¹¹C-OMAR and PET determined myocardial cannabinoid type 1 receptor upregulation in obesity: - a translational approach -

Objectives: To evaluate the feasibility of targeted imaging myocardial cannabinoid type 1 receptor (CB1-R) and its potential upregulation from normal weight to obese mice with translation to humans using [¹¹C]-OMAR and PET/CT.

Methods: Biodistribution measurements of the heart with [¹¹C]-OMAR was performed in male OB/OB homozygous mice. The animals were sacrificed by cervical dislocation at different time points 5, 15, 30, 60 and 90 min following injection of ≍200 μCi [¹¹C]-OMAR into the lateral tail vein and the heart (blood eliminated) was harvested from each mouse. The heart tissue was weighted and its radioactivity content determined in a γ-counter. In addition, in vivo binding specificity was determined by blocking studies, in which mice were pretreated with intraperitoneal (i.p.) injection of 1 to 5 mg/kg rimonabant or vehicle, followed by i.v. injection of ~7.4 MBq of [¹¹C]-OMAR (n=3/group). Further, applying the CB1-R ligand [¹¹C]-OMAR, dynamic PET/CT was carried out in seven obese and five normal weight mice. after CB1-R blocking study with 5mg/kg rimonabant. Ex vivo validation was performed by polymerase chain reaction. Subsequently, myocardial CB1-R expression was probed non-invasively with intravenous injection of CB1-R ligand [¹¹C]-OMAR and PET/CT in seven individuals with advanced obesity (AOB; BMI 蠅38kg/m²) and five normal weight controls (CON; BMI<25kg/m²), respectively.

Results: The biodistribution study of [¹¹C]-OMAR signified the highest accumulation in the heart after five 5 minutes (median (IQR): 4.63 (4.32, 4.90) %ID/g tissue) with a progressive decrease over 10, 15, to 30 minutes (3.31 (3.31, 3.52), 3.01 (3.31, 3.16), to 2.14 (2.12, 2.16) %ID/g tissue) and a plateau between 60-90 minutes (1.55 (1.52, 1.58) to 1.40 (1.32, 1.60) %ID/g tissue). Rimonabant significantly blocked OMAR uptake in the heart muscle as compared to vehicle (1.94 (1.92, 2.04) vs. 4.63 (4.31, 4.93) %ID/g tissue; p&#8804;0.05), signifying the specific binding of OMAR to the CB1-R in the heart muscle. Quantification of the myocardial OMAR retention with PET/CT in mice proved to significantly higher in obese than in normal weight mice [8.13 (2.87, 11.74) %/min vs. 0.10 (0.03, 0.11) %/min, p=0.015]. Absolute quantification of CB1-R gene expression with droplet digital PCR confirmed CB1-R up-regulation in obese [73.7 (68.1, 75.9)] copies/µL versus normal weight mice [54.5 (50.5, 57.8) copies/µL, p&#8804;0.05]. Obese mice also had elevated myocardial levels of endocannabinoids anandamide and 2-arachidonoylglycerol compared to lean mice. Translation to humans also signified a significantly higher OMAR retention in AOB than in CON [5.88 (2.06, 7.16) %/min vs. 0.51 (0.33, 1.19)%/min, p=0.015].

Conclusion: Noninvasive imaging of cardiac CB1-R expression with [¹¹C]-OMAR and PET/CT is feasible and signifies an upregulation of cardiac CB1-receptor expression in obesity. As the activation of myocardial CB1-receptor by endocannabinoids can lead to cardiac dysfunction, the observed up-regulation of myocardial CB1-receptors may provide a mechanistic link between obesity and the initiation of cardiomyopathy. Research Support: Departmental Fund from Johns Hopkins University (No. 175470 to T.H. Schindler), Baltimore, and the Intramural Research Program of NIAAA/NIH (to G.Kunos, and P. Pacher), Bethesda, Maryland.