Gerben M. Visser

Synthesis and preliminary evaluation of (R,S)-1-[2-((Carbamoyl-4-hydroxy)phenoxy)-ethylamino]-3-[4-(1-[11C]-methyl-4-trifluoromethyl-2-imidazolyl)phenoxy]-2-propanol ([11C]CGP 20712A) as a selective β1-adrenoceptor ligand for PET

The most selective beta 1-adrenoceptor ligand known at this moment is (S)-1-[2-((carbamoyl-4-hydroxy) phenoxy)ethylamino]-3-[4-(1-methyl-4-trifluoromethyl-2-imidazolyl) phenoxy]-2-propanol (CGP 26505), the S-isomer of CGP 20712A. We prepared the racemic 11C analogue by methylation with [11C]CH3I of the corresponding desmethyl compound using a microwave oven to accelerate the reaction. Several radioactive by-products (about 70% of the non-volatile radioactive products) were formed. After HPLC purification [11C]CGP 20712A with a specific activity of 35 TBq/mmol was dissolved in a propylene glycol-ethanol-saline mixture to prepare it for injection. The total preparation time was 35 min. The radiochemical yield was 5% (calculated from [11C]CH3I, not corrected for decay). The identity of [11C]CGP 20712A was proved by liquid chromatography-mass spectrometry (LC-MS). Tissue distribution studies in male Wistar rats have been performed. At 20 min after injection of the radioligand (0.1 nmol) the DAR [differential absorption ratio = (counts per minute recovered/g tissue)/(counts per min injected/g body weight)] in heart tissue decreased significantly (P < 0.005) from 1.84 +/- 0.11 to 1.21 +/- 0.12 after blocking of beta-adrenoceptors with 500 micrograms (R,S)-propranolol. A preliminary PET study in a Wistar rat showed maximal uptake in the time frame 10-20 min after injection. The ratio of specific/non-specific binding at this interval was 2.6.

The presence of progesterone receptors in arachnoid granulations and in the lining of arachnoid cysts: Its relevance to expression of progesterone receptors in meningiomas

Progesterone receptors (PR) were identified with an enzyme immunoassay in cytosols from human arachnoid granulations and arachnoid cysts. Meningiomas presumably originate from subdural endothelium which is abundantly present in these structures. In the three cases studied, oestrogen receptors were absent. The presence of PR in subdural endothelium may provide further ground for the expression of PR in meningiomas.Progesterone receptors (PR) were identified with an enzyme immunoassay in cytosols from human arachnoid granulations and arachnoid cysts. Meningiomas presumably originate from subdural endothelium which is abundantly present in these structures. In the three cases studied, oestrogen receptors were absent. The presence of PR in subdural endothelium may provide further ground for the expression of PR in meningiomas.

Synthesis and evaluation of 1′-[18F]fluorometoprolol as a potential tracer for the visualization of β-adrenoceptors with PET

Abstract (±)-1′-[ 18 F]Fluorometoprol 4 was prepared from desisopropylmetoprolol and [ 18 F]fluoroisopropyl tosylate 2 with a radiochemical yield of 2% [corrected for decay to end of bombardment (EOB), synthesis time 90 min]. Synthon 2 was prepared from (S)-1,2-propanediol di( p -toluenesulfonate) in 45% radiochemical yield (EOB, 40 min). Compound 4 shows in two in vitro assays a similar affinity at β-adrenoceptors (about 0.3 μM) as metoprolol 5 , but with a slightly higher β 1 β 2 -adrenoceptor selectivity ratio (48.6 vs 30.7). In vivo experiments with 4 showed almost no receptor-mediated uptake in the heart, probably because the affinity of (fluoro)metoprolol for the β 1 -adrenoceptors is too low for successful imaging. However, the in vitro experiments suggest that the fluoroisopropyl group is suitable for the synthesis of [ 18 F]fluorinated β 1 -adrenergic receptor binding ligands.

Suitability of CGP-12177 and CGP-26505 for quantitative imaging of β-adrenoceptors

[3H]CGP-12177, a non-selective beta-adrenoceptor antagonist, and [3H]CGP-26505, a beta 1-selective beta-adrenoceptor antagonist, were intravenously administered to rats. 94-97% of the injected radioactivity disappeared from plasma with t1/2 0.2 and 0.5 min. Total/non-specific binding ratios of 5.4 and 6.9 (CGP-12177) or 2.0 and 2.8 (CGP-26505) were maintained in heart and lung from 10 to 40 min post-injection. Labelled plasma metabolites appeared after greater than 20 min (CGP-12177) or within 2 min (CGP-26505). No metabolites were found in the heart. CGP-12177 binds to blood cells, but CGP-26505 does not. CGP-12177 can be used for PET imaging of total (beta 1 and beta 2) adrenoceptors in the heart and lung of experimental animals, but CGP-26505 is less suitable for in vivo analysis of the beta 1-subpopulation.

Synthesis and in vivo distribution in the rat of a dopamine agonist: N-([11C]methyl)norapomorphine.

A method for the rapid production and purification of 10,11-dihydroxy-N-([11C]methyl)norapomorphine ([11C]APO), a dopamine agonist (DA), is described. The potency of this ligand for studying the D2-receptors was examined. The label was introduced by N-methylation of norapomorphine hydrobromide with no-carrier-added (n.c.a) [11C]CH3I, produced from cyclotron-produced [11C]carbon dioxide. In 60 min (EOB) a radiochemical yield of 15% (corrected for decay) was achieved, based on [11C]CH3I. The specific activity ranged from 5 to 11 GBq/mumol. The distribution, after intravenous injection, was studied in rats. The radioactivity level in the striatum was higher than in the cerebellum and frontal cortex and was decreased after D2-blockade. The highest uptake ratio (1.47) was found at 30 min after injection. Dopamine depletion with reserpine did increase the striatum/cerebellum ratio at a low dosage of [11C]APO (10 nmol/kg). High uptakes of [11C]apomorphine were found in the lungs, liver and kidneys.

ROBOTIC SYNTHESIS OF L-[1-C-11]TYROSINE

Abstract l -[1-11C]tyrosine promises to become an important tracer for determination of the protein synthesis rate (PSR) in tumor tissue and brain. The commercially available Anatech RB-86 robotic system is utilized for the automation of the l -[1-11C]tyrosine production via the isocyanide method as reported by Bolster et al. (Eur. J. Nucl. Med. 12, 321–324 1986). The total synthesis time, including HPLC-purification and enantiomeric separation is 60 min. With a practical yield of 20 mCi l -[1-11C]tyrosine at a specific activity > 1000 Ci/mmol.

Studies of Cardiac Receptors by Positron Emission Tomography

New concepts regarding neurohumoral regulation of cardiac function by myocardial receptors in health and disease have become prominent during the last two decades. Radioligand binding studies have greatly advanced our knowledge of hormone and neurotransmitter binding sites. Changes in the number and/or affinity of cardiac receptors have been assessed by in vitro techniques and shown to be associated with congestive heart failure [1, 2, 3], myocardial ischemia and infarction [4, 5, 6, 7, 8, 9], cardiomyopathy [10,11], hypertension [12,13], chronic drug administration [14, 15, 16, 17] and ageing [18]. In vivo assays may improve our understanding of the time course of diseases and enable better prognosis.

Synthesis of 6α-[18F]Fluoroprogesterone: A first step towards a potential receptor-ligand for PET

6 alpha-[F-18]Fluoroprogesterone 3 was prepared by the BF3.Et(2) O-catalyzed reaction of progest-5 alpha,6 alpha-epoxy-3,20-bisketal 1 and [F-18]fluoride as a possible route for the in vivo visualization of progesterone receptors by PET. The radiochemical yield of 3 was 25% (EOB) and the sp. act. was 5 MBq/mu mol (100 Ci/mol, EOS).

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.