Understanding the Effect of Hydroxypropyl‐β‐Cyclodextrin on Fenebrutinib Absorption in an Itraconazole‐Fenebrutinib Drug‐Drug Interaction Study

Cyclodextrins are widely-used pharmaceutical excipients, particularly for insoluble compounds dosed orally such as the oral solution of itraconazole, which is frequently used in clinical drug-drug interaction studies to inhibit cytochrome P450 3A. Since cyclodextrins act by forming inclusion complexes with their co-formulated drug, they could have an unintended consequence of affecting absorption if they form a strong complex with the potential victim drug in an itraconazole drug-drug interaction study. This observation was made in a drug-drug interaction study with the BTK inhibitor fenebrutinib and itraconazole, in which, relative to the control group, the expected increase in fenebrutinib Cmax was not observed in the itraconazole group, and a delay in Tmax was observed in the itraconazole group. The in vitro binding constant between fenebrutinib and HP-β-CD was determined to be 2 x 105 M-1 , and the apparent permeability of fenebrutinib across an MDCK monolayer decreased in a cyclodextrin concentration-dependent manner. This observation was confirmed in vivo, in a pentagastrin-pretreated dog model, in which fenebrutinib was administered with or without cyclodextrin; a reduction in Cmax, a prolonged Tmax, and increased fenebrutinib recovery in feces replicated the previous observation in healthy volunteers and supported the hypothesis that complexation with cyclodextrin decreased rate and extent of fenebrutinib absorption. Physiologically-based pharmacokinetic modeling was used to translate the in vitro effect of cyclodextrin on fenebrutinib apparent permeability to the in vivo effect on absorption, which was then confirmed using the in vivo dog PK data.