P344 PET Imaging of P-glycoprotein Inhibition at the Human Blood-Brain Barrier by Quinidine

Li Liu , Pharmaceutics, University of Washington, Seattle, WA
Ann C. Collier , Medicine, University of Washington, Seattle, WA
Jeanne M. Link , Nuclear Medicine, University of Washington, Seattle, WA
Karen B. Domino , Anesthesiology & Pain Medicine, University of Washington, Seattle, WA
David A. Mankoff , Nuclear Medicine, University of Washington, Seattle, WA
Janet F. Eary , Nuclear Medicine, University of Washington, Seattle, WA
Peng Hsiao , Pharmaceutics, University of Washington, Seattle, WA
Anand K. Deo , Pharmaceutics, University of Washington, Seattle, WA
Jashvant D. Unadkat , Pharmaceutics, University of Washington, Seattle, WA
Permeability-glycoprotein (P-glycoprotein, P-gp), an efflux transporter at the blood-brain barrier (BBB), is believed to be one of the primary obstacles in the CNS delivery of many P-gp substrate drugs.  The functional importance of P-gp at the human BBB was first demonstrated by our laboratory using a model P-gp substrate (11C-verapamil), a model P-gp inhibitor (cyclosporine-A, CsA), and Positron Emission Tomography (PET) imaging.  CsA, at supertherapeutic blood concentration (~3 mM), modestly increased (by 79%) the brain-to-plasma AUC ratio of 11C-verapamil radioactivity1.   To determine if this interaction would be greater with another P-gp inhibitor, we determined the inhibition of P-gp at the human BBB using a FDA-approved potent P-gp inhibitor, quinidine.  After obtaining institutional review board approval and each subject’s consent, 7 healthy volunteers (4 women and 3 men) were administered 11C-verapamil (~0.1 mCi/kg, ~0.07 μg/kg) intravenously before and after 1 hour of quinidine IV infusion (0.153 mg/kg/min).  Prior to each 11C-verapamil administration, 15O-H2O (~0.5 mCi/kg) was administered to measure cerebral blood flow (Q).  During each PET-imaging session, brain images and frequent arterial blood samples were obtained for 5 (15O-H2O) or 20 (11C-verapamil) mins to measure radioactivity content in these tissues.  Plasma 11C-verapamil and metabolites’ radioactivity were quantified by rapid solid-phase extraction.  The tissue uptake (AUCtissue/AUCplasma ratio), distribution clearance (CL12) and extraction ratio, ER (CL12/Q), of 11C-radioactivity for the whole brain, gray and white matter during the first 10 mins (when metabolism of 11C-verapamil is minimal) were determined in the presence and absence of quinidine, and compared using the Student’s paired t-test.  In the presence of quinidine (at 3.1±0.4 µg/mL pseudo steady-state plasma concentration), the AUCtissue/AUCplasma ratios were significantly (p<0.005) increased for the whole brain (84±42%), gray matter (84±42%), or white matter (87±43%).  Quinidine did not affect the extent of 11C-verapamil metabolism.  A 2-tissue compartment model using a single input function of total plasma radioactivity was determined to be the best model for estimating CL12.  In the presence of quinidine, the average CL12 and ER estimates were significantly increased (p<0.05) for the brain (78±68%, 62±64%), gray matter (68±40%, 63±81%) and white matter (94±76%, 78±83%).  These data demonstrate that the inhibition of P-gp activity at the human BBB observed with CsA can be reproduced using another P-gp inhibitor, quinidine.  However, the level of inhibition was comparable to that obtained with CsA, indicating that quinidine at the plasma concentrations achieved is equipotent to CsA in inhibiting P-gp in vivo.  Therefore, at therapeutic concentrations, quinidine cannot be used to maximally inhibit P-gp at the human BBB in order to increase CNS delivery of P-gp substrate drugs.  More potent FDA-approved P-gp inhibitors, with a wider therapeutic window, are needed to maximally inhibit P-gp at the human BBB to significantly increase CNS delivery of P-gp substrate drugs. 

Supported by NIHRCNS06804 and TL1RR025016

1.       Sasongko L, Link JM, Muzi M, Mankoff DA, Yang X, Collier AC, Shoner SC, Unadkat JD. Imaging P-glycoprotein transport activity at the human blood-brain barrier with positron emission tomography. Clin Pharmacol Ther. 77(6):503-14. 2005