Tetrazine–trans-cyclooctene click reaction (Tetrazine–TCO click reaction)

The Tetrazine–trans-cyclooctene (TCO) click reaction is a highly efficient bioorthogonal chemical process used in drug delivery and molecular imaging (Blackman et al., 2008, J. Am. Chem. Soc.). It is based on the inverse electron-demand Diels-Alder (IEDDA) reaction, which occurs rapidly under physiological conditions without the need for a catalyst (Oliveira et al., 2017, Chem. Soc. Rev.). In therapeutic applications, such as the Click Activated Protodrugs Against Cancer (CAPAC) platform, a TCO-modified biopolymer is localized at a tumor site, followed by the administration of a tetrazine-linked protodrug (Wu et al., 2021, Chemical Science). When the two components meet, the click reaction triggers the release of the active drug specifically at the disease site, minimizing systemic toxicity (Shasqi, 2023). This technology is also widely used for pre-targeted PET imaging and radioimmunotherapy to improve the signal-to-noise ratio by decoupling the targeting agent's long circulation time from the short half-life of the effector molecule (Rossin et al., 2010, Angew. Chem. Int. Ed.). While not a biological target in the traditional sense (e.g., a receptor or enzyme), it serves as a critical chemical platform for the activation of therapeutics in vivo.

Other names

Inverse electron-demand Diels-Alder reactionIEDDA reactionBioorthogonal click chemistryTetrazine ligationIEDDA cycloaddition

The reaction proceeds via an inverse electron-demand Diels-Alder (IEDDA) [4+2] cycloaddition between an electron-deficient 1,2,4,5-tetrazine and a strained trans-cyclooctene (TCO), followed by a spontaneous retro-Diels-Alder reaction that eliminates nitrogen gas (N2) to yield a stable covalent dihydropyridazine or pyridazine linkage (Blackman et al., 2008, J. Am. Chem. Soc.).