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Human Brain Imaging and Radiation Dosimetry of ¹¹C-N-Desmethyl-Loperamide, a PET Radiotracer to Measure the Function of P-Glycoprotein

¹ Molecular Imaging Branch, National Institute of Mental Health, National Institutes of Health, Bethesda, Maryland; and ² PET Department, Clinical Center, National Institutes of Health, Bethesda, Maryland

Correspondence: For correspondence or reprints contact: Robert B. Innis, Molecular Imaging Branch, National Institute of Mental Health, 31 Center Dr., Bethesda, MD 20892-2035. E-mail: robert.innis{at}nih.gov

P-glycoprotein (P-gp) is a membrane-bound efflux pump that limits the distribution of drugs to several organs of the body. At the blood–brain barrier, P-gp blocks the entry of both loperamide and its metabolite, N-desmethyl-loperamide (N-dLop), and thereby prevents central opiate effects. Animal studies have shown that ¹¹C-dLop, compared with ¹¹C-loperamide, is an especially promising radiotracer because it generates negligible radiometabolites that enter the brain. The purposes of this study were to determine whether ¹¹C-dLop is a substrate for P-gp at the blood–brain barrier in humans and to measure the distribution of radioactivity in the entire body to estimate radiation exposure. Methods: Brain PET scans were acquired in 4 healthy subjects for 90 min and included concurrent measurements of the plasma concentration of unchanged radiotracer. Time–activity data from the whole brain were quantified using a 1-tissue-compartment model to estimate the rate of entry (K₁) of radiotracer into the brain. Whole-body PET scans were acquired in 8 healthy subjects for 120 min. Results: For brain imaging, after the injection of ¹¹C-dLop the concentration of radioactivity in the brain was low (standardized uptake value, 15%) and stable after approximately 20 min. In contrast, uptake of radioactivity in the pituitary was about 50-fold higher than that in the brain. The plasma concentration of ¹¹C-dLop declined rapidly, but the percentage composition of plasma was unusually stable, with the parent radiotracer constituting 85% of total radioactivity after approximately 5 min. The rate of brain entry was low (K₁ = 0.009 ± 0.002 mL·cm^–3·min^–1; n = 4). For whole-body imaging, as a measure of radiation exposure to the entire body the effective dose of ¹¹C-dLop was 7.8 ± 0.6 µSv/MBq (n = 8). Conclusion: The low brain uptake of radioactivity is consistent with ¹¹C-dLop being a substrate for P-gp in humans and confirms that this radiotracer generates negligible quantities of brain-penetrant radiometabolites. In addition, the low rate of K₁ is consistent with P-gp rapidly effluxing substrates while they transit through the lipid bilayer. The radiation exposure of ¹¹C-dLop is similar to that of many other ¹¹C-radiotracers. Thus, ¹¹C-dLop is a promising radiotracer to study the function of P-gp at the blood–brain barrier, at which impaired function would allow increased uptake into the brain.

Key Words: PET • N-desmethyl-loperamide • P-glycoprotein

COPYRIGHT © 2009 by the Society of Nuclear Medicine, Inc.

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JNM 2009 50: 11A-12A. [Full Text]