Interaction with hyperthermia of platinum complexes of triaminotriphenylmethane dyes

Abstract

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.

Key Words:

• Platinum complexes
• platinum-dye complexes
• hyperthermia