TISSUE SPECIFICITY OF HUMAN DRUG METABOLISM ENZYMES
Though most drug metabolism enzymes reside in the liver, other organs may also play an important role. All of the enzymes previously mentioned are found in the human liver, but other tissues and organs may have some complement of these enzymes. CYP3A4 and CYP3A5 have been found in the human gut and can contribute to substantial metabolism of orally adminis-tered drugs, even before the compound reaches the liver. For example, CYP3A4 may play a substantial role in the low bioavailability of cyclosporine. Drug-metab-olizing enzymes have also been found in measurable quantities in the kidney, brain, placenta, skin, and lungs.