Oxidation Potentials of N‐Modified Derivatives of the Analgesic Flupirtine Linked to Potassium K_V7 Channel Opening Activity But Not Hepatocyte Toxicity

Abstract

Openers of neuronal voltage-gated potassium channels (KV ) are of interest as therapeutic agents for treating pain (flupirtine) and epilepsy (retigabine). In an effort to better understand the mechanisms of action and toxicity of flupirtine, we synthesized nine novel analogues with varying redox behavior. Flupirtine can be oxidatively metabolized into azaquinone diimines; thus, the oxidation potentials of flupirtine and its analogues were measured by cyclic voltammetry. KV 7.2/3 (KCNQ2/3) opening activity was determined by an established assay with HEK293 cells overexpressing these channels. A link was found between the oxidation potentials of the compounds and their EC50 values for potassium channel opening activity. On the other hand, no correlation was observed between oxidation potentials and cytotoxicity in cultures of transgenic mouse hepatocytes (TAMH). These results support the idea that oxidative metabolites of flupirtine contribute to the mechanism of action, similar to what was recently proposed for acetaminophen (paracetamol), but not to hepatotoxicity.

TAMH: A Useful In Vitro Model for Assessing Hepatotoxic Mechanisms

“…To our knowledge, APAP-induced changes in Nrf2 levels and subcellular localization have yet to be investigated in TAMH. Interestingly, TAMH may also be able to differentiate between hepatocellular injury associated with oxidative stress and toxicity resulting from nonoxidative processes as in the case of flupirtine [72]. …”

“…The mechanism of flupirtine action is thought to be based on the drug's ability to open voltage gated potassium channels in the central nervous system [69]. Administration of flupirtine has been associated with hepatocellular injury and is thought to be related to flupirtine metabolism, which has previously been shown to undergo both oxidative and conjugative reactions in vivo and in vitro [70–72]. Oxidation of flupirtine is performed by peroxidases to form quinone diimine metabolites; however, oxidation potentials and toxicity were not correlated in studies using the TAMH line [72].…”

“…Administration of flupirtine has been associated with hepatocellular injury and is thought to be related to flupirtine metabolism, which has previously been shown to undergo both oxidative and conjugative reactions in vivo and in vitro [70–72]. Oxidation of flupirtine is performed by peroxidases to form quinone diimine metabolites; however, oxidation potentials and toxicity were not correlated in studies using the TAMH line [72]. When assessing hepatotoxicity of flupirtine in vivo, a double-blind study was conducted in which a group of patients received clinically relevant doses for either four or eight weeks [70].…”

“…Flupirtine's reactive quinone diimine metabolite resembles the reactive quinoneimine metabolite (NAPQI) that is generated during APAP oxidation [71]. However, differences in physiochemical properties between APAP (a hydrophilic, weak acid) and flupirtine (a lipophilic, weak base) likely contribute to the observed differences in hepatocyte uptake and compartmentalization of these agents [72]. In the case of flupirtine, unlike acetaminophen, hepatotoxicity was not directly linked to oxidative processes.…”

Flupirtine Analogues: Explorative Synthesis and Influence of Chemical Structure on KV7.2/KV7.3 Channel Opening Activity

“…In an initial test set of systematically alkylated flupirtine derivatives reported earlier, EC 50 values for their K V 7.2/K V 7.3 channel opening activity correlated with oxidation potentials . In order to evaluate this proposed connection, we followed a similar approach to increase stability towards oxidation.…”