Editorial
Histone Deacetylase Inhibitors in Cancer: What Have We Learnt? James R. Whittle, MBBS1 and Jayesh Desai, MBBS, FRACP1,2,3
The understanding that epigenetic modification of tumor suppressor genes by histone deacetylases (HDACs) plays a role in tumorigenesis has garnered significant interest in the development of therapeutic modulators. HDAC inhibitors have emerged over the past 15 years based on their ability to induce differentiation in cell culture as well as an observed clinical benefit in a few hematologic malignancies. Despite their proven anticancer benefit, many aspects of the HDAC inhibitors are not fully understood, and their progress highlights the difficulty in developing new targeted drugs to treat cancer. This problem is not unique to HDAC inhibitors, and the lack of predictive biomarkers when trying to target a complex biologic process is a major contributor to the high rate of attrition of new targeted drugs.1 Tumor development is associated with both genetic and epigenetic alterations, which include methylation, acetylation, histone modification, and gene silencing by small RNA.2 Acetylation of the N-terminal tail of core histones is tightly controlled by the antagonistic actions of histone acetyltransferases and HDACs.3 HDACs are critical regulators of gene expression and mediate their effect through both histone and nonhistone protein substrates. Histone acetylation status influences the access of transcription factors to DNA, and alteration of this process favors deacetylated histones, leading to gene expression in the multiple pathways involved in proliferation, migration, angiogenesis, differentiation, invasion, and metastasis. Histone acetyltransferases and HDACs also mediate their effect through nonhistone protein substrates, including nuclear transcription factors, p53 (tumor protein 53), nuclear factor-jB, c-Myc (v-myc avian myelocytomatosis viral oncogene homolog), and p21, that play important roles in tumorigenesis.3 In humans, 18 HDACs are described based on their homology to yeast proteins, subfamily-specific and subunitspecific roles, substrates, and localization. Eleven of these are zinc-dependent, termed classical HDACs, and are grouped into 3 classes: class I (HDAC1, HDAC1, HDAC3, and HDAC8), class IIa (HDAC4, HDAC5, HDAC7, and HDAC9), class IIb (HDAC6 and HDAC10), and class IV (HDAC11). Class III HDACs (sirutins 1-7) require nicotinamide adenine dinucleotide (NAD1) for their activity and are not inhibited by HDAC inhibitors.4,5 More recently, attempts have been made to describe the expression of HDACs in normal and tumor tissues. Class I HDACs are expressed ubiquitously in human tissues, as demonstrated in HDAC1 or HDAC2 knockout mice, which display embryonic and perinatal lethality.5 Functionally, class I HDACs participate in many cellular process, including proliferation, cell cycle, and apoptosis, whereas class II HDACs display more tissue specificity and are involved particularly in differentiation.5 Aberrant expression of HDAC serves as a marker of normal tissue versus tumor tissue, and the overexpression of individual HDACs in solid organ cancers has been correlated with reductions in disease-free and overall survival independent of other variables. However, information can be contradictory, because the overexpression of HDAC is not always a negative prognostic variable, and considerable difficulty in translating this into the clinic is further evidence highlighting the need to understand the target of interest. For example, elevated levels of HDAC6 confer an improved prognosis in estrogen receptor-positive breast cancer and cutaneous T-cell lymphoma.6 Moreover, and paradoxically, HDACs may have a tumor suppressor role in the growth of developing tumors but remain a therapeutic target in established tumors.6 HDAC inhibitors consist a broad class of compounds, including hydroxamic acids (trichostatin A, panobinostat, vorinostat, and abexinostat), cyclic peptides (trapoxin and depsipeptide), and benzamides (butyric acid, valproic acid, etinostat, and mocetinostat). These agents inhibit HDACs with various degrees of class specificity, promoting acetylation of
Corresponding author: Jayesh Desai, MBBS, FRACP, Royal Melbourne Hospital, Grattan Street, Parkville, VIC, 3050, Australia: Fax: (011) 613-9347-7508; jayesh. [email protected] 1 Department of Medical Oncology, Royal Melbourne Hospital, Parkville, Victoria, Australia; 2The Walter and Eliza Hall Institute of Medical Research, Parkville, Victoria, Australia; 3Peter MacCallum Cancer Center, Cancer Medicine, East Melbourne, Victoria, Australia
See related editorial on pages 000-000, this issue. DOI: 10.1002/cncr.29177, Received: November 2, 2014; Accepted: November 10, 2014, Published online Month 00, 2014 in Wiley Online Library (wileyonlinelibrary.com)
Cancer
Month 00, 2014
1
Editorial
histone and nonhistone proteins. Both isoform-specific HDAC inhibitors and pan-HDAC inhibitors are in development, and there is uncertainty about which will be more successful. Through alteration of chromatin structure, HDAC inhibitors can influence gene transcription and DNA repair and replication.3 Their activity also often extends more broadly, with inhibition of nonhistone proteins, including heat-shock proteins and transcription factors, leading to a multitude of effects, including tumor cell apoptosis, growth arrest, senescene, differentiation, immunogenicity, and inhibition of angiogenesis.6 Furthermore, HDAC inhibitors can induce cell death in both proliferative and nonproliferative phases of the cell cycle.7 Combined with the absence of toxicity in normal cells, these agents have the desirable properties of being tumor selective as well as targeting both proliferative and nonproliferative cells. However, the diverse mechanism of action implies that sensitivity to these agents will depend on the specific biologic changes driving tumor development. HDAC inhibitors have the potential for therapeutic use in combination therapy or monotherapy. The HDAC inhibitors vorinostat (suberoylanilide hydroxamic acid; Zolinza; Merck & Company, Inc., Whitehouse Station, NJ) and depsipeptide (romidepsin; Istodax; Celgene Corporation, Summit, NJ) are currently approved by the US Food and Drug Administration for the treatment of cutaneous and peripheral T-cell lymphoma, and, more recently, resminostat received US Food and Drug Administration “orphan drug” status for Hodgkin lymphoma and hepatocellular carcinoma. Although single-agent treatment with HDAC inhibitors has demonstrated efficacy in hematologic malignancies, a benefit in solid organ malignancies has yet to be established.8 The lack of significant single-agent activity may reflect the ability of HDAC inhibitors to modulate apoptosis as well as unknown mechanisms of resistance.8 Their acceptable toxicity profile has led to combinations with other currently approved agents, including cytotoxics, irradiation, and tyrosine kinase inhibitors, in which HDAC inhibitors appear to be a promising modulator, with multiple studies currently underway. These processes may cause epigenetic modulation that, in turn, is regulated by HDAC inhibitors, explaining the theoretical benefit of a combined approach. Alteration in chromatin structure through HDAC inhibitors may also make cells more vulnerable to DNA damage by reducing DNA repair. In this issue of Cancer, Choy and colleagues report on their phase 1 study of the HDAC inhibitor abexinostat (PCI-24781) in combination with doxorubicin in 2
patients with metastatic sarcomas.9 Although doxorubicin remains the standard-of-care agent for treating soft tissue sarcomas, its activity can be considered modest at best, resulting in a strong desire to partner doxorubicin with other synergistic cytotoxic and noncytotoxic agents. On the basis of the cell line sensitivity of abexinostat in combination with doxorubicin, including cell lines that are resistant to doxorubicin, Choy et al sought to elucidate the maximum tolerated dose (MTD), safety, and toxicity of abexinostat combined with doxorubicin in patients with sarcomas. To counter the expected dose-limiting effect of myelosuppression, combinations were assessed both with and without granulocyte-colony–stimulating factor (GCSF) support, a strategy that proved worthy of exploration given the significant difference observed in the MTD (75 mg/m2 with GCSF support compared with 45 mg/ m2 twice daily for patients who did not receive GCSF support). The majority of patients had received 2 prior regimens, and 10 had received prior doxorubicin. Seventeen of 20 patients were evaluable for response; however, as correctly stated by the authors, the small sample size and the nonrandomized nature of the study did not allow for a meaningful comparison. The current study by Choy et al sits within a growing portfolio of combination studies assessing the utility of HDAC inhibitors for patients with solid organ cancers, and the results from this trial are typical of what is being observed when agents are combined in a somewhat empirical manner. Critical review requires us to understand what we can learn from this phase 1 study and how can we better design early phase studies to reduce the attrition rate of new drugs. The authors propose the development of a more extensive phase 2 or 3 study in this population. However, we would advise caution before embarking on this costly enterprise, because several pertinent issues need to be clarified. It is clear that HDAC inhibitors can augment several therapeutics, including cytotoxics, radiation, and tyrosine kinase inhibitors; however, the molecular events that explain the additive benefit remain poorly understood. Reflecting on the nature of metastatic sarcoma trials and phase 1 trials in general, this was not a single, biologically defined population but, rather, 1 that was quite diverse. Eleven of the enrolled patients (52.4%) had a histologic sarcoma subtype described as “other." This is not unexpected, because the low prevalence of metastatic sarcoma often leads to the “lumping” together of patients on clinical trials despite significant heterogeneity. It is clear that a key element in improving the success of targeteddrug development is the identification of predictive Cancer
Month 00, 2014
Editorial/Whittle and Desai
biomarkers that enable clearer stratification with a molecular target and targeted drug. Too often, however, clinical trials are developed without an in-depth understanding of the drug target and without companion diagnostic tests to assist with patient selection.10 The challenges we face in targeting the complex biology of HDACs with HDAC inhibitors are perfect examples of this, because the growing list of HDAC inhibitors in development waiting to be brought forward through drug-development programs is yet to be matched by the development of a robust, predictive, clinically annotated biomarker. The counter argument put forward, of course, is that the requirement for companion diagnostic tests leads to unacceptable delays in drug development, thus suggesting that an empirical approach should continue.11 Although this may be true, it is reasonable to expect a plausible biologic rationale. Potential biomarkers were evaluated in the current study. The reported MTDs of abexinostat and doxorubicin exceeded the doses required for maximal histone acetylation. Because histone deacetylation is considered a biomarker for cancer, it is reasonable to conclude that histone acetylation could be considered as a surrogate biomarker. It can also be measured simply and conveniently in peripheral blood mononuclear cells. However, whereas histone acetylation is a biomarker of HDAC inhibition (ie, target specificity), it does not reflect clinical sensitivity.12 HDAC enzymes themselves are linked to tumorigenesis and have been proposed as biomarkers that can identify responsive tumors. However, the overexpression of HDAC does not necessarily predict a poor outcome; thus, the expression of HDAC may not indicate sensitivity to HDAC inhibitors or other anticancer drugs.6 Further work across tumors, including sarcomas, is required for this to be clinically relevant. In light of multiple published phase 1, 2, and 3 studies in solid tumors, it is surprising that this question has yet to be addressed. Because HDAC is involved in only a small number of genes, genetic screens at a genome-wide level or evaluating a specific subset are promising tools with which to identify prognostic and predictive biomarkers. Examples include breast cancer, in which short-term vorinostat reduced messenger RNA expression of proliferative genes.13 In addition, a genome-wide loss-of-function screen identified HR23B, a protein involved in DNA repair and protein targeting to the proteasome, as a determinant for sensitivity to HDAC inhibitors in cutaneous T-cell lymphoma. In cell culture, HR23B also sensitized osteosarcoma cells to HDAC inhibitors.14 Because of the variability of the enzymes, gene signatures are likely to vary for tumor type as well as duration of exposure and inhibitor combination.12 This Cancer
Month 00, 2014
leads to questions about the rationale for using HDAC inhibitors to target sarcoma. Choy et al cite a body of preclinical work in multiple sarcoma cell lines, in particular highlighting the understanding of the disruption of epigenetic regulation in synovial sarcoma cell lines. Accordingly, HDAC inhibition results in growth suppression and reduced invasion. However, a recently published review produced little data supporting the role of epigenetic modulation for the treatment of sarcoma, adding further to the controversy regarding how best to use these agents.15 The success of any clinical trial requires clear demonstrations of the benefit and safety of the therapeutic intervention in question. This raises an issue regarding the most appropriate endpoint for combination trials that use cytotoxics and biologics: that is, should we pursue to the MTD or to the optimal biologic dose?16 Therefore, it is critical both to select the most appropriate endpoint that accurately evaluates the biologic effect of that agent and to consider the ideal partners to synergize with that agent. With respect to the HDAC inhibitors, in vitro cell culture work suggests that HDAC inhibitors induce differentiation rather than causing apoptosis per se. Given our reliance on traditional endpoints, such as tumor response or delaying disease progression in signal-seeking clinical trials, we may need to more strongly consider this and perhaps even consider combining HDAC inhibitors with an apoptosis-inducing agent to induce such an effect. In vitro models demonstrate that HDAC inhibitors can increase the expression of proteins that transmit an apoptotic signal through death receptor pathways, providing a strong rationale for this model.17 However, we are still left with the challenge of the empirical approach, delivering drugs until reaching an MTD, versus a biologic approach. Although it is clear that HDAC inhibitors can augment a broad range of anticancer agents, the pharmacology and molecular mechanisms underpinning this remain poorly understood. Preclinical work will be required to inform future clinical trials about selection of the optimal drug and schedule. For example, chronic low-dose HDAC inhibitors as chemosensitizers may have a superior effect on tumor growth, which has been demonstrated using cell line and xenograft models.18,19 In light of the broad biologic effects described above, it may be expected that HDAC inhibitors would have a narrow therapeutic window; and, consequently, sequence-specific administration and timing of treatment combinations are likely to be important for both efficacy and toxicity. However, transformed cells are more sensitive to DNA damage from HDAC inhibitors than normal cells, and, as single agents, 3
Editorial
they demonstrate acceptable toxicity. Questions still remain regarding the long-term toxicity of HDAC inhibitors, with the risk that nontargeted epigenetic modification may promote enhanced oncogene expression and the risk of second malignancy. Furthermore, it remains to be discovered whether isoform-specific or selective HDAC inhibitors will be more effective, although, based on the diversity of targets and substrates, multiple agents are likely to play a role. In summary, the association between HDACs and cancer has led to the development of drugs targeting these enzymes as a new way to treat cancer. The progress of HDAC inhibitors demonstrates the ability to manipulate epigenetic mechanisms. HDAC inhibitors display several desirable properties for a novel agent, notably a broad range of biologic effects across cellular pathways, targeting of proliferative and nonproliferative cells as well as relative sparing of normal cells, a phenomenon that remains largely unanswered. Despite single-agent benefit in a small number of hematologic malignancies, evidence in solid organ cancers suggests that outcomes will be more promising with combination therapy and should be the focus of future studies. Finally, an understanding of the diseasespecific target and developing an appropriate biomarker must accompany further trials to help predict response and resistance. FUNDING SUPPORT No specific funding was disclosed.
CONFLICT OF INTEREST DISCLOSURES Dr. Desai reports grants from Novartis, GlaxoSmithKline, and Roche and reports service as a consultant to and on the advisory boards of Novartis, Pfizer, GlaxoSmithKline, Merck Serono, Circadian, Roche, and Bionomics outside the submitted work.
4
REFERENCES 1. Hait WN. Anticancer drug development: the grand challenges. Nat Rev Drug Discov. 2010;9:253-254. 2. Ellis L, Atadja PW, Johnstone RW. Epigenetics in cancer: targeting chromatin modifications. Mol Cancer Ther. 2009;8:1409-1420. 3. Ververis K, Hiong A, Karagiannis TC, et al. Histone deacetylase inhibitors (HDACIs): multitargeted anticancer agents. Biologics. 2013;7:47-60. 4. Bolden JE, Peart MJ, Johnstone RW. Anticancer activities of histone deacetylase inhibitors. Nat Rev Drug Discov. 2006;5:769-784. 5. Barneda-Zahonero B, Parra M. Histone deacetylases and cancer. Mol Oncol. 2012;6:579-589. 6. West AC, Johnstone RW. New and emerging HDAC inhibitors for cancer treatment. J Clin Invest. 2014;124:30-39. 7. Burgess A, Ruefli A, Beamish H, et al. Histone deacetylase inhibitors specifically kill nonproliferating tumour cells. Oncogene. 2004;23: 6693-6701. 8. Qiu T, Zhou L, Zhu W, et al. Effects of treatment with histone deacetylase inhibitors in solid tumors: a review based on 30 clinical trials. Future Oncol. 2013;9:255-269. 9. Choy E, Flamand Y, Balasubramanian S, et al. Phase 1 study of oral abexinostat, a histone deacetylase inhibitor, in combination with doxorubicin in patients with metastatic sarcoma. Cancer. 2014;000000-000. 10. Ellis LM, Bernstein DS, Voest EE, et al. American Society of Clinical Oncology perspective: raising the bar for clinical trials by defining clinically meaningful outcomes. J Clin Oncol. 2014;32:1277-1280. 11. Hirsch FR, Bunn PA Jr, Herbst RS. “Companion diagnostics”: has their time come and gone? Clin Cancer Res. 2014;20:4422-4424. 12. Stimson L, La Thangue NB. Biomarkers for predicting clinical responses to HDAC inhibitors. Cancer Lett. 2009;280:177-183. 13. Stearns V, Jacobs LK, Fackler M, et al. Biomarker modulation following short-term vorinostat in women with newly diagnosed primary breast cancer. Clin Cancer Res. 2013;19:4008-4016. 14. Khan O, Fotheringham S, Wood V, et al. HR23B is a biomarker for tumor sensitivity to HDAC inhibitor-based therapy. Proc Natl Acad Sci U S A. 2010;107:6532-6537. 15. Cote GM, Choy E. Role of epigenetic modulation for the treatment of sarcoma. Curr Treat Options Oncol. 2013;14:454-464. 16. Carraway HE, Gore SD. Addition of histone deacetylase inhibitors in combination therapy. J Clin Oncol. 2007;25:1955-1956. 17. Bots M, Johnstone RW. Rational combinations using HDAC inhibitors. Clin Cancer Res. 2009;15:3970-3977. 18. Chao H, Wang L, Hao J, et al. Low dose histone deacetylase inhibitor, LBH589, potentiates anticancer effect of docetaxel in epithelial ovarian cancer via PI3K/Akt pathway in vitro. Cancer Lett. 2013; 329:17-26. 19. Xia Q, Sung J, Chowdhury W, et al. Chronic administration of valproic acid inhibits prostate cancer cell growth in vitro and in vivo. Cancer Res. 2006;66:7237-7244.
Cancer
Month 00, 2014
Histone Deacetylase Inhibitors in Cancer: What Have We Learnt? James R. Whittle, MBBS1 and Jayesh Desai, MBBS, FRACP1,2,3
The understanding that epigenetic modification of tumor suppressor genes by histone deacetylases (HDACs) plays a role in tumorigenesis has garnered significant interest in the development of therapeutic modulators. HDAC inhibitors have emerged over the past 15 years based on their ability to induce differentiation in cell culture as well as an observed clinical benefit in a few hematologic malignancies. Despite their proven anticancer benefit, many aspects of the HDAC inhibitors are not fully understood, and their progress highlights the difficulty in developing new targeted drugs to treat cancer. This problem is not unique to HDAC inhibitors, and the lack of predictive biomarkers when trying to target a complex biologic process is a major contributor to the high rate of attrition of new targeted drugs.1 Tumor development is associated with both genetic and epigenetic alterations, which include methylation, acetylation, histone modification, and gene silencing by small RNA.2 Acetylation of the N-terminal tail of core histones is tightly controlled by the antagonistic actions of histone acetyltransferases and HDACs.3 HDACs are critical regulators of gene expression and mediate their effect through both histone and nonhistone protein substrates. Histone acetylation status influences the access of transcription factors to DNA, and alteration of this process favors deacetylated histones, leading to gene expression in the multiple pathways involved in proliferation, migration, angiogenesis, differentiation, invasion, and metastasis. Histone acetyltransferases and HDACs also mediate their effect through nonhistone protein substrates, including nuclear transcription factors, p53 (tumor protein 53), nuclear factor-jB, c-Myc (v-myc avian myelocytomatosis viral oncogene homolog), and p21, that play important roles in tumorigenesis.3 In humans, 18 HDACs are described based on their homology to yeast proteins, subfamily-specific and subunitspecific roles, substrates, and localization. Eleven of these are zinc-dependent, termed classical HDACs, and are grouped into 3 classes: class I (HDAC1, HDAC1, HDAC3, and HDAC8), class IIa (HDAC4, HDAC5, HDAC7, and HDAC9), class IIb (HDAC6 and HDAC10), and class IV (HDAC11). Class III HDACs (sirutins 1-7) require nicotinamide adenine dinucleotide (NAD1) for their activity and are not inhibited by HDAC inhibitors.4,5 More recently, attempts have been made to describe the expression of HDACs in normal and tumor tissues. Class I HDACs are expressed ubiquitously in human tissues, as demonstrated in HDAC1 or HDAC2 knockout mice, which display embryonic and perinatal lethality.5 Functionally, class I HDACs participate in many cellular process, including proliferation, cell cycle, and apoptosis, whereas class II HDACs display more tissue specificity and are involved particularly in differentiation.5 Aberrant expression of HDAC serves as a marker of normal tissue versus tumor tissue, and the overexpression of individual HDACs in solid organ cancers has been correlated with reductions in disease-free and overall survival independent of other variables. However, information can be contradictory, because the overexpression of HDAC is not always a negative prognostic variable, and considerable difficulty in translating this into the clinic is further evidence highlighting the need to understand the target of interest. For example, elevated levels of HDAC6 confer an improved prognosis in estrogen receptor-positive breast cancer and cutaneous T-cell lymphoma.6 Moreover, and paradoxically, HDACs may have a tumor suppressor role in the growth of developing tumors but remain a therapeutic target in established tumors.6 HDAC inhibitors consist a broad class of compounds, including hydroxamic acids (trichostatin A, panobinostat, vorinostat, and abexinostat), cyclic peptides (trapoxin and depsipeptide), and benzamides (butyric acid, valproic acid, etinostat, and mocetinostat). These agents inhibit HDACs with various degrees of class specificity, promoting acetylation of
Corresponding author: Jayesh Desai, MBBS, FRACP, Royal Melbourne Hospital, Grattan Street, Parkville, VIC, 3050, Australia: Fax: (011) 613-9347-7508; jayesh. [email protected] 1 Department of Medical Oncology, Royal Melbourne Hospital, Parkville, Victoria, Australia; 2The Walter and Eliza Hall Institute of Medical Research, Parkville, Victoria, Australia; 3Peter MacCallum Cancer Center, Cancer Medicine, East Melbourne, Victoria, Australia
See related editorial on pages 000-000, this issue. DOI: 10.1002/cncr.29177, Received: November 2, 2014; Accepted: November 10, 2014, Published online Month 00, 2014 in Wiley Online Library (wileyonlinelibrary.com)
Cancer
Month 00, 2014
1
Editorial
histone and nonhistone proteins. Both isoform-specific HDAC inhibitors and pan-HDAC inhibitors are in development, and there is uncertainty about which will be more successful. Through alteration of chromatin structure, HDAC inhibitors can influence gene transcription and DNA repair and replication.3 Their activity also often extends more broadly, with inhibition of nonhistone proteins, including heat-shock proteins and transcription factors, leading to a multitude of effects, including tumor cell apoptosis, growth arrest, senescene, differentiation, immunogenicity, and inhibition of angiogenesis.6 Furthermore, HDAC inhibitors can induce cell death in both proliferative and nonproliferative phases of the cell cycle.7 Combined with the absence of toxicity in normal cells, these agents have the desirable properties of being tumor selective as well as targeting both proliferative and nonproliferative cells. However, the diverse mechanism of action implies that sensitivity to these agents will depend on the specific biologic changes driving tumor development. HDAC inhibitors have the potential for therapeutic use in combination therapy or monotherapy. The HDAC inhibitors vorinostat (suberoylanilide hydroxamic acid; Zolinza; Merck & Company, Inc., Whitehouse Station, NJ) and depsipeptide (romidepsin; Istodax; Celgene Corporation, Summit, NJ) are currently approved by the US Food and Drug Administration for the treatment of cutaneous and peripheral T-cell lymphoma, and, more recently, resminostat received US Food and Drug Administration “orphan drug” status for Hodgkin lymphoma and hepatocellular carcinoma. Although single-agent treatment with HDAC inhibitors has demonstrated efficacy in hematologic malignancies, a benefit in solid organ malignancies has yet to be established.8 The lack of significant single-agent activity may reflect the ability of HDAC inhibitors to modulate apoptosis as well as unknown mechanisms of resistance.8 Their acceptable toxicity profile has led to combinations with other currently approved agents, including cytotoxics, irradiation, and tyrosine kinase inhibitors, in which HDAC inhibitors appear to be a promising modulator, with multiple studies currently underway. These processes may cause epigenetic modulation that, in turn, is regulated by HDAC inhibitors, explaining the theoretical benefit of a combined approach. Alteration in chromatin structure through HDAC inhibitors may also make cells more vulnerable to DNA damage by reducing DNA repair. In this issue of Cancer, Choy and colleagues report on their phase 1 study of the HDAC inhibitor abexinostat (PCI-24781) in combination with doxorubicin in 2
patients with metastatic sarcomas.9 Although doxorubicin remains the standard-of-care agent for treating soft tissue sarcomas, its activity can be considered modest at best, resulting in a strong desire to partner doxorubicin with other synergistic cytotoxic and noncytotoxic agents. On the basis of the cell line sensitivity of abexinostat in combination with doxorubicin, including cell lines that are resistant to doxorubicin, Choy et al sought to elucidate the maximum tolerated dose (MTD), safety, and toxicity of abexinostat combined with doxorubicin in patients with sarcomas. To counter the expected dose-limiting effect of myelosuppression, combinations were assessed both with and without granulocyte-colony–stimulating factor (GCSF) support, a strategy that proved worthy of exploration given the significant difference observed in the MTD (75 mg/m2 with GCSF support compared with 45 mg/ m2 twice daily for patients who did not receive GCSF support). The majority of patients had received 2 prior regimens, and 10 had received prior doxorubicin. Seventeen of 20 patients were evaluable for response; however, as correctly stated by the authors, the small sample size and the nonrandomized nature of the study did not allow for a meaningful comparison. The current study by Choy et al sits within a growing portfolio of combination studies assessing the utility of HDAC inhibitors for patients with solid organ cancers, and the results from this trial are typical of what is being observed when agents are combined in a somewhat empirical manner. Critical review requires us to understand what we can learn from this phase 1 study and how can we better design early phase studies to reduce the attrition rate of new drugs. The authors propose the development of a more extensive phase 2 or 3 study in this population. However, we would advise caution before embarking on this costly enterprise, because several pertinent issues need to be clarified. It is clear that HDAC inhibitors can augment several therapeutics, including cytotoxics, radiation, and tyrosine kinase inhibitors; however, the molecular events that explain the additive benefit remain poorly understood. Reflecting on the nature of metastatic sarcoma trials and phase 1 trials in general, this was not a single, biologically defined population but, rather, 1 that was quite diverse. Eleven of the enrolled patients (52.4%) had a histologic sarcoma subtype described as “other." This is not unexpected, because the low prevalence of metastatic sarcoma often leads to the “lumping” together of patients on clinical trials despite significant heterogeneity. It is clear that a key element in improving the success of targeteddrug development is the identification of predictive Cancer
Month 00, 2014
Editorial/Whittle and Desai
biomarkers that enable clearer stratification with a molecular target and targeted drug. Too often, however, clinical trials are developed without an in-depth understanding of the drug target and without companion diagnostic tests to assist with patient selection.10 The challenges we face in targeting the complex biology of HDACs with HDAC inhibitors are perfect examples of this, because the growing list of HDAC inhibitors in development waiting to be brought forward through drug-development programs is yet to be matched by the development of a robust, predictive, clinically annotated biomarker. The counter argument put forward, of course, is that the requirement for companion diagnostic tests leads to unacceptable delays in drug development, thus suggesting that an empirical approach should continue.11 Although this may be true, it is reasonable to expect a plausible biologic rationale. Potential biomarkers were evaluated in the current study. The reported MTDs of abexinostat and doxorubicin exceeded the doses required for maximal histone acetylation. Because histone deacetylation is considered a biomarker for cancer, it is reasonable to conclude that histone acetylation could be considered as a surrogate biomarker. It can also be measured simply and conveniently in peripheral blood mononuclear cells. However, whereas histone acetylation is a biomarker of HDAC inhibition (ie, target specificity), it does not reflect clinical sensitivity.12 HDAC enzymes themselves are linked to tumorigenesis and have been proposed as biomarkers that can identify responsive tumors. However, the overexpression of HDAC does not necessarily predict a poor outcome; thus, the expression of HDAC may not indicate sensitivity to HDAC inhibitors or other anticancer drugs.6 Further work across tumors, including sarcomas, is required for this to be clinically relevant. In light of multiple published phase 1, 2, and 3 studies in solid tumors, it is surprising that this question has yet to be addressed. Because HDAC is involved in only a small number of genes, genetic screens at a genome-wide level or evaluating a specific subset are promising tools with which to identify prognostic and predictive biomarkers. Examples include breast cancer, in which short-term vorinostat reduced messenger RNA expression of proliferative genes.13 In addition, a genome-wide loss-of-function screen identified HR23B, a protein involved in DNA repair and protein targeting to the proteasome, as a determinant for sensitivity to HDAC inhibitors in cutaneous T-cell lymphoma. In cell culture, HR23B also sensitized osteosarcoma cells to HDAC inhibitors.14 Because of the variability of the enzymes, gene signatures are likely to vary for tumor type as well as duration of exposure and inhibitor combination.12 This Cancer
Month 00, 2014
leads to questions about the rationale for using HDAC inhibitors to target sarcoma. Choy et al cite a body of preclinical work in multiple sarcoma cell lines, in particular highlighting the understanding of the disruption of epigenetic regulation in synovial sarcoma cell lines. Accordingly, HDAC inhibition results in growth suppression and reduced invasion. However, a recently published review produced little data supporting the role of epigenetic modulation for the treatment of sarcoma, adding further to the controversy regarding how best to use these agents.15 The success of any clinical trial requires clear demonstrations of the benefit and safety of the therapeutic intervention in question. This raises an issue regarding the most appropriate endpoint for combination trials that use cytotoxics and biologics: that is, should we pursue to the MTD or to the optimal biologic dose?16 Therefore, it is critical both to select the most appropriate endpoint that accurately evaluates the biologic effect of that agent and to consider the ideal partners to synergize with that agent. With respect to the HDAC inhibitors, in vitro cell culture work suggests that HDAC inhibitors induce differentiation rather than causing apoptosis per se. Given our reliance on traditional endpoints, such as tumor response or delaying disease progression in signal-seeking clinical trials, we may need to more strongly consider this and perhaps even consider combining HDAC inhibitors with an apoptosis-inducing agent to induce such an effect. In vitro models demonstrate that HDAC inhibitors can increase the expression of proteins that transmit an apoptotic signal through death receptor pathways, providing a strong rationale for this model.17 However, we are still left with the challenge of the empirical approach, delivering drugs until reaching an MTD, versus a biologic approach. Although it is clear that HDAC inhibitors can augment a broad range of anticancer agents, the pharmacology and molecular mechanisms underpinning this remain poorly understood. Preclinical work will be required to inform future clinical trials about selection of the optimal drug and schedule. For example, chronic low-dose HDAC inhibitors as chemosensitizers may have a superior effect on tumor growth, which has been demonstrated using cell line and xenograft models.18,19 In light of the broad biologic effects described above, it may be expected that HDAC inhibitors would have a narrow therapeutic window; and, consequently, sequence-specific administration and timing of treatment combinations are likely to be important for both efficacy and toxicity. However, transformed cells are more sensitive to DNA damage from HDAC inhibitors than normal cells, and, as single agents, 3
Editorial
they demonstrate acceptable toxicity. Questions still remain regarding the long-term toxicity of HDAC inhibitors, with the risk that nontargeted epigenetic modification may promote enhanced oncogene expression and the risk of second malignancy. Furthermore, it remains to be discovered whether isoform-specific or selective HDAC inhibitors will be more effective, although, based on the diversity of targets and substrates, multiple agents are likely to play a role. In summary, the association between HDACs and cancer has led to the development of drugs targeting these enzymes as a new way to treat cancer. The progress of HDAC inhibitors demonstrates the ability to manipulate epigenetic mechanisms. HDAC inhibitors display several desirable properties for a novel agent, notably a broad range of biologic effects across cellular pathways, targeting of proliferative and nonproliferative cells as well as relative sparing of normal cells, a phenomenon that remains largely unanswered. Despite single-agent benefit in a small number of hematologic malignancies, evidence in solid organ cancers suggests that outcomes will be more promising with combination therapy and should be the focus of future studies. Finally, an understanding of the diseasespecific target and developing an appropriate biomarker must accompany further trials to help predict response and resistance. FUNDING SUPPORT No specific funding was disclosed.
CONFLICT OF INTEREST DISCLOSURES Dr. Desai reports grants from Novartis, GlaxoSmithKline, and Roche and reports service as a consultant to and on the advisory boards of Novartis, Pfizer, GlaxoSmithKline, Merck Serono, Circadian, Roche, and Bionomics outside the submitted work.
4
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