Complexes of the tetrachoroplatinum(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)2 were only moderately cytotoxic to exponentially growing normally oxygenated or hypoxic EMT6 cells in vitro at pH 7.40 and 37 degrees C. At pH 7.40 and 42 degrees C and especially at 43 degrees 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 microM, 42 degrees C for 1 h). Pt(
neutral red)2 became much more cytotoxic at pH 6.45 and 42 degrees C or 43 degrees 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 degrees C with only 100 microM).
Tumor cell survival studies in the FSaIIC murine
fibrosarcoma demonstrated that both drugs killed in a dose-dependent log-linear manner.
Hyperthermia treatment (43 degrees 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 red)2 (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 microM of Pt(
Nile blue)2 or Pt(
neutral red)2 for 1 h at 37 degrees C than after exposure to the same concentration of
cis-diamminedichloroplatinum(II). Treatment of cells with the drugs at 42 degrees C (1 h) resulted in no change in
platinum levels with
cis-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.