Verapamil - inhibiting pgp drug efflux

Hi All,
I am hoping some one can give me some information on the p-glycoprotein inhibitor = verapamil and it’s cytotoxicity in C. elegans. I want to perform some experiments to inhibit drug efflux in C. elegans and need to first perform a cytotoxicity dose curve for verapamil, I’m wondering about appropriate dose ranges?

Many thanks!