Homologation of mexiletine alkyl chain and stereoselective blockade of skeletal muscle sodium channels

Voltage-Gated Sodium Channel Blockers: Synthesis of Mexiletine Analogues and Homologues

Mexiletine is an antiarrhythmic drug belonging to IB class, acting as sodium channel blocker. Besides its well-known activity on arrhythmias, its usefulness in the treatment of myotonia, myotonic dystrophy and amyotrophic lateral sclerosis is now widely recognized. Nevertheless, it has been retired from the market in several countries because of its undesired effects. Thus, several papers were reported in the last years about analogues and homologues of mexiletine being endowed with a wider therapeutic ratio and a more selectivity of action. Some of them showed sodium channel blocking activity higher than the parent compound. It is noteworthy that mexiletine is used in therapy as a racemate even though a difference in the activities of the two enantiomers was widely demonstrated, with (-)-(R)-enantiomer being more active: this finding led several research groups to study mexiletine and its analogues and homologues in their optically active forms. This review summarizes the different synthetic routes used to obtain these compounds. They could represent an interesting starting point to new mexiletine-like compounds without common side effects related to the use of mexiletine.

Synthesis and Evaluation of Voltage-Gated Sodium Channel Blocking Pyrroline Derivatives Endowed with Both Antiarrhythmic and Antioxidant Activities

Under the hypothesis that cardioprotective agents might benefit from synergism between antiarrhythmic activity and antioxidant properties, a small series of mexiletine analogues were coupled with the 2,2,5,5-tetramethylpyrroline moiety, known for its antioxidant effect, in order to obtain dual-acting drugs potentially useful in the protection of the heart against post-ischemic reperfusion injury. The pyrroline derivatives reported herein were found to be more potent as antiarrhythmic agents than mexiletine and displayed antioxidant activity. The most interesting tetramethylpyrroline congener, a tert-butyl-substituted analogue, was at least 100 times more active as an antiarrhythmic than mexiletine.

Design, synthesis and in vitro activity of 1,4-disubstituted piperazines and piperidines as triple reuptake inhibitors

Monoamine transporters regulate the concentration of monoamine neurotransmitters, which are essential for vital physiological processes, and their dysfunction can cause several central nervous system diseases. Monoamine transporters currently appear to be the potential target in the management of these disorders. In this study, homologation and bioisosterism techniques have been used in the designing of new 1,4-disubstituted piperazines and piperidines. These derivatives were synthesized and evaluated as potential triple reuptake inhibitors for studying the structure-activity relationships. The most advanced compound, 1-(4-(5-benzhydryl-1H-tetrazol-1-yl)butyl)-4-(3-phenylpropyl)piperazine (2i), was able to inhibit monoamine neurotransmitter reuptake in an in vitro test (IC₅₀=158.7nM for 5-HT, 99nM for NE and 97.5nM for DA). These novel potent triple reuptake inhibitor-based 1,4-disubstituted piperazine and piperidine scaffolds deserve further systematic optimization and pharmacological evaluation.

X-ray crystal structures of Enterococcus faecalis thymidylate synthase with folate binding site inhibitors

Infections caused by Enterococcus faecalis (Ef) represent nowadays a relevant health problem. We selected Thymidylate synthase (TS) from this organism as a potential specific target for antibacterial therapy. We have previously demonstrated that species-specific inhibition of the protein can be achieved despite the relatively high structural similarity among bacterial TSs and human TS. We had previously obtained the EfTS crystal structure of the protein in complex with the metabolite 5-formyl-tetrahydrofolate (5-FTHF) suggesting the protein role as metabolite reservoir; however, protein-inhibitors complexes were still missing. In the present work we identified some inhibitors bearing the phthalimidic core from our in-house library and we performed crystallographic screening towards EfTS. We obtained two X-ray crystallographic structures: the first with a weak phthalimidic inhibitor bound in one subunit and 5-hydroxymethylene-6-hydrofolic acid (5-HMHF) in the other subunit; a second X-ray structure complex with methotrexate. The structural information achieved confirm the role of EfTS as an enzyme involved in the folate pool system and provide a structural basis for structure-based drug design.

Synthesis of deuterium-labeled cyamemazine and monodesmethyl cyamemazine

A novel approach is presented for the synthesis of cyamemazine maleate and N-desmethyl cyamemazine maleate using a 10-(amino-2-methylpropyl)phenothiazine derivative. This method was successfully applied to the synthesis of [(2) H6 ]cyamemazine maleate and N-desmethyl-[(2) H3 ]cyamemazine maleate.

Combined modifications of mexiletine pharmacophores for new lead blockers of Na(v)1.4 channels

Previously identified potent and/or use-dependent mexiletine (Mex) analogs were used as template for the rational design of new Na(v)-channel blockers. The effects of the novel analogs were tested on sodium currents of native myofibers. Data and molecular modeling show that increasing basicity and optimal alkyl chain length enhance use-dependent block. This was demonstrated by replacing the amino group with a more basic guanidine one while maintaining a proper distance between positive charge and aromatic ring (Me13) or with homologs having the chirality center nearby the amino group or the aromatic ring. Accordingly, a phenyl group on the asymmetric center in the homologated alkyl chain (Me12), leads to a further increase of use-dependent behavior versus the phenyl Mex derivative Me4. A fluorine atom in paraposition and one ortho-methyl group on the xylyloxy ring (Me15) increase potency and stereoselectivity versus Me4. Charge delocalization and greater flexibility of Me15 may increase its affinity for Tyr residues influencing steric drug interaction with the primary Phe residue of the binding site. Me12 and Me15 show limited selectivity against Na(v)-isoforms, possibly due to the highly conserved binding site on Na(v). To our knowledge, the new compounds are the most potent Mex-like Na(v) blockers obtained to date and deserve further investigation.

Treatment in myotonia and periodic paralysis

The myotonic disorders, including the myotonic dystrophies (myotonic dystrophy type 1, DM1; myotonic dystrophy type 2, DM2/PROMM/PDM), the muscle channelopathies or non-dystrophic myotonias (chloride, sodium, calcium and potassium channelopathies) are all characterized by myotonia and muscle weakness despite different pathophysiology involved in these disorders. Myotonia may affect the eye, facial and jaw muscles as well as the hands and legs. It may be painful and disabling. Muscle weakness may be episodical as in the paralytic attacks of the sodium and calcium channelopathies or culminate in permanent muscle weakness as in the calcium channelopathies and some sodium channelopathies associated to specific point mutations. The severity of myotonia may fluctuate in the myotonic dystrophies, but weakness is usually fixed, affecting neck flexors, facial and jaw muscles as well as proximal and distal muscles of the limbs. Despite the recent progress in molecular genetics the precise mechanisms responsible for myotonia and weakness are not fully understood and there is no standardized treatment strategy. We present a review of selected treatment trials in the myotonic disorders and the muscle channelopathies, and discuss our experience in the treatment of myotonia and muscle weakness, with reference to the limits and advantages of treatment trials in this field.

Inhibition of skeletal muscle sodium currents by mexiletine analogues: specific hydrophobic interactions rather than lipophilia per se account for drug therapeutic profile

In striated fibers, the activity of mexiletine (Mex)-like sodium channel blockers is strongly modulated by the part of the molecule nearby the asymmetric carbon atom. A lipophilic aromatic phenyl group at this levels, as in 2-(2,6-dimethylphenoxy)-1-phenylethanamine (Me4), markedly increases drug potency, while an increased distance between the stereogenic center and the pharmacophore amino group, as in 3-(2,6-dimethylphenoxy)-2-methylpropan-1-amine (Me2), enhances the use-dependent behavior. In order to better evaluate the role of lipophilicity in drug potency in relation to the structural determinants for a specific binding, lipophilic analogs of Me2 and Me4 were synthesized. Compounds 3-[(2,6-dimethylphenyl)thio]-2-methylpropan-1-amine and 2-[(2,6-dimethylphenyl)thio]-1-phenylethanamine were obtained by isosteric substitution of the oxygen atom with sulfur, while the introduction of a chlorine atom in 4- position of the aryloxy ring lead to 3-(4-chloro-2,6-dimethylphenoxy)-2-methylpropan-1-amine and 2-(4-chloro-2,6-dimethylphenoxy)-1-phenylethanamine. The compounds were tested on nearly maximal Na(+) currents elicited with depolarizing steps at 0.3 Hz (tonic block) and 2-10 Hz (use-dependent block) by means of vaseline-gap voltage-clamp method on single frog muscle fibers.The augmented lipophilicity largely increase drug potency in Me2 analogues, the thio and chlorinated compounds being 20- and 10-fold more potent in producing the tonic block, respectively. However, both compounds showed a 2-fold lower use-dependent behavior vs. the high use-dependent Me2. Surprisingly, the same increase in lipophilicity brought about by the same substitutions, in the already high lipophilic and potent Me4 failed to further improve the potency, although both new analogs were more stereoselective than Me4. No correlation was found between logP and potency of all analogs tested. All compounds acted as inactivated channel blockers. In conclusion, lipophilicity differently influences drug profile based on the molecular determinants controlling drug-receptor interaction.