Andrew G. Horti

68Ga-Labeled Inhibitors of Prostate-Specific Membrane Antigen (PSMA) for Imaging Prostate Cancer

Gallium-68 is a generator-produced radionuclide for positron emission tomography (PET) that is being increasingly used for radiolabeling of tumor-targeting peptides. Compounds [(68)Ga]3 and [(68)Ga]6 are high-affinity urea-based inhibitors of the prostate-specific membrane antigen (PSMA) that were synthesized in decay-uncorrected yields ranging from 60% to 70% and radiochemical purities of more than 99%. Compound [(68)Ga]3 demonstrated 3.78 +/- 0.90% injected dose per gram of tissue (%ID/g) within PSMA+ PIP tumor at 30 min postinjection, while [(68)Ga]6 showed a 2 h PSMA+ PIP tumor uptake value of 3.29 +/- 0.77 %ID/g. Target (PSMA+ PIP) to nontarget (PSMA- flu) ratios were 4.6 and 18.3, respectively, at those time points. Both compounds delineated tumor clearly by small animal PET. The urea series of imaging agents for PSMA can be radiolabeled with (68)Ga, a cyclotron-free isotope useful for clinical PET studies, with maintenance of target specificity.

Quantification of cerebral cannabinoid receptors subtype 1 (CB1) in healthy subjects and schizophrenia by the novel PET radioligand [11C]OMAR

Several studies have examined the link between the cannabinoid CB1 receptor and several neuropsychiatric illnesses, including schizophrenia. As such, there is a need for in vivo imaging tracers so that the relationship between CB1 and schizophrenia (SZ) can be further studied. In this paper, we present our first human studies in both healthy control patients and patients with schizophrenia using the novel PET tracer, [(11)C]OMAR (JHU75528), we have shown its utility as a tracer for imaging human CB1 receptors and to investigate normal aging and the differences in the cannabinoid system of healthy controls versus patients with schizophrenia. A total of ten healthy controls and nine patients with schizophrenia were included and studied with high specific activity [(11)C]OMAR. The CB1 binding (expressed as the distribution volume; V(T)) was highest in the globus pallidus and the cortex in both controls and patients with schizophrenia. Controls showed a correlation with the known distribution of CB1 and decline of [(11)C]OMAR binding with age, most significantly in the globus pallidus. Overall, we observed elevated mean binding in patients with schizophrenia across all regions studied, and this increase was statistically significant in the pons (p<0.05), by the Students t-test. When we ran a regression of the control subjects V(T) values with age and then compared the patient data to 95% prediction limits of the linear regression, three patients fell completely outside for the globus pallidus, and in all other regions there were at least 1-3 patients outside of the prediction intervals. There was no statistically significant correlations between PET measures and the individual Brief Psychiatry Rating Score (BPRS) subscores (r=0.49), but there was a significant correlation between V(T) and the ratio of the BPRS psychosis to withdrawal score in the frontal lobe (r=0.60), and middle and posterior cingulate regions (r=0.71 and r=0.79 respectively). In conclusion, we found that [(11)C] OMAR can image human CB1 receptors in normal aging and schizophrenia. In addition, our initial data in subjects with schizophrenia seem to suggest an association of elevated binding specific brain regions and symptoms of the disease.

2-[18F]F-A-85380: PET imaging of brain nicotinic acetylcholine receptors and whole body distribution in humans

Noninvasive imaging of nicotinic acetylcholine receptors (nAChRs) in the human brain in vivo is critical for elucidating the role of these receptors in normal brain function and in the pathogenesis of brain disorders. Here we report the first in vivo visualization of human brain areas containing nAChRs by using PET and 2‐[18F]fluoro‐3‐(2(S)azetidinylmethoxy)pyridine (2‐[18F]FA). We acquired scans from six healthy non‐smoking volunteers after i.v. bolus administration of 2‐[18F]FA (1.6 MBq/kg or 0.043 ± 0.002 mCi/kg). This dose was sufficient for visualizing nAChRs in the thalamus up to 5 h after injection. There were no adverse effects associated with administration of no‐carrier‐added 2‐[18F]FA (1.3‐10 pmol/kg). Consistent with the distribution of nAChRs in human brain, accumulated radioactivity was greatest in thalamus, intermediate in the midbrain, pons, cerebellum, and cortex; and least in white matter. As ˜90% of the injected radioactivity was eliminated via the urine (biological half‐life ca. 4 h), the urinary bladder wall received the highest radiation dose. The estimate of radiation dose equivalent to the urinary bladder wall (ca. 180 ± 30 mSv/MBq or 0.7 rem/mCi with a 2.4 h void interval) suggests that multiple studies could be performed in a single subject. The results predict that quantitative PET imaging of nAChRs in human brain with 2‐[18F]FA is feasible.

Greater Nicotinic Acetylcholine Receptor Density in Smokers Than in Nonsmokers: A PET Study with 2-18F-FA-85380

Assays of human postmortem brain tissue have revealed that smokers have greater densities of high-affinity nicotinic acetylcholine receptors (nAChRs) in several brain regions than do nonsmokers or exsmokers. Quantitative PET imaging of nAChRs in humans has recently been reported using the α4β2* subtype–specific radioligand 2-18F-FA-85380 (2FA). Methods: We used PET and 2FA to measure total volumes of distribution corrected for the free fraction of 2FA in plasma (VT/fP) in 10 nonsmokers and 6 heavy smokers (>14 cigarettes/d; abstinent for >36 h). Dynamic PET scans were performed over 8 h, commencing immediately after a bolus injection of 2FA. Anatomic sampling was performed on PET images that were coregistered to MR images acquired from each volunteer. Data were analyzed by Logan plots and by 1- and 2-tissue-compartment models using unbound, unmetabolized arterial 2FA concentration as the input function. Results: All modeling methods yielded similar results. VT/fP was significantly higher in smokers than in nonsmokers in all brain regions tested, except the thalamus. We used measures of VT/fP and estimates of nondisplaceable volume of distribution and found 25%–200% higher values in smokers than in nonsmokers for the volume of distribution for the specific binding compartment in the frontal cortex, midbrain, putamen, pons, cerebellum, and corpus callosum. These findings were consistent with voxel-based analysis using statistical parametric mapping. Conclusion: Our findings suggest that PET with 2FA can be used to study the role of nicotine-induced upregulation of nAChRs in active smokers and during smoking cessation.

2-[18F]fluoro-A-85380, an in vivo tracer for the nicotinic acetylcholine receptors

6-[18F]Fluoro-3-(2(S)-azetidinylmethoxy)pyridine (6-[18F]fluoro-A-85380 or 6-[18F]FA), a new tracer for positron emission tomography, was synthesized by no-carrier-added [18F] fluorination of 6-iodo-3-((1-tert-butoxycarbonyl-2(S)-azetidinyl)methoxy)pyridine followed by acidic deprotection. 6-[18F]FA followed the regional densities of brain nicotinic acetylcholine receptors (nAChRs) reported in the literature. Evidence of binding to nAChRs and high specificity of the binding in vivo was demonstrated by inhibition with nAChR selective ligands as well as with unlabeled 6-FA. A preliminary toxicology study of the 6-FA showed a relatively low biological effect.

Synthesis and biodistribution of [11C]A-836339, a new potential radioligand for PET imaging of cannabinoid type 2 receptors (CB2)

Recently, A-836339 [2,2,3,3-tetramethylcyclopropanecarboxylic acid [3-(2-methoxyethyl)-4,5-dimethyl-3H-thiazol-(2Z)-ylidene]amide] (1) was reported to be a selective CB2 agonist with high binding affinity. Here we describe the radiosynthesis of [11C]A-836339 ([11C]1) via its desmethyl precursor as a candidate radioligand for imaging CB2 receptors with positron-emission tomography (PET). Whole body and the regional brain distribution of [11C]1 in control CD1 mice demonstrated that this radioligand exhibits specific uptake in the CB2-rich spleen and little specific in vivo binding in the control mouse brain. However, [11C]1 shows specific cerebral uptake in the lipopolysaccharide (LPS)-induced mouse model of neuroinflammation and in the brain areas with Abeta amyloid plaque deposition in a mouse model of Alzheimers disease (APPswe/PS1dE9 mice). These data establish a proof of principle that CB2 receptors binding in the neuroinflammation and related disorders can be measured in vivo.

The quest for Eldorado: Development of radioligands for in vivo imaging of nicotinic acetylcholine receptors in human brain

Neuronal nicotinic acetylcholine receptors (nAChRs), ubiquitously distributed in the human brain, are implicated in various neurophysiological processes and in the pathophysiology and/or treatment strategies of Alzheimers and Parkinsons diseases, Tourettes syndrome, epilepsy, schizophrenia, depression, and anxiety, as well as being particularly affected in tobacco dependence/withdrawal. In the past two decades, researchers have developed an extensive series of radioligands for the assessment of nAChRs in vivo through emission tomography, PET and SPECT. Several radioligands, derivatives of A-85380: 2-[(18)F]FA, 6-[(18)F]FA and 5-[(123)I]IA, are now being employed for the evaluation of nAChR in humans with PET and SPECT. Displaying better imaging properties than (11)C-nicotine and a better toxicity profile than epibatidine analogs, they have allowed quantification of thalamic nAChR in the human brain. Nevertheless, A-85380 derivatives still exhibit slow brain kinetics and a moderate signal-to-noise ratio. Current research efforts on the part of PET/SPECT radiochemists, therefore, have focused on development of new, highly specific and highly selective nAChR radioligands with improved brain kinetics that are able to localize high-affinity nAChRs in vivo. Key examples of new PET/SPECT ligands that are derived from several different structural classes are discussed along with a review of their chemical as well as their in vitro and/or in vivo properties. In particular, new PET nAChR radioligands will be examined that either present faster brain kinetics allowing simple and reliable quantification approaches or higher binding potentials suitable for the evaluation of extrathalamic nAChR.

Radiosynthesis and preliminary evaluation of 5-[123/125I]iodo-3-(2(S)-azetidinylmethoxy)pyridine: a radioligand for nicotinic acetylcholine receptors

The radiochemical syntheses of 5-[125I]iodo-3-(2(S)-azetidinylmethoxy)pyridine (5-[125I]-iodo-A-85380, [125I]1) and 5-[123I]-iodo-A-85380, [123I]1, were accomplished by radioiodination of 5-trimethylstannyl-3-((1-tert-butoxycarbonyl-2(S)-azetidinyl)metho xy)pyridine, 2, followed by acidic deprotection. Average radiochemical yields of [125I]1 and [123I]1 were 40-55%; and the average specific radioactivities were 1,700 and 7,000 mCi/mumol, respectively. Binding affinities of [125I]1 and [123I]1 in vitro (rat brain membranes) were each characterized by a Kd value of 11 pM. Preliminary in vivo assay and ex vivo autoradiography of mouse brain indicated that [125I]1 selectively labels nicotinic acetylcholine receptors (nAChRs) with very high affinity and specificity. These studies suggest that [123I]1 may be useful as a radioligand for single photon emission computed tomography (SPECT) imaging of nAChRs.

[125/123I]IPH: A radioiodinated analog of epibatidine for in vivo studies of nicotinic acetylcholine receptors

Tomographic imaging of central nicotinic acetylcholine receptors (nAChRs) via single photon emission computed tomography (SPECT) has been hampered by the lack of a radioligand with suitable in vivo binding characteristics. Therefore, a novel analog of epibatidine, (±)‐exo‐2‐(2‐iodo‐5‐pyridyl)‐7‐azabicyclo[2.2.1]heptane (IPH), labeled with [125I] or [123I] was evaluated as an in vivo marker of central nicotinic acetylcholine receptors (nAChRs). [125I]IPH showed substantial brain penetration (4.2% of the injected dose at 30 min) and a cerebral biodistribution in mice consistent with the in vivo labeling of nAChRs (% injected dose/gram of thalamus, superior colliculi ≫ cerebellum). [125I]IPH binding sites were shown to be saturable with unlabeled IPH (ED50 approximately 1 μg/kg). The uptake of [125I]IPH was blocked significantly by the nicotinic agonists, cytisine, lobeline, and (−)‐nicotine, but not by the noncompetitive nAChR antagonist, mecamylamine. Antagonists of muscarinic (scopolamine), serotonin (ketanserin), and opioid (naloxone) receptors had no significant effect on [125I]IPH binding. A preliminary SPECT imaging study with [123I]IPH in a baboon showed [123I]IPH to localize in nAChR‐rich areas of brain (thalamus > frontal cortex > cerebellum). [123I]IPH binding in baboon brain was also displaced (35–45% displacement) by a challenge dose of cytisine showing that a well‐characterized nicotinic agonist effectively competes for [123I]IPH binding sites. [123I]IPH seems well suited for imaging studies of nAChRs and, to our knowledge, is the first SPECT agent that has allowed for the visualization of nAChRs in primate brain. Synapse 26:392–399, 1997.

Synthesis of a radiotracer for studying nicotinic acetylcholine receptors: (+/−)-exo-2-(2-[18F]fluoro-5-pyridyl)-7-azabicyclo[2.2.1]heptane

The radiochemical synthesis of (+/-)-exo-2-(2-[ 18 F]fluoro-5-pyridyl)-7-azabicyclo[2.2.1]heptane ([ 18 F]1) was accomplished by Kryptofix® 222 assisted nucleophilic no-carrier-added [ 18 F]fluorination of (+/-)-exo-2-(2-bromo-5-pyridyl)-7-azabicyclo[2.2.1] heptane (3a). The average radiochemical yield of the final product was 10% and the average specific activity was greater than >2000 mCi/μmol, calculated at end-of-synthesis. The stable fluorine ligand ([ 19 F]1 was prepared by Kryptofix® 222 assisted nucleophilic fluorination of (+/-)-exo-2-(2-bromo-5-pyridyl) - 7- methoxycarbonyl - 7 - azabicyclo[2.2.1]heptane (3b) followed by acid deprotection.