In contrast, the drug permeability in the BA direction was decreased in presence of PSC833 in all cell layers (Table 2). In addition to its inhibitory properties on various MRP carriers [32], MK571 has been recently reported to interfere with the activity of OATP1B3 and OATP2B1 at a concentration as low as 1 μM [42] and [43]. Its modulatory effects on other OATP transporters present in Calu-3 layers (Table 1) are currently unknown. Nevertheless, the compound has been shown not to interact with MDR1 [33], which we confirmed in an IUC2 shift

assay. Although PSC833 was originally developed as a specific MDR1 inhibitor, it has since been reported to inhibit other PD0332991 concentration ABC transporters, such as the bile salt extrusion pump (BSEP) [44], MRP2 [45] or the breast cancer resistance protein (BCRP, Solvo Biotech

website) and its ability to inhibit OATP transporters has been suggested [46]. Taken together, 3H-digoxin permeability data in Calu-3 layers do not support an exclusive participation of the MDR1 transporter in its apparent efflux and suggest the involvement of one or several ATP-independent transport system(s). Similarly, it has previously been demonstrated that MDR1 was not the sole transporter responsible for digoxin asymmetric transport in Autophagy high throughput screening the Caco-2 intestinal absorption model [33] and in MDR1 transfected MDCK cell layers [47]. Although this/these transporter(s) remain(s) to be identified, OATP4C1 might be a possible candidate since

digoxin is a known substrate [20] and [21], the transporter is present in Calu-3 layers and a lower gene expression Casein kinase 1 was observed at a high passage number (Table 1). Assuming protein levels are in agreement with those of mRNA transcripts, this could explain the reduced digoxin apparent efflux in high passage cell layers. This assumption implies a basolateral location of OATP4C1 in Calu-3 layers in line with the basolateral presence of OATP transporters that has been recently postulated in the airway epithelium of foals [48]. However, there remains a possibility that digoxin is transported across bronchial epithelial cell layers by a transporter yet to be characterised, as suggested in other cell culture models [22], [23] and [47]. For instance, in addition to the apical MDR1 efflux pump, a basolaterally located uptake transporter was required to account for digoxin net secretory transport in MDCKII-MDR1 cell layers but this transporter could not be identified using a panel of inhibitors [47]. As previously debated for the MDCKII-MDR1 absorption model [47], the likely involvement of multiple transporters in digoxin bidirectional transport in Calu-3 layers questions its suitability for probing MDR1 activity in the bronchial epithelium.