Category: Preclinical Development
Purpose: Pitolisant is a selective histamine-3 receptor inverse agonist or antagonist and is a first in class drug to treat narcolepsy. The maximum daily dose of pitolisant is 36 mg. The objective of the study is to predict the drug interaction potential as a perpetrator and victim of CYP2D6 mediated drug interactions using in vitro data.
Methods: Inhibition potential towards major CYP isoforms (CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6 and CYP3A4) was determined using marker reactions in human liver microsomes. For measuring direct inhibition potential, pitolisant was incubated with HLM and probe substrates in the presence of NADPH followed by termination of reactions with acetonitrile. Time dependent inhibition potential of pitolisant was evaluated in a single point assay at 10 µM by incubating the compound with HLM and NADPH in the presence and absence of pre-incubation for 30min followed by dilution in to an activity assay containing probe substrate at 10x the Km. The enzymes involved in the metabolism of pitolisant were identified by chemical inhibition method in human liver microsomes. The drug interaction potential of pitolisant was predicted using mechanistic static model.
Results: Pitolisant did not inhibit the CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, and CYP3A4 (76 µM) mediated enzyme activities with IC50 values >100 µM. Pitolisant is potent inhibitor of CYP2D6 mediated dextromethorphan O-demethylation in human liver microsomes with an IC50 value of 0.68 µM. Pitolisant is not a time dependent inhibitor of the P450 enzymes in human liver microsomes. The Ki value against CYP2D6 mediated dextromethorphan O-demethylation was found to be 0.23 µM and the mode of inhibition is competitive. CYP2D6 and CYP3A4 were found to be involved in the metabolism of pitolisant with fm, CYP2D6 and fm, CYP3A4 values of 0.42 and 0.20 respectively. Methoxsalen a non-specific inactivator of the P450 enzymes inhibited the metabolism of pitolisant by ~ 80%. The predicted fold change in AUC of CYP2D6 substrates (fm, CYP2D6 = 0.85 – 0.95) when co-administered with pitolisant is in the range of 1.9 – 2.2. Co-administration of pitolisant with antidepressants (desipramine) and antipsychotics (Atomoxetine, Risperidone) should be done with caution. The predicted fold change in AUC of pitolisant when co-administered with paroxetine (a direct and time dependent inhibitor of CYP2D6) is 1.72 fold consistent with observed change in AUC of two fold. The predicted fold change in AUC of pitolisant in the presence of cinacalcet (potent reversible inhibitor of CYP2D6) is 1.67 fold.
Conclusion: Overall the results indicate that a caution is required when pitolisant is co-administered with either CYP2D6 substrates or inhibitors. Pitolisant can be a moderate inhibitor in vivo based on the magnitude of the drug interaction potential predicted from in vitro systems.