Technetium and Rhenium in Chemistry and Their Advanced Applications

Dear Colleagues,

Imaging and targeted therapy with radionuclides/radioactive probes continues to gain consensus in patient management, and to play an increasingly relevant role in precision medicine, where individuals can benefit from personalized treatments.

Despite the enhanced general emphasis on positron emission tomography (PET) technology, single photon emission computed tomography (SPECT) continues to be the first-line imaging modality in nuclear medicine. This is because SPECT imaging mostly relies on Technetium-99m (^99mTc), which, owing to its ideal physical decay properties, convenient availability, and rich and versatile coordination chemistry, still remains the most widely used radionuclide in clinical practice.

To add further value to this element is the existence of the radioactive rehenium-188 (¹⁸⁸Re) congener that, for its nuclear features, has been shown to be a very attractive candidate for endoradiotherapy application. The remarkable chemical similarities between technetium and rhenium, due to their position in the periodic table, result, for stable complexes characterized by equal molecular environments, in a similar biological behavior. This renders ^99mTc and ¹⁸⁸Re ideal for the development of a matching theranostic pair combining diagnosis and therapy.

However, looking at the ^99mTc/¹⁸⁸Re-radiopharmaceuticals scenario, it is evident that, although ^99mTc is the radionuclide of choice for SPECT, and a number of efficient approaches based on the Tc/Re-chemistry are available for the design and development of new target-specific radiopharmaceuticals, in the past two decades only a few new SPECT tracers have been approved by the FDA (Food Drug Administration) and/or the EMA (European Medicines Agency), and only a few re-labeled therapeutic agents have been investigated in early phase clinical trials for the management of primary tumors, bone metastasis and rheumatoid arthritis, pointing out a deadlock on the development of such classes of radiopharmaceuticals. Nevertheless, this fact can also be viewed as an incredible challenge and opportunity for researchers.

Over the last few years, the interruption of the global supply chain of reactor-produced ⁹⁹Mo needed for the production of ⁹⁹Mo/^99mTc generators has somehow brought out the key role of ^99mTc in clinical practice, forcing the scientific community to investigate alternative production routes for this important radionuclide. On the other hand, the recent advances in SPECT technology, which allow for rapid imaging acquisition, higher spatial resolution and sensitivity as compared to conventional anger-cameras, are gradually reducing the gap between SPECT and PET, also significantly changing the perspective of ^99mTc-housing radiopharmaceuticals and promoting the rediscovery and new imaging applications of this element after years of stagnation. In this situation, to fully exploit the potential of these new technologies, new categories of technetium complexes might be essential for opening a new era for this element.

This Special Issue is dedicated to all aspects of technetium and rhenium chemistry, and to the application of the corresponding radiometal-based compounds in radiopharmaceuticals applications as diagnostic, therapeutic or theranostic agents.

It is a pleasure to invite you to submit a manuscript to this Special Issue; articles and communications, as well as reviews, are all welcome.

Dr. Cristina Bolzati
Dr. Debora Carpanese
Dr. Laura Melendez-Alafort
Guest Editors

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