PLX51107 is a potent and selective BET inhibitor with Kds of 1.6, 2.1, 1.7, and 5 nM for BRD2-BD1, BRD3-BD1, BRD4-BD1, and BRDT-BD1, respectively.

BRDT BD2:120 nM (Kd), BRD2 BD2:5.9 nM (Kd), BRD3 BD2:6.2 nM (Kd), BRD2 BD1:1.6 nM (Kd), BRD4 BD2:6.1 nM (Kd), BRD4 BD1:1.7 nM (Kd), BRD3 BD1:2.1 nM (Kd), BRDT BD1:5 nM (Kd)

PLX51107 is a BET inhibitor, with Kds of 1.6, 2.1, 1.7, and 5 nM for BD1 and 5.9, 6.2, 6.1, and 120 nM for BD2 of BRD2, BRD3, BRD4, and BRDT, respectively. PLX51107 also interacts with the bromodomains of CBP and EP300 (Kd, in the 100 nM range). PLX51107 (0.156-10 μM) suppresses the CpG-induced proliferation of primary chronic lymphocytic leukemia (CLL) cells. PLX51107 also causes accumulation of p21 and IκBα, reduces c-MYC level, and modulates proapoptotic and antiapoptotic proteins.

PLX51107 (2 mg/kg, p.o.) inhibits splenomegaly by 75% in the Ba/F3 (murine IL3-dependent pro-B-cell line) splenomegaly mouse model, with the similar effect of 25 mg/kg OTX015. PLX51107 (20 mg/kg, q.d, p.o.) exhibits potent antileukemic effects in disease models of aggressive CLL and Richter transformation (RT) via oral administration once daily.

For engraftment studies, C57BL/6 WT mice are engrafted with 1E7 cells by tail-vein injection of splenocytes derived from Eμ-TCL1 or Eμ-Myc/TCL1 mice with active disease. At the onset of leukemia (Eμ-TCL1: ≥ 10% CD19/CD5/CD45-positive circulating cells; Eμ-Myc/TCL1: WBC count ≥ 8 and/or ≥ 5% CD19/CD5/CD45-positive circulating cells), mice are randomized to receive treatments as indicated. PLX51107 20 mg/kg, qd (once daily), oral gavage. Vehicle = 10% N-methyl-2-pyrrolidone plus diluent (40% PEG400, 5% TPGS, 5% Poloxamer 407, and 50% water). Mice are sacrificed when meeting early removal criteria (>20% weight loss, impaired motility, splenomegaly, and evident tumor masses), and tissues are collected for further analysis.