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 (I-Na) 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). I-Na 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 I-Na 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 greater than or equal to Benzyl-Toc > Benzyl-To5. At a holding potential of -100 mV and 10-Hz stimulation, Benzyl-To9 blocked I-Na with a half-maximal concentration of 0.5 muM, 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 pK(a) 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.