Eliminated without a trace: The fastest click reaction, the highly selective inverse‐electron‐demand Diels–Alder reaction, has been modified to enable selective bioorthogonal release. Thus, the click reaction of a tetrazine with a drug‐bound trans‐cyclooctene caused the instantaneous release of the drug and CO2 (see scheme). One possible application is the chemically triggered release, and thereby activation, of a drug from a tumor‐bound antibody–drug conjugate.

Mice and men: An antibody conjugate with trans‐cyclooctene was administered to tumor‐bearing mice, and the resulting chemically tagged tumors were subsequently treated with an 111In‐labeled tetrazine probe in an inverse‐electron‐demand Diels–Alder reaction. The adduct was formed in a remarkable 52–57 % yield in vivo and used for non‐invasive pretargeted tumor imaging in mice (see picture).

One of the challenges of pretargeted radioimmunotherapy, which centers on the capture of a radiolabeled probe by a preinjected tumor-bound antibody, is the potential immunogenicity of biological capturing systems. A bioorthogonal chemical approach may circumvent this drawback, but effective in vivo chemistry in mice, larger animals, and eventually humans, requires very high reagent reactivity, sufficient stability, and retained selectivity. We report here that the reactivity of the fastest bioorthogonal reaction, the inverse-electron-demand-Diels-Alder cycloaddition between a tetrazine probe and a trans-cyclooctene-tagged antibody, can be increased 10-fold (k2 = 2.7 × 10(5) M(-1) s(-1)) via the trans-cyclooctene, approaching the speed of biological interactions, while also increasing its stability. This was enabled by the finding that the trans-cyclooctene tag is probably deactivated through isomerization to the unreactive cis-cyclooctene isomer by interactions with copper-containing proteins, and that increasing the steric hindrance on the tag can impede this process. Next, we found that the higher reactivity of axial vs equatorial linked TCO can be augmented by the choice of linker. The new, stabilized, and more reactive tag allowed for improved tumor-to-nontumor ratios in pretargeted tumor-bearing mice.

The use of a bioorthogonal reaction for the selective cleavage of tumor-bound antibody-drug conjugates (ADCs) would represent a powerful new tool for ADC therapy, as it would not rely on the currently used intracellular biological activation mechanisms, thereby expanding the scope to noninternalizing cancer targets. Here we report that the recently developed inverse-electron-demand Diels-Alder pyridazine elimination reaction can provoke rapid and self-immolative release of doxorubicin from an ADC in vitro and in tumor-bearing mice.