What is the clinical significance:
WK500B could be a promising therapeutic drug candidate for DLBCL.
Introduction
Diffuse large B cell lymphoma (DLBCL) which arises from the germinal centre (GC) and exhibits highly aggressive and rapid progression is the most common form of non-Hodgkin lymphoma (NHL) (Basso & Dalla-Favera, 2015; Mlynarczyk, Fontan & Melnick, 2019; Teras, DeSantis, Cerhan, Morton, Jemal & Flowers, 2016). Approximately 60%-70% of DLBCL patients can be cured by using a first-line therapy regimen that consists of the combination of a CD20 monoclonal antibody with cyclophosphamide, hydroxyrubicin, oncovin and prednisone (R-CHOP) (Coiffier & Sarkozy, 2016). Despite major advances in the treatment of DLBCL, up to 50% of the patients will relapse or become refractory to this treatment and die of their disease (Coiffier & Sarkozy, 2016; Crump et al., 2017). The development of chimeric antigen receptor (CAR)-T cell therapy may be a boon for relapsed/refractory patients, however, not all patients respond to CAR-T therapy, and only approximately 50% of those progress within 12 months (Neelapu et al., 2017). Novel therapies that improve the outcome for DLBCL remain imperative.
The BTB/POZ transcriptional repressor BCL6, is a master regulator essential for the GC reaction, which is necessary for humoral immunity and has emerged as a critical therapeutic target for DLBCL (Cerchietti & Melnick, 2013; Leeman-Neill & Bhagat, 2018; Wagner, Ahearne & Ko Ferrigno, 2011). BCL6 contains six C2H2 zinc finger domain at its C-terminus, which interacts with DNA sequences (Ye et al., 1993). The N-terminal BTB-POZ domain mediates dimerization and recruits the corepressors silencing mediator of retinoid and thyroid receptor (SMRT), nuclear receptor corepressor (N-CoR) and BCL6 corepressor (B-CoR) to a specific groove motif (Ahmad et al., 2003; Dhordain et al., 1997; Huynh, Fischle, Verdin & Bardwell, 2000), which is required for the repression of target genes involved in DNA damage responses and proliferation checkpoints (Ci et al., 2009; Phan & Dalla-Favera, 2004; Ranuncolo et al., 2007; Ranuncolo, Polo & Melnick, 2008). The middle portion, RD2, interacts with the corepressor MTA3 and represses genes involved in plasma cell differentiation (Tunyaplin, Shaffer, Angelin-Duclos, Yu, Staudt & Calame, 2004). Dysregulation of BCL6 expression associated with promoter translocations or point mutations has been shown to play a central role in DLBCLs (Leeman-Neill & Bhagat, 2018; Pasqualucci, Migliazza, Basso, Houldsworth, Chaganti & Dalla-Favera, 2003; Polo et al., 2007; Ye et al., 1993). Constitutive expression of BCL6 in mice increases the risk of the development of DLBCL, similar to that observed in human disease (Baron et al., 2004; Cattoretti et al., 2005). Delivery of shRNA to DLBCL cells, which results in the loss of BCL6 biological activity, kills DLBCL cells, indicating that BCL6 is an attractive target for DLBCL treatment (Phan & Dalla-Favera, 2004). A peptidomimetic BCL6 inhibitor RI-BPI inhibits the recruitment of BCL6BTB corepressors and causes de-repression BCL6 target genes by binding to the BCL6 BTB domain (Cerchietti et al., 2009). More importantly, it can kill DLBCL cell lines and DLBCL patients cells (Cerchietti et al., 2009), demonstrating that inhibition of the BCL6BTB-corepressor interaction and reactivation of BCL6 target genes are a useful therapeutic strategy for DLBCL treatment.
At present, several BCL6 small molecule inhibitors have been reported, which demonstrates the feasibility of developing BCL6 inhibitors that directly bind to the BCL6 BTB domain (Cardenas et al., 2016; Cerchietti et al., 2010; Cheng et al., 2018; Kamada et al., 2017; Kerres et al., 2017; McCoull et al., 2017; Yasui et al., 2017). However, no such inhibitor has been enrolled in a clinical-trial. For example, the compounds 79-6 and FX1 directly bind to BCL6BTB, reactivate BCL6 target genes and inhibit the proliferation of DLBCL cells in vitro and in vivo (Cardenas et al., 2016; Cerchietti et al., 2010). However, the weak activities of 79-6 and FX1 and the fact that they are based on the skeleton of rhodamine may be detrimental and potential limitations for further drug development (Mendgen, Steuer & Klein, 2012; Tomasic & Peterlin Masic, 2012). Other reported small molecule inhibitors can bind to BCL6BTBand disrupt the interaction between BCL6 and corepressors in a micromolar range or even a nanomolar range in vitro , but their effects on the biological functions of BCL6 in vivo have not been further studied, which may be due to their poor pharmacokinetic/pharmacodynamic properties or other reasons (Cheng et al., 2018; Kamada et al., 2017; Kerres et al., 2017; McCoull et al., 2017; Yasui et al., 2017). Therefore, there is still a long way to go in terms of screening novel BCL6 inhibitors with high druggability for DLBCL therapy.
We showed that WK500B, a novel synthetic small molecule compound, directly bound to BCL6BTB, significantly inhibited the BCL6BTB/SMRT interaction, reactivated BCL6 target genes in a concentration-dependent manner, killed DLBCL cell lines and caused cell cycle arrest and apoptosis. More importantly, WK500B showed favourable pharmacokinetics, resulting in the disruption of the germinal centre formation and the affinity maturation of immunoglobulins in mice at a low dose, and it strongly suppressed the growth of DLBCL in vivo . Furthermore, no cases of toxic effects were noted. Taken together with the fact that WK500B is more potent both in vitro andin vivo , is orally available and shows superior druggability compared with other reported BCL6 inhibitors, it seems that WK500B could be a promising therapeutic drug candidate for DLBCL.
Methods