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though this is partly true for the highly conserved enzymes such as CYP 1A or CYP 2E1, it is not the case for many others. |
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The major CYP enzyme in the rat is CYP 2C, whilst in the human it is CYP 3A4. In the rat, CYP 3A2 and CYP 2C show sexual dimorphism, but not in man. The enzyme CYP 2C11 exists as 50% of all hepatic P450 in the male rat, whilst CYP 2C12 is the major enzyme in the female [27]. Similarly, CYP 2D6 is not present in the mouse or rat, and in general, rodents are poor models for polymorphism [28]. Thus, researchers are becoming increasingly aware that human tissues or cell lines must be used for identification of metabolic enzyme involvement during early drug development. Human microsomes are readily available and can be stored for several months, but hepatic slices and hepatocytes have limited viability and must be used within 48 hours. Until recently, this has severely restricted their use, but now with the availability of cadaver livers direct from hospitals or from nonprofit commercial organizations [29], this problem has been largely overcome. Whether this situation will continue, as transplant services become more efficient, is questionable. |
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In addition to predicting the possibility of intersubject differences in kinetics due to the variability in the expression of enzyme, these early in vitro methods will identify the possibility of drug-drug interactions since certain isoforms have been shown to be inhibited (CYP 2D6) or induced by drugs (CYP 2E1 and alcohol) or environmental factors (CYP 1A1 and smoking). |
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Although more than 65% of UK pharmaceutical companies are starting to undertake these studies [23], there is some concern as to the importance that certain academics and regulatory authorities are placing on these results. All those replying to this survey said that if a drug was shown to be metabolized by an enzyme under polymorphic control, they would still continue with its development because there are many other factors that must be taken into account before rejecting such a drug. Reasons given for continuing the development include a wide therapeutic margin, alternate, more important, pathways, novel activity and clinical need, relationship between therapeutic plasma levels, and enzyme affinity constant. Fluoxetine and other specific serotonin reuptake inhibitors (SSRI), which are all metabolized by CYP 2D6, are but one example of a class of drugs that could have been discarded if in vitro metabolism had been used for screening. Also, hepatic metabolism is only one of many elimination routes and it is known that the GI tract has large amounts of CYP 3A4 [30]. For example, cyclosporin is extensively metabolized in the gastrointestinal mucosa before reaching the liver [31]. The brain also has appreciable levels of CYP 2D6 and may contribute to the activity of CNS drugs such as the antidepressants, dextromethorphan and codeine [32]. |
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Alternative routes of elimination can become important in poor metabolizers, as has been shown for flecainide, which is extensively eliminated in the kidney thereby reducing potential side effects [33]. Similarly, alternative metabolic |
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