DEVELOPMENT AND VALIDATION OF A SENSITIVE SOLID-PHASE-EXTRACTION AND HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY ASSAY FOR THE BIOREDUCTIVE AGENT TIRAPAZAMINE AND ITS MAJOR METABOLITES IN MOUSE AND HUMAN PLASMA FORPHARMACOKINETICALLY GUIDED DOSE-ESCALATION

A sensitive solid-phase-extraction and high-performance liquid chromat ography (HPLC) method has been developed to investigate the pharmacoki netics and metabolism of the hypoxic-cell cytotoxic agent tirapazamine (1,2,4-benzotriazine-3-amine 1,4-di-N-oxide; WIN 59075, SR 4233), cur rently in phase I/II studies in the United Kingdom and United States. A sample extraction and concentration process was devised using strong cation-exchange Bond Elut cartridges. Tirapazamine, the mono and zero -N-oxide metabolites (WIN 64012, WIN 60109) were isocratically resolve d using a muBondapak phenyl HPLC column and measured using photodiode- array detection. The minimal quantifiable level (MQL) of tirapazamine was 40 ng/ml in mouse plasma and 20 ng/ml in human plasma. Recovery wa s consistently greater than 80% for all compounds over the concentrati on range of 20 ng/ml to 20 mug/ml. No significant decomposition was ob served following up to three freeze/thaw cycles and storage at -70 deg rees C for 52 days. The assay was accurate and reproducible, with meas ured values lying within the limits of defined acceptance criteria. Ad ditional studies to investigate the degree of plasma protein binding s howed that tirapazamine did not bind extensively to plasma proteins (b inding, 9.7% +/- 0.1% and 18.7% +/- 1.3% in mouse and human plasma, re spectively). These small species differences in protein binding are un likely to have any major impact on the extrapolation of pharmacokineti c data from mice to humans. The assay has now been successfully applie d to investigate the pharmacokinetics and metabolism of tirapazamine i n mice and patients as part of a pharmacokinetically guided dose-escal ation strategy for phase I clinical trials.