Impact of impurities on IC50 values of P450 inhibitors.

During early drug discovery, the synthetic pathways for test compounds are not well defined and impurities in the test compounds are inevitable. Compounds undergo serial screening tests at this stage to assess their biological activities and drug-like properties. Impurities in the test compounds can produce false positive results and therefore complicate the interpretation of data. P450 inhibition is one of the screens used in the early drug discovery process to assess the potential of drug-drug interactions caused by the inhibition of P450 enzymes. The impact of impurities on P450 inhibition has not been investigated. In this study, the impact of impurities on CYP2D6 IC(50) values was evaluated using model compounds. Cimetidine was chosen as the test compound. Quinidine, fluoxetine, fluvoxamine, and ibuprofen were chosen to represent impurities as they inhibit CYP2D6 to varying degrees. The IC(50) values of these model impurities for CYP2D6 were 0.11 µM, 0.98 µM, 13.4 µM, and >100 µM, respectively. Impurities with potent CYP2D6 inhibition, such as quinidine, can significantly decrease the apparent IC(50) value for the mixture. With the addition of only 2% quinidine to cimetidine (mol/mol), the apparent IC(50) value of cimetidine decreased from 98 µM to 4.4 µM. With the addition of 10% quinidine, the apparent IC(50) decreased to 1.04 µM. Such a significant decrease in apparent IC(50) values can produce a false alert and cause the inappropriate elimination of good compounds at an early stage. Impur6ities with low inhibitory potential, such as fluvoxamine and ibuprofen, did not cause a significant change in apparent IC(50) values. An impurity can have a similar effect on the IC(50) values for inhibition of other biological activities. The effect of an impurity on apparent IC(50) values can be predicted by using a simulation curve if the potency of the impurity is characterized.