Chemical stability, biological activity and cellular uptake of a cisplatin analogue having a 1,2-diarylethyleneamine ligand in cultures of human breast cancer cells.

The platinum(II) complex PtCl2(meso-6), which has the estrogenic ligand meso-1,2-bis(2,6-dichloro-4- hydroxyphenyl)ethylenediamine (meso-6), has been reported to be an effective antitumor drug for estrogen-receptor(ER)-positive tumors in animal experiments. The goal of this study was to investigate whether the observed biological effects could be ascribed to the intact PtCl2(meso-6). Cultures of the ER-positive human breast cancer cell line MCF-7 were used as the in vitro test system. In culture medium containing 10% fetal calf serum, PtCl2(meso-6) had a half-life of about 2 h, as determined by HPLC analysis, and no PtCl2(meso-6) was detectable after 10 h. The Pt complex bound irreversibly to serum protein. After 30 min, the diamine ligand was found released, with a maximum conversion of about 35% at 24 h. At this time the culture medium still had estrogenic activity, i.e. it induced ER processing in the MCF-7 cells. This indicates that the estrogenic effect was elicited by the released diamine ligand. In contrast, the growth-inhibitory activity of the medium preincubated with PtCl2(meso-6) was lost at a rate similar to the rate of loss of PtCl2(meso-6) from the medium. This accords with the platinum complex being the main cytotoxic entity. When MCF-7 cells were incubated with PtCl2([3H]meso-6), no free Pt complex could be identified in cellular extracts, and most of the cell-associated radioactivity coeluted with meso-6 in HPLC analysis. After 12 h, only 1.4% of the total cellular platinum was bound to DNA, but no tritium label could be detected. In conclusion, diamine ligand is released from the Pt(II) complex and can account for the estrogenic effects so far ascribed to PtCl2(meso-6).