Doubly Strapped Zwitterionic NIR‐I and NIR‐II Heptamethine Cyanine Dyes for Bioconjugation and Fluorescence Imaging

Heptamethine cyanine dyes enable deep tissue fluorescence imaging in the near infrared (NIR) window. Small molecule conjugates of the benchmark dye ZW800‐1 have been tested in humans. However, long‐term imaging protocols using ZW800‐1 conjugates are limited by their instability, primarily because the chemically labile C4′‐O‐aryl linker is susceptible to cleavage by biological nucleophiles. Here, we report a modular synthetic method that produces novel doubly strapped zwitterionic heptamethine cyanine dyes, including a structural analogue of ZW800‐1, with greatly enhanced dye stability. NIR‐I and NIR‐II versions of these doubly strapped dyes can be conjugated to proteins, including monoclonal antibodies, without causing undesired fluorophore degradation or dye stacking on the protein surface. The fluorescent antibody conjugates show excellent tumor‐targeting specificity in a xenograft mouse tumor model. The enhanced stability provided by doubly strapped molecular design will enable new classes of in vivo NIR fluorescence imaging experiments with possible translation to humans. A versatile and efficient synthetic method produces conjugatable sterically shielded heptamethine cyanine dyes for in vivo NIR‐I or NIR‐II fluorescence imaging. The fluorochromes are enclosed by two flanking straps which enhance fluorescence brightness and chemical stability, prevent dye stacking on the surface of an antibody, and enable high performance fluorescence imaging of a mouse tumor.