GP7 (4-[4"-(2", 2", 6", 6"-tetramethyl-l"-piperidinyloxy) amino]-4'-demethyl epipodophyllotoxin) is a promising anticancer drug of the podophyllotoxin class. However, little is known about its anti-multidrug resistance effects. In the present study, we investigated the effects of GP7 on P-glycoprotein (P-gp) overexpression multidrug-resistant human leukemia K562/ADM cells with the comparison of VP-16 and K562 cells. GP7 inhibited the proliferation of K562/ADM cells in a concentration- or time-dependent manner, and the inhibitory effect of GP7 on K562/ADM cells was 1.50-fold higher than that of VP-16. GP7 caused G2/M phase accumulation but VP-16 caused S phase accumulation in K562/ADM and K562 cells. GP7 could induce apoptosis of both K562/ADM and K562 cell lines, but there was no significant difference between GP7- and VP-16-induced apoptotic ratios. GP7 could also induce typical apoptotic morphological changes and internucleosomal DNA fragmentation of K562/ADM and K562 cells, but DNA fragmentation induced by GP7 in K562/ADM cells was weaker than that in K562 cells. When treated with GP7 or VP-16 for 48 h, 128-256 microM GP7 induced more DNA fragmentation than VP-16 did, but 32-64 microM GP7 induced less DNA fragmentation than VP-16 did. GP7 could down-regulate the expression of P-gp in K562/ADM cells but VP-16 could not. Our findings suggest that GP7 may reverse multidrug resistance in human leukemia K562/ADM cells via down-regulation of P-gp expression.