In vitro activity: P5091 is a trisubstituted thiophene with dichlorophenylthio, nitro, and acetyl substituents mediating anti-USP7 activity. P5091 exhibits potent, specific, and selective deubiquitylating activity against USP7. In contrast, P5091 does not inhibit other DUBs or other families of cysteine proteases tested (EC50> 100 mM). P5091 inhibits the labeling of USP7 with HA-UbVME in a concentration-dependent manner. USP7-mediated cleavage of high molecular weight polyubiquitin chains is inhibited in a dose-dependent manner by P5091. Moreover, P5091 inhibits USP7- but not USP2- or USP8-mediated cleavage of poly K48-linked ubiquitin chains. USP7 inhibition by P5091 induces HDM2 polyubiquitylation and accelerates degradation of HDM2. P5091 inhibits USP7 deubiquitylating activity, without blocking proteasome activity in MM Cells. P5091 inhibits growth in MM cells and overcomes bortezomib-resistance. P5091 induces a dose-dependent decrease in viability of various MM cell lines, including those that are resistant to conventional therapies dexamethasone (Dex) (MM.1R), doxorubicin (Dox-40), or melphalan (LR5) (IC50 range 6–14 μM). P5091 overcomes bone marrow stromal cell-induced growth of MM Cells. P5091 decreased HDM2 and HDMX, as well as upregulated p53 and p21 levels. Overall, P5091-induced cytotoxicity is mediated in part via HDM2-p21 signaling axis and although p53 is upregulated in response to P5091 treatment, the cytotoxic activity of P5091 is not dependent on p53.

Kinase Assay: Recombinant enzymes in 20 mM Tris-HCl (pH 8.0), 2 mM CaCl2, and 2 mM β-mercaptoethanol are incubated with dose ranges of P005091 for 30 min in a 96-well plate before the addition of Ub-PLA2 and NBD C6-HPC or Ub-EKL and EKL substrate. The liberation of a fluorescent product within the linear range of the assay is monitored using a Perkin Elmer Envision fluorescence plate reader. Vehicle (2% [v/v] DMSO) and 10 mM N-ethylmaleimide (NEM) are included as controls.

Cell Assay: P005091 exhibits growth inhibition with GI50 value of 1.82 μM in HL-60(TB) cell line and exhibits broad growth inhibition. In HCT-116 cells, P005091 shows cytotoxic activity with EC50 value of 9.21 μM.