Benzoxazolone-arylpiperazinyl scaffold-based PET ligand for 5-HT7 : Synthesis and biological evaluation

Efforts are underway to improve the diagnosis and treatment for neurological disorders like depression, anxiety, epilepsy, and schizophrenia. The G-protein-coupled receptors (GPCRs) 5-HT₇   receptor, the most recently identified member of 5-HT receptor family dysregulation has an association with various central nervous system (CNS) disorders and its ligands have an edge as potential therapeutics. Here, we report the synthesis, characterization, and biological evaluation of diversely substituted methoxy derivatives of 2-benzoxazolone arylpiperazine for targeting 5-HT₇  receptors. Out of all derivatives, only C-2 substituted derivative, 3-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)benzoxazol-2(3H)-one/ABO demonstrate a high affinity for human 5-HT₇ receptors. [¹¹ C]ABO was obtained by O-methylation of desmethyl-precursor using [¹¹ C]CH₃ OTf in the presence of NaOH giving a high radiochemical yield of 25 ± 12% (decay-corrected, n = 7) with stability up to 1.5 h postradiolabeling. In vitro autoradiography displays binding of [¹¹ C]ABO in accordance with 5-HT₇ distribution with a decrease of approximately 80% and 40% activity in the hippocampus and cerebellum brain region when administered with 10 µM cold ligand. Prefatory positron emission tomography scan results in Sprague-Dawley (SD) rat brain revealed fast and high radioactivity build-up in 5-HT₇ receptor-rich regions, namely, the hippocampus (2.75 ± 0.16 SUV) and the cerebral cortex (2.27 ± 0.02 SUV) establishing selective targeting of [¹¹ C]ABO. In summary, these pieces of data designate [¹¹ C]ABO as a promising 5-HT₇  receptor ligand that can have possible roles in clinics after its further optimization on different animal models.

Receptor mapping using methoxy phenyl piperazine derivative: Preclinical PET imaging

This study aimed at assessing 2-methoxyphenyl piperazine derivative for its binding specificity and suitability in mapping metabotropic glutamate receptor subtype 1, which is implicated in several neuropsychiatric disorders. N-(2-(4-(2-Methoxyphenyl)piperazin-1-yl)ethyl)-N-methylpyridin-2-amine was synthesised and evaluated for brain imaging subsequent to radiolabelling with [¹¹C] radioisotope via methylation process in 98.9% purity and 52 ± 6% yield (decay corrected). The specific activity was in the range of 72-93 GBq/µmol. The haemolysis of blood was 2-5% for initial 4 hr and remained < 10% after 24 h of incubation indicating low toxicity. In vitro autoradiograms after coincubation with unlabelled ligand confirmed the high uptake of the PET radioligand in the mGluR1 receptor rich regions. The PET as well as biodistribution studies also showed high activity in the brain with a direct correlation between receptor abundance distribution pattern and tracer activity. The biodistribution analyses revealed initial high brain uptake (4.18 ± 0.48). The highest uptake was found in cerebellum (SUV 4.7 ± 0.2), followed by thalamus (SUV 3.5 ± 0.1), and striatum (SUV 3 ± 0.1). In contrast, pons had negligible tracer activity. The high uptake observed in all the regions with known mGluR1 activity indicates suitability of the ligand for mGluR1 imaging.

Lanthanide (Ln3+) complexes of bifunctional chelate: Synthesis, physicochemical study and interaction with human serum albumin (HSA)

Bifunctional chelate EDTA-bis amide (N,N'-bis (tyramide)ethylenediamine-N,N'-diacetic acid) that has ability to mimic natural amino acids was synthesized and analyzed by various spectroscopic techniques. The physicochemical studies were performed to calculate the various thermodynamic and kinetic parameters for the synthesized poly-amino carboxylate ligand. The two protonation constant (pka's = 3.460 and 6.722) of the prepared ligand and stability constants (log K_ML's = 15.8, 18.1, 16.2, 18.4, 17.5, 18.9, 13.6 and 12.8) of the complexes formed with Ce³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Lu³⁺, Zn²⁺ and Cu²⁺ were determined by potentiometric titration using 0.1 M Me₄NOH as non-aqueous base. The formation kinetics of [EuEDTA-TA₂]⁺ and [CeEDTA-TA₂]⁺ was studied and the rate constants were found to be 2.95 × 10-5 s^-1 and 4.414 × 10-5 s^-1respectively including the exchange reaction of [EuEDTA-TA₂]⁺ with Zn²⁺ and Cu²⁺ spectrophotometrically. The Eu(III) complex of EDTA(TA)₂ gives three emission bands at 480 nm, 540 nm and 610 nm (λ_max = 270 nm, excitation) which shows efficacy of the ligand as an optical imaging agent. Molecular docking studies with Human Serum Albumin (HSA: PDB 1E78) showed binding pattern with the residues Arg218, Arg222, Lys195 and Lys444 in sub domain II A of site I via hydrogen bond and identifies the ligand-HSA interaction and specific insight for transportation to the target sites. Subsequently, fluorescence spectroscopy was performed at λ_ex = 350 nm binding constant for HSA was 5.847 × 10⁴ M^-1 which showed effective quenching effect.

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.

Computational prediction of interaction and pharmacokinetics profile study for polyamino-polycarboxylic ligands on binding with human serum albumin

Human serum albumin (HSA) is one of the most abundant plasma proteins available in blood and responsible for transport of fatty acids, drugs and metabolites at its binding sites which are very important for the assessment of pharmacokinetics profile of the polyamino-polycarboxylic ligands.

Radiometallic Complexes of DO3A-Benzothiazole Aniline for Nuclear Medicine Theranostics

To develop a radioactive metal complex platform for tumor theranostics, we introduced three radiopharmaceutical derivatives of 1,4,7,10-tetraazacyclododecane-1,4,7-trisacetic acid-benzothiazole aniline (DO3A-BTA, L1) labeled with medical radioisotopes for diagnosis (⁶⁸Ga/⁶⁴Cu) and therapy (¹⁷⁷Lu). The tumor-targeting ability of these complexes was demonstrated in a cellular uptake experiment, in which ¹⁷⁷Lu-L1 exhibited markedly higher uptake in HeLa cells than the ¹⁷⁷Lu-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid complex. According to in vivo positron emission tomography imaging, high accumulation of ⁶⁸Ga-L1 and ⁶⁴Cu-L1 was clearly visualized in the tumor site, while ¹⁷⁷Lu-L1 showed therapeutic efficacy in therapy experiments. Consequently, this molecular platform represents a useful approach in nuclear medicine toward tumor-theranostic radiopharmaceuticals when ⁶⁸Ga-L1 or ⁶⁴Cu-L1 is used for diagnosis, ¹⁷⁷Lu-L1 is used for therapy, or two of the compounds are used in conjunction with each other.

Amyloid β-targeted metal complexes for potential applications in Alzheimer's disease

Alzheimer's disease (AD) is currently an incurable neurodegenerative disorder that affects millions of people around the world. The aggregation of amyloid-β peptides (Aβ), one of the primary pathological hallmarks of AD, plays a key role in the AD pathogenesis. In this regard, Aβ aggregates have been considered as both biomarkers and drug targets for the diagnosis and therapy of AD. Various Aβ-targeted metal complexes have exhibited promising potential as anti-AD agents due to their fascinating physicochemical properties over the past two decades. This review classifies the complexes into three groups based on their potential applications in AD including therapy, diagnosis and theranosis. The recent representative examples are highlighted in terms of design rationale, working mechanism and potential applications.

A copper–amyloid-β targeted fluorescent chelator as a potential theranostic agent for Alzheimer's disease

A fluorescent chelator is able to specifically target and attenuate Cu²⁺–Aβ aggregates in the brain of mice with Alzheimer's disease, which can be visualized by fluorescence imaging of the chelator.

A simple approach to a new T8-POSS based MRI contrast agent

A simple synthesis of a new nanoglobular T₈-silsesquioxane based contrast agent for the application in MRI is reported.

Design, synthesis and relaxation studies of triazole linked gadolinium(iii)–DO3A-BT-bistriazaspirodecanone as a potential MRI contrast agent

Novel and facile synthesis of a triazole based bisconjugated triazaspirodecanone as a potential MRI contrast agent.