Tumor immunotherapy—the potential of epigenetic drugs to overcome resistance
Immune checkpoint inhibitors recently introduced into clinical practice have enriched available treatment options for a number of solid cancers. The efficacy of these treatments may however be impaired by tumor cell-intrinsic factors and tumor cell-extrinsic factors resulting in repressed tumor immunogenicity and host immune response. Accordingly, only a subgroup of patients responds to this treatment. Epigenetic drugs may offer an exciting novel approach to reverse immune suppression and to ‘prime’ tumors for immunotherapy, as DNA methyltransferase (DNMT) inhibitors, histone deacetylase (HDAC) inhibitors and bromodomain and extra-terminal motif (BET) inhibitors show immunomodulatory properties by affecting both cancer cells directly as well as the tumor microenvironment. Upregulation of tumor associated antigens (TAAs), ligands for natural killer (NK) cell receptors, chemokine expression, altered immune checkpoint molecules, antigen procession and presentation as well as induction of viral mimicry improve recognition of cancer cells. In the tumor microenvironment, suppression of myeloid-derived suppressor cells (MDSCs), modulation of regulatory T cells function and activation of NK cells may improve immunogenic efficacy. In conclusion, current preclinical data supports translation of combinatorial treatment approaches into clinical trials and practice.