One-pot and one-step automated radio-synthesis of [18F]AlF-FAPI-74 using a multi purpose synthesizer: a proof-of-concept experiment

Abstract

Background: Fibroblast activation protein (FAP) is overexpressed in the stroma of many types of cancer. [18F]AlF-FAPI-74 is a positron emission tomography tracer with high selectivity for FAP, which has already shown high accumulation within human tumors in clinical studies. However, [18F]AlF-FAPI-74 radiosynthesis has not been optimized using an automated synthesizer. Herein, we report a one-pot and one-step automated radiosynthesis method using a multi purpose synthesizer.

Results: Radiosynthesis of [18F]AlF-FAPI-74 was performed using a cassette-type multi purpose synthesizer CFN-MPS200. After the recovery rate of trapped [18F]fluoride onto the anion-exchange cartridge using a small amount of eluent was investigated manually, a dedicated [18F]AlF-FAPI-74 synthesis cassette and synthesis program for one-pot and one-step fluorination was developed. The solutions for the formulation of [18F]AlF-FAPI-74 synthesized using this were evaluated to obtain stable radiochemical purity. The recovery rate of [18F]fluoride with only 300 µL of eluent ranged 90 ± 9% by introduction from the male side and elution from the female side of the cartridge. In automated synthesis, the eluted [18F]fluoride and precursor solution containing aluminum chloride were mixed; then, fluorination was performed in a one-pot and one-step process at room temperature for 5 min, followed by 15 min at 95 °C. As a result, the radioactivity of [18F]AlF-FAPI-74 was 11.3 ± 1.1 GBq at the end of synthesis from 32 to 40 GBq of [18F]fluoride, and its radiochemical yield was 37 ± 4% (n = 10). The radiochemical purity at the end of the synthesis was ≥ 97% for all formulation solutions. When the diluent was saline, the radiochemical purity markedly decreased after 4 h of synthesis. In contrast, with phosphate-buffered saline (pH 7.4) or 10 mM phosphate-buffered saline (pH 6.7) containing 100 mg of sodium ascorbate, the radiochemical purity was stable at 97%. Non-radioactive AlF-FAPI-74 and total impurities, including non-radioactive AlF-FAPI-74, were 0.3 ± 0.1 µg/mL and 2.8 ± 0.6 µg/mL. Ethanol concentration and residual DMSO were 5.5 ± 0.2% and 21 ± 6 ppm, respectively.

Conclusions: We established a one-pot one-step automated synthesis method using a CFN-MPS200 synthesizer that provided high radioactivity and stable radiochemical purity for possible clinical applications.

Keywords: Automated synthesis; Fibroblast activation protein; Positron emission tomography; [18F]AlF-FAPI-74.

Conflict of interest statement

The authors declare no potential conflicts of interest relevant to this article. FLGs are inventors and have patent rights.

© 2021. The Author(s).

Figures

Fig. 1

Structures of the FAPI-74 precursor and the reference standard of AlF-FAPI-74. a The precursor was bound 1,4,7-triazacyclononane-N, N′,N′-triacetic acid (NOTA) as a chelating agent. b The reference standard of AlF-FAPI-74

Fig. 2

Systematic diagram of [18F]AlF-FAPI-74 with cupid system

Fig. 3

Reaction schema of [18F]AlF-FAPI-74 with one-pot and one-step radio-synthesis

Fig. 4

HPLC chromatograms of [18F]AlF-FAPI-74 with 10 mM phosphate buffered saline (pH 6.7) containing 100 mg sodium ascorbate as a formulation solution at the end of synthesis (EOS). a The typical radio-chromatogram of [18F]AlF-FAPI-74 solution. A similar chromatogram was shown in other formulation solutions and its radiochemical purity was ≥ 97%. b The typical ultraviolet (UV)-chromatogram (λ = 264 nm) of [18F]AlF-FAPI-74 solution. In the chromatogram, retention time (RT) 6.050 min was judged to be AlF-FAPI-74 based on the RT obtained from the AlF-FAPI-74 reference standard (c) and there is a time difference of approximately 0.3 min with the radioactivity detector. RT 6.519 min was judged to be FAPI-74 precursor based on prior analysis, and the remaining two peaks (RT 5.408 min and 5.878 min) were unknown impurity peaks. c The typical UV-chromatogram (λ = 264 nm) of AlF-FAPI-74 reference standard of 10 µg/mL (RT 6.039 min)

Fig. 5

TLC chromatograms of [18F]AlF-FAPI-74 with 10 mM phosphate buffered saline (pH 6.7) containing 100 mg sodium ascorbate as a formulation solution. [18F]fluoride or [18F]AlF peak is appeared near the origin (Rf = 0) and Rf of [18F]AlF-FAPI-74 is approximately 0.6–0.7