Microsome and S9 Stability

The liver accounts for a large amount of endogenous and xenobiotic metabolism. Microsomes are sub-cellular liver structures that contain a variety of enzymes responsible for phase I metabolism. The S9 fraction of liver preparations contains both phase I and phase II enzymes but yield lower enzyme activity. Liver microsome and S9 incubations are used to determine the potential metabolic stability of test compounds. Test compound is incubated with human, rat or mouse microsomes and sample aliquots are removed at pre-defined time intervals. The test compound in quantified by LC/MS/MS and the rate of compound disappearance can determine the compound’s metabolic half-life.

We routinely conduct both human and rat microsome studies on test compounds to determine relative stability in both species and correlate the in vivo rat studies with potential expected human stability. Compounds are incubated at pre-determined concentrations with liver microsomes and NADPH. The peak area of the test compound at each time point is compared to the peak area of the time zero sample. A control reaction (without NADPH co-factor) is used in order to assess the amount of thermal breakdown, insolubility and non-specific binding that contributes to the overall loss of the test compound.