Phase II metabolism usually follows phase I metabolism and generally plays a minor metabolic role. Lamotrigine, morphine, and lorazepam are primarily metabolized by phase II metabolism. These reactions are conjugation reactions in which water-soluble molecules bind with the drug to make it more easily excreted. The most common phase II enzymes are the uridine glucuronosyltransferases (UGTs). Classified into 1A and 2B, the UGT enzyme systems also have substrates, inhibitors, and inducers (e.g., glucuronida-tion of lorazepam is competitively inhibited by the nonsteroidal anti-inflammatory drugs [NSAIDs]).
P-glycoproteins participate in the transport of substances out of cells into the gastrointestinal tract, bile, blood, and urine (Hennessy and Spiers 2007). They are involved in blocking gastrointestinal absorption and are part of the first-pass effect, functioning as "gatekeepers" for CYP3A4 metabolism. P-glycoproteins may have a role in prevention of drug uptake from the blood to the brain (Lin and Yama-zaki 2003; Linnet and Ejsing 2008). The P-glycopro-tein transporter does not affect drug metabolism but rather influences drug bioavailability by removing P-glycoprotein substrates and returning them to the gut lumen or the bloodstream. P-glycoprotein inhibitors antagonize this process and precipitate retention of P-glycoprotein substrates. For example, omeprazole, which is an inhibitor of the P-glycopro-tein transporter system, may lead to increased serum concentrations of carbamazepine, a substrate for this system.
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