Comparison of in vivo and in vitro mouse bioassays for botulinum toxin antagonists

Measurements of the efficacy of novel botulinum toxin antagonists can be based on classical bioassays of toxin concentration. However, the relative sensitivities of in vivo and in vitro assays to the effects of antagonists are not necessarily correlated with the sensitivities of the assays to toxin. Comparisons of the sensitivity of an in vitro mouse muscle contraction assay with an in vivo mouse survival assay indicated that the in vivo assay was more sensitive to botulinum toxin serotype A by more than one order of magnitude at equivalent molar concentrations. However, in studies of toxin neutralization with equine antisera, the in vitro muscle assay was more than three times more sensitive to the presence of antisera than the equivalent mouse survival assay. For the development of new drugs to treat botulism, antagonist sensitivity is a primary consideration in determining relative efficacy during structure-activity studies. Thus, in studies of toxin antagonists, the in vitro assay appears to be superior for initial testing.