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Characterization of Glutathione Conjugates of the Remoxipride Hydroquinone Metabolite NCQ-344 Formed in Vitro and Detection following Oxidation by Human Neutrophils
journal contribution
posted on 2004-04-19, 00:00 authored by John C. L. Erve, Mats A. Svensson, Hans von Euler-Chelpin, Eva Klasson-WehlerRemoxipride is an atypical antipsychotic displaying selective binding to the dopamine D2
receptor. Several cases of aplastic anemia led to the withdrawal of remoxipride from the market
in December 1993. The remoxipride metabolite NCQ-344 is a hydroquinone while the structural
isomer NCQ-436 is a catechol, both of which have been suggested to be capable of forming a
reactive para- and ortho-quinone, respectively. Recently, these two remoxipride metabolites
were shown to induce apoptosis in human bone marrow progenitor cells. Furthermore, NCQ-344 also caused necrosis of these cells unlike NCQ-436. Although NCQ-344 has been detected
in plasma of humans dosed with remoxipride, to date, no experimental evidence for the
formation of the corresponding para-quinone has been obtained. Here, we report the detection
of three glutathione (GSH) conjugates of NCQ-344 in vitro that were formed following a chemical
reaction and characterized by tandem mass spectrometry and for a cyclized conjugate
additionally with derivatization and deuterium exchange. In contrast, NCQ-436 did not form
a GSH conjugate. Hypochlorous acid oxidized NCQ-344 to the para-quinone while NCQ-436
was resistant to oxidation. Upon incubation with NCQ-344, stimulated human neutrophils
produced from 2- to 5-fold greater amounts of glutathione conjugates than unstimulated
neutrophils. Ab initio calculations on these remoxipride metabolites indicated that the reaction
leading to the respective quinone was spontaneous for the para-quinone (e.g., from NCQ-344)
while ortho-quinone (e.g., from NCQ-436) formation was not. These results demonstrate that
NCQ-344 is capable of facile formation of a reactive para-quinone capable of reacting with
GSH and may rationalize previous findings regarding the biological effects observed in vitro
with these two remoxipride metabolites.