Preparation and pharmacological characterization of [76Br]-meta-bromobenzylguanidine ([76Br]MBBG)

Expanding PET-applications in life sciences with positron-emitters beyond fluorine-18

Positron-emission-tomography (PET) has become an indispensable diagnostic tool in modern nuclear medicine. Its outstanding molecular imaging features allow repetitive studies on one individual and with high sensitivity, though no interference. Rather few positron-emitters with near favourable physical properties, i.e. carbon-11 and fluorine-18, furnished most studies in the beginning, preferably if covalently bound as isotopic label of small molecules. With the advancement of PET-devices the scope of in vivo research in life sciences and especially that of medical applications expanded, and other than "standard" PET-nuclides received increasing significance, like the radiometals copper-64 and gallium-68. Especially during the last decades, positron-emitters of other chemical elements have gotten into the focus of interest, concomitant with the technical advancements in imaging and radionuclide production. With known nuclear imaging properties and main production methods of emerging positron-emitters their usefulness for medical application is promising and even proven for several ones already. Unfortunate decay properties could be corrected for, and β+-emitters, especially with a longer half-life, provided new possibilities for application where slower processes are of importance. Further on, (bio)chemical features of positron-emitters of other elements, among there many metals, not only expanded the field of classical clinical investigations, but also opened up new fields of application. Appropriately labelled peptides, proteins and nanoparticles lend itself as newer probes for PET-imaging, e.g. in theragnostic or PET/MR hybrid imaging. Furthermore, the potential of non-destructive in-vivo imaging with positron-emission-tomography directs the view on further areas of life sciences. Thus, exploiting the excellent methodology for basic research on molecular biochemical functions and processes is increasingly encouraged as well in areas outside of health, such as plant and environmental sciences.

European Association of Nuclear Medicine Practice Guideline/Society of Nuclear Medicine and Molecular Imaging Procedure Standard 2019 for radionuclide imaging of phaeochromocytoma and paraganglioma

PURPOSE

Diverse radionuclide imaging techniques are available for the diagnosis, staging, and follow-up of phaeochromocytoma and paraganglioma (PPGL). Beyond their ability to detect and localise the disease, these imaging approaches variably characterise these tumours at the cellular and molecular levels and can guide therapy. Here we present updated guidelines jointly approved by the EANM and SNMMI for assisting nuclear medicine practitioners in not only the selection and performance of currently available single-photon emission computed tomography and positron emission tomography procedures, but also the interpretation and reporting of the results.

METHODS

Guidelines from related fields and relevant literature have been considered in consultation with leading experts involved in the management of PPGL. The provided information should be applied according to local laws and regulations as well as the availability of various radiopharmaceuticals.

CONCLUSION

Since the European Association of Nuclear Medicine 2012 guidelines, the excellent results obtained with gallium-68 (68Ga)-labelled somatostatin analogues (SSAs) in recent years have simplified the imaging approach for PPGL patients that can also be used for selecting patients for peptide receptor radionuclide therapy as a potential alternative or complement to the traditional theranostic approach with iodine-123 (123I)/iodine-131 (131I)-labelled meta-iodobenzylguanidine. Genomic characterisation of subgroups with differing risk of lesion development and subsequent metastatic spread is refining the use of molecular imaging in the personalised approach to hereditary PPGL patients for detection, staging, and follow-up surveillance.

Radiobromine and radioiodine for medical applications

The halogens bromine and iodine have similar chemical properties and undergo similar reactions due to their closeness in Group 17 of the periodic chart. There are a number of bromine and iodine radionuclides that have properties useful for diagnosis and therapy of human diseases. The emission properties of radiobromine and radioiodine nuclides with half-lives longer than 1 h are summarized along with properties that make radionuclides useful in PET/SPECT imaging and β/Auger therapy, such that the reader can assess which of the radionuclides might be useful for medical applications. An overview of chemical approaches that have been used to radiolabel molecules with radiobromine and radioiodine nuclides is provided with examples. Further, references to a large variety of different organ/cancer-targeting agents utilizing the radiolabeling approaches described are provided.

Feasibility of myocardial PET imaging using a benzylguanidine analog: meta-(3-[18F]fluoropropyl)benzylguanidine ([18F]mFPBG)

INTRODUCTION

Global and regional sympathetic activity in the heart can be evaluated using [¹²³I]meta-iodobenzylguanidine ([¹²³I]mIBG) imaging. However, [¹²³I]mIBG is associated with low image spatial resolution and sensitivity in cardiac imaging. We investigated the capability of an F-18-labeled mIBG derivative, meta-(3-[¹⁸F]fluoropropyl)benzylguanidine ([¹⁸F]mFPBG), for identifying ischemic and viable myocardium in a rat model of myocardial infarction.

MATERIALS AND METHODS

The ex vivo biodistribution and in vivo metabolic stability of [¹⁸F]mFPBG were investigated in Sprague-Dawley rats. Selective cardiac adrenergic activation was confirmed via a blocking experiment involving pretreatment with desipramine (2 mg kg^-1), followed by the administration of [¹⁸F]mFPBG. Imaging properties of [¹⁸F]mFPBG were compared with those of traditional cardiac imaging radiotracers ([¹²³I]mIBG and [^99mTc]MIBI) in a rat model of myocardial infarction. Non-invasive image-based measurements of infarct sizes were then compared with histological findings by using Bland-Altman analysis.

RESULTS

The differences in infarct sizes determined using histological analysis and [¹⁸F]mFPBG PET were -2.55 ± 4.99% (range: -12.33 to 7.22), -2.35 ± 3.32% (range: -8.87 to 4.16), and -3.15 ± 6.16% (range: -15.24 to 8.93) at 5, 20, and 40 min, respectively. Furthermore, [¹⁸F]mFPBG PET was superior to traditional imaging methods in assessing the degree of ischemia in areas of myocardial infarction, as well as the actual infarct size.

CONCLUSION

Compared to [¹²³I]mIBG, [¹⁸F]mFPBG showed improved spatial resolution and sensitivity in a rat model of myocardial infarction. This result suggested that [¹⁸F]mFPBG is a promising cardiac PET imaging agent for potential diagnostic application in PET cardiology.

Synthesis and bioevaluation of [(18)F]4-fluoro-m-hydroxyphenethylguanidine ([(18)F]4F-MHPG): a novel radiotracer for quantitative PET studies of cardiac sympathetic innervation

A new cardiac sympathetic nerve imaging agent, [(18)F]4-fluoro-m-hydroxyphenethylguanidine ([(18)F]4F-MHPG), was synthesized and evaluated. The radiosynthetic intermediate [(18)F]4-fluoro-m-tyramine ([(18)F]4F-MTA) was prepared and then sequentially reacted with cyanogen bromide and NH4Br/NH4OH to afford [(18)F]4F-MHPG. Initial bioevaluations of [(18)F]4F-MHPG (biodistribution studies in rats and kinetic studies in the isolated rat heart) were similar to results previously reported for the carbon-11 labeled analog [(11)C]4F-MHPG. The neuronal uptake rate of [(18)F]4F-MHPG into the isolated rat heart was 0.68ml/min/g wet and its retention time in sympathetic neurons was very long (T1/2 >13h). A PET imaging study in a nonhuman primate with [(18)F]4F-MHPG provided high quality images of the heart, with heart-to-blood ratios at 80-90min after injection of 5-to-1. These initial kinetic and imaging studies of [(18)F]4F-MHPG suggest that this radiotracer may allow for more accurate quantification of regional cardiac sympathetic nerve density than is currently possible with existing neuronal imaging agents.

EANM 2012 guidelines for radionuclide imaging of phaeochromocytoma and paraganglioma

PURPOSE

Radionuclide imaging of phaeochromocytomas (PCCs) and paragangliomas (PGLs) involves various functional imaging techniques and approaches for accurate diagnosis, staging and tumour characterization. The purpose of the present guidelines is to assist nuclear medicine practitioners in performing, interpreting and reporting the results of the currently available SPECT and PET imaging approaches. These guidelines are intended to present information specifically adapted to European practice.

METHODS

Guidelines from related fields, issued by the European Association of Nuclear Medicine and the Society of Nuclear Medicine, were taken into consideration and are partially integrated within this text. The same was applied to the relevant literature, and the final result was discussed with leading experts involved in the management of patients with PCC/PGL. The information provided should be viewed in the context of local conditions, laws and regulations.

CONCLUSION

Although several radionuclide imaging modalities are considered herein, considerable focus is given to PET imaging which offers high sensitivity targeted molecular imaging approaches.

New Methods for Labeling RGD Peptides with Bromine-76

Direct bromination of the tyrosine residues of peptides and antibodies with bromine-76, to create probes for PET imaging, has been reported. For peptides that do not contain tyrosine residues, however, a prosthetic group is required to achieve labeling via conjugation to other functional groups such as terminal α-amines or lysine ε-amines. The goal of this study was to develop new strategies for labeling small peptides with Br-76 using either a direct labeling method or a prosthetic group, depending on the available functional group on the peptides. A new labeling agent, N-succinimidyl-3-[(76)Br]bromo-2,6-dimethoxybenzoate ([(76)Br]SBDMB) was prepared for cyclic RGD peptide labeling. N-succinimidyl-2, 6-dimethoxybenzoate was also used to pre-attach a 2, 6-dimethoxybenzoyl (DMB) moiety to the peptide, which could then be labeled with Br-76. A competitive cell binding assay was performed to determine the binding affinity of the brominated peptides. PET imaging of U87MG human glioblastoma xenografted mice was performed using [(76)Br]-BrE[c(RGDyK)](2) and [(76)Br]-BrDMB-E[c(RGDyK)](2). An ex vivo biodistribution assay was performed to confirm PET quantification. The mechanisms of bromination reaction between DMB-c(RGDyK) and the brominating agent CH(3)COOBr were investigated with the SCRF-B3LYP/6-31G* method with the Gaussian 09 program package. The yield for direct labeling of c(RGDyK) and E[c(RGDyK)](2) using chloramine-T and peracetic acid at ambient temperature was greater than 50%. The yield for [(76)Br]SBDMB was over 60% using peracetic acid. The conjugation yields for labeling c(RGDfK) and c(RGDyK) were over 70% using the prosthetic group at room temperature. Labeling yield for pre-conjugated peptides was over 60%. SDMB conjugation and bromination did not affect the binding affinity of the peptides with integrin receptors. Both [(76)Br]Br-E[c(RGDyK)](2) and [(76)Br]BrDMB-E[c(RGDyK)](2) showed high tumor uptake in U87MG tumor bearing mice. The specificity of the imaging tracers was confirmed by decreased tumor uptake after co-administration of unlabeled dimeric RGD peptides. The energy barrier of the transition state of bromination for the dimethoxybenzoyl group was about 9 kcal/mol lower than that for the tyrosine residue. In conclusion, the newly developed N-succinimidyl-2, 6-dimethoxybenzoate molecule can be used either for one step labeling through pre-conjugation or as the precursor for a Br-76 labeled prosthetic group for indirect labeling. Radiobromination on a dimethoxybenzoyl group has selectivity over radiobromination on tyrosine. The energy barrier difference of the transition states of bromination between the dimethoxybenzoyl group and the tyrosine residue may account for the reaction selectivity when both groups are present in the same molecule.

Modern PET imaging for paragangliomas: relation to genetic mutations

AIM AND METHODS

Paragangliomas (PGL) are neural crest-derived tumours that are found along the autonomic neural network throughout the body and can be multiple and/or metastatic. Nuclear medicine imaging in combination with conventional imaging is required to fully delineate the extent of the disease. The performance of molecular imaging modalities is widely dependent on tumour biology.

RESULTS

In the present paper we discuss the recent publications focused on the role of positron emission tomography (PET) imaging and the relationship between tracer uptake patterns and genetic mutations associated with the disease.

CONCLUSION

Recent advances in genetic and molecular pathogenesis of PGL have allowed for the identification of new molecular diagnostic and therapeutic radiopharmaceuticals tailored to genetic abnormalities. However, the optimal diagnostic imaging algorithm remains to be determined.

PET imaging of norepinephrine transporter-expressing tumors using 76Br-meta-bromobenzylguanidine

Meta-iodobenzylguanidine (MIBG) labeled with (123)I or (131)I has been widely used for the diagnosis and radiotherapy of norepinephrine transporter (NET)-expressing tumors. However, (123)I/(131)I-MIBG has limitations for detecting small lesions because of its lower spatial resolution than PET tracers. In this study, meta-bromobenzylguanidine (MBBG) labeled with (76)Br (half-life, 16.1 h), an attractive positron emitter, was prepared and evaluated as a potential PET tracer for imaging NET-expressing tumors.

METHODS

(76)Br-MBBG was prepared by a halogen-exchange reaction between the (76)Br and iodine of nonradioactive MIBG. The stability of MBBG was evaluated in vitro and in vivo by high-performance liquid chromatography analysis. Cellular uptake studies with or without NET inhibitors were performed in NET-positive PC-12 cell lines. Biodistribution studies were performed in PC-12 tumor-bearing nude mice by administration of a mixed solution of MBBG, MIBG, and (18)F-FDG. The tumor was imaged using (76)Br-MBBG and (18)F-FDG with a small-animal PET scanner.

RESULTS

MBBG was stable in vitro, but some time-dependent dehalogenation was observed after administration in mice. MBBG showed high uptake in PC-12 tumor cells that was significantly decreased by the addition of NET inhibitors. In biodistribution studies, MBBG showed high tumor accumulation (32.0 +/- 18.6 percentage injected dose per gram at 3 h after administration), and the tumor-to-blood ratio reached as high as 54.4 +/- 31.9 at 3 h after administration. The tumor uptake of MBBG correlated well with that of MIBG (r = 0.997) but not with that of (18)F-FDG. (76)Br-MBBG PET showed a clear image of the transplanted tumor, with high sensitivity, which was different from the lesion shown by (18)F-FDG PET.

CONCLUSION

(76)Br-MBBG showed high tumor accumulation, which correlated well with that of MIBG, and provided a clear PET image. These results indicated that (76)Br-MBBG would be a potential PET tracer for imaging NET-expressing neuroendocrine tumors and could provide useful information for determining the indications for (131)I-MIBG therapy.

Semi-remote production of Br-76 and preparation of high specific activity radiobrominated pharmaceuticals for PET studies

The PET radionuclide 76Br (t1/2=16.2 h) can be easily produced utilizing the nuclear reaction As(3He,2n)76Br. We use high-purity arsenic targets and isolate radioactive bromide by chromic acid oxidation followed by simple distillation of [76Br] hydrogen bromide using a semi-remote apparatus. Use of reagents with little or no carrier bromine yields high specific activity radiobrominated pharmaceuticals prepared from the distilled [76Br] hydrogen bromide.

Radioimmunoimaging with longer-lived positron-emitting radionuclides: potentials and challenges

Radioimmunoimaging and therapy has been an area of interest for several decades. Steady progress has been made toward clinical translation of radiolabeled monoclonal antibodies for diagnosis and treatment of diseases. Tremendous advances have been made in imaging technologies such as positron emission tomography (PET). However, these advances have so far eluded routine translation into clinical radioimmunoimaging applications due to the mismatch between the short half-lives of routinely used positron-emitting radionuclides such as (18)F versus the pharmacokinetics of most intact monoclonal antibodies of interest. The lack of suitable positron-emitting radionuclides that match the pharmacokinetics of intact antibodies has generated interest in exploring the use of longer-lived positron emitters that are more suitable for radioimmunoimaging and dosimetry applications with intact monoclonal antibodies. In this review, we examine the opportunities and challenges of radioimmunoimaging with select longer-lived positron-emitting radionuclides such as (124)I, (89)Zr, and (86)Y with respect to radionuclide production, ease of radiolabeling intact antibodies, imaging characteristics, radiation dosimetry, and clinical translation potential.

Radiolabeled phenethylguanidines: novel imaging agents for cardiac sympathetic neurons and adrenergic tumors

The norepinephrine transporter (NET) substrates [123I]-m-iodobenzylguanidine (MIBG) and [11C]-m-hydroxyephedrine (HED) are used as markers of cardiac sympathetic neurons and adrenergic tumors (pheochromocytoma, neuroblastoma). However, their rapid NET transport rates limit their ability to provide accurate measurements of cardiac nerve density. [11C]Phenethylguanidine ([11C]1a) and 12 analogues ([11C]1b-m) were synthesized and evaluated as radiotracers with improved kinetics for quantifying cardiac nerve density. In isolated rat hearts, neuronal uptake rates of [11C]1a-m ranged from 0.24 to 1.96 mL min-1 (g wet wt)-1, and six compounds had extremely long neuronal retention times (clearance T1/2 > 20 h) due to efficient vesicular storage. Positron emission tomography (PET) studies in nonhuman primates with [11C]1e, N-[11C]guanyl-m-octopamine, which has a slow NET transport rate, showed improved myocardial kinetics compared to HED. Compound [11C]1c, [11C]-p-hydroxyphenethylguanidine, which has a rapid NET transport rate, avidly accumulated into rat pheochromocytoma xenograft tumors in mice. These encouraging findings demonstrate that radiolabeled phenethylguanidines deserve further investigation as radiotracers of cardiac sympathetic innervation and adrenergic tumors.

A kit method for the high level synthesis of [211At]MABG

meta-[(211)At]Astatobenzylguanidine ([(211)At]MABG), an analogue of meta-iodobenzylguanidine (MIBG) labeled with the alpha-emitter (211)At, targets the norepinephrine transporter. Because MABG has been shown to have excellent characteristics in preclinical studies, it has been considered to be a promising targeted radiotherapeutic for the treatment of tumors such as micrometastatic neuroblastoma that overexpress the norepinephrine transporter. To facilitate clinical evaluation of this agent, a convenient method for the high level synthesis of [(211)At]MABG that is adaptable for kit formulation has been developed. A tin precursor anchored to a solid-support was treated with a methanolic solution of (211)At in the presence of a mixture of H(2)O(2)/HOAc as the oxidant; [(211)At]MABG was isolated by simple solid-phase extraction. By using C-18 solid-phase extraction, the radiochemical yield from 25 batches was 63+/-13%; however, loss of radioactivity during evaporation of the methanolic solution was a problem. This difficulty was avoided by use of a cation exchange resin cartridge for isolation of [(211)At]MABG, which resulted in radiochemical yields of 63+/-9% in a shorter duration of synthesis. The radiochemical purity was more than 90% and no chemical impurity has been detected. The final doses were sterile and apyrogenic. These results demonstrate that [(211)At]MABG can be prepared via a kit method at radioactivity levels anticipated for initiation of clinical studies.

Synthesis of copper octabromotetracarboranylphenylporphyrin for boron neutron capture therapy and its toxicity and biodistribution in tumour-bearing mice

Copper tetracarboranyltetraphenylporphyrin (CuTCPH) is a minimally toxic carborane-containing porphyrin that has safely delivered high concentrations of boron for experimental boron neutron capture therapy (BNCT). Copper octabromotetracarboranylphenylporphyrin (CuTCPBr), synthesized by bromination of CuTCPH, is one of several new minimally toxic analogues of CuTCPH being studied in our laboratory, which could possess comparable or better tumour-targeting properties with enhanced tumour cytotoxicity. Its biodistribution, biokinetics and toxicity in mice with subcutaneous EMT-6 (mammary) or SCCVII (squamous cell) carcinomas were compared with those of CuTCPH. The administration of approximately 200 mg kg(-1) of either porphyrin in six intraperitoneal injections over 2 days had no apparent effect, but administration of approximately 400 mg kg(-1) slightly lowered body weights, elevated alanine and aspartate transaminase activities in blood plasma, and depressed blood platelet counts for several days. Enzymes and platelets returned to normal within 5 days after those injections and body weights returned to normal within 2 weeks. High average concentrations of boron from either porphyrin were achieved in the two tumour models from a total dose of approximately 200 mg kg(-1). The high tumour boron concentration decreased slowly while concentrations in blood decreased rapidly. Boron concentrations in brain and skin were consistently lower than in tumour by a factor of 10 or more. Although either CuTCPH or CuTCPBr can be labelled with (64)Cu for imaging by positron emission tomography (PET), CuTCPBr can also be labelled by (76)Br, another PET-imageable nuclide.

Preparation of a bromine-76 labelled analogue of epibatidine: a potent ligand for nicotinic acetylcholine receptor studies

Epibatidine analogues have been labelled with I-123 for single photon emission computed tomography and with short half-life positron emitters (C-11 and F-18) for PET. For easier radiopharmacological studies the bromo analogue of epibatidine (norchlorobromoepibatidine or exo-7-azabicyclo-2-(2-bromo-5-pyridyl)-[2.2.1]heptane) was labelled with Br-76, a longer half-life positron emitter, (T1/2 = 16.2h). [76Br]-norchlorobromoepibatidine was prepared by using a Cu+ assisted bromodeiodination exchange from the iodo analogue in reducing conditions at 190 degrees C. The tracer purified by RP-HPLC was obtained in 70% radiochemical yield with a specific radioactivity of 20 GBq/micromol. Radiochemical and chemical purities measured by radio-TLC and HPLC were >98%.

Assessment of cardiac sympathetic nerve integrity with positron emission tomography

The autonomic nervous system plays a critical role in the regulation of cardiac function. Abnormalities of cardiac innervation have been implicated in the pathophysiology of many heart diseases, including sudden cardiac death and congestive heart failure. In an effort to provide clinicians with the ability to regionally map cardiac innervation, several radiotracers for imaging cardiac sympathetic neurons have been developed. This paper reviews the development of neuronal imaging agents and discusses their emerging role in the noninvasive assessment of cardiac sympathetic innervation.

General method to label antisense oligonucleotides with radioactive halogens for pharmacological and imaging studies

Evaluation of oligonucleotides for biomedical applications requires different in vivo and in vitro approaches (pharmacokinetics, biodistribution, macro- and microimaging, metabolism,.), that are performed with different radioisotopes according to the temporal and spatial resolution needed. A method to introduce radioactive isotopes of halogens (fluorine, bromine, and iodine) in a small and stable molecule has been developed. Radiosynthons can then be conjugated with any given oligonucleotide in one step to create the appropriate radiotracer. This general radiolabeling procedure for oligonucleotides is efficient to synthesize (18)F-, (76)Br-, and (125)I-oligonucleotides for biological needs. Applications of the method to biodistribution, metabolism, in vivo and ex vivo imaging of (125)I- and (18)F-labeled oligonucleotides are reported.

Characterization of bromine-76-labelled 5-bromo-6-nitroquipazine for PET studies of the serotonin transporter

The development of suitable radioligands for brain imaging of the serotonin transporter is of great importance for the study of depression and other affective disorders. The potent and selective serotonin transporter ligand, 5-iodo-6-nitro-2-piperazinylquinoline, has been labelled with iodine-123 and used as a radioligand for single photon emission computerized tomography. To evaluate the potential of the bromine-76-labelled analogue, 5-bromo-6-nitroquipazine, as a radioligand for positron emission tomography (PET), its brain distribution and binding characteristics were examined in rats. In vivo brain distribution and ex vivo autoradiography demonstrated that [76Br]5-bromo-6-nitroquipazine enters the brain rapidly. The regional brain distribution of [76Br]5-bromo-6-nitroquipazine was consistent with the known distribution of serotonin transporters in the midbrain, pons, thalamus, striatum, and neocortex. Specific binding was inhibited by the selective serotonin reuptake inhibitor citalopram. The peripheral metabolism in plasma was rapid, but more than 90% of the radioactivity in brain represented unchanged radioligand 2 h postinjection (p.i.). A preliminary PET study was also performed in a baboon. Following the intravenous injection of [76Br]5-bromo-6-nitroquipazine in a baboon, there was a conspicuous accumulation of radioactivity in thalamus, striatum, and pons. The radioactivity in these brain regions was 1.5 times higher than in the cerebellum at 3 h and 2.5-4 times higher at 24 h. A rapid metabolism of the radioligand in plasma was observed (38% unchanged after 5 min). The results indicate that [76Br]5-bromo-6-nitroquipazine has potential for PET imaging of the serotonin transporter.

Transcapillary transport of metaiodobenzylguanidine (MIBG) in isolated rat heart

A better understanding of transcapillary transport for tracer metaiodobenzylguanidine (MIBG) is desirable for development of tracer kinetic models that yield meaningful estimates of neuronal uptake function from tissue radioactivity time courses. This study utilized a multiple-indicator approach in Langendorff-perfused rat hearts to define transport mechanisms and determine the capillary permeability-surface area (PSc) over a broad range of flow (F). Multiple injections within the same heart at different flows allowed characterization of the PSc/F relationship within the same heart. The coefficient of variation of E for multiple injections within the same hearts at constant flow was 6 +/- 2% (3 to 6 injections in 9 hearts). In 10 hearts (4 to 6 injections per heart), flow was varied between 2.0-16.5 mL/min. PSc was found to be nearly proportional to flow in each heart (r = 0.88 +/- 0.14; slope = 0.23 +/- 0.10; intercept = 11 +/- 7 mL/min/g dry). Tissue hypoxia at low flows, as evidenced by enhanced lactate production, did not appear to influence the PSc/F relationship. Pharmacologic blockade of uptake-1 and uptake-2 had negligible affect on E or PSc as compared with flow-matched controls, although tissue retention was markedly reduced. The results show PSc of MIBG to be nearly proportional to flow but independent of specific neuronal and extraneuronal transport mechanisms and tissue hypoxia. The results are consistent with a passive diffusion process across the capillary endothelial barrier. The increase in PSc with increasing flow could reflect capillary recruitment and/or enhanced capillary permeability.

Kinetics of the norepinephrine analog [76Br]-meta-bromobenzylguanidine in isolated working rat heart

A related set of kinetic studies of the norepinephrine analog [76Br]-meta-bromobenzylguanidine (MBBG) were performed with an isolated working rat heart preparation. A series of constant infusion studies over a wide range of MBBG concentrations allowed estimation of the Michaelis-Menten constants for transport by the neuronal norepinephrine transporter (uptake1) and the extraneuronal uptake system (uptake2). Pharmacological blocking studies with inhibitors of uptake1, uptake2 and vesicular uptake were performed to delineate the relative importance of these norepinephrine handling mechanisms on the kinetics of MBBG in the rat heart. Bolus injection studies were done to assess the ability of compartmental modeling techniques to characterize the kinetics of MBBG. These studies demonstrate that MBBG shares many of the same uptake mechanisms as norepinephrine in the rat heart. PET imaging studies with MBBG would be useful for assessing sympathetic nerve status in the living human heart.

11C- and 76Br-labelled NNC 22-0010, selective dopamine D1 receptor radioligands for PET

NNC 22-0010 is a new dopamine antagonist with a high affinity and selectivity for D1 receptors in vitro. NNC 22-0010 has both an N-methyl group and a bromine, which allows radiolabelling with either 11C or 76Br. We labelled [11C]NNC 22-0010 by N-methylation of the free base of the secondary amine with [11C]methyl iodide in a total radiochemical yield of 40%. The total synthesis time was 30 min. The specific radioactivity at time of injection of the radioligand was 48 to 55 GBq/mumol. The [76Br]NNC 22-0010 was synthesized from the iodine precursor by an exchange reaction with 76Br using a Cu(+)-assisted nucleophilic substitution reaction. The radiochemical yield was 60% after purification. Specific radioactivity at time of injection of the radioligand was 6 to 20 GBq/mumol. In PET experiments with [11C]NNC 22-0010 and [76Br]NNC 22-0010 there was a rapid uptake of radioactivity in the monkey brain. The striatum-to-cerebellum ratio was 2-2.5 after 1 h. Binding in the striatum was displaced by SCH 23390, whereas binding in the cerebellum was not reduced. Metabolite studies showed that 1 h after injection about 20% of the radioactivity in plasma represented unchanged radioligand. This value was on the same level for at least 6 h. The results indicate that radiolabelled NNC 22-0010 has potential for imaging dopamine D1 receptors selectively in the human brain.

Pharmacological characterization and positron emission tomography evaluation of 4-[76Br]bromodexetimide and 4-[76Br]bromolevetimide for investigations of central muscarinic cholinergic receptors

4-[76Br]bromodexetimide and its inactive enantiomer 4-[76Br]bromolevetimide were prepared via electrophilic bromodesilylation using chloramine-T and no-carrier-added (NCA) [76Br]NH4. In vitro, Bmax measured on rat cortex membranes were 3.7 +/- 0.2 and < 0.07 pmol/mg protein for 4-[76Br]bromodexetimide and 4-[76Br]bromolevetimide, respectively. The kD of 4-[76Br]bromodexetimide was 1.9 +/- 0.3 nM. In vivo studies in rats showed specific uptake of 4-[76Br]bromodexetimide in cortex, striatum, thalamus and hippocampus. No specific uptake was observed with 4-[76Br]bromolevetimide. With [76Br]bromodexetimide, positron emission tomography (PET) studies in primates demonstrated a preferential accumulation of the radioactivity in the cortex and striatum which was displaced to the level of cerebellum by dexetimide. With 4-[76Br]bromolevetimide, the radioactivity concentrations in the cortex and striatum were similar to that of cerebellum.

No-carrier-added (4-fluoro-3-[131I]iodobenzyl)guanidine and (3-[211At]astato-4-fluorobenzyl)guanidine

With 3-bromo-4-fluorotoluene as starting material, [4-fluoro-3-(trimethylsilyl)benzyl]guanidine was prepared in five steps in 1.5% overall yield. Radioiodination of this silicon precursor using N-chlorosuccinimide in trifluoroacetic acid at room temperature for 5 min gave (4-fluoro-3-[131I]-iodobenzyl)guanidine ([131I]FIBG) in 50-60% radiochemical yield. A byproduct which had a retention time in two HPLC systems similar to that of (m-iodobenzyl)guanidine (MIBG) was formed in about 30% yield. [131I]FIBG was stable up to 3 h under these conditions of iodination, indicating that the byproduct is not generated as a result of [131I]FIBG degradation. Using hydrogen peroxide as the oxidant in aqueous medium and a reaction time of 30 min at 50 degrees C, yields of [131I]FIBG could be increased to 75-80%, with less than 7% of the byproduct formed under these conditions. Astatination of the silicon precursor using N-chlorosuccinimide in trifluoroacetic acid at 70 degrees C gave 65-70% radiochemical yield of (3-[211At]astato-4-fluorobenzyl)guanidine ([211At]AFBG) in 10-15 min; about 17% of the byproduct formation was seen. Astatination of the silicon precursor under aqueous conditions using hydrogen peroxide was not successful.

Uptake mechanisms of meta-[123I]iodobenzylguanidine in isolated rat heart

In order to clarify the uptake and retention mechanisms of radioiodinated meta-iodobenzylguanidine (MIBG) in heart, the kinetics of no-carrier-added [123I]MIBG were studied in the isolated working rat heart in interaction with pharmacologic agents. The tracer was administered in the perfusate as a 10-min pulse, followed by a 90-min washout period. Kinetic analysis of the externally monitored time-activity curves of control hearts showed avid uptake (Ki = 4.4 +/- 0.7 mL/min/g), and monoexponential clearance (ko = 0.0056 +/- 0.0017 l/min), indicating a distribution volume (Vd = Ki/ko) of 834 +/- 214 mL/g. Blocking experiments (n = 41) were performed with neuronal uptake (uptake-1) inhibitor desipramine (DMI; 50-100 nM) and the extraneuronal uptake (uptake-2) inhibitor N-(9-fluorenyl)-N-methyl-beta-chloroethylamine (SKF550; 0.4-0.8 microM). Uptake rate was 27% reduced (P < 0.05) by 50 nM DMI but not significantly affected by 0.4 microM SKF550. Distribution volume was 88% reduced (P < 0.0005) by 50 nM DMI and 28% reduced (P < 0.05) by 0.4 microM SKF550. In DMI-blocked hearts, uptake rate was dramatically decreased (-80%, P < 0.0005) by SKF550 (0.4 microM), indicating uptake-2 transport contributed predominantly to the extraneuronal uptake of the tracer. The slow uptake rate seen with concomitant inhibition of uptake-1 and uptake-2 was further decreased by addition of unlabeled MIBG (1-10 microM) in a concentration-dependent manner, yet unaffected by addition of the vesicular uptake inhibitor Ro 4-1284 (1 microM). Thus, the uptake rate of [123I]MIBG is primarily dependent on uptake-1 and uptake-2 activity. Other possible mechanisms of uptake such as passive diffusion in association with intracellular binding are significant only in conditions where uptake-1 and uptake-2 mechanisms are largely inhibited.