Fluorometric assay using high-pressure liquid chromatography for the microsomal metabolism of certain substituted aliphatics to 1,N6-ethenoadenine-forming metabolites

High-performance liquid chromatographic-fluorescent method to determine chloroacetaldehyde, a neurotoxic metabolite of the anticancer drug ifosfamide, in plasma and in liver microsomal incubations

Chloroacetaldehyde (CA) is a nephrotoxic and neurotoxic metabolite of the anticancer drug ifosfamide (IFA) and is a dose-limiting factor in IFA-based chemotherapy. Plasma levels of CA in IFA-treated cancer patients are often difficult to determine due to the lack of a sufficiently sensitive and specific analytical method. We have developed a simple and sensitive HPLC method with fluorescence detection to measure CA formation catalyzed by liver cytochrome P450 enzymes, either in vivo in IFA-injected rats or in vitro in liver microsomal incubations. This method is based on the formation of the highly fluorescent adduct 1-N(6)-ethenoadenosine from the reaction of CA with adenosine (10 mM) at pH 4.5 upon heating at 80 degrees C for 2 h. The derivatization mixture is directly injected onto a C18 HPLC column and is monitored with a fluorescence detector. Calibration curves are linear (r > 0.999) over a wide range of CA concentrations (5-400 pmol). The limit of detection of CA in plasma using this method is <0.1 microM and only 50 microl of plasma is required for the assay. By coupling this method with a recently described HPLC-fluorescent method to determine acrolein, a cytochrome P450 metabolite of IFA formed during the activation of the drug by 4-hydroxylation, the two major, alternative P450-catalyzed pathways of IFA metabolism can be monitored from the same plasma samples or liver microsomal incubations and the partitioning of drug between these two pathways thereby quantitated. This assay may prove to be useful for studies of IFA metabolism aimed at identifying factors that contribute to individual differences in CA formation and in developing approaches to minimize CA formation while maximizing IFA cytotoxicity.

Metabolic activation of vinyl chloride by rat liver microsomes: low-dose kinetics and involvement of cytochrome P450 2E1

The metabolism and pharmacokinetics of vinyl chloride (VC) have been extensively studied in rodents and humans, but the maximum velocity (Vmax) and Michaelis constant (K(m)) for the activation of VC by microsomal monooxygenases in vitro have not yet been determined. Using a new sensitive assay, the epoxidation of VC by rat liver microsomes (adult Sprague-Dawley) at concentrations from 1 ppm to 10(6) ppm in the gas phase was measured. In the assay, the reactive VC metabolites chloroethylene oxide and 2-chloroacetaldehyde were trapped with excess cAMP, yielding, 1,N6-etheno-cAMP (epsilon cAMP) which was quantitated by HPLC fluorimetry. The trapping efficiency of electrophilic VC metabolites by cAMP was close to 10%. The specificity of the method was confirmed by purification of epsilon cAMP on an immunogel. The VC concentration in the gas phase was measured by GC/flame ionization detection, while in the aqueous phase it was calculated from the partition coefficient between air and the microsomal suspension. Activation of VC by rat liver microsomes followed Michaelis-Menten kinetics with K(m) = 7.42 +/- 0.37 (+/- SD) microM and Vmax = 4674 +/- 46 pmol.mg protein-1.min-1. Inhibitor studies and immunoinhibition assays showed that VC was activated by cytochrome P450 (CYP) 2E1 down to 1 ppm in the air phase. Based on the metabolic parameters determined, the uptake of VC by rats in vivo can be accurately predicted.

Measurement of adenosine by capillary zone electrophoresis with on-column isotachophoretic preconcentration

An on-column isotachophoretic (ITP)-capillary electrophoresis (CE) system capable of preconcentrating polyhydroxyl species is reported. The ITP-CE system utilizes borate complexation of the neutral diol species to form anionic compounds that can be directly separated by CE. Borate buffer functions as both the terminating electrolyte for the ITP preconcentration and the operating buffer for the subsequent CE separation. Isotachophoretic preconcentration allows injection volumes as large as 50% of the column volume, without compromising separation integrity, to yield detection limits about 70-fold lower than direct CE separation (with borate operating buffer). In this paper we also present an application of the ITP-CE system, with laser-induced fluorescence (LIF) detection, to the quantitative analysis of adenosine from urine. Nanomolar concentration levels of adenosine are successfully derivatized with chloroacetaldehyde (CAA) to form a fluorescent derivative whose spectral characteristics match the He-Cd laser. The technique is shown to be capable of quantitative measurement of adenosine as low as 10(-9) M, the levels expected in plasma and urine.

Cytochrome P450 catalyzed metabolism of 1,2-dibromoethane in liver microsomes of differentially induced rats

The cytochrome P450 (P450) catalyzed oxidation of 1,2-dibromoethane (1,2-DBE) to 2-bromoacetaldehyde (2-BA) was measured in liver microsomes of both control and differentially induced rats. 2-BA formation was quantified by derivatization of 2-BA with adenosine (ADO), resulting in the formation of the highly fluorescent 1,N6-ethenoadenosine (epsilon-ADO), which was measured by HPLC. After microsomal incubation with 1,2DBE in the presence of ADO and removal of proteins by denaturation and centrifugation, derivatization by heating 4 h at 65 degrees C appeared necessary to ensure efficient formation of epsilon-ADO. Using this optimized derivatization method to quantitate 2-BA formation, the enzyme kinetics of the P450 catalyzed oxidation of 1,2-DBE to 2-BA were measured in liver microsomes prepared from untreated rats and rats pretreated with phenobarbital (PB), beta-naphtoflavone (beta NF) and pyrazole (PYR). P450 isoenzymes in PYR- and beta NF-induced microsomes showed linear enzyme kinetics while P450 isoenzymes in control and PB-induced microsomes showed non-linear enzyme kinetics. The apparent Vmax- and Km- values for the metabolism of 1,2-DBE to 2-BA were 2.5 nmol/min/mg protein and 144 microns for P450 isoenzymes in PYR-induced microsomes and 773 pmol/min/mg protein and 3.3 mM for P450 isoenzymes in beta NF-induced microsomes, respectively. Due to the non-linear enzyme kinetics of the P450 catalyzed oxidation of 1,2-DBE to 2-BA using control and PB-induced microsomes, no proper Vmax- and Km- values could be calculated. However, from Michaelis-Menten plots it was clear that the affinity of P450 isoenzymes for 1,2-DBE in control and PB-induced microsomes was in the same range when compared to beta NF-induced microsomes and thus much lower than the PYR-induced microsomes.

Determination of chloroacetaldehyde, a metabolite of oxazaphosphorine cytostatic drugs, in plasma

A derivatization high-performance liquid chromatographic method with ultraviolet detection to monitor the plasma concentration of chloroacetaldehyde, a neurotoxic metabolite of oxazaphosphorine drugs, is presented. To prevent the rapid degradation of chloroacetaldehyde, the plasma samples are stabilized with formaldehyde. The method is linear in the concentration range 1-250 nmol/ml. Blood samples from a patient who was treated with a ten-day continuous infusion of ifosfamide were assayed. The chloroacetaldehyde concentrations did not exceed 10 nmol/ml.

Determination of adenosine in fetal perfusates of human placental cotyledons using fluorescence derivatization and reversed-phase high-performance liquid chromatography

A simple, sensitive HPLC method using fluorescence detection was developed for determination of adenosine in fetal venous perfusates of dual-perfused cotyledons from human term placentas. Maternal and fetal circuits of in vitro placental cotyledons were perfused with physiological salt solution containing dextrose and dextran (Earle's medium). Conditions were established for optimal formation of fluorescent 1,N6-ethenoadenosine from adenosine and chloroacetaldehyde in Earle's medium and for optimal resolution of 1,N6-ethenoadenosine by reversed-phase HPLC of the reaction mixture. The yield of 1,N6-ethenoadenosine was enhanced by dilution and acidification of the sample matrix. Perfusate samples in autosampler vials were diluted 40% with water and reacted with chloroacetaldehyde for 40 min at 100 degrees C; replicate 100-microliters injections were made automatically from each reaction mixture for HPLC analysis with fluorescence detection on a column packed with 3 microns octadecylsilica (Hypersil). Calibration curves were prepared similarly from 4-100 nM adenosine in Earle's medium. Alternatively, perfusate samples were diluted twofold with dilute phosphoric acid to give a final pH of 5.4 before reaction with chloroacetaldehyde, and replicate 50-microliters injections were made automatically for HPLC; calibration curves were prepared from 2-400 nM adenosine in Earle's medium. 1,N6-Ethenoadenosine was well resolved from Earle's-derived artifactual peaks on chromatography with either a linear or a concave gradient of methanol in ammonium phosphate buffer. Total run times were 15 and 19 min, respectively. Sensitivity of measurement of adenosine was 2-4 nM. Derivatization of adenosine using the acidified reaction mixture gave a limit of detection of 100 fmol of adenosine per injection. Application of the method to analysis of adenosine in fetal venous perfusates of eight dual-perfused cotyledons, each from a different placenta, gave a range of 3.5-52 nM adenosine. Ischemia, imposed by cessation of maternal perfusion, caused a two- to sixfold increase in fetal venous perfusate adenosine concomitant with an increase in fetoplacental perfusion pressure; perfusion pressure and perfusate adenosine returned to baseline levels on reperfusion of the maternal circuit. This facile method of determination of perfusate adenosine should allow investigation of the role of placental adenosine release in regulation of fetoplacental vascular resistance and should be applicable to study of adenosine released by other isolated perfused organs.