Comparison of Ki and IC50 Values for Prototypical Inhibitors of ABC Transporters
Comparison of Ki and IC50 Values for Prototypical Inhibitors of the Human ABC Transporters P-gp and BCRP in Membrane Vesicles
P-gp (ABCB1/MDR1) and BCRP (ABCG2) are members of the ATP-binding cassette (ABC) superfamily of transporters that use ATP to actively transport compounds across a cell membrane. P-gp and BCRP are expressed on the luminal membrane of enterocytes, endothelial cells in the brain, the brush border membrane of renal proximal tubules and the canalicular membrane of hepatocytes where they limit intestinal absorption, blood-brain barrier penetration and facilitate excretion into the bile and urine. Compounds that inhibit these transporters may be perpetrators of drug-drug interactions [1,2]. While the FDA Draft Guidance for Industry (2012) recommends IC50 values for evaluating the inhibitory potential of transporters, the EMA’s Guideline on the Investigation of Drug Interactions (2013) recommends the determination of Ki values [3,4]. The EMA recommends the use of IC50 values only when Ki determinations are not possible.
Although Ki values have been reported for inhibitors of various transporters, the methodology differs between labs and oftentimes relies on mathematical extrapolation (e.g., using the IC50 value to determine the Ki value with the Cheng-Prusoff equation). In this study the inhibitory potential of various chemical inhibitors was evaluated with in vitro experiments in transporter-expressing membrane vesicles to determine both Ki and IC50 values. Prior to the conduct of inhibition experiments, Km values were experimentally determined with each probe substrate, namely N-methylquinidine (NMQ) for P-gp and estrone-3-sulfate (E3S) for BCRP. Subsequently, IC50 and Ki values were experimentally determined for the prototypical inhibitors verapamil (P-gp) and Ko143 (BCRP).