The inverse-electron-demand Diels-Alder cycloaddition between trans-cyclooctenes and tetrazines is biocompatible and exceptionally fast. We utilized this chemistry for site-specific fluorescence labeling of proteins on the cell surface and inside living mammalian cells by a two-step protocol. E. coli lipoic acid ligase site-specifically ligates a trans-cyclooctene derivative onto a protein of interest in the first step, followed by chemoselective derivatization with a tetrazinefluorophore conjugate in the second step. On the cell surface, this labeling was fluorogenic and highly sensitive. Inside the cell, we achieved specific labeling of cytoskeletal proteins with green and red fluorophores. By incorporating the Diels-Alder cycloaddition, we have broadened the panel of fluorophores that can be targeted by lipoic acid ligase.

Discussed herein is the development and advancement of trans-cyclooctene as a tool for facilitating bioorthogonal labeling through reactions with s-tetrazines. While a number of strained alkenes have been shown to combine with tetrazines for applications in bioorthogonal labeling, trans-cyclooctene enables fastest reactivity at low concentration with rate constants in excess of k2 = 106 M−1 s−1. In the present article, we describe advances in computation and synthesis that have enabled applications in chemical biology and nuclear medicine.

Labeling biomolecules with 18F is usually done through coupling with prosthetic groups, which generally requires several time-consuming radiosynthetic steps resulting in low labeling yield. Recently, the tetrazine-trans-cyclooctene ligation has been introduced as a method of bioconjugation that proceeds with fast reaction rates without need for catalysis. Herein, we report the development of an extremely fast and efficient method for generating 18F labeled probes based on the tetrazine-trans-cyclooctene ligation. Starting with only 30 μg (78 μM) of a tetrazine-RGD conjugate and 2 mCi (5 μM) of 18F-trans-cyclooctene, the 18F labeled RGD peptide could be obtained in more than 90% yield within five minutes. The 18F labeled RGD peptide demonstrated prominent tumor uptake in vivo. The receptor specificity was confirmed by blocking experiments. These results successfully demonstrate that the tetrazine-trans-cyclooctene ligation serves as an efficient labeling method for PET probe construction.

The fast kinetics and bioorthogonal nature of the tetrazine trans-cyclooctene (TCO) ligation makes it a unique tool for PET probe construction. In this study, we report the development of an 18F-labeling system based on a CF3-substituted diphenyl-s-tetrazine derivative with the aim of maintaining high reactivity while increasing in vivo stability. c(RGDyK) was tagged by a CF3-substituted diphenyl-s-tetrazine derivative via EDC-mediated coupling. The resulting tetrazine-RGD conjugate was combined with a 19F-labeled TCO derivative to give HPLC standards. The analogous 18F-labeled TCO derivative was combined with the diphenyl-s-tetrazine-RGD at μM concentration. The resulting tracer was subjected to in vivo metabolic stability assessment, and microPET studies in murine U87MG xenograft models. The diphenyl-s-tetrazine-RGD combines with an 18F-labeled TCO in high yields (>97% decay-corrected on the basis of TCO) using only 4 equiv of tetrazine-RGD relative to the 18F-labeled TCO (concentration calculated based on product’s specific activity). The radiochemical purity of the 18F-RGD peptides was >95% and the specific activity was 111 GBq/μmol. Noninvasive microPET experiments demonstrated that 18F-RGD had integrin-specific tumor uptake in subcutaneous U87MG glioma. In vivo metabolic stability of 18F-RGD in blood, urine and major organs showed two major peaks: one corresponded to the Diels-Alder conjugate and the other was identified as the aromatized analog. A CF3-substituted diphenyl-s-tetrazine displays excellent speed and efficiency in 18F-PET probe construction, providing nearly quantitative 18F labeling within minutes at low micromolar concentrations. The resulting conjugates display improved in vivo metabolic stability relative to our previously described system.

18F PET has a number of attributes that make it clinically attractive, including nearly 100% positron efficiency, very high specific radioactivity, and short half-life of ~110 min. However, the short half-life of 18F and the poor nucleophilicity of fluoride introduce challenges for the incorporation of 18F into complex molecules. Recently, the tetrazine-trans-cyclooctene ligation has been introduced as a novel 18F labeling method that proceeds with fast reaction rates without catalysis. Herein, we report an efficient method for 18F-labeling of free cysteines of peptides and proteins based on sequential ligation with a bifunctional tetrazinyl-maleimide and an 18F-labeled trans-cyclooctene. The newly developed method was tested for site specific labeling of both c(RGDyC) peptide and VEGF-SH protein. Starting with 4 mCi of 18F-trans-cyclooctene and only 10 μg of tetrazine-RGD (80–100 μM) or 15 μg of tetrazine-VEGF (6.0 μM), 18F labeled RGD peptide and VEGF protein could be obtained within five minutes in 95% yield and 75% yield, respectively. The obtained tracers were then evaluated in mice. In conclusion, a highly efficient method has been developed for site-specific 18F labeling of cysteine containing peptides and proteins. The special characteristics of the tetrazine-trans-cyclooctene ligation provide unprecedented opportunities to synthesize 18F-labeled probes with high specific activity for PET applications.