Epigenetics is a new and exciting territory to explore as we understand more about the role it plays in gene silencing and expression. Because epigenetic regulation of gene expression is caused by specific modification of histone proteins (e.g., methylation) that play a role in disease states like cancer, enzymes like histone deacetylases (HDACs) become viable drug targets. One drawback to inhibiting proteins that modify histones is even when selectively targeting HDACs, the effects can be far ranging with multiple HDAC-containing protein complexes found throughout the cell. These broad effects minimize the effectiveness of an inhibitor, caught between efficacy and toxicity. A recent article in Nature Communications explored how using a single compound to target two epigenetic enzymes was more effective than any individual inhibitor or combination of inhibitors.

Kalin et al. combined two known epigenetic inhibitors against HDAC and lysine-specific histone demethylase 1 (LSD1) to develop a single compound that targets both proteins. Using the purified core CoREST complex made of 1:1:1 HDAC1:LSD1:REST corepressor 1 (CoREST1), the inhibitory potency of the single compound with dual activity called corin was similar to each individual known inhibitors of HDAC and LSD. Interestingly, corin seemed to have a sustained HDAC inhibitory effect on the CoREST complex after dialysis and dilution experiments. This sustained inhibition may be related to the LSD1 binding as a HDAC1-containing complex without LSD1 did not show the same effect under dialysis.

The dual-action corin also inhibited proliferation of ten different melanoma cell lines and had minimal effect on primary melanocytes. In contrast, combining the individual HDAC and LSD inhibitors did not equal the antiproliferative effect of corin on the same cell lines. These cell lines were chosen because they are known epigenetic influencers in melanoma cancer. Furthermore, corin treatment of the WM983B melanoma cell line demonstrated an increase in histone acetylation and methylation at greater levels than that exerted by a single HDAC inhibitor.

When studying the changes in gene expression in the WM983B melanoma cell line, corin treatment induced expression of more genes compared to the HDAC inhibitor, including a larger number of tumor suppressor genes. These tumor suppressors are important as they were usually epigenetically silenced in cancer. When the single HDAC and LSD inhibitors were combined together, they did not induce expression of the tumor suppressor genes. For those genes whose expression was induced by corin, the promoter regions were highly occupied by HDAC1 and LSD1 compared to control uninduced genes. Again, these data suggest that because corin targeted both HDAC and LSD in the CoREST complex, this molecule was able to exert its effects.

When examining how individual and dual epigenetic inhibitors worked in two primary human cutaneous squamous cell carcinoma cell lines, the LSD inhibitors and HDAC inhibitors suppressed cell growth, but corin was more strongly antiproliferative. There was an additive effect when an HDAC inhibitor was paired with a LSD inhibitor, but corin was a stronger cell growth suppressor in both cell lines than the combination treatment. Similar to the primary melanocytes, corin was less toxic to primary human keratinocytes than a single HDAC inhibitor, primary cells that, unlike squamous cell carcinoma cell lines, were resistant to LSD inhibitors. Again, the data show the advantages of having two targets in a single inhibitor molecule rather than individual inhibitors.

Kalin et al. decided to test corin in a model animal. Mice treated daily for ten days with corin via intraperitoneal injection tolerated the compound well. When mouse xenografts of melanoma SK-MEL-5 cells were treated with corin, the tumors shrank by 61% compared to vehicle after 28 days. When an HDAC inhibitor was used to treat the xenograft mice on the same schedule, the toxicity was so great, six of the ten animals died in 1 week. Consistent with the cell culture experiments, corin increased tumor suppressor gene expression, reduced a proliferation biomarker and increased histone acetylation and methylation.

From the data generated using cultured cells and the melanoma xenograft mouse model, Kalin et al. have shown that the compound corin that targets both HDAC and LSD activities is more selectively effective against cancer cells with minimal effects on normal cells. As Danette Daniels, an R&D scientist at Promega Corporation explains, “Epigenetics is definitely an area where bivalent compounds are proving to be quite powerful.” This paper shows the promise of dual-targeting compounds and suggests that there is more than an additive effect compared to single inhibitors.

Reference
Kalin, J.H. et al. (2018) Targeting the CoREST complex with dual histone deacetylase and demethylase inhibitors. Nat. Commun. 9, 53. doi: 10.1038/s41467-017-02242-4