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