Crystalline preparation of botulinum toxin type A (Botox): degradation in potency with storage

Laryngeal injection of botulinum toxin type A is currently the most effective method of treating spasmodic dysphonia. Botox, a crystalline preparation of botulinum toxin type A, is the only toxin approved for clinical use in the United States and is packaged in vials of 100 mouse units (MU). One MU corresponds to the calculated median lethal intraperitoneal dose (LD50) injected in mice. The logistic problems arising from the need for repeated injections of small amounts of Botox have been addressed by several investigators by refreezing unused Botox for use at a later time. Although FDA labeling recommends that Botox not used within 4 hours of reconstitution be discarded, data regarding degradation in potency after reconstitution and refreezing are not currently available. Using the LD50 Swiss-Webster mouse bioassay and statistical analysis by the Probit procedure, a 69.8% loss in potency was found when Botox was reconstituted, immediately frozen, and then assayed 2 weeks later (p < 0.0001). Statistically significant degradation in potency was seen after refrigerator storage for 12 hours (p = 0.007), but not for 6 hours (p = 0.16). Clinical implications regarding the dilution, use, and storage of Botox are discussed.