P340 Loxapine P-glycoprotein interactions in vitro

Andrea Reed , PK/ADME, Alexza Pharmaceuticals Inc, Mountain View, CA
Elke Perloff , Dept of Discovery Labware, BD Sioscis, Woburn, MA
Keith Huie , PK/ADME, Alexza Pharmaceuticals Inc, Mountain View, CA
Lori H. Takahashi , PK/ADME, Alexza Pharmaceuticals Inc, Mountain View, CA
James Cassella , Alexza Pharmaceuticals Inc, Mountain View, CA
Loxapine is a dibenzoxazepine antipsychotic used in the treatment of schizophrenia. Because antipsychotic drugs such as risperidone, paliperidone, olanzapine, have been reported to have various degrees of interaction (substrate or inhibitor) with the multidrug resistance transporter, P-glycoprotein (P-gp), the interaction of loxapine with P-gp was evaluated. The permeability, bidirectional transport and P-gp interaction potential of loxapine was determined in vitro using Caco-2 cells.  Caco-2 cell monolayers were used after 21-23 days of culture in 24-well-format diffusion chambers. The apical (A) to basolateral (B) transport study was determined using a loxapine concentration of 1 µM in transport media with 1% DMSO in the donor (A) chamber (pH 7.4). The bidirectional transport of loxapine was conducted in the presence or absence of the known P-gp inhibitor ketoconazole.  For assessing P-gp inhibition, bidirectional transport of radiolabeled digoxin was determined in the presence and absence of loxapine at concentrations ranging from 0.05 - 50μM. Permeability comparators (propranolol and mannitol) were included in all studies, and ketoconazole, cyclosporine and verapamil were used as comparator controls for P-gp inhibition. Lucifer yellow and TEER values were used in all studies to assess monolayer integrity. Loxapine concentrations were determined by LC/MS/MS, and digoxin, comparator compounds and positive controls were measured by liquid scintillation. IC50 calculations were calculated by non-linear regression based on the Hill equation. The apparent permeability coefficient (Papp) for loxapine in the A to B direction was 67 nm/s, and 73 nm/s in the B to A direction, resulting in a polarization ratio close to unity. In addition, ketoconazole had no influence on the polarization ratio. Thus, unlike risperidone, olanzapine and aripiprazole, loxapine is not a substrate for P-gp. Loxapine demonstrated concentration dependent inhibition of digoxin efflux, reducing the digoxin efflux ratio to 1.4 at a loxapine concentration of 50 µM suggesting that loxapine, like other antipsychotics olanzapine, paliperidone and risperidone, is a moderate inhibitor of P-gp in vitro.