Abstract
It has been known for decades that as cancer progresses, tumors develop genetic alterations, making them highly prone to developing resistance to therapies. Classically, it has been thought that these acquired genetic changes are fixed. This has led to the paradigm of moving from one cancer therapy to the next while avoiding past therapies. However, emerging data on epigenetic changes during tumor progression and use of epigenetic therapies have shown that epigenetic modifications leading to chemotherapy resistance have the potential to be reversible with epigenetic therapy. In fact, promising clinical data exist that treatment with epigenetic agents can diminish chemotherapy resistance in a number of tumor types including chronic myelogenous leukemia, colorectal, ovarian, lung and breast cancer. The potential for epigenetic-modifying drugs to allow for treatment of resistant disease is exciting and clinical trials have just begun to evaluate this area.
It has been known for decades that as cancer progresses, tumors also develop genetic alterations, making them highly prone to developing resistance to therapies. Classically, it has been thought that these acquired genetic changes and subsequent resistance patterns are fixed and unalterable. This has led to the paradigm of moving from one cancer therapy to the next while avoiding past therapies. However, growing data on epigenetic changes during tumor progression and the use of epigenetic therapies may transform this paradigm.
Epigenetics describes a variety of strategies cells use to regulate the transcription of their DNA. Although numerous mechanisms are used, two of the best characterized and most studied are DNA methylation and histone modification. Histone modification may take place through a number of means, including phosphorylation, ubiquitination and methylation, but the most commonly seen changes occur through acetylation. DNA methylation is controlled by DNA methyltransferases (DNMTs) and histone acetylation is controlled by histone acetyltransferases (HATs) and histone deacetyltransferases (HDACs). Data on a myriad of tumor types including chronic myelogenous leukemia, lung, gastric, colorectal, breast, bladder, and ovarian cancer (1-10) have demonstrated that just as DNA mutations contribute to tumor progression, so do epigenetic alterations participate in tumor growth and chemotherapy resistance. In fact, treatment with DNMT inhibitors azacitidine or decitabine has been shown to produce responses in 13-18% of patients with myelodysplastic syndrome (MDS) lasting a median of 280-330 days (11, 12). In addition, a large randomized phase III trial comparing azacitidine to best conventional care for patients with MDS showed a 9.5-month survival benefit with the addition of azacitidine (13). Beyond the benefit seen with DNMT inhibitors for MDS, the HDAC inhibitors vorinostat and romidepsin have also shown clinical benefit in the treatment of cutaneous T-cell lymphoma (CTCL). In large randomized trials, these two agents produced responses lasting a median of more than 6 and up to 15 months in 29-35% of patients with CTCL (14,15). Taken together, these data led to Federal Drug Administration approval of these four agents, azacitidine and decitabine for MDS and vorinostat and romidepsin for CTCL.
While these agents have shown direct anti-neoplastic effects in MDS and CTCL, perhaps a greater role for epigenetic agents is as chemotherapy sensitizers to help overcome chemotherapy resistance in a wide array of malignancies. Although DNA mutations which lead to chemotherapy resistance may be irreversible, epigenetic alterations which lead to resistance have been found to be reversible with epigenetic therapy (10, 16). In fact, there exist recent data across a multitude of tumor types that epigenetic therapy has the potential to overcome chemotherapy resistance and re-sensitize cancer cells to previously ineffective therapy.
For example, RRx-001 is an epigenetic modifying agent belonging to a new class of drugs known as dinitroazetidines. It functions as an epigenetic agent by increasing the generation of reactive oxygen and nitrogen species under hypoxic conditions. These reactive species result in oxidation of critical cysteine residues at the catalytic sites of a number of epigenetic-modifying enzymes, including DNMTs and HDACs. Oxidation of these cysteine residues results in inhibition of these enzymes and an altered epigenetic profile in cancer cells. In a phase I trial of RRx-001 in patients with advanced refractory solid malignancies, activity was observed in some patients (one partial response and 14 out of 25 having stable disease) (17). In addition and more notably, four patients (three with colorectal and one with lung cancer) who had initially responded to chemotherapy regimens but whose disease then progressed went on to develop responses to these same regimens after receiving RRx-001.
Similar responses to previously ineffective therapy have also been found following treatment with other epigenetic-modifying agents. Hydralazine, most commonly known for its role as an anti-hypertensive medication, has also been found to act as a weak epigenetic agent by inhibiting non-nucleoside DNA methylation (18-20). In addition valproic acid, commonly used as a mood stabilizer and anti-epileptic, has been found to be a non-specific HDAC inhibitor (21,22). Similarly to RRx-001, valproic acid has been found to alter epigenetics through oxidative stress by generating reactive oxygen species that modulate and inhibit HDACs (23, 24). A phase II trial was conducted to evaluate epigenetic therapy with hydralazine and magnesium valproate in an effort to overcome chemotherapy resistance in patients with refractory solid tumors (25). Fifteen patients were evaluated who had cervical, breast, lung, testicular or ovarian cancer which was progressing on chemotherapy. These patients were treated with hydralazine and magnesium valproate daily for one week followed by addition of hydralazine and magnesium valproate daily to the same chemotherapy regimen their disease previously progressed on. Out of these 15 patients, four (three ovarian, one cervical) had a partial response and eight (four ovarian, and one each of cervical, lung, testicular and breast) were noted to have a stable disease confirmed 4 weeks after completing another one to two cycles of chemotherapy. In a separately published series, eight patients with imatinib-refractory chronic myelogenous leukemia (two in blast phase, five in accelerated phase, and one in the chronic phase) who had no access to second-line tyrosine kinase inhibitors, and for whom transplant was not an option, were given hydralazine and magnesium valproate daily in addition to their previous imatinib regimens (26). Of these eight patients, two had a complete hematological response, one had a complete cytogenetic response, one had a major cytogenetic response and three patients had stable disease after a median of 18 months of follow-up. This further supports the notion that epigenetic agents, including agents which induce oxidative stress, have the potential to re-sensitize refractory tumors to chemotherapy and possibly even targeted therapies.
Furthermore, in a phase I/II trial evaluating the epigenetic agents azacytadine and entinostat in patients with recurrent metastatic non small cell lung cancer, four out of 19 patients (21%) who had discontinued the study went on to have major responses to subsequent therapy (27). This occurred in spite of the fact that these patients' tumors were refractory to multiple standard therapies (median of three previous therapies). Moreover, the partial responses seen in these patients with subsequent therapy were dramatic, ranging from 45-85%. Of these four patients, three received chemotherapy as subsequent therapy (carboplatin/etoposide, cisplatin/docetaxel/bevacizumab, alimta) and one received an monoclonal antibody to PD1 (moAb). The response to these wide array of subsequent regimens was also noteworthy, suggesting that these epigenetic agents may have the potential to sensitize cancer cells to a wide array of chemotherapy regimens and possibly immunotherapies.
Very promising response data were also seen in a phase II trial evaluating low-dose (10 mg/m2) decitabine, a DNA methyltransferase inhibitor, in 17 women with heavily pre-treated and platinum-resistant ovarian cancer (28). Patients received low-dose decitabine on days 1 through 5 with carboplatin (area under the curve of 5) on day 8 of each 28-day cycle. Out of these patients, six experienced an objective response. Progression-free survival for this group was 10.2 months and nine patients (53%) were free of progression after 6 months. The response to carboplatin in that study was dramatically better than the historically reported response rates to carboplatin in similar patients with platinum-resistant ovarian cancer (<10%) (29).
In addition, another phase II trial also provides intriguing data that suggest that treatment with an epigenetic agent has the potential to improve tumor sensitivity to subsequent chemotherapy. A randomized phase II trial was conducted which evaluated exemestate with or without entinostat, a known HDAC inhibitor, in 130 post-menopasal women with locally recurrent or metastatic estrogen receptor-positive breast cancer previously progressing on non-steroidal aromatase inhibitors (30). The trial found that the addition of entinostat to exemestane significantly improved overall survival by 8.3 months (28.1 versus 19.8 months; hazard ratio=0.59; 95% confidence interval=0.36 to 0.97; p=0.036). However, despite this promising result, they also demonstrated that the progression-free survival was not significantly improved, nor was the response rate different with the addition of entinostat. Assessment of the post study treatments of both arms also showed that there were no significant differences. Based on these data, the authors hypothesized that the survival benefit seen may have been due to a sensitizing effect entinostat had on post study chemotherapies.
Given all of these emerging data, it appears that epigenetic agents may have the potential to help overcome chemotherapy resistance and improve sensitivity to chemotherapy, with promising clinical data on a number of malignancies, including chronic leukemia, and colorectal, ovarian, lung and breast cancer. This hypothesis is continuing to be evaluated in ongoing trials using multiple epigenetic agents against various tumor types.
Finally, while the above data are promising, it is worth noting that many of the trials mentioned above only enrolled patients with minimal residual toxicities from prior therapies. Unfortunately, in general practice, many patients come off prior chemotherapy not just due to progression but also to intolerance. While a possible role of epigenetic agents in overcoming chemotherapy resistance may not offer much benefit to patients intolerant to additional chemotherapy, these patients may still see a benefit from these therapies if these drugs are found to induce tumor-specific chemosensitivity, allowing for reduced doses and overall lower toxicities. Additionally, future studies should work to identify those patients who will benefit from these therapies by identifying and validating predictive biomarkers (such as by methylation or epigenomic profiling). All in all, we are excited about the direction the epigenetic field is exploring and hopeful that these avenues will alter the treatment armamentarium we can offer our patients.
- Received October 29, 2015.
- Revision received November 30, 2015.
- Accepted December 2, 2015.
- Copyright© 2016 International Institute of Anticancer Research (Dr. John G. Delinassios), All rights reserved
