Amid a growing understanding about the role of epigenetics as a driver of cancer, researchers have turned their attention to a key player in the process: histone deacetylases.
Amid a growing understanding about the role of epigenetics as a driver of cancer, researchers have turned their attention to a key player in the process: histone deacetylases (HDACs).
HDACs are enzymes responsible for acetylation, which essentially functions to repress gene transcription, and targeting this activity through small-molecule inhibitors has proved effective in hematologic malignancies.
In February 2015, the FDA approved panobinostat (Farydak) in combination with bortezomib and dexamethasone for the treatment of patients with multiple myeloma. That decision marked the fourth HDAC inhibitor that the regulatory agency has approved during the past decade.
Despite these demonstrable clinical successes in hematologic malignancies, HDAC inhibitors have proved impotent in the treatment of solid tumors, which has limited their utility. That may be changing, however, with a better understanding of the role of histone acetylation in the context of global epigenetic and genetic modifications in cancer.
More recently, researchers are seeing the first hints of clinical efficacy in solid tumors with next-generation, class-specific, and multitargeted HDAC inhibitors. Multiple agents are advancing in clinical development (Table).The classical view of cancer is as a genetic disease, driven by changes such as mutations to the sequence of genes involved in hallmark cellular processes such as growth and proliferation. In the past several decades, it has become clear that epigenetic abnormalities that alter gene expression without changing the DNA sequence are equally important. Epigenetics describes a secondary layer of regulation of the genetic material; while the specific sequence of DNA in a gene instructs the cell about what to make, epigenetics dictates when and where it will be made.
In cells that are not dividing, the genetic material is packaged up in the form of chromatin, composed of negatively charged DNA wound tightly around positively charged histone proteins like thread on a spool. Both thread (DNA) and spool (histones) can be modified by the addition and removal of chemical groups, mediated by groups of opposing enzymes.
At least 8 distinct types of modification have been described, including acetylation, methylation, phosphorylation, and ubiquitination. The specific pattern of modifications, attained by a balance between the different enzymes that govern them, acts as a kind of regulatory code for the genetic material, ultimately dictating gene expression. Disruption of this delicate balance has been associated with the development of a wide variety of human cancers.Acetylation and deacetylation, the addition and removal of acetyl groups, is governed by enzymes known as histone acetylases (HATs) and histone deacetylases (HDACs), respectively. Acetylation is among the best-characterized modifications of the histone proteins.
Although it is increasingly appreciated that the role of histone acetylation is vastly more complex than originally thought and likely acts in concert with other epigenetic modifications, researchers believe that acetylation changes the confirmation of the histone “spool,” loosening it and rendering the DNA more accessible and, in turn, more transcriptionally active.
This illustration captures the diverse proteins and processes that potentially could be affected by inhibiting HDAC enzymes.
HDAC indicates histone deacetylase; HR, homologous recombination; NHEJ, non-homologous end joining; ROS, reactive oxygen species.
Mottamal M, Zheng S, Huang TL, Wang G. Histone deacetylase inhibitors in clinical studies as templates for new anticancer agents. Molecules. 2015;20:3898-3941. doi:10.3390/molecules20033898.
The fact that epigenetic regulation plays such a key role in gene expression and that it may form part of the genomic manipulations of cancer cells has fueled significant interest in drugging the epigenetic enzymes, since these are readily targeted with small molecule drugs.
HDACs are a large family of 18 enzymes that are divided into 4 classes based on how similar they are to yeast HDACs, where they are found in the cell, and their function. Classes I, II, and IV are considered the classical HDACs, consisting of 11 zinc-dependent metalloproteins and have been the key focus of drug development efforts. The class III HDACs are known as sirtuins, and the 7 enzymes in this group are nicotinamide adenine dinucleotide (NAD+)-dependent proteins. This group is unaffected by currently available HDAC inhibitors; however, efforts to develop sirtuin-targeting agents are under way, although these studies are predominantly in the preclinical stages.
Inhibiting HDACs has had demonstrable anticancer success in the clinic, but the precise mechanisms of action are still being teased out. In general, HDACs are thought to be cancer permissive—that is, they predominantly repress the transcription of tumor suppressor genes—and that, therefore, inhibiting HDACs should reverse these effects.
However, some HDACs have been shown to have the opposite effect, thus acting as tumor suppressors themselves. Since the majority of HDAC inhibitors currently in development target multiple members of the HDAC family, understanding these nuances is becoming increasingly important.
Another key question is how HDAC inhibitors specifically target cancer cells. It is hypothesized that, in a manner analogous to oncogene addiction, cancer cells might become reliant on certain epigenetic pathways for maintaining the expression of a key set of genes that drive cancer hallmark processes. Alternatively, cancer cells may undergo epigenetic rewiring. In both cases, the result may be that cancer cells are more susceptible to HDAC inhibition.The clinical success of HDAC inhibitors has culminated in FDA approval of 4 drugs over the past decade, beginning with vorinostat (Zolinza) in 2006 for the treatment of patients with cutaneous T-cell lymphoma (CTCL).
The FDA approved romidepsin (Istodax) for the same patient population in 2009 and added second- line treatment of peripheral T-cell lymphoma (PTCL) two years later. In 2014, these agents were joined by the approval of belinostat (Beleodaq) for a PTCL indication. All approvals were based on the demonstration of response rates between 20% to 30% in heavily pretreated individuals.
Most recently, panobinostat became the first HDAC inhibitor to receive regulatory approval for the treatment of multiple myeloma. The approval came after an initial ruling against the drug in late 2014, when the FDA’s Oncologic Drugs Advisory Committee found that the benefits of panobinostat treatment did not outweigh the risks.
AML indicates acute myeloid leukemia; CLL, chronic lymphocytic leukemia; CTCL, cutaneous T-cell lymphoma; HCC, hepatocellular carcinoma; HDAC, histone deacetylase; MDS, myelodysplastic syndrome; PTCL, peripheral T-cell lymphoma.
Novartis subsequently submitted additional information supporting the use of panobinostat for a more specific indication—among patients with multiple myeloma who had received at least 2 prior therapies—and the drug was ultimately approved in February 2015 for use in combination with the proteasome inhibitor bortezomib and the corticosteroid dexamethasone.
The additional data submitted by Novartis was from a prespecified subset analysis of the PANORAMA- 1 trial comparing this combination with placebo plus bortezomib and dexamethasone.
Among 193 patients, 76% of whom had received 2 or more prior lines of therapy, median progression- free survival (PFS) was 10.6 months in the panobinostat arm, compared with 5.8 months in the placebo arm. Tumor shrinkage rate was 59% in the panobinostat arm versus 41% in the control group and objective response rate (ORR) was 55% versus 41%, respectively.
The most common nonhematologic adverse events (AEs) were diarrhea, peripheral neuropathy, and asthenia/fatigue, while common hematologic laboratory abnormalities were related to platelet and absolute lymphocyte count.The approval of panobinostat in combination with a proteasome inhibitor highlights our improved understanding of the complex mechanism of action of HDAC inhibitors. In reality, HDACs should more accurately be called lysine deacetylases since many of their targets in the cell are nonhistones—thus, HDAC inhibitors are likely to exhibit a range of cellular effects. The proposed mechanism of synergy between HDAC inhibitors and proteasome inhibitors is thought to be related to one of these nonhistone effects.
The ubiquitin—proteasome system is responsible for the degradation of the vast majority of regulatory proteins within a cell. Proteins that need to be removed from the cell are tagged with multiple ubiquitin molecules that are recognized by the proteasome. Preventing this process causes the accumulation of defective proteins and triggers cell death, a fact that was exploited for the development of proteasome inhibitors as anticancer drugs. Normal cells are able to adapt to proteasome inhibition by activating the unfolded protein response (UPR), whereby the undegraded proteins are organized into aggresomes and degraded by an alternative process known as autophagy. HDAC6 plays a particularly important, and histone-independent, role in the UPR, by mediating the transport of proteins to the aggressome.
Therefore, the synergy between proteasome inhibitors and HDAC inhibitors is thought to result from the fact that HDAC inhibition may block the cancer cell’s adaptive response to proteasome inhibition. Some examples of nonhistone protein substrates of HDACs that are prominently associated with cancer include p53, NFκB, heat-shock protein 90, androgen receptor, and estrogen receptor. Through their effects on these non-histone proteins, HDAC inhibitors may influence important processes such as the cell cycle, angiogenesis, immune modulation, and apoptosis.HDAC inhibitors have proved most successful in the treatment of hematologic malignancies, particularly lymphomas. Some clinical trials in solid tumors have been performed, but outcomes have been poor, although it remains unclear why this is the case. In general, 4 classes of HDAC inhibitors are in development: hydroxamic acids, benzamides, cyclic peptides, and short-chain fatty acids. Hydroxamic acid type inhibitors are the most widely studied class and include 3 of the currently approved drugs—vorinostat, panobinostat, and belinostat. Romidepsin is a naturally occurring cyclic peptide, derived from the bacteria Chromobacterium violaceum.Clinical development of the 4 FDA-approved HDAC inhibitors continues in a variety of tumor types, including several late-stage clinical trials. Vorinostat is being evaluated in a phase III trial in combination with thalidomide, lenalidomide, and bortezomib for the initial treatment of multiple myeloma (NCT01554852).
Most recently, vorinostat disappointed in a phase III trial as second- or third-line therapy in patients with advanced, malignant pleural mesothelioma. Among 661 patients evaluated there was no improvement in OS compared with placebo (30.7 weeks vs 27.1 weeks, respectively).
Romidepsin is undergoing phase III testing in combination with CHOP chemotherapy in patients with previously untreated PTCL (NCT01796002). Results from a phase I trial of this combination will be presented at the 2015 American Society of Hematology (ASH) Annual Meeting.
Onxeo, the European partner of Spectrum Pharmaceuticals, reported that the combination was well tolerated, with rates of AEs consistent with those typically observed with CHOP alone. The ORR was 89%, including 72% complete response (CR) among 23 patients evaluated.Numerous second-generation hydroxamic acid type inhibitors have subsequently been developed, including pracinostat, which was awarded an orphan drug designation for the treatment of myelodysplastic syndrome (MDS) in 2014.
Earlier this year, disappointing results from a phase II study of pracinostat in combination with the DNA methyltransferase inhibitor azacitidine in patients with previously untreated intermediate-2 or high-risk MDS were reported. No difference in the rate of CR was observed; however, data scheduled to be presented at 2015 ASH suggest that hazard ratios for the secondary endpoints of overall survival (OS), PFS, and event-free survival (EFS) strongly favor pracinostat compared with placebo (HRs, 0.59, 0.37, and 0.33, respectively). However, this combination is showing significantly more promise in elderly patients with acute myeloid leukemia, producing a high rate of durable responses, with 1-year OS estimated at 60% and median OS not yet reached in one study. Pracinostat is also being evaluated in a phase II trial in combination with the JAK inhibitor ruxolitinib in patients with myelofibrosis.
Another next-generation hydroxamic acid-based inhibitor is abexinostat, which is showing promise in the treatment of patients with various subtypes of non-Hodgkin lymphoma. In a phase II trial, single-agent abexinostat had a manageable toxicity profile and promising efficacy, with an ORR ≥30% among patients with follicular lymphoma, T-cell lymphoma, and diffuse large B-cell lymphoma (DLBCL).Thus far, the most success in solid tumors has been achieved with a number of benzamide-derived drugs. While the hydroxamic acid-based inhibitors are pan-HDAC inhibitors, those in the benzamide class tend to be more specific for particular HDAC classes. Furthest along in development are chidamide, which specifically targets HDACs 1, 2, 3, and 10; entinostat, a class I—specific agent; and mocetinostat, which is selective for class I and IV HDACs; entinostat, a class I–specific agent.
Chidamide recently received regulatory approval in China for the treatment of PTCL; however, both this agent and entinostat are showing promise in the treatment of advanced breast cancer in the United States, with phase III trials ongoing. Entinostat received orphan drug designation from the FDA for breast cancer, following promising results from the phase II ENCORE study in which a combination of entinostat and the aromatase inhibitor exemestane was well tolerated and demonstrated clinical activity in patients with advanced estrogen receptor—positive breast cancer.
Mocetinostat, which has been tested in about a dozen clinical trials, currently is undergoing separate phase II trials as monotherapy patients with bladder cancer and DLBCL that harbors mutations in the histone acetyl transferase genes CREBBP or EP300 has been generally well tolerated.
An even more specific benzamide-derived HDAC inhibitor is ricolinostat. This drug targets just HDAC6 and is currently being evaluated in phase I/II trials in patients with a variety of hematologic malignancies.
Preliminary data from a trial of ricolinostat in combination with the immunomodulatory drug pomalidomide and dexamethasone in patients with relapsed/refractory multiple myeloma were presented at the 20th Congress of the European Hematological Association in June.
Thus far, 28 of the planned 66 patients have been enrolled in the phase II portion of the trial. ORR was 29% at a median follow-up of 12 weeks and the clinical benefit rate (CBR) was 50%, with 68% of patients achieving stable disease or better. Common treatment-related AEs were fatigue, diarrhea, and neutropenia, with the majority being low-grade.Another possible strategy to boost the potency of HDAC inhibitors against solid tumors, one that recognizes the synergistic activity of HDAC inhibition in combination with other anticancer agents, is to develop multitargeted inhibitors.
Thus far, the majority of drug development has been in the preclinical stages. However, two drugs have been brought into clinical trials. CUDC-101, a combined HDAC inhibitor and inhibitor of the epidermal growth factor receptor family members EGFR and HER2, was examined in phase I trials, but issues with administration and bioavailability have halted its development.
CUDC-907 is a dual HDAC/PI3K inhibitor that is being evaluated in phase I clinical trials in patients with lymphoma and multiple myeloma and with advanced solid tumors. Curis is expected to present data on the efficacy of CUDC-907 in heavily pretreated patients with relapsed/refractory DLBCL at 2015 ASH, in which it demonstrated a manageable toxicity profile and sustained clinical activity, with an ORR of 55%.
Meanwhile, the short-chain fatty acid valproic acid, a less potent HDAC inhibitor, has also been tested as single agent and in combination with other therapies in numerous clinical trials, with several phase II trials ongoing.
Jane de Lartigue, PhD, is a freelance medical writer and editor based in New Haven, Connecticut