Stimuli‐Responsive Prodrug Linkers That Simultaneously Release Cargo and Neutralize In Situ Generated (Aza)Quinone Methides

Self-immolative linkers that use p-amino/hydroxy-benzyloxycarbonyl (PABC/PHBC) spacers are essential to the mechanism of many prodrugs. However, a highly reactive (aza)quinone methide is generated as a potential toxic byproduct. To remove the methide as it forms, we synthesized a series of novel tripartite prodrugs, comprising different triggers (nitro, amide, azide, boronate) and a PABC/PHBC-type self-immolative spacer with an integrated nucleophile (amine). Upon reductive, hydrolytic, or oxidative-trigger activation, the release of the cargo is facilitated via a 1,6-elimination that generates a reactive (aza)quinone methide. With the built-in nucleophile, the (aza)quinone methide is rapidly self-quenched to generate tetrahydroisoquinolines (THIQs). One of the selected THIQs does not exhibit an anti-proliferative effect on the A431 mammalian tumor cell line. The new prodrug strategy has broad scope, enabling the use of a trigger that matches the targeted stimulus, while allowing for a diverse range of drug/cargo attachment. This proof-of-concept study adds a new linker strategy that quenches the electrophilic (aza)quinone methide generated in many self-immolative linker systems and could find applications in prodrug and antibody-drug conjugate strategies, or as a linker for probes in chemical biology.