Biotransformation of Drugs

Metabolism The sum of anabolism and catabolism, although also used to describe the biotransformation of drugs in the liver. 

Caro I, Boulenc X, Rousset M, Meunier V, Bourne M, Julian B, Joyeux H, Roques C, Berger Y, Zweibaum A, Fabre G (1995) Characterisation of a newly isolated Caco-2 clone (TC-7), as a model of transport processes and biotransformation of drugs. Int J Pharm 116 147-158. 

Table 3.2. Biotransformation of drugs by phase 1 and phase 2 reactions...

Most drugs used in anaesthesia are metabolised in the liver by phase I reactions, mediated by cytochrome P-450 enzymes. These are susceptible to destruction by cirrhosis, so that the biotransformation of drugs, such as opioids (except morphine), benzodiazepines, barbiturates, and inhalational agents, may be markedly altered in severe liver disease. These enzymes are found in the centrilobular areas, which are more prone to hypoxia. In contrast, the enzymes responsible for phase II reactions, found predominantly in the peripheral areas, often function normally even in advanced disease. The disposition of benzodiazepines that are eliminated primarily by glucuronidation, e.g. lorazepam and oxazepam, are unaffected by chronic liver disease. For drugs with low hepatic extraction, advanced hepatocytic dysfunction decreases phase I and II biotransformation with a reduced clearance and prolongation of the elimination half-life. This is often partially offset by an increased free fraction due to decreased protein binding. 

Femandez-Metzler, C. L., Owens, K. G., Baillie, T. A., and King, R. C. (1999). Rapid liquid chromatography with tandem mass spectrometry-based screening procedures for studies on the biotransformation of drug candidates. Drug Metab. Dispos. 27 32-40. 

Smith, R.V., Rosazza, P. Microbial systems for study of the biotransformation of drugs. Biotechnol. Bioeng. 17 785-814. 

Biotransformation or metabolic inactivation of drugs occurs mainly in the liver and, to a lesser extent, in the plasma, kidney, and other tissues, depending on the enzyme system involved. In the liver, microsomal enzymes catalyze many of the metabolic processes involved in the biotransformation of drugs. These metabolic processes may involve nonsynthetic reactions such as oxidation, reduction, or hydrolysis, or synthetic reactions, including conjugation, whereby the drug is coupled with an endogenous substrate (53). 

Similar to many other cases of biologically active compounds, stereochemistry influences the pharmacological effect of a chiral drug. This can be explained by the fact that there is only one energeticaUy favorable (specific) interaction of an active molecule with its receptor, both being chiral structures. Qualitative and quantitative differences are caused by different receptor affinities as demonstrated in Fig, 1 (1). The metabolism (biotransformation) of drugs is mainly caused by enzymes, which are chiral macromolecules and discriminate between substrate molecules of different stereochemistry, This may result in metabolites of different activity and in different pharmacokinetics, resorption, and excretion. Therefore, racemic drugs should be looked on as a 1 1 mixture of two different compounds. 

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