Interaction with Hyperthermia of Tetrachloroplatinum(II)(Nile Blue)₂ and Tetrachloroplatinum(II)(Neutral Red)₂ in EMT6 Murine Cells and the Murine FSaIIC Fibrosarcoma

Complexes of the tetrachloroplatinum(II) dianion with positively charged nuclear dyes were prepared in an effort to produce agents which gain ready access into the nucleus and become very cytotoxic at clinically relevant hyperthermia temperatures. Pt(Nile blue)2 and Pt(neutral red)2 are complexes of tetrachloroplatinum(II) with two closely related p-quinonediamine dyes. Pt(Nile blue)2 and Pt(neutral red)j were only moderately cytotoxic to exponentially growing normally oxygenated or hypoxic EMT6 cells in vitro at pH 7.40 and 37°C. At pH 7.40 and 42°C and especially at 43°C, however, Pt(Nile blue)2 became far more cytotoxic. At pH 6.45 Pt(Nile blue)2 became more toxic toward hypoxic cells (cell kill of 3.5 logs at 500 mm, 42°C for 1 h). Pt(neutral red) became much more cytotoxic at pH 6.45 and 42°C or 43°C compared to pH 7.4, and the cell kill observed was similar in both euoxic and hypoxic cells (3 logs at pH 6.45, 43°C with only 100 mm). Tumor cell survival studies in the FSallC murine fibrosarcoma demonstrated that both drugs killed in a dose-dependent log-linear manner. Hyperthermia treatment (43°C, 30 min) immediately after either drug resulted in a dose modifying effect. The tumor growth delay produced by Pt(Nile blue)2 (100 mg/kg) was 4.6 days and by Pt(neutral redfe (100 mg/kg) was 3.8 days. Both drugs were markedly improved by hyperthermia (tumor growth delay 1.4 days for hyperthermia; tumor growth delay 10.9 days for Pt(Nile blue)2 and 8.0 days for Pt(neutral red)2. Intracellular platinum levels were approximately 200 times higher after exposure of EMT6 cells to 25 mm of Pt(Nile blue)2 or Pt(neutral red)2 for 1 h at 37°C than after exposure to the same concentration of c/5-diamminedichloroplatinum(II). Treatment of cells with the drugs at 42°C (1 h) resulted in no change in platinum levels with cfs-diamminedichloroplatinum(II), but with Pt(Nile blue)2 and Pt(neutral red)2 an increase of 2- to 3-fold was found. Since previous work has shown that both of these complexes are active radiosensitizing agents, these new drugs seem quite well suited for further development as antitumor agents for use against solid tumors alone and in conjunction with hyperthermia and/or radiation therapy.