Add time:07/19/2019         Source:sciencedirect.com

In developing technetium-99m–based radioligands for in vivo studies of cardiac adrenergic neurons, we compared the uptake characteristics of the 99mTc-labeled 1-(4-fluorobenzyl)-4-(2-mercapto-2-methyl-4-azapentyl)-4-(2-mercapto-2-methylpropylamino)-piperidine (99mTc-FBPBAT) with those of the clinically established meta-[123I]iodobenzylguanidine (123I-MIBG) in rat vascular smooth muscle cells and neonatal cardiac myocytes. Furthermore, the cardiac and extracardiac uptake of both radiopharmaceuticals was assessed in intact rats and in rats pretreated with various α- and β-adrenoceptor drugs, and adrenergic reuptake blocking agents. The uptake of 99mTc-FBPBAT and 123I-MIBG into vascular smooth muscle cells and neonatal cardiac myocytes was rapid; more than 85% of the radioactivity accumulation into the cells occurring within the first 3 minutes. Radioactivity uptake after a 60-minute incubation at 37°C (pH 7.4) varied from 15% to 65% of the total loaded activity per million cells. In all cases, 99mTc-FBPBAT showed the higher uptake, relative to 123I-MIBG, at any given cell concentration. The cellular uptake of 99mTc-FBPBAT was lower at 4°C and 20°C than at 37°C. In contrast, the 123I-MIBG uptake was only slightly temperature dependent. Inhibition experiments confirmed that the cellular uptake of 123I-MIBG is mediated by the uptake-I carrier, whereas α1- and β1-adrenoceptors were predominantly involved in the uptake of 99mTc-FBPBAT into the cardiovascular tissues. Biodistribution studies in rats showed that 99mTc-FBPBAT accumulated in myocardium after intravenous injection. Radioactivity in rat heart amounted to 2.32% and 1.91% of the injected dose per gram at 15 and 60 minutes postinjection, compared with 3.10% and 2.21% injected dose per gram of tissue (%ID/g) in the experiment with 123I-MIBG, respectively. Prazosin, urapidil, and metoprolol were as effective as treatment with other adrenergic drugs in lowering cardiac uptake of 99mTc-FBPBAT. Uptake reduction was more pronounced in myocardium than in other adrenergic-rich organs, including the lung, spleen, kidney, and adrenals, suggesting that the 99mTc-FBPBAT uptake in myocardium specifically reflects a high degree of α1/β1-receptor binding to cardiac adrenergic neurons. In comparison, reduction of cardiac and pulmonary uptake of 123I-MIBG was effective after pretreatment of rats with desipramine and reserpine, confirming distinct neuronal binding sites for 99mTc-FBPBAT and 123I-MIBG. 99mTc-FBPBAT was excreted via urine and to a lower degree via feces. Urine analysis 6 hours p.i. revealed that more than 40% of the total excreted radioactivity was unmetabolized 99mTc-FBPBAT. In conclusion, the uptake of 99mTc-FBPBAT in rat myocardium specifically reflects binding to cardiac adrenergic neurons. The 99mTc-FBPBAT uptake appears to be predominantly mediated via the α1/β1-adrenoceptor pathway. These data indicate that 99mTc-FBPBAT, like 123I-MIBG, may be suitable for mapping cardiac adrenergic innervation by SPET, especially for α1/β1-adrenoceptors as target in numerous heart diseases.

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