Kazunori Bando

Piperazine analog of vesamicol: In vitro and in vivo characterization for vesicular acetylcholine transporter

The probes to detect vesicular acetylcholine transporter (VAChT) in vivo are important to evaluate the mapping and function in cholinergic system. To develop high‐specific and high‐affinity radiotracer for single photon emission computed tomography, we investigated piperazine analogs which replaced the piperidine ring of (‐)‐vesamicol with a piperazine ring. We found that the piperazine analog of iodobenzovesamicol, trans‐5‐iodo‐2‐hydroxy‐3‐[4‐phenylpiperazinyl] tetralin (DRC140), had high affinity for VAChT in rat brain. We carried out binding assay in subcellular fraction of the rat brain. The highest Bmax for [125I]‐DRC140 binding was observed in the synaptic vesicle fraction (1,751 fmol/mg protein), followed by the crude vesicle (821 fmol/mg protein) and the P2 fraction (187 fmol/mg protein). These Kd values were similar to the affinity of highly purified synaptic vesicular fraction (Kd = 0.3 nM) with a one‐site model. The possibility that [125I]‐DRC140 recognizes sigma receptor was excluded by our finding large inhibition constants (Ki = 849 nM for haloperidol, Ki = 3,052 nM for 1,3‐di(2‐tolyl)guanidine). In vivo distribution studies with the [123I]‐DRC140 in rats showed a rapid brain uptake. The highest brain area was in striatum, followed by frontal cortex, occipital cortex, and hippocampus. The lowest brain area was cerebellum. The radioactivity of high‐accumulated areas in ex vivo autoradiography was reduced by a preinjection of (‐)‐vesamicol and these levels were reduced to the radioactivity in cerebellum. These results show that [125I]‐DRC140 can provide extremely high specific tracer with excellent brain permeability as a ligand for single photon emission computed tomography. Synapse 38:27–37, 2000.

Synthesis and evaluation of radiolabeled piperazine derivatives of vesamicol as SPECT agents for cholinergic neurons

To diagnose and investigate neurodegenerative diseases affecting cholinergic neuron density, piperazine derivatives of vesamicol were synthesized and evaluated. Previously, we reported that trans-5-iodo-2-hydroxy-3-[4-phenylpiperazinyl] tetralin (DRC140, 1) possessed high selectivity for vesicular acetylcholine transporter (VAChT). In present study of the effect of alkyl substituents, we observed that the introduction of a methyl group into the ortho or meta positions of the phenyl group of 1 increased affinity for VAChT. trans-5-Iodo-2-hydroxy-3-[4-[2-methylphenyl] piperazinyl]tetralin (2) displayed high affinity and specificity for VAChT. The regional distributions of radioactivity in the rat brain correlated well with known patterns of central cholinergic innervation. [(123)I]2 is a potentially useful compound for SPECT imaging.

In Vivo SPECT Imaging of Amyloid-β Deposition with Radioiodinated Imidazo[1,2-a]Pyridine Derivative DRM106 in a Mouse Model of Alzheimer's Disease

Noninvasive determination of amyloid-β peptide (Aβ) deposition has important significance for early diagnosis and medical intervention for Alzheimers disease (AD). In the present study, we investigated the availability of radiolabeled DRM106 (123/125I-DRM106 [6-iodo-2-[4-(1H-3-pyrazolyl)phenyl]imidazo[1,2-a]pyridine]), a compound with sufficient affinity for the synthesis of human Aβ fibrils and satisfactory metabolic stability, as a SPECT ligand in living brains. Method: The sensitivity of 125I-DRM106 for detecting Aβ deposition was compared with that of 125I-IMPY (2-(4′-dimethylaminophenyl)-6-iodo-imidazo[1,2-a]pyridine), a well-known amyloid SPECT ligand, by ex vivo autoradiographic analyses in 18-mo-old amyloid precursor protein transgenic mice. To verify the sensitivity and quantitation of radiolabeled DRM106 for in vivo imaging, we compared the detectability of Aβ plaques with 123I-DRM106 and a well-known amyloid PET agent, 11C-labeled Pittsburgh compound B (11C-PiB), in 29-mo-old transgenic mice and age-matched nontransgenic littermates. Additionally, we compared the binding characteristics of 125I-DRM106 with those of 11C-PiB and 11C-PBB3, which selectively bind to Aβ plaques and preferentially to tau aggregates, respectively, in postmortem AD brain sections. Results: Ex vivo autoradiographic analysis showed that measurement with 125I-DRM106 has a higher sensitivity for detecting Aβ accumulation than with 125I-IMPY in transgenic mice. SPECT imaging with 123I-DRM106 also successfully detected Aβ deposition in living aged transgenic mice and showed strong correlation (R = 0.95, P < 0.01) in quantitative analysis for Aβ plaque detection by PET imaging with 11C-PiB, implying that sensitivity and quantitation of SPECT imaging with 123I-DRM106 are almost as good as 11C-PiB PET for the detectability of Aβ deposition. Further, the addition of nonradiolabeled DRM106 fully blocked the binding of 125I-DRM106 and 11C-PiB, but not 11C-PBB3, to AD brain sections, and 125I-DRM106 showed a lower binding ratio of the diffuse plaque–rich lateral temporal cortex to the dense-cored/neuritic plaque–rich hippocampal CA1 area, compared with 11C-PiB. Conclusion: All of these data demonstrated the high potential of 123I-DRM106 for amyloid imaging in preclinical and clinical application, and it might more preferentially detect dense-cored/neuritic amyloid deposition, which is expected to be closely associated with neuropathologic changes of AD.

Indium-111-labeled liposomes: quality and biodistribution in mice bearing human tumor xenografts

AbstractPhospholipid vesicles containing the ionophore A23187 in the bilayer (VesCan) are the only liposomes produced for clinical imaging purposes. We undertook preliminary experiments with In-loaded VesCan (indium-111-V-liposome) using normal rabbits and nude mice bearing human gastric (MKN45) and colon (COL0205) tumor xenografts before clinical application of the product. Two V-liposome vials with different expiration dates were labeled with 111In and then injected into rabbits immediately or 24 h after loading. The blood clearances of 111In were not significantly different from rabbit to rabbit, indicating that 111In-V-liposomes did not vary among vial lots. Both MKN45 and COL0205 tumor xenografts in nude mice were successfully imaged with 111In-V-liposome; V-liposome delivered sufficient radioactivity to allow tumor visualization as early as 8 h after iv injection. It was also found that MKN45 tumors exhibited uptake of 6.5% ID/g. The present results indicated that the quality and shelf life of 111In...

Synthesis and biological evaluation of novel radioiodinated imidazopyridine derivatives for amyloid-β imaging in Alzheimer's disease.

Non-invasive detection for amyloid-β peptide (Aβ) deposition has important significance for the early diagnosis and medical intervention for Alzheimers disease (AD). In this study, we developed a series of imidazopyridine derivatives as potential imaging agents for single-photon emission computed tomography (SPECT). Two of them, compounds DRK092 and DRM106, showed higher affinity for synthetic human Aβ 1-40 fibrils than did the well-known amyloid-imaging agent IMPY. A metabolite analysis revealed brain-permeable radioactive metabolites of (125)I-labeled DRK092 and IMPY; no radioactive metabolites from (125)I-labeled DRM106 ([(125)I]DRM106) were detected. In addition, in vitro autoradiography clearly demonstrated specific binding of [(125)I]DRM106 in the hippocampal region of AD enriched with Aβ plaques. Thus, our results strongly suggested that compound DRM106 can be used as an imaging agent for SPECT to detect Aβ deposition in AD brain.

Distinct binding of amyloid imaging ligands to unique amyloid-β deposited in the presubiculum of Alzheimer's disease

Non‐invasive determination of amyloid‐β peptide (Aβ) deposition with radioligands serves for the early diagnosis and clarification of pathogenetic mechanisms of Alzheimers disease (AD). The polymorphic binding site on multimeric Aβ for current radioligands, however, is little understood. In this study, we investigated the binding of several radioligands including 11C‐Pittsburgh Compound B (11C‐PiB), 3H‐AZD2184, and two recently developed compounds, 125I‐DRM106 and 125I‐DRK092, with unique presubicular Aβ deposits lacking interaction with the commonly used amyloid dyes FSB. 11C‐PiB, 3H‐AZD2184, and 125I‐DRK092 showed overt binding to presubicular Aβ deposits, while 125I‐DRM106 barely bound to these aggregates despite its strong binding in the hippocampal CA1 sector. Unlike neuritic plaques in the CA1, Aβ lesions in the presubiculum were not accompanied by inflammatory gliosis enriched with 18‐kDa translocator protein (TSPO). Thus, there are at least two different components in Aβ aggregates providing distinct binding sites for the current amyloid radioligands, and one of these binding components is distinctly present in the presubicular Aβ deposits. Amyloid radioligands lacking affinity for this component, such as 125I‐DRM106, may selectively capture Aβ deposits tightly associated with TSPO neuroinflammation and neurodegeneration as exemplified by CA1 neuritic plaques. Hence, comparative autoradiographic assessments of radioligand binding in CA1 and presubiculum could serve for the development of an amyloid PET imaging agent visualizing neurotoxicity‐related Aβ pathologies.