Synthesis of new 2,6-prolylxylidide analogues of tocainide as stereoselective blockers of voltage-gated Na(+) channels with increased potency and improved use-dependent activity

Bioisosteric Modification of To042: Synthesis and Evaluation of Promising Use-Dependent Inhibitors of Voltage-Gated Sodium Channels

Three analogues of To042, a tocainide-related lead compound recently reported for the treatment of myotonia, were synthesized and evaluated in vitro as skeletal muscle sodium channel blockers possibly endowed with enhanced use-dependent behavior. Patch-clamp experiments on hNav1.4 expressed in HEK293 cells showed that N-[(naphthalen-1-yl)methyl]-4-[(2,6-dimethyl)phenoxy]butan-2-amine, the aryloxyalkyl bioisostere of To042, exerted a higher use-dependent block than To042 thus being able to preferentially block the channels in over-excited membranes while preserving healthy tissue function. It also showed the lowest active transport across BBB according to the results of P-glycoprotein (P-gp) interacting activity evaluation and the highest cytoprotective effect on HeLa cells. Quantum mechanical calculations and dockings gave insights on the most probable conformation of the aryloxyalkyl bioisostere of To042 in solution and the target residues involved in the binding, respectively. Both approaches indicated the conformations that might be adopted in both the unbound and bound state of the ligand. Overall, N-[(naphthalen-1-yl)methyl]-4-[(2,6-dimethyl)phenoxy]butan-2-amine exhibits an interesting toxico-pharmacological profile and deserves further investigation.

N-aryl-2,6-dimethylbenzamides, a new generation of tocainide analogues as blockers of skeletal muscle voltage-gated sodium channels

On the basis of a 3D-QSAR study, a new generation of tocainide analogues were designed and synthesized as voltage-gated skeletal muscle sodium channel blockers. Data obtained by screening new compounds by means of Hille-Campbell Vaseline gap voltage-clamp recordings showed that the elongation of the alkyl chain and the introduction of lipophilic and sterically hindered groups on the amino function enhance both potency and use-dependent block. The results provide additional indications about the structural requirement of pharmacophores for further increasing potency and state-dependent block and allowed us to identify a new tocainide analogue (6f) with a favorable pharmacodynamic profile to be proposed as a valid candidate for studies aimed at evaluating its usefulness in the treatment of myotonias.

Species-dependent binding of tocainide analogues to albumin: affinity chromatography and circular dichroism study

A series of novel tocainide analogues were characterized for their HSA and RSA binding, by using high-performance liquid affinity chromatography (HPLAC) and circular dichroism (CD). In this HPLAC study, HSA and RSA were covalently immobilized to the silica matrix of HPLC columns, with a procedure that maintained unaltered the binding properties of the proteins. The tocainide analogues were ranked for their affinity to HSA and RSA on the basis of their bound fractions measured by the two albumin-based columns. This technique was also applied to characterize the high affinity binding sites of these tocainide analogues to the protein. For this purpose displacement experiments were carried out by means of increasing concentrations in the mobile phase of competitors known to bind selectively to the main binding sites of HSA. The results obtained with the immobilized proteins were confirmed by investigating the same drug-protein systems in solution by circular dichroism. The comparison of the data collected with both methodologies highlighted the dramatic effect of small differences in the amino acidic sequences of the two proteins. In fact, despite their similar primary and secondary structures, a small difference in the amino acidic sequence leads to significant differences in their three-dimensional structure reflecting their different binding capacity and their stereoselectivity. Therefore, this study confirms how it is crucial to consider the significant differences among the animal models when performing pharmacokinetic studies. It is also clear that the knowledge of serum carrier binding parameters at an early stage of drug discovery represents a great advantage that may help to save time and efforts.

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

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.

Synthesis and Biological Evaluation of Chiral α-Aminoanilides with Central Antinociceptive Activity

Tocainide and related optically active chiral alpha-aminoanilides were synthesized and tested in vivo via the hot plate test to evaluate their central analgesic action. The aims of the study were to verify if a) the increase in lipophilicity, obtained by the introduction of an alkyl group on the steric center (3f-i), and the replacement of the C=O group with the C=S (10) group as well as the introduction of a methyl or ethyl group on the amidic nitrogen atom (8a-c) would produce an increase in central analgesic efficacy with respect to Tocainide; b) the 2,6-xylidide moiety is crucial for high analgesic activity (3b-e); c) the hydrogen atom bonded to the amidic nitrogen moiety is an essential pharmacophoric element for analgesic activity. Among all the synthesized compounds, 3f showed antinociceptive properties with a good enantioselective index.

New potent mexiletine and tocainide analogues evaluated in vivo and in vitro as antimyotonic agents on the myotonic ADR mouse

The antimyotonic activity of chiral derivatives of mexiletine and tocainide, selected as potent use-dependent blockers of skeletal muscle sodium channels, was evaluated in vivo acutely in myotonic ADR mice. The compounds had either aromatic (Me4 and Me6) or branched isopropyl groups (Me5 and To1) on the asymmetric centre, or had this latter one methylene apart from the amino group (Me2). Therapeutic doses of mexiletine (5-10 mg/kg) and tocainide (7-20 mg/kg) significantly reduced the long time of righting reflex (TRR), typical of ADR mice. Me4, Me5 and Me6 were 2-fold more potent than mexiletine. To1 fully normalised the TRR at 7 mg/kg. The electromyographic analysis confirmed a muscle-based activity for drug effectiveness on TRR. All the compounds reduced the myotonic hyperexcitability of intercostal muscle fibres when tested in vitro by current-clamp recordings, with a potency correlated with their action on sodium channels. On stimulus-evoked firing, the isopropyl analogues were 2-4-fold more potent than parent compounds, while the aromatic analogues were about 10-fold more potent than mexiletine. Patch-clamp recordings confirmed a normal-like pharmacological sensitivity of sodium channels of native ADR muscle fibres. Finally, the in vivo antimyotonic activity is due to the block of sodium channels and divergences with in vitro potency can be related to structure-based changes in drug pharmacokinetics.

Stereospecific synthesis of ?para-hydroxymexiletine? and sodium channel blocking activity evaluation

Both enantiomers of "para-hydroxymexiletine" (PHM), one of the main metabolites of mexiletine, were synthesized and fully characterized. Properties of (R)- and (S)-PHM, in terms of blocking potency and stereoselectivity on frog skeletal muscle Na(+) channels, were evaluated. The presence of a hydroxy group on the aryloxy moiety in the 4-position, as in PHM, reduced potency with respect to mexiletine in reducing I(Na max). However, PHM showed clear use-dependent behavior similar to that of mexiletine and, in contrast with what is observed with the parent compound, maintained its stereoselectivity during the use-dependent block. Chirality 16:72-78, 2004.

Optically Active Mexiletine Analogues as Stereoselective Blockers of Voltage-Gated Na+ Channels

Optically active mexiletine analogues were synthesized and evaluated in vitro as use-dependent blockers of skeletal muscle sodium channels. The mexiletine analogues were obtained by replacing either the methyl group on the stereogenic center of mexiletine [1-(2,6-dimethylphenoxy)propan-2-amine] with a phenyl group or modifying the phenoxy moiety (by removal of one or both of the methyl groups, or introducing a chlorine atom), or both. The voltage clamp recordings showed that, regardless of the substitution pattern of the aryloxy moiety, all the compounds bearing a phenyl group on the stereogenic center (3a-f) were more active than mexiletine both in tonic and phasic block. This observation was in contrast with what was observed for mexiletine, where the removal of both methyls from the aryloxy moiety caused a dramatic reduction of potency. The most potent congener, (R)-2-(2-methylphenoxy)-1-phenylethanamine [(R)-3b], was 27-fold more potent than (R)-mexiletine in producing a tonic block, i.e., the reduction of peak sodium current in resting conditions after application of the compound. (R)-3b maintained a use-dependent behavior, being 23-fold more potent in condition of high frequency of stimulation (phasic block). Despite what was observed with mexiletine, the stereoselectivity held in phasic block conditions. Stereoselectivity indexes were generally low, ranging from 1 to 4, but except for that of the 2,6-xylyloxy congener 3c, they were higher for the congeners bearing a phenyl ring on the stereogenic center than for mexiletine and its strictly related analogue 1-methyl-2-phenoxyethanamine (1). This finding was in agreement with Pfeiffer's rule. The introduction of a chlorine atom in the 4-position of the aryloxy moiety caused a reduction of potency and a reversal of stereoselectivity as well. On the basis of the model to date accepted for the sodium channel local anesthetic-like molecule receptor, some possible explanations of our observations will be proposed.

Optimal requirements for high affinity and use-dependent block of skeletal muscle sodium channel by N-benzyl analogs of tocainide-like compounds

Newly synthesized tocainide analogs were tested for their state-dependent affinity and use-dependent behavior on sodium currents (INa) of adult skeletal muscle fibers by means of the Vaseline-gap voltage clamp method. The drugs had the pharmacophore amino group constrained in position alpha [N-(2,6-dimethylphenyl)pyrrolidine-2-carboxamide (To5)] or beta [N-(2,6-dimethylphenyl)pyrrolidine-3-carboxamide (To9)] in a proline-like cycle and/or linked to a lipophilic benzyl moiety as in N-benzyl-tocainide (Benzyl-Toc), 1-benzyl-To5 (Benzyl-To5), and 1-benzyl-To9 (Benzyl-To9). INa were elicited with pulses to -20 mV from different holding potentials (-140, -100, and -70 mV) and stimulation frequencies (2 and 10 Hz). All compounds were voltage-dependent and use-dependent channel blockers. The presence of a proline-like cycle increased the potency; i.e., To5 was 3- and 10-fold more effective than Toc in blocking INa at the holding potential of -140 and -70 mV, respectively. The benzyl group on the amine further enhanced drug effectiveness with the following scale: Benzyl-To9 >/= Benzyl-Toc > Benzyl-To5. At a holding potential of -100 mV and 10-Hz stimulation, Benzyl-To9 blocked INa with a half-maximal concentration of 0.5 microM, being 60 and 400 times more potent than To9 and Toc, respectively. The similar effectiveness of Benzyl-Toc and Benzyl-To9 was paralleled by a similar spatial arrangement by equilibrium geometry modeling. In addition, the latter had a higher pKa value that probably contributed to a slow kinetic during its high use-dependent behavior. Benzyl-To5 had its lowest energy level at a more folded conformation that justifies the less favorable profile among the N-benzylated analogs. The new compounds are the most potent tocainide-like sodium channel blockers so far described and have high therapeutic potentials.

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.

Increased rigidity of the chiral centre of tocainide favours stereoselectivity and use-dependent block of skeletal muscle Na(+) channels enhancing the antimyotonic activity in vivo

1. Searching for the structural requirements improving the potency and the stereoselectivity of Na(+) channel blockers as antimyotonic agents, new derivatives of tocainide, in which the chiral carbon atom is constrained in a rigid alpha-proline or pyrrolo-imidazolic cycle, were synthesized as pure enantiomers. 2. Their ability to block Na(+) currents, elicited from -100 to -20 mV at 0.3 Hz (tonic block) and 2-10 Hz (use-dependent block) frequencies, was investigated in vitro on single fibres of frog semitendinosus muscle using the vaseline-gap voltage-clamp method. 3. The alpha-proline derivative, To5, was 5 and 21 fold more potent than tocainide in producing tonic and 10 Hz-use-dependent block, respectively. Compared to To5, the presence of one methyl group on the aminic (To6) or amidic (To7) nitrogen atom decreased use-dependence by 2- and 6-times, respectively. When methylene moieties were present on both nitrogen atoms (To8), both tonic and use-dependent block were reduced. 4. Contrarily to tocainide, all proline derivatives were stereoselective in relation to an increased rigidity. A further increase in the molecular rigidity as in pyrrolo-imidazolic derivatives markedly decreased the drug potency with respect to tocainide. 5. Antimyotonic activity, evaluated as the shortening of the time of righting reflexes of myotonic adr/adr mice upon acute drug in vivo administration was 3 fold more effective for R-To5 than for R-Tocainide. 6. Thus, constraining the chiral centre of tocainide in alpha-proline cycle leads to more potent and stereoselective use-dependent Na(+) channel blockers with improved therapeutic potential.