Notes: Abstract The effect of combining the oxygen-transport-modifying drug BW12C with mitomycin C was investigated in a phase 1 study of 26 patients with advanced gastrointestinal cancer. The dose of BW12C was increased from 20 mg/kg to 60 mg/kg. Dose-limiting toxicity of vomiting was experienced at doses greater than 50 mg/kg. This corresponded to whole blood levels ≥700 μg/ml and to 〉50% haemoglobin modification. Whole blood concentrations of BW12C and modification of the haemoglobin oxygen saturation curve were linearly dependent on dose. BW12C whole blood pharmacokinetics were best described by a one-compartment model and were clearly dose-dependent. The half-life increased from 2.1 h at a dose of 20 mg/kg to 7.2 h at a dose of 60 mg/kg. The AUC increased in a similar non-linear fashion with increasing dose. Mitomycin C was given at a fixed dose of 20 mg/m2 at the end of the BW12C infusion. Mitomycin C plasma pharmacokinetics fitted a two-compartment model, giving a mean beta half-life of 50±7 min and AUC of 1.1±0.08 μg/ml h, and were unaffected by the combined treatment. There was no evidence of increased mitomycin C toxicity.

Notes: Abstract  A sensitive solid-phase-extraction and high-performance liquid chromatography (HPLC) method has been developed to investigate the pharmacokinetics and metabolism of the hypoxic-cell cytotoxic agent tirapazamine (1,2,4-benzotriazine-3-amine 1,4-di-N-oxide; WIN 59075, SR 4233), currently 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 resolved using a μBondapak 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 was consistently greater than 80% for all compounds over the concentration range of 20 ng/ml to 20 μg/ml. No significant decomposition was observed following up to three freeze/thaw cycles and storage at –70°C for 52 days. The assay was accurate and reproducible, with measured values lying within the limits of defined acceptance criteria. Additional studies to investigate the degree of plasma protein binding showed that tirapazamine did not bind extensively to plasma proteins (binding, 9.7%±0.1% and 18.7%±1.3% in mouse and human plasma, respectively). These small species differences in protein binding are unlikely to have any major impact on the extrapolation of pharmacokinetic data from mice to humans. The assay has now been successfully applied to investigate the pharmacokinetics and metabolism of tirapazamine in mice and patients as part of a pharmacokinetically guided dose-escalation strategy for phase I clinical trials.

Notes: Abstract  Tirapazamine (3-amino-1,2,4-benzotriazine-1,4-di-N-oxide; SR 259075) is a selective hypoxic cell cytotoxic agent that is bioreductively activated in tumours to a reactive-drug free radical. Preclinically the agent has been shown to possess additive and synergistic anti-tumour activity in combination with radiotherapy and chemotherapy regimens. In the present study the pharmacokinetics and metabolism of tirapazamine were investigated in mice and patients as part of pre-clinical and phase I investigations. The objectives of this work were twofold; firstly, to evaluate retrospectively the utility of a pharmacokinetically guided 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 LD10 to the LD50. The AUC at the LD10 (2932 μg ml-1min) was used to determine a target AUC value of 1173 μg ml-1min (equivalent to 40% of the mouse LD10 AUC) for clinical studies. A phase I study to investigate the tolerance of a single i.v. infusion of tirapazamine (once every 3 weeks) was initiated with close pharmacokinetic monitoring. The starting dose (36 mg/m2) was based on toxicity data obtained in the mouse, rat and dog. Doses were escalated by increases in the volume and duration of infusion. A retrospective analysis of the pharmacokinetic and toxicity data was then made to determine the utility of a PGDE approach. The drug exhibited a steep dose-lethality relationship in mice (LD10 294 mg/m2, LD50 303 mg/m2). 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/m2. The plasma elimination of tirapazamine fitted a monoexponential open model, with rapid elimination from the plasma (t 1/2=36±0.65 min) occuring at the LD10 dose of 294 mg/m2. A 10.3-fold increase in dose resulted in a 25.0-fold increase in AUC. Clinically, doses were escalated over the range of 36–450 mg/m2. Ototoxicity (tinnitus and reversible hearing loss) was dose-limiting at 450 mg/m2 and the MTD was 390 mg/m2 for this schedule. Pharmacokinetic analyses in patients revealed that the elimination of tirapazamine in patients was generally bi-phasic, with low inter-patient variability being 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 metabolite formation between mice and humans with respect to the two- and four-electron bioreductive metabolites. AUC values recorded for tirapazamine at the MTD of 390 mg/m2 (range 1035–1611 μg ml-1min) were similar to the target AUC in mice. Importantly, these levels are consistent with the levels required for radiation-dose enhancement and effective 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 protein binding seen between species and (c) the relatively low inter-patient variability noted in drug clearance, a successful PGDE approach should have been feasible. The results also indicate that potentially therapeutic levels of tirapazamine are achievable in patients at tolerable doses.

Notes: Summary An understanding of lipophilicity and pharmacokinetics is important in developing alternative radiosensitizers to misonidazole (MIS). Analogues more hydrophilic that MIS, including its O-demethylated metabolite DEMIS (Ro 05-9963), appear promising candidates. In vivo testing is usually carried out in mice, and the present paper reports dose-dependence and related studies on the comparative kinetics of MIS and DEMIS in this species. The data are consistent with a model in which MIS is eliminated mainly by metabolism, including demethylation to DEMIS, and saturable (non-linear) kinetics are exhibited. Apparent t1/2 increased with dose. But DEMIS is eliminated mainly by renal clearance exhibiting first-order (linear) kinetics. Phenobarbitone reduced the t1/2 and toxicity of MIS but not of DEMIS. SKF 525A increased the t1/2 of MIS. The following were not responsible for the non-linear kinetics of MIS: short-term enzyme induction by MIS; potent enzyme inhibition by the product DEMIS; decrease in body temperature by MIS; injection volume; and protein binding. For both MIS and DEMIS peak blood concentrations were about 2-fold lower for IP than IV injection. The IP bioavailability of DEMIS (1.07 mmol/kg) was 100%, but that of MIS (1 mmol/kg) was 80%, suggesting some first-pass metabolism. Both MIS and DEMIS were absorbed more slowly and gave lower peak blood concentrations after IP injection in a large (40 ml/kg) as against a small (10ml/kg) volume. Peak concentrations were lower for equimolar IP DEMIS, but this was less marked at lower doses. With both MIS and DEMIS, tumour/blood and brain/blood ratios were slightly increased at higher doses. Some kinetic differences were also observed between male and female mice. The above findings, particularly the major differences in elimination kinetics between MIS and DEMIS, should be considered in in vivo experiments with nitroimidazoles.

Notes: Summary We have compared the mouse pharmacokinetics of six analogues of the hypoxic cell sensitizer misonidazole (MISO). The analogues were all uncharged and similar in redox potential, but widely different in octanol-water partition coefficient (range 0.026–1.5). Lipophilic analogues were cleared mainly by metabolism and non-linear elimination kinetics were seen at high doses. Hydrophilic analogues, including desmethylmisonidazole, SR-2508, and SR-2555, were removed principally by renal clearance exhibiting linear elimination kinetics. Lipophilic analogues were cleared more rapidly after hepatic microsomal enzyme induction by phenobarbitone, whereas the kinetics of hydrophilic analogues were unaffected. Low-dose clearance was similar for most of the analogues. But the hydrophilic SR-2555 was cleared twice as quickly as MISO, and the liphophilic Ro 07-0913 seven times faster than MISO. Plasma protein binding was low for all the analogues. The significance of these results for the predictive value of the mouse as a model for man is discussed.

Notes: Summary RSU 1069 is a leading compound in the class of mixed-function hypoxic cell sensitizers. Possessing an alkylating aziridine function as well as a nitro group, it represents an important prototype molecule for new sensitizer development. Using a novel HPLC assay for RSU 1069 and its metabolites with a cyanopropyl column, we studied the detailed pharmacokinetics and metabolism of this drug in mice. An i.v. dose of 100 mg kg-1 produced peak plasma concentrations of about 100 μg ml-1. Absorption was rapid after i.p. injection but peak plasma, concentrations were some three- to fourfold lower, giving an i.p. bioavailability of 55%. The elimination t1/2 was route-dependent; e.g. after 50 mg kg-1 the t1/2 was 37.2 and 22.4 min for the i.v. and i.p. routes respectively (P〈0.001). There was also an indication of dose-dependent kinetics, with a 37% increase in elimination t1/2 when the i.p. dose was doubled from 50 to 100 mg kg-1. Oral bioavailability was low. The volume of distribution was 0.65–1.31 ml g-1 at 50 mg kg-1, but tissue penetration was limited. Brain/plasma ratios ranged from 9.3% to 66.8%, while the mean steady-state tumour/plasma ratio was 28.4%, a value considerably less than the 80%–100% ratios occurring with the neutral 2-nitroimidazole misonidazole. About 18% and 8% of a dose were excreted as the parent drug and the ring-opened hydrolysis product (RSU 1137) in the 8 h urine, indicating the likelihood of extensive metabolism via aziridine-ring remooval and nitroreduction. RSU 1137 was also detected in mouse plasma and tissues and, in contrast to the aziridine ring-intact parent compound, gave tumour/plasma ratios of 100%. These studies should provide a pharmacokinetic basis for the evaluation and development of improved mixed-function sensitizers.

Notes: Summary A range of anthracyclines and related compounds were evaluated for activity against murine and human cell lines exhibiting multidrug resistance (MDR). Cell lines used were the NCI-H69 human small-cell lung cancer line and the EMT6 murine mammary tumour line, together with their multidrug-resistant counterparts produced by in vitro exposure to Adriamycin (ADM). Chemosensitivity testing was carried out using the tetrazolium (MTT) dye assay. Results were expressed as the ratio of the ID50 for the resistant line to that obtained in the parent, i.e. the resistance factor (RF). Compounds exhibiting much lower RF values than ADM in both resistant cell lines were identified as those anthracyclines with 9-alkyl substituents and those with certain changes to the amino sugar residue at position 3′ and 4′, together with the anthracenedione mitoxantrone (MIT). In a further attempt to overcome resistance, we used four of these compounds, Ro 31-1215, 4′-deoxy-4′-iodo-ADM (iodo-ADM), aclacinomycin A (ACL) and MIT (all yielding low RF values), in combination with the resistance modifiers verapamil (VRP) and cyclosporin A (CYA). Additional enhancement of chemosensitivity was achieved in the ADM-resistant sublines, as shown by the further decrease in RF values. At the concentrations used, the largest effects were generally seen with CYA, and the combination of this modifier with ACL and MIT was particularly effective. For the H69/LX4 resistant line, the latter combinations gave RF values approaching unity. These findings point to the use of analogues with the 9-alkyl substituent and/or specific changes to the sugar residue in combination with resistance modifiers as a therapeutic strategy for circumvention of the MDR phenotype.