Radiopharmaceuticals for CNS receptor imaging with SPECT

Evaluation of radioiodinated (R)-N-methyl-3-(2-iodophenoxy)-3-phenylpropanamine as a ligand for brain norepinephrine transporter imaging

(R)-N-methyl-3-(2-iodophenoxy)-3-phenylpropanamine (MIPP) was evaluated as a radiopharmaceutical for investigating brain norepinephrine transporters (NET) by single photon emission computed tomography (SPECT). (R)-[(125)I]MIPP was synthesized with high radiochemical yield (60%) and high radiochemical purity (> 98%). In biodistribution experiments, (R)-[(125)I]MIPP indicated that the brain uptake of (R)-[(125)I]MIPP was rapid and retained, and that the regional cerebral distribution was consistent with the density of NET. Moreover, the administration of desipramine decreased the accumulation of (R)-[(125)I]MIPP in the brain. HPLC analysis of brain radioactivity showed that more than 90% was intact (R)-MIPP. These results suggested that (R)-[(123)I]MIPP is a potential radiopharmaceutical for imaging brain NET.

In vivo spin-label murine pharmacodynamics using low-frequency electron paramagnetic resonance imaging

A novel, very-low-frequency electron paramagnetic resonance (EPR) technique is used to image the distribution of several nitroxides with distinct pharmacologic compartment affinities in the abdomens of living mice. Image acquisition is sufficiently rapid to allow a time sequence of the distribution for each compound. The spectra and concentrations of these nitroxides are imaged with the use of spectral-spatial imaging to distinguish a single spatial dimension. Liver and bladder of the mouse anatomy are distinguished by this technique. After an intraperitoneal injection of the spin-label probes, a shift in the distribution of the compounds from the upper abdomen (primarily liver) to the lower abdomen (primarily bladder) is observed. The time dependence of the shift in regional distribution depends on the structural properties of the side chain attached to the spin label. These results indicate that this application of in vivo electron paramagnetic resonance imaging will provide a new method of magnetic resonance imaging for determination of pharmacodynamics in the body of an intact animal.

Myocardial accumulation of a dopamine D2 receptor-binding radioligand, 2'-iodospiperone

125I-2'-iodospiperone (2'-ISP), which has a high and selective affinity for dopamine D2 receptors, produced a high myocardial accumulation of radioactivity in the early phase after intravenous injection into mice. A human scintigraphic study also showed that the myocardium was clearly visualized soon after intravenous injection of the tracer. Analysis of the myocardial homogenate obtained from mice showed that 125I-2'-ISP was metabolically stable and was taken up the myocardium in its intact form. Administration of spiperone significantly reduced the myocardial uptake of 125I-2'-ISP in mice. Treatment with haloperidol and (+) butaclamol, which have a high affinity for dopamine D2 receptors, also tended to reduce the myocardial uptake of radioactivity, while (-)-butaclamol, which has no affinity for dopamine D2 receptors, caused no change in uptake. These findings suggest that the myocardial accumulation of 2'-ISP occurred in association with dopamine D2 (DA2) receptors.

In vivo studies of [125I]iodobenzamide and [11C]iodobenzamide: a ligand suitable for positron emission tomography and single photon emission tomography imaging of cerebral D2 dopamine receptors

Iodobenzamide (IMB) labeled with either [11C] or [125I] was studied in mice and baboons. Pharmacological studies demonstrated an in vivo binding profile compatible with D2 dopamine receptors. Mouse biodistribution studies with both [11C]IMB and [125I]IMB showed a similar brain distribution of radioactivity. Mouse [125I]IMB studies with amphetamine and reserpine pretreatment suggested that IMB may be less susceptible to endogenous dopamine competition for D2 receptor binding in vivo as compared to raclopride. Preliminary baboon studies showed haloperidol competition for IMB binding sites.

Preparation and biodistribution in mice of [11C]carfentanil: a radiopharmaceutical for studying brain mu-opioid receptors by positron emission tomography

A potent mu-opioid agonist, [11C]carfentanil, was prepared by the methylation of carfentanil carboxylic acid with [11C]methyl iodide in order to study brain mu-opioid receptors by positron emission tomography. Synthesis (including purification) was completed within 25 min and the radiochemical yield was approximately 40%. The radiochemical purity of the product was more than 99% and its specific activity was 3.7-7.4 GBq/mumol. Biodistribution studies performed in mice after intravenous injection showed a high brain uptake and rapid blood clearance, so a high brain/blood ratio of 1.5-1.8 was found from 5 to 30 min. Regional cerebral distribution studies in the mouse showed a significantly higher uptake of [11C]carfentanil by the thalamus and striatum than by the cerebellum, with the radioactivity in the striatum disappearing more rapidly than that in the thalamus. Treatment with naloxone significantly reduced the uptake of [11C]carfentanil by the thalamus and striatum. These results indicate that [11C]carfentanil binds specifically to brain mu-opioid receptors.