PS1332 RIGOSERTIB MODULATES HEMATOPOIESIS GENE SIGNALING PATHWAYS ALONE OR IN SEQUENCED COMBINATION WITH AZACTIDINE ON MDS CELLS IN VITRO
Authors: R. Rai, S. Melana, S.C. Navada, R. Odchimar‐Reissig, E.P. Demakos, E.P. Reddy, L.R. Silverman
Journal: HemaSphere (2019)

Background: The Myelodysplastic Syndrome (MDS) is a disorder derived from an aberrant pluripotent stem cell, characterized clinically by hyperproliferative bone marrow and peripheral blood cytopenias involving one or more lineages. Azacitidine (AZA), a hypomethylating agent (HMA) is consideredfirst line therapy for patients (pts) with higher‐risk disease. AZA is associated with a response rate of 50% and is associated with an increase in overall survival of pts with higher‐risk MDS with a median survival of 14–24 months (Silverman et al. JCO 2002; Fenaux et al. Lancet Oncol 2009). Almost all pts treated with AZA develop resistance. Those pts who do not respond initially (primary resistance), and, almost all pts who do respond to AZA relapse (secondary resistance) and develop either bone marrow failure or transform to acute myeloid leukemia. Median survival after HMA failureis only 4–6 months (Prebet et al., 2011). Both primary and secondary resistance remain a significant challenge and result in poor survival. Rigosertib (RIGO) is a Ras‐mimetic that interferes with Ras/Raf signaling (Athuluri‐Divakar et al, 2016) and also acts as a histone deacetylase inhibitor with chromatin modifying activity (Chaurasia ASCO 2016). The combination of RIGO/AZA had been shown to have a synergistic effect in a sequence dependent manner in vitro (Skiddan et al 2006). A phase I/II study demonstrated that the combination RIGO/AZA can reverse the clinical resistance phenotype in pts failing an HMA with an overall response rate of 90% in HMA naïve pts and 54% in pts failing an HMA (Navada et al, ASH 2018). The ability to reverse the clinical resistance phenotype is a novel observation with clinical implications. Aims: To investigate the effect of AZA and RIGO alone or in sequential combination (RIGO/AZA; AZA/RIGO) (SC) on the MDS‐L cell line to identify the mechanism of action of the drugs. Methods: QPCR array was used to identify the differential gene expression profile. Results: Maximum gene alteration was observed (±2 fold change) following treatment with RIGO alone, AZA/RIGO and RIGO/AZA with differential expression of 20 (14up+6dn), 22 (16up+6dn) and 21 (16up+5dn) genes, respectively. AZA alone showed minimal effect on gene expression with altered expression of only 2 genes (1up+1dn). Gene expression with 1.5 fold change was used for the biological and functional analysis. Hematopoietic cell lineage and JAK‐STAT signaling pathways were most affected by RIGO alone, and the SCs. MAPK signaling is impacted in cells treated with RIGO alone and by AZA/RIGO suggesting that RIGO plays a crucial role in modulating these pathways. Cytokine‐cytokine receptor interactions were impacted only in cells treated by both SCs. The Retinoic acid inducible gene‐1 (RIG‐1)like receptor signaling was upregulated only by RIGO/AZA sequence. Activation of this pathway along with upregulation of Toll‐like receptor (TLR) signaling by RIGO/AZA suggests the involvement of viral mimicry. Moreover, TLR signaling dysregulation with AZA also suggests an association with viral mimicry and AZA also impacts the cell cycle pathway. Summary/Conclusion: These results indicate that RIGO either alone or in combination with AZA may act via the MAPK signaling pathway to impact hematopoietic signaling. RIGO in combination with AZA activates the anti‐viral defense pathway and affects TLR and RIG‐1 signaling which may explain the synergy seen in MDS pts. Further in‐depth study is needed to understand the mechanism of action of resistance to AZA in MDS pts and identify potential targets for the reversal of drug resistance.