Mark J. Bishton

Clinical Studies of Histone Deacetylase Inhibitors

Over the last 5 years, a plethora of histone deacetylase inhibitors (HDACi) have been evaluated in clinical trials. These drugs have in common the ability to hyperacetylate both histone and nonhistone targets, resulting in a variety of effects on cancer cells, their microenvironment, and immune responses. To date, responses with single agent HDACi have been predominantly observed in advanced hematologic malignancies including T-cell lymphoma, Hodgkin lymphoma, and myeloid malignancies. Therefore, in this review we focus upon hematologic malignancies. Generally HDACi are well tolerated with the most common acute toxicities being fatigue, gastrointestinal, and transient cytopenias. Of note, few patients have been treated for prolonged periods of time and little is known about long-term toxicities. The use of the biomarker of histone hyperacetylation has been useful as a guide to target specificity, but generally does not predict for response and the search for more clinically relevant biomarkers must continue.

Deciphering the molecular and biologic processes that mediate histone deacetylase inhibitor–induced thrombocytopenia

Histone deacetylase inhibitor (HDACI)-induced thrombocytopenia (TCP) is a major dose-limiting toxicity of this new class of drugs. Using preclinical models to study the molecular and biologic events that underpin this effect of HDACI, we found that C57BL/6 mice treated with both the HDAC1/2-selective HDACI romidepsin and the pan-HDACI panobinostat developed significant TCP. HDACI-induced TCP was not due to myelosuppression or reduced platelet lifespan, but to decreased platelet release from megakaryocytes. Cultured primary murine megakaryocytes showed reductions in proplatelet extensions after HDACI exposure and a dose-dependent increase in the phosphorylation of myosin light chain 2 (MLC2). Phosphorylation of MLC to phospho-MLC (pMLC) and subsequent proplatelet formation in megakaryocytes is regulated by the Rho-GTPase proteins Rac1, CDC42, and RhoA. Primary mouse megakaryocytes and the human megakaryoblastic cell line Meg-01 showed reductions in Rac1, CDC42, and RhoA protein levels after treatment with HDACIs. We were able to overcome HDACI-induced TCP by administering the mouse-specific thrombopoietin (TPO) mimetic AMP-4, which improved platelet numbers to levels similar to untreated controls. Our report provides the first detailed account of the molecular and biologic processes involved in HDACI-mediated TCP. Moreover, our preclinical studies provide evidence that dose-limiting TCP induced by HDACIs may be circumvented using a TPO mimetic.

Panobinostat (LBH589): a potent pan-deacetylase inhibitor with promising activity against hematologic and solid tumors.

The deacetylase inhibitors are a structurally diverse class of targeted antineoplastic agents that have demonstrated in vitro and in vivo preclinical activity in a wide range of malignancies. Based on this preclinical activity, several deacetylase inhibitors have undergone rapid clinical development in recent years. Among these, the deacetylase inhibitor panobinostat is one of the most widely studied, with extensive pharmacokinetic, pharmacodynamic and dose-finding data available across a wide variety of hematologic and solid tumors. Furthermore, panobinostat has demonstrated favorable clinical activity against various hematologic malignancies, most notably lymphomas and myeloid malignancies in Phase I and II studies. In this article, we discuss the preclinical data on panobinostat and emerging data from Phase I and II studies in cancer patients.

Preliminary evidence of disease response to the pan deacetylase inhibitor panobinostat (LBH589) in refractory Hodgkin Lymphoma

There are few treatment options for patients with Hodgkin Lymphoma (HL) who relapse after conventional therapies. Panobinostat is an orally available pan deacetylase inhibitor with evidence of activity in myeloid malignancies and cutaneous T cell lymphoma. Thirteen HL patients were treated with escalating doses of this novel agent in a phase IA/II multicentre study. A computed tomography partial response was achieved in 5/13(38%), and a metabolic response by 18F‐fluoro‐2‐deoxy‐d‐ glucose positron emission tomography scanning in 7/12 (58%) evaluable patients. This report describes the preliminary evidence of anti‐tumour activity seen in the early phase of this study, which recently closed to accrual.

Histone deacetylase inhibitors in lymphoma and solid malignancies

Histone deacetylase inhibitors (HDACi) are a new class of antineoplastic agents with demonstrable preclinical antitumor activity in both in vitro and in vivo studies in a wide range of malignancies. Based on these preclinical findings, in recent years HDACi have undergone a rapid phase of clinical development with many HDACi entering Phase I–III clinical trials, both as single agents and in combination with other therapies. Favorable clinical responses have been demonstrated in cutaneous T-cell lymphoma with emerging evidence of clinical activity in other types of lymphoma and, to date, a good toxicity profile. Solid tumor responses to single agent and combination therapies have also been reported, paving the way for larger studies in this field. In this review we discuss the recent advances in the clinical development of HDACi and their current therapeutic role in lymphoma and solid malignancies.

Epigenetic targets in hematological malignancies: combination therapies with HDACis and demethylating agents

Chromatin modeling in DNA is fundamental to gene expression, DNA repair and replication. Methylation of promoter regions of tumor-suppressor genes and histone deacetylation leads to gene silencing and transcriptionally repressive chromatin. Histone deacetylase inhibitors and hypomethylating agents allow upregulation of proapoptotic genes and downregulation of antiapoptotic genes, and show significant single-agent anticancer activity, predominantly in cutaneous T-cell lymphoma and myelodysplasia, respectively. Combinations of these drugs are being employed in clinical trials to target multiple biological pathways, with the hope of synergistic pharmacodynamics. Preclinical studies of combinations of these agents with chemotherapy, monoclonal antibodies and small-molecule inhibitors are ongoing and demonstrate synergy in multiple hematological cancers, raising the prospect of future treatment for these diseases having a multitargeted approach.

The role of granulocyte transfusions in neutropenic patients

The precise role for donor granulocyte infusions remains to be delineated, partly because of the lack of defined clinical trials. The aim of this article is to summarize the studies undertaken so far and highlight the logistical problems associated with undertaking future studies. We also aim to provide a practical guide to the application of this therapeutic approach.

A focus on the preclinical development and clinical status of the histone deacetylase inhibitor, romidepsin (depsipeptide, Istodax(®)).

Romidepsin (Istodax(®), depsipeptide, FR901228, FK228, NSC 630176) is a cyclic peptide, broad-spectrum, potent histone deacetylase inhibitor, with activity mainly against class I histone deacetylase enzymes. In this article, we give an overview of the putative modes of action, such as effects on gene expression, cell cycle regulation, apoptosis induction, DNA repair, protein acetylation and induction of autophagy. Romidepsin has mainly been developed as a therapy for hematologic malignancies and is approved by the US FDA for the treatment of cutaneous T-cell lymphomas. This report outlines the laboratory and clinical development of the compound as a single agent that has more recently been evaluated in combination with other anticancer therapeutics, such as proteasome inhibitors.

Ifosphamide, etoposide and epirubicin is an effective combined salvage and peripheral blood stem cell mobilisation regimen for transplant-eligible patients with non-Hodgkin lymphoma and Hodgkin disease.

A total of 143 patients with relapsed (n =  90), primary refractory (n = 32) and first line chemotherapy responsive (n = 21) non‐Hodgkin lymphoma (NHL) and Hodgkin disease (HD) were treated with IVE (ifosphamide, etoposide and epirubicin) chemotherapy with the intent to proceed to high‐dose therapy with either autologous or allogeneic transplantation, following peripheral blood stem cell mobilisation. A major response (complete/partial response) to IVE was seen in 115 patients (80·4%) with 5‐year overall survival (OS) and event free survival (EFS) of 53% and 43%, respectively. Subgroup analysis showed overall response rates of 93·1% for HD with a 5‐year OS and EFS of 62% and 52% respectively, while NHL showed response rates of 78·0% with 5‐year OS and EFS of 50% and 39% respectively. The median number of CD34 +ve cells mobilised following IVE was 7·86 × 106 (range 1·72–42·91 × 106), with 60% mobilising >2 × 106/kg in a single collection. Grade IV neutropenia was seen in 79·6% patients and 77/270 cycles required intravenous antibiotic treatment. We conclude that IVE has a high response rate across a range of refractory and relapsed lymphoma with acceptable toxicity and excellent PBSC mobilising characteristics.

IVE (ifosfamide, epirubicin and etoposide) is a more effective stem cell mobilisation regimen than ICE (ifosphamide, carboplatin and etoposide) in the context of salvage therapy for lymphoma

Two commonly used chemotherapy regimens for lymphoma salvage therapy were compared: ICE (ifosphamide, carboplatin and etoposide) ± rituximab and IVE (ifosfamide, epirubicin and etoposide) ± rituximab, for their efficacy in mobilising peripheral blood stem cells for autologous transplantation. Significant differences were observed between the cohorts in terms of number of patients mobilising the stipulated minimum >2 × 106 CD34+/kg (99·2% in IVE group versus 83% in ICE group: P = 0·0002) and also in terms of the number of patients achieving the predetermined target of >5 × 106 CD34+/kg, both in total and during the first apheresis procedure (72% in IVE versus 51% in ICE group and 49% in IVE versus 7% in ICE group: P = 0·02 and P < 0·0001 respectively). This analysis of two similar groups of patients treated within a single‐centre appears to demonstrate that the IVE regimen is a more effective stem cell mobilisation regimen than ICE in the context of salvage therapy for Hodgkin and non‐Hodgkin lymphoma, allowing more patients to achieve the target CD34+ cell collection and proceed to high‐dose therapy and autologous stem cell transplantation.