Kit-Like ¹⁸F-Labeling of Proteins: Synthesis of 4-(Di-tert-butyl[¹⁸F]fluorosilyl)benzenethiol (Si[¹⁸F]FA-SH) Labeled Rat Serum Albumin for Blood Pool Imaging with PET

Abstract

Radiosyntheses of 18F-radiopharmaceuticals for positron emission tomography (PET) normally require an extraordinarily high effort of technical equipment and specially trained personnel. We recently reported a novel method for the introduction of fluorine-18 into peptides for PET-imaging based on silicon-18F-chemistry (SiFA technique). We herewith introduce the first SiFA-based Kit-like radio-fluorination of a protein (rat serum albumin,RSA) and demonstrate its usefulness for in vivo imaging with microPET in normal rats as well as in a rat heterotropic transplanted heart model. As a labeling agent, we prepared 4-(di-tert-butyl[18F]fluorosilyl)benzenethiol (Si[18F]FASH)by simple isotopic exchange in 40-60% radiochemical yield (RCY) and coupled it directly to a Sulfo-SMCC derivatized RSA in an overall RCY of 12% within 20-30 min. The technically simple labeling procedure does not require any elaborated purification procedures and is a straightforward example of a successful application of Si-18F chemistry for in vivo imaging with PET.

In Vitro and Initial In Vivo Evaluation of 68Ga-Labeled Transferrin Receptor (TfR) Binding Peptides as Potential Carriers for Enhanced Drug Transport into TfR Expressing Cells

“…The resin for peptide synthesis (Fmoc-Pro-NovaSyn® TGT resin), Fmoc-protected amino acids and p-SCN-Bn-NOTA, were purchased from NovaBiochem (Nottingham, UK) and Macrocyclics (Dallas, USA), respectively. Thiol-DOTA (2,2′,2″-(10-(2-(2-mercaptoethylamino)-2-oxoethyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid) and the silicon fluoride acceptor (SiFA)-SH (4-(di-tert-butylfluorosilyl) benzenethiol) were synthesized according to published procedures [28,29].…”

“…2). It was lately shown that SiFAs represent a favorable strategy for simple and convenient introduction of 18 F into various biomolecules by simple isotopic exchange [29,33,34]. The peptide synthesis of THRPPMWSPVWP was carried out using standard Fmoc-solid-phase-peptide synthesis [30], and the derivatization reactions were performed by reacting the protected on-resin peptide with hexanoic acid followed by p-SCN-Bn-NOTA and maleimido hexanoic acid, respectively.…”

Synthesis of [18F]RGD-K5 by catalyzed [3+2] cycloaddition for imaging integrin αvβ3 expression in vivo

“…A disadvantage of the 68 Ga is its high maximum positron energy (1.9 MeV compared to 0.96 MeV for 18 F) limiting the spatial resolution of the images and increasing the radiation dose to the patient. Although both the SiFA and the AlF methodology tend more to be true-kit labeling procedures [29,50] compared to the click chemistry approach, the metal-binding ligands that need to be coupled to the peptide are large building blocks, which may influence the molecular integrity, binding properties and immunoreactivity of the peptide or protein. The CuAAC reaction requires less derivatization of the peptide.…”

18F-Labeled Silicon-Based Fluoride Acceptors: Potential Opportunities for Novel Positron Emitting Radiopharmaceuticals

“…Figure 4 presents various synthesized SiFA building blocks bearing reactive groups for peptide conjugation (for proteins and small molecules vide infra ) [6, 32, 41–45]. The coupling of those SiFAs to peptides prior to the IE labeling would in theory allow for a direct and mild 18 F-incorporation without subsequent HPLC purification.…”

“…As an alternative approach, Iovkova et al designed a prefunctionalization strategy involving protein derivatization with 2-iminithiolane ( 57 ) followed by the reaction with the SiFA maleimide [ 18 F] 45 for the labeling of rat serum albumin (RSA) used for blood pool PET imaging (Scheme 9) [45]. The derivatization strategy was also applied with success to RSA labeling with [ 18 F]SiFA-SH ([ 18 F] 38 ) [6]. Protein functionalization with sulfo-SMCC ( 55 ) followed by treatment with [ 18 F]SiFA-SH obtained by IE allowed for the isolation of [ 18 F]SiFA-RSA in overall 12% RCY within 20–30 minutes.…”