Cytotoxicity, radiosensitization, and DNA interaction of platinum complexes of thiazin and xanthene dyes

Complexes of the platinum(II) tetrachlorodianion with positively charged nuclear dyes have been prepared in an effort to produce neutral molecules which could gain ready access to the nuclear DNA where the platinum(II) tetrachlorodianion could function as a radiosensitizing and a bifunctional alkylating agent. The thiazin dyes Thionin, Azure B, and Methylene Blue, the aminoxanthene dye Pyronin Y, and the thiazole dye Thioflavin have each been complexed to the platinum(II) tetrachlorodianion(PtCl4) in a ratio of 2:1(dye:PtCl4). Studies of the interaction of these complexes and of the dyes with the pBR322 plasmid superhelical DNA demonstrated that while each complex and dye readily associated with the DNA in a dose-dependent manner, only Pt(Thioflavin)2 and Thioflavin produced irreversible DNA changes (single-strand breaks). In exponentially growing EMT6 cells the cytotoxicity of these drugs was assessed in normally oxygenated and hypoxic cells at both pH 7.4 and 6.45. At concentrations ranging from 1 to 500 microM, Pt(Methylene Blue)2 was significantly more cytotoxic than the other thiazin dye complexes Pt(Thionin)2 and Pt(Azure B)2. The cytotoxicity of Pt(Thionin)2 and Pt(Methylene Blue)2 was increased in normally oxygenated and hypoxic cells at low pH. Both Pt(Pyronin Y)2 and Pt(Thioflavin)2 were more toxic than the thiazin complexes. Pt(Pyronin Y)2 was most cytotoxic to normally oxygenated cells at normal pH and hypoxic cells at low pH, while Pt(Thioflavin)2 was most cytotoxic to cells at low pH under both oxygenation conditions. In vitro studies of the radiosensitizing properties of these agents in EMT6 cells demonstrated that exposure to 100 microM for 1 h before and during irradiation (except for Pt[Thioflavin]2, which was assayed at 25 microM) resulted in enhancement rations of 2.5, 1.9, 1.5, and 1.5 for Pt(Azure B)2, Pt(Thionin)2, Pt(Pyronin Y)2, and Pt(Thioflavin)2, respectively, in hypoxic cells. In contrast, Pt(Methylene Blue)2 (and Methylene Blue) proved to be a radioprotector of normally oxygenated cells and did not sensitize hypoxic cells to the cytotoxic effects of radiation. In the FSaIIC fibrosarcoma in vivo administration of each drug at 100 mg/kg intraperitoneally (ip) 15 min prior to irradiation (except for Pt[Thioflavin]2, which was given at 1 mg/kg ip) showed that, with single radiation fractions of 10 and 20 Gy, dose-modifying factors of 2.1, 1.8, 1.5, and 1.2 were produced by Pt(Azure B)2, Pt(Thionin)2, Pt(Pyronin Y), and Pt(Methylene Blue)2, respectively, after correcting for growth delays induced by the drug alone. In comparison, misonidazole at 1 g/kg ip produced a dose-modifying factor of 1.4.(ABSTRACT TRUNCATED AT 400 WORDS)