The HSP90-dependent bioorthogonal PROTAC prodrug system enables tumor-selective and enhanced protein degradation

Proteolysis targeting chimeras (PROTACs) possess significant therapeutic potential; however, they encounter challenges related to a lack of tumor specificity and normal-tissue toxicity. We developed a heat shock protein 90 (HSP90)-dependent bioorthogonal PROTAC prodrug system (HBPROTAC) to achieve tumor-specific target proteolysis. The HBPROTAC system consists of two components: (1) Tz-PU, a tetrazine-conjugated HSP90 inhibitor designed for tumor-selective accumulation, and (2) TCO-caged PROTAC prodrugs (TCO-MZ1 or TCO-DT2216), which release active PROTACs (MZ1 or DT2216) through inverse electron demand Diels-Alder (IEDDA) reactions with Tz-PU. We demonstrated that HBPROTAC exhibited tumor-specific activation and degradation of BRD4 and Bcl-xL. Moreover, inhibition by Tz-PU synergistically enhanced the degradation efficiency of the target proteins through HSP90-mediated signaling. The tumor-specific and enhanced degradation character of HBPROTAC was confirmed in various tumor cell lines and the melanoma mouse model, demonstrating that this strategy establishes a broadly applicable platform for tumor-specific spatiotemporal control of targeted protein degradation and diminished off-tissue on-target toxicity.