THE 2,2'-BIPYRIDYL-6-CARBOTHIOAMIDE COPPER(II) COMPLEX DIFFERS FROM THE IRON(II) COMPLEX IN ITS BIOCHEMICAL EFFECTS IN TUMOR-CELLS, SUGGESTING POSSIBLE DIFFERENCES IN THE MECHANISM LEADING TO CYTOTOXICITY

2,2'-bipyridyl-6-carbothioamide (BPYTA) is an antitumor agent with che lating properties. It has previously been shown that the R2 subunit of ribonucleotide reductase (RR) is its major cellular target, but RR in hibition is observed only in the presence of ferrous iron (BPYTA-Fe, m olar ratio 2:1). Because the copper (II) complex of BPYTA (BPYTA-Cu, m olar ratio 1:1)) has in vitro antitumor activity comparable to that of BPYTA-Fe, we studied the mechanism of action of this new metal comple x. Spectorphotometric and HPLC studies demonstrated that, at pH 7.5, B PYTA Cu is stable at molar ratio 2:1 and copper is in its favored oxid ized form [BPYTA Cu(II)]. Electron paramagnetic resonance (EPR) studie s with mouse recombinant R2 demonstrated that BPYTA-Cu destroys the R2 tyrosyl radical at the same concentration at which BPYTA-Fe does (78% vs 73% destruction at 200 mu M, With 5 min of contact), but R2 inhibi tion is not time-dependent. Studies of the metabolism of [C-14] cytidi ne suggest that the cytotoxic activity of BPYTA-Cu can be explained in terms of RR inhibition. However, the significant inhibition of RNA sy nthesis and the lack of cross-resistance to BPYTA-Cu of cell lines res istant to other RR inhibitors suggest that BPYTA-Cu may have more than one cellular target. Moreover, cell proliferation studies suggest tha t, unlike BPYTA-Fe, BPYTA-Cu displays its activity immediately after c ontact with the target cells. Our study demonstrates that significant differences in the biochemical effects of BPYTA and, perhaps, also its mechanism of action are due solely to the bonded transition metalloel ement. This might also be the case with other chelators that demonstra te cytotoxic activity following metalloelement chelation.