Mapping neuroreceptors with metal-labeled radiopharmaceuticals

Optical molecular imaging technology and its application in precise surgical navigation of liver cancer

Recent innovations in medical imaging technology have placed molecular imaging techniques at the forefront of diagnostic advancements. The current research trajectory in this field aims to integrate personalized molecular data of patients and diseases with traditional anatomical imaging data, enabling more precise, non-invasive, or minimally invasive diagnostic options for clinical medicine. This article provides an in-depth exploration of the basic principles and system components of optical molecular imaging technology. It also examines commonly used targeting mechanisms of optical probes, focusing especially on indocyanine green-the FDA-approved optical dye widely used in clinical settings-and its specific applications in diagnosing and treating liver cancer. Finally, this review highlights the advantages, limitations, and future challenges facing optical molecular imaging technology, offering a comprehensive overview of recent advances, clinical applications, and potential impacts on liver cancer treatment strategies.

Development of amphiphilic self-assembled nucleolipid as BBB targeting probe based on SPECT

Several approaches have been utilised to deliver therapeutic nanoparticles inside the brain but rendered by certain limitation such as active efflux, non-stability, toxicity of the nanocarrier, transport, physicochemical properties and many more. In this context use of biocompatible nano carriers is currently investigated. We herein present the hypothesis that the nucleoside-lipid based conjugates (nucleolipids) which are biocompatible in nature and have molecular recognition can be tuned for improved permeation across blood-brain barrier (BBB). In this work, a di-C15-palmitoyl-ketal nucleolipid nanoparticle bearing an acyclic chelator has been formulated, radiolabeled with 99mTc and evaluated for in vivo fate using SPECT imaging. The mean particle size of particles was 113 nm and found to be nontoxic as depticted through haemolytic assay (2.33% erythrocyte destruction) and 75 ± 0.3% HEK(Human Embryonic Kidney) cells survived at 72 h as depicted in SRB (Sulforhodamine B) toxicity assay. The encapsulation efficiency (68 ± 2.75%) and drug loading capacity (22 ± 1.8%.) was calculated for nanoparticles using Methotrexate as model anti-cancer drug. The mathematical models indicate fickian release with a release constant KH = 20.70. With 98 ± 0.75% radiolabelling efficiency and established in vitro stability, nanoparticles showed brain uptake in normal mice as 0.91 times in comparison to BBB compromised mice (1.6% ± 0.03 ID/g)indicating higher brain uptake with rapid clearance as depicted through blood kinetics.

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 and evaluation of technetium-99m labelled 1-(2-methoxyphenyl)piperazine derivative for single photon emission computed tomography imaging for targeting 5-HT1A

Quantitative changes in expression level of 5HT_1A are somewhere related to common neurological disorders such as anxiety, major depression and schizophrenia. We have designed EDTA conjugated SPECT imaging probe for localization of 5HT_1A receptor in brain. For designing SPECT probe we have employed the concept of bivalent approach and a homodimeric system with desirable pharmacokinetics of 5HT_1A imaging. ^99mTc-EDHT was also evaluated for its stability through serum stability assay and glutathione challenge experiment. Biodistribution study showed the highest accumulation of radioactivity in kidney which depicted the renal mode of excretion from the body. However in brain the uptake of 1.21% ID per gram was observed in initial 5 min of drug administration. On blocking the receptor this percent get decreased to 0.97% ID per gram. The regional distribution in brain was also performed which showed the accumulation of drug in cerebellum, cortex and hippocampus part, which are already known for 5HT_1A expression. Dynamic study in rabbit is also in support of results derived from biodistribution and blood kinetics experiment. These finding suggest that ^99mTc-EDHT holds promising place for further optimization before nuclear medicine applications in different animal species.

68 Ga-Labelled Tropane Analogues for the Visualization of the Dopaminergic System

The development of radiometal-labelled pharmaceuticals for neuroimaging could offer great potential due to easier handling during labelling and availability through radionuclide generator systems. Nonetheless, to date, no such tracers are available for positron emission tomography, primarily owing to the challenge of crossing the blood-brain barrier (BBB) and loss of affinity through chelator attachment. We have prepared a variety of 68 Ga-labelled phenyltropanes showing that, through a simple hydrocarbon-linker, it is possible to introduce a chelator onto the lead structure while maintaining its high affinity for hDAT (human dopamine transporter) and simultaneously achieving adequate lipophilicity. One of the candidates, [68 Ga]Ga-HBED-hexadiyne-tropane, showed an IC50 value of 66 nM, together with a log D7.4 of 0.96. A μPET study in a hemi-parkinsonian rat model showed a fast wash-out of the tracer, and no specific uptake in the brain, thus implying an inability to penetrate the BBB.

5-HT1A targeting PARCEST agent DO3AM-MPP with potential for receptor imaging: Synthesis, physico-chemical and MR studies

Contrast enhancement in MRI using magnetization or saturation transfer techniques promises better sensitivity, and faster acquisition compared to T₁ or T₂ contrast. This work reports the synthesis and evaluation of 5-HT_1A targeted PARACEST MRI contrast agent using 1,4,7,10-tetraazacycloDOdecane-4,7,10-triacetAMide (DO3AM) as the bifunctional chelator, and 5-HT_1A-antagonist methoxyphenyl piperazine (MPP) as a targeting unit. The multi-step synthesis led to the MPP conjugated DO3AM with 60% yield. CEST-related physicochemical parameters were evaluated after loading DO3AM-MPP with paramagnetic MRI active lanthanides: Gadolinium (Gd-DO3AM-MPP) and Europium (Eu-DO3AM-MPP). Luminescence lifetime measurements with Eu-DO3AM-MPP and computational DFT studies using Gd-DO3AM-MPP revealed the coordination of one water molecule (q = 1.43) with metal-water distance (r_M-H₂O) of 2.7 Å and water residence time (τ_m) of 0.23 ms. The dissociation constant of K_d 62 ± 0.02 pM as evaluated from fluorescence quenching of 5-HT_1A (protein) and docking score of -4.81 in theoretical evaluation reflect the binding potential of the complex Gd-DO3AM-MPP with the receptor 5-HT_1A. Insights of the docked pose reflect the importance of NH₂ (amide) and aromatic ring in Gd-DO3AM-MPP while interacting with Ser 374 and Phe 370 in the antagonist binding pocket of 5-HT_1A. Gd-DO3AM-MPP shows longitudinal relaxivity 5.85 mM^-1s^-1 with a water residence lifetime of 0.93 ms in hippocampal homogenate containing 5-HT_1A. The potentiometric titration of DO3AM-MPP showed strong selectivity for Gd³⁺ over physiological metal ions such as Zn²⁺ and Cu²⁺. The in vitro and in vivo studies confirmed the minimal cytotoxicity and presential binding of Gd-DO3AM-MPP with 5-HT_1A receptor in the hippocampus region of the mice. Summarizing, the complex Gd-DO3AM-MPP can have a potential for CEST imaging of 5-HT_1A receptors.

Microenvironment Stimulated Bioresponsive Small Molecule Carriers for Radiopharmaceuticals

The widespread and successful use of radiopharmaceuticals in diagnosis, treatment, and therapeutic monitoring of cancer and other ailments has spawned significant literature. The transition from untargeted to targeted radiopharmaceuticals reflects the various stages of design and development. Targeted radiopharmaceuticals bind to specific biomarkers, get fixed, and highlight the disease site. A new subset of radioprobes, the bioresponsive radiopharmaceuticals, has been developed in recent years. These probes generally benefit from signal enhancement after undergoing molecular changes due to the fluctuations in the environment (pH, redox, or enzymatic activity) at the site of interest. This review presents a comprehensive overview of bioresponsive radioimaging probes covering the basis, application, and scope of development.

Acetylated Benzothiazolone as Homobivalent SPECT Metallo-Radiopharmaceutical 99mTc-(6-AcBTZ)2DTPA: Design, Synthesis, and Preclinical Evaluation for Mapping 5-HT1A/7 Receptors

Mapping different structural forms of serotonin subtypes 5-HT1A-5-HT7 using a selective-specific ligand with good pharmacokinetics and brain permeability can open avenues for personalized medication in depressed population. Herein, the selective 5-HT1A/7 antagonist, modified for enhanced brain permeation, is developed as a homobivalent ligand, (6-AcBTZ)2DTPA. After in-depth computational studies to probe the binding mechanism, two-step synthesis lead to (6-AcBTZ)2DTPA. Biocompatibility studies indicated cytocompatibility with 3.6-1.64% cell death (0.1 mM-1 pM) and hemocompatibility with 2.33% hemolysis of human erythrocytes. When 99mTc-radiolabeled in a quantitative yield (98%), a stable preparation was obtained with 7.4 and 3.5% dissociation upon incubation with human serum and excess cysteine. The single-photon-emission computed tomography (SPECT) tracer 99mTc-(6-AcBTZ)2DTPA showed biphasic clearance (t 1/2, distribution = 0.5 min and t 1/2, elimination = 482 min) and maximum brain uptake of 0.42 ± 0.02% ID/g with the regional localization (hippocampus: 11.38% ID/g; cortex: 26.42% ID/g; cerebellum: 25.23% ID/g). Thus, the 99mTc-metal-based SPECT neurotracer holds potential for neuroreceptor mapping.

Design, physico-chemical and pre-clinical evaluation of a homo-bivalent 99mTc-(BTZ)2DTPA radioligand for targeting dimeric 5-HT1A/5-HT7 receptors

A ^99mTc-labelled bis-benzothiazolone-DTPA radio-complex as a SPECT neuroimaging agent.

New Trends and Current Status of Positron-Emission Tomography and Single-Photon-Emission Computerized Tomography Radioligands for Neuronal Serotonin Receptors and Serotonin Transporter

The critical role of serotonin (5-hydroxytryptamine; 5-HT) and its receptors (5-HTRs) in the pathophysiology of diverse neuropsychiatric and neurodegenerative disorders render them attractive diagnostic and therapeutic targets for brain disorders. Therefore, the in vivo assessment of binding of 5-HT receptor ligands under a multitude of physiologic and pathologic scenarios may support more-accurate identification of disease and its progression and the patient's response to therapy as well as the screening of novel therapeutic strategies. The present Review aims to focus on the current status of radioligands used for positron-emission tomography (PET) and single-photon-emission computerized tomography (SPECT) imaging of human brain serotonin receptors. We further elaborate upon and emphasize the attributes that qualify a radioligand for theranostics on the basis of its frequency of use in clinics, its benefit to risk assessment in humans, and its continuous evolution, along with the major limitations.