Selective binding of 2-[125I]iodo-nisoxetine to norepinephrine transporters in the brain

Nucl Med Biol. 2004 Jul;31(5):533-41. doi: 10.1016/j.nucmedbio.2004.03.001.

Abstract

A radioiodinated ligand, (R)-N-methyl-(2-[(125)I]iodo-phenoxy)-3-phenylpropylamine, [(125)I]2-INXT, targeting norepinephrine transporters (NET), was successfully prepared. A no-carrier-added product, [(125)I]2-INXT, displayed a saturable binding with a high affinity (K(d)=0.06 nM) in the homogenates prepared from rat cortical tissues as well as from LLC-PK(1) cells expressing NET. A relatively low number of binding sties (B(max)=55 fmol/mg protein) measured with [(125)I]2-INXT in rat cortical homogenates is consistent with the value reported for a known NET ligand, [(3)H]nisoxetine. Competition studies with various compounds on [(125)I]2-INXT binding clearly confirmed the pharmacological specificity and selectivity for NET binding sites. Following a tail-vein injection of [(125)I]2-INXT in rats, a good initial brain uptake was observed (0.56% dose at 2 min) followed by a slow washout from the brain (0.2% remained at 3 hours post-injection). The hypothalamus (a NET-rich region) to striatum (a region devoid of NET) ratio was 1.5 at 3 hours post-i.v. injection. Pretreatment of rats with nisoxetine significantly inhibited the uptake of [(125)I]2-INXT (70-100% inhibition) in locus coeruleus, hypothalamus and raphe nuclei, regions known to have a high density of NET; whereas escitalopram, a serotonin transporter ligand, did not show a similar effect. Ex vivo autoradiography of rat brain sections of [(125)I]2-INXT (at 3 hours after an i.v. injection) displayed an excellent regional brain localization pattern corroborated to the specific NET distribution in the brain. The specific brain localization was significantly reduced by a dose of nisoxetine pretreatment. Taken together, the data suggest that [(123)I]2-INXT may be useful for mapping NET binding sites in the brain.

Publication types

  • Evaluation Study
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Brain / diagnostic imaging*
  • Brain / metabolism*
  • Fluoxetine / analogs & derivatives*
  • Fluoxetine / pharmacokinetics*
  • LLC-PK1 Cells
  • Male
  • Metabolic Clearance Rate
  • Norepinephrine Plasma Membrane Transport Proteins
  • Organ Specificity
  • Protein Binding
  • Radionuclide Imaging
  • Radiopharmaceuticals / pharmacokinetics
  • Rats
  • Rats, Sprague-Dawley
  • Reproducibility of Results
  • Sensitivity and Specificity
  • Swine
  • Symporters / metabolism*
  • Tissue Distribution

Substances

  • 2-iodonisoxetine
  • Norepinephrine Plasma Membrane Transport Proteins
  • Radiopharmaceuticals
  • Slc6a2 protein, rat
  • Symporters
  • Fluoxetine