Efficient ^sup 18^F-Labeling of Large 37-Amino-Acid pHLIP Peptide Analogues and Their Biological Evaluation

• Published in: Bioconjugate chemistry 2012-08, Vol.23 (8), p.1557
• Main Authors: Daumar, Pierre, Wanger-Baumann, Cindy A, Pillarsetty, NagaVaraKishore, Fabrizio, Laura, Carlin, Sean D, Andreev, Oleg A, Reshetnyak, Yana K, Lewis, Jason S
• Format: Article
• Language: English
• Subjects: Amino acids; Biochemistry; Chemical reactions; Peptides; Radioactive materials; Tumors
• ISSN: 1043-1802; 1520-4812

Solid tumors often develop an acidic microenvironment, which plays a critical role in tumor progression and is associated with increased level of invasion and metastasis. The 37-residue pH (low) insertion peptide (pHLIP) is under study as an imaging platform because of its unique ability to insert into cell membranes at a low extracellular pH (pH... < 7). Labeling of peptides with [...F]-fluorine is usually performed via prosthetic groups using chemoselective coupling reactions. One of the most successful procedures involves the alkyne-azide copper(I) catalyzed cycloaddition (CuAAC). However, none of the known "click" methods have been applied to peptides as large as pHLIP. We designed a novel prosthetic group and extended the use of the CuAAC "click chemistry" for the simple and efficient 18F-labeling of large peptides. For the evaluation of this labeling approach, a D-amino acid analogue of WT-pHLIP and an L-amino acid control peptide K-pHLIP, both functionalized at the N-terminus with 6-azidohexanoic acid, were used. The novel 6-[...F]fluoro-2-ethynylpyridine prosthetic group, was obtained via nucleophilic substitution on the corresponding bromo-precursor after 10 min at 130 ...C with a radiochemical yield of 27.5 ± 6.6% (decay corrected) with high radiochemical purity ...98%. The subsequent Cu...-catalyzed "click" reaction with the azido functionalized pHLIP peptides was quantitative within 5 min at 70 ...C in a mixture of water and ethanol using Cu-acetate and sodium l-ascorbate. [...F]-d-WT-pHLIP and [...F]-l-K-pHLIP were obtained with total radiochemical yields of 5-20% after HPLC purification. The total reaction time was 85 min including formulation. In vitro stability tests revealed high stability of the [...F]-d-WT-pHLIP in human and mouse plasma after 120 min, with the parent tracer remaining intact at 65% and 85%, respectively. PET imaging and biodistribution studies in LNCaP and PC-3 xenografted mice with the [...F]-d-WT-pHLIP and the negative control [...F]-l-K-pHLIP revealed pH-dependent tumor retention. This reliable and efficient protocol promises to be useful for the ...F-labeling of large peptides such as pHLIP and will accelerate the evaluation of numerous [...F]-pHLIP analogues as potential PET tracers. (ProQuest: ... denotes formulae/symbols omitted.)