Immunoproteasome as a Target for Prodrugs.

Immunoproteasome (iCP) is a proteasome isoform that is expressed under inflammatory conditions such as cytokine interferon-γ exposure. The iCP has different catalytic subunits other than the standard CP (standard core particle), allowing the production of major histocompatibility complex class I (MHC-I) compatible peptides for eventual T-cell activation. We have previously reported the design of a fluorescent probe that monitors iCP activity in cells called TBZ-1, and we applied TBZ-1's iCP recognition sequence for prodrug release into iCP-active cells. Here, we demonstrate a proof-of-concept of the iCP as a prodrug release enzyme. The "payload" we utilized was a toxic moiety, doxorubicin, and a degrader for transcription factor, BRD4. Both examples show that iCP activity is required to elicit cell death or degradation of BRD4. This report highlights that the iCP is a viable prodrug target, and its activity can be used to release a variety of cargo in cells expressing the iCP.