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

Abstract

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.

Synthesis and in vitro sodium channel blocking activity evaluation of novel homochiral mexiletine analogs

“…Homochiral 2, 3, and 5 were tested on sodium currents (I Na ) of single frog skeletal muscle fibers by means of voltage-clamp recordings. 10,18 Depolarizing steps to 220 mV from the holding potential of 2100 mV, at two different stimulation frequencies (0.3 and 10 Hz), were applied in order to evaluate tonic and use-dependent blocks (TB and UDB, respectively) exerted by drugs. Positional isomers 2 and 3 showed about 1.522.5 fold increased potency for both TB and UDB with respect to compound 1 (Fig.…”

“…Then the phenyl analog 5 was designed in order to evaluate the steric requirements for potent and use-dependent block of skeletal muscle sodium channels. Given that studies on adult skeletal muscle fibers evidenced the stereoselective behavior of Mex and its analogs toward the binding sites 18,19 we prepared compounds 2, 3, and 5 in their homochiral forms. The effects of the newly synthesized compounds were evaluated on single fibers of frog skeletal muscle, under both tonic and phasic conditions.…”

Molecular insights into the local anesthetic receptor within voltage-gated sodium channels using hydroxylated analogs of mexiletine

“…The structure of mexiletine is related to that of local anesthetic (LA) drugs, presenting a protonable amine group connected to a hydrophobic aromatic ring through an intermediate ether link (Table 1). We have shown that little modification of these three components can substantially affect sodium channel blockade in vitro and antimyotonic effects in vivo (Desaphy et al, 1999, 2001; De Luca et al, 2000, 2003, 2004; De Bellis et al, 2006). The use of mexiletine analogs has allowed to get new important information on the molecular dynamic interaction between the drug and its receptor within the sodium channel pore (De Luca et al, 2000, 2003).…”

2-Chloro-2-phenylethylammonium chloride

“…In this respect, 2-phenylaziridine was of interest as it is an analogue of, and exhibits similar properties to, mexiletine (De Luca et al, 2000, 2003. In this synthetic sequence, 2-chloro-2-phenylethylaminium chloride, (I), was produced as an intermediate.…”