The interaction between aminoquinolines and presynaptically acting neurotoxins.

Chloroquine and hydroxychloroquine block neuromuscular transmission in isolated tissues from mouse, rat, guinea pig and chick. Blockade is associated with depressed muscle responses to potassium and abolished muscle responses to nicotinic cholinergic agonists. Within certain time and concentration limits, the blocking effects of chloroquine and hydroxychloroquine are reversible. Both drugs antagonize the onset of paralysis caused by botulinum neurotoxin types A and B, but neither drug antagonizes tetanus toxin or beta-bungarotoxin. The ability of chloroquine and hydroxychloroquine to antagonize botulinum toxin is not due to blockade of nicotinic cholinergic receptors. At concentrations that produce neuromuscular blockade, d-tubocurarine does not antagonize botulinum toxin types A and B. Chloroquine causes botulinum toxin to remain at an antitoxin sensitive site. These data could mean that chloroquine acts at the cell membrane to inhibit toxin binding or internalization, or that it acts in the cell interior to inhibit lysosomal processing of toxin. Whatever its action, chloroquine is the most effective antagonist of botulinum toxin yet described.