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

Abstract

IntroductionThe 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 [11C]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[11C]Methyl-losartan and [11C]methyl-EXP3174 were synthesized by [11C]methylation of the tetrazole-protected analogs using [11C]methyl iodide. Tissue uptake and binding selectivity of [11C]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, [11C]methyl-EXP3174 was further investigated via PET by co-injection with AT1R antagonist candesartan or the AT2R antagonist PD123,319. ResultsBinding selectivity to renal AT1 over AT2 and Mas receptors was demonstrated for [11C]methyl-losartan. Plasma metabolite analysis at 10min revealed stability of [11C]methyl-losartan and [11C]methyl-EXP3174 with the presence of unchanged tracer at 70.8±9.9% and 81.4±6.0%, of total radioactivity, respectively. Contrary to [11C]methyl-losartan, co-injection of candesartan with [11C]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 [11C]methyl-EXP3174 uptake in the kidney, whereas no difference was observed following PD123,319 indicating binding selectivity for AT1R. Conclusions[11C]Methyl-EXP3174 displayed a favorable binding profile compared to [11C]methyl-losartan for imaging renal AT1Rs supporting further studies to assess its full potential as a quantitative probe for AT1R via PET.