SYNTHESIS AND BIOLOGICAL-ACTIVITY OF NOVEL THYMIDINE DERIVATIVES OF PODOPHYLLOTOXIN AND 4'-DEMETHYLEPIPODOPHYLLOTOXIN

We have synthesized a number of novel derivatives of podophyllotoxin ( POD)and 4'-demethylepipodophyllotoxin (DMEP) in which the nucleoside t hymidine has been conjugated at the C4 position. To investigate the st ructure-activity relationship among these compounds, the cross-resista nce patterns of these derivatives towards a set of either POD-resistan t (Pod(R)) or VP16/VM26-resistant (Vpm(R)) mutants of Chinese hamster ovary (CHO) cells were determined. These mutants exhibit highly specif ic cross-resistance patterns toward compounds that show either POD- or VP16/VM26-like activity. The observed cross-resistance patterns of th e thymidine derivatives suggests that these compounds display POD-like activity in vivo and show no VP16/VM26-like activity. Further, treatm ent of Chinese hamster cells with these compounds caused a dose-depend ent increase in the mitotic index similar to the patterns observed wit h POD and DMEP, supporting the data from the cross-resistance assay. M ost thymidine derivatives exhibited much lower activity in comparison to POD or DMEP, suggesting that the thymidine moiety interferes with t he interaction of these compounds with the receptor site on the tubuli n molecule. One of these derivatives which was most active in the afor ementioned assays was also found to be a competitive inhibitor of radi olabelled POD binding to purified bovine brain tubulin. All other comp ounds were insoluble at concentrations required to perform the competi tion assay. Molecular modelling studies provide valuable insight regar ding the three-dimensional structural requirements that distinguish PO D-like compounds from their VP16/VM26-like counterparts. There appears to be a very limited spatial and electrostatic requirement for the bu lky glycosidic moiety at C4 which is essential for VP16/VM26-like acti vity.