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  • The small intestine in addition to

    2019-10-08

    The small intestine, in addition to its primary role of absorption, has the ability to metabolize drugs and other xenobiotics through phase 1 and phase 11 metabolisms [25] since almost all the cytochrome P450 this page present in the liver are also present in the small intestine although, in lower concentrations. The higher microsomal protein detected in the intestinal microsome in this study may not necessarily indicate higher cytochrome P450 content than the liver fraction. Constructed on the principle that CYP3A converts erythromycin to N-demethyl erythromycin and formaldehyde which determines CYP 3A activity based on the quantity of formaldehyde formed, the result of the study on intestinal microsomes indicated that ADT and AAM significantly inhibited intestinal CYP 3A activity when compared to the negative control. However, the greater inhibition achieved with the 50 μg concentration of AAM compared to the 100 μg concentration may be due to saturable inhibition that may result in saturable first pass metabolism with the rate of inhibition being concentration dependent. This saturable inhibition has been shown to occur more with intestinal enzymes [26]. The inhibitions observed with the 100 μg concentrations of PEDT and EAM with no inhibitions with their 50 μg concentrations was indicative of dose dependent activity. Similarly, the result of this study on liver microsomes indicated that all the extracts exhibited a direct dose dependent liver CYP 3A inhibition with the 100 μg of each of the extracts inhibiting the liver microsomal activity more than their corresponding 50 μg concentrations. The greater inhibition of the intestinal microsomal CYP 3A activity compared to that of the liver similar to other studies [17] may be due to higher concentrations of CYP enzymes in liver microsomes. Hence, higher metabolic clearance is seen in the liver preparations than intestinal preparations [26], [27]. The effect of the extracts on CYP 3A was further assessed using the in vivo method. Using simvastatin as the substrate of choice [21], [22], the in vivo assay method was built on the principle that inhibition of a major metabolising enzyme responsible for the metabolism of an orally administered agent will lead to an enhanced plasma concentration of the agent. Simvastatin is majorly metabolised by CYP3A4 with minor contribution of CYP3A5 [28], [29]. Since AUC and Cmax are used to measure the extent of drugs bioavailability [30], the significant (P < 0.05) increases in AUC, AUMC and Cmax of simvastatin observed in the extracts treated groups were indicative of enhanced absorption and bioavailability of the drug with possible suppression of CYP 3A enzyme responsible for the drug\'s metabolism. The significant increase in these parameters in the vehicle and simvastatin treated group compared to the dexamethasone, vehicle and simvastatin treated group is indicative of the role of dexamethasone in inducing CYP 3A enzymes in the rats. Dexamethasone is a potent CYP 3A inducer and is used in inducing this enzyme family in animal studies [31]. Although dexamethasone treated liver microsomes has been an accepted model for evaluating drug interactions involving CYP450 3A enzyme inhibition [20] as reported by Nduka et al[18]; findings by Perloff et al[32] likewise indicated that dexamethasone also induces P-glycoprotein in rats in addition to CYP3A. Hence, possible effect of dexamethasone on P-glycoprotein may have also contributed to the observed differences in simvastatin dispositions in these two groups. On the other hand, ketoconazole is a potent and well-studied inhibitor of CYP 3A enzyme in humans and animals and often used for in vitro and in vivo studies as the CYP 3A inhibitor of choice [33]. Therefore, ketoconazole\'s use as a positive control in dexamethasone and simvastatin treated group showed Cmax, AUC and AUMC significantly higher than the vehicle treated group showing its potent CYP 3A inhibition. This inhibition of the metabolizing enzyme family by the extracts was further shown by the decreased clearance of the drug in the extracts treated groups and their corresponding increased MRT though not all were significant. Hence, these suggested the in vivo cytochrome P450 3A inhibition by the extracts though, at different degrees. Furthermore, it is important to state that P-glycoprotein may have also been inhibited by these extracts because CYP 3A enzymes and P-glycoproteins have been reported to share similar modulators [34].