The voltage-gated sodium ion channel inhibitory activities of four tetrodotoxin analogues were evaluated for their ability to reduce the cytotoxicity of ouabain and veratridine in mouse neuroblastoma Neuro-2a cells. EC 50 of the novel analogue, 4,4a-anhydrotetrodotoxin purified from pufferfish, was 750 fold larger than that of tetrodotoxin, supporting the implication of 4-OH in activity. The high activity of 11-oxotetrodotoxin was confirmed. Modification of C-6 of 11-nortetrodotoxin-6,6-diol to form an oxime derivative decreased the activity to 1/22.
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