Development of 123I-labelled NNC 13-8241 as a radioligand for SPECT visualization of benzodiazepine receptor binding

Discovery and labeling of high-affinity 3,4-diarylpyrazolines as candidate radioligands for in vivo imaging of cannabinoid subtype-1 (CB1) receptors

Imaging of cannabinoid subtype-1 (CB1) receptors in vivo with positron emission tomography (PET) is likely to be important for understanding their role in neuropsychiatric disorders and for drug development. Radioligands for imaging with PET are required for this purpose. We synthesized new ligands from a 3,4- diarylpyrazoline platform of which (-)-12a ((-)-3-(4-chlorophenyl)-N'-[(4-cyanophenyl)sulfonyl]-4-phenyl- 4,5-dihydro-1H-pyrazole-1-carboxamidine) was found to have high-affinity and selectivity for binding to CB1 receptors. (-)-12a and its lower affinity enantiomer ((+)-12a) were labeled with carbon-11 (t1/2 ) 20.4 min) using [11C]cyanide ion as labeling agent and evaluated as PET radioligands in cynomolgus monkeys. After injection of [11C](-)-12a, there was high uptake and retention of radioactivity across brain according to the rank order of CB1 receptor densities. The distomer, [11C](+)-12a, failed to give a sustained CB1 receptor-specific distribution. Polar radiometabolites of [11C](-)-12a appeared moderately slowly in plasma. Radioligand [11C](-)-12a is promising for the study of brain CB1 receptors and merits further investigation in human subjects.

2-Substitution of adenine nucleotide analogues containing a bicyclo[3.1.0]hexane ring system locked in a northern conformation: enhanced potency as P2Y1 receptor antagonists

Preference for the northern (N) ring conformation of the ribose moiety of adenine nucleotide 3',5'-bisphosphate antagonists of P2Y(1) receptors was established by using a ring-constrained methanocarba (a bicyclo[3.1.0]hexane) ring as a ribose substitute (Nandanan et al. J. Med. Chem. 2000, 43, 829-842). We have now combined the ring-constrained (N)-methanocarba modification with other functionalities at the 2-position of the adenine moiety. A new synthetic route to this series of bisphosphate derivatives was introduced, consisting of phosphorylation of the pseudoribose moiety prior to coupling with the adenine base. The activity of the newly synthesized analogues was determined by measuring antagonism of 2-methylthio-ADP-stimulated phospholipase C (PLC) activity in 1321N1 human astrocytoma cells expressing the recombinant human P2Y(1) receptor and by using the radiolabeled antagonist [(3)H]2-chloro-N(6)-methyl-(N)-methanocarba-2'-deoxyadenosine 3',5'-bisphosphate 5 in a newly developed binding assay in Sf9 cell membranes. Within the series of 2-halo analogues, the most potent molecule at the hP2Y(1) receptor was an (N)-methanocarba N(6)-methyl-2-iodo analogue 12, which displayed a K(i) value in competition for binding of [(3)H]5 of 0.79 nM and a K(B) value of 1.74 nM for inhibition of PLC. Thus, 12 is the most potent antagonist selective for the P2Y(1) receptor yet reported. The 2-iodo group was substituted with trimethyltin, thus providing a parallel synthetic route for the introduction of an iodo group in this high-affinity antagonist. The (N)-methanocarba-2-methylthio, 2-methylseleno, 2-hexyl, 2-(1-hexenyl), and 2-(1-hexynyl) analogues bound less well, exhibiting micromolar affinity at P2Y(1) receptors. An enzymatic method of synthesis of the 3',5'-bisphosphate from the corresponding 3'-monophosphate, suitable for the preparation of a radiophosphorylated analogue, was explored.

Pharmacological evaluation of (S)-8-[123I]iodobretazenil: a radioligand for in vivo studies of central benzodiazepine receptors

In vitro studies on cortical membranes indicated (S)-8-[(123)I]iodobretazenil bound saturably to a single population of binding sites (B(max) = 2.33 pmol/mg protein) with a dissociation constant K(d) = 1.9 nM. (R)-8-[(123)I]Iodobretazenil displayed only non-specific binding. In vivo biodistribution of (S)-8-[(123)I]iodobretazenil in rats indicated high accumulation in regions of high BZR density. Radioactivity was blocked by preadministration with iodobretazenil and flumazenil, while non-BZR drugs had no effect on the uptake of activity in any brain region. (S)-8-[(123)I]Iodobretazenil uptake was saturable in a dose dependent manner (ID(50) = 0.13 mg/kg) in all brain regions. With the (R)-enantiomer no specific uptake was observed. Metabolite studies at 1-3 h p.i. indicated that greater than 95% of activity extracted from brain tissue corresponded to unchanged radiotracer while that in plasma was over 70%. (S)-8-[(123)I]Iodobretazenil potently and selectively labels BZR in vivo and deserves further investigation as a possible SPECT radiotracer.

Could automated template based quantification of benzodiazepine receptors in brain single photon emission tomography with 123I NNC 13-8241 be used to demonstrate neuronal damage in traumatic brain injury?

Automated methods are required for the analysis of brain single photon emission tomography images. We applied an automated method to assess the benzodiazepine receptor distribution in the brain. Images of 19 patients with mild traumatic brain injury who had received I NNC 13-8241 were compared with a mean brain template accumulated from 18 healthy volunteers. To obtain more information, we calculated the neuronal benzodiazepine receptor binding in the brain by using pre-defined anatomical regions and a voxel-by-voxel technique. The group of patients with mild traumatic brain injury differed significantly (P =0.015) from the group of healthy volunteers in the distribution of benzodiazepine receptors. This methodological work suggests that a reference based template and a three-dimensional brain model help in regional analysis and quantification and could be useful in demonstrating permanent neuronal damage after head injury.

In vitro and in vivo characterisation of [3H]ANSTO-14 binding to the sigma 1 binding sites

N-(4-phenylbutyl)-3-hydroxy-4-azahexacyclo[5.4.1.0(2,6).0(3, 10).0(5,9) .0(8,11)]dodecane (ANSTO-14) showed the highest activity for the sigma 1 site (Ki = 9.4 nM) and 19-fold sigma 1/sigma 2 selectivity. The present study showed that [3H]ANSTO-14 binds to a single high-affinity site in guinea pig brain membranes with an equilibrium Ki of 8.0 +/- 0.3 nM, in good agreement with the kinetic studies (Kd = 13.3 +/- 5.4 nM, n = 4), and a Bmax of 3.199 +/- 105 fmol/mg protein (n = 4). The in vivo biodistribution of [3H]ANSTO-14 showed a high uptake in the diencephalon. Pretreatment of rats with sigma ligands including (+)-pentazocine (sigma 1), ANSTO-14 (sigma 1), and DTG (sigma 1 and sigma 2) did not significantly reduce radiotracer uptake in the brain, but did in the spleen. A labelled metabolite was found in the liver and brain. Due to its insensitivity to sigma ligands, the accumulation of [3H]ANSTO-14 in the brain indicates high nonspecific binding. Therefore, [3H]ANSTO-14 is a suitable ligand for labelling sigma 1 sites in vitro but is not suitable for brain imaging of sigma binding sites in vivo.

Iodine-123 labelled radiopharmaceuticals and single-photon emission tomography: a natural liaison

Most nuclear medicine departments possess one or more imaging apparatuses for single-photon emission tomography (SPET). Molecules of biological interest to assess metabolism and receptor function are often labelled with 123I, which allows proper SPET imaging. The various methods for radiolabelling are reviewed. As the biological integrity of these agents has been demonstrated for numerous radiopharmaceuticals, the purpose of this review is to summarize the efficacy in various fields of medicine, including the imaging of tumours, infection, myocardium and cerebrum.

Initial human studies with single-photon emission tomography using iodine-123 labelled 3-(5-cyclopropyl-1,2,4-oxadiazo-3-yl)- 7-iodo-5, 6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]-benzodiazepine (NNC 13-8241)

The iodine-123 labelled ligand 3-(5-cyclopropyl-1,2, 4-oxadiazo-3-yl)-7-iodo-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1, 5-a][1,4]-benzodiazepine ([123I]NNC 13-8241) was evaluated as a probe for in vivo imaging of benzodiazepine receptor sites in the human brain. Four healthy volunteers were imaged with a high-resolution single-photon emission tomography (SPET) scanner. The metabolism of [123I]NNC 13-8241 in plasma was slow. The total brain uptake was about 1.5-fold higher than that of [123I]iomazenil. The specific binding in the cortical areas was high and less intense in the thalamus. The most intense uptake was seen in the occipital cortex. The peak cortical uptake of [123I]NNC 13-8241 was observed 6-10 h after the injection of tracer. The radiation burden to the patient was moderate, being 2.5 middle dot10(-2 )mSv/MBq (effective dose equivalent). A slow metabolism together with favourable kinetics indicates that [123I]NNC 13-8241 is a specific and promising SPET ligand for imaging benzodiazepine receptor sites in the living human brain.