HNODThia: A Promising Chelator for the Development of 64Cu Radiopharmaceuticals.

Herein, we present the synthesis and coordination chemistry of copper(II) and zinc(II) complexes of two novel heterocyclic triazacyclononane (tacn)-based chelators (HNODThia and NODThia-AcNHEt). The chelator HNODThia was further derivatized to obtain a novel PSMA-based bioconjugate (NODThia-PSMA) and a bifunctional photoactivatable azamacrocyclic analogue, NODThia-PEG3-ArN3, for the development of copper-64 radiopharmaceuticals. 64Cu radiolabeling experiments were performed on the different metal-binding chelates, whereby quantitative radiochemical conversion (RCC) was obtained in less than 10 min at room temperature. The in vitro stability of NODThia-PSMA in human plasma was assessed by ligand-challenge and copper-exchange experiments. Next, we investigated the viability of the photoactivatable analog (NODThia-PEG3-ArN3) for the light-induced photoradiosynthesis of radiolabeled proteins. One-pot photoconjugation reactions to human serum albumin (HSA) as a model protein and the clinically relevant monoclonal antibody formulation MetMAb were performed. [64Cu]Cu-7-azepin-HSA and [64Cu]Cu-7-azepin-onartuzumab were prepared in less than 15 min by irradiation at 395 nm, with radiochemical purities (RCP) of >95% and radiochemical yields (RCYs) of 42.7 ± 5.3 and 49.6%, respectively. Together, the results obtained here open the way for the development of highly stable 64Cu-radiopharmaceuticals by using aza-heterocyclic tacn-based chelators, and the method can easily be extended to the development of 67Cu radiopharmaceuticals for future applications in molecularly targeted radio(immuno)therapy.

The Influence of a Polyethylene Glycol Linker on the Metabolism and Pharmacokinetics of a 89Zr-Radiolabeled Antibody.

Most experimental work in the space of bioconjugation chemistry focuses on using new methods to construct covalent bonds between a cargo molecule and a protein of interest such as a monoclonal antibody (mAb). Bond formation is important for generating new diagnostic tools, yet when these compounds advance to preclinical in vitro and in vivo studies, and later for translation to the clinic, understanding the fate of potential metabolites that arise from chemical or enzymatic degradation of the construct is important to obtain a full picture of the pharmacokinetic performance of a new compound. In the context of designing new bioconjugate methods for labeling antibodies with the positron-emitting radionuclide 89Zr, we previously developed a photochemical process for making 89Zr-mAbs. Experimental studies on [89Zr]ZrDFO-PEG3-azepin-mAb constructs revealed that incorporation of the tris-polyethylene glycol (PEG3) linker improved the aqueous phase solubility and radiochemical conversion. However, the use of a PEG3 linker also has an impact on the whole-body residence time of the construct, leading to a more rapid excretion of the 89Zr activity when compared with radiotracers that lack the PEG3 chain. In this work, we investigated the metabolic fate of eight possible metabolites that arise from the logical disconnection of [89Zr]ZrDFO-PEG3-azepin-mAb at bonds which are susceptible to chemical or enzymatic cleavage. Synthesis combined with 89Zr-radiolabeling, small-animal positron emission tomography imaging at multiple time points from 0 to 20 h, and measurements of the effective half-life for whole-body excretion are reported. The conclusions are that the use of a PEG3 linker is non-innocent in terms of its impact on enhancing the metabolism of [89Zr]ZrDFO-PEG3-azepin-mAbs. In most cases, degradation can produce metabolites that are rapidly eliminated from the body, thereby enhancing image contrast by reducing nonspecific accumulation and retention of 89Zr in background organs such as the liver, spleen, kidney, and bone.

Microfluidic Preparation of 89Zr-Radiolabelled Proteins by Flow Photochemistry.

89Zr-radiolabelled proteins functionalised with desferrioxamine B are a cornerstone of diagnostic positron emission tomography. In the clinical setting, 89Zr-labelled proteins are produced manually. Here, we explore the potential of using a microfluidic photochemical flow reactor to prepare 89Zr-radiolabelled proteins. The light-induced functionalisation and 89Zr-radiolabelling of human serum albumin ([89Zr]ZrDFO-PEG3-Et-azepin-HSA) was achieved by flow photochemistry with a decay-corrected radiochemical yield (RCY) of 31.2 ± 1.3% (n = 3) and radiochemical purity >90%. In comparison, a manual batch photoreactor synthesis produced the same radiotracer in a decay-corrected RCY of 59.6 ± 3.6% (n = 3) with an equivalent RCP > 90%. The results indicate that photoradiolabelling in flow is a feasible platform for the automated production of protein-based 89Zr-radiotracers, but further refinement of the apparatus and optimisation of the method are required before the flow process is competitive with manual reactions.

Light-Activated Protein Conjugation and 89 Zr-Radiolabelling with Water-Soluble Desferrioxamine Derivatives.

Protein-conjugates are vital tools in biomedical research, drug discovery and imaging science. For example, functionalised monoclonal antibodies (mAbs) coupled to the desferrioxamine B (DFO) chelate and radiolabelled with 89 Zr4+ ions are used as radiopharmaceuticals for diagnostic positron emission tomography (PET). In this context, protein functionalisation requires the formation of a covalent bond that must be achieved without compromising the biological properties of the mAb. Photochemistry offers new synthetic routes toward protein conjugates like 89 Zr-mAbs but to harness the potential of light-induced conjugation reactions new photoactivatable reagents are required. Herein, we introduce two photoactivatable DFO-derivatives functionalised with an aryl azide (ArN3 ) for use in light-activated conjugation and radiosynthesis of 89 Zr-mAbs. Incorporation of a tris-polyethylene glycol (PEG)3 linker between DFO and the ArN3 group furnished water-soluble chelates that were used in the one-pot, photoradiosynthesis of different 89 Zr-radiolabelled protein conjugates with radiochemical yields up to 72.9±1.9 %. Notably, the DFO-PEG3 chelates can be readily synthesised in accordance with Good Laboratory Practice (GLP), which will facilitate clinical trials with photoradiolabelled 89 Zr-mAbs.

Photochemical Reactions in the Synthesis of Protein-Drug Conjugates.

The ability to modify biologically active molecules such as antibodies with drug molecules, fluorophores or radionuclides is crucial in drug discovery and target identification. Classic chemistry used for protein functionalisation relies almost exclusively on thermochemically mediated reactions. Our recent experiments have begun to explore the use of photochemistry to effect rapid and efficient protein functionalisation. This article introduces some of the principles and objectives of using photochemically activated reagents for protein ligation. The concept of simultaneous photoradiosynthesis of radiolabelled antibodies for use in molecular imaging is introduced as a working example. Notably, the goal of producing functionalised proteins in the absence of pre-association (non-covalent ligand-protein binding) introduces requirements that are distinct from the more regular use of photoactive groups in photoaffinity labelling. With this in mind, the chemistry of thirteen different classes of photoactivatable reagents that react through the formation of intermediate carbenes, electrophiles, dienes, or radicals, is assessed.

Expanding the Chemistry Palette for Radiotracer Synthesis.

The synthesis, characterisation and application of radiolabelled compounds for use in diagnostic and therapeutic medicine requires a diverse skill set. This article highlights a selection of our ongoing projects that aim to provide new synthetic methods and radiochemical tools for building molecular imaging agents with various radionuclides.

Methylthiazolyl Tacn Ligands for Copper Complexation and Their Bifunctional Chelating Agent Derivatives for Bioconjugation and Copper-64 Radiolabeling: An Example with Bombesin.

We present here the synthesis of two new bifunctionalized azachelators, no2th-EtBzNCS and Hno2th1tha, as bioconjugable analogues of two previously described di- and trimethylthiazolyl 1,4,7-triazacyclononane (tacn) ligands, no2th and no3th, for potential uses in copper-64 (64Cu) positron emission tomography imaging. The first one bears an isothiocyanate group on the remaining free nitrogen atom of the tacn framework, while the second one presents an additional carboxylic function on one of the three heterocyclic pendants. Their syntheses required regiospecific N-functionalization of the macrocycles. In order to investigate their suitability for in vivo applications, a complete study of their copper(II) chelation was performed. The acid-base properties of the ligands and their thermodynamic stability constants with copper(II) and zinc(II) cations were determined using potentiometric techniques. Structural studies were conducted in both solution and the solid state, consolidated by theoretical calculations. The kinetic inertness in an acidic medium of both copper(II) complexes was determined by spectrophotometry, while cyclic voltammetry experiments were performed to evaluate the stability at the copper(I) redox state. UV-vis, NMR (of the zinc complexes), electron paramagnetic resonance spectroscopy, and density functional theory studies showed excellent agreement between the solution structures of the complexes and their crystallographic data. These investigations unambiguously prove that these bifunctional derivatives display similar coordination properties as their no2th and no3th counterparts, opening the door to targeted bioapplications. The no2th-EtBzNCS and Hno2th1tha ligands were then conjugated to a bombesin antagonist peptide for targeting the gastrin-releasing peptide receptor (GRPr). To highlight the potential of the two chelators for radiopharmaceutical development, the 64Cu-radiolabeling properties, in vitro stability, and binding affinity to GRPr of the corresponding bioconjugates were determined. Altogether, the results of this work warrant the further development of 64Cu-based radiopharmaceuticals comprising our novel bifunctional chelators.

Heptadentate chelates for 89Zr-radiolabelling of monoclonal antibodies.

Herein, we report the synthesis of three new bifunctional heptadentate metal ion binding chelates derived from desferrioxamine B (DFO) linked to a tripeptide unit that comprises of a glutamic acid and two glycine residues. The three DFO derivatives were also functionalised with a photoactivatable aryl azide unit for light-triggered labelling of proteins. The chelates were obtained in 3 synthetic steps in good overall yields by using solid phase peptide synthesis (SPPS). Density Functional Theory (DFT) calculations were used to estimate thermodynamic formation constants (log ß) of the corresponding Zr4+ complexes. Quantitative zirconium-89 radiolabelling (>95%) was obtained in <5 min at room temperature, and the stability of the radioconjugates toward different competitors (human serum, EDTA and Fe3+) was assessed in vitro. One-pot 89Zr-photoradiosynthesis produced [89Zr]Zr-2-onartuzumab directly from the formulated, clinical-grade sample MetMAb™, without pre-purifying the monoclonal antibody (mAb) component, with an isolated decay-corrected radiochemical yield of 36.4 ± 2.4%. PET imaging and biodistribution studies were performed in female athymic nude mice bearing subcutaneous xenografts derived from the MKN-45 human gastric cancer cell line to assess the pharmacokinetic profile and tumour binding of [89Zr]Zr-2-onartuzumab. Specific tumour uptake of [89Zr]Zr-2-onartuzumab was confirmed by using competitive inhibition (blocking) studies and bone uptake was significantly reduced compared to the parent DFO analogue.

Photoactivatable Fluorescent Tags for Dual-Modality Positron Emission Tomography Optical Imaging.

Fluorescent protein conjugates are vital tools in a wide range of scientific disciplines from basic biochemical research to applications in clinical pathology and intraoperative surgery. We report the synthesis and characterization of photoactivatable fluorophores (PhotoTags) based on the functionalization of coumarin, fluorescein, BODIPY, rhodamine B, and cyanine dyes with a photochemically active aryl azide group. Photochemical labeling experiments using human serum albumin produced fluorescent proteins in high yields under irradiation with ultraviolet light for <15 min. We also synthesized DFO-RhodB-PEG3-ArN3─a photoactivatable compound that can be radiolabeled with 89Zr for applications in optical imaging and positron emission tomography. One-pot 89Zr-radiolabeling and light-induced protein conjugation produced [89Zr]ZrDFO-RhodB-PEG3-azepin-trastuzumab. Proof-of-concept studies in vitro and in vivo confirmed that [89Zr]ZrDFO-RhodB-PEG3-azepin-trastuzumab is a potential dual-modality agent for detecting human epidermal growth factor receptor 2 (HER2/neu) expression. Overall, the PhotoTag technology represents a rapid, synthetically versatile, and user-friendly approach for generating novel protein conjugates.

Photoactivatable bis(thiosemicarbazone) derivatives for copper-64 radiotracer synthesis.

In recent years, copper-64 and copper-67 have been considered as a useful theranostic pair in nuclear medicine, due to their favourable and complementary decay properties. As 67Cu and 64Cu are chemically identical, development of both existing and new bifunctional chelators for 64Cu imaging agents can be readily adapted for the 67Cu-radionuclide. In this study, we explored the use of photoactivatable copper chelators based on the asymmetric bis(thiosemicarbazone) scaffold, H2ATSM/en, for the photoradiolabelling of protein. Photoactivatable 64CuATSM-derivatives were prepared by both direct synthesis and transmetallation from the corresponding natZn complex. Then, irradiation with UV light in the presence of a protein of interest in a pH buffered aqueous solution afforded the 64Cu-labelled protein conjugates in decay-corrected radiochemical yield of 86.9 ± 1.0% via the transmetallation method and 35.3 ± 1.7% from the direct radiolabelling method. This study successfully demonstrates the viability of photochemically induced conjugation methods for the development of copper-based radiotracers for potential applications in diagnostic positron emission tomography (PET) imaging and targeted radionuclide therapy.

Bioconjugated chelates based on (methylpyridinyl)tacn: synthesis, 64Cu labeling and in vitro evaluation for prostate cancer targeting.

Three new bifunctional copper chelators based on the 1,4,7-triazacyclononane (tacn) platform have been synthesized and conjugated to peptides. The first one is constituted of the tacn with two methylpyridinyl and one methylthiazolyl carboxylic acid pendant arms, while, in the second and third ones, the macrocycle is functionalized by three methylpyridinyl groups, with an additional hexynoic acid chain on a carbon of one or two pyridine rings. These three bifunctional chelators have been conjugated to the antagonist DPhe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2 peptide for targeting the gastrin-releasing peptide receptor, which is overexpressed in prostate cancer. The resulting monomeric bioconjugates have shown their efficiency to be radiolabeled with ß+ emitter 64Cu, and the hydrophilicity and PC-3 cell internalization properties of these radiolabeled conjugates have been studied. PC-3 cell binding affinity of mono- and dimeric metal-free and natCu metallated conjugates have been evaluated by IC50 measurements. The results demonstrate the potential of these methylpyridinyl tacn derivatives for radiopharmaceutical applications.

Charting the Chemical and Mechanistic Scope of Light-Triggered Protein Ligation.

The creation of discrete, covalent bonds between a protein and a functional molecule like a drug, fluorophore, or radiolabeled complex is essential for making state-of-the-art tools that find applications in basic science and clinical medicine. Photochemistry offers a unique set of reactive groups that hold potential for the synthesis of protein conjugates. Previous studies have demonstrated that photoactivatable desferrioxamine B (DFO) derivatives featuring a para-substituted aryl azide (ArN3) can be used to produce viable zirconium-89-radiolabeled monoclonal antibodies (89Zr-mAbs) for applications in noninvasive diagnostic positron emission tomography (PET) imaging of cancers. Here, we report on the synthesis, 89Zr-radiochemistry, and light-triggered photoradiosynthesis of 89Zr-labeled human serum albumin (HSA) using a series of 14 different photoactivatable DFO derivatives. The photoactive groups explore a range of substituted, and isomeric ArN3 reagents, as well as derivatives of benzophenone, a para-substituted trifluoromethyl phenyl diazirine, and a tetrazole species. For the compounds studied, efficient photochemical activation occurs inside the UVA-to-visible region of the electromagnetic spectrum (∼365-450 nm) and the photochemical reactions with HSA in water were complete within 15 min under ambient conditions. Under standardized experimental conditions, photoradiosynthesis with compounds 1-14 produced the corresponding 89ZrDFO-PEG3-HSA conjugates with decay-corrected isolated radiochemical yields between 18.1 ± 1.8% and 62.3 ± 3.6%. Extensive density functional theory (DFT) calculations were used to explore the reaction mechanisms and chemoselectivity of the light-induced bimolecular conjugation of compounds 1-14 to protein. The photoactivatable DFO-derivatives operate by at least five distinct mechanisms, each producing a different type of bioconjugate bond. Overall, the experimental and computational work presented here confirms that photochemistry is a viable option for making diverse, functionalized protein conjugates.

Light-induced synthesis of protein conjugates and its application in photoradiosynthesis of 89Zr-radiolabeled monoclonal antibodies.

Efficient methods to functionalize proteins are essential for the development of many diagnostic and therapeutic compounds, such as fluorescent probes for immunohistochemistry, zirconium-89 radiolabeled mAbs (89Zr-mAbs) for positron emission tomography and antibody-drug conjugates (ADCs). This protocol describes a step-by-step procedure for the light-induced functionalization of proteins with compounds bearing the photochemically active aryl azide group. As an illustration of the potential utility of our approach, this protocol focuses on the synthesis of 89Zr-mAbs using photoactivatable derivatives of the metal ion binding chelate desferrioxamine B (DFO). The light-induced synthesis of 89Zr-mAbs is a unique, one-pot process involving simultaneous radiolabeling and protein conjugation. The photoradiochemical synthesis of purified 89Zr-mAbs, starting from unmodified proteins, [89Zr][Zr(C2O4)4]4- (89Zr-oxalate), and a photoactivatable DFO derivative, can be performed in <90 min. The method can be easily adapted to prepare other radiolabeled proteins, ADCs or fluorescently tagged proteins by using drug molecules or fluorophores functionalized with photoactive moieties.

Picolinate-appended tacn complexes for bimodal imaging: Radiolabeling, relaxivity, photophysical and electrochemical studies.

Based on our previous works involving two 1,4,7-triazacyclononane (tacn)-based ligands Hno2py1pa (1-Picolinic acid-4,7-bis(pyridin-2-ylmethyl)-1,4,7-triazacyclononane) and Hno1pa (1-Picolinic acid-1,4,7-triazacyclononane), we report here the synthesis of analogues bearing picolinate-based π-conjugated ILCT (Intra-Ligand Charge Transfer) transition antenna (HL1, HL2), using regiospecific N-functionalization of the tacn skeleton and their related transition metal complexes (e.g. Cu2+, Zn2+ and Mn2+). Coordination properties as well as their photophysical and electrochemical properties were investigated in order to quantify the impact of such antenna on the luminescent or relaxometric properties of the complexes. The spectroscopic properties of the targeted ligands and metal complexes have been studied using UV-Vis absorption and fluorescence spectrocopies. While the zinc complex formed with HL1 possesses a moderate quantum yield of 5%, complexation of Cu2+ led to an extinction of the luminescence putatively attributed to a photo-induced electron transfer, as supported by spectroscopic and electrochemical evidences. The [Mn(L2)]+ complex is characterized by a fluorescence quantum yield close to 8% in CH2Cl2. The potential interest of such systems as bimodal probes has been assessed from radiolabeling experiments conducted on HL1 and 64Cu2+ as well as confocal microscopy analyses and from relaxometric studies carried out on the cationic [Mn(L2)]+ complex. These results showed that HL1 can be used for radiolabeling, with a radiochemical conversion of 40% in 15 min at 100 °C. Finally, the relaxivity values obtained for [Mn(L2)]+, r1p = 4.80 mM-1·s-1 and r2p = 8.72 mM-1·s-1, make the Mn(II) complex an ideal candidate as a probe for Magnetic Resonance Imaging.

endo- versus exo-Cyclic coordination in copper complexes with methylthiazolylcarboxylate tacn derivatives.

Three tacn (1,4,7-triazacyclononane)-based ligands substituted by methylthiazolylcarboxylate (tha) and/or methylthiazolyl (th) arms have been examined for copper complexation with the aim to study the impact of carboxylate groups on the complexation of Cu(ii), which can present an endo- or exo-cyclic coordination. Two new ligands have been synthesised: H3no3tha, tacn bearing three methylthiazolylcarboxylate arms, and H2no1th2tha, tacn with one methylthiazolyl and two methylthiazolylcarboxylate arms, while Hno2th1tha had already been described. Their complexation behaviour with 1 or 1.5 equivalents of metal was studied on the basis of preliminary results showing the tendency of tha arms to form exocyclic polynuclear species. The solid state studies of the Cu(ii) and Zn(ii) complexes were investigated and some of their structures were characterised by X-ray diffraction. The physicochemical properties of the complexes in solution were also investigated by means of potentiometric measurements, UV-vis spectroscopy, EPR and computational studies, NMR characterisation of the corresponding Zn(ii) complexes and redox behaviour by electrochemistry. Mono- and tri-nuclear complexes ML and M3L2 were formed and isolated, highlighting the tendency of methylthiazolylcarboxylate arms, when carried by a tacn platform, to form exo-cyclic and polynuclear complexes. However, this exhaustive study evidences that the "out of cage" and "in cage" present different behaviour in terms of stability.

Final step gallium-68 radiolabelling of silica-coated iron oxide nanorods as potential PET/MR multimodal imaging agents.

The investigation of iron oxide-based positron emission tomography/magnetic resonance (PET/MR) multimodal imaging agents is an expanding field in which a variety of nanoparticle sizes, shapes, surface coatings and radioisotopes are open for exploration. This study develops iron oxide nanorods which are coated with various mixtures of poly(ethylene glycol) (PEG) and macrocyclic ligand (DO3A) via the formation of a silica layer on the surface. Gallium-68 radiolabelling of the nanorods was carried out in high radiochemical yields (RCY) and their stability in human serum was demonstrated for all constructs, even in the absence of the macrocyclic chelating unit. Further studies were carried out in an attempt to determine the appropriate amount of PEG coating to give optimal properties for future in vivo studies.