Molecular imaging is an important scientific discipline that plays a major role in clinical medicine and pharmaceutical development. While several imaging modalities including X-ray computed tomography (CT) and magnetic resonance imaging (MRI) generate high-resolution anatomical images, positron emission tomography (PET) and single photon emission computed tomography (SPECT) offer insight into the physiological processes that occur within a living organism. Of these two nuclear medicine imaging techniques, PET has advantages with respect to sensitivity and resolution, and this has led to the production and development of many positron emitting radionuclides that include non-traditional radionuclides of the transition metals. Copper-64 (t1/2 = 12.7 h, β+: 17.4%, E β+max = 656 keV; -:39%, E β-max = 573 keV) has emerged as an important positron emitting radionuclide that has the potential for use in diagnostic imaging and radiotherapy. However, 64Cu must be delivered to the living system as a stable complex that is attached to a biological targeting molecule for effective imaging and therapy. Therefore, significant research has been devoted to the development of ligands that can stably chelate 64Cu. This review discusses the necessary characteristics of an effective 64Cu chelator, while highlighting the development and evaluation of 64Cu-complexes attached to biologically-targeted ligands.

Targeted alpha-particle therapy (TAT) aims to selectively deliver radionuclides emitting α-particles (cytotoxic payload) to tumors by chelation to monoclonal antibodies, peptides or small molecules that recognize tumor-associated antigens or cell-surface receptors. Because of the high linear energy transfer (LET) and short range of alpha (α) particles in tissue, cancer cells can be significantly damaged while causing minimal toxicity to surrounding healthy cells. Recent clinical studies have demonstrated the remarkable efficacy of TAT in the treatment of metastatic, castration-resistant prostate cancer. In this comprehensive review, we discuss the current consensus regarding the properties of the α-particle-emitting radionuclides that are potentially relevant for use in the clinic; the TAT-mediated mechanisms responsible for cell death; the different classes of targeting moieties and radiometal chelators available for TAT development; current approaches to calculating radiation dosimetry for TATs; and lead optimization via medicinal chemistry to improve the TAT radiopharmaceutical properties. We have also summarized the use of TATs in pre-clinical and clinical studies to date.

The interest in zirconium-89 (⁸⁹Zr) as a positron-emitting radionuclide has grown considerably over the last decade due to its standardized production, long half-life of 78.2 h, favorable decay characteristics for positron emission tomography (PET) imaging and its successful use in a variety of clinical and preclinical applications. However, to be utilized effectively in PET applications it must be stably bound to a targeting ligand, and the most successfully used ⁸⁹Zr chelator is desferrioxamine B (DFO), which is commercially available as the iron chelator Desferal^®. Despite the prevalence of DFO in ⁸⁹Zr-immuno-PET applications, the development of new ligands for this radiometal is an active area of research. This review focuses on recent advances in zirconium-89 chelation chemistry and will highlight the rapidly expanding ligand classes that are under investigation as DFO alternatives.

⁸⁹Zr–Tetraazamacrocycle complexes display extraordinary stability.

The theranostic potential of ²²⁵Ac-based radiopharmaceuticals continues to increase as researchers seek innovative ways to harness the nuclear decay of this radioisotope for therapeutic and imaging applications. This communication describes the evaluation of ²²⁵Ac-DOTA-c(RGDyK) in both biodistribution and Cerenkov luminescence imaging (CLI) studies. Initially, La-DOTA-c(RGDyK) was prepared as a non-radioactive surrogate to evaluate methodologies that would contribute to an optimized radiochemical synthetic strategy and estimate the radioactive conjugate's affinity for α_vβ₃, using surface plasmon resonance spectroscopy. Surface plasmon resonance spectroscopy studies revealed the IC₅₀ and K_i of La-DOTA-c(RGDyK) to be 33 ± 13 nM and 26 ± 11 nM, respectively, and suggest that the complexation of the La³⁺ ion to the conjugate did not significantly alter integrin binding. Furthermore, use of this surrogate allowed optimization of radiochemical synthesis strategies to prepare ²²⁵Ac-DOTA-c(RGDyK) with high radiochemical purity and specific activity similar to other ²²⁵Ac-based radiopharmaceuticals. This radiopharmaceutical was highly stable in vitro. In vivo biodistribution studies confirmed the radiotracer's ability to target α_vβ₃ integrin with specificity; specificity was detected in tumor-bearing animals using Cerenkov luminescence imaging. Furthermore, tumor growth control was achieved using non-toxic doses of the radiopharmaceutical in U87mg tumor-bearing nude mice. To our knowledge, this is the first report to describe the CLI of α_vβ₃⁺ tumors in live animals using the daughter products derived from ²²⁵Ac decay in situ. This concept holds promise to further enhance development of targeted alpha particle therapy.

Speciation leads to adaptive changes in organ cellular physiology and creates challenges for studying rare cell-type functions that diverge between humans and mice. Rare cystic fibrosis transmembrane conductance regulator (CFTR)-rich pulmonary ionocytes exist throughout the cartilaginous airways of humans^1,2, but limited presence and divergent biology in the proximal trachea of mice has prevented the use of traditional transgenic models to elucidate ionocyte functions in the airway. Here we describe the creation and use of conditional genetic ferret models to dissect pulmonary ionocyte biology and function by enabling ionocyte lineage tracing (FOXI1-Cre^ERT2::ROSA-TG), ionocyte ablation (FOXI1-KO) and ionocyte-specific deletion of CFTR (FOXI1-Cre^ERT2::CFTR^L/L). By comparing these models with cystic fibrosis ferrets^3,4, we demonstrate that ionocytes control airway surface liquid absorption, secretion, pH and mucus viscosity—leading to reduced airway surface liquid volume and impaired mucociliary clearance in cystic fibrosis, FOXI1-KO and FOXI1-Cre^ERT2::CFTR^L/L ferrets. These processes are regulated by CFTR-dependent ionocyte transport of Cl⁻ and HCO₃⁻. Single-cell transcriptomics and in vivo lineage tracing revealed three subtypes of pulmonary ionocytes and a FOXI1-lineage common rare cell progenitor for ionocytes, tuft cells and neuroendocrine cells during airway development. Thus, rare pulmonary ionocytes perform critical CFTR-dependent functions in the proximal airway that are hallmark features of cystic fibrosis airway disease. These studies provide a road map for using conditional genetics in the first non-rodent mammal to address gene function, cell biology and disease processes that have greater evolutionary conservation between humans and ferrets.