**2. Expression of CYP Enzymes in Extrahepatic Tissues**

Although the metabolic capacity of extrahepatic CYPs is relatively low compared to the capacity of hepatic CYPs, extrahepatic CYPs may still influence local tissue function and drug exposure, as well as drug biotransformation and bioactivation at these sites. Further, extrahepatic CYP enzymes may alter overall systemic exposure to xenobiotics, with corresponding elevations in toxicity [19,31]. Interestingly, some CYPs are expressed preferentially in extrahepatic tissues, which may lead to unique extrahepatic metabolites and tissue-specific consequences in cellular toxicity and organ pathology. The CYPs expressed in extrahepatic tissues are shown in Table 1.


**Table 1.** Select xenobiotic metabolizing cytochrome P450 enzymes expressed in human extrahepatic tissues.

Key: + mRNA, ++ protein, +++ activity.
