P Doze

Enhanced cerebral uptake of receptor ligands by modulation of P-glycoprotein function in the blood-brain barrier.

Low cerebral uptake of some therapeutic drugs can be enhanced by modulation of P‐glycoprotein (P‐gp), an ATP‐driven drug efflux pump at the blood–brain barrier (BBB). We investigated the possibility of increasing cerebral uptake of the β‐adrenergic ligands S‐1′‐[18F]‐fluorocarazolol (FCAR) and [11C]‐carazolol (CAR) in P‐gp knockout mice (mdr1a (−/−)) and by modulation of P‐gp with cyclosporin A (CsA) in rats. Specific and nonspecific binding of FCAR in the rat brain were doubled by CsA, while target/nontarget ratios and clearance from plasma (area under curve (AUC)) were not affected. Cerebral uptake of CAR in rats was much lower than FCAR and nonspecific. CsA increased this uptake 5–6‐fold, not only due to P‐gp modulation in the BBB but also to a 2‐fold higher plasma AUC. In the CNS of mdr1a (−/−) mice, uptake of FCAR and CAR was, respectively, 2‐fold and 3‐fold higher than in mdr1a (+/+) mice. These results indicate that the cerebral uptake of β‐adrenoceptor ligands can be increased by administration of P‐gp modulators such as CsA without affecting regional distribution in the brain. P‐gp modulation could improve the count statistics in PET studies of the CNS. Synapse 36:66–74, 2000.

A novel beta-adrenoceptor ligand for positron emission tomography: Evaluation in experimental animals

Myocardial and pulmonary beta-adrenoceptors can be imaged and quantified with the antagonist (S)-4-[3[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-b enzimidazol-2-[11C]-one (S-[11C]CGP-12177). The synthesis of this ligand (based on the reaction of a precursor with [11C]phosgene) is laborious and in many centers the final product has a low and variable specific activity. This prevents widespread use of S-[11C]CGP-12177 for studies in patients. We prepared S-[11C]CGP-12388, the isopropyl analogue of CGP-12177, by a reliable one-pot procedure and evaluated the radiopharmaceutical for beta-adrenoceptor imaging. Blocking experiments with subtype-selective beta-adrenergic drugs showed that myocardial and pulmonary uptake of S-[11C]CGP-12388 in anesthetized rats reflects ligand binding to beta1- and beta2-adrenoceptors. In this animal model, clearance, metabolism and tissue/plasma ratios of S-[11C]CGP-12388 were similar to those of S-[11C]CGP-12177. A [18F]fluoroisopropyl analogue of CGP-12177 showed less favorable characteristics. S-[11C]CGP-12388 was therefore selected for evaluation in humans and it may become the tracer of choice for clinical studies since it is easily prepared.

Synthesis and evaluation of radiolabeled antagonists for imaging of beta-adrenoceptors in the brain with PET

Five potent, lipophilic beta-adrenoceptor antagonists (carvedilol, pindolol, toliprolol and fluorinated analogs of bupranolol and penbutolol) were labeled with either carbon-11 or fluorine-18 and evaluated for cerebral beta-adrenoceptor imaging in experimental animals. The standard radioligand for autoradiography of beta-adrenoceptors, [125I]-iodocyanopindolol, was also included in this survey. All compounds showed either very low uptake in rat brain or a regional distribution that was not related to beta-adrenoceptors, whereas some ligands did display specific binding in heart and lungs. Apparently, the criteria of a high affinity and a moderately high lipophilicity were insufficient to predict the suitability of beta-adrenergic antagonists for visualization of beta-adrenoceptors in the central nervous system.

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.

Imaging of β-adrenoceptors in the human thorax using (S)-[11C]CGP12388 and positron emission tomography

We report positron emission tomography studies of β-adrenoceptors in the human thorax with (S)-[11C]CGP12388 (4-(3-(2′-[11C]-isopropylamino)-2-hydroxypropoxy)-2H-benzimidazol-2-one). β-Adrenoceptors have previously been quantified using (S)-[11C]CGP12177 (4-(3-tert-butylamino-2-hydroxypropoxy)-2H-benzimidazol-2[11C]-one), but (S)-[11C]CGP12388 is more easily prepared and therefore more suitable in a clinical setting. (S)-[11C]CGP12388 was administered to five healthy volunteers on two separate days (control and pindolol block study). Arterial plasma samples were used to determine clearance, metabolites, and protein binding of the radioligand. Heart, lung and spleen showed high uptake of radioactivity, which was strongly suppressed (68–77%) by pindolol. Plasma clearance of (S)-[11C]CGP12388 was rapid, binding to plasma proteins was low (53±4%), and the radioligand was slowly metabolized. (S)-[11C]CGP12388 produces high-quality images of the human thorax. Uptake of (S)-[11C]CGP12388 in heart, lung and spleen represents binding to β-adrenoceptors. (S)-[11C]CGP12388 seems useful for imaging of β-adrenoceptors in a clinical setting.

Preclinical testing of N-[11c]-methyl-piperidin-4-yl 2-cyclohexyl-2-hydroxy-2-phenylacetate, a novel radioligand for detection of cerebral muscarinic receptors using PET

The muscarinic antagonist N‐[11C]methyl‐piperidin‐4‐yl 2‐cyclohexyl‐2‐hydroxy‐2‐phenylacetate (VC‐004) 1 was tested for visualization of muscarinic receptors in the brain. The active (R)‐isomer (pKb = 10.92) was labeled by reacting [11C]‐CH3I with the secondary amine precursor (40–60% decay‐corrected radiochemical yield, specific activity 13.0–34.3 TBq/mmol, 45 min after end of bombardment). Biodistribution studies were performed in male Wistar rats. Brain uptake of (R)‐[11C]‐VC‐004 was high, standard uptake values (SUVs) ranging from 1.6 in cerebellum to 3.3 in frontal cortex. In all brain regions, the nonsubtype selective muscarinic antagonist scopolamine (2.5 mg/kg) blocked (R)‐[11C]‐VC‐004 binding to the same extent (84.6 ± 3.3%) as nonlabeled (R)‐VC‐004 (2.0 mg/kg, 83.2 ± 4.6%). In contrast, the fraction of [11C]VC‐004 binding which was blocked by atropine (2.5 mg/kg) was significantly smaller (54 ± 17%). The reduction of (R)‐[11C]‐VC‐004 binding by low‐dose atropine (0.5 mg/kg) was not significantly different from that caused by (R)‐(‐)‐QNB (20 μg/kg). The decrease in specific binding of (R)‐[11C]VC‐004 after (R)‐(‐)‐QNB block correlated well with literature values for the percentages of M2 receptors in the brain regions studied. (R)‐[11C]VC‐004 was rapidly cleared from plasma (92% with a half‐life of 0.27 min) and the fraction of total plasma radioactivity representing parent compound decreased from 99% to 42% at 10 min postinjection. Although (R)‐[11C]VC‐004 can visualize muscarinic receptors in the brain, it does not show selectivity for the M2‐subtype. A low dose (0.5 mg/kg) of atropine seems to preferentially block M2‐receptors in vivo, as has been reported for (R)‐(‐)‐QNB. Synapse 35:62–67, 2000.