In Vivo Positron Emission Tomography (PET) Imaging with an α_vβ₆ Specific Peptide Radiolabeled using ¹⁸F-“Click” Chemistry: Evaluation and Comparison with the Corresponding 4-[¹⁸F]Fluorobenzoyl- and 2-[¹⁸F]Fluoropropionyl-Peptides

Abstract

Numerous radiolabeled peptides have been utilized for in vivo imaging of a variety of cell-surface receptors. For applications in PET using [ 18 F]fluorine, peptides are radiolabeled via a prosthetic group approach. We previously developed solution-phase 18 F-"click" radiolabeling and solid-phase radiolabeling using 4-[ 18 F]fluorobenzoic and 2-[ 18 F]fluoropropionic acids. Here we compare the 3 different radiolabeling approaches and report the effects on PET imaging and pharmacokinetics. The prosthetic groups did have an influence; metabolites with significantly different polarities were observed.Non-invasive PET a imaging has become a widely used tool for the detection of many diseases. 1 Among the available positron emitting nuclides [ 18 F]fluorine is particularly widely used because it can be produced on demand in medical cyclotrons and combines favorable decaycharacteristics (T 1/2 = 110 min, mode of decay: 97% β + , maximum β + energy = 0.64 MeV) with relative chemical versatility. 2 As new disease-specific imaging targets (e.g. cell surface receptors) are being identified, there is an increased demand for targeted radiotracers. 3 Peptides are receiving much attention for in vivo cancer detection because excellent, tissue-specific uptake can be achieved. Relying on well-established synthetic chemistries, peptides are readily produced and modified. 3 Strategies include automated syntheses with incorporation of unnatural amino acids, peptidomimetics, and cyclization, among others, to develop compounds with desirable pharmacokinetic properties. To make peptides amendable for PET imaging the [ 18 F]fluorine-radiolabel is introduced using small molecules (prosthetic groups). Examples of [ 18 F]-labeled peptides for PET imaging include octreotide, 4 vasoactive intestinal peptide, 5 integrin specific peptides, 6-8 N ε -(γ-glutamyl)lysine, 9 neurotensin analogs, 10 human Cpeptide, 11 and insulin. 12The prosthetic group approach involves at least two synthetic steps: Incorporation of [ 18 F] fluorine into the prosthetic group, and attachment to the peptide. Generally, the prosthetic group itself should not negatively affect receptor binding, and the synthetic approach should be applicable to many different peptide substrates with minimal synthetic modifications. For this, Recently, our group successfully used the copper-catalyzed Huisgen 1,3-dipolar cycloaddition ("click" chemistry) 20, 21 to conjugate ω-[ 18 F]fluoroalkynes to peptides functionalized with 3-azidopropionic acid (Table 1 , Scheme 1). 22 The formation of 1,4-disubstituted 1,2,3-triazoles proceeded smoothly under mild conditions and the radiolabeled peptides were obtained in a short period of time. Subsequently, this approach has been applied for radiolabeling of different substrates with [ 18 F]fluorine. 10, 23-25 As a result of ongoing improvements in 1,3-dipolar cycloaddition-chemistry and because of its versatility and short reaction times "click" radiochemistry promises to become a widely used tool for preparation of radiotracers.Here we...

Characterization and Evaluation of 64Cu-Labeled A20FMDV2 Conjugates for Imaging the Integrin αvβ6

“…It has previously been demonstrated that the addition of bulky chelators or prosthetic groups can affect the binding, internalization, and metabolism of receptor targeting peptides [25–28]. The in vitro cell binding and internalization assays revealed that all four radiotracers show binding directed towards the integrin α v β 6 with fairly comparable internalization levels, indicating that the addition of a chelator and the radiolabeling with 64 Cu did not drastically affect the binding potential of the PEG 28 -A20FMDV2 peptide.…”

Solid-phase synthesis and fluorine-18 radiolabeling of cycloRGDyK

“…[31,32] Additionally, fluoride capturing reactions based on peptide modifications allow for stable peptide-Si- 18 F, peptide-B- 18 F, and peptide-chelator-Al- 18 F bonds to be generated rapidly and efficiently. [23,33–38] Collectively, these studies indicate a correlation between the prosthetic group and the biodistribution of the radiotracer, [39,40] highlighting the importance of the ability to test different prosthetic groups on a radiolabeled peptide.…”

Evaluation of [64Cu]Cu-DOTA and [64Cu]Cu-CB-TE2A Chelates for Targeted Positron Emission Tomography with an αvβ6-Specific Peptide

“…For the present study, solid-phase peptide synthesis and radiolabeling of the 18 F and 64 Cu tracers followed robust protocols, 15,21,25 yielding the radiotracers in high radiochemical purity (≥ 99%).…”