Site-Specific 68Ga Radiolabeling of Trastuzumab Fab via Methionine for ImmunoPET Imaging.

Bioconjugates of antibodies and their derivatives radiolabeled with ß+-emitting radionuclides can be utilized for diagnostic PET imaging. Site-specific attachment of radioactive cargo to antibody delivery vectors provides homogeneous, well-defined immunoconjugates. Recent studies have demonstrated the utility of oxaziridine chemistry for site-specific labeling of methionine residues. Herein, we applied this approach to site-specifically radiolabel trastuzumab-derived Fab immunoconjugates with 68Ga, which can be used for in vivo PET imaging of HER2-positive breast cancer tumors. Initially, a reactive azide was introduced to a single solvent-accessible methionine residue in both the wild-type Fab and an engineered derivative containing methionine residue M74, utilizing the principles of oxaziridine chemistry. Subsequently, these conjugates were functionalized with a modified DFO chelator incorporating dibenzocyclooctyne. The resulting DFO-WT and DFO-M74 conjugates were radiolabeled with generator-produced [68Ga]Ga3+, to yield the novel PET radiotracers, [68Ga]Ga-DFO-WT and [68Ga]Ga-DFO-M74. In vitro and in vivo studies demonstrated that [68Ga]Ga-DFO-M74 exhibited a higher affinity for HER2 receptors. Biodistribution studies in mice bearing orthotopic HER2-positive breast tumors revealed a higher uptake of [68Ga]Ga-DFO-M74 in the tumor tissue, accompanied by rapid renal clearance, enabling clear delineation of tumors using PET imaging. Conversely, [68Ga]Ga-DFO-WT exhibited lower uptake and inferior image contrast compared to [68Ga]Ga-DFO-M74. Overall, the results demonstrate that the highly facile methionine-oxaziridine modification approach can be simply applied to the synthesis of stable and site-specifically modified radiolabeled antibody-chelator conjugates with favorable pharmacokinetics for PET imaging.

To chelate thallium(I) - synthesis and evaluation of Kryptofix-based chelators for 201Tl.

While best known for its toxic properties, thallium has also been explored for applications in nuclear diagnostics and medicine. Indeed, [201Tl]TlCl has been used extensively for nuclear imaging in the past before it was superceded by other radionuclides such as 99mTc. One reason for this loss of interest is the severe lack of suitable organic chelators able to effectively coordinate ionic forms of Tl and deliver it to specific diseased tissue by means of attached biological vectors. Herein, we describe the synthesis and characterisation of a series of Kryptofix 222-based chelators that can be radiolabelled with 201Tl(I) in high radiochemical yields at ambient temperature. We demonstrate that from these simple chelators, targeted derivatives are readily accessible and describe the synthesis and preliminary biological evaluation of a PSMA-targeted 201Tl-labelled Kryptofix 222-peptide conjugate. While the Kryptofix system is demonstrably capable of binding the thallium cation, no PSMA-mediated cell-uptake could be detected with the PSMA conjugate, suggesting that this targeting moiety may not be ideal for use in conjunction with 201Tl.

Synthesis and ex vivo biological evaluation of gallium-68 labelled NODAGA chelates assessing cardiac uptake and retention.

Radiolabelled lipophilic cations can be used to non-invasively report on mitochondrial dysfunction in diseases such as cardiovascular disease, cardiotoxicity and cancer. Several such lipophilic cations are currently used clinically to map myocardial perfusion using SPECT imaging. Since PET offers significant advantages over SPECT in terms of sensitivity, resolution and the capacity for dynamic imaging to allow pharmacokinetic modelling, we have synthesised and radiolabelled a series of NODAGA-based radiotracers, with triarylphosphonium-functionalisation, with gallium-68 to develop PET-compatible cationic complexes. To evaluate their capacity to report upon mitochondrial membrane potential, we assessed their pharmacokinetic profiles in isolated perfused rat hearts before and after mitochondrial depolarisation with the ionophore CCCP. All three tracers radiolabel with over 96% RCY, with log D7.4 values above -0.4 observed for the most lipophilic example of this family of radiotracers. The candidate tracer [68Ga]Ga4c exhibited non-preferential uptake in healthy cardiac tissue over CCCP-infused cardiac tissue. While this approach does show promise, the lipophilicity of this family of probes needs improving in order for them to be effective cardiac imaging agents.

Cyanoferrocenes as redox-active metalloligands for coordination-driven self-assembly.

Ferrocene-based Lewis bases have found utility as metalloligands in a wide variety of applications. The coordination chemistry of cyanoferrocenes however, is underexplored. Herein, we describe a new synthetic protocol for the generation of cyanoferrocenes. The coordination chemistry of these metalloligands to [Cu(NCMe)4][PF6], [(PPh3)2Cu(NCMe)2][PF6] and [(dppf)Cu(NCMe)2][PF6] salts has been explored, providing crystallographic evidence of cluster and polymeric forms of 1,1'- and 1,2-dicyanoferrocene complexes. The stability of the complexes and ligand dissociation were found to be strongly solvent-dependent.