Multiplicity of biliary excretion mechanisms for the camptothecin derivative irinotecan (CPT-11), its metabolite SN-38, and its glucuronide: role of canalicular multispecific organic anion transporter and P-glycoprotein

Predictive Value of ABCC2 and UGT1A1 Polymorphisms on Irinotecan-Related Toxicities in Patients with Cancer

Background: There is extensive interindividual variability in response and tolerance to anticancer drugs. This heterogeneity provides a major limitation to the "rational" use of cytotoxic drugs, and it becomes a major problem in oncology giving a narrow therapeutic window with a vital risk. Among these anticancer drugs, irinotecan can cause dose-limiting toxicities, commonly diarrhea and neutropenia. Interaction among pathways of activation/inactivation (UGT1A1) and hepatobiliary transport of irinotecan and its metabolites could, in part, explain its interindividual variability. The objective of this study was to perform an exploratory analysis to evaluate the correlation between the genetic polymorphisms of UGT1A1 and ABCC2 with the different toxicities associated with irinotecan treatment. Materials and Methods: Seventy-five patients with solid cancers were included, all were administered an irinotecan-based regimen in both Mission Bay Medical Center; and Zuckerberg San Francisco General Hospital from May 2016 to December 2016. The patients' genotyping was performed for both the UGT1A1*28 polymorphism, and the ABCC2 - 1549G>A, and ABCC2 - 1249G>A single nucleotide polymorphism. Comparisons among qualitative data were assessed using the χ²-test, and Fisher's exact test in the case of small group sizes. Results: Diarrhea was observed in 40 patients (53.3%), among them only 9 patients had high grades diarrhea (grades III and IV). Grades III/IV of nausea were more frequently associated with the ABCC2-1549 AA genotype (83.3% p = 0.004) in patients with colorectal cancer. In pancreatic cancer, a significant absence of diarrhea grades III-IV was noted in patients with the ABCC2 1249 GG genotype compared to the other ABCC2 1249 genotypes.

TROP2 expression and SN38 antitumor activity in malignant pleural mesothelioma cells provide a rationale for antibody-drug conjugate therapy

OBJECTIVES

Malignant pleural mesothelioma (MPM) is an aggressive cancer which at large is not amenable to curative surgery. Despite the recent approval of immune checkpoint inhibitor therapy, the response rates and survival following systemic therapy is still limited. Sacituzumab govitecan is an antibody-drug conjugate targeting the topoisomerase I inhibitor SN38 to trophoblast cell-surface antigen 2 (TROP-2)-positive cells. Here we have explored the therapeutic potential of sacituzumab govitecan in MPM models.

MATERIALS AND METHODS

TROP2 expression was analyzed in a panel of two well established and 15 pleural effusion derived novel lines by RT-QPCR and immunoblotting, TROP2 membrane-localization was studied by flow cytometry and immunohistochemistry. Cultured mesothelial cells and pneumothorax pleura served as controls. The sensitivity of MPM cell lines to irinotecan and SN38 was studied using cell viability, cell cycle, apoptosis and DNA damage assays. Drug sensitivity of cell lines was correlated with RNA expression of DNA repair genes. Drug sensitivity was defined as an IC50 below 5 nM in the cell viability assay.

RESULTS

TROP2 expression was detected at RNA and protein level in 6 of the 17 MPM cell lines, but not in in cultured mesothelial control cells or in the mesothelial layer of the pleura. TROP2 was detectable on the cell membrane in 5 MPM lines and was present in the nucleus in 6 cell models. Ten of 17 MPM cell lines showed sensitivity to SN38 treatment, among those 4 expressed TROP2. High AURKA RNA expression and high proliferation rate correlated with sensitivity to SN38-induced cell death, DNA damage response, cell cycle arrest and cell death. Sacituzumab govitecan treatment effectively induced cell cycle arrest and cell death in TROP2-positive MPM cells.

CONCLUSION

TROP2 expression and sensitivity to SN38 in MPM cell lines support biomarker-selected clinical exploration of sacituzumab govitecan in patients with MPM.

Multidrug resistance proteins (MRPs): Structure, function and the overcoming of cancer multidrug resistance

ATP-binding cassette (ABC) transporters mediate the ATP-driven translocation of structurally and mechanistically distinct substrates against steep concentration gradients. Among the seven human ABC subfamilies namely ABCA-ABCG, ABCC is the largest subfamily with 13 members. In this respect, 9 of the ABCC members are termed "multidrug resistance proteins" (MRPs1-9) due to their ability to mediate cancer multidrug resistance (MDR) by extruding various chemotherapeutic agents or their metabolites from tumor cells. Furthermore, MRPs are also responsible for the ATP-driven efflux of physiologically important organic anions such as leukotriene C₄, folic acid, bile acids and cAMP. Thus, MRPs are involved in important regulatory pathways. Blocking the anticancer drug efflux function of MRPs has shown promising results in overcoming cancer MDR. As a result, many novel MRP modulators have been developed in the past decade. In the current review, we summarize the structure, tissue distribution, biological and pharmacological functions as well as clinical insights of MRPs. Furthermore, recent updates in MRP modulators and their therapeutic applications in clinical trials are also discussed.

High Doses of Δ9-Tetrahydrocannabinol Might Impair Irinotecan Chemotherapy: A Review of Potentially Harmful Interactions

This review proposes the hypothesis that the effectiveness of irinotecan chemotherapy might be impaired by high doses of concomitantly administered Δ⁹-tetrahydrocannabinol (THC). The most important features shared by irinotecan and THC, which might represent sources of potentially harmful interactions are: first-pass hepatic metabolism mediated by cytochrome P450 (CYP) enzyme CYP3A4; glucuronidation mediated by uridine diphosphate glycosyltransferase (UGT) enzymes, isoforms 1A1 and 1A9; transport of parent compounds and their metabolites via canalicular ATP-binding cassette (ABC) transporters ABCB1 and ABCG2; enterohepatic recirculation of both parent compounds, which leads to an extended duration of their pharmacological effects; possible competition for binding to albumin; butyrylcholinesterase (BChE) inhibition by THC, which might impair the conversion of parent irinotecan into the SN-38 metabolite; mutual effects on mitochondrial dysfunction and induction of oxidative stress; potentiation of hepatotoxicity; potentiation of genotoxicity and cytogenetic effects leading to genome instability; possible neurotoxicity; and effects on bilirubin. The controversies associated with the use of highly concentrated THC preparations with irinotecan chemotherapy are also discussed. Despite all of the limitations, the body of evidence provided here could be considered relevant for human-risk assessments and calls for concern in cases when irinotecan chemotherapy is accompanied by preparations rich in THC.

Variant discovery using next-generation sequencing and its future role in pharmacogenetics

Next-generation sequencing (NGS) has enabled the discovery of a multitude of novel and mostly rare variants in pharmacogenes that may alter a patient’s therapeutic response to drugs. In addition to single nucleotide variants, structural variation affecting the number of copies of whole genes or parts of genes can be detected. While current guidelines concerning clinical implementation mostly act upon well-documented, common single nucleotide variants to guide dosing or drug selection, in silico and large-scale functional assessment of rare variant effects on protein function are at the forefront of pharmacogenetic research to facilitate their clinical integration. Here, we discuss the role of NGS in variant discovery, paving the way for more comprehensive genotype-guided pharmacotherapy that can translate to improved clinical care.

Individualization of Irinotecan Treatment: A Review of Pharmacokinetics, Pharmacodynamics, and Pharmacogenetics

Since its clinical introduction in 1998, the topoisomerase I inhibitor irinotecan has been widely used in the treatment of solid tumors, including colorectal, pancreatic, and lung cancer. Irinotecan therapy is characterized by several dose-limiting toxicities and large interindividual pharmacokinetic variability. Irinotecan has a highly complex metabolism, including hydrolyzation by carboxylesterases to its active metabolite SN-38, which is 100- to 1000-fold more active compared with irinotecan itself. Several phase I and II enzymes, including cytochrome P450 (CYP) 3A4 and uridine diphosphate glucuronosyltransferase (UGT) 1A, are involved in the formation of inactive metabolites, making its metabolism prone to environmental and genetic influences. Genetic variants in the DNA of these enzymes and transporters could predict a part of the drug-related toxicity and efficacy of treatment, which has been shown in retrospective and prospective trials and meta-analyses. Patient characteristics, lifestyle and comedication also influence irinotecan pharmacokinetics. Other factors, including dietary restriction, are currently being studied. Meanwhile, a more tailored approach to prevent excessive toxicity and optimize efficacy is warranted. This review provides an updated overview on today’s literature on irinotecan pharmacokinetics, pharmacodynamics, and pharmacogenetics.

Irinotecan and Δ⁸-Tetrahydrocannabinol Interactions in Rat Liver: A Preliminary Evaluation Using Biochemical and Genotoxicity Markers

There is growing interest regarding the use of herbal preparations based on Cannabis sativa for medicinal purposes, despite the poorly understood interactions of their main constituent Δ⁸-tetrahydrocannabinol (THC) with conventional drugs, especially cytostatics. The objective of this pilot study was to prove whether the concomitant intake of THC impaired liver function in male Wistar rats treated with the anticancer drug irinotecan (IRI), and evaluate the toxic effects associated with this exposure. IRI was administered once intraperitoneally (at 100 mg/kg of the body weight (b.w.)), while THC was administered per os repeatedly for 1, 3, and 7 days (at 7 mg/kg b.w.). Functional liver impairments were studied using biochemical markers of liver function (aspartate aminotransferase-AST, alanine aminotransferase-ALP, alkaline phosphatase-AP, and bilirubin) in rats given a combined treatment, single IRI, single THC, and control groups. Using common oxidative stress biomarkers, along with measurement of primary DNA damage in hepatocytes, the degree of impairments caused at the cellular level was also evaluated. THC caused a time-dependent enhancement of acute toxicity in IRI-treated rats, which was confirmed by body and liver weight reduction. Although single THC affected ALP and AP levels more than single IRI, the levels of liver function markers measured after the administration of a combined treatment mostly did not significantly differ from control. Combined exposure led to increased oxidative stress responses in 3- and 7-day treatments, compared to single IRI. Single IRI caused the highest DNA damage at all timepoints. Continuous 7-day oral exposure to single THC caused an increased mean value of comet tail length compared to its shorter treatments. Concomitant intake of THC slightly affected the levels of IRI genotoxicity at all timepoints, but not in a consistent manner. Further studies are needed to prove our preliminary observations, clarify the underlying mechanisms behind IRI and THC interactions, and unambiguously confirm or reject the assumptions made herein.

Improved Progression-Free Survival in Irinotecan-Treated Metastatic Colorectal Cancer Patients Carrying the HNF1A Coding Variant p.I27L

Hepatocyte nuclear factor 1-alpha (HNF1A) is a liver-enriched transcription factor that plays a key role in many aspects of hepatic functions including detoxification processes. We examined whether HNF1A polymorphisms are associated with clinical outcomes in two independent cohorts combining 417 European ancestry patients with metastatic colorectal cancer (mCRC) treated with irinotecan-based chemotherapy. The intronic rs2244608A>G marker was predictive of an improved progression-free survival with a trend in the Canadian cohort and reaching significance in the Italian cohort, with hazard ratios (HR) of 0.74 and 0.72, P = 0.076 and 0.038, respectively. A strong association between rs2244608A>G and improved PFS was found in the combined analysis of both cohorts (HR = 0.72; P = 0.002). Consistent with an altered HNF1A function, mCRC carriers of the rs2244608G minor allele displayed enhanced drug exposure by 45% (P = 0.032) compared to non-carriers. In Caucasians, rs2244608A>G is in strong linkage with the coding variant rs1169288c.79A>C (HNF1A p.I27L). In healthy donors, we observed an altered hepatic (ABCC1, P = 0.009, ABCC2, P = 0.048 and CYP3A5, P = 0.001; n = 89) and intestinal (TOP1, P = 0.004; n = 75) gene expression associated with the rs1169288C allele. In addition, the rs1169288C polymorphism could significantly increase the ABCC1 promoter activity by 27% (P = 0.008) and 15% (P = 0.041) in the human kidney HEK293 and the human liver HepG2 cell lines, respectively. Our findings suggest that the HNF1A rs2244608, or the tightly linked functional coding variant p.I27L, might be a potential prognostic marker with irinotecan-based regimens.

Pharmacokinetic interactions in mice between irinotecan and MBL-II-141, an ABCG2 inhibitor

PURPOSE

The chromone derivative MBL-II-141, specifically designed to inhibit ABCG2, was previously demonstrated to combine strong inhibition potency, low toxicity and good efficiency in reversing resistance to irinotecan in a xenografted mouse model. Here, the pharmacokinetic interactions in mice between irinotecan, its active metabolite SN-38 and MBL-II-141 were characterized quantitatively in the blood and in the brain.

METHODS

Compartmental models were used to fit the data. Goodness-of-fit was assessed by simulation-based diagnostic tools.

RESULTS

Irinotecan increased the MBL-II-141 apparent clearance and Vss 1.5-fold, probably by increasing the MBL-II-141 unbound fraction. MBL-II-141 decreased the total apparent clearance of irinotecan by 23%, by decreasing its biliary clearance. MBL-II-141 increased 3-fold the brain accumulation of irinotecan, as a result of the rise of systemic exposure combined with the inhibition of ABCG2-mediated efflux at the blood-brain barrier. Finally, SN-38 exposure was increased by 1.16-fold under treatment with MBL-II-141, owing to the higher irinotecan exposure with increased metabolism towards the formation of SN-38.

CONCLUSIONS

These results may help to anticipate the pharmacokinetic interactions between MBL-II-141 and other ABCG2 substrates. The irinotecan-MBL-II-141 interaction is also expected to occur in humans. Copyright © 2017 John Wiley & Sons, Ltd.

Chemotherapeutic agent CPT-11 eliminates peritoneal resident macrophages by inducing apoptosis

CPT-11 (Irinotecan) is a first-line chemotherapeutic agent in clinic, but it may induce side effects including diarrhea and enteritis in patients. The underlying mechanism of CPT-11's intestinal toxicity is unclear. Peritoneal resident macrophages have been reported to be important for the maintenance of intestinal homeostasis. In this study, we evaluated the cytotoxic effects of CPT-11 on mouse peritoneal resident macrophages. CPT-11 was administered intraperitoneally to mice and their peritoneal exudate cells were isolated for evaluation. CPT-11 treatment strikingly decreased the ratio of F4/80(hi)MHCII(low) large peritoneal macrophages (LPMs), which are regarded as prenatally-originated peritoneal resident macrophages. Consistent with this, the transcription factor GATA6 specifically expressed in LPMs was barely detectable in the macrophages from CPT-11-treated mice, indicative of elimination of LPMs. Such elimination of LPMs was at least partly due to CPT-induced apoptosis in macrophages, because inhibition of apoptosis by caspase-3 inhibitor z-DEVD-fmk significantly diminished the loss of GATA6(+) LPMs. As GATA6 is a transcription factor that controls expression of multiple genes regulating peritoneal B-1 cell development and translocation, elimination of GATA6(+) LPMs led to a great reduction in B-1 cells in the peritoneal cavity after CPT-11 treatment. These results indicated that CPT-11-induced apoptosis contributed to the elimination of peritoneal resident macrophages, which might in turn impair the function of peritoneal B-1 cells in maintaining intestinal homeostasis. Our findings may at least partly explain why CPT-11 treatment in cancer patients induces diarrhea and enteritis, which may provide a novel avenue to prevent such side effects.

Effect of hesperidin on the pharmacokinetics of CPT-11 and its active metabolite SN-38 by regulating hepatic Mrp2 in rats

The usage of irinotecan hydrochloride (CPT-11) chemotherapy is hindered by its dose-limiting diarrhea which appears to be associated with the intestinal exposure to SN-38, the active metabolite of CPT-11. Hesperidin, a safe and natural food ingredient flavonoid, exhibits various biological properties. Accumulated evidence showed that the regulatory effect of hesperidin on the expression of Mrp2 in the liver may be one of the critical factors controlling the biliary excretion of SN-38. This study examined the effect of hesperidin on the pharmacokinetics of CPT-11 and SN-38 as well as the regulatory effect on the hepatic expression of Mrp2. Compared with the control group, the AUC_5-t was increased to 115% of CPT-11 and 122% of SN-38; the CL was decreased to 87% for CPT-11; the tissue concentration was increased in the liver, kidney and colon; and the accumulated biliary excretion was significantly decreased to 77% for CPT-11 and 76% for SN-38 in hesperidin-treated rats. Furthermore, the expression of Mrp2 in the liver was significantly decreased to 37% in the hesperidin-treated rats compared with that of the control group. These results indicate that oral administration of hesperidin significantly increased the AUC_5-t and reduced the clearance of CPT-11 and SN-38, possibly by decreasing the hepatic expression of Mrp2, and thus inhibiting the biliary excretion of CPT-11 and SN-38. The results from this present study suggest that hesperidin may reduce the exposure of CPT-11 and SN-38 in the intestine by reducing the amount of biliary excretion of CPT-11 and SN-38. Copyright © 2016 John Wiley & Sons, Ltd.

Development and validation of an UPLC-MS/MS method for the quantification of irinotecan, SN-38 and SN-38 glucuronide in plasma, urine, feces, liver and kidney: Application to a pharmacokinetic study of irinotecan in rats

The objective of this research is to develop and validate a sensitive and reproducible UPLC-MS/MS method to quantify irinotecan, its active metabolite SN-38 and SN-38 glucuronide (phase II metabolite of SN-38) simultaneously in different bio-matrices (plasma, urine, feces), tissues (liver and kidney) and to use the method to investigate its pharmacokinetic behavior in rats. Irinotecan, SN-38 and SN-38 glucuronide has been resolved and separated by C18 column using acetonitrile and 0.1% formic acid in water used as the mobile phases. Triple quadruple mass spectrometer using multiple reaction monitoring (MRM) with positive scan mode were employed to perform mass analysis. The results showed that the linear response range of irinotecan and SN-38 in plasma, feces, liver and kidney is 4.88-10000 nM, 39-5000 nM, 48.8-6250 nM and 48.8-6250 nM, respectively (R(2)>0.99). In case of SN-38 glucuronide, the standard curves were linear in the concentration range of 6.25-2000 nM, 4.88-1250 nM, 9.8-1250 nM and 9.8-1250 nM in plasma, feces, liver and kidney homogenates, respectively. The lower limit of detection (LLOD) of irinotecan, SN-38 and SN-38 glucuronide was determined to be less than 25 nM in all bio-matrices as well as tissue homogenates. Recoveries of irinotecan, SN-38 and SN-38 glucuronide at three different concentrations (low, medium and high) were not less than 85% at three different concentrations in plasma and feces. The percentage matrix factors in different bio-matrices and tissues were within 20%. The UPLC-MS/MS method was validated with intra-day and inter-day precision of less than 15% in plasma, feces, liver and kidney. Owing to the high sensitivity of this method, only 20 μl of plasma, urine and homogenates of liver, kidney and feces is needed. The validated method has been successfully employed for pharmacokinetic evaluation of irinotecan in male wistar rats to quantify irinotecan, SN-38 and SN-38 glucuronide in plasma, feces, and urine samples.

The role of glucuronidation in drug resistance

The final therapeutic effect of a drug candidate, which is directed to a specific molecular target strongly depends on its absorption, distribution, metabolism and excretion (ADME). The disruption of at least one element of ADME may result in serious drug resistance. In this work we described the role of one element of this resistance: phase II metabolism with UDP-glucuronosyltransferases (UGTs). UGT function is the transformation of their substrates into more polar metabolites, which are better substrates for the ABC transporters, MDR1, MRP and BCRP, than the native drug. UGT-mediated drug resistance can be associated with (i) inherent overexpression of the enzyme, named intrinsic drug resistance or (ii) induced expression of the enzyme, named acquired drug resistance observed when enzyme expression is induced by the drug or other factors, as food-derived compounds. Very often this induction occurs via ligand binding receptors including AhR (aryl hydrocarbon receptor) PXR (pregnane X receptor), or other transcription factors. The effect of UGT dependent resistance is strengthened by coordinate action and also a coordinate regulation of the expression of UGTs and ABC transporters. This coupling of UGT and multidrug resistance proteins has been intensively studied, particularly in the case of antitumor treatment, when this resistance is "improved" by differences in UGT expression between tumor and healthy tissue. Multidrug resistance coordinated with glucuronidation has also been described here for drugs used in the management of epilepsy, psychiatric diseases, HIV infections, hypertension and hypercholesterolemia. Proposals to reverse UGT-mediated drug resistance should consider the endogenous functions of UGT.

Clinical validity of new genetic biomarkers of irinotecan neutropenia: an independent replication study

The overall goal of this study was to provide evidence for the clinical validity of nine genetic variants in five genes previously associated with irinotecan neutropenia and pharmacokinetics. Variants associated with absolute neutrophil count (ANC) nadir and/or irinotecan pharmacokinetics in a discovery cohort of cancer patients were genotyped in an independent replication cohort of 108 cancer patients. Patients received single-agent irinotecan every 3 weeks. For ANC nadir, we replicated UGT1A1*28, UGT1A1*93 and SLCO1B1*1b in univariate analyses. For irinotecan area under the concentration-time curve (AUC0-24), we replicated ABCC2 -24C>T; however, ABCC2 -24C>T only predicted a small fraction of the variance. For SN-38 AUC0-24 and the glucuronidation ratio, we replicated UGT1A1*28 and UGT1A1*93. In addition to UGT1A1*28, this study independently validated UGT1A1*93 and SLCO1B1*1b as new predictors of irinotecan neutropenia. Further demonstration of their clinical utility will optimize irinotecan therapy in cancer patients.

Influence of multidrug resistance and drug transport proteins on chemotherapy drug metabolism

INTRODUCTION

Chemotherapy involving the use of anticancer drugs remains an important strategy in the overall management of patients with metastatic cancer. Acquisition of multidrug resistance remains a major impediment to successful chemotherapy. Drug transporters in cell membranes and intracellular drug metabolizing enzymes contribute to the resistance phenotype and determine the pharmacokinetics of anticancer drugs in the body.

AREAS COVERED

ATP-binding cassette (ABC) transporters mediate the transport of endogenous metabolites and xenobiotics including cytotoxic drugs out of cells. Solute carrier (SLC) transporters mediate the influx of cytotoxic drugs into cells. This review focuses on the substrate interaction of these transporters, on their biology and what role they play together with drug metabolizing enzymes in eliminating therapeutic drugs from cells.

EXPERT OPINION

The majority of anticancer drugs are substrates for the ABC transporter and SLC transporter families. Together, these proteins have the ability to control the influx and the efflux of structurally unrelated chemotherapeutic drugs, thereby modulating the intracellular drug concentration. These interactions have important clinical implications for chemotherapy because ultimately they determine therapeutic efficacy, disease progression/relapse and the success or failure of patient treatment.

Gefitinib enhances the antitumor activity of CPT-11 in vitro and in vivo by inhibiting ABCG2 but not ABCB1: a new clue to circumvent gastrointestinal toxicity risk

BACKGROUND

Despite the potent antitumor activity of CPT-11, late-onset diarrhea owing to enterohepatic circulation of SN-38 is a critical issue.

METHODS

We aimed to evaluate the inhibitory potency of gefitinib against the ABCB1- or ABCG2-mediated excretion of CPT-11 and its active metabolite SN-38 in vitro and in vivo.

RESULTS

Gefitinib dose-dependently enhanced the antiproliferation activity of SN-38 in vitro by inhibiting ABCG2. The inhibitory effect of gefitinib on ABCB1 was marginal. When both CPT-11 and gefitinib were administered orally to nude mice bearing human lung cancer PC-6 cells, tumor growth was markedly suppressed. By gefitinib coadministration, the lactone forms of both CPT-11 and SN-38 in the tumor tissue increased more than 2-fold.

CONCLUSIONS

Gefitinib significantly enhances the antitumor efficacy of CPT-11 and its tumor distribution in vivo. Coadministration of gefitinib may provide a new means to reduce the dose of CPT-11 and to circumvent the gastrointestinal toxicity risk.

Multidrug resistance: Physiological principles and nanomedical solutions

Multidrug resistance (MDR) is a pathophysiological phenomenon employed by cancer cells which limits the prolonged and effective use of chemotherapeutic agents. MDR is primarily based on the over-expression of drug efflux pumps in the cellular membrane. Prominent examples of such efflux pumps, which belong to the ATP-binding cassette (ABC) superfamily of proteins, are Pgp (P-glycoprotein) and MRP (multidrug resistance-associated protein), nowadays officially known as ABCB1 and ABCC1. Over the years, several strategies have been evaluated to overcome MDR, based not only on the use of low-molecular-weight MDR modulators, but also on the implementation of 1-100(0) nm-sized drug delivery systems. In the present manuscript, after introducing the most important physiological principles of MDR, we summarize prototypic nanomedical strategies to overcome multidrug resistance, including the use of carrier materials with intrinsic anti-MDR properties, the use of nanomedicines to modify the mode of cellular uptake, and the co-formulation of chemotherapeutic drugs together with low- and high-molecular-weight MDR inhibitors within a single drug delivery system. While certain challenges still need to be overcome before such constructs and concepts can be widely applied in the clinic, the insights obtained and the progress made strongly suggest that nanomedicine formulations hold significant potential for improving the treatment of multidrug-resistant malignancies.

The role of human carboxylesterases in drug metabolism: have we overlooked their importance?

Carboxylesterases are a multigene family of mammalian enzymes widely distributed throughout the body that catalyze the hydrolysis of esters, amides, thioesters, and carbamates. In humans, two carboxylesterases, hCE1 and hCE2, are important mediators of drug metabolism. Both are expressed in the liver, but hCE1 greatly exceeds hCE2. In the intestine, only hCE2 is present and highly expressed. The most common drug substrates of these enzymes are ester prodrugs specifically designed to enhance oral bioavailability by hydrolysis to the active carboxylic acid after absorption from the gastrointestinal tract. Carboxylesterases also play an important role in the hydrolysis of some drugs to inactive metabolites. It has been widely believed that drugs undergoing hydrolysis by hCE1 and hCE2 are not subject to clinically significant alterations in their disposition, but evidence exists that genetic polymorphisms, drug-drug interactions, drug-disease interactions and other factors are important determinants of the variability in the therapeutic response to carboxylesterase-substrate drugs. The implications for drug therapy are far-reaching, as substrate drugs include numerous examples from widely prescribed therapeutic classes. Representative drugs include angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, antiplatelet drugs, statins, antivirals, and central nervous system agents. As research interest increases in the carboxylesterases, evidence is accumulating of their important role in drug metabolism and, therefore, the outcomes of pharmacotherapy.

Significance of Kampo, traditional Japanese medicine, in supportive care of cancer patients

The current standard treatment for cancer is a multidisciplinary therapy whereby various types of treatment are properly combined. Chemotherapy with multiple anticancer drugs is now common, and traditional, complementary, and alternative therapies are adopted as supportive measures. Medical care in Japan is distinguished by the ability for patients to access both Western and Kampo medical cares at the same time. There is a high degree of trust in the safety of Kampo therapies because they are practiced by medical doctors who are educated with fundamental diagnosis of Western medicine. Highly reliable clinical studies are being published, demonstrating that palliative or supportive care for cancer patients using Kampo preparations alleviates adverse effects of chemotherapy or radiotherapy. This paper reports the circumstances around cancer care in Japan where traditional therapeutic Kampo formulas are used for patients undergoing cancer treatment with cutting-edge chemotherapy, specifically to alleviate adverse effects of anticancer drugs.

A perspective on efflux transport proteins in the liver

Detailed knowledge regarding the influence of hepatic transport proteins on drug disposition has advanced at a rapid pace over the past decade. Efflux transport proteins located in the basolateral and apical (canalicular) membranes of hepatocytes play an important role in the hepatic elimination of many endogenous and exogenous compounds, including drugs and metabolites. This review focuses on the role of these efflux transporters in hepatic drug excretion. The impact of these proteins as underlying factors for disease is highlighted, and the importance of hepatic efflux proteins in the efficacy and toxicity of drugs is discussed. In addition, a brief overview of methodology to evaluate the function of hepatic efflux transport proteins is provided. Current challenges in predicting the impact of altered efflux protein function on systemic, intestinal, and hepatocyte exposure to drugs and metabolites are highlighted.

Contribution of tumoral and host solute carriers to clinical drug response

Members of the solute carrier family of transporters are responsible for the cellular uptake of a broad range of endogenous compounds and xenobiotics in multiple tissues. Several of these solute carriers are known to be expressed in cancer cells or cancer cell lines, and decreased cellular uptake of drugs potentially contributes to the development of resistance. As result, the expression levels of these proteins in humans have important consequences for an individual's susceptibility to certain drug-induced side effects, interactions, and treatment efficacy. In this review article, we provide an update of this rapidly emerging field, with specific emphasis on the direct contribution of solute carriers to anticancer drug uptake in tumors, the role of these carriers in regulation of anticancer drug disposition, and recent advances in attempts to evaluate these proteins as therapeutic targets.

Metabolic pathways of the camptothecin analog AR-67

7-tert-Butyldimethylsilyl-10-hydroxycamptothecin (AR-67; also known as DB-67) is a novel lipophilic camptothecin analog in early-phase anticancer clinical trials. In support of these studies, we evaluated the metabolism of AR-67 in vitro and identified potential metabolites in patient samples. The lactone form of AR-67 was found to be preferentially metabolized over AR-67 carboxylate in human microsomes. Subsequently, the lactone form was tested as a substrate in a panel of CYP450 and UDP-glucuronosyltransferase (UGT) enzymes known to metabolize the majority of clinically approved molecules. AR-67 was metabolized by CYP3A5, CYP3A4, CYP1A1, and CYP1A2, in order of activity. Extrahepatic UGT1A8 and UGT1A7 possessed at least 6-fold higher metabolizing activity than UGT1A1 and other UGT enzymes tested. CYP1A1 and UGT1A7 displayed Michaelis-Menten kinetics, whereas CYP3A4, CYP3A5, and UGT1A8 displayed kinetics consistent with substrate inhibition. Chromatographic analysis of representative patient plasma and urine samples demonstrated the presence of AR-67 glucuronides and oxidized products in the urine but only in very minimal amounts. We conclude that limited in vivo metabolism of AR-67 by UGT1A1 may partly explain the absence of AR-67 glucuronides in plasma and hypothesize that UGT1A8- and CYP3A-mediated biotransformation within the gastrointestinal epithelium may provide protective mechanisms against AR-67 gastrointestinal toxicity.

Irinotecan and its active metabolite, SN-38: review of bioanalytical methods and recent update from clinical pharmacology perspectives

The introduction of irinotecan has revolutionized the applicability of camptothecins as predominant topoisomerase I inhibitor for anti-cancer therapy. The potent anti-tumor activity of irinotecan is due to rapid formation of an in vivo active metabolite, SN-38. Therefore, irinotecan is considered as a pro-drug to generate SN-38. Over the past decade, side-by-side with the clinical advancement of the use of irinotecan in the oncology field, a plethora of bioanalytical methods have been published to quantify irinotecan, SN-38 and other metabolites. Because of the availability of HPLC, LC-MS and LC-MS/MS methods, the pharmacokinetic profiling of irinotecan and its metabolites has been accomplished in multiple species, including cancer patients. The developed assays continue to find use in the optimization of newly designed delivery systems with regard to pharmacokinetics to promote safe and effective use of either irinotecan or SN-38. This review intends to: firstly, provide an exhaustive compilation of the published assays for irinotecan, SN-38 and other metabolite(s) of irinotecan, as applicable; secondly, to enumerate the validation parameters and applicable conclusions; and thirdly, provide some recent perspectives in the clinical pharmacology arena pertaining to efflux transporters, pediatric profiling, role of kidney function in defining toxicity, drug-drug interaction potential of irinotecan, etc.

Effects of oral administration of S-1 on the pharmacokinetics of SN-38, irinotecan active metabolite, in patients with advanced colorectal cancer

Previous studies have assessed the efficacy and safety of combined treatment with irinotecan (7-ethyl-10-[4-[1-piperidino]-1-piperidino]carbonyloxycamptothecin, CPT-11) and S-1, containing tegafur, a prodrug of 5-fluorouracil, in the treatment of colorectal and gastric cancer. The objective of this study was to describe the interaction between CPT-11 and S-1 in 4 patients with colorectal cancer. Coadministration of S-1 changed the pharmacokinetic behavior of CPT-11 and its metabolites. In particular, maximum plasma concentration (Cmax) and area under the plasma concentration curve (AUC) of 7-ethyl-10-hydroxycampothecin (SN-38) was markedly decreased by coadministration of S-1. For SN-38, the median ratio of Cmax and AUC with S-1 to those without S-1 was median 0.34 (range 0.24-0.78) and 0.56 (range 0.23-0.68), respectively. A markedly difference in drug interaction among individual patients was observed. We conclude that the plasma concentration of SN-38 was decreased by oral administration of S-1 in patients with colorectal cancer. This observation might be important for clinical decisions regarding combination therapy.

Pharmacogenomics of drug-metabolizing enzymes and drug transporters in chemotherapy

There is wide variability in the response of individuals to standard doses of drug therapy. This is an important problem in clinical practice, where it can lead to therapeutic failures or adverse drug events. Polymorphisms in genes coding for metabolizing enzymes and drug transporters can affect drug efficacy and toxicity. Pharmacogenomics aims to identify individuals predisposed to high risk of toxicity and low response from standard doses of anticancer drugs. This chapter focuses on the clinical significance of polymorphisms in drug-metabolizing enzymes and drug transporters in influencing efficacy and toxicity of anticancer therapy. The most important examples to demonstrate the influence of pharmacogenomics on anticancer therapy are thiopurine methyltransferase (TPMT), UGT (uridine diphosphate glucuronosyltransferase) 1A1*28, and DPD (dihydropyrimidine dehydrogenase) *2A, respectively, for 6-mercaptopurine, irinotecan, and 5-fluorouracil therapy. However, in most other anticancer therapies no clear association has been found for polymorphisms in drug-metabolizing enzymes and drug transporters and pharmacokinetics or pharmacodynamics of anticancer drugs. Evaluation of different regimens and tumor types showed that polymorphisms can have different, sometimes even contradictory, pharmacokinetic and pharmacodynamic effects in different tumors in response to different drugs. The clinical application of pharmacogenomics in cancer treatment therefore requires more detailed information regarding the different polymorphisms in drug-metabolizing enzymes and drug transporters. A greater understanding of complexities in pharmacogenomics is needed before individualized therapy can be applied on a routine basis.

[Irinotecan and liver dysfunctions]

During last years, irinotecan has become registered as a major cytotoxic drug in several tumor types. Since the metabolism of this drug is predominantly made in the liver, administration to patients with liver dysfunctions remains a major problem. Hyperbilirubinemia has been shown to require dose reduction. In addition, gene polymorphism of UGT1A1 was shown to be associated with a higher risk of toxicity. However, studies are still required to optimise the use of irinotecan in patients with liver dysfunctions.

Pharmacogenetics/genomics of membrane transporters in cancer chemotherapy

Inter-individual variability in drug response and the emergence of adverse drug reactions are main causes of treatment failure in cancer therapy. Recently, membrane transporters have been recognized as an important determinant of drug disposition, thereby affecting chemosensitivity and -resistance. Genetic factors contribute to inter-individual variability in drug transport and targeting. Therefore, pharmacogenetic studies of membrane transporters can lead to new approaches for optimizing cancer therapy. This review discusses genetic variations in efflux transporters of the ATP-binding cassette (ABC) family such as ABCB1 (MDR1, P-glycoprotein), ABCC1 (MRP1), ABCC2 (MRP2) and ABCG2 (BCRP), and uptake transporters of the solute carrier (SLC) family such as SLC19A1 (RFC1) and SLCO1B1 (SLC21A6), and their relevance to cancer chemotherapy. Furthermore, a pharmacogenomic approach is outlined, which using correlations between the growth inhibitory potency of anticancer drugs and transporter gene expression in multiple human cancer cell lines, has shown promise for determining the relevant transporters for any given drugs and predicting anticancer drug response.

Involvement of up-regulation of hepatic breast cancer resistance protein in decreased plasma concentration of 7-ethyl-10-hydroxycamptothecin (SN-38) by coadministration of S-1 in rats

The safety and efficacy of combination therapy with 7-ethyl-10-[4-[1-piperidino]-1-piperidino]carbonyloxycamptothecin (CPT-11, irinotecan) and S-1 composed of tegafur, a prodrug of 5-fluorouracil, gimeracil, and potassium oxonate, have been confirmed in patients with colorectal cancer. Previously, we showed that p.o. coadministration of S-1 decreased the plasma concentration of both CPT-11 and its metabolites in a patient with advanced colorectal cancer. The aim of this study was to clarify the mechanism of drug interaction using both in vivo and in vitro methods. Rats were administered S-1 p.o. (10 mg/kg) once a day for 7 consecutive days. On day 7, CPT-11 (10 mg/kg) was administered by i.v. injection. Coadministration of S-1 affected the pharmacokinetic behavior of CPT-11 and its metabolites, with a decrease in the C(max) and area under the plasma concentration curve (AUC) of the active metabolite 7-ethyl-10-hydroxycampothecin (SN-38) lactone form. Furthermore, the rate of biliary excretion of the SN-38 carboxylate form increased on coadministration of S-1. In the liver, the level of breast cancer resistance protein (BCRP), but not P-glycoprotein and multidrug resistance-associated protein 2, was elevated after administration of S-1. Enzymatic conversion of CPT-11 to SN-38 by carboxylesterase (CES) was unaffected by the liver microsomes of rats treated with S-1. In addition, components of S-1 did not inhibit the hydrolysis of p-nitrophenylacetate, a substrate of CES, in the S9 fraction of HepG2 cells. Therefore, the mechanism of drug interaction between CPT-11 and S-1 appears to involve up-regulation of BCRP in the liver, resulting in an increased rate of biliary excretion of SN-38 accompanied by a decrease in the C(max) and AUC of SN-38.

Irinotecan-induced diarrhea: functional significance of the polymorphic ABCC2 transporter protein

Interindividual pharmacokinetic variability of the anticancer agent irinotecan is high. Life-threatening diarrhea is observed in up to 25% of patients receiving irinotecan and has been related with irinotecan pharmacokinetics and UGT1A1 genotype status. Here, we explore the association of ABCC2 (MRP2) polymorphisms and haplotypes with irinotecan disposition and diarrhea. A cohort of 167 Caucasian cancer patients who were previously assessed for irinotecan pharmacokinetics (90-min infusion given every 21 days), toxicity, and UGT1A1*28 genotype were genotyped for polymorphisms in ABCC2 using Pyrosequencing. Fifteen ABCC2 haplotypes were identified in the studied patients. The haplotype ABCC2*2 was associated with lower irinotecan clearance (28.3 versus 31.6 l/h; P=0.020). In patients who did not carry a UGT1A1*28 allele, a significant reduction of severe diarrhea was noted in patients with the ABCC2*2 haplotype (10 versus 44%; odds ratio, 0.15; 95% confidence interval, 0.04-0.61; P=0.005). This effect was not observed in patients with at least one UGT1A1*28 allele (32 versus 20%; odds ratio, 1.87; 95% confidence interval, 0.49-7.05; P=0.354). This study suggests that the presence of the ABCC2*2 haplotype is associated with less irinotecan-related diarrhea, maybe as a consequence of reduced hepatobiliary secretion of irinotecan. As the association was seen in patients not genetically predisposed at risk for diarrhea due to UGT1A1*28, confirmatory studies of the relationships of ABCC2 genotypes and irinotecan disposition and toxicity are warranted.

[Pharmacokinetic study of cancer chemotherapy]

We reported that the rate of conversion of lactone to carboxylate forms of irinotecan (CPT-11) and its metabolites plays a major role in the biliary excretion of these compounds. Sulfobromophthalein partially inhibited the secretion of SN-38-glucronide into the gastrointestinal lumen, whereas little change was seen in that of active metabolite SN-38. Co-administration of sulphobromophthalein with CPT-11 might lower the late-onset gastrointestinal toxicity observed during treatment with CPT-11 without lowering anticancer activity. In the ileum, the level of transport in the direction form the serosal layer to mucosal layer was significantly greater than that in the direction form the mucosal layer to serosal layer, whereas a significant difference was not observed in the jejunum. This secretory transport required metabolic energy was diminished by sulfobromophthalein. A specific transport system plays a major role in the secretion of SN-38 and that this secretory transport system predominantly exists in the ileum. Uptake of SN-38 was significantly reduced at 4 degrees C. Baicalin inhibited the uptake of SN-38. A specific transport system mediates the uptake of SN-38 across the apical membrane in Caco-2 cells. Inhibition of this transporter would be a useful means for reducing late-onset diarrhea.

Molecular modeling of new camptothecin analogues to circumvent ABCG2-mediated drug resistance in cancer

Irinotecan (CPT-11) is a widely used potent antitumor drug that inhibits mammalian DNA topoisomerase I (Topo I). However, overexpression of ABCG2 (BCRP/MXR/ABCP) reportedly confers cancer cells resistance to SN-38, the active form of CPT-11. To circumvent the ABCG2-associated drug resistance, the structure-activity-relationship (SAR) of 14 new camptothecin (CPT) analogues has been studied with respect to the substrate specificity of ABCG2. While the lactone E ring is a prerequisite for anticancer activity, modifications of the A or B rings do not significantly affect Topo I inhibition. Based on the substrate specificity of ABCG2, it is strongly suggested that CPT analogues with a hydroxyl group at position 10 or 11 of the A ring are recognized by ABCG2 and are thereby effectively extruded from cancer cells. To develop a platform for the molecular modeling to circumvent anticancer drug resistance, we have carried out quantum chemical calculations and neural network SAR analysis. Electrostatic potential iso-surfaces generated by ab initio MO calculations using restricted Hartree-Fock method have revealed that negative potential localized at positions 10 or 11 in the A ring is important for recognition by ABCG2.

Expression and function of efflux drug transporters in the intestine

A variety of drug transporters expressed in the body control the fate of drugs by affecting absorption, distribution, and elimination processes. In the small intestine, transporters mediate the influx and efflux of endogenous or exogenous substances. In clinical pharmacotherapy, ATP-dependent efflux transporters (ATP-binding cassette [ABC] transporters) expressed on the apical membrane of the intestinal epithelial cells determine oral bioavailability, intestinal efflux clearance, and the site of drug-drug interaction of certain drugs. The expression and functional activity of efflux transporters exhibit marked interindividual variation and are relatively easily modulated by factors such as therapeutic drugs and daily foods and beverages. In this article, we will summarize the recent findings regarding the intestinal efflux transporters, especially P-glycoprotein (P-gp or human multidrug resistance gene [MDR] 1), multidrug resistance-associated protein 2 (MRP2), and breast cancer resistance protein (BCRP).

Determination of thalidomide by high performance liquid chromatography: Plasma pharmacokinetic studies in the rat

A sensitive and simple high performance liquid chromatography (HPLC) method was developed and validated for the determination of thalidomide in rat plasma. Chromatography was accomplished with a reversed-phase Hypersil C18 column. Mobile phase consisted of acetonitrile-10 mM ammonium acetate buffer (pH 5.50) (28:72, v/v), at a flow rate of 0.8 ml/min. Thalidomide was monitored by ultraviolet detector at 220 nm and it gave a linear response as a function of concentration over 0.02-50 microM. The limit of quantitation in rat plasma was 0.50 ng (0.02 microM plasma concentration) with an aliquot of 20 microl. Results from a 3-day validation study indicated that this method allows for simple and rapid quantitation of thalidomide with excellent accuracy and reliability. Using this validated assay, the effect of coadministered irinotecan (CPT-11) on the plasma pharmacokinetics of thalidomide in rats was determined. Coadministration of CPT-11 (intravenously, 60 mg/kg) increased the maximum plasma concentration (C(max)) and area under the plasma concentration-time curve (AUC(0-10h)) of thalidomide by 32.29 and 11.66%, respectively, as compared to the control, but none of the effect of CPT-11 was of statistical significance (P > 0.05). Concomitant CPT-11 also caused a 10.04% decrease in plasma clearance (CL) and 14.51% decrease in volume of distribution (V(d)) (P > 0.05). These results suggest that coadministered CPT-11 did not significantly alter the plasma pharmacokinetics of thalidomide in rats. Further studies are warranted to explore the pharmacokinetic and pharmacodynamic interactions between CPT-11 and thalidomide.

Benefit-risk assessment of irinotecan in advanced colorectal cancer

Irinotecan exerts its cytotoxic activity through inhibition of the nuclear enzyme topoisomerase I. It has been approved in most countries worldwide for treatment of patients with advanced colorectal cancer (CRC). Activity is seen in previously untreated patients and in patients refractory to fluorouracil treatment, whether it is given alone or in combination with other cytotoxic drugs. Irinotecan was first developed in patients refractory to fluorouracil. Activity in terms of tumour responses and patient benefit was seen in several phase II trials that used either a weekly or a three-weekly schedule. In two randomised trials (irinotecan vs best supportive care, and irinotecan vs an infused fluorouracil-based regimen), irinotecan prolonged median survival by approximately 2.5 months without any deterioration in quality-of-life. It was later studied in previously untreated patients with advanced CRC in combination with fluorouracil/folinic acid (leucovorin). In three large randomised trials, median time to tumour progression was prolonged by approximately 2.5 months and overall survival by about 2.5 months compared with fluorouracil/folinic acid alone. Tumour responses were also seen more frequently in the irinotecan arm (35-40% vs 20%). Again, quality-of-life scores were not deteriorated by the addition of irinotecan. Irinotecan has many acute adverse effects. The most prominent and dose limiting being diarrhoea and neutropenia. With irinotecan monotherapy, diarrhoea was seen in 80% of patients and severe grade 3 to 4 diarrhoea occurred in 30-40% of the patients. The severity of diarrhoea can be diminished by preventive actions. Less risk of diarrhoea is generally seen when irinotecan is combined with fluorouracil. Neutropenia is generally short-lived, but may be severe if diarrhoea is also present. This has been noticed particularly when irinotecan has been given in combination with a bolus fluorouracil/folinic acid regimen. Other toxicities include acute cholinergic-like symptoms, nausea and vomiting, and alopecia. In spite of these adverse effects, irinotecan has been accepted as an important first-line treatment for patients with advanced CRC, in combination with, preferably, an infused fluorouracil-based regimen, and has been approved for use as monotherapy in the second-line indication.

Mechanism-based inhibition of cytochrome P450 3A4 by therapeutic drugs

Consistent with its highest abundance in humans, cytochrome P450 (CYP) 3A is responsible for the metabolism of about 60% of currently known drugs. However, this unusual low substrate specificity also makes CYP3A4 susceptible to reversible or irreversible inhibition by a variety of drugs. Mechanism-based inhibition of CYP3A4 is characterised by nicotinamide adenine dinucleotide phosphate hydrogen (NADPH)-, time- and concentration-dependent enzyme inactivation, occurring when some drugs are converted by CYP isoenzymes to reactive metabolites capable of irreversibly binding covalently to CYP3A4. Approaches using in vitro, in silico and in vivo models can be used to study CYP3A4 inactivation by drugs. Human liver microsomes are always used to estimate inactivation kinetic parameters including the concentration required for half-maximal inactivation (K(I)) and the maximal rate of inactivation at saturation (k(inact)). Clinically important mechanism-based CYP3A4 inhibitors include antibacterials (e.g. clarithromycin, erythromycin and isoniazid), anticancer agents (e.g. tamoxifen and irinotecan), anti-HIV agents (e.g. ritonavir and delavirdine), antihypertensives (e.g. dihydralazine, verapamil and diltiazem), sex steroids and their receptor modulators (e.g. gestodene and raloxifene), and several herbal constituents (e.g. bergamottin and glabridin). Drugs inactivating CYP3A4 often possess several common moieties such as a tertiary amine function, furan ring, and acetylene function. It appears that the chemical properties of a drug critical to CYP3A4 inactivation include formation of reactive metabolites by CYP isoenzymes, preponderance of CYP inducers and P-glycoprotein (P-gp) substrate, and occurrence of clinically significant pharmacokinetic interactions with coadministered drugs. Compared with reversible inhibition of CYP3A4, mechanism-based inhibition of CYP3A4 more frequently cause pharmacokinetic-pharmacodynamic drug-drug interactions, as the inactivated CYP3A4 has to be replaced by newly synthesised CYP3A4 protein. The resultant drug interactions may lead to adverse drug effects, including some fatal events. For example, when aforementioned CYP3A4 inhibitors are coadministered with terfenadine, cisapride or astemizole (all CYP3A4 substrates), torsades de pointes (a life-threatening ventricular arrhythmia associated with QT prolongation) may occur.However, predicting drug-drug interactions involving CYP3A4 inactivation is difficult, since the clinical outcomes depend on a number of factors that are associated with drugs and patients. The apparent pharmacokinetic effect of a mechanism-based inhibitor of CYP3A4 would be a function of its K(I), k(inact) and partition ratio and the zero-order synthesis rate of new or replacement enzyme. The inactivators for CYP3A4 can be inducers and P-gp substrates/inhibitors, confounding in vitro-in vivo extrapolation. The clinical significance of CYP3A inhibition for drug safety and efficacy warrants closer understanding of the mechanisms for each inhibitor. Furthermore, such inactivation may be exploited for therapeutic gain in certain circumstances.

St. John's Wort modulates the toxicities and pharmacokinetics of CPT-11 (irinotecan) in rats

CPT-11 is a DNA topoisomerase I inhibitor for the therapy of colorectal cancer, whereas St. John's Wort (Hypericum perforatum, SJW) is a widely used herbal anti-depressant. This study aimed to investigate the effects of co-administered SJW on the toxicities and pharmacokinetics of CPT-11 and the underlying mechanisms. The body weight loss, gastrointestinal and hematological toxicities induced by CPT-11, and the pharmacokinetic parameters of CPT-11 were evaluated in rats pretreated with SJW or vehicle. Rats treated with CPT-11 alone experienced rapid decrease in body weight, whereas co-administration of SJW with CPT-11 resulted in lesser body weight loss. The gastrointestinal and hematological toxicities following CPT-11 injection were both alleviated in the presence of SJW. The rat pharmacokinetics of both CPT-11 and its metabolite SN-38 were significantly altered in presence of SJW. In conclusion, co-administered SJW significantly ameliorated the toxicities induced by CPT-11. The protective effect of SJW may be partially due to pharmacokinetic interaction between CPT-11 and SJW.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Phase I Pharmacokinetic, Food Effect, and Pharmacogenetic Study of Oral Irinotecan Given as Semisolid Matrix Capsules in Patients with Solid Tumors

PURPOSE

To characterize the maximum-tolerated dose, recommended dose, dose-limiting toxicities (DLT), pharmacokinetic profile, and food effect of orally administered irinotecan formulated as new semisolid matrix capsules.

EXPERIMENTAL DESIGN

Irinotecan was given orally in fasted patients once daily for 5 consecutive days and repeated every 3 weeks. Patients were randomly assigned to take the drug along with a high-fat, high-calorie breakfast for the administration at day 1 of the first or second cycle. Dosages tested were 70 and 80 mg/m(2)/day.

RESULTS

Twenty-five patients received 101 cycles of therapy (median two cycles, range 1-15). During the first cycle, grade 3 delayed diarrhea and grade 3 fever were the DLTs at the dosage of 80 mg/m(2)/day in three out of five patients. Hematologic and nonhematologic toxicities were mild to moderate. Exposure to the active metabolite SN-38 was relatively high compared with i.v. infusion, but no relevant accumulation was observed. Food had no significant effect on irinotecan pharmacokinetics. One confirmed partial remission and 10 disease stabilizations were observed in previously treated patients. No association was found between the UGT1A1*28 genotype and the risk of severe irinotecan-induced toxicity.

CONCLUSIONS

For oral irinotecan, a dose of 70 mg/m(2)/day for 5 consecutive days every 3 weeks is recommended for further studies. Delayed diarrhea was the main DLT, similar to that observed with intravenously administered irinotecan. This study confirms that oral administration of irinotecan is feasible and may have favorable pharmacokinetic characteristics.

Characterization of p53 wild-type and null isogenic colorectal cancer cell lines resistant to 5-fluorouracil, oxaliplatin, and irinotecan

To elucidate mechanisms of resistance to chemotherapies currently used in the first-line treatment of advanced colorectal cancer, we have developed a panel of HCT116 p53 wild-type (p53(+/+)) and null (p53(-/-)) isogenic colorectal cancer cell lines resistant to the antimetabolite 5-fluorouracil (5-FU), topoisomerase I inhibitor irinotecan (CPT-11), and DNA-damaging agent oxaliplatin. These cell lines were generated by repeated exposure to stepwise increasing concentrations of each drug over a period of several months. We have demonstrated a significant decrease in sensitivity to 5-FU, CPT-11, and oxaliplatin in each respective resistant cell line relative to the parental line as determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide analysis, with increases in IC(50 (72 h)) concentrations ranging from 3- to 65-fold. Using flow cytometry, we have also demonstrated compromised apoptosis and cell cycle arrest in 5-FU-, oxaliplatin-, and CPT-11-resistant cell lines compared with the parental lines after exposure to each drug. In addition, we found that resistance to 5-FU and oxaliplatin was higher in parental p53(-/-) cells compared with parental p53(+/+) cells, with an approximately 5-fold increase in IC(50 (72 h)) for each drug. In contrast, the IC(50 (72 h)) doses for CPT-11 were identical in the p53 wild-type and null cell lines. Furthermore, apoptosis after treatment with 5-FU and oxaliplatin, but not CPT-11, was significantly reduced in parental p53(-/-) cells compared with parental p53(+/+) cells. These data suggest that p53 may be an important determinant of sensitivity to 5-FU and oxaliplatin but not CPT-11. Using semiquantitative reverse transcription-PCR, we have demonstrated down-regulation of thymidine phosphorylase mRNA in both p53(+/+) and p53(-/-) 5-FU-resistant cells, suggesting that decreased production of 5-FU active metabolites may be an important resistance mechanism in these lines. In oxaliplatin-resistant cells, we noted increased mRNA levels of the nucleotide excision repair gene ERCC1 and ATP-binding cassette transporter breast cancer resistance protein. In CPT-11-resistant cells, we found reduced mRNA levels of carboxylesterase, the enzyme responsible for converting CPT-11 to its active metabolite SN-38, and topoisomerase I, the SN-38 target enzyme. In addition, we noted overexpression of breast cancer resistance protein in the CPT-11-resistant lines. These cell lines are ideal tools with which to identify novel determinants of drug resistance in both the presence and absence of wild-type p53.

The MDR1 (ABCB1) gene polymorphism and its clinical implications

There has been an increasing appreciation of the role of drug transporters in the pharmacokinetic and pharmacodynamic profiles of certain drugs. Among various drug transporters, P-glycoprotein, the MDR1 gene product, is one of the best studied and characterised. P-glycoprotein is expressed in normal human tissues such as liver, kidney, intestine and the endothelial cells of the blood-brain barrier. Apical (or luminal) expression of P-glycoprotein in these tissues results in reduced drug absorption from the gastrointestinal tract, enhanced drug elimination into bile and urine, and impeded entry of certain drugs into the central nervous system. The clinical relevance of P-glycoprotein depends on the localisation in human tissues (i.e. vectorial or directional movement), the therapeutic index of the substrate drug and the inherent inter- and intra-individual variability. With regard to the variability, polymorphisms of the MDR1 gene have recently been reported to be associated with alterations in disposition kinetics and interaction profiles of clinically useful drugs, including digoxin, fexofenadine, ciclosporin and talinolol. In addition, polymorphism may play a role in patients who do not respond to drug treatment. Moreover, P-glycoprotein is an important prognostic factor in malignant diseases, such as tumours of the gastrointestinal tract.A growing number of preclinical and clinical studies have demonstrated that polymorphism of the MDR1 gene may be a factor in the overall outcome of pharmacotherapy for numerous diseases. We believe that further understanding the physiology and biochemistry of P-glycoprotein with respect to its genetic variations will be important to establish individualised pharmacotherapy with various clinically used drugs.

Novel camptothecin analogues that circumvent ABCG2-associated drug resistance in human tumor cells

Irinotecan (7-ethyl-10-[4-(1-piperidino)-1-piperidino]-carbonyloxycamptothecin; CPT-11) is a widely used potent antitumor drug that inhibits mammalian DNA topoisomerase I (Topo I); however, overexpression of ABCG2 (BCRP/MXR/ABCP) can confer cancer cell resistance to SN-38, the active form of CPT-11. We have recently demonstrated that plasma membrane vesicles prepared from ABCG2-overexpressing PC-6/SN2-5H cells transported SN-38 and its glucuronide conjugate in an ATP-dependent manner (Nakatomi et al., Biochem Biophys Res Commun 2001;288:827-32). In the present study, we have characterized a total of 14 new camptothecin (CPT) analogues with respect to both the inhibition of Topo I and the substrate specificity of ABCG2. All of the tested CPT analogues, which have different substitutions at positions 10 and 11, strongly inhibited the Topo I activity in a cell-free system, as did SN-38. Their antitumor activities in the SN-38-resistant PC-6/SN2-5H2 cell line greatly varied, however, being correlated with intracellular accumulation levels. We have examined ATP-dependent transport of those CPT analogues by using plasma membrane vesicles prepared from both PC-6/SN2-5H2 cells and ABCG2-transfected HEK-293 cells. Based on the substrate specificity of ABCG2 thus evaluated, it is strongly suggested that CPT analogues with high polarity are good substrates for ABCG2 and are therefore effectively extruded from cancer cells. In this context, to circumvent ABCG2-associated drug resistance, low-polarity CPT analogues are considered to be potent lead compounds. The present study provides a practical approach to discover new CPT-based drugs for the chemotherapy of drug-resistant human cancer.

Membrane transport of camptothecin: facilitation by human P-glycoprotein (ABCB1) and multidrug resistance protein 2 (ABCC2)

BACKGROUND

The purpose of the present study was to continue the investigation of the membrane transport mechanisms of 20-(S)-camptothecin (CPT) in order to understand the possible role of membrane transporters on its oral bioavailability and disposition.

METHODS

The intestinal transport kinetics of CPT were characterized using Caco-2 cells, MDCKII wild-type cells and MDCKII cells transfected with human P-glycoprotein (PGP) (ABCB1) or human multidrug resistance protein 2 (MRP2) (ABCC2). The effects of drug concentration, inhibitors and temperature on CPT directional permeability were determined.

RESULTS

The absorptive (apical to basolateral) and secretory (basolateral to apical) permeabilities of CPT were found to be saturable. Reduced secretory CPT permeabilities with decreasing temperatures suggests the involvement of an active, transporter-mediated secretory pathway. In the presence of etoposide, the CPT secretory permeability decreased 25.6%. However, inhibition was greater in the presence of PGP and of the breast cancer resistant protein inhibitor, GF120918 (52.5%). The involvement of additional secretory transporters was suggested since the basolateral to apical permeability of CPT was not further reduced in the presence of increasing concentrations of GF120918. To investigate the involvement of specific apically-located secretory membrane transporters, CPT transport studies were conducted using MDCKII/PGP cells and MDCKII/MRP2 cells. CPT carrier-mediated permeability was approximately twofold greater in MDCKII/PGP cells and MDCKII/MRP2 cells than in MDCKII/wild-type cells, while the apparent Km values were comparable in all three cell lines. The efflux ratio of CPT in MDCKII/PGP in the presence of 0.2 microM GF120918 was not completely reversed (3.36 to 1.49). However, the decrease in the efflux ratio of CPT in MDCKII/MRP2 cells (2.31 to 1.03) suggests that CPT efflux was completely inhibited by MK571, a potent inhibitor of the Multidrug Resistance Protein transporter family.

CONCLUSIONS

The current results provide evidence that PGP and MRP2 mediate the secretory transport of CPT in vitro. However, the involvement of other transporters cannot be ruled out based on these studies. Since these transporters are expressed in the intestine, liver and kidney variations in their expression levels and/or regulation may be responsible for the erratic oral absorption and biliary excretion of CPT observed in human subjects.

Transport characteristics of 9-nitrocamptothecin in the human intestinal cell line Caco-2 and everted gut sacs

The intestinal absorptive characteristics and the efflux mechanisms of 9-nitrocamptothecin (9-NC), a novel water-insoluble camptothecin (CPT) derivative, were investigated. The Caco-2 cells and the everted gut sacs were used as models of the intestinal mucosa to assess transepithelial transport of 9-NC. The determination of 9-NC was performed by HPLC. In the Caco-2 cells, the absorptive transport of 9-NC was pH dependent and the transport was enhanced at weakly acidic pH on the apical side. No concentration dependence and saturation were observed for the absorptive transport of 9-NC at concentrations up to 250 microM, while secretory transport were concentration dependent and saturable process (K(m) was 49.8 +/- 1.2 microM, V(max) was 38.28 +/- 0.8 ng/cm(2)/min). In the presence of verapamil (100 microM) and CsA (10 microM), potent inhibitors of P-glyprotein (P-gp)/MRP2 (cMOAT), the P(appBL-AP)/P(appAP-BL) ratio was decreased from 3.4 to 1.4 and 1.3, respectively, and permeation of apical to basolateral was enhanced approximately two-fold. In the everted gut sacs, the absorption of 9-NC was passive diffusion and had no significant difference in different gut regions. Adding verapamil in the everted gut sacs over a concentration ranging from 10 to 100 microM, the absorption of 9-NC was significantly enhanced, especially more markedly in lower small intestine (P < 0.05). Overall, the current study suggests that pH and efflux transporters are capable of mediating the absorption and efflux of 9-NC, and they may play significant roles in limiting the oral absorption of 9-NC.