D-verapamil and L-verapamil are equally effective in increasing vincristine accumulation in leukemic cells in vitro.

Leukemic cells isolated from peripheral blood from 6 patients with chronic lymphocytic leukemia, immunocytoma and prolymphocytic leukemia were incubated with vincristine (10 nM) with and without racemic verapamil and its L- and D-isomers (6.6 microM). Verapamil increased vincristine accumulation in all cells, the increase varying between 100 and 700%. Racemic verapamil and the L- and D-isomer increased cellular vincristine accumulation to the same extent. The verapamil effect could not be reversed by increasing the calcium concentration in the incubation medium (final concentration 2.5-5.0 mM). The results indicate that the mechanism behind the effect of verapamil on cellular accumulation of vincristine does not depend upon changes in calcium transport.