Pharmacokinetics and Toxicology of the Hypoxia-Activated Prodrug SN30000 and its Major Metabolites

Abstract

SN30000 is a second generation analogue of the hypoxia-activated prodrug tirapazamine, currently under advanced pre-clinical testing. This thesis explores aspects of the pharmacology, metabolism, toxicity and redox cycling of SN30000 and its 1-oxide metabolite. Pharmacokinetics (PK) of SN30000 and 1-oxide was investigated in liver by rapid or delayed sampling under anaesthesia and by delayed post-mortem freezing identifying post-mortem metabolism as being a significant issue for SN30000 but not its 1-oxide metabolite, necessitating rapid sample recovery from mice in studies of SN30000. Plasma PK of SN30000 and its 1-oxide metabolite was well-fitted by a preliminary two-compartment open model. PK of intraperitoneal (i.p.) SN30000 in NIH-III nude mice demonstrated increasing plasma exposure relative to dose especially at doses of 50-100% MTD. The i.p. bioavailability of SN30000 at low dose was 26% while renal excretion was low and clearance of co-administered mannitol decreased with SN30000 dose. These results suggest the dosedependent PK of SN30000 may be due to suppression of its first-pass metabolism in liver at high doses partly by physiological mechanisms. High concentrations of SN30000 1-oxide in the brain after its administration indicates uptake or partitioning from plasma is a major mechanism for the high concentration of this metabolite observed in brain after SN30000 administration. In addition, a previously unidentified metabolite of SN30000 (M9) was identified as the corresponding carboxylic acid. M9 had a long plasma half-life, with higher concentrations in plasma than liver, tumour and brain and was shown to have very low toxicity or cytotoxicity. The redox cycling of SN30000 was investigated in CHO cells and its rate was shown to be dependent on SN30000 concentration, length of time elapsed since exposure and the cell’s reductase activity. In contrast, little redox cycling was found for SN30000 1-oxide metabolite. SN30000 redox cycling was demonstrated to affect cellular reactive oxygen species (ROS) defence pathways by significantly decreasing both reduced-to-oxidised glutathione and dimer-to-monomer peroxiredoxin (Prx) ratios. These biomarkers have potential to monitor normal tissue toxicities and tumour cytotoxicity associated with SN30000 redox cycling.