KZR-504

The epoxide KZR-504 appears to be a suitable chemical probe of the LMP2 subunit of the immunoproteasome. It is a selective and potent inhibitor, as assessed in the ProCISE platform, which was also used to measure pharmacology in live cells (IC₅₀=136 nM). No direct evidence was provided that confirmed covalent inhibition, although this could be inferred. The probe possesses poor permeability in MDCK as one would expect based on the physicochemistry of the molecule. An IV bolus in mouse produced the expected pharmacodynamic effect. Selectivity and potency is comparable to a previously disclosed probe called LU-001i, which is also a peptidic keto-epoxide, although the latter was not assessed in vivo.

KZR-504 is a selective inhibitor of LMP2 and, therefore, of the immunoproteasome. It has been optimized for in vivo use, which has led to some decrease in cellular permeability. Based on the published selectivity and lack of cytotoxicity, as well as the demonstrated pharmacology in mice, this is a good probe for selective inhibition of LMP2. No assessment has been made of what other targets this compound may bind to or inhibit. Given the understood pharmacology of compounds in this class, significant pharmacology from unknown other targets is a relatively low risk at appropriate doses. The ability to use this same molecule in vitro and in vivo is an advantage and a reason to recommend this molecule over other analogs.

KZR-504 is a potent and highly selective inhibitor for the LMP2 proteosome subunit with ca 50 nM IC₅₀ and >100-fold selectivity over other subunits (80-fold for mouse LMP7) and fair cell permeability and metabolic stability. The compound selectively inhibits LMP2 in mouse in all tissues except brain at 1 mg/kg (IV, 1-hr post dose). Unlike non-selective proteosome inhibitors, such as ONX-0914, KZR-504 has no effect on release of inflammatory cytokines released from human PBMCs treated with LPS.