Chemoselective hydrazone formation between HYNIC-functionalized peptides and (18)F-fluorinated aldehydes.
BACKGROUND
Since the demand for (18)F-fluorinated peptides for quantitative in vivo receptor imaging using PET has increased, a new chemoselective two-step (18)F-labeling strategy based on hydrazone formation between an unprotected hydrazine-functionalized peptide and an (18)F-labeled aldehyde was developed.


METHODS
First, 4-[(18)F]fluorobenzaldehyde ([(18)F]FB-CHO) was prepared from 4-formyl-N,N,N-trimethylanilinium triflate via direct no-carrier-added (18)F-fluorination (dimethyl sulfoxide, 90 degrees C, 5 min) and purified by RP-HPLC. Hydrazone formation between [(18)F]FB-CHO and 6-hydrazinonicotinic acid (HYNIC) and the unprotected HYNIC-functionalized peptides (HYNIC-d-Phe(1))-Tyr(3)-Thr(8)-octreotide and (HYNIC-Arg(1))-substance P was evaluated with respect to the dependence of radiochemical yield on pH, precursor concentration and temperature. The stability of [(18)F]FB-CH=N-HYNIC-Tyr(3)-Thr(8)(NH(2))-octreotide in aqueous solution at various pH (4.0, 5.5 and 7.5) as well as the in vivo stability of [(18)F]FB-CH=N-HYNIC-Tyr(3)-Thr(8)-octreotide in mouse blood (30 min p.i.) was investigated.


RESULTS
Yields of the hydrazone formation were independent of pH between pH 0.5 and 5.5. Optimal labeling yields of 85% were obtained with a precursor concentration of 2.1 mM at 70 degrees C for 10 min. The labeling products were stable at pH 7.5 at 37 degrees C, while in more acidic media (pH 4.0) the product slowly decomposed to form up to 31+/-2% [(18)F]FB-CHO within 5 h. Metabolite studies showed no detectable degradation of [(18)F]FB-CH=N-HYNIC-Tyr(3)-Thr(8)-octreotide in mouse blood (30 min p.i.).


CONCLUSIONS
In conclusion, chemoselective hydrazone formation between unprotected HYNIC-functionalized peptides and [(18)F]FB-CHO is a fast and straightforward radiolabeling method leading to high yields under mild acidic conditions. In addition, it represents a powerful and versatile radiolabeling strategy that is applicable to a variety of radionuclides and peptide precursors already available for (99m)Tc labeling.
Bruus-Jensen
Kjerstin
K
Department of Nuclear Medicine, Klinikum rechts der Isar, Technischen Universität München, D-81675 Munich, Germany.
Poethko
Thorsten
T
Schottelius
Margret
M
Hauser
Andrea
A
Schwaiger
Markus
M
Wester
Hans-Jürgen
HJ
D000447:Aldehydes / Q000097:blood / Q000737:chemistry; D000818:Animals; D005462:Fluorine Radioisotopes / Q000097:blood* / Q000737:chemistry*; D006834:Hydrazines / Q000097:blood* / Q000737:chemistry*; D006835:Hydrazones / Q000097:blood / Q000737:chemistry*; D007553:Isotope Labeling / Q000379:methods; D008657:Metabolic Clearance Rate; D051379:Mice; D009539:Nicotinic Acids / Q000097:blood* / Q000737:chemistry*; D010455:Peptides / Q000097:blood* / Q000737:chemistry*; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics
C120392:6-hydrazinopyridine-3-carboxylic acid; D000447:Aldehydes; D005462:Fluorine Radioisotopes; D006834:Hydrazines; D006835:Hydrazones; D009539:Nicotinic Acids; D010455:Peptides; D019275:Radiopharmaceuticals