Intracellular distribution of a platinum-rhodamine 123 complex in cis-platinum sensitive and resistant human squamous carcinoma cell lines.

The platinum(II) tetrachlorodianion and two molecules of rhodamine-123 associate to form a neutral tight ion pair. To examine the intracellular fate of this ionic complex, the levels of uptake after a 1-hr exposure to a 100 microM concentration of each component of the complex, the complex itself and cis-diamminedichloroplatinum(II) (CDDP) were measured in SCC-25 cells. The uptake of Pt(Rh-123)2 was measured by two independent methods: fluorescence and 195mPt gamma-counting. There was excellent agreement between these two methods as to the amount of Pt(Rh-123)2 which was taken up by the cells, indicating that the Pt(Rh-123)2 is probably entering cell intact. Association with Rh-123 increased the amount of platinum which entered the cells by about 70-fold compared to CDDP and increased by about 700-fold the amount of platinum which entered the cells compared to K2PtCl4. The subcellular distributions of Pt(Rh-123)2, Rh-123, CDDP and K2PtCl4 were also examined. When measured by fluorescence or 195mPt gamma-counting, 40-54% of the Pt(Rh-123)2 was in the nuclei of the SCC-25 or SCC-25/CP cells and 27-35% was in the cytosol of the cells. There was excellent agreement between the findings of fluorescence and 195mPt gamma-counting regarding the amount of Pt(Rh-123)2 in each of the subcellular fractions immediately after incubation with the drug and over the time course of observation after drug removal, indicating that the Pt(Rh-123)2 is probably remaining largely intact intracellularly. On a per mg protein basis, there was about a 55-fold greater amount of platinum in the nuclei of the SCC-25 cells exposed to Pt(Rh-123)2 compared to cells exposed to CDDP. In the SCC-25/CP cells, there was about 258-fold greater platinum in the nuclei of cells exposed to Pt(Rh-123)2 than those exposed to CDDP because CDDP was taken up to a much lesser extent by the SCC-25/CP cells. Association of Rh-123 with potassium tetrachlorodianion forms a tight ion pair, which enters cells in relatively high amounts and is selectively concentrated in the nuclei of the cells.