2-Amino-3-(1-(4-(4-(2-methoxyphenyl) piperazine-1-yl)butyl)-1H-1,2,3-triazol-4-yl) propanoic acid: synthesized,99mTc-tricarbonyl labeled, and bioevaluated as a potential 5HT1Areceptor ligand

Insights into the development of 99mTc-radioligands for serotonergic receptors imaging: Synthesis, labeling, In vitro, and In vivo studies

Serotonergic (5-hydroxytryptamine; 5-HT) receptors play critical roles in neurological and psychological disorders such as schizophrenia, anxiety, depression, and Alzheimer's diseases. Therefore, it is particularly important to develop novel radioligands or modify the existing ones to identify the serotonergic receptors involved in psychiatric disorders. Among the 16 subtypes of serotonergic systems, only technetium-99m based radiopharmaceuticals have been evaluated for serotonin-1A (5-HT1A), serotonin-2A (5-HT2A), 5-HT1A/7 heterodimers and serotonin receptor neurotransmitter (SERT). This review focuses on recent efforts in the design, synthesis and evaluation of 99mTc-radioligands used for single photon emission computerized tomography (SPECT) imaging of serotonergic (5-HT) receptors. Additionally, the discussion will cover aspects such as chemical structure, in vitro/vivo stability, affinity toward serotonin receptors, blood-brain barrier permeation (BBB), and biodistribution study.

Synthesis, biological evaluation and preclinical study of a novel 99mTc-peptide: A targeting probe of amyloid-β plaques as a possible diagnostic agent for Alzheimer's disease

With respect to the main role of amyloid-β (Aβ) plaques as one of the pathological hallmarks in the brain of Alzheimer's patients, the development of new imaging probes for targeted detection of Aβ plaques has attracted considerable interests. In this study, a novel cyclopentadienyl tricarbonyl Technetium-99 m (^99mTc) agent with peptide scaffold, ^99mTc-Cp-GABA-D-(FPLIAIMA)-NH₂, for binding to the Aβ plaques was designed and successfully synthesized using the Fmoc solid-phase peptide synthesis method. This radiopeptide revealed a good affinity for Aβ42 aggregations (K_d = 20 µM) in binding affinity study and this result was confirmed by binding to Aβ plaques in brain sections of human Alzheimer's disease (AD) and rat models using in vitro autoradiography, fluorescent staining, and planar scintigraphy. Biodistribution studies of radiopeptide in AD and normal rats demonstrated a moderate initial brain uptake about 0.38 and 0.35% (ID/g) 2 min post-injection, respectively. Whereas, AD rats showed a notable retention time in the brain (0.23% ID/g at 30 min) in comparison with fast clearance in normal rat brains. Normal rats following treatment with cyclosporine A as a p-glycoprotein inhibitor showed a significant increase in the radiopeptide brain accumulation compared to non-treated ones. There was a good correlation between data gathered from single-photon emission computed tomography/computed tomography (SPECT/CT) imaging and biodistribution studies. Therefore, these findings showed that this novel radiopeptide could be a potential SPECT imaging agent for early detection of Aβ plaques in the brain of patients with AD.

Mapping neuroreceptors with metal-labeled radiopharmaceuticals

The growing epidemiological and economic burden of neurological diseases on society is tremendous. A correct and timely diagnosis can help in lowering the burden and improving the life quality of both the diseased person and the caretaker. Imaging of the brain (neuroimaging) using CT, MRI, and nuclear imaging methods can provide anatomical and functional information. Neuroreceptors are central to neurotransmission and neuromodulation in the CNS. In vivo imaging of receptors in the brain provides powerful tools for the functional study of the central nervous system (CNS) in normal or diseased states. Presently, PET imaging using non-metallic radiotracers dominates the imaging of neuroreceptors. Metal-based probes for SPECT and PET can be economical and logistically easier to use without compromising the information. This review focuses on the development of metallic radiotracers for (99mTc) SPECT and (68Ga) PET along with future directions based on the metallic probes developed for other imaging modalities namely MRI.

Click-to-Chelate: development of technetium and rhenium-tricarbonyl labeled radiopharmaceuticals

The Click-to-Chelate approach is a highly efficient strategy for the radiolabeling of molecules of medicinal interest with technetium and rhenium-tricarbonyl cores. Reaction of azide-functionalized molecules with alkyne prochelators by the Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC; click reaction) enables the simultaneous synthesis and conjugation of tridentate chelating systems for the stable complexation of the radiometals. In many cases, the functionalization of (bio)molecules with the ligand system and radiolabeling can be achieved by convenient one-pot procedures. Since its first report in 2006, Click-to-Chelate has been applied to the development of numerous novel radiotracers with promising potential for translation into the clinic. This review summarizes the use of the Click-to-Chelate approach in radiopharmaceutical sciences and provides a perspective for future applications.