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.

PET radioligands for dopamine receptors and re-uptake sites: chemistry and biochemistry

This report, based on the past experience of European centres, offers practical guidance on the chemistry and biochemistry of PET radioligands used for the in vivo imaging of dopamine receptors and re-uptake sites. It mainly summarizes methods for the preparation of D1 and D2 receptor ligands labelled with positron-emitting radioisotopes. Some of these ligands (11C-labelled SCH23390, raclopride and nomifensine, 18F-labelled butyrophenones, [76Br]bromolisuride), which have been found useful in PET clinical investigations, have been emphasized. This report is intended as an introduction and guideline for new PET-groups who want to start research in the dopaminergic neurotransmission imaging field.

Where have we got to with neuroreceptor mapping of the human brain?

In the past two decades, tritiated radioligand receptor binding, a tool commonly used to investigate the site of action of drugs in laboratory animals, has provided a vast body of information on neuropharmacology and neurobiology. Several neurological and psychiatric diseases have been related to neurotransmitter and receptor disorders. In order to study ligand interactions with receptors in vivo in humans, new tracers capable of carrying a gamma-emitting radionuclide to the receptor have been designed. Emission computerized tomography (ECT) techniques such as positron (PET) or single photon emission tomography (SPET) allow monitoring of the time-course of regional tissue concentration of these radiolabelled ligands. PET and SPET each have their inherent advantages and drawbacks. The cyclotron-based technology of PET is a demanding and expensive technique that, to date, is still mainly reserved for research purposes. It is hoped that once the scientific basis of a physiopathological study is established using PET, diagnostic information might be provided by the more readily available SPET technology. The purpose of this article is to review the current state of receptor-binding gamma-emitting radioligands and to present the clinical potential of these new kinds of radiopharmaceuticals in clinical investigation.

3-N-(2-[18F]-fluoroethyl)-spiperone: a novel ligand for cerebral dopamine receptor studies with pet

3-N-(2-[18F]-fluoroethyl)-spiperone [( 18F]-FESP) was synthesized at high specific activity by condensation with 2-[18F]-fluoroethyltosylate (35 TBq/mmol). In vivo binding studies in baboons by positron emission tomography exhibited regio-selective uptake in the striatum which was saturable with the cold ligand and prevented by pretreatment with (+)-butaclamol. The pharmacokinetic behaviour, i.e. the absolute uptake in tissue and the striatum-to-cerebellum ratio, was very similar to that of methylspiperone. Analysis of the radioactivity in mouse brain after administration of [18F]-FESP indicated a high in-vivo stability (greater than 90% after 210 min in the striatum). Comparative distribution studies of other N-fluoroalkylspiperones in mice suggest that FESP and the N-fluoropropyl analogue are the most potent D2 receptor ligands.

Radiobrominated spiroperidol for the study of dopamine D2 receptors

Some neuropsychiatric disorders have been suggested to be related to the CNS dopaminergic system. In order to probe this neurotransmitter system, radiobrominated spiroperidol, a potent dopamine (DA) D2-receptor antagonist, was developed. This review deals with the routine synthesis of [75, 76, or 77Br]-p-bromospiroperidol (BrSP), its validation as a radiopharmaceutical directed to DA D2 receptors, and the imaging studies which have been done using this radioligand with either single photon tomography (SPECT) or positron tomography (PET). [*Br]BrSP will be compared to other D2 ligands currently available. The present state and future direction of DA receptor research, in particular, the quantitative aspects, will be discussed.

No-carrier-added 3-(2'-[18F]fluoroethyl)spiperone, a new dopamine receptor-binding tracer for positron emission tomography

No-carrier-added (NCA)3-(2'-[18F]fluoroethyl)spiperone (5), a new dopamine receptor-binding radiopharmaceutical for positron emission tomography, was synthesized by two different methods. Alkylation of the amide nitrogen in spiperone by NCA [18F]fluorobromoethane in the presence of a strong base gave 5 (Method A). Experimental methods were also developed for the syntheses of functional 3-N-alkylderivatives of spiperone such as 3-(2'-bromoethyl)- or 3-(2'-methylsulfonyloxyethyl)spiperone (4a and 4b, respectively). These derivatives (4) reacted with NCA Ag18F, Cs18F or K18F/Kryptofix 222 in acetonitrile or DMSO to give 5 (Method B). Method B, using K18F/Kryptofix 222 in acetonitrile provided 5 in multimillicure amounts (30-40% isolated radiochemical yield) with a specific activity of 2-10/mumol (EOS) in less than 60 min. This one-step, one-pot synthesis is simple, and the high radiochemical yield of 5, as well as the 110 min half-life of 18F, permit multiple tomographic studies a day with one preparation. Tomographic results in monkey brain with 5 are consistent with the labeling of dopamine-D2 receptor systems.

In vivo quantitative imaging of dopamine receptors in human brain using positron emission tomography and [76Br]bromospiperone

The brain regional distribution and kinetics of [76Br]bromospiperone, a derivative of a neuroleptic (spiperone) labeled with the positron emitter bromine-76, were studied by time-of-flight tomography after i.v. injection in man. In a control subject the kinetic distribution study showed an accumulation of radioactivity which reached a maximum 3 h postinjection in the frontal cortex and cerebellum regions and 4-5 h postinjection in the basal ganglia. Thereafter the striatal activity remained essentially constant over a period of 25 h. In a group of 13 control subjects, the mean value for the striatum-to-cerebellum ratio, at 4.5 h postinjection, was 1.84 (S.D. 0.21). In two schizophrenics treated with high doses of haloperidol, this ratio was found to be only 1.22. These data indicate that radiolabeled bromospiperone is very suitable for human pharmacological or pathological investigations of the central dopaminergic system.

Effect of lipophilicity on the in vivo localization of radiolabelled spiperone analogues

A series of N-alkylated and para-brominated analogues of spiperone were synthesized to ascertain the effect of lipophilicity on the in vivo localization of neuroleptics. While the IC50 for D2 receptor binding was within the same order of magnitude for these compounds, the calculated octanol-water partition coefficients varied 300-fold. When the in vivo distribution data for 77Br-labelled compounds were compared with previous data for 18F- and 11C-labelled analogues it was seen that the highest cerebral uptake was for bromospiperone, but the optimum striatum-to-cerebellum and brain-to-blood concentrations were achieved by N-methyl spiperone. The implications of these results for radiopharmaceutical design or medicinal chemistry are discussed.

In vivo binding of spiroperidol and bromospiroperidol at low drug loadings

The binding of spiroperidol and bromospiroperidol, in vivo, was studied over a wide range of drug dosages. It was found that while spiroperidol and bromospiroperidol bind selectively in vivo to tissues known to be high in dopamine receptor binding sites, this specificity of binding does not persist at very low doses. Such anomalous binding behavior can have implications for the non-invasive imaging of these drugs.

76Br-bromospiroperidol: a new tool for quantitative in-vivo imaging of neuroleptic receptors

Bromine-76 labeled bromospiperone has been prepared with a very high specific activity (greater than 1 Ci/mumole). In-vivo studies in rat corroborated by PET studies in baboon have shown that the regional concentration of this radioligand parallels the morphologic distribution of dopamine receptors and that its binding in the striatum is saturable and displaceable. Tomographic images show a clear delineation of the striatal region 2.5 hours after administration of the radioligand.