Synthesis of fluorine-18 radio-labeled serum albumins for PET blood pool imaging

Abstract

We sought to develop a practical, reproducible and clinically translatable method of radiolabeling serum albumins with fluorine-18 for use as a PET blood pool imaging agent in animals and man. Fluorine-18 radiolabeled fluoronicotinic acid-2,3,5,6-tetrafluorophenyl ester, [18F]F-Py-TFP was prepared first by the reaction of its quaternary ammonium triflate precursor with [18F]tetrabutylammonium fluoride ([18F]TBAF) according to a previously published method for peptides, with minor modifications. The incubation of [18F]F-Py-TFP with rat serum albumin (RSA) in phosphate buffer (pH 9) for 15 min at 37–40 °C produced fluorine-18-radiolabeled RSA and the product was purified using a mini-PD MiniTrap G-25 column. The overall radiochemical yield of the reaction was 18–35% (n = 30, uncorrected) in a 90-min synthesis. This procedure, repeated with human serum albumin (HSA), yielded similar results. Fluorine-18-radiolabeled RSA demonstrated prolonged blood retention (biological half-life of 4.8 hours) in healthy awake rats. The distribution of major organ radioactivity remained relatively unchanged during the 4 hour observation periods either by direct tissue counting or by dynamic PET whole-body imaging except for a gradual accumulation of labeled metabolic products in the bladder. This manual method for synthesizing radiolabeled serum albumins uses fluorine-18, a widely available PET radionuclide, and natural protein available in both pure and recombinant forms which could be scaled up for widespread clinical applications. These preclinical biodistribution and PET imaging results indicate that [18F]RSA is an effective blood pool imaging agent in rats and might, as [18F]HSA, prove similarly useful as a clinical imaging agent.

Facile room temperature synthesis of fluorine-18 labeled fluoronicotinic acid-2,3,5,6-tetrafluorophenyl ester without azeotropic drying of fluorine-18

“…[27, 28] Fluorine-18 labeling was achieved on the sep-pak (scheme 1). Specifically, fluorine-18 containing target water from cyclotron was passed through an anion exchange cartridge (PS-HCO 3 ) and washed with 3 mL anhydrous acetonitrile.…”

“…The precursor N,N,N-trimethyl-5-((2,3,5,6-tetrafluorophenoxy)carbonyl)pyridin-2-aminium fluoromethanesulfonate ( 1 ) and cold standard fluoronicotinic acid-2,3,5,6-tetrafluorophenyl ester were prepared according to a previously described method. [27, 28] Fluorine-18 was purchased from National Institutes of Health cyclotron facility (Bethesda, MD, USA). Chromafix 30-PS-HCO 3 anion-exchange cartridge was purchased from Macherey-Nagel (Düren, Germany).…”

“…This was first reported by Olberg et al[27] Since then it has been used by our and other group to radiolabel protein and peptides. [28, 29] However, the precursor was not stable in K 222 /K 2 CO 3 . This issue was overcame by using less basic TBA-HCO 3 but due to the limited amount of base used in this radiolabeling there is a significant amount of loss of radioactivity.…”

Fast indirect fluorine-18 labeling of protein/peptide using the useful 6-fluoronicotinic acid-2,3,5,6-tetrafluorophenyl prosthetic group: A method comparable to direct fluorination

“…All other chemicals and solvents were received from Sigma‐Aldrich (St. Louis, MO, USA) and used without further purification. The precursor N,N,N‐trimethyl‐5‐((2,3,5,6‐tetrafluorophenoxy)carbonyl)pyridin‐2‐aminium fluoromethanesulfonate ( 1 ) and cold standard 6‐fluoronicotinic acid‐2,3,5,6‐tetrafluorophenyl ester were prepared as previously described . Fluorine‐18 was obtained from National Institutes of Health cyclotron facility (Bethesda, MD, USA).…”

“…Moreover, the stability of tetrafluorophenyl ester is better at higher pH. This prosthetic group has been used by us and others to radiolabel peptides/proteins . Recently, we reported a simple preparation of 6‐[ 18 F]fluoronicotinic acid‐2,3,5,6‐tetrafluorophenyl ester on an anion‐exchange cartridge (Chromafix 30‐PS‐HCO 3 ) without azeotropic drying of fluorine‐18.…”