Structure-Activity Relationships of Flupirtine Analogues for Liver Esterase-Mediated Cleavage of the 4-Fluorobenzylamine Moiety and Its Possible Relevance to Liver Toxicity.

Flupirtine and retigabine were essential drugs to combat pain and epilepsy. However, the Kv 7 potassium channel openers are fraught with hepatotoxicity and tissue discoloration, respectively, limiting their therapeutic value. Both adverse events are likely due to reactive metabolites arising from oxidative metabolism. Designing safer analogues lacking the structural elements leading to described side effects is an active area of current research. One of the main metabolites of flupirtine is the biologically inactive 4-fluorohippuric acid. Hitherto unexplained, the proposed metabolic pathway leading to the formation of 4-fluorohippuric acid from flupirtine is verified here. Through the use of eighteen flupirtine analogues, mechanistic details of this pathway could be elucidated. A possible connection with the in vitro hepatotoxicity of the flupirtine analogues and the levels of 4-fluorobenzoic acid formed in enzyme incubations was examined by correlation analysis. These findings provide important information for the design of new flupirtine analogues as potential drug candidates.

A graphite furnace-atomic absorption spectrometry-based rubidium efflux assay for screening activators of the Kv 7.2/3 channel.

For the characterization of Kv 7.2/3 channel activators, several analytical methods are available that vary in effort and cost. In addition to the technically elaborate patch-clamp method, which serves as a reference method, there exist several medium to high-throughput screening methods including a rubidium efflux flame-atomic absorption spectrometry (F-AAS) assay and a commercial thallium uptake fluorescence-based assay. In this study, the general suitability of a graphite furnace atomic absorption spectrometry (GF-AAS)-based rubidium efflux assay as a screening method for Kv 7.2/3 channel activators was demonstrated. With flupirtine serving as a reference compound, 16 newly synthesizedcompounds and the known Kv 7.2/3 activator retigabine were first classified as either active or inactive by using the GF-AAS-based rubidium (Rb) efflux assay. Then, the results were compared with a thallium (Tl) uptake fluorescence-based fluorometric imaging plate reader (FLIPR) potassium assay. Overall, 16 of 17 compounds were classified by the GF-AAS-based assay in agreement with their channel-activating properties determined by the more expensive Tl uptake, fluorescence-based assay. Thus, the performance of the GF-AAS-based Rb assay for primary drug screening of Kv 7.2/3-activating compounds was clearly demonstrated, as documented by the calculated Z'-factor of the GF-AAS-based method. Moreover, method development included optimization of the coating of the microtiter plates and the washing procedure, which extended the range of this assay to poorly adherent cells such as the HEK293 cells used in this study.

Replacing the oxidation-sensitive triaminoaryl chemotype of problematic KV 7 channel openers: Exploration of a nicotinamide scaffold.

KV 7 channel openers have proven their therapeutic value in the treatment of pain as well as epilepsy and, moreover, they hold the potential to expand into additional indications with unmet medical needs. However, the clinically validated but meanwhile discontinued KV 7 channel openers flupirtine and retigabine bear an oxidation-sensitive triaminoraryl scaffold, which is suspected of causing adverse drug reactions via the formation of quinoid oxidation products. Here, we report the design and synthesis of nicotinamide analogs and related compounds that remediate the liability in the chemical structure of flupirtine and retigabine. Optimization of a nicotinamide lead structure yielded analogs with excellent KV 7.2/3 opening activity, as evidenced by EC50 values approaching the single-digit nanomolar range. On the other hand, weighted KV 7.2/3 opening activity data including inactive compounds allowed for the establishment of structure-activity relationships and a plausible binding mode hypothesis verified by docking and molecular dynamics simulations.

Carba Analogues of Flupirtine and Retigabine with Improved Oxidation Resistance and Reduced Risk of Quinoid Metabolite Formation.

The KV 7 potassium channel openers flupirtine and retigabine have been valuable options in the therapy of pain and epilepsy. However, as a result of adverse reactions, both drugs are currently no longer in therapeutic use. The flupirtine-induced liver injury and the retigabine linked tissue discolouration do not appear related at first glance; nevertheless, both events can be attributed to the triaminoaryl scaffold, which is affected by oxidation leading to elusive reactive quinone diimine or azaquinone diimine metabolites. Since the mechanism of action, i. e. KV 7 channel opening, seems not to be involved in toxicity, this study aimed to further develop safer replacements for flupirtine and retigabine. In a ligand-based design strategy, replacing amino substituents of the triaminoaryl core with alkyl substituents led to carba analogues with improved oxidation resistance and negligible risk of quinoid metabolite formation. In addition to these improved safety features, some of the novel analogues exhibited significantly improved KV 7.2/3 channel opening activity, indicated by an up to 13-fold increase in potency and an efficacy of up to 176 % compared to flupirtine, thus being attractive candidates for further development.

Modifications of the Triaminoaryl Metabophore of Flupirtine and Retigabine Aimed at Avoiding Quinone Diimine Formation.

The potassium channel opening drugs flupirtine and retigabine have been withdrawn from the market due to occasional drug-induced liver injury (DILI) and tissue discoloration, respectively. While the mechanism underlying DILI after prolonged flupirtine use is not entirely understood, evidence indicates that both drugs are metabolized in an initial step to reactive ortho- and/or para-azaquinone diimines or ortho- and/or para-quinone diimines, respectively. Aiming to develop safer alternatives for the treatment of pain and epilepsy, we have attempted to separate activity from toxicity by employing a drug design strategy of avoiding the detrimental oxidation of the central aromatic ring by shifting oxidation toward the formation of benign metabolites. In the present investigation, an alternative retrometabolic design strategy was followed. The nitrogen atom, which could be involved in the formation of both ortho- or para-quinone diimines of the lead structures, was shifted away from the central ring, yielding a substitution pattern with nitrogen substituents in the meta position only. Evaluation of KV7.2/3 opening activity of the 11 new specially designed derivatives revealed surprisingly steep structure-activity relationship data with inactive compounds and an activity cliff that led to the identification of an apparent "magic methyl" effect in the case of N-(4-fluorobenzyl)-6-[(4-fluorobenzyl)amino]-2-methoxy-4-methylnicotinamide. This flupirtine analogue showed potent KV7.2/3 opening activity, being six times as active as flupirtine itself, and by design is devoid of the potential for azaquinone diimine formation.

Flupirtine and retigabine as templates for ligand-based drug design of KV7.2/3 activators.

Drug induced liver injury (DILI) and tissue discoloration led to the recent discontinuation of the therapeutic use of the closely related drugs flupirtine and retigabine, respectively. Experience gained with these drugs strongly suggests that heterotetramer, voltage-gated potassium channels 2 and 3 (KV7.2/3) are valid targets for effective treatment of pain and epilepsy. Because the adverse effects are not related to the mechanism of action, it appears promising to investigate chemical modifications of these clinically validated, drug-like leads. In the present retro-metabolic drug design study, a series of 43 compounds were synthesized and characterized with regard to KV7.2/3 opening activity and efficacy. The most active compound 22d displays excellent potency (EC50 = 4 nM) and efficacy (154%) as a KV7.2/3 opener. Limited aqueous solubility hampered toxicity testing at concentrations higher than 63 µM, but this concentration was nontoxic to two hepatocellular cell lines (HEP-G2 and TAMH) in culture. The slightly less active but more soluble compound 25b (EC50 = 11 nM, efficacy 111%) showed an improved toxicity/activity ratio compared to flupirtine by three orders of magnitude and represents an attractive lead structure for the development of safer analgesics and antiepileptics.