Key points

Introduction

Cutaneous T-cell lymphomas (CTCLs) are non-Hodgkin’s T-cell lymphomas that present as skin lesions. The most common is mycosis fungoides (MF) and its leukemic variant, Sézary syndrome (SS). CTCLs are currently rarely cured and may have an indolent clinical course. MF with large cell transformation, defined as greater than 25% atypical lymphocytes with nuclei 4 times the normal size, has poor survival, similar to that of peripheral T-cell lymphoma (PTCL) with a 5-year overall survival of only 32% with involved skin and 7% with extracutaneous involvement. The failure of MF with large cell transformation and PTCL to respond to multiagent chemotherapy with cytoxan, adriamycin, oncovin, and prednisone (CHOP) or CHOP-based chemotherapy has resulted in a search for novel targeted agents including antibodies and gene modulators. Histone deacetylase (HDAC) inhibitors are small molecules that seem to be particularly active for T-cell lymphoma.

Introduction

Cutaneous T-cell lymphomas (CTCLs) are non-Hodgkin’s T-cell lymphomas that present as skin lesions. The most common is mycosis fungoides (MF) and its leukemic variant, Sézary syndrome (SS). CTCLs are currently rarely cured and may have an indolent clinical course. MF with large cell transformation, defined as greater than 25% atypical lymphocytes with nuclei 4 times the normal size, has poor survival, similar to that of peripheral T-cell lymphoma (PTCL) with a 5-year overall survival of only 32% with involved skin and 7% with extracutaneous involvement. The failure of MF with large cell transformation and PTCL to respond to multiagent chemotherapy with cytoxan, adriamycin, oncovin, and prednisone (CHOP) or CHOP-based chemotherapy has resulted in a search for novel targeted agents including antibodies and gene modulators. Histone deacetylase (HDAC) inhibitors are small molecules that seem to be particularly active for T-cell lymphoma.

Mechanism of action of histone deacetylase inhibitors

Histone acetyltransferases and HDACs are enzymes capable of modifying histone and nonhistone acetylation sites in proteins. Histone acetyltransferases and HDACs regulate a broad range of pathways and processes that are dysregulated in cancer, including the cell cycle, apoptosis, and protein folding. The balance between histone acetylation effected by acetylases and deacetylation is mediated by HDAC inhibitors and is abnormal in cancer cells. Acetylation of histones promotes opened chromatin, transcription factor binding to promoters, and initiation of mRNA synthesis encoding genes. The action of deacetylase inhibitors is not limited to histones and can prevent deacetylation of other proteins, such as the tumor promoter p53.

HDAC inhibitors are small molecules that interact with HDAC’s catalytic sites preventing the removal of acetyl groups, counteracting the effects of HDACs. Valproic acid was the first HDAC inhibitor investigated for malignancy followed by development of pan-HDAC and more selective inhibitors to improve efficacy and safety. Whether HDAC inhibitors with selected specificity are more effective with fewer side effects and thus are superior to pan-inhibitors is still under debate. Wells and colleagues showed that selective HDAC3 inhibition by the first HDAC3 inhibitor, RGFP966, resulted in decreased cell growth in CTCL lines owing to increased apoptosis associated with DNA damage and impaired S phase progression. HDAC3 was present around the DNA replication forks and significantly decreased the speed of replication.

The 18 known HDACs are classified into 4 main groups based on their homology to yeast HDACs and dependence on the essential cofactor, zinc, present at the active sites. Zinc-dependent HDACs include those of class I that target histones and includes HDACs 1, 2, 3, and 8; class IIa HDACs that target both histones and nonhistone proteins and includes HDACs 4, 5, 7, 9; Class IIb HDAC 6, which targets the mitogen-activated protein kinase pathway, and 10; and class IV HDAC targeting only HDAC 11. The class 3 HDACs, called sirtulins (1–7), are zinc independent and nicotinamide adenine dinucleotide dependent.

HDAC inhibitors have been classified by 4 major chemical structures and their ability to interact with specific HDACs. By clinical structure, HDAC inhibitors are short-chain fatty acids (valproic acid), hydroxamic acids (vorinostat [suberoylanilide hydroxamic acid (SAHA)], panobinostat [LBH589], and quisinostat [JNJ-26481585]), depsipeptide (romidepsin [FK228]), and benzamides (entinostat [MS-275] and mocetinostat [MGCD-0103]). The chemical structures of the hydroxamic acids and romidepsin are shown in Fig. 1 . Vorinostat was the first HDAC inhibitor approved by US Food and Drug Administration (FDA) for use in cancer based on 2 phase II studies in refractory CTCL patients. In a study of 45 PTCL patients, HDAC 1, 2, and 6 were overexpressed compared with normal lymphoid tissue making T-cell lymphoma a logical target for HDAC inhibition.

Chemical structures of histone deacetylase inhibitors.

HDAC inhibitors inhibit cell cycle progression by upregulating p21, p27, and p16, which bind to and deactivate CDK2 and CDK4 leading to G1 arrest. HDAC inhibitors may also inhibit S-phase progression through inhibition of cytidine triphosphate synthase and thymidylate synthetase, involved in DNA synthesis. They also induce generation of reactive oxygen species while inhibiting DNA repair.

The balance between proapoptotic and antiapoptotic factors is influenced by HDAC inhibitors. HDAC inhibitors increase expression of genes encoding death receptors and their respective ligands (Fas and Apo 2 L/TRAIL receptors, DR4 and DR5). They also downregulate c-FLIP (a negative regulator of caspase-8), and modulate the mitogen-activated protein kinase pathway, a possible mechanism of resistance. HDAC inhibitors also upregulate expression of proapoptic BH3 domain proteins while downregulating the antiapoptotic proteins BCL-2, BCL-XL, and MCL-1. In leukemia cells, apoptosis-involved pathways independent of p53, regulated by Bcl-2/Bxl-XL, c-Jun, and p21CIP1.

The STAT family of transcription factors (STAT-3, -4, -5, and -6) are thought to have a key role in driving T-cell proliferation in CTCL. STAT-4, associated with a T helper cell (Th) 1 phenotype, is downregulated in CTCL cell lines, whereas STAT-6, which is associated with Th2 phenotype, is upregulated. Change in STAT expression could contribute to the TH1 to TH2 phenotype switch seen in disease progression. Increasing STAT-5 signaling may upregulate Th2 cytokines and synergize to promote the malignancy. Downregulation of STAT-4 may be owing to upregulation of STAT 5 signaling during the early stages of CTCL. HDAC inhibitors may act through STATs to increase the Th1>TH2 response.

Growing evidence shows that STAT5 is important for the expression of anti-apoptotic proteins (including bcl-2 and bcl-x), cell cycle genes (Cyclin D and c-myc), and the oncogenic miR-155 microRNA (which has a putative binding site on the STAT4 3′UTR). Evidence suggests that with STAT5 upregulation early in CTCL, the resulting miR-155 increase promotes proliferation of malignant T-cells and may be responsible for STAT-4 decline and conversion to TH2 phenotype. The imbalance between STAT-4 and STAT-6 is thought to be owing to aberrant histone acetylation. MyLa cells treated with romidepsin or vorinostat, show downregulation of STAT-6 and upregulation of STAT-4, which is beneficial for CTCL. HDAC inhibitors may restore this balance because treatment with HDAC inhibitors have been shown to upregulate STAT-4 and decrease STAT-6 in vitro.

Vorinostat (Zolinza; suberoylanilide hydroxamic acid)

The mechanisms of action of vorinostat have been studied extensively with complexity related to the ability of HDAC inhibitors to modulate multiple genes and cell types. Vorinostat induces differentiation and growth arrest in a wide range of cancer cells studied.

We reported that CTLC lines treated in vitro with vorinostat underwent apoptosis selectively compared with normal T cells. The level of acetylated histone protein does not predict response, but did increase in duration with higher doses of vorinostat. In MF skin lesions, phospho-stat 3 localized in the cytoplasm at baseline became nuclear in distribution in patients who had clinical responses to vorinostat. Vorinostat and romidepsin were reported to downregulate expression of interleukin-10 (a Th2 cytokine) in CTCL cells.

Vorinostat (suberoyl + anilide + hydroxamic acid or SAHA), is a class I/II HDAC inhibitor with a hydroxamic structure. After activity was shown in cancer cell lines, vorinostat was evaluated in phase I trials as both an oral and intravenous formulation with initial responses seen in hematopoietic malignancies, including T-cell lymphoma. Vorinostat was the first HDAC inhibitor to receive FDA approval in 2006 for the treatment of relapsed/refractory CTCL. Approval was based on the overall response seen in 2 phase II single-arm clinical trials. A phase I study was also completed in patients with advanced leukemia, and the drug was active in nodal lymphoma.

The initial phase II trial was a single-center, open-label, dose-ranging study of 33 advanced CTCL patients, all with either MF or SS. Patients with relapsed/refractory MF/SS and a median of 5 prior therapies were included. Response to vorinostat in the first group of patients suggested that an optimal dose of 400 mg given orally once daily and an overall response rate (ORR) of 24% with no complete remissions (CR). The median duration of response was only about 4 months. Intermittent dose schedules of 300 mg by mouth twice daily for 2 weeks with a 2 week rest were also clinically active, especially for SS patients, but the highest dose was associated with more frequent and severe dose-limiting thrombocytopenia. The most common drug-related adverse events (AEs) were fatigue (73%), thrombocytopenia (54%), diarrhea (49%), nausea (49%), and dysgeusia (46%); the most common grade 3 or 4 AE was thrombocytopenia (19%), followed by anemia, deep vein thrombosis, dehydration, and pyrexia (each experienced by 8% of patients).

The second phase II multicenter, open-label, clinical trial enrolled 74 patients with MF/SS who had at least 2 prior systemic therapies. The ORR of 29% was similar to the first study and the median duration of response was longer than 6 months. Of note, 6 patients had long-term disease control exceeding 2 years. The recommended dose of vorinostat for the phase IIb trial was 400 mg by mouth daily with dose reductions taken for gastrointestinal or thrombocytopenia side effects. The most common drug-related AEs were diarrhea (49%), fatigue (46%), nausea (43%), anorexia (26%), and dysgeusia (24%); the most common grade 3 or 4 AEs were thrombocytopenia (5%), nausea (4%), anorexia (3%), and muscle spasms (3%).

HDAC inhibitors have in general similar side effect and safety profiles, but vorinostat given orally has gastric side effects of diarrhea, nausea, and loss of taste, which can cause weight loss and dehydration, especially in older patients. Other common symptoms of HDAC inhibitors are fatigue and thrombocytopenia owing to platelet maturation arrest. The initial warning of alterations in cardiac rhythm with HDAC inhibitors was removed based on lack of evidence. Elimination is primarily hepatic, with total mean plasma clearance and elimination half-life of 1240 mL/min and 1.1 hours, respectively.

Vorinostat has been combined with other agents, especially with protease inhibitors and lenolidamide, in the setting of multiple myeloma. With respect to CTCL, a single phase I safety and efficacy multicenter clinical trial evaluated 23 advanced MF/SS patients treated with vorinostat (200, 300, and 400 mg/d) combined with bexarotene (150, 225, and 300 mg/m ² ). Patients were allowed to stay on treatment as long as their disease was stable or improved. The maximum tolerated dose for the therapies when combined were determined to be vorinostat 200 mg/day plus bexarotene 300 mg/m ² per day, lower doses for vorinostat than standard monotherapy. A confirmed objective responses was observed in 4 patients, unconfirmed response was observed in 2 patients, and stable disease was observed in 15 patients. The most common treatment-related AEs were hypothyroidism (35%), fatigue (30%), and hypertriglyceridemia (30%). Five drug-related serious AEs were reported in 4 patients (lymphangitis, lymph node abscess, skin necrosis, gastroenteritis, and fall). Dose-limiting toxicities experienced were grade 3 hypertriglyceridemia and grade 1 diarrhea in patients taking vorinostat 300 mg/d plus bexarotene 150 mg/m ² , and grade 3 neutropenia in patients taking vorinostat 400 mg/d plus bexarotene 225 mg/m ² . The study was discontinued before meeting its endpoints.