EDO-S101: First alkylating HDAC inhibitor works in synergy with proteasome inhibitors for MM

EDO-S101 is a new class of chemotherapy drug called an A-DAC, that combines the active portions of an alkylating agent and pan-histone deacetylase (HDAC) inhibitor. The rationale behind the development of such molecules is to overcome drug resistance and to deliver synergistic modes of action. EDO-S101 combines two commonly used chemotherapeutics: bendamustine and vorinostat. Bendamustine drives DNA damage, whilst vorinostat is a functional pan-HDACi and therefore blocks DNA repair. Preclinical studies in vivo have shown that EDO-S101 is effective against Multiple Myeloma (MM), leukemia and B-cell Lymphomas and is currently in phase I trials to treat relapsed and refractory hematological malignancies, including MM.

In a letter to the editor published in Blood Cancer Journal, Lenka Besse from the department of Oncology and Hematology, Cantonal Hospital St Gallen, Switzerland, and colleagues, describe the molecular mechanism for EDO-S101 when used in combination with a proteasome inhibitor (PI).

Key highlights

• PI activity in MM relies on activation of the unfolded protein response (UPR)
• Addition of a PI to a HDAC inhibitor leads to interference with the alpha-tubulin-mediated transport of polyubiquitinated proteasome substrates for lysosomal destruction
• EDO-S101 therefore has potential for synergy with PI’s
• The strongest cytotoxic effect was observed when EDO-S101 was combined with the PI bortezomib, when compared with vorinostat or bendamustine alone, or vorinostat or bendamustine plus bortezomib
• EDO-S101 also showed superior cytotoxicity compared to melphalan, cyclophosphamide or bendamustine, and more effective synergistic cytotoxicity in combination with bortezomib and carfilzomib, compared to bendamustine or melphalan
• Synergy was also observed with all clinically available PIs at 4µM across a number of cells lines and with primary hematological cancer cells
• Acetylation of α-tubulin controls the transport of polyubiquitinated protein to aggresomal proteolysis and is a major molecular mechanism for synergy between PIs and HDACi’s
• EDO-S101 led to superior histone acetylation compared to vorinostat and induced robust acetylation of α-tubulin
• EDO-S101 led to an accumulation of polyubiquitinated protein, with simultaneous inhibition of proteasomal and aggresomal pathways
• EDO-S101 induced strong activation of UPR-regulators XBP1 and IRE1; this effect was more pronounced when combined with bortezomib
• The hypothesis that alternative proteolytic systems such as autophagy are activated was supported by the accumulation of MAP1LC3A/B protein after EDO-S101 treatment either alone or with PIs
• EDO-S101 also induced marked S-phase arrest and further enhanced with bortezomib (an effect not observed with bendamustine or vorinostat used either alone or with bortezomib)
• EDO-S101 leads to 70% reduced c-myc expression after 4 hours and >90% reduction when used with bortezomib; Bcl-2 was also down-regulated and Noxa up-regulated

Comnclusion

This study highlights the potent synergistic activity of EDO-S101 when used in combination with a PI and offers a mechanistic explanation. EDO-S101 causes DNA-alkylation leading to double strand breaks in the DNA, which is further enhanced by vulnerability caused by histone/protein acetylation and increased transcriptional activity. In addition, bortezomib drives degradation of p21 leading to S-phase arrest. Enhanced transcriptional activation leads to accumulation of polyubiqitinated proteins driving ER stress and initiating apoptosis, and at the same time proteolytic stress drives autophagy.

This potent synergy may in part may be related to different and superior activity when compared to vorinostat and bendamustine used either together or alone. EDO-S101 was highly effective when used with the PI bortezomib, and therefore offers encouraging data to support further studies to assess efficacy and safety in patients. The specific down-regulation of c-myc would suggest high efficacy in aggressive MM where c-myc overexpression has been observed, and warrants further investigation.