Treatment of cancer by targeting epigenetic pathways has been referred to as epigenetic therapy20
. The principle of this approach is to reverse pathologic gene expression changes in malignant cells, which is presumed to elicit a therapeutic effect that culminates in tumor responses. The specific therapeutic effect induced by these agents is still a matter of debate, and there are data favoring many differing pathways as affected by this therapy, including differentiation, senescence, apoptosis, immune recognition etc.40
In addition, many epigenetic drugs show dose-dependent cytotoxicity8;80
, which could also be a factor in responses. The therapeutic index of this approach lies, in principle, on the fact that cancer cells are more dependent on continued silencing tumor-suppressor genes and like molecules than normal cells. There have been concerns that epigenetic therapy could have dramatic effects on normal cells through reactivating, for example, imprinted genes, and may even lead to cancer formation. In-vivo, however, these fears have not been substantiated in studies so far, with no evidence of unusual side-effects, new chromosomal defects or secondary malignancies107
. It is likely that the doses of drugs that elicit such effects are not achievable in-vivo.
Targeting epigenetics at the present time essentially equates to targeting silencing pathways in cancer and their potential interactions. A central pathway () involves DNA methylation, methyl-binding proteins, histone deacetylation, histone (H3K9) methylation and eventual binding of a silencing protein complex that, itself, may trigger more methylation57
. This “silencing loop” is stable, as evidenced by imprinted genes and the inactive X-chromosome, which remain turned off for decades in adult cells. There is also a distinct silencing mechanism in development and cancer that involves the PCG complexes PRC1 and PRC2, resulting in H3K27me3 modification. These complexes also involve histone deacetylases. Each step in these silencing cascades is a potential target for therapeutic intervention. Combining inhibition of multiple epigenetic targets will likely be synergistic, as demonstrated for DNA methylation and HDAC inhibition14
. There are two epigenetic targets for which drugs are available in clinical trials – DNA methylation and histone deacetylation.
5-Azacytidine (azacitidine) and 5-aza-2′-deoxycytidine (DAC, decitabine) are two hypomethylating cytosine analogues with activity in leukemia65;91
. Both drugs were synthesized in the 1960’s as cytosine analogues, and were shown in the early 1980’s to be potent DNA methylation inhibitors and in-vitro differentiation inducers50
. DNA methylation inhibition is related to the shared modified structure of the cytosine ring with a C to N substitution at the 5 position, resulting in trapping and eventual degradation of DNA methyltransferases. Azacitidine incorporates into RNA and, after intracellular conversion to DAC, incorporates into DNA and inhibits DNA methylation. Unlike azacitidine, DAC does not incorporate into RNA and is directly incorporated into DNA, resulting in 10 fold higher demethylating activity at equimolar concentrations in-vitro50
. Both drugs have activity in MDS demonstrated in randomized studies65;70;94
, and both are now approved by the United States Food and Drug Administration. A feature of both azacitidine and DAC studies in MDS has been delayed clearance of blasts, delayed myelosuppression, slow responses (median number of cycles to best response was >3) and eventual cytogenetics responses. A similar phenomenon has been seen with CML patients treated with DAC54
. These studies have suggested that the mechanism of action of these drugs is not cytotoxicity, although the exact mechanism of achieving complete remissions is not known.
There are a variety of Histone deacetylase inhibitors (HDACi) in clinical trials currently71
. These include drugs discovered through activity in the NCI-60 cell panel (romidepsin), drugs discovered through in-vitro differentiation screens (vorinostat), or drugs discovered to be HDACi serendipitously (Valproic Acid78
). For all these drugs, in-vitro inhibition of HDAC activity was demonstrated, as well as gene reactivation and induction of apoptosis. Once again, the mechanism of downstream action of these drugs is unclear, with recent data pointing towards DNA damage and effects on reactive oxygen species102
(in addition to gene expression activation). Clinically, several of these drugs are in phase I/II studies, with activity demonstrated for depsipeptide and SAHA in lymphomas79
, and both drugs are now approved for the treatment of cutaneous T-cell lymphoma. Less data is available in MDS and AML, though anecdotal responses have been reported26;61
Drugs targeting other epigenetic pathways are currently in development or pre-clinical studies. There is particular interest in drugs that can inhibit the activity of various histone methyltransferases60;98
, as these could work independently of (and complement) DNA methylation and histone deacetylation inhibitors. It is likely that several drugs targeting epigenetic pathways will enter clinical trials in MDS in the next few years.