In vivo evaluation of [¹¹C]methyl-Losartan as a selective AT₁R radioligand in rats

Objectives The Angiotensin II type 1 receptor (AT₁R) is responsible for the physiological effects of the renin-angiotensin system, and its expression pattern is altered in cardiac and renal diseases. O-methyl-Losartan (IC₅₀=32nM) and Losartan (IC₅₀=19nM) display high affinity and similar antagonistic activity for AT₁Rs. We present here the tracer kinetics and binding selectivity of [¹¹C]Methyl-Losartan in rats.

Methods Seven male Sprague-Dawley rats (n=15 scans) received 0.5-2.5mCi (0.2-15μg) [¹¹C]Methyl-Losartan IV, and dynamic PET scans (60min) were performed. Data was reconstructed into a dynamic histogram. Time-activity curves were normalized to peak blood activity. Distribution volumes (DV) were generated for the kidneys using a Logan graphical analysis and the left atrial cavity as input function. Biodistribution studies were performed 10min post-injection (0.7-1.2mCi, 1-5ug). To assess binding selectivity, animals received either tracer alone or antagonists of AT₁R (Losartan 20mg/kg, Candesartan 5mg/kg), AT₂R (PD-123,319 5mg/kg) or Mas (A-779 100ug/kg) receptors.

Results High tracer uptake and contrast to surrounding tissues were observed in the AT₁R-rich kidneys, with retained activity levels peaking at 5min and reaching background levels after 30min. Logan DVs for the right and left kidneys were 1.69±0.25ml/g and 1.61±0.23ml/g respectively. At current tracer doses and specific activity, low uptake was observed in the heart and none in the brain. Biodistribution showed highest radioactive signal in the liver, kidney cortex and outer medulla. Tracer uptake was significantly reduced by AT₁R antagonists Losartan and Candesartan in the kidney cortex (86% and 79%), outer medulla (77% and 74%), and in the liver (76%) with Losartan (p<0.05); but not with AT₂R or Mas blockers.

Conclusions High uptake, tissue contrast and selective binding of [¹¹C]Methyl-Losartan to AT₁R in the kidneys demonstrates the potential for non-invasive in vivo PET imaging and quantification of AT₁R binding.

Research Support This research is supported by CIHR MOP-80203