Fluoride-18 radiolabeling of peptides bearing an aminooxy functional group to a prosthetic ligand via an oxime bond

Abstract

We have developed a novel F-18 prosthetic ligand named fluoro-PEG-benzaldehyde (FPBA) 1. [(18)F]-FPBA 1 is formed in situ from its radiolabeled precursor [(18)F]6. Compound 6 is efficiently synthesized in four steps starting from commercially available 6-bromo-3-pyridine carbaldehyde 2. [(18)F]-FPBA was evaluated as a prosthetic ligand to radiolabel three cyclic peptides bearing an aminooxy functional group at the N-terminus position. Acetal [(18)F]6 is purified by either solid-phase extraction (SPE) or reverse-phase HPLC with the overall radiochemical yields (RCY) and radiochemical purity (RCP) in very close agreement. The SPE purification process has the advantage of shorter reaction times (71-87 min for entire reaction sequence), while the use of the reverse-phase HPLC purification process allows the use of up to fifty times less of the expensive synthetic peptides (~ 50 nmol) in the oxime coupling reaction. We have demonstrated an efficient methodology in the production of [(18)F]-FPBA 1 and demonstrated its use as a prosthetic ligand for the labeling of peptides possessing an aminooxy functional group.

Figure 1

Novel prosthetic ligand [¹⁸F]-fluoro-PEG-benzaldehyde ([¹⁸F]-FPBA) 1.

Figure 2

Peptides 7, 8, and 9. AOAC-Tyr-Asp-Cys³-Hyp-Tyr(3-Cl)-Gly-Leu-Cys⁸-Tyr-Ile-OH, disulfide bond: 3 → 8 (7); AOAC-Arg-Hyp-Cys³-Asp-Tyr(3-Cl)-Tyr(3-Cl)-Gly-Thr-Cys⁸-Phe-Asp-OH, disulfide bond: 3 → 8 (8); AOAC-Leu-Hyp-Cys³-Asp-Tyr(3-Cl)-Tyr(3-Cl)-Gly-Thr-Cys⁹-Leu-Asp-OH, disulfide bond: 3 → 9 (9).

Scheme 1

Synthetic pathway for the preparation of ligand 6. Reagents and conditions: (i) ethylene glycol, p-TsOH, benzene, reflux (98 %); (ii) triethylene glycol, Cs₂CO₃, DMF, microwave, 150 °C, 50 min (61 %); (iii) p-TsCl, Et₃N, DMAP, CH₂Cl₂ (82 %); (iv) TBAF.H₂O, THF, reflux (57 %).

Scheme 2

General coupling between ligand 6 and peptides 7, 8, and 9. Reagents and conditions: (i) peptide 7, 8, or 9, TFA, EtOH/H₂O, 70 °C, 30 min.

Scheme 3

Radiolabeling of ligand [¹⁸F]6. Reagents and conditions: (i) K[222]¹⁸F, K₂CO₃, ACN, 110 °C, 10 min (RCY = 71 ± 2 %; RCP = 99 ± 1 %; n = 3, for SPE purification).

Scheme 4

General coupling between ligand [¹⁸F]6 and peptides [¹⁸F]7-ox, [¹⁸F]8-ox, and [¹⁸F]9-ox. Reagents and conditions: (i) TFA, EtOH/H₂O, 70 °C, 2 min, then peptide 7, 8, or 9, EtOH/H₂O, 70 °C, 30 min.