BET-BD1 inhibition is also being investigated as a potential strategy over
inhibiting both bromodomains for reducing potential off-target effects. Early studies
in mice showed that deletion of domain 1 of BET, BRDT, which is expressed
predominantly in the testis, was sufficient to impair sperm function [69]. The Zhou
lab has since developed molecules MS-436 [61], MS-611 [62], and Olinone [62]
with BD1 selectivity (Fig. 6). Despite being potent BRD4 BD1 inhibitors, however,
MS-436 and MS-611 are not selective for BD1 over BD2 in other BET proteins.
Alternatively, Olinone is a weak-affinity ligand for BRD4 BD1 (K d ¼ 3.3 μM) but
maintains selectivity against non-BET bromodomains and only exhibits off-target
activity against CBP at high micromolar levels. Divakaran has now reported a
slightly improved inhibitor BD1-selective inhibitor, V, with a 1.2 μM K d and
>55-fold selectivity of BRD4 BD1 over BD2 and the highest affinity for BRD4
BD1 relative to the other BET bromodomains tested [30]. Improved BET BD1
inhibitors, and ultimately single BET bromodomain inhibitors for BRD2, 3, 4,
and T, will allow researchers to further dissect BET bromodomain interactions
with chromatin, acetylated transcription factors, and their effects on transcription.
As an alternative to improved selectivity, polypharmacology has also been
explored for dual kinase and bromodomain inhibition. The BET bromodomain,
BRD4, was recently identified as an atypical kinase capable of phosphorylating
RNA Pol II [70]. Such behavior led Schönbrunn and co-workers to evaluate a cyclindependent kinase inhibitor, dinaciclib, for interactions with BD1 of BRDT (Fig. 7)
[59]. Although the binding interaction was weak, K d ¼ 37 μM, selectivity remained
high for BETs. A high-resolution x-ray co-crystal structure identified the pyridine
oxide of dinaciclib to serve as the acetylated lysine mimic. Moreover, comparison of
binding modes with kinase inhibitor co-crystal structures led the researchers to
speculate the hinge-binding kinase inhibitor scaffolds could serve as optimal inhibitors for bromodomains.
This observation of kinase hinge binders serving as bromodomain inhibitors
was validated the following year with two independent publications from the
Schönbrunn lab and the Knapp lab [57, 58]. In these cases, library screening and
crystallization studies identified a variety of known kinase inhibitors with affinity for
BET bromodomains. Most notably both groups identified PLK1 inhibitor, BI-2536,
and JAK2 inhibitor TG-101348 to inhibit BRD4 and BRDT with nanomolar potency
(Fig. 6). A subset of the published kinase inhibitor sets I and II from GSK was also
used in a tandem bromodomain NMR screen against BPTF and BRD4 BD1 [71],
identifying ligands for both proteins including a p38α inhibitor analog of the 1,4,5trisubstituted imidazole scaffold identified by the Schönbrunn lab for BRD4 BD1.
Although a monotherapy using a dual kinase/bromodomain inhibitor has yet to
be investigated in the clinic, promising results of a potent PI3K and BET inhibitor
with nanomolar potency have been reported using a thienopyranone scaffold
[72]. Supporting a polypharmacology approach, dual inhibition of Myc degradation
through PI3K inhibition and Myc transcription via BET bromodomain inhibition,
led to less toxic effects in an orthotopic pancreatic cancer model for spontaneous
tumor metathesis than when treated with a combination of (+)-JQ1 and PI3K
inhibitor BKM120. Of note, their lead thienopyranone, SF2523, also displayed
304
W. C. K. Pomerantz et al.
inhibiting both bromodomains for reducing potential off-target effects. Early studies
in mice showed that deletion of domain 1 of BET, BRDT, which is expressed
predominantly in the testis, was sufficient to impair sperm function [69]. The Zhou
lab has since developed molecules MS-436 [61], MS-611 [62], and Olinone [62]
with BD1 selectivity (Fig. 6). Despite being potent BRD4 BD1 inhibitors, however,
MS-436 and MS-611 are not selective for BD1 over BD2 in other BET proteins.
Alternatively, Olinone is a weak-affinity ligand for BRD4 BD1 (K d ¼ 3.3 μM) but
maintains selectivity against non-BET bromodomains and only exhibits off-target
activity against CBP at high micromolar levels. Divakaran has now reported a
slightly improved inhibitor BD1-selective inhibitor, V, with a 1.2 μM K d and
>55-fold selectivity of BRD4 BD1 over BD2 and the highest affinity for BRD4
BD1 relative to the other BET bromodomains tested [30]. Improved BET BD1
inhibitors, and ultimately single BET bromodomain inhibitors for BRD2, 3, 4,
and T, will allow researchers to further dissect BET bromodomain interactions
with chromatin, acetylated transcription factors, and their effects on transcription.
As an alternative to improved selectivity, polypharmacology has also been
explored for dual kinase and bromodomain inhibition. The BET bromodomain,
BRD4, was recently identified as an atypical kinase capable of phosphorylating
RNA Pol II [70]. Such behavior led Schönbrunn and co-workers to evaluate a cyclindependent kinase inhibitor, dinaciclib, for interactions with BD1 of BRDT (Fig. 7)
[59]. Although the binding interaction was weak, K d ¼ 37 μM, selectivity remained
high for BETs. A high-resolution x-ray co-crystal structure identified the pyridine
oxide of dinaciclib to serve as the acetylated lysine mimic. Moreover, comparison of
binding modes with kinase inhibitor co-crystal structures led the researchers to
speculate the hinge-binding kinase inhibitor scaffolds could serve as optimal inhibitors for bromodomains.
This observation of kinase hinge binders serving as bromodomain inhibitors
was validated the following year with two independent publications from the
Schönbrunn lab and the Knapp lab [57, 58]. In these cases, library screening and
crystallization studies identified a variety of known kinase inhibitors with affinity for
BET bromodomains. Most notably both groups identified PLK1 inhibitor, BI-2536,
and JAK2 inhibitor TG-101348 to inhibit BRD4 and BRDT with nanomolar potency
(Fig. 6). A subset of the published kinase inhibitor sets I and II from GSK was also
used in a tandem bromodomain NMR screen against BPTF and BRD4 BD1 [71],
identifying ligands for both proteins including a p38α inhibitor analog of the 1,4,5trisubstituted imidazole scaffold identified by the Schönbrunn lab for BRD4 BD1.
Although a monotherapy using a dual kinase/bromodomain inhibitor has yet to
be investigated in the clinic, promising results of a potent PI3K and BET inhibitor
with nanomolar potency have been reported using a thienopyranone scaffold
[72]. Supporting a polypharmacology approach, dual inhibition of Myc degradation
through PI3K inhibition and Myc transcription via BET bromodomain inhibition,
led to less toxic effects in an orthotopic pancreatic cancer model for spontaneous
tumor metathesis than when treated with a combination of (+)-JQ1 and PI3K
inhibitor BKM120. Of note, their lead thienopyranone, SF2523, also displayed
304
W. C. K. Pomerantz et al.