Synthesis and murine biodistribution of a para and ortho iodinated radiopharmaceutical directed towards the muscarinic receptor
Creators: name: Alcorn, Laura N.
Publisher: name: University of British Columbia
Year: 2009

description: Ortho and para N-(iodobenzyl)(cyclopentylmethyl)-t-butylamine (o-ICTB A and p-lCTBA) were synthesized and radioiodinated. The analogs were designed to bind specifically to the muscarinic receptor for use as radiopharmaceuticals in the diagnosis and monitoring of myocardial disease with standard nuclear medicine techniques. Uptake in receptor-bearing tissues was assessed by preliminary biodistribution studies in mice. The distribution of the muscarinic receptor is postulated to alter in response to disease. There may be an increase or decrease in the density and number of receptors. Receptors and their ligands typically interact with high sensitivity and specificity. The use of a radiolabeled compound which binds to the receptor could allow external imaging of its distribution by detection of the radioactivity in target tissue compared to non-target organs. This would permit early detection of changes in receptor density and monitoring of the onset and progress of disease. Cardiovascular disease remains a significant cause of morbidity and mortality in man. Techniques which assist in its early detection would be expected to significantly contribute to both prevention and treatment. The muscarinic receptor exists in at least five genomic forms. Various tissues typically express several muscarinic subtypes. This is not the case in the heart where predominantly only the ml subtype is expressed. Therefore, this m2 subtype if bound by a radiopharmaceutical, might be expected to demonstrate the health of the receptor population in the heart with little interference from background radioactivity in surrounding tissues. A simple, reliable synthetic method was devised involving five reaction steps. The overall yield for the synthesis of o-ICTBA was 39% and for p-ICTBA was 37%. Identity testing was done for o-ICTBA and p-ICTBA using normal and reverse phase thin layer chromatography, proton nuclear magnetic resonance spectroscopy and elemental analysis. Additionally, the ortho analog underwent Fourier transform infrared spectroscopy; positive electrospray ionization and atmospheric pressure chemical ionization mass spectrometry; high resolution mass spectrometry and; high performance liquid chromatography. Radioiodination using the Cu-I assisted exchange technique was used to prepare the final radiopharmaceuticals. The radiochemical purity for the ortho and para analogs was 98.8% and 98.9%), respectively. The radiochemical yields for each analog was 42% and 98.9%) for o-ICTBA and p-ICTBA, respectively. The specific activity of the final preparation was low at a maximum of 122 GBq/mmol (3.34 Ci/mmol). Biodistribution studies with each analog were done in mice. Ten tissues were assessed at 0.25, 1, 2, 4 and 6 hours post-injection. The following organs were harvested: blood, liver, kidneys, small intestine, adrenal gland, stomach, lung, heart, muscle and the tail. In an attempt to specifically block the muscarinic receptor, atropine was preinjected in a second series of mice who underwent an otherwise identical biodistribution protocol. Uptake in the selected tissues was compared with and without the receptor blocking ligand. The maximal heart to blood ratio obtained with the ortho derivative was 1.63 at 15 minutes post-injection and with the para was 1.54 at the same time point. These values are insufficient for efficient external imaging. For both analogs the highest targetto- nontarget ratio was observed in the adrenal gland at 4.35 for the ortho and 12.0 for the para. A marked difference in the distributions of the ortho and para analogs was observed. Organ uptake was consistently much higher with the para species. Atropine pre-injection did not result in notable attenuation of radioligand accumulation in receptorbearing tissues after administration of the ortho analog. For the para species, uptake by the heart and brain was attenuated by approximately 40% at 0.25, 1, 4 and 6 hours postinjection.