Ton J. Visser

Characterisation of beta(2)-adrenoceptors, using the agonist [C-11]formoterol and positron emission tomography

Abstract The agonist radioligand N -[2-hydroxy-5-[1-hydroxy-2-[[2-(4- [ 11 C ] -methoxyphenyl)-1-methylethyl]amino]ethyl]phenyl]formamide ( [ 11 C ] formoterol) was synthesised in order to test its ability to visualise pulmonary β 2 -adrenoceptors in vivo, with positron emission tomography (PET). Formoterol was labelled via reaction of a dibenzyl-protected precursor with [ 11 C ] CH 3 I. Subsequent deprotection with Pd/C and H 2 yielded [ 11 C ] formoterol in 5–15% (corrected for decay) and the specific activity ranged from 5.5–22.2 TBq mmol −1 (150–600 Ci mmol −1 ), 60–70 min after end of bombardment. Biodistribution studies with [ 11 C ] formoterol were performed in male Wistar rats which were either untreated or predosed with ( d , l )-propranolol hydrochloride (2.5 mg kg −1 , β-adrenoceptor antagonist), erythro- dl -1-(7-methylindan-4-yloxy)-3-isopropylaminobutan-2-ol hydrochloride (ICI 118551, 0.15 mg kg −1 , β 2 -adrenoceptor antagonist), isoprenaline (15 mg kg −1 , non-subtype selective β-adrenoceptor agonist) or (±)-(2-hydroxy-5-[2-((2-hydroxy-3-(4-((1-methyl-4-trifluoromethyl)1 H -imidazol-2-yl-)phenoxy)propyl)amino)ethoxy]benzamide)monomethane sulfonate (CGP 20712A, 0.15 mg kg −1 , β 1 -adrenoceptor antagonist). Lungs, heart, liver and plasma were analysed for radioactive metabolites. The kinetics of [ 11 C ] formoterol in the lungs of male Wistar rats were investigated by means of a dynamic PET study. The biodistribution studies showed significant specific binding in tissues known to contain β 2 -adrenoceptors (lungs, spleen, and heart). Binding in these organs was blocked by ICI 118551 and isoprenaline, but not by CGP 20712A. [ 11 C ] Formoterol was rapidly metabolised in rats but lungs and heart did not substantially take up the labelled metabolites. The binding of [ 11 C ] formoterol in various tissues of rats is consistent with the β 2 -selectivity of formoterol. Whether [ 11 C ] formoterol selectively binds to the high affinity state of β 2 -adrenoceptors remains to be elucidated. [ 11 C ] Formoterol is potentially useful for studying β 2 -adrenoceptors with PET and this radioligand may provide new insights in the mechanisms underlying prolonged sympathomimetic action.

Visualization of beta-Adrenoceptors Using PET

PET-investigations of beta-adrenoceptors in heart, lungs, and brain are of great clinical interest. Receptor densities are altered under various pathophysiological conditions. This review gives an overview of PET-research of beta-adrenoceptors. The clinical relevance of in vivo PET-investigations of beta-adrenoceptors is described for heart, lungs, and brain. Results obtained with the available PET-ligands are discussed. [(11)C]CGP 12177 has been successfully applied in patients with cardiac and pulmonary diseases. The complicated labelling procedure prevents widespread use of the radiopharmaceutical. Due to its lipophilicity, [(18)F]-fluorocarazolol is a suitable ligand for cardiac and pulmonary as well as for cerebral sites. Non-specific binding is higher as compared to [(11)C]CGP 12177. Unfavorable toxicity data prevents human use of this ligand. [(11)C]Carazolol shows comparable distribution in heart and lung as [(18)F]fluorocarazolol, but it hardly enters the brain. [(11)C]CGP 12388 displays comparable pharmacokinetics in rats and is much easier prepared than [(11)C]CGP 12177. Therefore [(11)C]CGP 12388 seems a promising PET-ligand for clinical studies of cardiac and pulmonary beta-adrenoceptors. Pilot studies with the long-acting agonist [(11)C]formoterol have indicated that it may be possible to investigate the high affinity state of the beta(2)-subtype.

Study of Cardiac Receptor Ligands by Positron Emission Tomography

Changes in receptor populations may be early markers of disease, or indicators of the therapeutic success. A large research effort is therefore directed towards tomographic imaging of receptors and quantitative interpretation of these images in terms of receptor densities. Before human studies are possible, putative receptorbinding radiotracers should be thoroughly tested.

Rodent biodistribution and metabolism of tritiated 4-DAMP, a M3 subtype-selective cholinoceptor ligand

The biodistribution of [3H]4-DAMP (a M3-selective cholinoceptor antagonist) was studied in rats which had received either saline or saline containing atropine (to block cholinoceptors). Specific binding of the radioligand was observed in the urinary bladder, ileum, pancreas, stomach, submandibular gland and trachea. Maximal ratios of total-to-non-specific uptake reached values of 1.8 (trachea), 3.2 (bladder), 4.0 (stomach), 4.8 (ileum), 6.6 (pancreas) and 6.9 (submandibular gland) at 5-10 min post-injection; this rank order reflects the tissue densities of M3 cholinoceptors, 4-DAMP did not bind to blood cells and it was rapidly cleared from the circulation (> 90% with a half-life of 0.2 min, the remainder with a half-life of 9.4 min). Labelled metabolites appeared within 5 min in plasma, but metabolite uptake by the target organs was low (< 15% of total radioactivity 40 min post-injection). Although 4-DAMP binds to M3-cholinoceptors in vivo, its potential use as a radiopharmaceutical appears limited since the compound does not cross the blood-brain barrier and it does not show measurable specific binding in airways.

Development of Ligands for Receptor Imaging

The noninvasive characterization of the cardiac nervous system by positron emission tomography (PET) may provide important pathophysiological information in various cardiac diseases. Changes in numbers of neurotransmitter receptors have been associated with myocardial ischemia and infarction, congestive heart failure, cardiomyopathy, as well as diabetes or thyroid-induced heart muscle disease. Knowledge on these receptor densities is based on post mortem or biopsy material. With PET it is possible to investigate the whole heart in vivo. A prerequisite for imaging of the cardiac nervous system is the availability of appropriate radioligands, which includes its radiolabeling and evaluation in animals and human volunteers. This chapter gives an overview of the process of development of a radioligand for the β-adrenergic receptor, which is illustrative for the development of radioligands in general.