Guest Edited Collection: Radioisotopes and radiochemistry in health science

Radioisotopes can be produced artificially from stable nuclei through the interaction with particles or highly energetic photons. In combination with modern detection and counting techniques, radioisotopes and radiochemical methods uniquely contribute to the health sciences. This Collection showcases salient aspects of medical radioisotope science ranging from the production, recovery and purification of radioisotopes to the methods used to attach them to biomolecules. The Collection also presents studies that highlight the importance of radiochemistry in the assessment of environmental radioactivity.

Tc (half-life 2.1·10 5 y) to mention here-and dangerous levels of radioactivity.
Decades later there was the "Chernobyl disaster" 8,9 . Still a high schooler in Cold-War era West Germany, I vividly recall the news streaming in that stoked the populace's fears of radioactivity like no other event before. And the event on April 26 th , 1986, made us Europeans think twice about what produce to eat and to better ask where it came from. Children were barred from entering playgrounds; the time span "eight days", the half-life of volatile fission product 131 I, kept circulating in people's heads. The "Chernobyl" and "Fukushima" 10 disasters are certainly entries in the history annals of nuclear science and technology that bear witness to the blight of radioactivity.
The blessings of radioactivity, on the other hand, became apparent from the work of another radiochemist and Nobel Prize laureate, George de Hevesy. Hevesy pioneered the concept of radioactive tracers to study biological processes in vivo 11 , and he is widely regarded as the "father of modern nuclear medicine" 12 . Other sources 13 name the Donner Laboratory at the University of California, Berkley, the "birthplace of nuclear medicine" and rather ascribe nuclear fatherhood to John Lawrence. The discovery of several well-known medical isotopes was claimed by the Donner Laboratory, including 14 C, 18 May 1946 15 . Ironically, the later "Chernobyl scare" 131 I had become the first U.S. Food and Drug Administration approved radiopharmaceutical in 1951 16 . Seaborg -along with Emilio Segré -also isolated metastable 99m Tc (half-life 6.0 h) for the first time 17 . This isotope has become an invaluable tool for single photon computer tomography (SPECT). The last radioisotopes to mention in my almost chiastic introductory narrative are alpha emitters 223 Ra and 224 Ra 18 (half-lives 11.4 d and 3.7 d), thereby taking the reader's attention back to the element that Madame Curie discovered. One of them, 223 Ra, has recently become a U.S. FDA approved pharmaceutical for the treatment of bone metastases.

Collection Overview
As the 'Radioisotopes and radiochemistry in health science' Collection launches, applications of short-lived positron emitter fluorine-18 (half-life 109.8 min) naturally find the strongest representation. Certainly due to fluorine's ability to rapidly form extremely stable F-C bonds via nucleophilic or electrophilic fluorination of organic molecules, 18 F is undisputedly the most widely used positron emission tomography (PET) labelling agent. Advances in 18 F labelling chemistry also shed light on the more fundamental chemistry of this most reactive halogen. For instance, a new route to [ 18 F]fluoroform has been demonstrated 19 , where the trifluoromethyl synthon is formed in the gas phase by passing [ 18 F]fluoromethane over heated CoF 3 , one of the few binary phases that release elemental fluorine (F 2 ) upon heating. The introduction of new 18 F-labelled triazolyl-linked argininamides to target neuropeptides for the imaging of mammary carcinoma 20 serves as an apt example of click chemistry-based 18 F-labelling, and a Design of Experiments (DoE) approach has been reported to optimize copper-mediated radiofluorination 21 . The evaluation of new 18 F-labelled melanoma xenograft targeting peptides is the subject of an imaging study 22 that is also published as part of this Collection.
While the lightest halogen has become the workhorse of PET imaging, the heaviest (natural) halogen, astatine, has been advancing as a therapy agent. This Collection includes reports on optimized 209 Bi(α,2n) cyclotron production and recovery of targeted alpha therapy (TAT) isotope 211 At (7.2 h) 23,24 and an investigation into astatine's solvent extraction behaviour 25 . Radioiodine still plays an important role; beyond historic 131 I, other iodine isotopes 26 are now utilized for SPECT imaging ( 123 I, 13.3 h) and treatment (Auger emitter 125 I, 59.4 d).
Radiometals have their deserved place in the toolbox of nuclear medicine: cobalt ( 57 Co, 271.8 d; as a surrogate for 55 Co, 17.5 h) and indium ( 111 In, 2.8 d) were used as labels for affibody monomers targeting tumours in BxPC-3 xenografted mice 27 . A further contribution broaches the application of nanospheres labelled with 89 Zr (78.4 h) 28 , a positron emitter with a half-life roughly matching the in vivo circulation of antibodies.
Lanthanides are represented, as many of their easily accessible chemically similar -and thus interchangeableradioisotopes constitute versatile agents for diagnostics and therapy. The purification of SPECT isotope 155 Tb (5.3 d) is featured 29 , and a method for the production and recovery of the theranostic pair 132,135 La (4.8 h and 19.5 h) with initial imaging and biodistribution evaluation is reported 30 . Alpha emitter 225 Ac (10.0 d) has emerged as a TAT isotope: one contribution looks into the in-vivo redistribution of 225 Ac daughter isotopes in a mouse model 31 ; another study reports the purification of accelerator produced 225 Ac using a silicotitanate sorbent 32 .
Surveys into the distribution and detection of naturally occurring radioactivity are an important part of the Collection as well, as they demonstrate how radioisotope science helps assess radiation doses from natural radioactivity; the "isodose" concept to Swedish residential buildings was applied to optimize topsoil removal concepts 33 .
As the Collection is still open for submissions on a rolling basis, this is only the beginning! May the interested colleague find in this "living" Collection a one-stop overview of the current research that puts radioactivity and radiochemistry to work for the sake of human health.
Published: xx xx xxxx