Radiofluorination and docking studies for [18F]FEt-Tyr-urea-Glu ([18F]FEt-TUG) as a new potential PSMA ligand

Objectives For PET-imaging, various PSMA ligands have been synthesized based on glutamate-urea-amino acids moieties. We prepared a peptide, Tyr-urea-Glu, for labeling it by using [18F]FEt-Tosyl as prosthetic group. The synthesis was to be prepared for automation while docking studies were supposed to substantiate the possible application as PSMA ligand.

Methods The precursor i.e. (S)-2-[3-[(S)-1-Carboxy-2-(4-hydroxyphenyl)ethyl]-ureido]pentanedioic acid was prepared by coupling of bis(tert-butyl)-L-glutamate.HCl and L-tyrosine tert-butyl ester. Radiofluorination was performed by [18F]FEt-Tosyl produced in a module. After elution from C18-cartridge (ca. 510 MBq), it was heated at 80°C (15min) with peptide (3mg/15µL of 10M NaOH, 18eq). Then, peak of [18F]FEt-TUG (HPLC: Phenomenex Luna; 100%water/0.1%TFA to 40%MeCN/0.1%TFA at 10min to 20min; 5mL/min, Rt: 12min) was fixed on C18-cartridge, eluted by EtOH, evaporated and diluted with saline (450µL) ([18F]FEt-TUG, radiochemical purity: ≥98%, ca.100MBq). For calculating binding energies with protein, analysis was performed using Discovery Studio 3.1.

Results Efficiency of coupling between [18F]FEt-Tosyl and -OH group of peptide depended on base. With 14mg (20eq) of NaHCO3, 40±1% (15min) RCY was obtained at 90°C. Using 15µL of 10M NaOH (18eq), RCY increased to 77±0.8%. Binding affinity data of [18F]FEt-TUG (Calc. BE: 278kcal/mol) was compared to existing compounds like ZJ38, ARM-P, and DCFPyL (Fig 1). Also, docking studies showed that it filled the PSMA tunnel (70Å) with urea moiety reaching near Zn-atoms. In addition, LogD values were calculated by ACD-Freeware (Fig 1).

Conclusions An efficient method for radiosynthesis of a new PSMA ligand i.e. [18F]FEt-TUG was developed. The calculations showed similar binding affinities when compared to known PSMA-ligands indicating a promising applicability.