F-18 Labeling Protocol of Peptides Based on Chemically Orthogonal Strain-Promoted Cycloaddition under Physiologically Friendly Reaction Conditions

Abstract

We introduce the high-throughput synthesis of various (18)F-labeled peptide tracers by a straightforward (18)F-labeling protocol based on a chemo-orthogonal strain-promoted alkyne azide cycloaddition (SPAAC) using aza-dibenzocyclootyne-substituted peptides as precursors with (18)F-azide synthon to develop peptide based positron emission tomography (PET) molecular imaging probes. The SPAAC reaction and subsequent chemo-orthogonal purification reaction with azide resin proceeded quickly and selectively under physiologically friendly reaction conditions (i.e., toxic chemical reagents-free, aqueous medium, room temperature, and pH ≈7), and provided four (18)F-labeled tumor targetable bioactive peptides such as cyclic Arg-Gly-Asp (cRGD) peptide, bombesin (BBN), c-Met binding peptide (cMBP), and apoptosis targeting peptide (ApoPep) in high radiochemical yields as direct injectable solutions without any HPLC purification and/or formulation processes. In vitro binding assay and in vivo PET molecular imaging study using the (18)F-labeled cRGD peptide also demonstrated a successful application of our (18)F-labeling protocol.

Click Reaction: An Applicable Radiolabeling Method for Molecular Imaging

“…In addition, they optimized the SPAAC reaction with various cyclooctynes and proposed that these reactions represent an attractive strategy for application-specific tuning in pretargeting systems ( The Kim group developed a labeling protocol based on the SPAAC reaction followed by a chemo-orthogonal purification reaction with an azide-bound resin [36]. This process provides a rapid and selective isolation method without the need for HPLC purification and formulation (Table 1, entry 7).…”

18F-Labeling Using Click Cycloadditions

“…In 2011, Arumugam et al [9] investigated the direct 18 F-labeling of azadibenzocyclooctyne (DBCO) yielding the 18 F-labeled prosthetic group (RCY = 36%). The radiolabeling was followed by a click reaction with an azido -octreotide leading to the 18 F-labeled octreotide in a RCY of 95% within a total reaction time of 1.5 h. In contrast, other working groups used 18 F-cyclooctynes for labeling RDG-derivatives [11] as well as further integrin-specific peptides [10, 13]. …”

F-18 Labeled RGD Probes Based on Bioorthogonal Strain-Promoted Click Reaction for PET Imaging

“…Specific activity is one of the most important factors for determining PET imaging quality and the toxicity or side-effects of these radiotracers. 3,4,26 In this regard, the development of a more reliable F-18 labeling protocol remains an interesting research topic for the preparation of F-18 labeled RGD peptide-based probes. Scheme 1.…”

“…Moreover, in that radiosynthetic method, 18 F-labeled peptides were efficiently purified by solid phase extraction using an azide-substituted resin as an ADIBO precursor-scavenger without an HPLC purification step. 26 This paper introduces the straightforward synthesis of fluorine (including F-18) substituted monomeric and dimeric RGD peptides based on the SPAAC conjugation reaction, and reports the results of in vitro and in vivo evaluation of the PET imaging of integrin expression. Scheme 1 presents the synthesis of non-radioactive fluorinesubstituted monomeric and dimeric RGD peptide derivatives based on the SPAAC conjugation reaction.…”