CPT-11 sensitivity in relation to the expression of P170-glycoprotein and multidrug resistance-associated protein

Uremic serum residue decreases SN-38 sensitivity through suppression of organic anion transporter polypeptide 2B1 in LS-180 colon cancer cells

Pharmacokinetics of SN-38 in patients with end-stage kidney disease (ESKD) is partially varied because of fluctuations in transporters expression and/or function by high protein bound-uremic toxins concentration. The fluctuations may induce variations in anticancer drugs sensitivity to cancer cells. We aimed to clarify the variations in sensitivity of SN-38 to cancer patients with ESKD and investigate this mechanism, by human colon cancer cells exposed to uremic serum residue. LS180 cells were exposed to normal or uremic serum residue (LS/NSR or LS/USR cells) for a month. IC₅₀ values of SN-38 in LS/NSR or LS/USR cells were calculated from viability of each cells treated SN-38. mRNA expression and intracellular SN-38 accumulation was evaluated by RT-PCR and HPLC-fluorescence methods, respectively. The IC₅₀ value in LS/USR cells was higher than that in LS/NSR cells. Organic anion transporter polypeptide (OATP) 2B1 mRNA expression was lower in LS/USR cells than in LS/NSR cells, and SN-38 accumulation in LS/USR cells was lower than that in LS/NSR cells. Only co-treatment baicalin, which is OATP2B1 inhibitor, almost negated the difference in SN-38 accumulation between LS/NSR and LS/USR. Anticancer effects of substrates of OATP2B1, such as SN-38, were reduced in ESKD patients at the same plasma substrate concentration.

MALDI Mass Spectrometry Imaging for Evaluation of Therapeutics in Colorectal Tumor Organoids

Patient-derived colorectal tumor organoids (CTOs) closely recapitulate the complex morphological, phenotypic, and genetic features observed in in vivo tumors. Therefore, evaluation of drug distribution and metabolism in this model system can provide valuable information to predict the clinical outcome of a therapeutic response in individual patients. In this report, we applied matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) to examine the spatial distribution of the drug irinotecan and its metabolites in CTOs from two patients. Irinotecan is a prodrug and is often prescribed as part of therapeutic regimes for patients with advanced colorectal cancer. Irinotecan shows a time-dependent and concentration-dependent permeability and metabolism in the CTOs. More interestingly, the active metabolite SN-38 does not co-localize well with the parent drug irinotecan and the inactive metabolite SN-38G. The phenotypic effect of irinotecan metabolism was also confirmed by a viability study showing significantly reduced proliferation in the drug treated CTOs. MALDI-MSI can be used to investigate various pharmaceutical compounds in CTOs derived from different patients. By analyzing multiple CTOs from a patient, this method could be used to predict patient-specific drug responses and help to improve personalized dosing regimens. Graphical Abstract ᅟ.

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.

The role of ABC transporters in ovarian cancer progression and chemoresistance

Over 80% of ovarian cancer patients develop chemoresistance which results in a lethal course of the disease. A well-established cause of chemoresistance involves the family of ATP-binding cassette transporters, or ABC transporters that transport a wide range of substrates including metabolic products, nutrients, lipids, and drugs across extra- and intra-cellular membranes. Expressions of various ABC transporters, shown to reduce the intracellular accumulation of chemotherapy drugs, are increased following chemotherapy and impact on ovarian cancer survival. Although clinical trials to date using ABC transporter inhibitors have been disappointing, ABC transporter inhibition remains an attractive potential adjuvant to chemotherapy. A greater understanding of their physiological functions and role in ovarian cancer chemoresistance will be important for the development of more effective targeted therapies. This article will review the role of the ABC transporter family in ovarian cancer progression and chemoresistance as well as the clinical attempts used to date to reverse chemoresistance.

DEK is a potential marker for aggressive phenotype and irinotecan-based therapy response in metastatic colorectal cancer

BACKGROUND

DEK is a transcription factor involved in stabilization of heterochromatin and cruciform structures. It plays an important role in development and progression of different types of cancer. This study aims to analyze the role of DEK in metastatic colorectal cancer.

METHODS

Baseline DEK expression was firstly quantified in 9 colorectal cell lines and normal mucosa by WB. SiRNA-mediated DEK inhibition was carried out for transient DEK silencing in DLD1 and SW620 to dissect its role in colorectal cancer aggressiveness. Irinotecan response assays were performed with SN38 over 24 hours and apoptosis was quantified by flow cytometry. Ex-vivo assay was carried out with 3 fresh tumour tissues taken from surgical resection and treated with SN38 for 24 hours. DEK expression was determined by immunohistochemistry in 67 formalin-fixed paraffin-embedded tumour samples from metastatic colorectal cancer patients treated with irinotecan-based therapy as first-line treatment.

RESULTS

The DEK oncogene is overexpressed in all colorectal cancer cell lines. Knock-down of DEK on DLD1 and SW620 cell lines decreased cell migration and increased irinotecan-induced apoptosis. In addition, low DEK expression level predicted irinotecan-based chemotherapy response in metastatic colorectal cancer patients with KRAS wild-type.

CONCLUSIONS

These data suggest DEK overexpression as a crucial event for the emergence of an aggressive phenotype in colorectal cancer and its potential role as biomarker for irinotecan response in those patients with KRAS wild-type status.

Old drug new use--amoxapine and its metabolites as potent bacterial β-glucuronidase inhibitors for alleviating cancer drug toxicity

PURPOSE

Irinotecan (CPT-11) induced diarrhea occurs frequently in patients with cancer and limits its usage. Bacteria β-glucuronidase (GUS) enzymes in intestines convert the nontoxic metabolite of CPT-11, SN-38G, to toxic SN-38, and finally lead to damage of intestinal epithelial cells and diarrhea. We previously reported amoxapine as a potent GUS inhibitor in vitro. To further understand the molecular mechanism of amoxapine and its potential for treatment of CPT-11-induced diarrhea, we studied the binding modes of amoxapine and its metabolites by docking and molecular dynamics simulation, and tested the in vivo efficacy on mice in combination with CPT-11.

EXPERIMENTAL DESIGN

The binding of amoxapine, its metabolites, 7-hydroxyamoxapine and 8-hydroxyamoxapine, and a control drug loxapine with GUS was explored by computational protocols. The in vitro potencies of metabolites were measured by Escherichia coli GUS enzyme and cell-based assay. Low-dosage daily oral administration was designed to use along with CPT-11 to treat tumor-bearing mice.

RESULTS

Computational modeling results indicated that amoxapine and its metabolites bound in the active site of GUS and satisfied critical pharmacophore features: aromatic features near bacterial loop residue F365' and hydrogen bond toward E413. Amoxapine and its metabolites were demonstrated as potent in vitro. Administration of low dosages of amoxapine with CPT-11 in mice achieved significant suppression of diarrhea and reduced tumor growth.

CONCLUSIONS

Amoxapine has great clinical potential to be rapidly translated to human subjects for irinotecan-induced diarrhea.

Reactive oxygen species responsive nanoprodrug to treat intracranial glioblastoma

Chemotherapy for intracranial gliomas is hampered by limited delivery of therapeutic agents through the blood brain barrier (BBB). An optimal therapeutic agent for brain tumors would selectively cross the BBB, accumulates in the tumor tissue and be activated from an innocuous prodrug within the tumor. Here we show brain tumor-targeted delivery and therapeutic efficacy of a nanometer-sized prodrug (nanoprodrug) of camptothecin (CPT) to treat experimental glioblastoma multiforme (GBM). The CPT nanoprodrug was prepared using spontaneous nanoemulsification of a biodegradable, antioxidant CPT prodrug and α-tocopherol. The oxidized nanoprodrug was activated more efficiently than nonoxidized nanoprodrug, suggesting enhanced therapeutic efficacy in the oxidative tumor microenvironment. The in vitro imaging of U-87 MG glioma cells revealed an efficient intracellular uptake of the nanoprodrug via direct cell membrane penetration rather than via endocytosis. The in vivo study in mice demonstrated that the CPT nanoprodrug passed through the BBB and specifically accumulated in brain tumor tissue, but not in healthy brain tissue and other organs. The accumulation preferably occurred at the periphery of the tumor where cancer cells are most actively proliferating, suggesting optimal therapeutic efficacy of the nanoprodrug. The nanoprodrug was effective in treating subcutaneous and intracranial tumors. The nanoprodrug inhibited subcutaneous tumor growth more than 80% compared with control. The median survival time of mice implanted with an intracranial tumor increased from 40.5 days for control to 72.5 days for CPT nanoprodrug. This nanoprodrug approach is a versatile method for developing therapeutic nanoparticles enabling tumor-specific targeting and treatment. The nontoxic, tumor-specific targeting properties of the nanoprodrug system make it a safe, low cost, and versatile nanocarrier for pharmaceuticals, imaging agents, and diagnostic agents.

Differential toxicity biomarkers for irinotecan- and oxaliplatin-containing chemotherapy in colorectal cancer

PURPOSE

Oxaliplatin or irinotecan is usually administered jointly with fluoropyrimidines in colorectal cancer patients treated with chemotherapy. Both drugs have different toxicity patterns. Biomarkers for predicting high-risk severe adverse reactions can help select the best treatment.

METHODS

A retrospective analysis of 106 colorectal cancer patients receiving an oxaliplatin-based treatment and 56 receiving an irinotecan-based treatment was performed. One copy number variant (GSTT1) and nine polymorphisms in irinotecan and oxaliplatin metabolism, transport or DNA repair genes (ABCB1, UGT1A1, XRCC1, ERCC1, ERCC2, GSTP1) were genotyped by SNaPshot, polymerase chain reactions' length fragments, or copy number assays.

RESULTS

In irinotecan-treated patients, T allele of ABCB1C1236T SNP was associated with a lower risk of asthenia(OR = 0.047; 95 % CI = 0.004–0.493; P = 0.011) and Tallele of ABCB1 C3435T SNP was associated with a lower risk of diarrhea (OR = 0.177; 95 % CI = 0.034–0.919;P = 0.039), and individuals with two copies of GSTT1 gene had a lower risk for asthenia (OR = 0.093; 95 %CI = 0.011–0.794; P = 0.030). In oxaliplatin-treated patients, carriers of one or two T variants of Asn118Asn ERCC1 SNP had a lower risk for neutropenia(OR = 0.205; 95 % CI = 0.061–0.690; P = 0.01) [corrected].

CONCLUSIONS

These biomarkers could help oncologists select the best treatment by reducing toxicity associated with irinotecan or oxaliplatin in colorectal cancer patients, thus increasing their quality of life.

Nuclear receptors in the multidrug resistance through the regulation of drug-metabolizing enzymes and drug transporters

Chemotherapy is one of the three most common treatment modalities for cancer. However, its efficacy is limited by multidrug resistant cancer cells. Drug metabolizing enzymes (DMEs) and efflux transporters promote the metabolism, elimination, and detoxification of chemotherapeutic agents. Consequently, elevated levels of DMEs and efflux transporters reduce the therapeutic effectiveness of chemotherapeutics and, often, lead to treatment failure. Nuclear receptors, especially pregnane X receptor (PXR, NR1I2) and constitutive androstane activated receptor (CAR, NR1I3), are increasingly recognized for their role in xenobiotic metabolism and clearance as well as their role in the development of multidrug resistance (MDR) during chemotherapy. Promiscuous xenobiotic receptors, including PXR and CAR, govern the inducible expressions of a broad spectrum of target genes that encode phase I DMEs, phase II DMEs, and efflux transporters. Recent studies conducted by a number of groups, including ours, have revealed that PXR and CAR play pivotal roles in the development of MDR in various human carcinomas, including prostate, colon, ovarian, and esophageal squamous cell carcinomas. Accordingly, PXR/CAR expression levels and/or activation statuses may predict prognosis and identify the risk of drug resistance in patients subjected to chemotherapy. Further, PXR/CAR antagonists, when used in combination with existing chemotherapeutics that activate PXR/CAR, are feasible and promising options that could be utilized to overcome or, at least, attenuate MDR in cancer cells.

Pregnane X Receptor (PXR) expression in colorectal cancer cells restricts irinotecan chemosensitivity through enhanced SN-38 glucuronidation

Background

Clinical efficacy of chemotherapy in colorectal cancer is subjected to broad inter-individual variations leading to the inability to predict outcome and toxicity. The topoisomerase I inhibitor irinotecan (CPT-11) is worldwide approved for the treatment of metastatic colorectal cancer and undergoes extensive peripheral and tumoral metabolism. PXR is a xenoreceptor activated by many drugs and environmental compounds regulating the expression of drug metabolism and transport genes in detoxification organs such as liver and gastrointestinal tract. Considering the metabolic pathway of irinotecan and the tissue distribution of Pregnane × Receptor (PXR), we hypothesized that PXR could play a key role in colon cancer cell response to irinotecan.

Results

PXR mRNA expression was quantified by RT-quantitative PCR in a panel of 14 colon tumor samples and their matched normal tissues. PXR expression was modulated in human colorectal cancer cells LS174T, SW480 and SW620 by transfection and siRNA strategies. Cellular response to irinotecan and its active metabolic SN38 was assessed by cell viability assays, HPLC metabolic profiles and mRNA quantification of PXR target genes. We showed that PXR was strongly expressed in colon tumor samples and displayed a great variability of expression. Expression of hPXR in human colorectal cancer cells led to a marked chemoresistance to the active metabolite SN38 correlated with PXR expression level. Metabolic profiles of SN38 showed a strong enhancement of SN38 glucuronidation to the inactive SN38G metabolite in PXR-expressing cells, correlated with an increase of UDPglucuronosyl transferases UGT1A1, UGT1A9 and UGT1A10 mRNAs. Inhibition of PXR expression by lentivirus-mediated shRNA, led to SN38 chemoresistance reversion concomitantly to a decrease of UGT1A1 expression and SN38 glucuronidation. Similarly, PXR mRNA expression levels correlated to UGT1A subfamily expression in human colon tumor biopsies.

Conclusion

Our results demonstrate that tumoral metabolism of SN38 is affected by PXR and point to potential therapeutic significance of PXR quantification in the prediction of irinotecan response. Furthermore, our observations are pharmacologically relevant since many patients suffering from cancer diseases are often exposed to co-medications, food additives or herbal supplements able to activate PXR. A substantial part of the variability observed among patients might be caused by such interactions

Dofequidar fumarate sensitizes cancer stem-like side population cells to chemotherapeutic drugs by inhibiting ABCG2/BCRP-mediated drug export

The ATP-binding cassette (ABC) transporters (ABC-T) actively efflux structurally and mechanistically unrelated anticancer drugs from cells. As a consequence, they can confer multidrug resistance (MDR) to cancer cells. ABC-T are also reported to be phenotypic markers and functional regulators of cancer stem/initiating cells (CSC) and believed to be associated with tumor initiation, progression, and relapse. Dofequidar fumarate, an orally active quinoline compound, has been reported to overcome MDR by inhibiting ABCB1/P-gp, ABCC1/MDR-associated protein 1, or both. Phase III clinical trials suggested that dofequidar had efficacy in patients who had not received prior therapy. Here we show that dofequidar inhibits the efflux of chemotherapeutic drugs and increases the sensitivity to anticancer drugs in CSC-like side population (SP) cells isolated from various cancer cell lines. Dofequidar treatment greatly reduced the cell number in the SP fraction. Estimation of ABC-T expression revealed that ABCG2/breast cancer resistance protein (BCRP) mRNA level, but not the ABCB1/P-gp or ABCC1/MDR-associated protein 1 mRNA level, in all the tested SP cells was higher than that in non-SP cells. The in vitro vesicle transporter assay clarified that dofequidar had the ability to suppress ABCG2/BCRP function. Dofequidar treatment sensitized SP cells to anticancer agents in vitro. We compared the antitumor efficacy of irinotecan (CPT-11) alone with that of CPT-11 plus dofequidar in xenografted SP cells. Although xenografted SP tumors showed resistance to CPT-11, treatment with CPT-11 plus dofequidar greatly reduced the SP-derived tumor growth in vivo. Our results suggest the possibility of selective eradication of CSC by inhibiting ABCG2/BCRP.

MDR1 polymorphism role in patients treated with cetuximab and irinotecan in irinotecan refractory colorectal cancer

The aim of the study was to evaluate the influence of the MDR1 C3435T polymorphism on the therapeutic response in 23 patients treated with cetuximab plus irinotecan for irinotecan refractory liver metastatic colorectal cancer considering their KRAS status. Indeed, irinotecan and its active metabolite (SN-38) are both substrates of P-glycoprotein (P-gp) encoded by MDR1. Patients received cetuximab and irinotecan up to progression. The overall survival was 55% at 10 months. Overall, four patients had an undetermined KRAS status and two patients with mutated KRAS were in progression disease. The response to treatment was observed after 3 months among the 17 wild-type KRAS patients. Two patients presented a progressive disease (1 TT and 1 CT), eight patients had a stable disease (5 CC and 3CT) and five patients had a partial response (3 CC and 2 CT). Importantly, 2 patients (2 TT) were in complete response and still alive 5 years after starting the treatment, which suggests that the combination of wild-type KRAS and MDR1 3435 TT may be a factor of good prognosis. These results suggest that EGFR inhibition by cetuximab may overcome this irinotecan resistance by abrogating drug efflux depending on MDR1 3435 polymorphism. Among patients resistant to irinotecan, it is still possible to use the association of cetuximab plus irinotecan to obtain a complete resection of hepatic metastases that is necessary to improve their survival.

Rhodamine inhibitors of P-glycoprotein: an amide/thioamide "switch" for ATPase activity

We have examined 46 tetramethylrosamine/rhodamine derivatives with structural diversity in the heteroatom of the xanthylium core, the amino substituents of the 3- and 6-positions, and the alkyl, aryl, or heteroaryl group at the 9-substituent. These compounds were examined for affinity and ATPase stimulation in isolated MDR3 CL P-gp and human P-gp-His(10), for their ability to promote uptake of calcein AM and vinblastine in multidrug-resistant MDCKII-MDR1 cells, and for transport in monolayers of MDCKII-MDR1 cells. Thioamide 31-S gave K(M) of 0.087 microM in human P-gp. Small changes in structure among this set of compounds affected affinity as well as transport rate (or flux) even though all derivatives examined were substrates for P-gp. With isolated protein, tertiary amide groups dictate high affinity and high stimulation while tertiary thioamide groups give high affinity and inhibition of ATPase activity. In MDCKII-MDR1 cells, the tertiary thioamide-containing derivatives promote uptake of calcein AM and have very slow passive, absorptive, and secretory rates of transport relative to transport rates for tertiary amide-containing derivatives. Thioamide 31-S promoted uptake of calcein AM and inhibited efflux of vinblastine with IC(50)'s of approximately 2 microM in MDCKII-MDR1 cells.

Modulation of Oral Drug Bioavailability: From Preclinical Mechanism to Therapeutic Application

Currently, more than one fourth of all anticancer drugs are developed as oral formulations, and it is expected that this number will increase substantially in the near future. To enable oral drug therapy, adequate oral bioavailability must be achieved. Factors that have proved to be important in limiting the oral bioavailability are the presence of ATP-binding cassette drug transporters (ABC transporters) and the cytochrome P450 enzymes. We discuss the tissues distribution and physiological function of the ABC transporters in the human body, their expression in tumors, currently known polymorphisms and drugs that are able to inhibit their function as transporter. Furthermore, the role of the ABC transporters and drug-metabolizing enzymes as mechanisms to modulate the pharmacokinetics of anticancer agents, will be reviewed. Finally, some clinical examples of oral drug modulation are discussed. Among these examples are the coadministration of paclitaxel with CsA, a CYP3A4 substrate with P-glycoprotein (P-gp) modulating activity, and topotecan combined with the BCRP/P-gp transport inhibitor elacridar. Both are good examples of improvement of oral drug bioavailability by temporary inhibition of drug transporters in the gut epithelium.

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.

Detection of the MDR1 P-glycoprotein expression and function

Acquired and intrinsic multidrug resistance is the major reason for the failure of anticancer chemotherapy. The most important component of clinical multidrug resistance is mediated by P-glycoprotein (Pgp), an ABC transporter encoded by the MDR1 gene and expressed on the membrane of tumor and normal cells. Sensitive and reproducible detection of Pgp expression and function are critical for the development of new MDR1 drugs and clinical protocols aimed at modulating Pgp-mediated multidrug resistance. The most commonly used methods for detecting Pgp distribution and functional activity have major flaws when used in routine clinical diagnostics. In this chapter, we describe and compare these techniques and introduce a new method for simultaneous detection of Pgp expression and function--the UIC2 Shift assay.

Phase I study of the combination of topotecan and irinotecan in children with refractory solid tumors

PURPOSE

We have shown in xenograft studies that the antitumor activities of topotecan and irinotecan are highly schedule- and dose-dependent, with a high frequency of response at low, protracted dose schedules. Preclinical and clinical data suggest that topotecan and irinotecan have different antitumor activities and mechanisms of resistance, and non-overlapping toxicities, providing a rationale for their combination. Combining both agents may increase the amount of camptothecin delivered to the tumor, without additive toxicity.

METHODS

We conducted a phase I study in children with refractory solid tumors to determine the maximum tolerated dose (MTD) of irinotecan when administered with a targeted systemic exposure (TSE) of topotecan and to define the dose-limiting toxicity (DLT) of this combination. Irinotecan was administered IV over 60 min followed by topotecan over 30 min daily for 5 days for two consecutive weeks. We initially fixed the topotecan-TSE to 80+/-10 ng*h/ml and investigated the ability to escalate irinotecan (starting dose 16 mg/m2/d). Topotecan and irinotecan pharmacokinetics were determined.

RESULTS

Eleven patients (median age 10 years) were enrolled. Owing to DLT, irinotecan was de-escalated to 12 (level -1; n = 3) and 9 (level -2; n = 3) mg/m2/day, and topotecan-TSE was reduced to 60+/-10 ng*h/ml (level -3; n = 2). DLTs were neutropenia (n = 8), typhlitis (n = 5), and skin rash (n = 1). MTD could not be reached. Median (range) irinotecan and topotecan lactone systemic clearances were 50.3 (16.6-76.2) l/h/m2 and 27.6 (14.7-55.9) l/h/m2, respectively. The pharmacokinetics profile of each agent was similar to that seen in previous single agent studies. One patient with neuroblastoma and one with rhabdomyosarcoma had a partial and a complete response, respectively.

CONCLUSION

Despite promising antitumor activity, the combination of topotecan and irinotecan given on a protracted schedule does not warrant further development in children due to unacceptable toxicity.

Irinotecan pharmacokinetic and pharmacogenomic alterations induced by methylselenocysteine in human head and neck xenograft tumors

The combination of methylselenocysteine and irinotecan (CPT-11) is synergistic against FaDu and A253 xenografts. Methylselenocysteine/CPT-11 increased tumor cure rate to 100% in FaDu and to 60% in A253. In this study, the effect of methylselenocysteine on pharmacokinetic and pharmacogenetic profiles of genes relevant to CPT-11 metabolic pathway was evaluated to identify possible mechanisms associated with the observed combinational synergy. Nude mice bearing tumors (FaDu and A253) were treated with methylselenocysteine, CPT-11, and a combination of methylselenocysteine/CPT-11. Samples were collected and analyzed for plasma and intratumor concentration of CPT-11 and 7-ethyl-10-hydroxyl-camptothecin (SN-38) by high-performance liquid chromatography. The intratumor relative expression of genes related to the CPT-11 metabolic pathway was measured by real-time PCR. After methylselenocysteine treatment, the intratumor area under the concentration-time curve of SN-38 increased to a significantly higher level in A253 than in FaDu and was associated with increased expression of CES1 in both tumors. Methylselenocysteine/CPT-11 treatment, compared with CPT-11 alone, resulted in a significant decrease in levels of ABCC1 and DRG1 in FaDu tumors and an increase in levels of CYP3A5 and TNFSF6 in A253 tumors. No statistically significant changes induced by methylselenocysteine/CPT-11 were observed in the levels of other investigated variables. In conclusion, the significant increase in the cure rate after methylselenocysteine/CPT-11 could be related to increased drug delivery into both tumors (CES1), reduced resistance to SN-38 (ABCC1 and DRG1) in FaDu, and induced Fas ligand apoptosis (TNFSF6) in A253. No correlation was observed between cure rate and other investigated variables (transporters, degradation enzymes, DNA repair, and cell survival/death genes) in either tumor.

Mechanisms of resistance to anticancer drugs: the role of the polymorphic ABC transporters ABCB1 and ABCG2

ATP-binding cassette (ABC) genes play a role in the resistance of malignant cells to anticancer agents. The ABC gene products, including ABCB1 (P-glycoprotein) and ABCG2 (breast cancer-resistance protein [BCRP], mitoxantrone-resistance protein [MXR], or ABC transporter in placenta [ABCP]), are also known to influence oral absorption and disposition of a wide variety of drugs. As a 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. Naturally occurring variants in ABC transporter genes have been identified that might affect the function and expression of the protein. This review focuses on recent advances in the pharmacogenetics of the ABC transporters ABCB1 and ABCG2, and discusses potential implications of genetic variants for the chemotherapeutic treatment of 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.

Cytotoxicity, apoptosis induction and downregulation of MDR-1 expression by the anti-topoisomerase II agent, salvicine, in multidrug-resistant tumor cells

Salvicine, a novel topoisomerase II inhibitor and a diterpenoid quinone compound, exerts potent in vitro and in vivo antitumor effects. In our study, we show that salvicine effectively kills multidrug-resistant (MDR) sublines, such as K562/A02, KB/VCR and MCF-7/ADR, and parental K562, KB and MCF-7 cell lines to an equivalent degree. These cytotoxic activities of salvicine were much more potent than those of several classical anticancer drugs (average resistance factor: 1.42 for salvicine vs. 344.35, 233.19 and 71.22 for vincristine, doxorubicin and etoposide, respectively). Flow cytometry and DNA agarose gel electrophoresis demonstrated that salvicine induced similar levels of apoptosis in MDR K562/A02 and parental cells. The compound activated caspase-1 and -3 (but not caspase-8) and increased the ratio of bax to bcl-2 mRNA via reduction of bcl-2 mRNA expression in the same cells. Furthermore, salvicine induced the downregulation of mdr-1 gene and P-gp expression but had no effect on MRP and LRP gene expression in MDR K562/A02 cells. These results suggest that the reduction of mdr-1 and bcl-2 expression by salvicine possibly contributes to its cytotoxicity and apoptotic induction in this system. The effectiveness, broad-spectrum activity and possibly novel mechanism of killing MDR tumor cells in vitro of salvicine signify promising in vivo and clinical activity. The novel chemical structure of this compound further implies a role for salvicine in future MDR tumor therapy.

CPT-11-induced cell death in leukemic cells is not affected by the MDR phenotype

CPT-11 is a topoisomerase I (Topo I) inhibitor which was initially described as active in multi-drug resistance (MDR) tumors. The MDR phenomenon is characterized by the overexpression of efflux pumps which are able to extrude a range of drugs non-related chemical or functionally. In this work, we treated leukemic cells with CPT-11 300 microM at 24h and compared its cytotoxicity with the activity of efflux pumps and with cell cycle phase. Our findings show that CPT-11 has a potent anti-tumor activity in leukemic cells regardless MDR phenotype and the cell cycle phase, suggesting new avenues to be explored in leukemia treatment.

Considerations in the design and development of transport inhibitors as adjuncts to drug therapy

With the realization of the importance of drug efflux transporters in disease processes and treatment, development of inhibitors to these transporters has been sought for use as adjuncts to therapy. To date, inhibitors that have been best studied are modulators of P-glycoprotein, a transporter important in the removal of anticancer agents from cells and overexpression of this transporter results in multidrug resistance. There is a delicate balance between efficacy and toxicity. This review summarizes key learning points in the development of P-glycoprotein inhibitors. Topics covered include specificity of the inhibitor for the target transporter, effect on metabolism of coadministered drugs, pharmacokinetic interactions, toxicity and the salient features needed for efficacy. These points will have general application to the development of inhibitors of transporters.

The activity profile of the hexacyclic camptothecin derivative DX-8951f in experimental human colon cancer and ovarian cancer

DX-8951f or exatecan mesylate ((1S,9S)-1-amino-9-ethyl-5-fluoro-2,3-dihydro-9-hydroxy-4-methyl-1H,12H-benzo[de]pyrano[3',4':6,7]indolizino[1,2-b]quinoline-10-13(9H,15H)-dione methanesulfonate dihydrate), is a new water-soluble derivative of camptothecin. We determined the activity of DX-8951f in experimental human colon cancer and ovarian cancer, being tumor types sensitive to camptothecins. With the use of the MTT assay, DX-8951f was more potent than SN-38 in four out of five human colon cancer cell lines and three out of four human ovarian cancer cell lines (P<0.05). DX-8951f was considerably more potent than topotecan in all cell lines tested (P<0.05). Prolonged exposure to DX-8951f resulted in a greater increase in inhibition of cell proliferation as compared to that obtained with SN-38 or topotecan (P<0.05). Overexpression of Pgp, MRP1, and LRP did not affect the in vitro activity of DX-8951f. DX-8951f administered daily x 5 or weekly x 2 resulted in growth inhibition <50% in two human colon cancer xenografts grown s.c. in nude mice. In three human ovarian cancer xenografts, however, >50% growth inhibition was observed at both schedules. In the OVCAR-3 human ovarian cancer model, DX-8951f showed considerably greater activity than topotecan (P<0.01). DX-8951f combined with cisplatin or paclitaxel did not indicate the presence of a pharmacological interaction. In OVCAR-3 xenografts the combination was clearly more effective than DX-8951f alone, as the number of complete remissions increased substantially. In conclusion, this study shows that DX-8951f is highly potent in vitro and highly effective in experimental human ovarian cancer in vivo. Prolonged exposure to DX-8951f in vitro greatly increased the antiproliferative effects, which may be a rationale for testing a continuous infusion schedule in the clinic. Addition of cisplatin or paclitaxel improved the in vivo antitumor effects of DX-8951f.

Reversal of multidrug resistance-associated protein-mediated daunorubicin resistance by camptothecin

The multidrug-resistance (MR) status of camptothecin (CPT) was investigated in colon adenocarcinoma HT29 cells, leukemia K562, and breast carcinoma MCF7 cells expressing P-glycoprotein (Pgp) and/or MR-associated protein (MRP1). The concentration that induced 50% growth inhibition (IC(50)) against CPT was 0.14 and 0.20 microM in parental K562/WT and MCF7/WT cells, respectively. The drug resistant subline KH30 and MCF7/VP cells, which both overexpress MRP1, presented IC(50) values of 0.63 and 3.10 microM, respectively. The resulting resistance indexes were 3.80 and 12.50, respectively. However, in KH300 cells, a cell line that preferentially overexpresses Pgp, the IC(50) of CPT was 0.08 microM and thus did not exhibit resistance against CPT. In MCF7/DoX cells, preferentially overexpressing Pgp, but also a significant level of MRP1, the IC(50) of CPT was 0.64 microM and thus presented a resistance index of 3.26 against CPT. The cytotoxic effect of CPT was modulated in cells expressing MRP1 (MCF7/VP, HT29 cells) by the specific MRP1 modulators, probenecid and MK571. These results led us to consider CPT as a substrate for MRP1 and a potential modulator of MRP1 activity. To test this hypothesis, we examined the ability of nontoxic concentrations of CPT to sensitize MRP1-overexpressing cells to daunorubicin (DNR). In MCF7/VP and KH30 cells, nontoxic concentrations of CPT were able to enhance cytotoxicity of DNR and its nuclear accumulation. Sequential and simultaneous associations of CPT (100 nM) and DNR provided complete reversal of resistance, thus showing a synergistic effect in KH30 cells. However, simultaneous association (with 10 or 20 nM CPT) had an additive effect in MCF7/VP. These data suggest that CPT could be proposed as a candidate for the reversal of the MRP1 phenotype at clinically achievable concentrations.

Intestinal transport of irinotecan in Caco-2 cells and MDCK II cells overexpressing efflux transporters Pgp, cMOAT, and MRP1

Irinotecan (CPT-11) is a water-soluble camptothecin (CPT) derivative that has been recently approved in the United States for patients as a first-line therapy in advanced colorectal cancer. Phase I clinical trials using oral CPT-11 have shown poor and variable oral bioavailability. The present study was designed to investigate the intestinal absorption and efflux mechanisms of CPT-11 using in vitro cell culture models, Caco-2 cells, and engineered Madine-Darby canine kidney (MDCK) II cells overexpressing P-glycoprotein (Pgp), canalicular multispecific organic anion transporter (cMOAT), and multidrug resistance-associated protein (MRP1). The intestinal absorptive and secretory transport of CPT-11 was investigated using Caco-2 cell monolayers. Secretory transport was concentration-dependent and saturable. The secretory efflux permeability (P(eff)) of CPT-11 decreased with decreasing temperature, with an estimated activation energy of 19.6 +/- 2.9 kcal/mol suggesting the involvement of active transporters. The involvement of potential secretory transporters was further characterized in MDCK II cells. The secretory efflux carrier permeability (P(c)) was approximately 4- and approximately 2-fold greater in MDCK II/Pgp and MDCK II/cMOAT cells than that in MDCK II/wild-type cells. Furthermore, the secretory efflux P(eff) of CPT-11 was significantly decreased by Pgp inhibitors, elacridar (GF120918) (IC50 = 0.38 +/- 0.06 microM) and verapamil (IC(50) = 234 +/- 48 microM) in MDCK II/Pgp cells and by cMOAT inhibitor 3-([(3-(2-[7-chloro-2-quinolinyl]ethyl)phenyl]-[(3-dimethylamino-3-oxoprphyl)-thio)-methyl]-thio) propanoic acid (MK571) (IC50) = 469 +/- 60 micro;M) in MDCK II/cMOAT cells. Overall, the current study suggests that Pgp and cMOAT are capable of mediating the efflux of CPT-11 in vitro. Since both Pgp and cMOAT are expressed in the intestine, liver, and kidney, it is likely that these efflux transporters play a significant role limiting the oral absorption and disposition of this important anticancer drug.

Homocamptothecin-daunorubicin association overcomes multidrug-resistance in breast cancer MCF7 cells

The multidrug-resistance (MDR) status of a novel camptothecin analogue, homocamptothecin (hCPT), was investigated in human colon adenocarcinoma HT29 cells, myelogenous leukemia K562 cells and breast carcinoma MCF7 cells. The cytotoxicity of hCPT was not sensitive to the MDR status in K562 cell lines. However, its cytotoxicity was altered by MRP1, but not Pgp, in naturally MRP1-expressing HT29 cells, and etoposide- and doxorubicin-resistant MCF7/VP and MCF7/DOX cells, respectively. These cells were sensitized to hCPT in presence of MK571, probenecid but not verapamil. These results led to consider hCPT as a substrate for MRP1 and a potential modulator of MRP1 activity. The relationship between the cytotoxic effect of anthracyclines and their nuclear localization had been previously demonstrated. We show that MRPI mediated the daunorubicin (DNR) efflux in MCF7/VP and MCF7/DOX cells. The combination of sub-toxic doses of hCPT with DNR resulted in the potentiation of DNR activity, well-correlated with an increase in its nuclear accumulation in MCF7/VP cells. Simultaneous pattern was shown to provide higher cytotoxic response than sequential one. In agreement, hCPT increased also the DNR nuclear accumulation in low MRP1-expressing MCF7/DOX cells. However, the enhancement of cytotoxicity in the DNR-hCPT combination was poorly correlated with the nuclear concentration of DNR in MCF7/DOX cells. In addition to the increase in DNR accumulation, the potentiation of DNR activity by hCPT in MCF7/DOX cells implied a synergistic mechanism between both drugs. These data suggest that the present topoisomerase I/II inhibitors combination may be of clinical interest to overcome MDR phenotype in DNR-treated breast cancer patients.

Irinotecan in the treatment of colorectal cancer: clinical overview

PURPOSE AND METHODS

For more than three decades, the therapeutic options for patients with advanced colorectal cancer have almost exclusively been based on fluoropyrimidines. With the recognition that topoisomerase-I (TOP-I) is an important therapeutic target in cancer therapy, irinotecan, a semisynthetic TOP-I-interactive camptothecin derivative, has been clinically established in the treatment of colorectal cancer.

RESULTS

Irinotecan was investigated as second-line chemotherapy after prior treatment with fluorouracil (FU)-based regimens in two large randomized phase III trials comparing irinotecan with either best supportive care or an infusional FU/leucovorin (LV) regimen. The outcomes of these trials established irinotecan as the standard therapy in the second-line treatment of colorectal cancer. The therapeutic value of irinotecan in the first-line treatment of metastatic colorectal cancer was investigated in two large randomized phase III trials comparing the combination of irinotecan and FU/LV with FU/LV alone. Both trials demonstrated significant superior efficacy for the combination of irinotecan and FU/LV in terms of response rate, median time to disease progression, and median survival time. Consequently, the combination of irinotecan and FU/LV has been approved as first-line chemotherapy for patients with metastatic colorectal cancer and constitutes the reference therapy against which other treatment options must be tested in the future.

CONCLUSION

In this review, the clinical rationale and update of the present clinical status of irinotecan in the treatment of colorectal cancer and future prospects of irinotecan-based combinations are discussed.

The clinical development of 9-aminocamptothecin

9-Aminocamptothecin (9-AC) is a topoisomerase I-targeting agent first synthesized by Wani and Wall in 1986. Because of its potent in vitro effects and promising preclinical activity in colorectal cancer animal models, it was designated a high-priority compound for further drug development by the NCI. In 1993, 9-AC first entered clinical trials as a 72-hour intravenous (i.v.) infusion. Predictable myelosuppression was the major dose-limiting toxicity, and pharmacokinetic studies showed a relatively short plasma half-life and unstable lactone ring. Unfortunately, phase II studies using this schedule showed minimal or no activity in tumors such as colorectal and lung cancer. Modest activity was observed in ovarian cancer and in refractory lymphomas. Efforts to improve systemic drug exposure by utilizing alternative schedules of administration of 9-AC such as prolonged, continuous intravenous infusions have also been tested. However, phase II studies of 120-hour weekly infusions of 9-AC have not shown improved activity against solid tumors such as colorectal cancer. More recently, a daily times 5 days i.v. administration schedule has been tested. Currently, further development of intravenously administered 9-AC for the treatment of colorectal cancer is not promising. Thus, topotecan and irinotecan remain the only two successfully developed topoisomerase I-targeting drugs in the United States. This experience with 9-AC raises important questions regarding how to best select new topoisomerase I-targeting drugs for future clinical development.

New highly lipophilic camptothecin BNP1350 is an effective drug in experimental human cancer

BNP1350, 7-[(2-trimethylsilyl)ethyl]-20(S)-camptothecin, is a novel semi-synthetic, highly lipophilic, silicon-containing camptothecin and an inhibitor of topoisomerase I. It has been supercomputer engineered for superior oral bioavailability, superior lactone stability, broad anti-tumor activity, increased potency and insensitivity to Pgp/MRP/LRP drug resistance. We determined the efficacy of BNP1350 in experimental human colon cancer and compared its anti-tumor effects with those of CPT-11/SN-38. We also determined a possible influence of Pgp, MRP and LRP on the efficacy of BNP1350. The in vitro anti-proliferative capacity of the compounds using various exposure times was assessed in five colon cancer cell lines and indicated that BNP1350 was similarly effective or slightly more potent than SN-38. Four cell lines of other origin with sublines expressing Pgp, MRP and/or LRP showed that BNP1350 was significantly more effective than SN-38 (p < 0.05) and that the activity of BNP1350 was not reduced in multidrug-resistant cells. For in vivo experiments, BNP1350 was given 1.0 mg/kg i.p. or 1.5 mg/kg p.o. daily x 5 and CPT-11 20 mg/kg i.p. daily x 5 being equitoxic schedules in nude mice bearing s.c. human tumor xenografts. The schedules were studied in colon cancer xenografts COLO320, COLO205 or WiDr as well as in two Pgp-positive xenografts 2780AD and BRO/mdr1.1 and the parental Pgp-negative A2780 ovarian cancer xenografts and BRO melanoma xenografts. Growth inhibition of >50% was obtained for BNP1350 given i.p. in six out of the seven xenografts studied. BNP1350 was similarly effective when given i.p. or p.o. CPT-11 was as effective as BNP1350, except in BRO and BRO/mdr1.1 xenografts. Pgp expression in xenografts in vivo confirmed that there was no negative influence on the efficacy of BNP1350. In conclusion, BNP1350 shows a broad spectrum of activity in experimental human tumors and is a suitable candidate for oral treatment of cancer.

Clinical use of topoisomerase I inhibitors in anticancer treatment

The camptothecin analogs topotecan and irinotecan have shown to be among the most effective anticancer agents and, as S-phase specific agents, their antitumor effect is maximized when they are administered in protracted schedules. The documented activity as single agents in many adult and pediatric malignancies has been followed by their use in combination with other anticancer agents. These studies have shown promising results, and have placed topotecan and irinotecan in the first line treatment for some malignancies. However, studies to better determine the optimal schedules and sequence of combinations are needed.

The oral route for the administration of cytotoxic drugs: strategies to increase the efficiency and consistency of drug delivery

There is an increasing interest to administer cytotoxic drugs to patients by the oral route. Quality of life issues, treatment advantages and pharmaco-economics are major arguments in favor of oral therapy. However, low or moderate bioavailability in combination with considerable interpatient variability are frequently observed which may reduce the feasibility of the oral route for this class of drugs with a generally narrow therapeutic window. Until recently, investigators focused on absorption enhancers which slightly damage the intestinal surface such as salicylates, methylxanthines and surfactants to improve the oral bioavailability of drugs. To date, a shift can be seen towards more subtle mechanisms to enhance the absorption. This review article focuses on two important mechanisms that determine the oral bioavailability of cytotoxic drugs. These include the presence of drug transporters in the intestinal epithelium pumping drugs into the intestinal lumen, such as MDR1 type P-glycoproteins, and first-pass elimination by cytochrome P450 isoenzymes (e.g. 3A4 and 3A5) or other enzymes in the intestines and/or liver. Currently preclinical and clinical studies are being performed to explore the feasibility of blocking these transporters/enzymes in order to achieve higher and less variable systemic drug levels after oral dosing. This review gives an update of the results of these studies. It is concluded however, that further research to unravel the processes involved in oral drug uptake is warranted to make the oral route a more efficient and consistent way of drug administration.

Sensitivity to CPT-11 of xenografted human colorectal cancers as a function of microsatellite instability and p53 status

Biological parameters influencing the response of human colorectal cancers (CRCs) to CPT-11, a topoisomerase 1 (top1) inhibitor, were investigated using a panel of nine CRCs xenografted into nude mice. CRC xenografts differed in their p53 status (wt or muf) and in their microsatellite instability phenotype (MSI+ when altered). Five CRC xenografts were established from clinical samples. All five had a functional p53, two were MSI+ and three were MSI-. Tumour-bearing nude mice were treated intraperitonealy (i.p.) with CPT-11. At 10 mg kg(-1) of CPT-11, four injections at 4-day intervals, four of the five xenografts responded to CPT-11 (growth delay of up to 10 days); the non-responder tumour was MSI-. At 40 mg kg(-1) of CPT-11, six injections at 4-day intervals, the five CRCs displayed variable but marked responses with complete regressions. In order to assess the role of p53 status in CPT-11 response, four CRC lines were used. HT29 cell line was MSI-/Ala273-mutp53, its subclone HT29A3 being transfected by wtp53. LoVo cell line was MSI+/wtp53, its subclone X17LoVo dominantly expressed Ala273-mutp53 after transfection. LoVo tumours (MSI+/mutp53) were more sensitive than X17LoVo (MSI+/mutp53. HT 29 tumours (MSI-Imutp53), were refractory to CPT-11 while HT29A3 tumours (MSI-/wtp53) were sensitive, showing that wtp53 improves the drug-response in these MSI- tumours. Levels of mRNA expression of top1, fasR, TP53 and mdr1 were semi-quantified by reverse transcription polymerase chain reaction. None of these parameters correlated with CPT-11 response. Taken together, these observations indicate that MSI and p53 alterations could be associated with different CPT-11 sensitivities; MSI phenotype moderately influences the CPT-11 sensitivity, MSI+ being more sensitive than MSI(-)CRC freshly obtained from patients, mutp53 status being associated with a poor response to CPT-11.

Reversal of MDR1-associated resistance to topotecan by PAK-200S, a new dihydropyridine analogue, in human cancer cell lines

Recent data suggest that expression of the membrane P170-glycoprotein (P-gp) may confer resistance to the topoisomerase-I-interactive agent topotecan. The present study describes the cellular effects of a new dihydropyridine analogue, PAK-200S, on P-gp-mediated resistance to topotecan in human breast and ovarian tumour cells. PAK-200S at a non-cytotoxic concentration of 2.0 microM completely reversed resistance to topotecan in P-gp-expressing MCF-7/adr (breast) and A2780/Dx5 (ovarian) tumour cells, respectively, with no effects on parental cells. Cellular pharmacokinetic studies by reversed-phase high-performance liquid chromatography analysis showed significantly lower cellular drug concentrations of the pharmacologically active closed-ring lactone of topotecan in multidrug-resistant cells than in parental cells. PAK-200S was effective in restoring the cellular lactone concentrations of topotecan in resistant MCF-7/adr cells to levels comparable to those obtained in parental cells. Furthermore, exposure of MCF-7/adr cells to topotecan in the presence of PAK-200S significantly increased the induction of protein-linked DNA breaks. PAK-200S did not alter nuclear topoisomerase I-mediated ex vivo pBR322 DNA plasmid unwinding activity and topoisomerase-I protein expression. These results suggest that reversal of P-gp-mediated resistance to topotecan by PAK-200S was related to the restoration of cellular drug concentrations of the active lactone form of topotecan rather than a direct effect on topoisomerase-I function.

Topoisomerase I inhibitors: selectivity and cellular resistance

Topoisomerase I (top1) inhibitors (camptothecins and other structurally diverse compounds) are effective and promising anticancer agents. Determinants of selectivity toward cancer cells and resistance are multifactorial. These factors can be separated in three groups. The first is related to alterations in drug distribution and metabolism. The second group includes both quantitative and qualitative (mutations) differences in top I. The third group includes resistance and sensitivity factors downstream from the cleavage complexes. They include DNA repair, cell cycle checkpoints and apoptosis, and are probably key to the relative selectivity of camptothecins toward cancer cells and to clinical resistance. Copyright 1999 Harcourt Publishers Ltd.

Topoisomerase-I inhibitors in gynecologic tumors

The first section of this article reviews recent studies that have clarified both the cellular role of topoisomerase I and the mechanisms of cytotoxicity of the topoisomerase inhibitors, the camptothecins. Different analogs of this new class of antitumor drug have been studied using various dose schedules in the treatment of refractory or recurrent gynecologic cancer. Response rates are between 13% and 25%. The main toxic effects are hematologic and gastrointestinal, the latter remains problematic. Radiotherapy, alkylate, platinum analogues, and topoisomerase II inhibitors are currently being studied in combination with camptothecins.

Determinants of CPT-11 and SN-38 activities in human lung cancer cells

Irinotecan (CPT-11) is a semisynthetic camptothecin derivative with a broad spectrum of anti-tumour activity. Carboxylesterase (CE) catalyses the conversion of CPT-11 to SN-38 (7-ethyl-10-hydroxycamptothecin), the active form of CPT-11. The antiproliferative effects of CPT-11 and SN-38, CE-activity and topoisomerase I protein expression were investigated in five human small-cell lung cancer (SCLC) cell lines and four human non-small-cell lung cancer (NSCLC) cell lines. Antiproliferative activity, expressed as IC50 values, was determined using the MTT assay. CPT-11 was significantly more active in SCLC than in NSCLC cell lines (P = 0.0036), whereas no significant difference between histological types was observed with SN-38. A significant correlation (r2 = 0.52, P = 0.028) was observed between CE activity and chemosensitivity to CPT-11 but not to SN-38, and significantly higher CE activity was observed in SCLC compared with NSCLC cell lines (P = 0.025). Western blotting experiments showed topoisomerase I protein expressions within a factor of 2, and a granular nuclear staining was detectable in all cell lines by immunocytochemistry of cytospins. No correlation was observed between protein expression and sensitivity to CPT-11 or SN-38. Cellular and medium concentrations of CPT-11 and SN-38 were measured by high-performance liquid chromatography (HPLC) in one SCLC cell line with high CE activity and high sensitivity to CPT-11, and one NSCLC cell line with low sensitivity to CPT-11 and CE activity. Intracellular concentrations of CPT-11 and SN-38 were higher in the SCLC cell line, and this was associated with an increase in cellular uptake of CPT-11 compared with the medium, and an increased intracellular formation of SN-38. In conclusion, CE activity appears to be associated with higher sensitivity to CPT-11 in human lung cancer cell lines and may partly explain the difference in the in vitro sensitivity to CPT-11 between SCLC and NSCLC cells. The assessment of CE activity in clinical material of lung cancer patients undergoing treatment with CPT-11 may be warranted. However, other mechanisms may influence sensitivity to CPT-11, possibly including drug transport.