| 研究実績の概要 |
Radiolabeled receptor-binding peptides/proteins represent a class of promising positron emission tomography (PET) tracers. However, labeling unmodified peptides/proteins with 18F for PET imaging is a formidable challenge. In 2017, Toste and Chang et al. reported the innovative strategy for chemoselective methionine bioconjugation through redox reactivity, using oxaziridine-based reagents to achieve highly selective, rapid, and robust methionine labelling under a range of biocompatible reaction conditions (Science, 2017, 355, 597-602). Inspired by this study, I and my coworkers successfully applied this reaction to label the thio-ether-tethered peptides (Bioconjug Chem. 2018, 29, 2904-2908). Based on these achievements, I proposed here a strategy for chemoselective radio-methionine bioconjugation.
I firstly explored the synthetic routs of oxaziridine-based 18F-reagents. I successfully obtained several kinds of oxaziridine compounds, and checked their reactivities with methionine-containing peptides. Moreover, I have examined the 18F labeling efficiency though two strategies, one is “18F labeling before S-conjugation” and another is “S-conjugation before 18F labeling”. The total yields of the two-step reactions of each strategy were compared. As a result, we found the former strategy a better option for 18F labeling. Then I synthesized several model peptides, and tested the feasibility and sequence tolerability of the strategy. Besides, I also synthesized several Met-containing bioactive peptides. Currently, labeling of these peptides is underway.
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
2: おおむね順調に進展している
理由
The purpose of this project is to develop a convenient 18F labeling method for methionine-containing moieties, and examine the feasibility of this method in developing peptide/protein PET tracers. The rate-determining step of this project is considered to be the screening of labeling conditions. Focusing on this goal, I have firstly prepared several kinds of oxaziridine analogues and tested their reactivity with Met. Furthermore, I have examined the impact of the order of “18F labeling” and “S-conjugation” in the whole synthetic efficiency. The results suggested that the “conjugation before labeling” strategy shows advantages on total radiochemical yield and synthetic time than “labeling before conjugation” strategy. Based on this finding, I have further investigated the feasibility of the method on labeling Met-containing model peptides by manual. The reaction kinetics, the stability of the S=N centers, and the purification methods were all carefully examined. Taken together, I have demonstrated that Met-oriented 18F-labeling based on oxaziridine chemistry is a viable and shows comparable capability with previously reported methods in peptide 18F labeling. This important stage-progress ensures further research on the value of this method for PET tracer development. Except the above works, I have also prepared several bioactive peptides containing Met, which will be used to test for Met-18F labeling and in vivo PET imaging in the next fiscal year.
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| 今後の研究の推進方策 |
Based on the current achievement, the future work will be conducted on the following aspects. First, examine the stability of the 18F-labeling peptides in PBS and mouse serum. Second, study the selectivity of the labeling reaction when the peptides present different reactive functional groups, such as thiol or carboxyl groups. Third, try to label with 18F on several basic proteins, such as BSA and integrin. Fourth, study the in vivo pharmacokinetics and distribution of several selected bioactive peptides in experimental mice by using microPET scanner. A CD-133-targeting peptide presenting an inherent Met residue in the sequence will be labeled with our method and compared it with that using the classical [18F]AlF labeling method.
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