A Systematic In Vitro Investigation of the Inhibitor Preincubation Effect on Multiple Classes of Clinically Relevant Transporters

Preincubation of a drug transporter with its inhibitor in a cell-based assay may result in the apparent enhancement of the inhibitory potency. Currently, limited data are available on potentiation of transporter inhibition by preincubation (PTIP) for clinically relevant solute-carrier transporters other than OATP1B1 and OATP1B3. Therefore, PTIP was examined systematically using OATP1B1, OATP1B3, OAT1, OAT3, OCT1, OCT2, MATE1, and MATE2-K cell lines. IC₅₀ values of 30 inhibitors were determined with or without 3 hours of preincubation, and compounds with a PTIP ≥2.5× were further characterized by assessing the time course of transport inhibition potency and cellular concentration. For each compound, correlations were calculated between highest observed PTIP and physicochemical properties. PTIP was prevalent among organic cation transporters (OCTs) and organic anion-transporting polypeptides (OATPs) but not among organic anion transporters (OATs) or multidrug and toxin extrusion transporters (MATEs), and most instances of PTIP persisted after controlling for toxicity and nonspecific binding. Occasionally, preincubation in excess of 2 hours was required to attain full inhibitory potency. For four drugs examined, preincubation had the potential to change the in vitro drug-drug interaction risk prediction from "no risk" to "risk" on the basis of current regulatory criteria. Molecular weight and LogD_7.4, as well as the ratio of passive cellular accumulation and cellular uptake rate correlated with PTIP; thus, low cellular permeation and a slow build-up of unbound intracellular inhibitor concentration may contribute to PTIP. Taken together, our data suggest that PTIP is partly determined by the physicochemical properties of the perpetrator drug, and preincubation may affect the in vitro predicted drug-drug interaction risk for OCTs as well as OATPs. SIGNIFICANCE STATEMENT: During the development of a novel pharmaceutical drug, in vitro studies are conducted to assess the risk of potential adverse interactions between existing medications a patient may already be taking and the novel compound. The exact way these in vitro assays are performed may influence the outcome of risk assessment. Here we suggest that the interaction risk may be underestimated unless specific assay protocols are modified to include an additional incubation step that allows the test drug to accumulate inside the cells, and demonstrate that adding this step is particularly important for large and hydrophobic drug molecules.