) and then detoxified to SN-38 glucuronide (SN-38G) by UDP-glucuronosyltransferase (UGT) in the liver. SN-38G is excreted via bile and de-conjugated to SN-38 by β-glucuronidase (β-GLU) in the intestinal content. In order to clarify the alleviative effect of antibiotics on CPT-11-induced diarrhea, we examined whether penicillin G and streptomycin (SM) alleviate CPT-11-induced delayed-onset diarrhea using three diarrheal models, i.e., Wistar rats with repeated dosing of CPT-11 (60 mg/kg/day i.v. for 4 consecutive days) and Wistar and Gunn rats with a single dosing of CPT-11 (200 and 20 mg/kg i.v., respectively). Gunn rats have an inherited deficiency of UGT1A and cannot conjugate SN-38 to SN-38G. Therefore, onset of CPT-11-induced diarrhea in Gunn rats is not affected by β-GLU activity. SM alleviated diarrhea in all three diarrheal models. The alleviation of diarrhea by SM in Gunn rats indicated that the effect of SM occurred by a mechanism other than the inhibition of β-GLU activity. SM decreased CPT-11 and/or SN-38 concentrations in intestinal tissues and alleviated epithelial damage from the ileum to colon. SM did not inhibit β-GLU activity in the cecal content. SM also inhibited the intestinal absorption of CPT-11 and decreased CES activity and increased UGT activity in the intestinal epithelium. These findings indicated that SM decreased the exposure of CPT-11 and SN-38 to the intestinal epithelium by inhibiting the absorption of CPT-11 from the intestinal lumen and the change of CES and UGT activities in the intestinal epithelium and alleviated delayed-onset diarrhea.     

Irinotecan pharmacokinetics-pharmacodynamics: the clinical relevance of prolonged exposure to SN-38

We have shown previously that the terminal disposition half-life of SN-38, the active metabolite of irinotecan, is much longer than earlier thought. Currently, it is not known whether this prolonged exposure has any relevance toward SN-38-induced toxicity. Here, we found that SN-38 concentrations present in human plasma for up to 3 weeks after a single irinotecan infusion induce significant cytotoxicity in vitro. Using pharmacokinetic data from 26 patients, with sampling up to 500 h, relationships were evaluated between systemic exposure (AUC) to SN-38 and the per cent decrease in absolute neutrophil count (ANC) at nadir, or by taking the entire time course of ANC into account (AOC). The time course of SN-38 concentrations (AUC(500 h)) was significantly related to this AOC (P<0.001). Based on these findings, a new limited-sampling model was developed for SN-38 AUC(500 h) using only two timed samples: AUC(500 h)=(6.588 x C(2.5 h))+(146.4 x C(49.5 h))+15.53, where C(2.5 h) and C(49.5 h) are plasma concentrations at 2.5 and 49.5 h after start of infusion, respectively. The use of this limited-sampling model may open up historic databases to retrospectively obtain information about SN-38-induced toxicity in patients treated with irinotecan.     

Inhibition of intestinal microflora β-glucuronidase modifies the distribution of the active metabolite of the antitumor agent, irinotecan hydrochloride (CPT-11) in rats

Purpose: SN-38, a metabolite of irinotecan hydrochloride (CPT-11), is considered to play a key role in the development of diarrhea as well as in the antitumor activity of CPT-11. We have previously found that the inhibition of β-glucuronidase, which hydrolyzes detoxified SN-38 (SN-38 glucuronide) to reform SN-38, in the lumen by eliminating the intestinal microflora with antibiotics, markedly ameliorates the intestinal toxicity of CPT-11 in rats. In this study we compared the disposition of CPT-11 and its metabolites in rats treated with and without antibiotics. Methods: Rats were given drinking water containing 1 mg/ml penicillin and 2 mg/ml streptomycin from 5 days before the administration of CPT-11 (60 mg/kg i.v.) and throughout the experiment. CPT-11, SN-38 glucuronide and SN-38 concentrations in the blood, intestinal tissues and intestinal luminal contents were determined by HPLC. Results: Antibiotics had little or no effect on the pharmacokinetics of CPT-11, SN-38 glucuronide or SN-38 in the blood, or in the tissues or contents of the small intestine, which has less β-glucuronidase activity in its luminal contents. In contrast, antibiotics markedly reduced the AUC_1–24 h of SN-38 (by about 85%) in the large intestine tissue without changing that of CPT-11, and this was accompanied by a complete inhibition of the deconjugation of SN-38 glucuronide in the luminal contents. Conclusions: These results suggest that SN-38, which results from the hydrolysis of SN-38 glucuronide by β-glucuronidase in the intestinal microflora, contributes considerably to the distribution of SN-38 in the large intestine tissue, and that inhibition of the β-glucuronidase activity by antibiotics results in decreased accumulation of SN-38 in the large intestine. Received: 8 August 1997 / Accepted: 16 January 1998     

Severe CPT-11 toxicity in patients with Gilbert's syndrome: Two case reports

Background: CPT-11 is hydrolyzed to its active metabolite SN-38, which is mainly eliminated through conjugation by hepatic uridine diphosphate glucuronosyl transferases (UGTs) to the glucuronide (SN-38G) derivative. Preclinical studies showed that UGT*1.1 is the isozyme responsible for SN-38 glucuronidation. Patients with Gilbert's syndrome have deficient UGT*1.1 activity, therefore may have an increased risk for related CPT-11 toxicity.Patients and methods: Two patients with metastatic colon cancer and Gilbert's syndrome were treated with CPT-11 based chemotherapy. CPT-11, SN-38 and SN-38G pharmacokinetics parameters were obtained. Serum bilirubin was analysed by alkaline methanolysis and HPLC.Results: Both patients presented grade 4 neutropenia and/or diarrhea (NCI-CTC) in every treatment cycle. Biliary index (after Gupta et al) values were well above 4000.Conclusion: We present the first clinical evidence linking bilirubin glucuronidation status and CPT-11 related toxicity. The severe toxicity experienced by the two patients with Gilbert's syndrome treated with CPT-11 based chemotherapy has a genetic basis. Individuals with Gilbert's syndrome have an enhanced risk for CPT-11 toxicity. Unconjugated serum bilirubin could be predictive parameter of CPT-11 toxicity.     

Pharmacokinetic, metabolic, and pharmacodynamic profiles in a dose-escalating study of irinotecan and cisplatin.

PURPOSE: To investigate the pharmacokinetics and pharmacodynamics of irinotecan and cisplatin administered once every 3 weeks in a dose-escalating study in patients with solid tumors. PATIENTS AND METHODS: Fifty-two cancer patients were treated with irinotecan administered as a 90-minute infusion at doses ranging from 175 to 300 mg/m(2) followed by cisplatin administered as a 3-hour intravenous infusion at doses ranging from 60 to 80 mg/m(2). After reaching the maximum-tolerated dose, the sequence of drug administration was revised. For pharmacokinetic analysis, serial plasma samples were obtained on days 1 through 3 of the first cycle. Forty-five patients were assessable for irinotecan pharmacokinetics, and 46 were assessable for cisplatin pharmacokinetics. RESULTS: Irinotecan and cisplatin demonstrated linear pharmacokinetics comparable to that observed with single-agent administration, which suggests an absence of pharmacokinetic interaction. SN-38G constituted the major plasma metabolite of irinotecan, whereas 7-ethyl-10-[4-N-(1-piperidino)1-amino]-carbonyloxycamptothecine (NPC) was only a minor metabolite in plasma, possibly indicating a rapid conversion of NPC to SN-38. The terminal elimination phases of SN-38 and SN-38G were similar and relatively delayed when compared with the elimination of irinotecan. Maximal DNA adduct formation did not significantly differ from that observed with single-agent administration. The percentage decrease in WBC was significantly related to the areas under the plasma concentration-time curve (AUCs) of the lactone form of irinotecan (P =.0245) and SN-38 (P =. 0123). The severity of diarrhea was not significantly related to the AUCs of irinotecan and SN-38, nor to the systemic glucuronidation rate of SN-38. CONCLUSION: There was no apparent pharmacokinetic interaction between irinotecan and cisplatin in this study. Reversion of the administration sequence of the drugs did not seem to have any influence on the pharmacokinetics. The incidence and severity of delayed-type diarrhea was not related to any of the studied parameters.