Is Protein‐Mediated Uptake Of Drugs By OATPs, Via Protein‐Lipid Interaction, A Real Phenomenon Or An Artifact?
Authors: Yin Mengyue , Storelli Flavia , Unadkat Jashvant 
Published: May 2022

There is considerable interest in the observation that OATP‐mediated transport is significantly increased in the presence of albumin or plasma proteins, a phenomenon called the “protein‐mediated uptake (PMU)” effect (Miyauchi et al., 2018; Liang et al., 2020; Bi et al., 2021). PMU proposes an interaction between the drug‐albumin complex and the cell surface leading to enhanced dissociation of the drug‐albumin complex resulting in increased local unbound drug concentration and therefore an “apparent” increased uptake intrinsic clearance (CLint) (Miyauchi et al., 2018; Kim et al., 2019). This PMU effect helps overcome some (but not all) of the underprediction of hepatic clearance (CL) of OATP drugs by IVIVE. If the PMU hypothesis is correct, the following should be observed in the presence vs. absence of plasma proteins: 1) the active (e.g. by OATP1B1) and passive uptake CLint of the drug should increase to the same extent; and 2) an increase in the slope (i.e. the uptake rate), but not the intercept of the uptake vs. time curve. To determine if the above is true, we determined the total, active (OATP1B1‐mediated) and passive CLint of a cocktail of OATP1B1 drugs [fluvastatin (FLV), pitavastatin (PTV), cerivastatin (CRV), atorvastatin (ATV) and rosuvastatin (RSV)], in the absence and presence of 1%, 2%, or 5% human serum albumin (HSA), by HEK293 MOCK and OATP1B1‐transfected cells at concentrations much less than their reported OATP1B1 Km values. In addition, the unbound drug concentration of each drug, in the absence and presence of HSA, was maintained approximately the same in all experiments. We found: 1) In the presence of HSA, the increase in the passive CLint(i.e. of the unbound drug) was greaterthan for either the total or active uptake CLint of the drug; and 2) in the presence of HSA, the intercept of the uptake curve increased in proportion to the HSA concentration used. An increase in the intercept is usually interpreted as non‐specific binding (NSB) of the drug to the cells/labware; in this case, NSB of the albumin‐drug complex to the cells. Therefore, we asked if PMU can be explained in whole or part by NSB of the albumin‐drug complex to cells or labware? We found that: 1) PMU on passive/active uptake of ATV, FLV and RSV was largely due to NSB (i.e. an artifact). However, it could only partially explain the effect of 5% HSA on OATP1B1‐mediated PTV uptake or the passive uptake of CRV. If the PMU of PTV is a real phenomenon, the unbound inhibitory capacity (IC50,u) of PTV towards OATP1B1 should decrease as the IC50,u is determined ONLY by the interaction between the drug and the transporter and independent of NSB or passive uptake. Indeed, the OATP1B1 IC50,u of PTV (on RSV uptake) in the presence of 5%HSA decreased by 80%, but as expected, that of ATV was unaffected. In conclusion, our results suggests that, except for PTV, the PMU of the drugs studied is likely an artifact. Therefore, mechanisms other than the presence of plasma proteins should be investigated to explain the underprediction of hepatic CL by IVIVE.Reference: Bi YA. et al. (2021) Drug Metabolism and Disposition 49:188–201. Kim SJ. et al. (2019) Drug Metabolism and Disposition 47:94–103. Liang X. et al. (2020) Drug Metabolism and Disposition 48:1283–1292. Miyauchi S. et al. (2018) Drug Metabolism and Disposition 46:259–267.