PHARMACOKINETICS OF THE HYPOXIC CELL CYTOTOXIC AGENT TIRAPAZAMINE ANDITS MAJOR BIOREDUCTIVE METABOLITES IN MICE AND HUMANS - RETROSPECTIVEANALYSIS OF A PHARMACOKINETICALLY GUIDED DOSE-ESCALATION STRATEGY IN A PHASE-I TRIAL

Tirapazamine (3-amino-1,2,4-benzotriazine-1,4-di-N-oxide; SR 259075) i s a selective hypoxic cell cytotoxic agent that is bioreductively acti vated in tumours to a reactive-drug free radical. Preclinically the ag ent has been shown to possess additive and synergistic anti-tumour act ivity in combination with radiotherapy and chemotherapy regimens. In t he present study the pharmacokinetics and metabolism of tirapazamine w ere investigated in mice and patients as part of pre-clinical and phas e I investigations. The objectives of this work were twofold; firstly, to evaluate retrospectively the utility of a pharmacokinetically guid ed dose-escalation (PGDE) strategy for tirapazamine, and secondly, to investigate if pharmacologically relevant plasma concentrations could be achieved at tolerable doses. Pharmacokinetic studies for PGDE were conducted in mice at four dose levels ranging from one-tenth of the LD 10 to the LD50. The AUC at the LD10 (2932 mu g ml(-1) min) was used to determine a target AUC value of 1173 mu g ml(-1) min (equivalent to 4 0% of the mouse LD10 AUC) for clinical studies. A phase I study to inv estigate the tolerance of a single i.v. infusion of tirapazamine (once every 3 weeks) was initiated with close pharmacokinetic monitoring. T he starting dose (36 mg/m(2)) was based on toxicity data obtained in t he mouse, rat and dog. Doses were escalated by increases in the volume and duration of infusion. A retrospective analysis of the pharmacokin etic and toxicity data was then made to determine the utility of a PGD E approach. The drug exhibited a steep dose-lethality relationship in mice (LD10 294 mg/m(2), LD50 303 mg/m(2)). The major gross toxicities were body-weight loss (15-20%), pilo-erection and hypoactivity at all dose levels. Sporadic ptosis and conjunctivitis were observed at doses of >300 mg/m(2). The plasma elimination of tirapazamine fitted a mono -exponential open model, with rapid elimination from the plasma (t(1/2 ) = 36 +/- 0.65 min) occuring at the LD10 dose of 294 mg/m(2). A 10.3- fold increase in dose resulted in a 25.0-fold increase in AUC. Clinica lly, doses were escalated over the range of 36-450 mg/m(2). Ototoxicit y (tinnitus and reversible hearing loss) was dose-limiting at 450 mg/m (2) and the MTD was 390 mg/m(2) for this schedule. Pharmacokinetic ana lyses in patients revealed that the elimination of tirapazamine in pat ients was generally bi-phasic, with low inter-patient variability bein g found in clearance. A 12.5-fold increase in dose resulted in a 19.0- fold increase in AUC. There was good quantitative agreement in metabol ite formation between mice and humans with respect to the two- and fou r-electron bioreductive metabolites. AUC values recorded for tirapazam ine at the MTD of 390 mg/m(2) (range 1035-1611 mu g ml(-1) min) were s imilar to the target AUC in mice. Importantly, these levels are consis tent with the levels required for radiation-dose enhancement and effec tive combination with cisplatin in mice. Given (a) the similarities in plasma pharmacokinetics and metabolism observed at the target AUC/MTD in mice, rats, dogs and humans, (b) the similar degree of plasma prot ein binding seen between species and (c) the relatively low inter-pati ent variability noted in drug clearance, a successful PGDE approach sh ould have been feasible. The results also indicate that potentially th erapeutic levels of tirapazamine are achievable in patients at tolerab le doses.