Hepatic Metabolism

Hepatic metabolism is the liver's contribution to clearance, and it is bounded by arithmetic: hepatic clearance equals liver blood flow multiplied by the extraction ratio, so nothing can be cleared hepatically faster than blood delivers it, on the order of 90 litres per hour in an adult. For small molecules the chemistry is usually cytochrome P450 oxidation followed by conjugation. Peptides take a different route, cut by peptidases in plasma, kidney, liver and gut into fragments and amino acids that rejoin normal metabolism.

The consequence is visible on labels. Semaglutide is degraded by proteolytic cleavage of the backbone and beta-oxidation of its fatty diacid chain, processes spread across tissues rather than concentrated in the liver, and no cytochrome-mediated interaction drives its dosing. Small molecules in the same therapeutic space behave differently: saxagliptin is metabolised by CYP3A4 and 3A5 to an active metabolite, while sitagliptin is largely excreted unchanged by the kidney.

Knowing the route tells you which organ failure matters and which interaction questions are worth asking. A peptide cleared by ubiquitous peptidases has little scope for a classical enzyme-inhibition interaction, one of the genuine advantages of the class. A compound with a high extraction ratio, by contrast, has clearance sensitive to hepatic blood flow and loses heavily to first pass when given by mouth.

Two misreadings recur. The first treats any hepatic-impairment statement on a label as proof of hepatic metabolism, when such statements often exist because impairment alters plasma proteins, blood flow or fluid distribution. The second reads transaminase elevations as evidence that the liver is clearing the drug; enzyme release reports injury, not clearance, and the two are independent.