Time-dependent inhibition (TDI) of drug transporters is an emerging area of interest in drug development. Despite previous reports the frequency, mechanism, and overall clinical importance of transporter TDI are currently unknown. Time-dependent inhibition of OATP1B1 by cyclosporine A is the best characterized, displaying effects in in vitro, preclinical, and clinical settings. HIV proteasome inhibitors have also been implicated in OATP1B1 TDI. Mechanisms of transporter TDI have included drug permeability/intracellular binding domains, drug metabolites, and alteration of protein expression and localization. The purpose of this work is to investigate TDI potential for the canonical uptake and efflux liver transporters OATP1B1 and MDR1 (P-gp). Briefly, Caco-2, LLC-PK1-MDR1 and HEK-293-OATP1B1 cell lines were utilized to determine the effects of an inhibitor pre-incubation period on the IC50, compared to traditional inhibitor/substrate co-culture. We have also investigated whether changes in IC50 were significant enough to alter predicted drug-drug interaction (DDI) potentials [digoxin DDI for MDR1, and statin DDI for OATP1B1] based on FDA and EMA DDI Guidance documents. Elacridar, a potent and low permeability drug, was the sole inhibitor to display MDR1 TDI in both Caco-2 and LLC-PK1 cell systems. Two other low permeability MDR1 inhibitors, vinblastine and clarithromycin, did not result in TDI, suggesting the role of permeability may be limited or overridden by the relatively long (2 hr.) culture time. Additionally, digoxin DDI prediction was not altered for any of the tested inhibitors. Similar to previous reports, HIV protease inhibitors Saquinavir and Ritonavir showed the greatest OATP1B1 TDI potential, but no changes in statin DDI predictions were altered for any inhibitors. Since the cell lines used are not metabolism competent, studies are ongoing in plated hepatocytes to explore the potential role metabolites may play in transporter TDI.