Tc-99m und deuterium markierte gehirn-affine radiodiagnostika - vergleich von HMPAO mit cytectren-derivaten cyclischer amine

Kinetic isotope effects and synthetic strategies for deuterated carbon-11 and fluorine-18 labelled PET radiopharmaceuticals

The deuterium labelling of pharmaceuticals is a useful strategy for altering pharmacokinetic properties, particularly for improving metabolic resistance. The pharmacological effects of such metabolites are often assumed to be negligible during standard drug discovery and are factored in later at the clinical phases of development, where the risks and benefits of the treatment and side-effects can be wholly assessed. This paradigm does not translate to the discovery of radiopharmaceuticals, however, as the confounding effects of radiometabolites can inevitably show in preliminary positron emission tomography (PET) scans and thus complicate interpretation. Consequently, the formation of radiometabolites is crucial to take into consideration, compared to non-radioactive metabolites, and the application of deuterium labelling is a particularly attractive approach to minimise radiometabolite formation. Herein, we provide a comprehensive overview of the deuterated carbon-11 and fluorine-18 radiopharmaceuticals employed in PET imaging experiments. Specifically, we explore six categories of deuterated radiopharmaceuticals used to investigate the activities of monoamine oxygenase (MAO), choline, translocator protein (TSPO), vesicular monoamine transporter 2 (VMAT2), neurotransmission and the diagnosis of Alzheimer's disease; from which we derive four prominent deuteration strategies giving rise to a kinetic isotope effect (KIE) for reducing the rate of metabolism. Synthetic approaches for over thirty of these deuterated radiopharmaceuticals are discussed from the perspective of deuterium and radioisotope incorporation, alongside an evaluation of the deuterium labelling and radiolabelling efficacies across these independent studies. Clinical and manufacturing implications are also discussed to provide a more comprehensive overview of how deuterated radiopharmaceuticals may be introduced to routine practice.

Expanding the Cyclopentadienyl Framework: 99mTc/Re Complexes with Orthogonal Functions for Bioconjugation

A series of bifunctional cyclopentadienes of the type 1,3-EtOCO-HCp-linker-NH₂ were synthesized. In this series, the linker length (distance between the amine functionalities and the cyclopentadiene) has been systematically varied (CH₂)_n (n = 1-3). The corresponding Re complexes [(η⁵-C₅H₃RR')Re(CO)₃] (R = -COOEt, R' = -linker-NH₂) were synthesized and structurally characterized. They exhibit extraordinary stability toward water and air. All bifunctional cyclopentadienes have been labeled with the [^99mTc(CO)₃]⁺ moiety. Whereas the reactions with ethylene and propylene linked cyclopentadiene under mild reaction conditions led to the products in high radiochemical purity (>96%) without applying further purification protocols, harsher reaction conditions were required for the synthesis of the methylene-linked cyclopentadiene compound. Masking the amine in the methylene-linked cyclopentadiene by an amide bond bypasses this problem. The very hydrophilic characters of these complexes were assessed by K_OW analysis. The reported cyclopentadienes and their complexes offer a robust and versatile platform for (radio)metal incorporation into biologically active lead structures.