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

Blockers of Skeletal Muscle Nav1.4 Channels: From Therapy of Myotonic Syndrome to Molecular Determinants of Pharmacological Action and Back

The voltage-gated sodium channels represent an important target for drug discovery since a large number of physiological processes are regulated by these channels. In several excitability disorders, including epilepsy, cardiac arrhythmias, chronic pain, and non-dystrophic myotonia, blockers of voltage-gated sodium channels are clinically used. Myotonia is a skeletal muscle condition characterized by the over-excitability of the sarcolemma, resulting in delayed relaxation after contraction and muscle stiffness. The therapeutic management of this disorder relies on mexiletine and other sodium channel blockers, which are not selective for the Na_v1.4 skeletal muscle sodium channel isoform. Hence, the importance of deepening the knowledge of molecular requirements for developing more potent and use-dependent drugs acting on Na_v1.4. Here, we review the available treatment options for non-dystrophic myotonia and the structure-activity relationship studies performed in our laboratory with a focus on new compounds with potential antimyotonic activity.

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.

Dual Action of Mexiletine and Its Pyrroline Derivatives as Skeletal Muscle Sodium Channel Blockers and Anti-oxidant Compounds: Toward Novel Therapeutic Potential

Mexiletine (Mex) has been recently appointed as an orphan-drug in myotonic-syndromes, being a potent use-dependent blocker of skeletal-muscle sodium channels (Na_V1.4). Available evidences about a potential anti-oxidant effect of Mex and its tetramethyl-pyrroline-derivatives in vivo, suggest the possibility to further enlarge the therapeutic potential of Mex-like compounds in myopathies in which alteration of excitation-contraction coupling is paralleled by oxidative stress. In line with this and based on our previous structure-activity-relationship studies, we synthesized new compounds with a tetramethyl-pyrroline-ring on the amino-group of both Mex (VM11) and of its potent use-dependent isopropyl-derivative (CI16). The compounds were tested for their ability to block native Na_V1.4 and to exert cyto-protective effects against oxidative-stress injury in myoblasts. Voltage-clamp-recordings on adult myofibers were performed to assess the tonic and use-dependent block of peak sodium-currents (I_Na) by VM11 and CI16, as well as Mex, VM11 and CI16 were 3 and 6-fold more potent than Mex in producing a tonic-block of peak sodium-currents (I_Na), respectively. Interestingly, CI16 showed a 40-fold increase of potency with respect to Mex during high-frequency stimulation (10-Hz), resulting the strongest use-dependent Mex-like compound so far. The derivatives also behaved as inactivated channel blockers, however the voltage dependent block was modest. The experimental data fitted with the molecular-modeling simulation based on previously proposed interaction of main pharmacophores with Na_V1.4 binding-site. CI16 and VM11 were then compared to Mex and its isopropyl derivative (Me5) for the ability to protect C₂C₁₂-cells from H₂O₂-cytotoxicity in the concentration range effective on Na_v1.4. Mex and Me5 showed a moderate cyto-protective effect in the presence of H₂O₂, Importantly, CI16 and VM11 showed a remarkable cyto-protection at concentrations effective for use-dependent block of Na_V1.4. This effect was comparable to that of selected anti-oxidant drugs proved to exert protective effect in preclinical models of progressive myopathies such as muscular dystrophies. Then, the tetramethyl-pyrroline compounds have increased therapeutic profile as sodium channel blockers and an interesting cyto-protective activity. The overall profile enlarges therapeutic potential from channelopathies to myopathies in which alteration of excitation-contraction coupling is paralleled by oxidative-stress, i.e., muscular dystrophies.

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, antiarrhythmic activity, and toxicological evaluation of mexiletine analogues

Four mexiletine analogues have been tested for their antiarrhythmic, inotropic, and chronotropic effects on isolated guinea pig heart tissues and to assess calcium antagonist activity, in comparison with the parent compound mexiletine. All analogues showed from moderate to high antiarrhythmic activity. In particular, three of them (1b,c,e) were more active and potent than the reference drug, while exhibiting only modest or no negative inotropic and chronotropic effects and vasorelaxant activity, thus showing high selectivity of action. All compounds showed no cytotoxicity and 1b,c,d did not impair motor coordination. All in, these new analogues exhibit an interesting cardiovascular profile and deserve 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.

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.

Pharmacologically active drug metabolites: impact on drug discovery and pharmacotherapy

Metabolism represents the most prevalent mechanism for drug clearance. Many drugs are converted to metabolites that can retain the intrinsic affinity of the parent drug for the pharmacological target. Drug metabolism redox reactions such as heteroatom dealkylations, hydroxylations, heteroatom oxygenations, reductions, and dehydrogenations can yield active metabolites, and in rare cases even conjugation reactions can yield an active metabolite. To understand the contribution of an active metabolite to efficacy relative to the contribution of the parent drug, the target affinity, functional activity, plasma protein binding, membrane permeability, and pharmacokinetics of the active metabolite and parent drug must be known. Underlying pharmacokinetic principles and clearance concepts are used to describe the dispositional behavior of metabolites in vivo. A method to rapidly identify active metabolites in drug research is described. Finally, over 100 examples of drugs with active metabolites are discussed with regard to the importance of the metabolite(s) in efficacy and safety.

Molecular Insights into the Local Anesthetic Receptor within Voltage-Gated Sodium Channels Using Hydroxylated Analogs of Mexiletine

We previously showed that the β-adrenoceptor modulators, clenbuterol and propranolol, directly blocked voltage-gated sodium channels, whereas salbutamol and nadolol did not (Desaphy et al., 2003), suggesting the presence of two hydroxyl groups on the aromatic moiety of the drugs as a molecular requisite for impeding sodium channel block. To verify such an hypothesis, we synthesized five new mexiletine analogs by adding one or two hydroxyl groups to the aryloxy moiety of the sodium channel blocker and tested these compounds on hNav1.4 channels expressed in HEK293 cells. Concentration–response relationships were constructed using 25-ms-long depolarizing pulses at −30 mV applied from an holding potential of −120 mV at 0.1 Hz (tonic block) and 10 Hz (use-dependent block) stimulation frequencies. The half-maximum inhibitory concentrations (IC₅₀) were linearly correlated to drug lipophilicity: the less lipophilic the drug, minor was the block. The same compounds were also tested on F1586C and Y1593C hNav1.4 channel mutants, to gain further information on the molecular interactions of mexiletine with its receptor within the sodium channel pore. In particular, replacement of Phe1586 and Tyr1593 by non-aromatic cysteine residues may help in the understanding of the role of π–π or π–cation interactions in mexiletine binding. Alteration of tonic block suggests that the aryloxy moiety of mexiletine may interact either directly or indirectly with Phe1586 in the closed sodium channel to produce low-affinity binding block, and that this interaction depends on the electrostatic potential of the drug aromatic tail. Alteration of use-dependent block suggests that addition of hydroxyl groups to the aryloxy moiety may modify high-affinity binding of the drug amine terminal to Phe1586 through cooperativity between the two pharmacophores, this effect being mainly related to drug lipophilicity. Mutation of Tyr1593 further impaired such cooperativity. In conclusion, these results confirm our former hypothesis by showing that the presence of hydroxyl groups to the aryloxy moiety of mexiletine greatly reduced sodium channel block, and provide molecular insights into the intimate interaction of local anesthetics with their receptor.

Evaluation of the pharmacological activity of the major mexiletine metabolites on skeletal muscle sodium currents

BACKGROUND AND PURPOSE

Mexiletine (Mex), an orally effective antiarrhythmic agent used to treat ventricular arrhythmias, has also been found to be effective for myotonia and neuropathic pain. It is extensively metabolized in humans but little information exists about the pharmacodynamic properties of its metabolites.

EXPERIMENTAL APPROACH

To determine their contribution to the clinical activity of Mex, p-hydroxy-mexiletine (PHM), hydroxy-methyl-mexiletine (HMM), N-hydroxy-mexiletine (NHM) (phase I reaction products) and N-carbonyloxy beta-D-glucuronide (NMG) (phase II reaction product) were tested on sodium currents (I(Na)) of frog skeletal muscle fibres. Sodium currents were elicited with depolarizing pulses from different holding potentials (HP=-140, -100, -70 mV) and stimulation frequencies (0.25, 0.5, 1, 2, 5, 10 Hz) using the vaseline-gap voltage-clamp method.

KEY RESULTS

All the hydroxylated derivatives blocked the sodium channel in a voltage- and use-dependent manner. The PHM, HMM and NHM metabolites were up to 10-fold less effective than the parent compound. However, HMM showed a greater use-dependent behaviour (10 Hz), compared to Mex and the other metabolites. Similar to Mex, these products behaved as inactivating channel blockers. Conjugation with glucuronic acid (NMG) resulted in almost complete abolition of the pharmacological activity of the parent compound.

CONCLUSIONS AND IMPLICATIONS

Thus, although less potent, the phase I metabolites tested demonstrated similar pharmacological behaviour to Mex and might contribute to its clinical profile.

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.