Characterization of Glutathione Conjugates of the Remoxipride Hydroquinone Metabolite NCQ-344 Formed in Vitro and Detection following Oxidation by Human Neutrophils

Abstract

Remoxipride 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.

Identification of a Novel Glutathione Adduct of Diclofenac, 4′-Hydroxy-2′-Glutathion-Deschloro-Diclofenac, Upon Incubation With Human Liver Microsomes

“…The ipso substitution of a halogen from a hydroquinone derivative by GSH has been previously reported (Erve et al, 2004). Formation of 4Ј-OH-2Ј-GS-DDF through a similar ipso substitution is mechanistically feasible, as evidenced by the chemical synthesis.…”

Synthesis and Cyclic Voltammetry Studies of 3,4-Methylenedioxymethamphetamine (MDMA) Human Metabolites

“…Since 2-aducts are more stereochemical constrained than 5 adducts the adopted conformation of GSH chain should promote the stabilization of the catechol structure and prevent its oxidation to the quinone as described for the catechol NCQ-436, a metabolite of the antipsychotic remoxipride. 44) Noteworthy, the lower oxidation potential observed for the 5-GSH adducts of α-MeDA relative to the parent catecholamine correlates well with the higher toxicity of this adduct in rat cortical neurons. Accordingly, unlike N-Me-α-MeDA and α-MeDA, their systemically formed thioether conjugates are easily transported across the blood-brain barrier.…”

Computational Methods in Drug Discovery

“…This information was then used to redesign remoxipride (Erve et al, 2004). Density functional theory calculations were used to eliminate the formation of reactive metabolites from a series of tyrosine kinase-2 inhibitors.…”