The role of aldehyde oxidase in the in vivo metabolism of benzothiazole

The Role of Aldehyde Oxidase in the Metabolic Clearance of Substituted Benzothiazoles

BACKGROUND

A group of substituted benzothiazoles from a research project was found to have low microsomal clearance. However, these compounds had very high clearance in vivo.

METHODS

In the present study, the clearance mechanism of two of the structural analogs, was investigated in vitro and in vivo.

RESULTS

In vitro studies showed the formation of corresponding non-P450 dependent oxidative metabolites in S9, cytosol, and hepatocytes. The in vitro formation of these metabolites was observed in mice, rats, non-human primates, and humans. The dog did not form the corresponding metabolites in any of the matrices. Inhibition studies with S9 fraction and incubation with human recombinant aldehyde oxidase (AO) showed that the formation of the corresponding metabolites was AO dependent. To investigate the role of this pathway in vivo, mice were dosed with compound A and bile and plasma were analyzed. Most of the metabolites in bile contained the AO-dependent oxidized benzothiazole moiety, indicating that metabolism involving AO was probably the main pathway for clearance. The same metabolites were also observed circulating in plasma. Mass spectrometric analysis of the metabolite showed that the oxidation was on the benzothiazole moiety, but the exact position could not be identified. Isolation of the metabolite of compound A and analysis by NMR confirmed the structure of the metabolite as C2 carbon oxidation of the thiazole ring resulting in carboxamide moiety. Further comparison of both metabolites with corresponding authentic standards confirmed the structures.

CONCLUSION

To our knowledge, such an observation of in vitro and in vivo oxidation of substituted benzothiazole by AO has not been reported before. The results helped the medicinal chemists design compounds that avoid AO-mediated metabolism and with better ADME property.

Metabolism by Aldehyde Oxidase: Drug Design and Complementary Approaches to Challenges in Drug Discovery

Aldehyde oxidase (AO) catalyzes oxidations of azaheterocycles and aldehydes, amide hydrolysis, and diverse reductions. AO substrates are rare among marketed drugs, and many candidates failed due to poor pharmacokinetics, interspecies differences, and adverse effects. As most issues arise from complex and poorly understood AO biology, an effective solution is to stop or decrease AO metabolism. This perspective focuses on rational drug design approaches to modulate AO-mediated metabolism in drug discovery. AO biological aspects are also covered, as they are complementary to chemical design and important when selecting the experimental system for risk assessment. The authors' recommendation is an early consideration of AO-mediated metabolism supported by computational and in vitro experimental methods but not an automatic avoidance of AO structural flags, many of which are versatile and valuable building blocks. Preferably, consideration of AO-mediated metabolism should be part of the multiparametric drug optimization process, with the goal to improve overall drug-like properties.

Drug-Metabolizing Ability of Molybdenum Hydroxylases

Molybdenum hydroxylases, which include aldehyde oxidase and xanthine oxidoreductase, are involved in the metabolism of some medicines in humans. They exhibit oxidase activity towards various heterocyclic compounds and aldehydes. The liver cytosol of various mammals also exhibits a significant reductase activity toward nitro, sulfoxide, N-oxide and other moieties, catalyzed by aldehyde oxidase. There is considerable variability of aldehyde oxidase activity in liver cytosol of mammals: humans show the highest activity, rats and mice show low activity, and dogs have no detectable activity. On the other hand, xanthine oxidoreductase activity is present widely among species. Interindividual variation of aldehyde oxidase activity is present in humans. Drug-drug interactions associated with aldehyde oxidase and xanthine oxidoreductase are of potential clinical significance. Drug metabolizing ability of molybdenum hydroxylases and the variation of the activity are described in this review.

Molecular characterization of the amplified aldehyde oxidase from insecticide resistant Culex quinquefasciatus

Primary structural information including the complete nucleotide sequence of the first insect aldehyde oxidase (AO) was obtained from the common house mosquito Culex quinquefasciatus (Say) through cloning and sequencing of both genomic DNA and cDNA. The deduced amino-acid sequence encodes a 150-kDa protein of 1266 amino-acid residues, which is consistent with the expected monomeric subunit size of AO. The Culex AO sequence contains a molybdopterin cofactor binding domain and two iron-sulfur centres. A comparison of the partial sequences of AO from insecticide resistant and susceptible strains of C. quinquefasciatus shows two distinct alleles of this enzyme, one of which is amplified in the insecticide resistant strain on a 30-kb DNA amplicon alongside two resistance-associated esterases. The amplified AO gene results in elevated AO activity in all life stages, but activity is highest in 3rd instar larvae. The elevated enzyme can be seen as a separate band on polyacrylamide gel electrophoresis. The role of AO in xenobiotic oxidation in mammals and the partial inhibition of elevated AO activity by a range of insecticides in Culex, suggest that this AO may play a role in insecticide resistance.