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

A series of tocainide chiral analogues were designed, synthesized, and evaluated in vitro, in pure enantiomeric form, as use-dependent blockers of skeletal muscle sodium channels to better understand the structural requirements responsible for the antimyotonic activity. The voltage clamp recordings showed a remarkable increase of both potency and use-dependent behavior with the analogue N-(2, 6-dimethylphenyl)-2-pyrrolidinecarboxamide (1a). In fact (R)-1a was 5-fold more potent than (R)-tocainide in producing the tonic block, i.e., the reduction of peak sodium current in resting conditions after application of the compound, but it was 21-fold more potent in condition of high frequency of stimulation (phasic block). Furthermore, as opposite to tocainide, this compound was also stereoselective, (S)-1a being 2-3-fold less potent than (R)-1a. The introduction in 1a of a methyl group in place of the hydrogen bonded to either the aminic nitrogen atom [N-(2, 6-dimethylphenyl)-1-methyl-2-pyrrolidinecarboxamide (2a)] or the amidic nitrogen atom [N-(2, 6-dimethylphenyl)-N-methyl-2-pyrrolidinecarboxamide (3a)] led unexpectedly to an inversion of stereoselectivity, the (S)-enantiomers being 3-fold more potent than the (R)-ones. The comparison between eutomers showed that (S)-2a and (S)-3a are almost equieffective to (R)-1a in producing a tonic block, the half-maximal concentrations being about 100 microM; however, the use-dependent behavior was remarkably decreased by the presence of the methyl group: i.e., the gain of potency observed at high frequency of stimulation amounted to 3 and 1.6 times for 2a and 3a, respectively. The replacement of both hydrogens bonded to the aminic and amidic nitrogen atoms resulted in N-(2,6-dimethylphenyl)-N, 1-dimethyl-2-pyrrolidinecarboxamide (4a) in which the (S)-isomer was still twice as potent as the (R)-one, but the absolute potency and mostly the use-dependent behavior were strongly reduced, showing therefore no clear advantages with respect to tocainide. The use-dependent behavior, which plays a pivotal role for antimyotonic activity, is strongly reduced by the presence of methyl groups on the nitrogen atoms, likely for modification of pK(a) and/or for constraint of molecular conformation.