Satraplatin activation by haemoglobin, cytochrome C and liver microsomes in vitro.

BACKGROUND: Satraplatin is thought to require reduction to a reactive Pt(II) complex (JM118) before exerting chemotherapeutic activity. In this study, we investigated the role of heme proteins in this reductive activation of satraplatin. METHODS: Satraplatin was incubated in solution with heme proteins and liver microsomes. The oxidation state of heme iron was monitored by visible absorption spectrometry. Satraplatin and JM118 were detected using a sensitive and specific HPLC-ICPMS assay. RESULTS: Satraplatin was stable in solutions containing haemoglobin, cytochrome c, glutathione, liver microsomes or NADH alone. However, in solutions containing haemoglobin plus NADH, satraplatin disappeared with a half-life of 35.8 mins. Under these conditions, satraplatin was reduced to JM118 and haemoglobin was oxidised to methaemoglobin. The reaction between haemoglobin and satraplatin was inhibited by carbon monoxide or by cooling the reaction solution. Cytochrome c and liver microsomes also reduced satraplatin to JM118 in a manner that depended upon the presence of NADH and was inhibited by carbon monoxide. CONCLUSION: This study has identified a mechanism of satraplatin activation involving metal-containing redox proteins and the transfer of electrons to the Pt(IV) drug from protein-complexed metal ions. Heme proteins may act by this mechanism as reducing agents for the activation of satraplatin in vivo.

Rapid biotransformation of satraplatin by human red blood cells in vitro.

PURPOSE: Satraplatin is an orally administered platinum complex that has demonstrated clinical activity and manageable toxicity in phase II trials. The presence of several different platinum-containing species and very little intact parent drug in the systemic circulation indicates extensive biotransformation of satraplatin in vivo. To investigate the basis for the biotransformation of satraplatin, studies were carried out into the stability of the drug in whole blood and various other biological fluids in vitro. METHODS: Concentrations of satraplatin and platinum-containing biotransformation products in incubation fluids were measured using high-performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICPMS). The fate of satraplatin-derived platinum in whole blood in vitro was determined by analysis of blood fractions for platinum by ICPMS. RESULTS: In fresh human whole blood in vitro, satraplatin concentrations fell very rapidly, resulting in a half-life for the disappearance of the drug of only 6.3 min (95% CI, 5.9 to 6.7 min). After the addition of drug to red blood cells that had been prepared from whole blood and suspended in 0.9% NaCl, satraplatin also disappeared very rapidly. Satraplatin was much more stable in fresh human plasma (t(1/2) 5.3 h) and fully supplemented cell culture medium (t(1/2) 22 h). Two new platinum-containing species appeared on HPLC-ICPMS platinum chromatograms of methanol extracts of plasma after the addition of the drug to whole blood. Their identities were assigned as the platinum(II) complex known as JM118 and a platinated protein with similar electrophoretic mobility to that of serum albumin. During the incubation of satraplatin in blood, platinum associated with red blood cells at an accumulation half-life of 9.5 min (95% CI, 7.1 to 14.2 min). At equilibrium, 62% of the added platinum was associated with red blood cells in a form that was not exchangeable in methanol or 0.9% NaCl. CONCLUSIONS: The rapid disappearance of satraplatin from human blood in vitro depends upon the presence of red blood cells. Generation of JM118 and irreversibly bound membrane- and protein-associated platinum indicates that satraplatin undergoes rapid biotransformation in whole blood.