Fluorine-18 Labeling of S100 Proteins for Small Animal Positron Emission Tomography

The interaction of S100 proteins (S100s), a multigenic family of Ca²⁺-binding and Ca²⁺-modulated proteins, with pattern recognition receptors, e.g., Toll-like receptors (TLRs), the receptor for advanced glycation end products (RAGE), or scavenger receptors (SR), is hypothesized to be of high relevance in the pathogenesis of various diseases. This includes chronic inflammatory conditions, atherosclerosis, cardiomyopathies, neurodegeneration, and progression of cancers. However, data concerning the role of circulating S100s in these pathologies are scarce. One reason for this is the shortage of suitable radiolabeling methods for direct assessment of the metabolic fate of circulating S100s in vivo. We report a radiotracer approach using radiolabeling of recombinant human S100s with the positron emitter fluorine-18 (¹⁸F) by conjugation with N-succinimidyl 4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB). The methodological radiochemical part focuses on an optimized and automated synthesis of [¹⁸F]SFB comprising HPLC purification to achieve higher chemical purity. The respective radioligands, [¹⁸F]fluorobenzoylated S100s ([¹⁸F]FB-S100s), were obtained with appropriate radiochemical purities, yields, and effective molar activities. Biological applications comprise cell and tissue binding experiments in vitro, biodistribution and metabolite studies in rodents in vivo/ex vivo, and dynamic positron emission tomography studies using dedicated small animal PET systems. Radiolabeling of S100s with ¹⁸F and, particularly, the use of small animal PET provide novel probes to delineate both their metabolic fate and the functional expression of their specific receptors under normal and pathophysiological conditions in rodent models of disease.