microsomal assay

A microsomal assay is a way to ask, in a test tube, how quickly the liver would chew up a new compound — without dosing any animal or person. It uses microsomes, tiny membrane bubbles harvested from liver cells that are packed with the major drug-oxidizing enzymes, giving a convenient, reproducible stand-in for liver metabolism.

In a typical run, the compound is incubated with liver microsomes plus the cofactor NADPH, which powers cytochrome P450 oxidation. Samples are taken at several time points, and the amount of intact parent compound remaining is measured by mass spectrometry. The rate at which the parent disappears gives a half-life in the incubation, from which an intrinsic clearance is calculated and used to rank or predict metabolic stability.

The assay is fast, cheap, and ideal for screening many analogues during lead optimization, but it has honest limits. Standard microsomes capture P450-mediated oxidation well but miss enzymes located outside the microsomal fraction — for instance cytosolic enzymes and most non-P450 pathways — unless extra cofactors or hepatocytes are used. So a compound can look stable in microsomes yet still be cleared by routes the assay does not see.