Searching for novel anti-myotonic agents: Pharmacophore requirement for use-dependent block of skeletal muscle sodium channels by N-benzylated cyclic derivatives of tocainide

Abstract

Drug screening on sodium currents of native myofibers by means of voltage-clamp recordings is predictive of pre-clinical anti-myotonic activity in vivo and ex vivo. By this approach we identified the N-benzylated beta-proline derivative of tocainide (To10) as the most potent use-dependent blocker of Nav1.4 so far. We tested novel analogs with modifications on the pharmacophore groups of To10. The substitution of the proline cycle with less planar piperidine or piperazine rings disclosed the importance of a two carbon atom distance and/or an additional nitrogen atom for potency. Structural changes on the xylididic group corroborated the role of a proper electronic cloud for hydrophobic interactions with the binding site. The N-benzylated moiety lead to a stereoselective behavior only in the rigid alpha-proline analog To11 vs. To10 and N-benzylated tocainide (To12). The results confirm the strict structural requirements of Nav1.4 blockers and allow to refine the drug design toward novel anti-myotonic drugs.

Combined Modifications of Mexiletine Pharmacophores for New Lead Blockers of Nav1.4 Channels

“…1), decreased its potency in producing tonic block (TB) of resting Na v 1.4 and, importantly, produced a corresponding increase in UDB (12)(13)(14). Whether this increase is due to the change in physicochemical properties such as pK a or the simple lengthening of the chain is not yet established (15,16). However, an increase in UDB is pivotal to the therapeutic usefulness of all LA-type compounds in countering abnormal membrane hyperexcitability (17).…”

Increased sodium channel use-dependent inhibition by a new potent analogue of tocainide greatly enhances in vivo antimyotonic activity

“…Both these drugs display antimyotonic activity in patients; while mexiletine has recently received orphan designation in myotonic syndromes, tocainide has been withdrawn because of side effects. The benzyl-β-proline tocainide derivative, To10 (also called NeP1), showed a significant improvement of sodium channel inhibition and analgesic activity in animal models (Ghelardini et al., 2010, De Luca et al., 2012). In this study, the new To040 compound showed IC 50 values very similar to To10, while To042 compound further improved both tonic and phasic block.…”

“…Through a series of previous SAR studies, we obtained two tocainide analogues, namely To040 and To042, showing greatly enhanced potency and use-dependent block of sodium currents recorded in frog skeletal muscle fibers (Carrieri et al., 2009, De Luca et al., 2004, De Luca et al., 2012, Muraglia et al., 2014, Talon et al., 2001). Here we show that these two compounds are also very potent blockers of human skeletal muscle voltage-gated sodium channels.…”

“…We found that the antimyotonic activity of drugs in vivo was closely parallel to the in vitro inhibition of sodium currents elicited by high-frequency voltage-clamp protocols in mammalian cells transfected with hNav1.4 cDNA. Beside the study of marketed drugs, we also explored the possibility to increase potency and use-dependent behavior through chemical optimization of mexiletine and tocainide in a series of SAR studies (De Bellis et al., 2013, De Luca et al., 2000, De Luca et al., 2003a, De Luca et al., 2003b, De Luca et al., 2012). We have identified beta-proline derivatives of tocainide with a 10-fold increase in potency and use dependence for blockade of sodium channels in frog muscle fibers (Catalano et al., 2008).…”

The anti‐convulsants lacosamide, lamotrigine, and rufinamide reduce myotonia in isolated human and rat skeletal muscle

“…Pharmacologically, such Nav1.4 blockade has been accomplished with off‐label use of mexiletine or tocainide, but, although at least mexiletine has been shown to be a well‐tolerated and efficient treatment for myotonia in some patients, the use of these agents for anti‐myotonic treatment has several drawbacks. First, considerable neurological, ocular, and gastrointestinal side effects have been reported for both drugs, and tocainide has been withdrawn from the market for this reason . Related to these adverse effects, mexiletine treatment for ventricular tachycardia was discontinued in 40% of enrolled patients in 1 study .…”