No abstract

High-energy radiation has been utilized for decades, however, the role of low-energy electrons created during irradiation has only recently begun to be appreciated Nuclear decay is one of the most extreme processes and is central to a range of fields including energy, medicine, imaging, labelling, archaeology and sensing. Radiation in the form of alpha particles, beta particles and gamma rays have fundamentally different interactions with matter and therefore exhibit different mean-free paths (∼1 μm, 1 mm and 1 cm, respectively). These forms of primary radiation deposit their energy over the course of their trajectory by ionizing their surroundings and producing non-thermal secondary electrons. Only very recently has the ability of low-energy secondary electrons to induce chemical reactions and biological damage begun to be appreciated 1 , because they have energies below the typical ionization threshold of organic matter. For example, low-energy electrons (3-20 eV) have been shown to be effective at causing DNA cleavage 2,4,7 . This ability stems from their high cross-section for breaking chemical bonds, and as a consequence they have a very short meanfree path of ∼1-10 nm in solution 8,9 . Furthermore, hot electrons that are not captured by surrounding molecules become thermalized as solvated electrons which are known to be chemically and biological active [9][10][11][12] . To harness these unique properties, the design of radioactive materials that increase and localize the flux of short-range lowenergy electrons to target sites is crucial for their application in targeted cancer therapies that minimize damage to healthy cells. Thus far, it has not been possible to design atomically precise radioactive materials that maximize these effects due to self-destruction arising from nuclear recoil, Coulomb explosion and self-irradiation [13][14][15][16] . We report a straightforward method for synthesizing monolayer films of radioactive 125 I atoms on gold-coated mica substrates under ambient conditions, and characterize their composition and their electron emission. Despite being synthesized from radioactive 125 I (> 99.9% purity) they are robust with respect to self-destruction, and provide well-defined, intense planar sources of secondary electrons. 125I decays by electron capture (EC) of a core shell electron to produce a nuclear excited state of 125 Te (Figure 1a), the majority of which eject another core

Stromal cell-derived factor (SDF) 1 is a potent chemoattractant for leukocytes through activation of the receptor CXCR4/Fusin/LESTR, which is a fusion co-factor for the entry of T lymphocytotropic human immunodeficiency virus type 1 (HIV-1). This CXCR4-mediated HIV-1 fusion can be inhibited by SDF-1. Because of its importance in the study of immunity and AIDS, large scale production of SDF-1 is desirable. In addition to recombinant technology, chemical synthesis provides means by which biologically active proteins can be produced not only in large quantity but also with a variety of designed modifications. In this study, we investigated the binding and function of an SDF-1␣ analogue, N33A, synthesized by a newly developed native chemical ligation approach. Radioiodinated N33A showed high affinity binding to human monocytes, T lymphocytes, as well as neutrophils, and competed equally well with native recombinant SDF-1␣ for binding sites on leukocytes. N33A also showed equally potent chemoattractant activity as native recombinant SDF-1␣ for human leukocytes. Further study with CXCR4/Fusin/LESTR transfected HEK 293 cells showed that N33A binds and induces directional migration of these cells in vitro. These results demonstrate that the chemically synthesized SDF-1␣ analogue, N33A, which can be produced rapidly in large quantity, possesses the same capacity as native SDF-1␣ to activate CXCR4-expressing cells and will provide a valuable agent for research on the host immune response and AIDS.

The ADP-activated P2Y1 receptor is broadly expressed and plays a crucial role in ADP-promoted platelet aggregation. We previously synthesized 2-iodo-N6-methyl–(N)-methanocarba-2′-deoxyadenosine 3′,5′-bisphosphate (MRS2500), as a selective, high affinity, competitive antagonist of this receptor. Here we report utilization of a trimethylstannyl precursor molecule for the multistep radiochemical synthesis of a [125I]-labeled form of MRS2500. [125I]MRS2500 bound selectively to Sf9 insect cell membranes expressing the human P2Y1 receptor but did not specifically bind to membranes isolated from empty vector-infected cells. Binding of [125I]MRS2500 to P2Y1 receptors was saturable with a Kd of 1.2 nM. Known agonists and antagonists of the P2Y1 receptor inhibited [125I]MRS2500 binding to P2Y1 receptor-expressing membranes with potencies in agreement with those previously observed in functional assays of this receptor. A high-affinity binding site for [125I]MRS2500 also was observed on intact human platelets (Kd = 0.61 nM) and mouse platelets (Kd = 1.20 nM) that exhibited the pharmacological selectivity of the P2Y1 receptor. The densities of sites observed were 151 sites/platelet and 229 sites/platelet in human and mouse platelets, respectively. In contrast, specific binding was not observed in platelets isolated from P2Y1 receptor (−/−) mice. Taken together, these data illustrate the synthesis and characterization of a novel P2Y1 receptor radioligand and its utility for examining P2Y1 receptors natively expressed on human and mouse platelets.

A recently reported selective agonist of the human A3 adenosine receptor (hA3AR), MRS5127, (1′R,2′R,3′S,4′R,5′S)-4′-[2-chloro-6-(3-iodobenzylamino)-purine]-2′,3′-O-dihydroxy-bicyclo-[3.1.0]hexane, was radioiodinated and characterized pharmacologically. It contains a rigid bicyclic ring system in place of a 5′-truncated ribose moiety, and was selected for radiolabeling due to its nanomolar binding affinity at both human and rat A3ARs. The radioiodination of the N6-3-iodobenzyl substituent by iododestannylation of a 3- (trimethylstannyl)benzyl precursor was achieved in 73% yield, measured after purification by HPLC. [125I]MRS5127 bound to the human A3AR expressed in membranes of stably transfected HEK 293 cells. Specific binding was saturable, competitive, and followed a one-site binding model, with a Kd value of 5.74 ± 0.97 nM. At a concentration equivalent to its Kd, non-specific binding comprised 27±2% of total binding. In kinetic studies, [125I]MRS5127 rapidly associated with the hA3AR (t1/2 = 0.514 ± 0.014 min), and the affinity calculated from association and dissociation rate constants was 3.50 ± 1.46 nM. The pharmacological profile of ligands in competition experiments with [125I]MRS5127 was consistent with the known structure-activity-relationship profile of the hA3AR. [125I]MRS5127 bound with similar high affinity (Kd, nM) to recombinant A3ARs from mouse (4.90 ± 0.77), rabbit (2.53 ± 0.11), and dog (3.35 ± 0.54). For all of the species tested, MRS5127 exhibited A3AR agonist activity based on negative coupling to cAMP production. Thus, [125I]MRS5127 represents a new species-independent agonist radioligand for the A3AR. The major advantage of [125I]MRS5127 compared with previously used A3AR radioligands is its high affinity, low degree of non-specific binding, and improved A3AR selectivity.