THE ASSAY OF CURARE BY THE RABBIT "HEAD-DROP" METHOD

¹ Development and Quality Control Divisions, E. R. Squibb and Sons, New Brunswick, N. J.

1. The head-drop assay in the rabbit has been employed for the biological determination of potency of curares having an onset and duration of action comparable to that of d-tubocurarine chloride. Qualitatively similar crude or purified curare preparations standardized by this method show uniform activity in clinical use, although the relative activity of various curare compounds in the rabbit and man may be different.

2. The endpoint of the test, head-drop, is relaxation of the neck muscles to such a degree that the animal's head cannot be raised or turned in response to a physical stimulus.

3. In the assay the reference standard and test sample of curare are each injected intravenously into at least eight rabbits in a crossover pattern on two consecutive days at the rate of 0.1 cc. of a 2 unit per cc. solution every fifteen seconds without regard to body weight. Each animal must have received within seven days prior to use in the test at least two such titrations to head-drop. For routine assays the solution of test sample to be injected should have a potency within ±10 per cent of the 2 units per cc. reference solution prepared from a standard sample of d-tubocurarine chloride pentahydrate of which 0.155 mgm. equals 1 unit of curare activity.

4. The relative potency of the diluted sample is calculated as the geometric mean of the ratios of the volume of standard to that of sample necessary to produce head-drop in each animal. The expected standard error of the geometric mean of the individual ratios, when sixteen rabbits are used in a crossover design, is 1.1 per cent.

5. The effect of rate of injection on head-drop dosage, recovery time and sharpness of the endpoint have been analyzed, and the limitations of the head-drop method and of other procedures studied have been discussed.