Table 4.

Inhibition of Tubulin Assembly and Inhibition of Colchicine Binding.

Compd	Inhibition of tubulin assembly IC50 ± SD (µM)	Inhibition of colchicine binding (% inhibition ± SD) at 5 µM
6	1.4 ± 0.007	84 ± 0.5
14	1.7 ± 0.2	76 ± 3
15	1.7 ± 0.07	82 ± 1
1	2.6 ± 0.05	70 ± 2
CA-4	1.0 ± 0.09	99 ± 0.2

Inhibition of tubulin assembly was performed as described previously,²⁶ with tubulin at 10 µM (1.0 mg/mL) and varying concentrations of potential inhibitors. Extent of assembly at 20 min was the parameter measured. The CA-4 control is performed twice a year, and IC₅₀ values obtained range from 1 to 1.2 µM. The most recent value is shown in the table. Inhibition of colchicine binding was performed as described previously,²⁷ with tubulin at 1.0 µM and both [³H]colchicine and potential inhibitor at 5.0 µM. Incubation was for 10 min at 37 °C.