van Aren Waarde

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.

Adenosine A1 Receptors in the Central Nervous System: Their Functions in Health and Disease, and Possible Elucidation by PET Imaging

Adenosine is a neuromodulator with several functions in the central nervous system (CNS), such as inhibition of neuronal activity in many signaling pathways. Most of the sedating, anxiolytic, seizure-inhibiting and protective actions of adenosine are mediated by adenosine A(1) receptors (A(1)R) on the surface of neurons and glia. Positron Emission Tomography (PET) is a powerful in vivo imaging tool which can be applied to investigate the physiologic and pathologic roles of A(1)R in the human brain, and to elucidate the mechanism of action of therapeutic drugs targeting adenosine receptors, nucleoside transporters and adenosine-degrading enzymes. In this review article, we discuss (i) functions of adenosine and its receptors in cerebral metabolism; (ii) radioligands for A(1)R imaging: xanthine antagonists, non-xanthine antagonists, and agonists; (iii) roles of A(1)R in health and disease, viz. sleep-wake regulation, modulation of memory retention and retrieval, mediating the effects of alcohol consumption, protecting neurons during ischemia and reperfusion, suppression of seizures, modulating neuroinflammation and limiting brain damage in neurodegenerative disorders. The application of PET imaging could lead to novel insights in these areas. Finally (iv), we discuss the application of PET in pharmacodynamic studies and we examine therapeutic applications of adenosine kinase inhibitors, e.g. in the treatment of pain, inflammation, and epilepsy.

UPTAKE OF RADIOLIGANDS BY RAT-HEART AND LUNG INVIVO - CGP-12177 DOES AND CGP-26505 DOES NOT REFLECT BINDING TO BETA-ADRENOCEPTORS

The biodistribution of (-)-4-(3-t-butylamino-2-hydroxypropoxy)-[5,7-3H-benzimidazol-2-one (CGP12177, a non-selective beta-adrenoceptor antagonist) and 1-[2-(3-carbamoyl-4-hydroxy)-(5-3H-phenoxy)]-2-propanol methanesulfonate, (CGP26505, a beta 1-adrenoceptor antagonist) was studied in rats pretreated with various alpha- and beta-adrenoceptor blocking drugs (5 min before 3H injection, in dosages at which the drugs demonstrated the expected selectivity). Cardiac and pulmonary radioactivity were measured after 10 min, when specific binding was maximal. Uptake of [3H]CGP12177 was linked to binding to beta-adrenoceptors since it was not affected by prazosin or yohimbine, and was equally well inhibited by propranolol, unlabelled CGP12177 and isoprenaline. Moreover, atenolol and CGP20712A inhibited [3H]CGP12177 uptake in heart (predominantly beta 1-adrenoceptors) more potently than ICI 118,551, while in lungs (predominantly beta 2-adrenoceptors) ICI 118,551 was more potent than atenolol or CGP20712A. In contrast, [3H]CGP26505 uptake in the target organs was equally effectively inhibited by propranolol and ICI 118,551, and significantly lowered by alpha-adrenoceptor antagonists. We conclude that [11C]CGP12177, but not [11C]CGP2605 will be suitable for positron emission tomography imaging of beta-adrenoceptors in animals.

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.

[18F]FHPG Positron Emission Tomography for Detection of Herpes Simplex Virus (HSV) in Experimental HSV Encephalitis

ABSTRACT Herpes simplex virus type 1 (HSV-1) is one of the most common causes of sporadic encephalitis. The initial clinical course of HSV encephalitis (HSE) is highly variable, and the infection may be rapidly fatal. For effective treatment with antiviral medication, an early diagnosis of HSE is crucial. Subtle brain infections with HSV may be causally related to neuropsychiatric disorders such as Alzheimers dementia. We investigated the feasibility of a noninvasive positron emission tomography (PET) imaging technique using [18F]FHPG as a tracer for the detection of HSE. For this purpose, rats received HSV-1 (infected group) or phosphate-buffered saline (control group) by intranasal application, and dynamic PET scans were acquired. In addition, the distribution of tracer accumulation in specific brain areas was studied with phosphor storage imaging. The PET images revealed that the overall brain uptake of [18F]FHPG was significantly higher for the infected group than for control animals. Phosphor storage images showed an enhanced accumulation of [18F]FHPG in regions known to be affected after intranasal infection with HSV. High-performance liquid chromatography metabolite analysis showed phosphorylated metabolites of [18F]FHPG in infected brains, proving that the increased [18F]FHPG uptake in infected brains was due to HSV thymidine kinase-mediated trapping. Freeze lesion experiments showed that damage to the blood-brain barrier could in principle induce elevated [18F]FHPG uptake, but this nonspecific tracer uptake could easily be discriminated from HSE-derived uptake by differences in the tracer kinetics. Our results show that [18F]FHPG PET is a promising tool for the detection of HSV encephalitis.

Measurement of myocardial β-adrenoceptor density in clinical studies: a role for positron emission tomography?

Abstract. The β-adrenoceptor (β-AR) plays an important role in the regulation of heart function and has been extensively studied in recent decades. In vitro studies have shown down-regulation of β-AR density in heart failure and cardiac conditions that may lead to heart failure. As in vitro measurements on cardiac tissue samples do not allow longitudinal and regional assessment of myocardial β-ARs in humans, new methods are being developed to measure β-ARs in vivo using positron emission tomography (PET). Studies using PET and the radioligand [11C]CGP 12177 have shown promising results that are in agreement with those of in vitro studies. However, the radiochemical synthesis of [11C]CGP 12177 is very demanding, preventing its widespread use. Hence, new radioligands are being developed using simpler methods of radiochemical synthesis. (S)-[11C]CGP 12388 has been presented as a promising new radioligand. So far, in vivo measurements of β-AR density using PET have mainly been performed to confirm in vitro studies. Using the full potential of PET, performance of regional measurements and longitudinal studies might add further knowledge on the pathophysiological role of the β-AR in cardiac disease and the effect of interventions. Furthermore, PET might gain a role in the clinical management of patients with abnormalities of cardiac contractile function.

Synthesis and organ distribution of [18F]fluoro-org 6141 in the rat: A potential glucocorticoid receptor ligand for positron emission tomography

For the synthesis of [18F]Fluoro-Org 6141 via a nucleophilic substitution reaction with 18F-, the tosyl group was chosen as the leaving group because of its stability and excellent leaving group ability. The biodistribution of the high affinity and moderate lipophilicity (log P = 2.66, calculated value) ligand [18F]Fluoro-Org 6141 (specific activity 8.2 to 37 TBq/mmol, yield 10% at EOB) was examined in sham adrenalectomized (sADX) and adrenalectomized (ADX) male Wistar rats. Two days after ADX or sADX, the animals were anesthetized and 0.37 to 1.85 MBq of [18F]Fluoro-Org 6141 was administered intravenously. Kinetics of 18F activity uptake were monitored for 3 h using a stationary double-headed positron emission tomography (PET) camera, and the biodistribution was assessed by ex vivo determination of radioactivity in several tissues and different brain areas. One hour after injection of the radioligand, the bladder, kidney, liver, trachea, and bone of sADX animals contained more concentration on a wet weight basis than blood. Three hours post injection, radioactivity was retained in bladder, trachea, and bone. The accumulation of radioactivity in brain corresponded to the concentration of activity in the blood within the first hours after injection. ADX animals showed a higher uptake of 18F activity in spleen, testes, and brain areas (hippocampus and brainstem) but a lower uptake in bone than sADX rats. PET scans suggested that 18F activity uptake in the brain had not yet reached a maximum at this interval. Although [18F]Fluoro-Org 6141 is not useful for PET studies of glucocorticoid receptors (GRs), the results obtained with this compound indicate a synthetic strategy suitable for the synthesis of high-affinity radioligands for GRs.

Population pharmacokinetics of cutamesine in rats using NONMEM, C-11-SA4503, and microPET

C. Asferg, R. Møgelvang, A. Flyvbjerg, A. Frystyk, J.S. Jensen, J.L. Marott, M. Appleyard, P. Schnoh, G.B. Jensen, J. Jeppesen. Copenhagen University Hospital Glostrup, Department of Clinical Physiology and Nuclear Medicine, Glostrup, Denmark, Copenhagen University Hospital Gentofte, Department of Cardiology, Gentofte, Denmark, Aarhus University Hospital Aarhus, Medical Research Laboratories, Clinical Institute and Medical Department M, Aarhus, Denmark, Copenhagen University Hospital Bispebjerg, The Copenhagen City Heart Study, Bispebjerg, Denmark, Copenhagen University Hospital Hvidovre, Department of Cardiology, Hvidovre, Denmark, Copenhagen University Hospital Glostrup, Department of Medicine, Glostrup, Denmark, Faculty of Health Sciences, University of Aarhus, Aarhus, Denmark, Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark

Development of an inexpensive, low attenuation styrofoam primate chair for use in a PET scanner

Abstract:  Pharmacokinetic modelling of radiotracers for positron emission tomography (PET) imaging of neuroreceptors can be performed with time‐activity data for brain and blood. We aimed to develop an alternative to withdrawal of arterial blood samples for acquisition of a blood curve. A supportive primate chair was constructed out of styrofoam and fixed to the head portion of the bed of a PET scanner. A lightly anaesthetised rhesus monkey was positioned in the chair in a sitting position and injected with the radiotracer. The styrofoam chair provided sufficient support for the monkey. The presence of the chair in the PET scanner caused negligible attenuation of radiation, allowing simultaneous acquisition of dynamic data from the subjects brain and heart. We conclude that a styrofoam primate chair is an ideal tool to measure blood and brain data from a rhesus monkey with PET. Invasiveness to the animal is reduced, as well as experimenter time.