An evaluation of the dilution method for identifying metabolism-dependent inhibitors of cytochrome P450 enzymes.

Metabolism-dependent inhibition (MDI) of cytochrome P450 is usually assessed in vitro by examining whether the inhibitory potency of a drug candidate increases after a 30-min incubation with human liver microsomes (HLMs). To augment the IC(50) shift, many researchers incorporate a dilution step whereby the samples, after being preincubated for 30 min with a high concentration of HLMs (with and without NADPH), are diluted before measuring P450 activity. In the present study, we show that the greater IC(50) shift associated with the dilution method is a consequence of data processing. With the dilution method, IC(50) values for direct-acting inhibitors vary with the dilution factor unless they are based on the final (postdilution) inhibitor concentration, whereas the IC(50) values for MDIs vary with the dilution factor unless they are based on the initial (predilution) concentration. When the latter data are processed on the final inhibitor concentration, as is commonly done, the IC(50) values for MDI (shifted IC(50) values) decrease by the magnitude of the dilution factor. The lower shifted IC(50) values are a consequence of data processing, not enhanced P450 inactivation. In fact, for many MDIs, increasing the concentration of HLMs actually leads to considerably less P450 inactivation because of inhibitor depletion and/or binding of the inhibitor to microsomes. A true increase in P450 inactivation and IC(50) shift can be achieved by assessing MDI by a nondilution method and by decreasing the concentration of HLMs. These results have consequences for the conduct of MDI studies and the development of cut-off criteria.