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

Abstract

A series of fluorine-substituted monomeric and dimeric cRGD peptide derivatives, such as cRGD-ADIBOT-F (ADIBOT = azadibenzocyclooctatriazole), di-cRGD-ADIBOT-F, cRGD-PEG5-ADIBOT-F, and di-cRGD-PEG5-ADIBOT-F, were prepared by strain-promoted alkyne azide cycloaddition (SPAAC) reaction of the corresponding aza-dibenzocyclooctyne (ADIBO) substituted peptides with a fluorinated azide 3. Among these cRGD derivatives, di-cRGD-PEG5-ADIBOT-F had the highest binding affinity in a competitive binding assay compared to other derivatives and even the original cRGDyk. On the basis of the in vitro study results, di-cRGD-PEG5-ADIBOT-(18)F was prepared from a SPAAC reaction with (18)F-labeled azide and subsequent chemo-orthogonal scavenger-assisted separation without high performance liquid chromatography (HPLC) purification in 92% decay-corrected radiochemical yield (dcRCY) with high specific activity for further in vivo positron emission tomography (PET) imaging study. In vivo PET imaging study and biodistribution data showed that this radiotracer allowed successful visualization of tumors with good tumor-to-background contrast and significantly higher tumor uptake compared to other major organs.

Click Chemistry and Radiochemistry: The First 10 Years

“…61 Following a similar strategy, another laboratory modified a series of α ν β 3 -targeting RGD peptides with DBCO and radiolabeled them using an [ 18 F]fluoro–PEG 4 –azide prosthetic group. 50,62 In a creative twist, the authors scavenged excess unlabeled peptide using an azide-grafted resin, allowing them to achieve specific activities of up to 62.5GBq/ μ mol. Critically, all of the 18 F-labeled peptides bore biological affinity comparable to their unlabeled cousins and were shown to be effective for the visualization α ν β 3 -expressing U87MG xenografts (Figure 3).…”

A review of approaches to 18F radiolabelling affinity peptides and proteins

“…An additional, and very important, category of click‐chemistry approaches are cycloadditions in which new σ‐bonds are formed at the expense of π‐bonds to form a cyclic compound; examples include copper catalysed and strain‐promoted azide‐alkyne cycloadditions and inverse electron demand Diels‐Alder cycloadditions …”

“…The bioorthogonality of SPAAC click‐chemistry enabled use of ambient temperature and neutral pH; quantitative conjugation of the azide‐bearing prosthetic group was reported and [ 18 F]RGD RCY of 92% (d.c.) were attained . In vivo analyses of [ 18 F]RGD, in tumour‐bearing mice, showed good tumour accumulation and tumour to background ratios and thus highlighted the preservation of the APP's biological activity …”

Responsive hybrid (poly)peptide–polymer conjugates

“…128 In another example, a difunctional cRGD peptide derivative was functionalized with an N 3 -PEG 4 -azadibenzocyclooctatriazole (ADIBOT) via SPAAC for positron emission tomography (PET); the addition of the small PEG linker increased the binding affinity to the α v β 3 receptor. 129 In terms of reactivity, ring strain as well as electron withdrawing functionality (such as fluorine substituents) can induce a rapid reaction of cyclooctynes with azide. The most important limitation of this reaction is the rate of the reaction, which is 10-to-100 times slower than a typical CuAAC reaction.…”