Metabolism of tocainide in the rat

Bioisosteric Modification of To042: Synthesis and Evaluation of Promising Use-Dependent Inhibitors of Voltage-Gated Sodium Channels

Three analogues of To042, a tocainide-related lead compound recently reported for the treatment of myotonia, were synthesized and evaluated in vitro as skeletal muscle sodium channel blockers possibly endowed with enhanced use-dependent behavior. Patch-clamp experiments on hNav1.4 expressed in HEK293 cells showed that N-[(naphthalen-1-yl)methyl]-4-[(2,6-dimethyl)phenoxy]butan-2-amine, the aryloxyalkyl bioisostere of To042, exerted a higher use-dependent block than To042 thus being able to preferentially block the channels in over-excited membranes while preserving healthy tissue function. It also showed the lowest active transport across BBB according to the results of P-glycoprotein (P-gp) interacting activity evaluation and the highest cytoprotective effect on HeLa cells. Quantum mechanical calculations and dockings gave insights on the most probable conformation of the aryloxyalkyl bioisostere of To042 in solution and the target residues involved in the binding, respectively. Both approaches indicated the conformations that might be adopted in both the unbound and bound state of the ligand. Overall, N-[(naphthalen-1-yl)methyl]-4-[(2,6-dimethyl)phenoxy]butan-2-amine exhibits an interesting toxico-pharmacological profile and deserves further investigation.

Preparative high-performance liquid chromatography and preparative thin-layer chromatography isolation of tocainide carbamoyl-O-beta-D-glucuronide: structural characterization by gas chromatography-mass spectrometry and fast atom bombardment-mass spectrometry

Tocainide carbamoyl-O-beta-D-glucuronide, a major urinary metabolite of the antiarrhythmic drug tocainide [2-amino-N-(2',6'-xylyl)propanoxylidide], was isolated by preparative-TLC and preparative-HPLC. The isolated glucuronide was hydrolyzed in sodium hydroxide (pH greater than 12) to 3-(2',6'-xylyl)-5-methylhydantoin. This hydantoin product was also identified when tocainide was reacted with urea in urine. Structural characterization of the isolated tocainide glucuronide was carried out using GC-MS of the permethylated derivative. The molecular ion of the permethylated glucuronide was not observed, but ion fragments at m/z 232(244), 277(288), and 334(349) were found to correspond to the postulated novel carbamoyl ester structure of the permethylated (perdeuteromethylated) glucuronide. Structural evidence for the underivatized tocainide glucuronide was obtained using fast atom bombardment-MS. The [M + H]+ ion at m/z 413 was observed. Characteristic sodium ion adducts [M + Na]+ and [M-H + 2Na]+ were also observed at m/z 435 and 457, respectively.

Phosgene as a derivatizing reagent prior to gas and liquid chromatography

The use of phosgene as a derivatizing agent for bifunctional compounds prior to gas and liquid chromatographic analysis is reviewed. Applications include gas chromatographic determinations of metoprolol and its metabolites in biological fluids, enantiomeric separations of beta-blocking drugs and sympathomimetic agents on a chiral stationary phase and liquid chromatographic enantiomer separations.

Stereoselective disposition of RS-tocainide in man

The disposition of RS-tocainide in three healthy volunteers has been studied after oral administration of a pseudoracemic mixture containing S(+) [3H] tocainide as a radioactive tracer together with a therapeutic dose of the racemate. Analytical methods based on HPLC have been developed to measure S(+) and R(-) tocainide in urine samples. Selected ion detection has been used for quantification of a tocainide conjugate. The radioactive dose was efficiently absorbed and mainly cleared via the kidneys. The elimination half-life of RS-tocainide was found to be 14.3 hours. The elimination half-lives of the two stereoisomers of tocainide differed significantly, i.e. R(-) tocainide 10 hours, and S(+) tocainide 16.7 hours. The observed t1/2 for the tocainide conjugate of 10.3 hours was close to that of R(-) tocainide, indicating that the metabolite was preferably formed from the R(-) stereoisomer of tocainide. Of the given dose, between 45 and 70% can be accounted for.

Species differences in the urinary excretion of the novel primary amine conjugate: tocainide carbamoyl O-beta-D-glucuronide

The metabolism of the antiarrhythmic drug tocainide (I) has been shown previously to occur via a novel pathway involving the addition of carbon dioxide to the primary amine nitrogen of I followed by conjugation with glucuronic acid. The product of this reaction, tocainide carbamoyl O-beta-D-glucuronide (II), the principal metabolite of I in humans, has been found to cyclize under strongly basic conditions to form 3-(2,6-xylyl)-5-methylhydantoin (III). Thus, evidence for the existence of II can be obtained by two different procedures: conversion of II to III in the presence of strong base and by hydrolysis of II with beta-glucuronidase. The principal purpose of the present investigation was to identify suitable species for studies of the mechanism involved in the formation of II, as well as to find an animal model suitable for toxicological evaluation of tocainide and structurally related compounds. Eight animal species were examined to identify those capable of metabolizing I into II. The fraction of an intraperitoneal dose excreted in urine as II was estimated by measurement of tocainide released by beta-glucuronidase mediated hydrolysis of urine and by the quantitation of III formed after alkalinization of urine samples. Urinary recovery of unchanged drug ranged from 9.5% of the dose in the gerbil to 48.7% in the cat. The percent of the dose excreted in urine as acid hydrolyzable conjugates ranged from less than 1% in the gerbil to a mean of 13% in the rabbit. Guinea pigs, dogs, cats, rabbits, and pigtail monkeys excreted amounts of II ranging from 0.2 to 2.4% of the dose. Thus, none of the species appeared to be a suitable model for the study of the mechanism of formation of II because of the quantitative insignificance of this pathway.