Electrochemistry-coupled to mass spectrometry in simulation of metabolic oxidation of methimazole: Identification and characterization of metabolites

Abstract

Methimazole (MMI), an antithyroid drug, is associated with idiosyncratic toxicity. Reactive metabolites resulting from bioactivation of the drug have been implicated in these adverse drug reactions. Mimicry of enzymatic oxidation of MMI was carried out by electrochemically oxidizing MMI using a coulometric flow-through cell equipped with a porous graphite working electrode. The cell was coupled on-line to electrospray ionization mass spectrometry (EC/ESI-MS. ESI spectra were acquired in both negative and positive modes. In acidic medium, ESI spectral analysis showed that the dimer was the main product, while in neutral and basic media, methimazole sulfenic acid, methimazole sulfinic acid and methimazole sulfonic acid were observed as the major electrochemical oxidation products. Oxidation of MMI and subsequent trapping with nucleophiles resulted in formation of adducts with N-acetylcysteine. Some of the electrochemically generated species observed in these experiments were similar to metabolites that have been observed from in vitro and in vivo studies. Trapping studies also showed that bioactivation of MMI proceeds predominantly through the S-oxide and not through formation of thyil radicals. These results show that electrochemistry coupled to mass spectrometry can be used in mimicry of oxidative metabolism and subsequent high throughput screening of metabolites.

Electrochemical Aspects of Mass Spectrometry: Atmospheric Pressure Ionization and Ambient Ionization for Bioanalysis

“…EC‐MS cells are particularly advantageous since a wide range of conditions can be introduced or manipulated within the cell in a timely and high‐throughput manner. In one case, a porous graphite flow‐through cell operated at either basic, neutral or acidic pH was used to monitor electrochemical products of methimazole (MMI), an antithyroid drug that displays adverse reactions upon oxidative metabolism . MMI is a thiol‐containing compound, which makes metabolic pathways especially difficult to elucidate, since sulfur‐containing compounds often undergo radical chemistry and polymerization .…”

“…In one case, a porous graphite flow‐through cell operated at either basic, neutral or acidic pH was used to monitor electrochemical products of methimazole (MMI), an antithyroid drug that displays adverse reactions upon oxidative metabolism . MMI is a thiol‐containing compound, which makes metabolic pathways especially difficult to elucidate, since sulfur‐containing compounds often undergo radical chemistry and polymerization . Reactive sulfur metabolites have been implicated in rare adverse reactions .…”

“…Reactive sulfur metabolites have been implicated in rare adverse reactions . Low flow rates and high interaction with the porous electrode allowed detection of different MMI products and intermediates under acidic conditions than neutral and basic conditions . Especially notable for the flow‐through cell was MS detection of an S‐oxide metabolite, as compared to similar studies with convective methods in which the S‐oxide eluded detection …”

“…[100] MMI is a thiol-containing compound, which makes metabolic pathways especially difficult to elucidate, since sulfur-containing compounds often undergo radical chemistry and polymerization. [100] Reactive sulfur metabolites have been implicated in rare adverse reactions. [101] Low flow rates and high interaction with the porous electrode allowed detection of different MMI products and intermediates under acidic conditions than neutral and basic conditions.…”

“…[101] Low flow rates and high interaction with the porous electrode allowed detection of different MMI products and intermediates under acidic conditions than neutral and basic conditions. [100] Especially notable for the flow-through cell was MS detection of an S-oxide metabolite, as compared to similar studies with convective methods in which the S-oxide eluded detection. [100] While complete interaction with a porous flow-through working electrode is one way to ensure high mass transport of ionic species, microfluidic cells, which can have even smaller total volumes, can demonstrate high conversion efficiencies and can often present a low-cost, disposable means to analyze samples.…”