Effects of podophyllotoxin and VP-16-213 on microtubule assembly in vitro and nucleoside transport in HeLa cells.

VP-16-213, a semisynthetic derivative of podophyllotoxin, is an active antitumor agent. In this paper, the effects of VP-16-213 and podophyllotoxin on microtubule assembly in vitro and nucleoside transport in HeLa cells are compared. At 100 muM, VP-16-213 does not inhibit microtubule assembly in vitro, while 5 muM podophyllotoxin completely prevents the formation of microtubules. The presence of the glucoside moiety in VP-16-213 is responsible for the inactivity of VP-16-213 in this system because 4'-demethylepipodo-phyllotoxin, the nonglucoside congener of VP-16-213, inhibits microtubule assembly. In HeLa cells, VP-16-213 and podophyllotoxin share a common biological property; both agents inhibit the uptake of thymidine and uridine into cells by inhibiting the facilitated diffusional component of nucleoside transport. The conncentrations of drug necessary to inhibit thymidine and uridine uptake into HeLa cells by 50% are 10 and 5 muM, respectively, for podophyllotoxin, and 25 and 20 muM for VP-16-213. The action of podophyllotoxin on nucleoside transport appears unrelated to its effect on microtubule assembly, since VP-16-213, which does not inhibit microtubule assembly, inhibits nucleoside transport.