Indolequinone-based hypoxia-activated PROTACs (HAP-TACs) selectively degrade BRD4 in hypoxic cancer cells

Proteolysis targeting chimeras (PROTACs) have helped to establish proximity induction as an exciting strategy in drug discovery, and there are multiple clinical trials focused on this modality. However, degradation of a full protein in a physiological setting might lead to dose-limiting toxicities, giving rise to the need for PROTACs that are activated in a context dependent nature. Here we report the development of hypoxia-activated PROTACs (HAP-TACs), which are selectively activated in conditions of low oxygen (hypoxia), such as those found in solid tumors. To develop HAP-TACs, we have attached an indolequinone bioreductive group to an essential functional group of either the VHL- or cereblon-recruiting component of the PROTAC, reducing affinity for its cognate E3 ligase, and preventing protein of interest degradation. Using BRD4, we have conducted proof-of-concept studies, which demonstrate that the indolequinone group is bioreduced in hypoxic conditions releasing the active PROTAC, resulting in selective degradation of BRD4 in hypoxia. As the bioreductive group is attached to the VHL or cereblon ligand, this approach is potentially applicable to all PROTACs that recruit these commonly employed E3 ligases.