Imatinib disposition and ABCB1 (MDR1, P-glycoprotein) genotype.

The aim of this study was to explore the impact of individual variation in drug elimination on imatinib disposition. Twenty-two patients with gastrointestinal stromal tumor or chronic myeloid leukemia initially received imatinib 600 mg daily with dosage subsequently toxicity adjusted. Pharmacokinetic parameters on day 1 and at steady-state were compared with elimination phenotype and single-nucleotide polymorphisms of CYP3A5 and ABCB1. A fivefold variation in estimated imatinib clearance (CL/F) was present on day 1 and mean CL/F had fallen by 26% at steady state. This reduction in imatinib CL/F was associated with ABCB1 genotype, being least apparent in thymidine homozygotes at the 1236T>C, 2677G>T/A and 3435C>T loci. Toxicity-related dose reduction also tended to be less common in these individuals. ABCB1 genotype was associated with steady-state CL/F due to an apparent genotype-specific influence of imatinib on elimination. Further evaluation of ABCB1 genotype and imatinib dosage is warranted.

Kinetics of the in vivo interconversion of the carboxylate and lactone forms of irinotecan (CPT-11) and of its metabolite SN-38 in patients.

The kinetics of the in vivo interconversion of the carboxylate and lactone forms of the prodrug irinotecan, 7-ethyl-10-[4-(1-piperidino)-1- piperidino]carbonyloxycamptothecin (CPT-11), and its active metabolite SN-38 were studied in five patients using a HPLC method that allows the simultaneous determination of all four compounds and detects any hydrolysis of lactones due to inadequate sample handling and storage. The apparent conversion of CPT-11 lactone to the carboxylate in vivo was rapid with a mean half-life of 9.5 min; the carboxylate became the predominant form of plasma CPT-11 soon after the end of the infusion. The ratio of the area under the plasma concentration-time curves of the lactone to total CPT-11 was 36.8 +/- 3.5% (SD). In contrast, SN-38 was present predominantly as the lactone at all times and with little interpatient variability (lactone/total area under the plasma concentration-time curve ratio, 64.0 +/- 3.4%). This may explain in part the promising activity of CPT-11 because CPT derivatives are active against their target, topoisomerase I, only in their lactone form.

Identification and properties of a major plasma metabolite of irinotecan (CPT-11) isolated from the plasma of patients.

Irinotecan [7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin (CPT-11)] is a promising water-soluble analogue of camptothecin [S. Sawada et al., Chem. & Pharm. Bull. (Tokyo), 39: 1446-1454, 1991]. We have reported previously the presence of an important polar metabolite, in addition to 7-ethyl-10-hydroxycamptothecin (SN-38) beta-glucuronide, in samples of plasma taken from patients undergoing treatment with CPT-11 (L.P. Rivory and J. Robert, Cancer Chemother. Pharmacol. 36: 176-179, 1995; L. P. Rivory and J. Robert, J. Cromatogr., 661: 133-141, 1994). Plasma samples (0.5 ml) containing comparatively large amounts of this metabolite were extracted by solid-phase columns and subjected to high-performance liquid chromatography and mass spectrometry in parallel to fluorometric detection. The metabolite yielded [M + 1] ions with a m/z of 619, representing the addition of 32 atomic mass units to CPT-11. Purified fractions were subjected to proton nuclear magnetic resonance, and the structure determined, 7-ethyl-10-[4-N-(5-aminopentanoic acid)-1-piperidino]carbonyloxycampothecin (APC), was further validated following synthesis. Like CPT-11, APC was found to be only a weak inhibitor of the cell growth of KB cells in culture (IC50, 2.1 versus 5.5 micrograms/ml for CPT-11 and 0.01 microgram/ml for SN-38, the active metabolite of CPT-11) and was a poor inducer of topoisomerase I DNA-cleavable complexes (100-fold less potent than SN-38). In contrast to CPT-11, APC was not hydrolyzed to SN-38 by human liver microsomes or purified human liver carboxylesterase. Furthermore, APC did not inhibit the hydrolysis of CPT-11 in these preparations. Interestingly, APC was only a weak inhibitor of acetylcholinesterase in comparison to CPT-11 and neostigmine. It appears likely, therefore, that APC does not contribute directly to the activity and toxicity profile of CPT-11 in vivo.

Molecular, cellular, and clinical aspects of the pharmacology of 20(S)camptothecin and its derivatives.

The discovery of the plant alkaloid 20(S)camptothecin (CPT), which displayed potent antitumor activity in preclinical trials, has led to the identification of a novel target of cancer chemotherapy: the nuclear enzyme topoisomerase I. The mechanism by which CPT induces cytotoxicity is the topic of continued research, but appears to be mediated by the stabilisation of transient "cleavable" topoisomerase I-DNA complexes. The pharmacology of CPT and its derivatives is complicated by the apparent requirement of an alpha-hydroxy-delta-lactone ring, which, unfortunately, is hydrolysed reversibly to form inactive carboxylates. Recent research has shown that the extent of hydrolysis in vivo varies between the various derivatives and that this may be an important factor in determining antitumoral activity. In this review, we discuss recent developments in our understanding of the molecular, cellular, and clinical pharmacology of CPT and several of the more promising derivatives.

Pharmacokinetic interrelationships of irinotecan (CPT-11) and its three major plasma metabolites in patients enrolled in phase I/II trials.

Irinotecan (CPT-11) is an analogue of 20(S)-camptothecin with promising activity against several tumor types. In patients, CPT-11 is metabolized to 7-ethyl-10-hydroxycamptothecin (SN-38) and to the beta-glucuronide of SN-38. Recently, we identified an additional metabolite of CPT-11, 7-ethyl-10-[4-N-(5-aminopentanoic acid)-1-piperidino] carbonyloxycamptothecin (APC; L. P. Rivory et al. , Cancer Res., 56: 3689-3694, 1996). The aim of this study was to investigate the interrelationships of all four compounds to identify factors that might be responsible for the large interpatient variability in CPT-11 and SN-38 kinetics. The plasma kinetics of CPT-11, SN-38, the beta-glucuronide of SN-38, and APC were studied in 19 patients for a total of 33 cycles (115-600 mg/m2). Although the area under the concentration curves (AUCs) of all compounds studied increased with dose, there was considerable variability. Ratios of the AUCs of the appropriate compounds were used as estimates of the major routes of metabolism (conversion of CPT-11 to SN-38, metabolism of CPT-11 to APC, and glucuronidation of SN-38). Each ratio varied more than 10-fold across the patient population, and the apparent extent of conversion of CPT-11 to SN-38 was highest at the 115 mg/m2 dose level. Interestingly, AUCSN-38 was greater in patients with both high AUCCPT-11 and AUCAPC. We conclude that the variability of the pharmacokinetics of CPT-11 and SN-38 is likely to be due to extensive interpatient differences in the pathways implicated in the metabolism of CPT-11.

Optimizing the erythromycin breath test for use in cancer patients.

The erythromycin breath test (EBT) is a putative in vivo probe for drug metabolism by cytochrome P450 3A4 (CYP3A4). Because many anticancer drugs are metabolized by this system, we sought to further develop the EBT as a tool for predicting the clearance, in cancer patients, of drugs metabolized by CYP3A4. Sixteen adult patients with incurable cancer were studied. The EBT was performed on day 1 and breath sampled after the i.v. injection of 4 microCi of 14C-erythromycin. The breath 14CO2 flux (CERt) was estimated at 11 time points over 2 h. On day 2, the EBT was repeated midway through a 10-min infusion of 100 mg of erythromycin lactobionate, and the plasma pharmacokinetics of erythromycin were determined. The infusion of 100 mg of erythromycin did not modify the EBT results significantly. The values of the conventional EBT parameter CER20 min obtained on day 1 were comparable for most subjects (0.03-0.06% dose/min), with the exception of an individual receiving the known CYP3A4 inducers dexamethasone and phenytoin who returned a value of 0.14% dose/min. There was no significant correlation between any of the conventional EBT parameters and erythromycin clearance. However, two parameters reflecting early emergence of breath radioactivity (1/TMAX and CER3 min/CERMAX) correlated significantly with erythromycin clearance (P = 0.005 and 0.006, respectively). Novel parameters derived from the EBT are significantly correlated with the clearance of erythromycin even in the presence of confounding factors, such as metastatic liver disease, altered protein binding, and comedication. These parameters may enable dose optimization of cytotoxics metabolized by CYP3A4.

The effects of aging and nutritional state on hypoxia-reoxygenation injury in the perfused rat liver.

During liver transplantation, donor livers are subject to hypoxia and reoxygenation. Recently, because of a shortage of suitable donors, the livers of older donors have been used for transplantation. In this study, we examined the influence of aging and nutritional state on hepatic intracellualr pH (pHi) and the susceptibility of the rat liver to hypoxia-reoxygenation injury. Perfused livers from fasted and fed, young (2-3 months) and aged (24-28 months) male Wistar rats were compared during 30 min of nitrogen hypoxia followed by 20 min of reoxygenation. Under control conditions, pHi was significantly lower and glucose release higher in livers from fed young rats (7.24 +/- 0.04, 3.4 +/- 1.8 mumol/min/g) than in those from fasted young (7.33 +/- 0.04, 0.0 +/- 1.3), fasted aged (7.32 +/- 0.03, -0.1 +/- 0.5), and fed aged rats (7.29 +/- 0.06, 0.9 +/- 1.0). In all groups, pHi fell by approximately 0.15 U during hypoxia. Lactate dehydrogenase (LDH) release from the livers of fed young livers was unaffected by hypoxia-reoxygenation as was that from the livers of fed aged rats, despite features that would be expected to predispose to injury (reduced glucose output and more alkaline pHi). In contrast, livers from fasted young and aged rats had substantially increased LDH release and reduced bile flow during hypoxia. There were no age-related differences in these parameters, but, during reoxygenation, LDH release was significantly less in the aged livers. These results indicate that the livers of fed rats are resistant to hypoxia-reoxygenation injury and that aging does not increase the susceptibility of the liver to injury in the fasted state.

Clinical pharmacokinetics of docetaxel.

Docetaxel (Taxotere), a semi-synthetic analog of paclitaxel (Taxol), is a promoter of microtubule polymerization leading to cell cycle arrest at G2/M, apoptosis and cytotoxicity. Docetaxel has significant activity in breast, non-small-cell lung, ovarian and head and neck cancers. Docetaxel has undergone phase I study in a number of schedules, including different infusion durations and various treatment cycles. Doses studied in adults have ranged from 5 to 145 mg/m2 and those in children from 55 to 235 mg/m2. The most frequently used regimen in adults is 100 mg/m2 every 3 weeks. A 1-hour infusion every 3 weeks has been favoured in phase II and III studies, and the disposition of docetaxel after such treatment is best described by a 3 compartment model with alpha, beta and gamma half-lives of 4.5 minutes, 38.3 minutes and 12.2 hours, respectively. The disposition of docetaxel appears to be linear, the area under the plasma concentration-time curve (AUC) increasing proportionately with dose. Docetaxel is widely distributed in tissues with a mean volume of distribution of 74 L/m2 after 100 mg/m2, every 3 weeks. The mean total body clearance after this schedule is approximately 22 L/h/m2, principally because of hepatic metabolism by the cytochrome P450 (CYP)3A4 system and biliary excretion into the faeces. Renal excretion is minimal (< 5%). Docetaxel is > 90% bound in plasma. Population pharmacokinetic studies of docetaxel have demonstrated that clearance is significantly decreased with age, decreased body surface area, increased concentrations of alpha 1-acid glycoproteinand albumin. Importantly, patients with elevated plasma levels of bilirubin and/or transaminases have a 12 to 27% decrease in docetaxel clearance and should receive reduced doses. Although docetaxel is metabolised by CYP3A4, phase I combination studies have not shown major evidence of significant interaction between docetaxel and other drugs metabolised by the same pathway. Nevertheless, care should be taken with the use of known CYP3A4 inhibitors such as erythromycin, ketoconazole and cyclosporin. Conversely, increased doses may be required for patients receiving therapy known to induce this cytochrome (e.g. anticonvulsants). Perliminary data suggest the erythromycin breath test, an indicator of CYP3A4 function, is a predictor of toxicity after treatment with docetaxel. Such methodologies may eventually enable clinicians to individualise doses of docetaxel for patients with cancer.

Clinical and preclinical pharmacokinetics of raltitrexed.

Raltitrexed is a specific, folate-based inhibitor of thymidylate synthase with activity in advanced colorectal cancer comparable with that of fluorouracil (5-fluorouracil) plus folinic acid. Its activity is enhanced by rapid cellular entry and polyglutamation, with the polyglutamated derivatives having approximately 100-fold greater inhibitory potency than the parent compound. A number of phase I/pharmacokinetic studies have been performed, including schedules involving a 15-minute infusion every 3 weeks, weekly x 6 every 8 weeks, and every 2 weeks. The maximum tolerated dose (MTD) for the 3-weekly schedule was 3.5 to 4.5 mg/m2 in adults and 6 mg/m2 in a paediatric population. The MTDs for the other schedules have not yet been reported. The disposition of raltitrexed in patients is best described by a 3-compartment model with a terminal half-life (t1/2gamma) of 260 hours, the latter being subject to significant interpatient variability. A similar protracted t1/2gamma has been detected in all of the animal species studied. Together with evidence from the mass-balance studies performed, this delayed elimination suggests considerable sequestration of raltitrexed in tissues, predominantly as polyglutamate forms. Nevertheless, there has been no pharmacokinetic evidence of drug accumulation in plasma following repeated administration. On the basis of animal experiments, the oral bioavailability and penetration of raltitrexed into cerebrospinal fluid are both likely to be limited in the clinical setting. Raltitrexed is over 90% bound to plasma protein over the concentration range of 20 to 100 micromol/L. Apart from poly-glutamation, raltitrexed does not appear to be metabolised to a significant extent, and most of the excreted drug (approximately 20% of the administered dose) is recovered unchanged in the urine within the first 24 hours post-administration. The average clearance of raltitrexed is 2.4 L/h (40 ml/min), and this value is significantly reduced in patients with compromised renal function (glomerular filtration rate of 25 to 65 ml/min). These patients are more likely to experience severe antiproliferative toxicity with raltitrexed. A careful evaluation of renal function, particularly in the elderly, is warranted. It has not been possible to establish strong correlations between the plasma pharmacokinetics of raltitrexed and toxicity, and the cellular pharmacokinetics of raltitrexed may be more predictive. Studies in mice have demonstrated that delayed administration of folinic acid can assist in the recovery of animals from antiproliferative toxicity, possibly by promoting the release of polyglutamated drug from tissues. This approach should be evaluated as a rescue regimen in patients with severe proliferative toxicity.

The erythromycin breath test for the prediction of drug clearance.

The erythromycin breath test (EBT) is a putative probe of cytochrome P450 (CYP) 3A4 activity in vivo. Therefore, the EBT might prove useful for the individualisation of doses of drugs that have a low therapeutic window (for example the immunosuppressants or cytotoxics) and are metabolised by CYP3A4. However, there is a lack of consensus as to how the EBT should be used to predict total body clearance (CL), and the results so far have been largely disappointing. We argue that the required assumption that individuals produce 5 mmol of CO2/min per m2 at rest is one of the problems with the existing EBT, as the literature suggests significant variability and possible gender differences in this parameter. An examination of the EBT with a simple compartment model suggests that alternative parameters could be more useful in the prediction of CL. In particular, there is theoretical support for the use of the time-point at which breath radioactivity is maximal (tmax) as a correlate for CL. This is in agreement with our recent study of the pharmacokinetics of erythromycin in patients with cancer.

Conversion of irinotecan (CPT-11) to its active metabolite, 7-ethyl-10-hydroxycamptothecin (SN-38), by human liver carboxylesterase.

We have investigated the conversion of the novel anti-topoisomerase I agent CPT-11 (irinotecan; 7-ethyl-10[4-(1-piperidino)-1-piperidno]carbonyloxycamptothecin ) to its active metabolite, SN-38 (7-ethyl-10-hydroxycamptothecin), by human liver carboxylesterase (HLC). Production of SN-38 was relatively inefficient and was enzyme deacylation rate-limited with a steady-state phase occurring after 15-20 min of incubation. This later phase followed Michaelis-Menten kinetics with an apparent Km of 52.9 +/- 5.9 microM and a specific activity of 200 +/- 10 mumol/sec/mol. However, the total enzyme concentration estimated from the intercept concentrations of SN-38 was much lower than that estimated directly from the titration of active sites with paraoxon (0.65 vs. 2.0 microM, respectively). Because deacylation rate-limiting kinetics result in the accumulation of inactive acyl-enzyme complex, we postulated that incubation of CPT-11 with HLC would result in an inhibition of the HLC-catalysed hydrolysis of p-nitrophenylacetate (p-NPA), an excellent substrate for this enzyme. Indeed, this was found to be the case although complete inhibition could not be attained. Analysis of possible kinetic schemes revealed that the most likely explanation for the disparity in estimated enzyme concentrations and the incomplete inhibition of p-NPA hydrolysis is that CPT-11 also interacts at a modulator site on the enzyme, which profoundly reduces substrate hydrolysis. Furthermore, loperamide, a drug often used for the treatment of CPT-11-associated diarrhea, was found to inhibit both CPT-11 and p-NPA HLC-catalysed hydrolysis, most likely by a similar interaction. These observations have direct implications for the clinical use of CPT-11.

The influence of pH on the interaction of lipophilic anthracyclines with bovine serum albumin. Quantitative characterization by measurement of fluorescence quenching.

We have investigated the interaction of the lipophilic anthracyclines 4'-iodo-4'-deoxydoxorubicin (IDX) and 4-demethoxy-daunorubicin (DDN) with bovine serum albumin by the quantitation of fluorescence quenching. The protein binding of IDX was extremely sensitive to the pH of the solution in which the complex was formed and paralleled the effect of pH on dimerization of the drug. The effect of pH on the protein binding and self-association of DDN was less extensive. Both compounds exhibited curvilinear Scatchard plots indicating apparent cooperativity in the binding process. Because of the self-association of the drugs in aqueous solution, we attempted to resolve this cooperativity in terms of the preferential binding of the dimer to the acceptor. However, we found that similar Scatchard plots could be simulated by using slightly erroneous estimates of the fluorescence yield of the complex, rendering any such analysis inconclusive. Consequently, the relationship between acceptor concentration and the fraction of ligand bound was considered to be fitted adequately in terms of a single acceptor site per albumin molecule. The pH dependence of the association constants for bovine serum albumin was described best by the hydrophobic interaction of neutral drug monomer with a binding site with titratable affinity. We postulate that the pH-dependent binding of some anthracyclines with albumin may lead to their enhanced uptake, relative to that of non-target organs, into tumours with an acidotic extracellular milieu.

Carbon monoxide disposition and permeability-surface area product in the foetal circulation of the perfused term human placenta.

In order to estimate the placental barrier to gas transfer, a novel carbon monoxide (CO) wash-in method was used to estimate the permeability-surface area (PS) product for the transfer of gas across the foetal circulation in the perfused human term placenta. The PS product for CO was 0.0096+/-0.006 ml/s/g or 0.012+/-0.007 ml/s/g using compartmental or Crone-Renkin analysis, respectively. Using this result and a published estimate of the placental capillary surface area, the permeability coefficient to CO across the foetal circulation was found to be approximately 4 x 10(-5)cm/s. This result is compatible with the hypothesis that the foetal circulation of the human placenta imposes a potentially significant barrier to gas transfer.

Metabolism of CPT-11. Impact on activity.

Irinotecan (CPT-11) is a semi-synthetic camptothecin with a broad spectrum of clinical activity. It is a prodrug that is cleaved by esterases to the potent topoisomerase I poison, SN-38. In humans, this activation is relatively inefficient, but this may result in a more protracted formation of SN-38 lactone. Some intratumoral activation may also occur, but the significance of this process is uncertain. CPT-11 is metabolized by cytochrome P450 3A to yield a number of comparatively inactive compounds. SN-38 is glucurono-conjugated in the liver, and this metabolite, although inactive, may participate in the enterohepatic cycling of SN-38 after hydrolysis in the intestinal lumen. Overall, the production of SN-38 from CPT-11 is the result of the complex interplay of several metabolic pathways and the source of considerable interpatient variability.

Identification and kinetics of a beta-glucuronide metabolite of SN-38 in human plasma after administration of the camptothecin derivative irinotecan.

Irinotecan (7-ethyl-10-[4-[1-piperidino]-1-piperidino]carbonyloxycampothecin) , also known as CPT-11, is a promising semi-synthetic derivative of camptothecin with significant activity against a range of tumor types. The pharmacokinetic behaviour of its principal and presumedly active metabolite, SN-38 (7-ethyl-10-hydroxy-camptothecin), displays wide inter-patient variation. During the high-performance liquid chromatographic (HPLC) analysis of plasma samples collected from a patient given CPT-11, we observed several unidentified peaks that were not present in pre-infusion samples. In this paper we describe the manner in which one of these was determined to be a beta-glucuronide of SN-38. The total plasma concentrations of this metabolite were quantified following digestion with beta-glucuronidase and were found to be greater than those of SN-38 in the patient studied. The elimination phases of the plasma concentration profile of SN-38 and its glucuronide were parallel, suggesting that the transformation of SN-38 to the glucuronide is the rate-limiting step in the elimination of SN-38 and could play a key role in its pharmacokinetics.

Effects of lipophilicity and protein binding on the hepatocellular uptake and hepatic disposition of two anthracyclines, doxorubicin and iododoxorubicin.

The anthracyclines, in particular doxorubicin (DOX), have been used for the intra-arterial locoregional therapy of liver tumours for over two decades. However, the results obtained with this form of therapy have been disappointing. It is widely recognised that DOX has a slow and limited tissue uptake, and we hypothesised that lipophilic analogues could be more suitable for locoregional administration. Using rat hepatocyte suspensions and the isolated rat liver, we examined the effects of lipophilicity, as determined from the octanol: buffer partition coefficient (Koct:buf), and protein binding of several anthracyclines on hepatocellular uptake. In particular, we compared DOX with 4'-iodo-4'-deoxy-doxorubicin (IDX), which differs only in the substitution of the daunosamine hydroxyl by an iodine molecule. Using a direct spectrofluorimetric method to evaluate cell uptake, we found that the influx rates correlated with the logarithm of Koct:buf and that IDX had the highest rate. However, the addition of bovine serum albumin (BSA) to the medium reduced the hepatocellular uptake of IDX more extensively than that of DOX such that the DOX uptake exceeded that of IDX with 4% BSA. Experiments in the isolated perfused rat liver confirmed these findings. We suggest that a trade-off of cellular uptake for reduced protein binding is desirable in the selection of drugs for intrahepatic administration. This may be accomplished by choosing anthracyclines with intermediate lipophilicity.

Clinical pharmacokinetics of the irinotecan metabolite 4-piperidinopiperidine and its possible clinical importance.

PURPOSE: To investigate the clinical relevance of 4-piperidinopiperidine (4PP) in the activity of irinotecan (CPT-11), a high-performance liquid chromatography-turboionspray-tandem mass spectrometry assay for plasma 4PP was developed. METHODS: Plasma samples were prepared for analysis following C18 solid-phase extraction. Chromatography was performed on a Waters Nova-Pak Phenyl column. Selected reaction monitoring with the mass transitions m/z 169.2 --> 84.2 and 139.2 --> 98.1 was used for the detection of 4PP and the internal standard (IS), 1-piperidineproprionitrile, respectively. RESULTS: The assay was linear from 14.8 to 591.0 nM with absolute recoveries of 4PP (59.1 nM) and IS (143.7 nM) of 85.7% (n = 10) and 86.7% (n = 10), respectively. The accuracy and imprecision of the method (total) was > or = 96.8% and < or = 8.5% over the concentration range studied, respectively. 4PP was detectable in plasma following the administration of 125, 350, 500 mg/m2 and 600 mg/m2 CPT-11 to patients, with AUC(4PP) correlated with the dose (r2 = 0.66). Plasma concentrations of 4PP declined slowly with a long terminal half-life (33.4 +/- 17.1 h). CONCLUSIONS: Overall, the concentrations of 4PP in plasma were in the sub-micromolar range (< 206.9 nM) and substantially lower than those capable of inducing apoptosis of cancer cells.

The transformation of irinotecan (CPT-11) to its active metabolite SN-38 by human liver microsomes. Differential hydrolysis for the lactone and carboxylate forms.

Irinotecan (CPT-11) is a new camptothecine derivative presently in development for the treatment of several advanced malignancies. It is converted in vivo to a highly potent metabolite, SN-38, by carboxylesterases. All camptothecine derivatives undergo lactonolysis in a pH-dependent reversible manner, generating inactive carboxylate forms. We have investigated in vitro the kinetics of transformation of CPT-11 to SN-38 by human liver microsomes originating from several donors. Microsomes from seven livers were studied individually or as a pooled preparation. CPT-11, either in its lactone or its carboxylate form, was added at a range of concentrations. The SN-38 formed was measured by HPLC with fluorometric detection. In the deacylation-limited carboxylesterase reaction, the linear steady-state kinetics between 10 and 60 min were determined. At all concentrations of CPT-11, the steady-state velocity of SN-38 formation as well as the intercept concentrations of SN-38 were about 2-fold higher when the substrate was under the lactone form than under the carboxylate form. We estimated the values (+/-SD) of K'm and Vmax to be 23.3 +/- 5.3 microM and 1.43 +/- 0.15 pmol/min/mg for the lactone and 48.9 +/- 5.5 microM and 1.09 +/- 0.06 pmol/min/mg for the carboxylate form of CPT-11, respectively. We conclude that the greater rate of conversion of CPT-11 lactone may contribute to the plasma predominance of SN-38 lactone observed in vivo. The inter-individual variation of SN-38 formation was relatively high (ratio of 4 between extreme values) but no large age- or gender-related differences were seen. The effect of twelve drugs of different therapeutic classes (antibiotics, antiemetics, antineoplastics, antidiarrhoeics, analgesics), which could be administered in association with irinotecan in the clinical setting, was evaluated in this system (drug concentration: 100 microM; CPT-11 lactone concentration: 10 microM). Loperamide and ciprofloxacine where the only drugs exerting a weak inhibition of CPT-11 conversion to SN-38.

Photodegradation of irinotecan (CPT-11) in aqueous solutions: identification of fluorescent products and influence of solution composition.

The photodegradation of irinotecan (CPT-11), the semi-synthetic derivative of the antitumor alkaloid 20(S)-camptothecin, has been investigated. The drug was exposed to laboratory light for up to 5 days in 0.9% saline solution (pH 8.5). Five significant photodegradation products were observed and a high-performance liquid chromatography (HPLC) assay was employed to isolate them from CPT-11 using gradient conditions. The structures were elucidated by nuclear magnetic resonance spectroscopy and tandem mass spectrometry and shown to be the result of extensive modifications of the lactone ring of CPT-11. Three of the compounds were found to belong to the mappicine group of alkaloids. In addition, the effect of light on the stability of CPT-11 in aqueous solutions and biological fluids was also assessed. Potassium phosphate buffers (0.05 M, pH 5.0-8.2) and saline, plasma, urine, and bile solutions containing 20 microM CPT-11 were equilibrated in the dark for 24 h before being exposed to laboratory light for up to 171 h at ambient temperature. Four of the five identified photodegradation products were observed and quantitated by isocratic HPLC, using a different detection mode (fluorescence) than the one used for gradient elution. In general, CPT-11 was found to be unstable under neutral and alkaline conditions for all solutions investigated, with the exception of bile. We conclude that CPT-11 is photolabile and that care should be taken to protect samples, particularly those intended for the isolation and identification of novel metabolites of CPT-11.

The detection of photodegradation products of irinotecan (CPT-11, Campto, Camptosar), in clinical studies, using high-performance liquid chromatography/atmospheric pressure chemical ionisation/mass spectrometry.

A method for the detection of the photodegradation products of irinotecan (CPT-11, Campto, Camptosar) was developed using high-performance liquid chromatography (HPLC) with fluorescence detection and HPLC/atmospheric pressure chemical ionisation/mass spectrometry (HPLC/APCI/MS). Remnants of infusion solution as well as samples of urine and plasma collected at the end of the infusion of CPT-11 to cancer patients were screened for the five principal known photodegradation products (PDPs) of CPT-11. The concurrent use of standards of the PDPs with ion-extract HPLC/APCI/MS chromatograms enabled the identification of trace quantities of two PDPs in most samples analysed. However, similar analyses of fresh clinical drug solutions revealed that the PDPs were not generated significantly by exposure to light during the infusion period, but were already present in the drug ampoules. Furthermore, this appears to be the source of traces of PDPs detectable in urine and plasma of patients rather than metabolism, per se.