Constrained analogues of tocainide as potent skeletal muscle sodium channel blockers towards the development of antimyotonic agents

doi:10.1016/j.ejmech.2008.01.023

European Journal of Medicinal Chemistry

Volume 43, Issue 11, November 2008, Pages 2535-2540

https://doi.org/10.1016/j.ejmech.2008.01.023 Get rights and content

Abstract

1-Benzyl-N-(2,6-dimethylphenyl)piperidine-3-carboxamide and 4-benzyl-N-(2,6-dimethylphenyl)piperazine-2-carboxamide, two conformationally restricted analogues of tocainide, were designed and synthesized as voltage-gated skeletal muscle sodium channel blockers. They showed, with respect to tocainide, a marked increase in both potency and use-dependent block.

Graphical abstract

Introduction

Sodium channels are involved in many cellular functions and are altered in many pathological conditions, as in the channelopathies. Therefore, drugs targeting sodium channels constitute important therapeutic interventions for a number of diseases. Among these, tocainide (1a, Fig. 1), a well-known sodium channel blocker, is a class Ib antiarrhythmic drug once used in the treatment of symptomatic life-threatening ventricular arrhythmias [1], [2]. It has also a marked analgesic effect in trigeminal neuralgia in humans [3], [4] and antinociceptive effect in rats [5]. Furthermore, tocainide has been proposed as a clinically useful antimyotonic drug being able to block sodium channels in a use-dependent manner, i.e., with an increased potency in condition of high-frequency discharges of action potentials [6]. Myotonic syndromes are hereditary disorders of the skeletal muscle caused by missense mutations in the human skeletal muscle sodium channel Na_v1.4 that prevent the normal fast inactivation of sodium channel [7], [8], [9]. Currently, tocainide is among the few drugs clinically used for the symptomatic treatment of muscle hyperexcitability in myotonic syndromes. However, its use as antimyotonic is hindered by unwanted adverse side-effects [10]. Thus, there is a need to develop new safer use-dependent sodium channel blockers with an improved pharmacological profile. A few years ago, a comprehensive model of the sodium channel was reported, showing that the increase in lipophilicity and molecular size of antiarrhythmic drugs can reinforce hydrophobic interactions with the binding site during use-dependent block [11], [12]. As a part of our program aimed at developing new antimyotonic drugs using tocainide as a “lead compound”, a series of tocainide analogues were designed with the purpose of identifying novel potent voltage- and use-dependent skeletal muscle sodium channel blockers with very high affinity constants for the inactivated channels [13], [14]. In particular, potency and use-dependent behaviour were found to be strongly increased by constraining the amino terminal group of 1a in both a rigid alpha- and beta-proline cycle (2a and 3a, Fig. 1). A further improvement was still achieved by introducing a benzyl on the amino group of proline-derived compounds (2c and 3b) [15], [16]. To confirm the importance of both molecular rigidity and presence of the benzyl group, we designed and synthesized two compounds, 4 and 5, having a chemical structure that combines alpha- and beta-proline features (2 and 3) in a six-membered ring.

The introduction of an N-benzyl versus the more classic N-alkyl was designed as an attempt to investigate the effect of the presence of a second aromatic moiety in the molecule, which could establish specific additional hydrophobic interactions with the binding site in the protein. As we have previously reported [16], lipophilicity seems to affect the tonic block; in fact, all the N-alkyl derivatives showed higher tonic block values than their corresponding unsubstituted precursors. This, in turn, is related to the increase of the block potency of the channel. Recently, docking studies [17] on tocainide (1a) and N-benzyltocainide (1b) showed that the absence of an alkyl linked to the amino group of the tocainide led to the reduction of the van der Waals interactions with the side chain of Phe1579. In fact, the benzyl group in 1b adopts an off-centered parallel orientation relative to the aromatic ring of the side chain of Phe1579. The additional energy of interaction explains the higher binding affinity of 1b than tocainide (1a). We found that N-benzyltocainide analogues 2c and 3b showed a remarkable increase of potency, 3b being the most active, also with respect to the corresponding N-methyl derivative [15]. Thus, to better clarify the role of the amino group as a pharmacophore in the drug–sodium channel interaction and confirm our previous findings, we designed two anilide derivatives, 4b and 5b that simultaneously resemble tocainide and alpha- and beta-proline. This has been accomplished by including the pharmacophore amino group into N-benzyl substituted piperidine or piperazine rings. Thus, the two tocainide derived compounds were designed with the aim to merge the characteristics of 2c and 3b, and with the consideration that several local anaesthetics currently used in therapy, such as mepivacaine, bupivacaine and ropivacaine, are substituted 2-piperidine carboxanilides, structurally related to our compounds [18] and that 2-piperazine carboxanilides were already investigated as anaesthetic and antiarrhythmic drugs [19], [20].

Section snippets

Chemistry

Compounds 4a,b and 5a,b were synthesized as depicted in Scheme 1. N-t-Boc derivatives 8 and 9 were prepared by reacting 6 (or 7) and 2-tert-butoxycarbonylimino-2-phenylacetonitrile (Boc-ON) in a mixture of dioxane/water in the presence of Et₃N. Then, 8 (or 9) was reacted with 2,6-dimethylaniline in the presence of IIDQ (2-isobutoxy-1-isobutoxycarbonyl-1,2-dihydroquinoline) to afford the corresponding carboxamides 4c and 5c, which were, in turn, deprotected by treatment with 3 N HCl or 48% HBr to

Pharmacology

The effects of the newly synthesized N-benzyl analogues of tocainide in which the asymmetric carbon atom is constrained in a piperidine or piperazine ring were then evaluated on Na⁺ currents (I_Na) of native frog skeletal muscle fibers. Na⁺ current (I_Na) measurements executed in the presence of 4a or 5a were not performed because the corresponding compounds in the set of alpha- and beta-proline (2a and 3a, respectively) were less active than the corresponding N-benzyl derivatives (2c and 3b) in

Results and discussion

The research herein described is based upon our previous works dealing with tocainide derivatives with enhanced potency and selectivity for use-dependent inhibition of the skeletal muscle Na_v1.4 channel. In particular, additional tonic potency was attained by introduction of the hydrophobic benzyl group, whereas ring constraint (proline ring) affected both potency and use-dependence of channel block. The present work shows that similar potency and use-dependence inhibition values are still

Chemistry

Yields refer to purified products and were not optimized. All chemicals were purchased from Sigma–Aldrich or Lancaster in the highest quality commercially available. The structures of the compounds were confirmed by routine spectrometric analyses. For compounds not previously described, complete spectroscopic characterization is given; for known compounds only spectra not described in the literature are given. Melting points were determined on a Gallenkamp melting point apparatus in open glass

Acknowledgment

This work was carried out under the framework of the National Projects “Progettazione, Sintesi e Valutazione Biologica di Nuovi Farmaci Cardiovascolari” supported by the Ministero dell'Università e della Ricerca (MiUR, Rome) shared also with the University of Bari.

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Original articleConstrained analogues of tocainide as potent skeletal muscle sodium channel blockers towards the development of antimyotonic agents

Abstract

Graphical abstract

Introduction

Section snippets

Chemistry

Pharmacology

Results and discussion

Chemistry

Acknowledgment

Pain

Pain

Pain

Neuron

Neuromuscul. Disord.

Neurotherapeutics

J. Biol. Chem.

J. Biol. Chem.

Eur. J. Med. Chem.

Pharmacol. Ther.

New Engl. J. Med.

Physiol. Rev.

Original article
Constrained analogues of tocainide as potent skeletal muscle sodium channel blockers towards the development of antimyotonic agents