Interaction with hyperthermia of platinum complexes of triaminotriphenylmethane dyes

Complexes of the tetrachloroplatinum(II) dianion PtCl₄ with positively charged nuclear dyes have been designed in an effort to create new anticancer drugs for use with hyperthermia and/or radiation. The PtCl₄ complexes with die monocationic triaminotriphenylmethane dye basic fuchsin [Pt(basic fuchsin)₂] and the dicatrionic triaminotriphenylmethane dye methyl green [Pt(methyl green)], as well as the free dyes, were tested in exponentially growing EMT6 cells in vitro. Both the platinum complexes and free dyes were only moderately cytotoxic at pH 7.40 and 37°C in normally oxygenated and hypoxic cells where cell killing by these drugs ranged from 0.5 to 1.5 logs at 500 μM. Each agent, however, became more cytotoxic at hyperthermic temperatures and pH 7.40. Pt(methyl green) and Pt(basic fuchsin)₂ were slightly more cytotoxic to euoxic as opposed to hypoxic cells. Bom platinum complexes became even more cytotoxic at pH 6.45 and 43°C. Under these conditions, Pt(basic fuchsin)₂ killed more hypoxic than euoxic cells (4.5 versus 2.5 logs at 500 μM), but Pt(methyl green) killed more euoxic than hypoxic cells (4.5 versus 2.5 logs at 100 μM). Methyl green was less cytotoxic than Pt(methyl green) at pH 6.45 and 43°C, but basic fuchsin was the most cytotoxic drug under these conditions (cell kill of 3.5 logs in both euoxic and hypoxic cells at 100 μM). Intracellular platinum levels measured after 1 h exposure to 25 μM cisplatin, K₂PtCl₄, PT(methyl green), and PT(basic fuchsin)₂ showed that approximately 1 ng of platinum per 10⁵ cells was present after treatment with CDDP at pH 7 40 and pH 6 45 and at 37°C and 42°C; and approximately 0 2 ng was present after exposure to K₂PtCl₄ under each of these conditions. After exposure to Pt(methyl green), approximately 2-5 ng of platinum per 10⁶ cells at pH 7.40, 37°C, and 42°C were present but increased to about 6.5 ng at pH 6.45 and 42°C. With Pt(basic fuchsin)₂, 726 ng of platinum were present at 37°C, pH 7.40; 920 ng at 42°C, pH 7.40; 313 ng at 37°C, pH 6.45; and 413 ng at 42°C, pH 6.45. Since Pt(methyl green) was more cytotoxic to cells at pH 6.45 and 42°C, some of this effect could be attributed to increased uptake under these conditions. In addition, the cytotoxicity of Pt(basic fuchsin)₂ was inhibited at 42°C and pH 6.45 relative to 42°C and, pH 7.40 which, again, may be partially explained by a decrease in uptake in the acidic conditions. Pt(methyl green) shows an impressive increase in cytotoxicity at elevated temperatures, and may be a candidate as a drug specifically for use with local hyperthermia.