Cytotoxicity and mode of action of the potential cytostatic drug oracin.

Primary screening in vitro and study on the mode of action of oracin in Ehrlich ascites carcinoma cells have been performed. The measure of the cytotoxic effect was the degree of inhibition of 14C-adenine and 14C-valine incorporation into TCA insoluble fraction of Ehrlich ascites carcinoma (EAC) cells. The inhibitory effect was characterized by IC50 values. The biosynthesis of nucleic acides indicated by the incorporation of 14C-adenine was more sensitive (IC50 = 66 micromol/ l) than the biosynthesis of proteins indicated by the incorporation of 14C-valine (IC50 = 196 micromol/l). To elucidate the biochemical mode of action, the effect of oracin on dynamics of biosynthesis of macromolecules indicated by the incorporation rate of [14C] labeled precursors (adenine, thymidine, uridine, valine) into appropriate macromolecules of EAC cells was studied. Oracin inhibited incorporation of all four precursors into the trichloracetic acid - insoluble fraction of Ehrlich ascites cells. The extent of inhibition was dependent on both time and drug concentration. We found that oracin inhibited activity of topoisomerase II by 100% at concentration 5 to 15 micromol/l.