Synthesis of C12-Keto Saxitoxin Derivatives with Unusual Inhibitory Activity Against Voltage-Gated Sodium Channels.

A novel series of C12-keto-type saxitoxin (STX) derivatives bearing an unusual nonhydrated form of the ketone at C12 has been synthesized, and their Na _V -inhibitory activity has been evaluated in a cell-based assay as well as whole-cell patch-clamp recording. Among these compounds, 11-benzylidene STX (3 a) showed potent inhibitory activity against neuroblastoma Neuro 2A in both cell-based and electrophysiological analyses, with EC ₅₀ and IC ₅₀ values of 8.5 and 30.7 nm, respectively. Interestingly, the compound showed potent inhibitory activity against tetrodotoxin-resistant subtype of Na _V 1.5, with an IC ₅₀ value of 94.1 nm. Derivatives 3 a-d and 3 f showed low recovery rates from Na _V 1.2 subtype (ca 45-79 %) compared to natural dcSTX (2), strongly suggesting an irreversible mode of interaction. We propose an interaction model for the C12-keto derivatives with Na _V in which the enone moiety in the STX derivatives 3 works as Michael acceptor for the carboxylate of Asp ¹⁷¹⁷ .
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