DNA-Targeted Complexes of Tc and Re for Biomedical Applications

Novel technetium-99m-labelled ribociclib isocyanide derivatives for imaging cyclin-dependent kinase 4/6 (CDK4/6) expression in cancer

Cyclin-dependent kinase 4/6 (CDK4/6) plays a crucial role in cell cycle regulation, is overexpressed in various cancers and is an important target in the development of radiotracers for tumour imaging. Despite the increasing recognition of CDK4/6 inhibitors in cancer therapy, their application is limited by the lack of suitable biomarkers. Herein, we developed a series of technetium-99m-labelled CDK4/6 radiotracers and utilized a linker optimization strategy to reduce their abdominal uptake and enhance their imaging properties. By introducing polyethylene glycol chains (PEGn, n = 2, 3, or 4) of different lengths, we successfully prepared the first technetium-99m-labelled ribociclib isocyanide derivatives via a one-step method. After rapid screening, we selected [99mTc]Tc-RIB-PEG4-CN (LogD7.4 = 0.01 ± 0.01) because of its superior uptake in the cell lines and suitable nontarget uptake in vivo. Additionally, it displayed nanomolar affinity (5.887 ± 0.3579 nM). In HCT116 xenograft models, the probe exhibited significant tumour uptake (2.44 ± 0.29 % ID/g at 4 h p.i.) while maintaining reduced abdominal uptake. Moreover, the probe showed specificity in HCT116 xenograft models, as evidenced by a 49.2 % decrease in the tumour-to-muscle ratio in the presence of excess ribociclib for blocking. Micro-SPECT/CT images of HCT116 and MCF-7 xenografts revealed the liver metabolism of [99mTc]Tc-RIB-PEG4-CN, with robust tumour retention and comparatively low abdominal uptake at 4 h p.i. This novel radiotracer enables the noninvasive evaluation of CDK4/6 expression, providing valuable insights for clinical treatment strategies and further mechanistic studies.

Synthesis and Biological Evaluation of Novel Cationic Rhenium and Technetium-99m Complexes Bearing Quinazoline Derivative for Epidermal Growth Factor Receptor Targeting

Background/Objectives: Epidermal growth factor receptor (EGFR) plays a vital role in cell proliferation and survival, with its overexpression linked to various malignancies, including non-small cell lung cancer (NSCLC). Although EGFR tyrosine kinase inhibitors (TKIs) are a key therapeutic strategy, acquired resistance and relapse remain challenges. This study aimed to synthesize and evaluate novel rhenium-based complexes incorporating EGFR TKIs to enhance anticancer efficacy, particularly in radiosensitization. Methods: We synthesized a rhenium tricarbonyl complex (Complex 2) and its 99mTc analog (Complex 2') by incorporating triphenylphosphine instead of bromine as the monodentate ligand and PF6- as the counter-ion, resulting in a positively charged compound that forms cationic structures. Cytotoxicity and EGFR inhibition were evaluated in A431 cells overexpressing EGFR using MTT assays, Western blotting, and flow cytometry. Radiosensitization was tested through MTT and clonogenic assays. The 99mTc complex's radiochemical yield, stability, and lipophilicity were also assessed. Results: Complex 2 exhibited significant cytotoxicity with an IC50 of 2.6 μM and EGFR phosphorylation inhibition with an IC50 of 130.6 nM. Both complex 1 and 2 induced G0/G1 cell cycle arrest, with Complex 2 causing apoptosis. Radiosensitization was observed at doses above 2 Gy. Complex 2' demonstrated high stability and favorable lipophilicity (LogD7.4 3.2), showing 12% cellular uptake after 30 min. Conclusions: Complexes 2 and 2' show promise as dual-function anticancer agents, offering EGFR inhibition, apoptosis induction, and radiosensitization. Their potential as radiopharmaceuticals warrants further in-depth investigation in preclinical models.

Cancer phototherapy by CO releasing terpyridine-based Re(I) tricarbonyl complexes via ROS generation and NADH oxidation

Here, we have synthesized and characterized three visible light responsive terpyridine based-Re(I)-tricarbonyl complexes; [Re(CO)3(ph-tpy)Cl] (Retp1), [Re(CO)3(an-tpy)Cl] (Retp2), and [Re(CO)3(py-tpy)Cl] (Retp3) where ph-tpy = 4'-phenyl-2,2':6',2″-terpyridine; an-tpy = 4'-anthracenyl-2,2':6',2″-terpyridine, py-tpy = 4'-pyrenyl-2,2':6',2″-terpyridine. The structures of Retp1 and Retp2 were confirmed from the SC-XRD data, indicating distorted octahedral structures. Unlike traditional PDT agents, these complexes generated reactive oxygen species (ROS) via type I and type II pathways and oxidized redox crucial NADH (reduced nicotinamide adenine dinucleotide) upon visible light exposure. Retp3 showed significant mitochondrial localization and demonstrated photoactivated anticancer activity (IC50 ∼ 2 µM) by inducing ROS-mediated cell death in cancer cells selectively (photocytotoxicity Index, PI > 28) upon compromising mitochondrial function in A549 cells. Their diagnostic capabilities were ultimately assessed using clinically relevant 3D multicellular tumor spheroids (MCTs).

Two rhenium compounds with benzimidazole ligands: synthesis and DNA interactions

Two rhenium compounds: cis-tetrachlorotetrabenzimidazoldirhenium(iii) chloride - I and tetrabenzimidazoldioxorhenium(v) - II have been synthesized and characterized. X-ray data are presented for the new complex II. I and II show strong emission that has been used to investigate their interaction with several non-canonical DNA structures. Both compounds have a quenching effect on the fluorescence intensity upon addition of the investigated oligonucleotides; I was more selective for binding G4-than II. Association constant values obtained for I and II with G-quadruplexes reached 106 M-1, which suggests a strong interaction between both complexes and these sequences. FRET-melting assays show that I and II have a rather high level of stabilization of ckit1 and ckit2 quadruplexes. I is toxic against macrophages RAW267.7 only in high concentrations, while complex II shows no toxicity against these cells. I and II accumulate inside cells in different degrees. Molecular dynamic simulation studies have provided insights into the binding modes of II with ckit1 and ckit2 G-quadruplexes. The results obtained show the DNA binding activity of the rhenium complexes and their ability to be players in the anti-cancer fight since they can bind to non-canonical DNA forms in oncogene promoters, accumulate in some cancer cells, and influence the cancer cells microenvironment.

Targeting of G-quadruplex DNA with 99mTc(I)/Re(I) Tricarbonyl Complexes Carrying Pyridostatin Derivatives

The main goal of this work was to elucidate the potential relevance of (radio)metal chelates of 99mTc and Re targeting G-quadruplex structures for the design of new tools for cancer theranostics. 99mTc provides the complexes with the ability to perform single-photon-emission computed tomography imaging studies, while the Re complexes should act as anticancer agents upon interaction with specific G4 DNA or RNA structures present in tumor tissues. Towards this goal, we have developed isostructural 99mTc(I) and Re(I) tricarbonyl complexes anchored by a pyrazolyl-diamine (Pz) chelator carrying a pendant pyridostatin (PDS) fragment as the G4-binding motif. The interaction of the PDF-Pz-Re (8) complex with different G4-forming oligonucleotides was studied by circular dichroism, fluorescence spectroscopy and FRET-melting assays. The results showed that the Re complex retained the ability to bind and stabilize G4-structures from different DNA or RNA sequences, namely those present on the SRC proto-oncogene and telomeric RNA (TERRA sequence). PDF-Pz-Re (8) showed low to moderate cytotoxicity in PC3 and MCF-7 cancer cell lines, as typically observed for G4-binders. Biodistribution studies of the congener PDF-Pz-99mTc (12) in normal mice showed that the complex undergoes a fast blood clearance with a predominant hepatobiliary excretion, pointing also for a high in vitro stability.