- DNA topoisomerases (topo) are the cellular targets of several anticancer drugs used today in the clinic. Our previous work demonstrated that certain tyrphostin derivatives, known as protein tyrosine kinase antagonists, are catalytic inhibitors of DNA topoisomerases I (topo I) in vitro. In this study, we examined the ability of tyrphostin derivatives to affect the activity of topo I in the cell (in vivo) and determined their in vivo mode of action. Two approaches were used; in the first, we examined the direct effect of the treatment of tumor cells with tyrphostins on the activity, level, and post-translational modifications of the cellular topo I. The second approach was to determine the influence of pretreatment of tumor cells with tyrphostin on the cellular induced effects of camptothecin (CPT), a known inhibitor of topo I. The results show that treatment of fibrosarcoma cells with tyrphostin inhibited the DNA relaxation activity of topo I but did not reduce the level of topo I protein. Tyrphostin treatments caused conformational changes of the cellular topo I probably by binding to the enzyme. Pretreatment of the cells with tyrphostin before CPT prevented the CPT-induced degradation of topo I and reduced the enzyme-DNA cleavable complexes and the ubiquitination/sumoylation of the enzyme. These data suggest that topo I is one of the cellular targets of tyrphostin and that this drug acts in vivo (in the cell) as a catalytic inhibitor of the enzyme that alters the binding of the enzyme to the DNA.