A Bioorthogonally Cleavable N-Oxide Linker for Prodrug Design: Elucidating the Structure-Cyclization Kinetics Relationships for Controlled Drug Release.

Bioorthogonal bond cleavage reactions that exhibit rapid kinetics in both the initial click and subsequent release steps are essential for in vivo applications. In this study, we focus on the bioorthogonal decaging reaction between an N-oxide and silylborane, and systematically modify the structure of an N-oxide-masked ethylenediamine linker, which undergoes cyclization to release the attached payload upon silylborane activation. Our results uncovered significant structure-cyclization kinetics relationships, leading to the identification of several novel linkers that combine favorable stability with rapid cyclization kinetics (T1/2 on the scale of minutes) following activation. Using one representative linker, we constructed a bioorthogonal prodrug of CA-4 that displayed potent antitumor activity only in combination with silylborane both in vitro and in vivo. We anticipate that these ethylenediamine linkers will have potential application in the design of prodrugs.