In vitro CYP inhibition

The additional recommendations contained in the draft guidance and on the FDA s website as they relate specifically to the design of in vitro CYP inhibition studies can be summarized as follows ... 

Additional Industry Perspectives on the Conduct of In Vitro CYP Inhibition Studies... 

All three assumptions can be violated in the case of CYP enzymes, depending on the design of the in vitro CYP inhibition study. The first assumption can be potentially violated if the drug being tested is a time-dependent inhibitor (e.g., one with a slow on rate see below). The potency of some inhibitors (e.g., the CYP3A inhibitors ketoconazole and clotrimazole) is such that the free concentration of the inhibitor tends to approach the concentration of the enzyme (40), a violation of the second assumption. In the case of such tight-binding inhibition, an apparent A) value (A i a ) )) can be estimated, as follows ... 

For most cases encountered in drug development, it is not necessary to definitively prove all of the above criteria. Ideally, the experimental design of a definitive in vitro CYP inhibition study should allow the following questions to be answered in a single initial experiment ... 

PREDICTION OF HUMAN DRUG-DRUG INTERACTIONS FROM IN VITRO CYP INHIBITION DATA... 

DDI CYP inhibition (rCYP, HLM) (see Section 6.3.2.3) CYP induction (PXR-TA, Fa2N-4, hepatocytes) (see Section 6.3.2.3) CYP reaction phenotyping Assessing in vitro CYP inhibition and induction potentials Predicting in vivo DDIs Assay (HLM CYP inhibition) ° Human hve microsomes ° 0.5-100 rM probe concentration depending on each isozyme Analysis ° PPT followed by LC-MS/MS or onhne SPE-MS/MS... 

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