Use of Microorganisms as Models of Mammalian Drug Metabolism

USE  OF  MICROORGANISMS  AS  MODELS  OF MAMMALIAN  DRUG  METABOLISM

The safety and efficacy of a drug must be exhaustively evaluated before its approval for use in the treatment of human diseases. Investigations of the manner in which a drug is metabolized are extremely valuable as they potentially provide information on its mode of action, why it exhibits toxicity and how it is distributed, excreted and stored in the body. Traditionally, drug metabolism studies have relied on the use of animal models and, to a lesser extent, liver microsomal preparations, tissue culture and perfused organ systems. Each of these models has certain advantages and disadvantages. Animals in particular are expensive to purchase and maintain, and there is considerable pressure from animal welfare groups to curb the use of animals in scientific research.

The many similarities between certain microbial enzyme systems and mammalian liver enzyme systems has led to the utilization of various microbial models for the exploration of mammalian drug metabolism. The major advantages of using microorganisms are their ability to produce significant quantities of metabolites that would otherwise be difficult to obtain from animal systems or by chemical synthesis, and the considerable reduction in operating costs compared with animal studies.

Microbial drug metabolism studies are usually carried out by firstly screening a large number of microorganisms for their ability to metabolize a drug substrate. The organism is usually grown in a medium such as peptone glucose in flasks that are shaken to ensure good aeration. Drugs as substrates are generally added after 24 hours of growth and are then sampled for the presence of metabolites at intervals up to 14 days after substrate addition. Once it has been determined that a microorganism can metabolize a drug, the whole process can be scaled up for the production of large quantities of metabolites for the determination of their structure and biological properties.

As an example of this the metabolism of the antidepressant drug imipramine can be considered. In mammalian systems, this is metabolized to five major metabolites:2-hydroxyimipramine,10-hydroxyimipramine, iminodibenzyl, imipramine-N-oxide and desipramine (Figure 26.10).

For microbial metabolism studies, a large number of fungi are screened, from which several are chosen for the preparative scale production of imipramine metabolites. Cunninghamella blakesleeana produces the hydroxylated metabolites2-hydroxyimipramine and 10-hydroxyimipramine; Aspergillus flavipes and Fusarium oxysporum f. sp. cepae yield the N-oxide derivative and iminodibenzyl, respectively; while the pharmacologically active metabolite desipramine is produced by Mucor griseocyanus together with the 10-hydroxy and N-oxide metabolites. By scaling up this procedure, significant quantities of the metabolites that are formed during mammalian metabolism can be obtained.

Microorganisms thus have considerable potential as tools in the study of drug metabolism. Although they cannot completely replace animals, they are extremely useful as predictive models for initial studies.