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Evaluation of [(11)C]methyl-losartan and [(11)C]methyl-EXP3174 for PET imaging of renal AT1receptor in rats. 2015

Basma Ismail, and Tayebeh Hadizad, and Rawad Antoun, and Mireille Lortie, and Robert A deKemp, and Rob S B Beanlands, and Jean N DaSilva
National Cardiac PET Centre, Division of Cardiology (Department of Medicine) University of Ottawa Heart Institute, 40 Ruskin St., Ottawa, ON, Canada, K1Y 4W7; Department of Cellular and Molecular Medicine, Faculty of Medicine, University of Ottawa, 451 Smyth Road, Ottawa, ON, Canada, K1H 8M5.

BACKGROUND The angiotensin II type 1 receptor (AT1R) is responsible for the main effects of the renin-angiotensin system (RAS), and its expression pattern is altered in several diseases. The [(11)C]methylated derivatives of the clinically used AT1R blocker (ARB) losartan and its active metabolite EXP3174, that binds with higher affinity to AT1R, were evaluated as potential PET imaging tracers in rat kidneys. METHODS [(11)C]Methyl-losartan and [(11)C]methyl-EXP3174 were synthesized by [(11)C]methylation of the tetrazole-protected analogs using [11C]methyl iodide. Tissue uptake and binding selectivity of [(11)C]methyl-losartan were assessed by ex-vivo biodistribution and in-vitro autoradiography. Radiolabeled metabolites in rat plasma and kidneys were analysed by column-switch HPLC. Both tracers were evaluated with small animal PET imaging. Due to better pharmacokinetics, [(11)C]methyl-EXP3174 was further investigated via PET by co-injection with AT1R antagonist candesartan or the AT2R antagonist PD123,319. RESULTS Binding selectivity to renal AT1 over AT2 and Mas receptors was demonstrated for [(11)C]methyl-losartan. Plasma metabolite analysis at 10 min revealed stability of [(11)C]methyl-losartan and [(11)C]methyl-EXP3174 with the presence of unchanged tracer at 70.8 ± 9.9% and 81.4 ± 6.0%, of total radioactivity, respectively. Contrary to [(11)C]methyl-losartan, co-injection of candesartan with [(11)C]methyl-EXP3174 reduced the proportion of unchanged tracer (but not metabolites), indicating that these metabolites do not bind to AT1R in rat kidneys. MicroPET images for both radiotracers displayed high kidney-to-background contrast. Candesartan significantly reduced [(11)C]methyl-EXP3174 uptake in the kidney, whereas no difference was observed following PD123,319 indicating binding selectivity for AT1R. CONCLUSIONS [(11)C]Methyl-EXP3174 displayed a favorable binding profile compared to [(11)C]methyl-losartan for imaging renal AT1Rs supporting further studies to assess its full potential as a quantitative probe for AT1R via PET.

UI MeSH Term Description Entries
D007093 Imidazoles Compounds containing 1,3-diazole, a five membered aromatic ring containing two nitrogen atoms separated by one of the carbons. Chemically reduced ones include IMIDAZOLINES and IMIDAZOLIDINES. Distinguish from 1,2-diazole (PYRAZOLES).
D007668 Kidney Body organ that filters blood for the secretion of URINE and that regulates ion concentrations. Kidneys
D008297 Male Males
D011854 Radiochemistry The study of the chemical and physical phenomena of radioactive substances. Radiochemistries
D002250 Carbon Radioisotopes Unstable isotopes of carbon that decay or disintegrate emitting radiation. C atoms with atomic weights 10, 11, and 14-16 are radioactive carbon isotopes. Radioisotopes, Carbon
D000818 Animals Unicellular or multicellular, heterotrophic organisms, that have sensation and the power of voluntary movement. Under the older five kingdom paradigm, Animalia was one of the kingdoms. Under the modern three domain model, Animalia represents one of the many groups in the domain EUKARYOTA. Animal,Metazoa,Animalia
D013777 Tetrazoles
D014018 Tissue Distribution Accumulation of a drug or chemical substance in various organs (including those not relevant to its pharmacologic or therapeutic action). This distribution depends on the blood flow or perfusion rate of the organ, the ability of the drug to penetrate organ membranes, tissue specificity, protein binding. The distribution is usually expressed as tissue to plasma ratios. Distribution, Tissue,Distributions, Tissue,Tissue Distributions
D017207 Rats, Sprague-Dawley A strain of albino rat used widely for experimental purposes because of its calmness and ease of handling. It was developed by the Sprague-Dawley Animal Company. Holtzman Rat,Rats, Holtzman,Sprague-Dawley Rat,Rats, Sprague Dawley,Holtzman Rats,Rat, Holtzman,Rat, Sprague-Dawley,Sprague Dawley Rat,Sprague Dawley Rats,Sprague-Dawley Rats
D044140 Receptor, Angiotensin, Type 1 An angiotensin receptor subtype that is expressed at high levels in a variety of adult tissues including the CARDIOVASCULAR SYSTEM, the KIDNEY, the ENDOCRINE SYSTEM and the NERVOUS SYSTEM. Activation of the type 1 angiotensin receptor causes VASOCONSTRICTION and sodium retention. Angiotensin II Type 1 Receptor,Angiotensin Type 1 Receptor,Angiotensin Type 1a Receptor,Angiotensin Type 1b Receptor,Receptor, Angiotensin, Type 1a,Receptor, Angiotensin, Type 1b,Angiotensin AT1 Receptor,Angiotensin AT1a Receptor,Angiotensin AT1b Receptor,Angiotensin II Type 1a Receptor,Angiotensin II Type 1b Receptor,Receptor, Angiotensin II Type 1,Receptor, Angiotensin II Type 1a,Receptor, Angiotensin II Type 1b,AT1 Receptor, Angiotensin,AT1a Receptor, Angiotensin,AT1b Receptor, Angiotensin,Receptor, Angiotensin AT1,Receptor, Angiotensin AT1a,Receptor, Angiotensin AT1b

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