Metabolism experimental models

Before discussing the animal data it is important to note that caffeine is metabolized differently in experimental animals than it is in humans. This is particularly so in the rat, which is the most common experimental model used. Therefore results in animals, either positive or negative, cannot be directly applied to humans. However, since caffeine itself and some of its metabolites are present in both the animal experiments and during human exposure, an adverse effect of caffeine in an animal model should be verified or excluded in the human. 

Systems to study the role of intestinal oxidative metabolism (CYP3A4) have been developed and appear to have adequate enzyme activity levels. Although there appears to be a relatively limited need for additional system development in this area, there is still a fundamental question as to whether any synergistic interplay exists between metabolic enzymes and transporters (i.e., does the presence of an efflux transporter influence the extent of metabolism ) and co-expression of CYP3A4 and transporters provides a pivotal experimental model. 

Ward et al. [125] investigated the disposition of 14C-radiolabeled primaquine in the isolated perfused rat liver preparation, after the administration of 0.5, 1.5, and 5 mg doses of the drug. The pharmacokinetics of primaquine in the experimental model was dependent on dose size. Increasing the dose from 0.5 to 5 mg produced a significant reduction in clearance from 11.6 to 2.9 mL/min. This decrease was accompanied by a disproportionate increase in the value of the area under the curve from 25.4 to 1128.6 pg/mL, elimination half-life from 33.2 to 413 min, and volume of distribution from 547.7 to 1489 mL. Primaquine exhibited dose dependency in its pattern of metabolism. While the carboxylic acid derivative of primaquine was not detected perfusate after the 0.5 mg dose, it was the principal perfusate metabolite after 5 mg dose. Primaquine was subject to extensive biliary excretion at all doses, the total amount of 14C-radioactivity excreted in the bile decreased from 60 to 30%i as the dose of primaquine was increased from 0.5 to 5 mg. 

The student should be aware that a pathway is essentially a conceptual model developed by biochemists in order to represent the flow of compounds and energy through metabolism. Such models are simply ways of trying to explain experimental data. A potential problem in representing metabolic pathways as in Figure 1.1 is that there is an implication that they are physically and/or topographically organized sequences. This is not necessarily true. With some exceptions (described in Section 1.3), most enzymes are likely to be found free within the cytosol or a compartment of a cell where reactions occur when an enzyme and its substrate meet as a result of their own random motion. Clearly this would be very inefficient were it not for the fact that cells contain many copies of each enzyme and many molecules of each type of substrate. 

Lung metabolism studies would greatly benefit from the availability of reliable in vitro experimental models that are able to reproduce biochemical and toxicological processes that occur in the human lung. Because of differences in the expression profiles of biotransformation enzymes between animal species, conclusions drawn from experimental animals may not be representative of what occurs in the human lung. 

Bromoethylamine (11.133, R = Br, Fig. 11.18) is a potent nephrotoxin used to create an experimental model of nephropathy. Its mechanism of toxicity is postulated to involve perturbation of mitochondrial function, and its metabolism was investigated in a search for toxic metabolites. In rat plasma, 2-bromoethylamine was converted to aziridine (11.134), formed by intramolecular nucleophilic substitution and bromide elimination [155], Another major metabolite was oxazolidin-2-one (11.136). This peculiar metabolite resulted from the reaction of 2-bromoethylamine with endogenous carbonate to form carbamic acid 11.135, followed by cyclization-elimination to oxazoli-din-2-one. In aqueous media containing excess carbonate, the formation of... 

While the amine hypothesis is undoubtedly too simplistic, it has provided the major experimental models for the discovery of new antidepressant drugs. As a result, all the currently available antidepressant drugs—except bupropion—are classified as having their primary actions on the metabolism, reuptake, or selective receptor antagonism of serotonin, norepinephrine, or both. 

Our insight into the development, evolution and the mechanisms of damage in cerebral ischemia is mainly based on animal studies. A large variety of experimental models have been developed that imitate conditions of stroke and cardiac arrest (Hossmann 1991). In the past, experiments had to be terminated at certain timepoints to obtain invasive measurements of lesion size, blood flow, metabolism or other markers of injury. Therefore, longitudinal observations required large animal numbers and the inter-individual differences complicated the analysis of results. The advent of MR techniques of imaging... 

Musil J (1977) Physiological characteristics of various experimental models for the study of disorders in purine metabolism. In Muller MM, Kaiser E, SeegmiUer JE (eds) Purine Metabolism in Man II - Physiology, Pharmacology and Clinical Aspects. Plenum Publishing Corp., New York, pp 179-188... 

Cryopreserved human hepatocytes Cryopreserved human hepatocytes are extremely useful for the evaluation of drug metabolism, but in general cannot be cultured due to their impaired cell-attachment. There are preparations of cryopreserved human hepatocytes with high plating efficiency and therefore can be cultured for induction studies. It has been shown that hepatocytes cultured after cryopreserva-tion are responsive to CYP1A and 3A inducers, but they have a significantly lower basal (uninduced) levels of these enzymes. Cryopreserved human hepatocytes therefore represent a more convenient experimental model than freshly isolated human hepatocytes for enzyme induction studies (Roymans etal. 2005). 

Langlais PJ (1995) Pathogenesis of diencephalic lesions in an experimental model of Wernicke s encephalopathy. Metabolic Brain Diseases 10, 31-44. 

Myelin disorders can have a genetic, toxic, or infectious origin and some such as MS even an immunological component. For most of these diseases, there are animal models. Whatever the causes, the targets are structural or metabolic elements necessary for intact myelin. Diseases involving myelin are often related to specific myelin constituents whether common to CNS and PNS, or different. One can believe that these specialized compounds play significant roles in the pathology of myelin. Thus there are diseases or experimental models that involve only CNS or PNS, and others that involve both systems. 

Figure 8.6 shows the projection of 1346 compounds from Johnson Johnson on the VolSurf metabolic stability model. The projected compounds are color-coded according to their percentage experimental metabolic stability (%MS) the red color defines compounds with %MS > 95, while the blue color defines compounds with %MS < 40. The figure shows that the great majority of projected compounds with %MS > 95 are predicted to be of medium or high stability for the metabolic activity of the CYP3A4, while compounds with low %MS < 40 are predicted to be unstable. The presence of outliers may be explained by the fact that the experimental %MS of the test set compounds is obtained from the activity of all CYP family enzymes (2C9, 3A4, 2D6 etc.), while the model uses only 3A4 mediated information. 

The mode of action of protopine on rabbit platelet aggregation was investigated in the metabolic system of adenosine 3 ,5 -cyclic monophosphate (cyclic AMP) in vitro experimental models. The inhibitory activity of the alkaloid on adenosine S -diphosphate induced platelet... 

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