Histone deacetylase inhibitors (HDAC inhibitors) are a group of chemotherapy agents used to treat hematological cancers. The Food and Drug Administration has approved the use of four HDAC inhibitors to treat T-cell lymphoma and myeloma.

HDAC inhibitors have shown promising results as targeted cancer therapies because of their limited side effects and their ability to selectively kill cancer cells [1]. This is an epigenetic therapy.  Histones are proteins that DNA wrap around in chromosomes.  In normal functioning of the cell, histones are sometimes modified by acetylation; this an epigenetic regulation of gene expression.  The body has both histone deacetylases and histone acetyltransferases.  The HDAC inhibitor drugs works at this level of biochemistry.

Interest in these drugs has been so high that there are over 100 clinical trials running on HDAC inhibitory drugs [2] for treatment of blood and solid cancers.

How do histone deacetylase inhibitors work?

Histone deacetylase inhibitors are thought to act by inhibiting DNA repair, altering gene expression and making post-translational modifications to proteins [3]. In addition, HDAC inhibitors have been shown to stop the proliferation of cancer cells, stimulate apoptosis (cell death) and arrest the cell cycle [3].  Some have antiangiogenic properties.  These actions collectively result in the disruption of vital cell functions and stop the ability of cancerous cells to grow and multiply. Yet unlike other anti-cancer drugs, research shows that HDAC inhibitors may at times act selectively.

For example, Vorinostat (approved by the FDA for treatment of multiple myeloma) causes DNA strands to break in both normal and cancerous cells [4]. However, normal cells can recover from this destruction and will repair themselves while the cancerous cells cannot.

These multiple mechanisms make HDAC inhibitors an attractive area for medicinal chemists to pursue when looking for new anti-cancer drugs.

Specific Histone deacetylase inhibitors

The US FDA has approved the use of four HDACs inhibitors: Vorinostat (Zolinza) and Romidepsin (Istodax) for the treatment of cutaneous T-cell lymphoma [7] and Belinostat (Beleodaq) and Panobinstat (Farydak) for the treatment of peripheral T-cell lymphoma and multiple myeloma respectively.

Broadly speaking, Vorinostat and Romidepsin are used in cases where the cancer is still present in the body, if the cancer gets worse or if it comes back during or after treatment with other medications [5].

Belinostat is approved for use in relapsed or refractory peripheral T-cell lymphoma. This is cancer that has returned or has been resistant to other treatments.

Panobinstat is marketed as the first HDAC inhibitor approved to treat multiple myeloma. It is intended for patients who have received at least two prior standard therapies and only in combination with bortezomib (another anti-cancer drug), chemotherapy and dexamethasone, an anti-inflammatory medication.

Oncologists have the option of using HDAC inhibitors either as monotherapy (not with other drugs) or in combination with other chemotherapy agents or in combination with radiotherapy.

Tumors treated by Histone deacetylase inhibitors

To date histone deacetylase inhibitors have been used in the treatment of cancers like lymphoma and myeloma. For reasons still not fully understood by scientists these inhibitors don’t appear to work as well against solid tumors.

However some of the clinical trials currently in operation are testing HDAC inhibitors in treatment of solid tumors like lung, breast and prostate cancer [6].

Vorinostat is given orally in the treatment of peripheral T-cell lymphoma. Like other HDAC inhibitors, Vorinostat is fairly well tolerated.  All drugs have potential side effects; the most common side effects this drug produces are fatigue, diarrhea and nausea.

Romidepsin and Belinostat have similar side effect profiles and are administered intravenously (into a vein). They are usually given on tightly scheduled days over a 28-day cycle.

Panobinstat is given orally to treat myeloma.

Although much is known about the actions of HDAC inhibitors and their future as cancer-fighting drugs appears promising, scientists still do not have a perfect understanding of how these drugs work. Observers hope research in this area will bring about further developments of more HDAC drugs that can be used in the fight against cancer.

Right now there are four HDAC inbibitors on the market.  PDF List of HDAC Inhibitors

Belinostat

Brand/Trade Names: Beleodaq, PXD101

Manufacturers: Biophore India Pharmaceuticals Pvt Ltd, Fresenius Kabi Oncology Limited, MSN Laboratories Pvt Ltd,   Sionc Pharmaceuticals,  Spectrum Pharmaceuticals

Formula: C₁₅H₁₄N₂O₄S

Mechanism: HDAC inhibitor

Class:

Administration: Intravenous

Notes:  Approved by the FDA in 2014.  Used to treat lymphoma.

Panobinostat

Brand/Trade Names: Farydak

Manufacturer: Novartis Pharmaceuticals

Formula: C₂₁H₂₃N₃O₂

Mechanism: HDAC inhibitor

Class:

Administration: Oral

Notes:  Approved by the FDA in 2015.  Used to treat multiple myeloma.

Romidepsin

Brand/Trade Names: Istodax, Chromadax

Manufacturers: Celgene Corporation, Concord Biotech, MSN Laboratories Pvt Ltd, Mylan Inc 

Formula: C₂₄H₃₆N₄O₆S₂

Mechanism: HDAC inhibitor

Class:

Administration: Intravenous

Notes: Approved by the FDA in 2009.  Used to treat lymphoma. Romidepsin for the treatment of relapsed/refractory peripheral T-cell lymphoma

Vorinostat

Brand/Trade Names: Zolinza

Manufacturers: Merck Sharp & Dohme Corp., AMRI,  Mylan Inc,   Unipex

Formula: C₁₄H₂₀N₂O₃

Mechanism: HDAC inhibitor

Class:

Administration: Oral

Notes: Also known as suberanilohydroxamic acid.  Approved by the FDA in 2006.  Used for treatment of lymphoma.

References

1) Lee, J. H., Choy, M. L., Ngo, L., Foster, S. S., & Marks, P. A. (2010). Histone deacetylase inhibitor induces DNA damage, which normal but not transformed cells can repair. Proceedings of the National Academy of Sciences, 107(33), 14639-14644.
http://www.pnas.org/content/107/33/14639.full

2) Tan, J., Cang, S., Ma, Y., Petrillo, R. L., & Liu, D. (2010). Novel histone deacetylase inhibitors in clinical trials as anti-cancer agents. Journal of hematology & oncology, 3(1), 5.
https://jhoonline.biomedcentral.com/articles/10.1186/1756-8722-3-5

3) Katoch O, Dwarakanath B and Agrawala P K: HDAC inhibitors: applications in oncology and beyond. HOAJ Biology 2013, 2:2 http://dx.doi.org/10.7243/2050-0874-2-2
http://www.hoajonline.com/hoajbiology/2050-0874/2/2

4) Lee, J. H., Choy, M. L., Ngo, L., Foster, S. S., & Marks, P. A. (2010). Histone deacetylase inhibitor induces DNA damage, which normal but not transformed cells can repair. Proceedings of the National Academy of Sciences, 107(33), 14639-14644.
http://www.pnas.org/content/107/33/14639.full

5) Bubna, A. K. (2015). Vorinostat—An Overview. Indian Journal of Dermatology, 60(4), 419. http://doi.org/10.4103/0019-5154.160511
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4533557/

6) Halsall, J. A. and Turner, B. M. (2016), Histone deacetylase inhibitors for cancer therapy: An evolutionarily ancient resistance response may explain their limited success. BioEssays, 38: 1102–1110. doi:10.1002/bies.201600070
http://onlinelibrary.wiley.com/doi/10.1002/bies.201600070/full

7) Science Direct. Histone deacetylase inhibitor
https://www.sciencedirect.com/topics/neuroscience/histone-deacetylase-inhibitor