Preliminary Comparison of Competitive Efflux and Historical Transwell Assays for Assessing Canine P-Glycoprotein Substrate Status of Eleven Drugs Representing Ten Different Drug Classes.

Because P-glycoprotein (P-gp) influences the disposition of substrate drugs, regulatory agencies recommend ascertaining the P-gp substrate status of active pharmaceutical ingredients (API's) intended for humans. Transwell-based assays using P-gp-overexpressing human cell lines are routinely used to predict the API's P-gp substrate status. Although ABCB1-1Δ-mediated P-gp deficiency contributes to serious canine adverse drug reactions, assessment of canine P-gp status is not currently recommended by regulatory agencies for canine API's. We have previously employed a competitive efflux assay to assess drugs as canine P-gp substrates as it affords some advantages over transwell assays. Our objective was to preliminarily compare results generated from these two methods using historical data for transwell assays. Eleven drugs were assessed as canine P-gp substrates by both methods, with two identified as non-P-gp substrates (erythromycin, propranolol) and nine identified as canine P-gp substrates (clarithromycin, daunorubicin, digoxin, etoposide, paclitaxel, quinidine, ritonavir, verapamil, vinblastine). Erythromycin was identified as a canine P-gp substrate by competitive efflux assay at roughly 10-fold higher concentrations than was used for transwell assays. Concordance of these methods assessing canine P-gp substrate status using historical data supports the need for head-to-head studies comparing these two methods for identifying canine P-gp substrates.