Tetraplatin (
Ormaplatin) has good antitumor activity against some
cisplatin-resistant cells and is currently being studied in clinical trials. We have studied the effect of extracellular
reduced glutathione (GSH) on the cytotoxicity and biochemical pharmacology of
tetraplatin in
L1210 leukemia cells. Parent L1210/0 cells were exposed to
tetraplatin for 2 hr with or without GSH in
Hanks' balanced salt solution (HBSS), and cytotoxicity was assessed by a soft
agar clonogenic assay. GSH (10 or 100 microM) increased
tetraplatin (10 microM)-induced cell kill by about 2 logs; concentrations of the
thiol 10-fold below or above these levels increased cell kill to a lesser degree. GSH-mediated increases in the cytotoxicity of
tetraplatin were also observed against
cisplatin-resistant L1210/DDP and
tetraplatin-resistant L1210/
DACH cells. An equimolar concentration of 1,2-diaminocyclohexane-platinum(II) dichloride [
DACH-Pt(II)Cl2] alone was as cytotoxic as the combination of
tetraplatin and GSH. Intracellular accumulations of
tetraplatin in both L1210/0 and L1210/DDP cells were increased by GSH, whereas in L1210/
DACH cells
platinum uptake decreased in the presence of the
thiol. Reactions between
tetraplatin and salmon sperm
DNA in the presence or absence of GSH (1 or 100 microM), performed at 37 degrees in HBSS, revealed that levels of total and interstrand
DNA-
platinum adducts were minimal in the absence of GSH, whereas in the presence of GSH
DNA adducts of
tetraplatin were substantial and similar to those seen with DACH-Pt(II)Cl2.
Tetraplatin (10 microM) incubated at 37 degrees in HBSS with GSH (10 microM-1 mM) was reduced chemically to the
DACH-Pt(II) species within 5 min; a 200-microM
tetraplatin solution required a GSH concentration of at least 100 microM for substantial reduction to occur. This chemical reduction of
tetraplatin appears to be a prerequisite for its
biological activity. Thus, extracellular GSH can modulate the
biological activity of
tetraplatin, and the combination may prove useful in specific clinical applications, such as intracavitary
platinum therapy.