Inhibition of neuromuscular transmission in isolated mouse phrenic nerve-diaphragm by the enterotoxin of Clostridium perfringens type A

Comparison of effects of botulinum toxin subtype A1 and A2 using twitch tension assay and rat grip strength test

Botulinum toxin type A is used as a therapeutic agent for some spastic neurological disorders. Type A organisms have been classified into four subtypes (A1 to A4) based on the amino acid sequence variability of the produced neurotoxin. At present, commercially available preparations of the toxin belong to subtype A1. To date, no study has compared the characteristics of the biological activity of toxins from different subtypes. We compared the efficacy of A1 toxin (LL toxin or neurotoxin: NTX) with that of A2 toxin (NTX) employing the twitch tension assay using the mouse phrenic nerve hemidiaphragm and grip strength test in rats. The inhibitory effects on neuromuscular transmission of A2NTX at pH 7.4 and pH 6.8 were 1.95 and 3.73 times more potent than those of A1LL, respectively. The 50% effective doses for the administered limb, the dose which caused a 50% reduction in grip strength, i.e. ED(50), of A1LL, A1NTX, and A2NTX were calculated as 0.087, 0.060, and 0.040 U/head, respectively. These doses for the contralateral limb, i.e. TD(50), of A1LL, A1NTX, and A2NTX were calculated as 6.35, 7.54, and 15.62 U/head, respectively. In addition, the time required for A2NTX-injected rats to recover the grip strength of the contralateral limb was 17 days, while that for rats injected with A1LL was 35 days. The results indicated that A2NTX is a more potent neuromuscular blocker than A1 toxins, and suggested that A2NTX will provide a preferentical therapeutic agent for neurological disorders.

Comparison of the therapeutic indexes of different molecular forms of botulinum toxin type A

Botulinum toxin is produced by Clostridium botulinum in three different molecular-weight forms: LL toxin, 900 kDa; L toxin, 500 kDa; and M toxin, 300 kDa. We isolated the M toxin, then compared its muscle-weakening efficacy with those of L+LL toxin and BOTOX both in vitro and in vivo. The twitch tension of the mouse isolated phrenic nerve-hemidiaphragm was used for the in vitro study. For the in vivo study, grip strength was measured in the toxin-injected legs. Undesirable muscle weakening was evaluated by grip-strength measurement in the contralateral leg. Concentration-response curves for effects on the phrenic nerve-hemidiaphragm showed that M toxin was 10 times more potent than L+LL toxin. The therapeutic index in vivo was 3- to 5-times higher for M toxin than for L+LL toxin or BOTOX, indicating a greater separation for M toxin between doses with local efficacy and systemic toxicity. These findings indicate that the M toxin preparation may have a better pharmacological profile than the conventional preparation.