highest induced stability for the first bromodomain of BRD4 [9]. X-ray crystallography (PDBID 3mxf) established the binding mechanism, in which the methylated
1,2,4-triazole serving as the acetylated lysine mimetic effectively engaged N140 and
Y97 via a water-mediated hydrogen bond.
One advantageous feature of a chemical probe molecule is a suitable negative
control with similar physicochemical properties to the active compound. The enantiomer (À)-JQ1 has served this role and has led to robust results describing the
biological activity of (+)-JQ1 across a range of disease indications including cancer,
cardiac hypertrophy, and inflammation [14]. In the first report, the authors demonstrated the efficacy of (+)-JQ1 for treating patient-derived NMC cells, driven by a
BRD4-NUT fusion protein [9]. Treatment led to the cellular differentiation and G1
cell cycle arrest, demonstrating the potential of bromodomain inhibitors for epigenetic reprogramming of cell identity. PET imaging of NMC-derived mouse xenografts also showed tumor reduction, further validating anticancer activity.
(+)-JQ1 is a widely shared chemical probe which has been distributed to
researchers across the globe. One of the first collaborative studies to use this probe
in combination with an RNAi screen of chromatin regulators was with the Vakoc
and Lowe labs demonstrating that BRD4 inhibition with (+)-JQ1 can reduce transcription of the oncogene c-Myc in acute myeloid leukemia, leading to differentiation of terminal myeloid cells [66]. At the same time, similar effects were also seen
against multiple myeloma, supporting a broader potential for a BET-targeted therapy
for hematological diseases [67]. Young and co-workers shed light on how (+)-JQ1
inhibition of a ubiquitous coactivator protein like BRD4, typically involved in
transcriptional elongation through association with the positive transcription elongation factor B, P-TEFB, can induce a therapeutic response [68]. In this work, they
showed BRD4 associated with Mediator at large closely spaced enhancer regions
more than 10 kilobases in total size, termed super-enhancers. Inhibiting transcription
of c-Myc in blood cancers due to sensitivity to super-enhancer regulation is one of
the several mechanisms for which BET bromodomains have now been shown to
regulate oncogenic pathways.
Since the disclosure of the triazolodiazepine inhibitor scaffold, numerous inhibitors have since been reported and are actively being investigated in the clinic briefly
discussed below. Selectivity within the BET family still remains a significant
challenge. Currently the majority of inhibitors in clinical trials are pan-BET inhibitors. Bayer has developed a BRD4 inhibitor BAY123807 with >10fold selectivity
over other BETs, but the structure has not been disclosed. In the published literature,
Ouyang et al. reported a BRD4-selective inhibitor (FL-411, Fig. 6) with high
nanomolar affinity for inducing autophagy-mediated cell death in breast cancer
[60]. In lieu of other specific BET family member inhibitors, selectivity between
the two terminal bromodomains (BD1 and BD2) within the BET family of proteins
has been established in several cases (Fig. 6). RVX-208 [63] and RVX-297 [64],
reported by the Structural Genomics Consortium in Oxford and Zenith Epigenetics
Corp., now allow for the study of BD2-dependent processes. RVX-208 affected a
smaller set of BET-dependent genes when compared to pan-BET inhibitors
[63]. However, RVX-208 testing for treating cardiovascular disease is now in
phase III clinical trials (NCT02586155).
Applied Biophysics for Bromodomain Drug Discovery
303
1,2,4-triazole serving as the acetylated lysine mimetic effectively engaged N140 and
Y97 via a water-mediated hydrogen bond.
One advantageous feature of a chemical probe molecule is a suitable negative
control with similar physicochemical properties to the active compound. The enantiomer (À)-JQ1 has served this role and has led to robust results describing the
biological activity of (+)-JQ1 across a range of disease indications including cancer,
cardiac hypertrophy, and inflammation [14]. In the first report, the authors demonstrated the efficacy of (+)-JQ1 for treating patient-derived NMC cells, driven by a
BRD4-NUT fusion protein [9]. Treatment led to the cellular differentiation and G1
cell cycle arrest, demonstrating the potential of bromodomain inhibitors for epigenetic reprogramming of cell identity. PET imaging of NMC-derived mouse xenografts also showed tumor reduction, further validating anticancer activity.
(+)-JQ1 is a widely shared chemical probe which has been distributed to
researchers across the globe. One of the first collaborative studies to use this probe
in combination with an RNAi screen of chromatin regulators was with the Vakoc
and Lowe labs demonstrating that BRD4 inhibition with (+)-JQ1 can reduce transcription of the oncogene c-Myc in acute myeloid leukemia, leading to differentiation of terminal myeloid cells [66]. At the same time, similar effects were also seen
against multiple myeloma, supporting a broader potential for a BET-targeted therapy
for hematological diseases [67]. Young and co-workers shed light on how (+)-JQ1
inhibition of a ubiquitous coactivator protein like BRD4, typically involved in
transcriptional elongation through association with the positive transcription elongation factor B, P-TEFB, can induce a therapeutic response [68]. In this work, they
showed BRD4 associated with Mediator at large closely spaced enhancer regions
more than 10 kilobases in total size, termed super-enhancers. Inhibiting transcription
of c-Myc in blood cancers due to sensitivity to super-enhancer regulation is one of
the several mechanisms for which BET bromodomains have now been shown to
regulate oncogenic pathways.
Since the disclosure of the triazolodiazepine inhibitor scaffold, numerous inhibitors have since been reported and are actively being investigated in the clinic briefly
discussed below. Selectivity within the BET family still remains a significant
challenge. Currently the majority of inhibitors in clinical trials are pan-BET inhibitors. Bayer has developed a BRD4 inhibitor BAY123807 with >10fold selectivity
over other BETs, but the structure has not been disclosed. In the published literature,
Ouyang et al. reported a BRD4-selective inhibitor (FL-411, Fig. 6) with high
nanomolar affinity for inducing autophagy-mediated cell death in breast cancer
[60]. In lieu of other specific BET family member inhibitors, selectivity between
the two terminal bromodomains (BD1 and BD2) within the BET family of proteins
has been established in several cases (Fig. 6). RVX-208 [63] and RVX-297 [64],
reported by the Structural Genomics Consortium in Oxford and Zenith Epigenetics
Corp., now allow for the study of BD2-dependent processes. RVX-208 affected a
smaller set of BET-dependent genes when compared to pan-BET inhibitors
[63]. However, RVX-208 testing for treating cardiovascular disease is now in
phase III clinical trials (NCT02586155).
Applied Biophysics for Bromodomain Drug Discovery
303