Positron Emission Tomography Imaging Reveals an Importance of Saturable Liver Uptake Transport for the Pharmacokinetics of Metoclopramide

Positron emission tomography (PET) imaging using [(11)C]metoclopramide, a P-glycoprotein (P-gp) substrate, was used to investigate the contribution of transport processes to metoclopramide liver clearance. The liver kinetics obtained after injection of [(11)C]metoclopramide were measured using PET in rats (n=4‐5) in the absence (tracer dose) and the presence of a pharmacologic dose of metoclopramide (3 mg/kg), with or without P-gp inhibition using i.v. tariquidar (8 mg/kg). Corresponding [(11)C]metoclopramide kinetics and metabolism in plasma (n=3) were measured using radio-HPLC analysis. [(11)C]metoclopramide exposure to the liver and plasma was described by the area under the time-activity curve (AUC) of the radioactivity kinetics in the liver and parent [(11)C]metoclopramide kinetics in plasma, respectively. The pharmacologic dose of metoclopramide resulted in a ∼2.2-fold increase in [(11)C]metoclopramide AUC(plasma), while P-gp inhibition did not. AUC(liver) was lower using the pharmacologic dose (42.9 ± 13.8 SUV·min) compared with the tracer dose (210.0 ± 32.4 SUV·min). P-gp inhibition enhanced the liver exposure in the pharmacologic condition only (81.0 ± 3.1 SUV·min). [(11)C]metoclopramide PET imaging suggests an unpredicted role for hepatocyte uptake transporter(s) in controlling metoclopramide pharmacokinetics in addition to the known contribution of the metabolic enzymes and the P-gp.