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Rapid progress in the development of BET inhibitors has created a surge of enthusiasm.
Bromodomains, a group of structurally similar proteins, serve as epigenetic “readers”— they recognize and bind acetylated lysine residues. The human body’s 40-plus bromodomains play a wide range of roles, including signal transduction in inflammatory pathways and transcription mediation.1
In 1992, researchers discovered the bromodomain and extraterminal domain (BET) family, responsible for controlling expression of key immune signals and oncogenic pathways.2,3 This family of proteins, with each member characterized by 2 bromodomains, includes bromodomain-containing protein 2 (BRD2), BRD3, BRD4, and bromodomain testis-associated protein.2
BET proteins were quickly recognized as possible therapeutic targets in cancer, but at the time, epigenetic therapy was viewed with skepticism. Could widespread regulators of transcription be blocked without also inflicting collateral damage on normal gene function? This question would be answered, years later, with the arrival of BET inhibitors, a type of small-molecule inhibitor.
In 2010, the BET inhibitor JQ1 showed activity in nuclear protein in testis midline carcinoma (NMC), a rare and highly aggressive cancer involving BRD4 or BRD3 activity.4 Instead of widespread downregulation with severe toxicity, the activity of JQ1 was contextual and specific to a small number of genes, suggesting that BET inhibitors could be a viable reality. Since then, the landscape has become increasingly active.At present, at least 10 BET inhibitors are under investigation, with over a dozen active clinical trials.5 Early research suggested that BET inhibitors would have the most impact on hematologic malignancies, but clinical trials show that restricted doses may be necessary for safety, particularly due to thrombocytopenia. This has triggered development of a second generation of BET inhibitors, which are more selective for particular bromodomains. These agents appear to be better tolerated but may be more specific to certain cancer types.6
Although solid tumors may not respond as markedly as hematologic malignancies, research suggests that BET inhibitors could work well in combination with existing therapies to overcome resistance mechanisms.7 Combination studies are under way for breast cancer, prostate cancer, melanoma, renal cell carcinoma, lung cancer, and more.8-10
Rapid progress in the development of BET inhibitors has created a surge of enthusiasm, but Guillaume Andrieu, PhD, and colleagues at the Cancer Research Center at Boston University School of Medicine in Massachusetts, have urged caution. In a 2016 journal article, they described the rapid transition into clinical trials as “reckless and likely to lead to adverse events [AEs].”2
The authors explained that a lack of basic science could prove risky, particularly because BET proteins have many roles beyond cancer, including repression of latent viruses such as HIV, insulin production, and T-cell differentiation. Each BET protein controls unique transcriptional pathways, sometimes with opposing effects from another BET protein. Early BET inhibitors block activity of all BET proteins (pan-BET inhibitors) with low selectivity, possibly increasing the likelihood of AEs.
Ac indicates acetylation; BET, bromodomain and extra-terminal; BD, bromodomain; BRD, bromodomain-containing proteins; CTM, C-terminal motif; ET, extraterminal; NPS, N-terminal cluster of phosphorylation sites; P, phosphorylation.
The BET family members are “readers” of acetylated lysines that activate the transcription of aberrant genes, leading to carcinogenic processes affecting the cell cycle, proliferation, stemness properties, metastatic spreading, and angiogenesis. Inhibition of BET activity is believed to disrupt these processes.
Ocaña A, Nieto-Jiménez C, Pandiella A. Oncotarget. 2017;8(41):71285-71291. doi: 10.18632/oncotarget.19744.
“Providing selective compounds of individual BET proteins is a crucial challenge to understand...their biological roles,” Andrieu and colleagues added. Such compounds could “unravel the molecular mechanisms of their signaling.” Fortunately, upcoming research is focused on developing such highly selective agents, so these ambitions may soon be realized.7BET inhibitors competitively bind bromodomains on a BET protein, thereby blocking localization to chromatin, which in turn suppresses recruitment of transcriptional proteins and resultant neoplastic processes.7 These processes include angiogenesis, cell cycle control, cancer stem cells, proliferation, metabolism, and metastasis11,12 (Figure12). Studies have shown that BET inhibitors alter numerous pathways and expression of hundreds of genes, including MYC, E2F, FOSL1, WNT5A, JAG1, NOTCH1, AURKB, and cyclin D family genes. Emerging key areas of interest in BET inhibitor research include aspects of MYC downregulation and angiogenic control.BET inhibitors gained early recognition by downregulating MYC, a known oncogenic driver in most human cancers.13 MYC amplification promotes cell survival and division through induction of multiple pathways. Hematologic malignancies, in particular, are characterized by pathologic activation of c-Myc, an oncoprotein. Outside of transgenic models, c-Myc inhibition was unknown prior to the introduction of JQ1.
A 2011 study using a murine model of multiple myeloma (a c-Myc—dependent cancer) showed that JQ1 could downregulate MYC transcription and Myc-dependent pathways.13 Treatment with JQ1 significantly reduced cell proliferation via cell senescence and cell cycle arrest, prolonging overall survival of treated mice. One animal had a complete response (CR), which had been previously achieved just by bortezomib (Velcade), an FDA-approved proteasome inhibitor.
By downregulating MYC, BET inhibitors may have even more potential as a means of overcoming resistance mechanisms. For example, in estrogen receptor—positive breast cancer, resis- tance to everolimus can be overcome by adding a BET inhibitor, since resistance is mediated by MYC.14 Other breast cancer studies have had similar results using BET inhibitors to overcome resistance to tamoxifen, PI3K inhibitors, and lapatinib (Tykerb).12 In a similar vein, early research suggests that BET inhibitors also downregulate PD-L1 expression, pointing to possible synergisms with checkpoint inhibitors.15BET inhibitors such as JQ1 downregulate VEGF by targeting multiple angiogenic pathways, including the interleukin 1 beta (IL-1β), hypoxia-inducible factor, and activator protein 1 pathways.16 In each, the end result is similar: VEGF expression decreases, and so does angiogenic activity. Other angiogenic pathways affected include those of carbonic anhydrase 9, which regulates tumor pH, and PI3K/AKT, which is also known to promote cancer growth and proliferation.
AML indicates acute myeloid leukemia; BET, bromodomain and extraterminal; DLBCL, diffuse large B-cell lymphoma; ER, estrogen receptor; mCRPC, metastatic castration-resistant prostate cancer; MDS, myelodysplastic syndrome; NHL, non-Hodgkin lymphoma; NUT, nuclear protein of the testis; TNBC, triple-negative breast cancer.
aTrial is active but not recruiting participants.
bNot yet recruiting.
It is widely accepted that downregulating angiogenesis could have a significant effect on solid tumor growth and survival, a concept demonstrated by JQ1. In a study involving a xenograft model of triple-negative breast cancer, JQ1 affected 44% of hypoxia-regulated genes, of which two-thirds were downregulated.11 The result was a reduction in both tumor growth and xenograft vascularization.Due to a short half-life in plasma (about 1 hour), JQ1 is not being pursued in clinical trials. However, more than 10 other BET inhibitors are in various stages of development, with frontrunners now into phase II (Table). Below, results are described for some leading agents.In a first-in-human phase I trial, 64 patients with relapsed or refractory lymphoma were treated with escalating doses of CPI-0610, resulting in early activity and an acceptable safety profile.3
Thrombocytopenia, a reversible and noncumulative AE, occurred in 42.2% of patients. This was dose dependent, occurring just with doses over 50 mg. Nausea (17.2%) and fatigue (17.2%) were other common AEs.
Within the same study,5 patients (13.2%) had objective responses, including 3 partial responses (PRs) and 2 CRs. One CR was in a patient with activated B-cell diffuse large B-cell lymphoma (DLBCL); the other patient achieving a CR had T-cell/histio- cyte-rich B-cell lymphoma.
Phase II trials are under way for CPI-0610 with or without ruxolitinib (Jaka ) in myelofibrosis as second-line therapy (NCT02158858).In a phase I/II study, 54 patients with relapsed or refractory advanced malignancies were treated with INCB057643. Fifty patients had solid tumors, and 4 had lymphoma.17
One patient (2%) with non—small cell lung cancer (NSCLC) had a PR, 3 (6%) achieved stable disease (SD) lasting more than 6 months, 14 (28%) achieved SD lasting less than 6 months, and 25 (50%) had radiographic progression. Remaining patients had clinical progression, withdrew consent, or were lost to follow-up.
Researchers concluded that INCB057643 was reasonably well tolerated. As in the CPI-0610 study, thrombocytopenia was a common AE (26%) and was also dose dependent. Other AEs included fatigue (28%), decreased appetite (24%),and neutropenia (24%).
Researchers described high pharmacokinetic variability between patients. Clinical trials of INCB057643 are ongoing.Birabresib has been evaluated in patients with both solid tumors and hematologic malignancies, with 2 completed phase I trials.
In a recent phase Ib study, birabresib was administered to 46 patients with NMC, castrate-resistant prostate cancer, or NSCLC. Three patients with NMC had a PR (duration, 1.4-8.4 months).18
Eighty-three percent of patients had treatment-related AEs, including nausea (37%), diarrhea (37%), anorexia (30%), vomiting (26%), and thrombocytopenia (22%). Again, higher doses were correlated with grade 3 or 4 thrombocytopenia.
An ongoing phase I trial with an estimated enrollment of 9 patients is testing birabresib in acute myeloid leukemia and DLBCL (NCT02698189). Researchers suggest that tolerability may improve with an intermittent dosing regimen.Since thrombocytopenia seems to restrict dose levels, researchers may turn toward more selective BET-targeting strategies. For example, ABBV-744 is a first-in-class, highly selective BET inhibitor being developed by AbbVie in an effort to improve tolerability.6
Early results suggest that tolerability may come at the cost of efficacy. Xenograft models show that ABBV-744 has antiproliferative activity in a narrower range of cancer cell lines compared with earlier pan-BET inhibitors. Perhaps the tradeoff will be worthwhile if clinical results live up to the potential that many researchers believe BET inhibitors have yet to reveal.