Block of human Na_V 1.5 sodium channels by novel α-hydroxyphenylamide analogues of phenytoin

Voltage-gated sodium (Na) channels are a critical component of electrically excitable cells. Phenytoin (diphenylhydantoin, DPH) is an established sodium channel blocker and is a useful anticonvulsant and class 1b antiarrhythmic, and has been effectively used in the treatment of neuropathic pain. In this study, we have synthesized novel α-hydroxyphenylamide analogues of diphenylhydantoin and examined their ability to inhibit human Na_v1.5 sodium channels expressed in Chinese Hamster Ovary (CHO-K1) cells. Phenyl ring substitutions were examined including para-methyl, para-fluoro, para-chloro, ortho-chloro and meta-chloro. We have found that phenyl ring substitutions with electron withdrawing properties resulted in compounds with greater activity. In comparison to diphenylhydantoin, the novel chloro-substituted α-hydroxyphenylamide compounds produced as much as a 20-fold greater tonic and frequency-dependent blockade of Na_v1.5 channels with an IC₅₀ value of 14.5μM. In addition, the chloro-substitutions have position specific state dependent blocking properties. The ortho-, meta- and para-chloro substitutions have an 8-, 13- and 3-fold increased affinity for the inactivated state, respectively. Molecular modeling suggests that these differences in affinity are due to a direct interaction with the receptor. Comparing models of diphenylhydantoin to the novel α-hydroxyphenlyamide compound suggests that the increased activity may be due to an optimized phenyl ring position and increased molecular volume. This information may be useful in the development of more potent sodium channel blockers.