Synthesis and evaluation of [N-methyl-11C]N-desmethyl-loperamide as a new and improved PET radiotracer for imaging P-gp function

J Med Chem. 2008 Oct 9;51(19):6034-43. doi: 10.1021/jm800510m. Epub 2008 Sep 11.


[(11)C]Loperamide has been proposed for imaging P-glycoprotein (P-gp) function with positron emission tomography (PET), but its metabolism to [N-methyl-(11)C] N-desmethyl-loperamide ([(11)C]dLop; [(11)C]3) precludes quantification. We considered that [(11)C]3 might itself be a superior radiotracer for imaging brain P-gp function and therefore aimed to prepare [(11)C]3 and characterize its efficacy. An amide precursor (2) was synthesized and methylated with [(11)C]iodomethane to give [(11)C]3. After administration of [(11)C]3 to wild-type mice, brain radioactivity uptake was very low. In P-gp (mdr-1a(-/-)) knockout mice, brain uptake of radioactivity at 30 min increased about 3.5-fold by PET measures, and over 7-fold by ex vivo measures. In knockout mice, brain radioactivity was predominantly (90%) unchanged radiotracer. In monkey PET experiments, brain radioactivity uptake was also very low but after P-gp blockade increased more than 7-fold. [(11)C]3 is an effective new radiotracer for imaging brain P-gp function and, in favor of future successful quantification, appears free of extensive brain-penetrant radiometabolites.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, N.I.H., Intramural

MeSH terms

  • ATP Binding Cassette Transporter, Subfamily B / deficiency
  • ATP Binding Cassette Transporter, Subfamily B / metabolism*
  • Animals
  • Brain / diagnostic imaging
  • Brain / metabolism*
  • Carbon Radioisotopes
  • Injections, Intravenous
  • Loperamide / analogs & derivatives*
  • Loperamide / chemical synthesis*
  • Loperamide / chemistry
  • Loperamide / pharmacokinetics*
  • Macaca mulatta
  • Male
  • Mice
  • Mice, Knockout
  • Molecular Structure
  • Positron-Emission Tomography / methods
  • Radiopharmaceuticals / chemical synthesis*
  • Radiopharmaceuticals / chemistry
  • Radiopharmaceuticals / pharmacokinetics*
  • Stereoisomerism
  • Time Factors


  • ATP Binding Cassette Transporter, Subfamily B
  • Carbon Radioisotopes
  • Radiopharmaceuticals
  • N-demethylloperamide
  • Loperamide