Abstract: Background: Based on a model suggesting leukemia can be driven by combined effect of mutations in an epigenetic gene (DNMT3) and Ras, the combination of a hypomethylating agent (HMA) such as azacitidine (AZA) and a Ras mimetic such as rigosertib (RIG) may have enhanced activity in both MDS and AML. The mechanism of action for RIG (Athuluri-Divakar et al, Cell 2016) documents its interference with the RAS-binding domains of RAF kinases and inhibition of the RAS-RAF-MEK and the PI3Ks pathways. In vitro, the combination of RIG with AZA was found to act synergistically to inhibit growth and to induce apoptosis of leukemic cells in a sequence-dependent manner (exposure to RIG first, followed by AZA) (Skidan et al, AACR 2006). Rigosertib's low bone marrow toxicity in pre-clinical assays, effective inhibition of human hematopoietic tumor cell lines, and its synergy with AZA suggests the potential value of combination treatment for patients (pts) with MDS. Phase I results of the current clinical study in pts with MDS or AML showed the combination of oral RIG and standard-dose AZA to be well-tolerated with evidence of efficacy (Navada et al, Blood 2014). The phase II portion of the study was initiated to further evaluate the combination in pts with MDS. Methods: Phase II results are presented for HMA-treatment-naïve MDS pts and for those with MDS failing to respond to or progressed on a prior HMA. Oral RIG was administered twice daily on Day 1-21 of a 28-day cycle at the recommended Phase II dose (RPTD: 560 mg qAM and 280 mg qPM). AZA 75 mg/m2/d SC or IV was administered for 7 days starting on Day 8. A CBC was performed weekly and a bone marrow aspirate and/or biopsy were performed at baseline, D29, and then every 8 weeks thereafter. Results: The combination of oral RIG and injectable AZA has been administered to a total of 54 pts, of whom 40 were pts with MDS including HMA-treatment-naïve (N=23) and previously HMA treated pts (N=17). Median age was 66 years (range 25-85); 73% of pts were male; and ECOG performance status was 0, 1, and 2 in 23%, 73%, and 5%, respectively. 17 pts received prior HMA therapy: 12 AZA, 4 decitabine, and 1 both. Patients have received 1-36+ cycles of treatment (median, 6 cycles), with a median duration of treatment of 25 weeks (range 4 to 145+ weeks). 8 (20%) and 2 (5%) of pts have been treated for more than 1 and 2 years, respectively. Table 1 shows the response per IWG 2006 criteria (Cheson, Blood 2006) among 33 evaluable patients. The response per IWG 2006 was complete remission (CR) in 8 (24%), concurrent marrow CR and hematologic improvement (HI) in 9 (27%), marrow CR alone in 7 (21%), and HI alone in 1 (3%). When overall response is defined as CR plus PR plus HI - responses with improvement in marrow function and thus either normalization of the peripheral blood count or lineage improvement - defined here as Clinical Benefit Response - 55% of all evaluable pts and 70% of the evaluable HMA-treatment-naïve patients showed responses meeting these criteria. Median time to initial response was 2 cycles (2.2 months), and median time to best response was 3 cycles (3.3 months). Median duration of response was 8 months for CR, 14.3 months for marrow CR, 7.4 months for erythroid response, 8 months for platelet response, and 6.2 months for neutrophil response. Clinical response is classified by IPSS-R risk categories below. The most frequently reported adverse events are nausea (41%), fatigue (39%), diarrhoea (37%), constipation (37%), dysuria (28%), decreased appetite (28%), haematuria (24%, 8% Grade 3), pyrexia (24%), dizziness (22%), thrombocytopenia (20%), back pain (20%), dyspnoea (20%), and cough (20%). Eight deaths were reported on study with most common causes including infection and progression of disease. Conclusions: The combination oforalRIG and standard-dose AZA was well tolerated in repetitive cycles in pts with MDS. Response per IWG 2006 criteria was observed both in HMA-treatment-naïve patients (85%) and in patients after failure of prior HMA therapy (62%); employing Clinical Benefit Response as the criteria, these groups had 70% and 31% response, respectively. These clinical results confirm the preclinical synergistic interaction with the combination of RIG and AZA reported by Skidan et al, and suggest that the combination can overcome clinical resistance to HMAs. Based on these results, a Phase III study of the combination of oral RIG and AZA in patients with MDS is planned. Disclosures Navada: Onconova Therapeutics, Inc.: Research Funding. Daver:Karyopharm: Honoraria, Research Funding; Pfizer: Consultancy, Research Funding; Sunesis: Consultancy, Research Funding; Ariad: Research Funding; Otsuka: Consultancy, Honoraria; Kiromic: Research Funding; BMS: Research Funding. DiNardo:Agios: Other: advisory board, Research Funding; Novartis: Other: advisory board, Research Funding; Celgene: Research Funding; Abbvie: Research Funding; Daiichi Sankyo: Other: advisory board, Research Funding. Konopleva:Reata Pharmaceuticals: Equity Ownership; Abbvie: Consultancy, Research Funding; Genentech: Consultancy, Research Funding; Stemline: Consultancy, Research Funding; Eli Lilly: Research Funding; Cellectis: Research Funding; Calithera: Research Funding. Fenaux:Celgene, Janssen,Novartis, Astex, Teva: Honoraria, Research Funding. Petrone:Onconova Therapeutics, Inc.: Employment. Zbyszewski:Onconova Therapeutics, Inc.: Employment. Fruchtman:Onconova: Employment. Silverman:Onconova Therapeutics, Inc.: Patents & Royalties: Co-Patent holder for the combination of azacitidine and rigosertib, Research Funding.

Abstract: Background: Rigosertib (RIG) is a Ras-mimetic that inhibits the PI3K and PLK cellular signaling pathways by binding directly to the Ras-binding Domain found in Ras effector proteins. It has been tested as a single agent in patients (pts) after failure of hypomethylating agents (HMAs). In vitro, the combination of RIG with azacitidine (AZA) inhibits growth and induces apoptosis of leukemic cells in a sequence-dependent fashion (RIG administered prior to AZA) (Skidan et al 2006). Phase I results of this study in pts with MDS or AML showed combination of oral RIG and standard-dose AZA to be well-tolerated with evidence of efficacy (Navada et al, Blood 2014). Phase II was initiated to further study the combination in pts with MDS. Methods: Results from pts in Phase II with MDS previously untreated with an HMA, or who had failed to respond to or progressed on a prior HMA, are presented, while response data from Phase I MDS pts are updated. Pts with CMML are analyzed separately. Oral RIG was administered twice daily on Day 1-21 of a 28-day cycle at the recommended Phase II dose (RPTD: 560 mg qAM and 280 mg qPM). AZA 75 mg/m2/d SC or IV was administered for 7 days starting on Day 8. A CBC was performed weekly and a bone marrow aspirate and/or biopsy was performed at baseline, day 29, and then every 8 weeks thereafter. Results: The combination of oral RIG and AZA has been administered to a total of 45 pts within Phase I (N=18) and Phase II (N=27). Pts were classified into the following MDS risk categories per the IPSS (Greenberg et al, Blood 1997): intermediate-1 (4), intermediate-2 (10), high-risk (14), and IPSS classification pending (4). Five pts had CMML and 8 had AML. Median age was 66 years; 69% of pts were male; and ECOG performance status was 0, 1, and 2 in 27%, 67%, and 6%, respectively. Twelve pts [MDS (9), CMML (3)] received prior HMA therapy: AZA (11 pts), decitabine (1 pts). Patients have received 1-21+ cycles of treatment to date (median, 3 cycles), with median duration of treatment of 14 weeks. Among 15 evaluable MDS pts treated with the RPTD (1 pt in Phase I, 14 pts in Phase II), marrow responses were observed in 10: marrow CR (mCR) (8), marrow PR (mPR) (2). Responses according to IWG criteria were observed in 10 pts: complete remission (CR) (1), mCR (7), hematologic improvement (HI) (2). Table 1. Responses for MDS Patients Treated at the Recommended Phase II Dose Pt Prior HMA Best BMBL at Nadir1 IWG Response2 Hematologic Improvement 102-008 None mCR mCR Platelet 101-010 None mCR CR Erythroid & Neutrophil 101-011 None mCR mCR None 101-013 None mCR mCR Erythroid 102-010 None SD SD None 101-014 AZA PD PD None 102-011 AZA mPR HI Erythroid & Platelet 101-016 AZA SD SD None 101-017 AZA mCR mCR None 102-013 None NE NE NE 101-019 None SD SD None 101-021 None PD PD None 101-024 None mCR mCR None 101-022 AZA mCR mCR None 101-025 None mCR mCR None 101-026 AZA NE NE NE 101-027 None NE NE NE 102-016 None mPR HI Platelet 1 Silverman et al, Hematol Oncol 2014 2 IWG = International Working Group (Cheson et al, Blood 2006) NE = not evaluable BMBL = bone marrow blast Overall, in pts with MDS treated on Phase I and Phase II, marrow responses were observed in 15 out of 20 evaluable pts: mCR (13), mPR (2). Responses according to IWG 2006 criteria were observed in 14 out of 19 evaluable MDS pts: CR (2), mCR (10), HI (2). Among the 7 evaluable pts with MDS in both the Phase I and Phase II who had failed to respond or progressed on prior treatment with an HMA, 5 had a response after RIG was added: CR (1), mCR (3), HI (1). Analyzed as a separate subgroup, 2 out of 5 (40%) pts with CMML had a mCR. The most frequent adverse events (AEs) in Cycle 1 included nausea (21%) and fatigue (15%), which were also the most frequent AEs in all cycles (fatigue, 28%; nausea, 26%). Six deaths have been observed so far. Three pts were treated for more than 1 year and continue on study. Conclusions: The combination oforalrigosertib and standard-dose AZA was well tolerated in repetitive cycles in pts with MDS. Marrow CR was observed in 65% of pts, both with de novo MDS and after failure of prior HMA therapy. In pts who received the RPTD, 67% of pts with MDS had a bone marrow blast and IWG response. These results suggest potential synergistic interaction of the combination and support continued study of this unique combination in patients with MDS. Disclosures Silverman: Onconova Therapeutics Inc: Honoraria, Patents & Royalties: co-patent holder on combination of rigosertib and azacitdine, Research Funding. Daver:ImmunoGen: Other: clinical trial, Research Funding. DiNardo:Novartis: Research Funding. Konopleva:Novartis: Research Funding; AbbVie: Research Funding; Stemline: Research Funding; Calithera: Research Funding; Threshold: Research Funding. Pemmaraju:Stemline: Research Funding; Incyte: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding; LFB: Consultancy, Honoraria. Fenaux:CELGENE: Honoraria, Research Funding; JANSSEN: Honoraria, Research Funding; AMGEN: Honoraria, Research Funding; NOVARTIS: Honoraria, Research Funding. Fruchtman:Onconova Therapeutics Inc: Employment. Azarnia:Onconova Therapeutics Inc: Employment.

Abstract: Background:Rigosertib is a small molecule anti-cancer agent targeting PI3/polo-like kinase pathways that promotes G2/M arrest and has effects on the B-Raf and Ras pathways. It is currently being tested as a single agent with the intravenous (IV) formulation in patients (pts) who have relapsed or are refractory to hypomethylating agents (HMAs) as well as with the oral formulation in lower-risk, red-cell transfusion-dependent MDS patients. Azacitidine (AZA) is first-line therapy for pts with higher-risk MDS. In vitro, the combination of rigosertib with AZA acts synergistically to inhibit growth and induce apoptosis of leukemic cells (Skidan et al 2006). This effect appears to be sequence dependent, requiring exposure to rigosertib first, followed by AZA. These nonclinical results provided the rationale to combine the 2 agents in a phase I/II study in pts with MDS and AML. Methods: Pts with MDS and non-proliferative AML, who were previously untreated or had failed or progressed on an HMA were included in the phase I component of the study. Oral rigosertib was administered twice daily from day 1 through day 21 of a 28-d cycle. AZA 75 mg/m2/d was administered for 7 days starting on day 8 of the 28-d cycle. Pts were entered in 3 escalating-dose cohorts of rigosertib in a classic 3+3 design: [1] 140 mg twice daily; [2] 280 mg twice daily; [3] 560 mg qAM and 280 mg qPM. A CBC was performed weekly and a bone marrow (BM) aspirate and/or biopsy was performed at baseline and every 4-8 weeks afterwards. Results: Eighteen pts have been treated with the combination of oral rigosertib and AZA. Pts had diagnoses of intermediate-1 MDS (3), intermediate-2 MDS (6), high-risk MDS (2), CMML (1), and AML (6); median age was 70.5 years; 61% of pts were male. Pts have received 1-10+ cycles of treatment with the total number of cycles administered thus far being 58. Cytogenetic profiles by IPSS were good (8 pts), poor (8 pts), and intermediate (2 pts). 11of 18 patients were transfusion dependent at baseline [RBC (11), platelet (6)]. One patient became RBC transfusion independent after 3 cycles of treatment. 5 additional patients have had a reduction in their RBC and platelet transfusion requirements. 56% of patients received prior treatment with HMAs: AZA (6 pts), decitabine (4 pts). The most frequent adverse events (AEs) in Cycle 1 included constipation, diarrhea, nausea, fatigue, hypotension, and pneumonia. The AEs did not differ significantly among the 3 cohorts. Elevation in creatinine in 1 pt in cohort 1 was a possibly related grade 3 dose-limiting toxicity that required subsequent expansion of the cohort. Drug-related dysuria/cystitis was not reported in this pt population. Responses according to IWG 2006 criteria were observed in the BM and peripheral blood: Complete Response (CR) (1 pt), Cri (CR with incomplete blood count recovery) (4 pts), stable disease (2), hematologic improvement-erythroid (1). Six pts received fewer than 4 cycles of treatment and are too early to evaluate. Six pts came off study for the following reasons: progression of disease (1), pt request (1), death from pneumonia (2), received stem cell transplant (1), persistent fungal pneumonia (1). Two evaluable pts have responded to the combination after progression or failure on HMA alone. Conclusions: The combination oforalrigosertib at 560/280 mg BID (recommended phase II dose) and standard-dose AZA can be safely administered and appears to be well tolerated in repetitive cycles in pts with MDS and non-proliferative AML. The AE profile does not differ significantly from that of AZA alone. Data from the Phase I component of this study suggest activity in patients with MDS after HMA failure. Additional data are required to evaluate this observation. The Phase II segment of this study is underway to further assess the response of the combination. Table Patient ID Diagnosis Prior HMA % Blasts in BM at Baseline % Blasts in BM after Treatment IWG Response 1 MDS No 2 1 CRi 2 AML No 40 0 CRi 3 AML No 22 N/A NE 4 MDS Azacitidine 0 0 NE 5 AML No 59 N/A NE 6 MDS No 21 〈 5 CRi 7 MDS No 2 1 CR 8 MDS No 2.5 2 SD 9 AML Decitabine 25 N/A NE 10 MDS Decitabine 12 3 CRi 11 CMML Azacitidine 2 3 SD 12 MDS Azacitidine 4 1 HI-E 13 AML Azacitidine 47 40 TE 14 AML Decitabine 7 7 TE 15 MDS No 9 5 TE 16 AML No 25 6 TE 17 AML No 15 19 TE 18 AML Azacitidine 64 45 TE IWG = International Working Group CR = Complete Response CRi = Complete Response with incomplete blood count recovery NE = Not Evaluable SD = Stable Disease HI-E = Hematologic Improvement - Erythroid TE = Too Early Disclosures Wilhelm: Onconova Therapeutics, Inc: Employment, Equity Ownership. Demakos:Onconova: Consultancy. Azarnia:Onconova Therapeutics, Inc: Employment. Silverman:Onconova: with Icahn School of Medicine at Mount Sinai Patents & Royalties.

Abstract: Background: MDS is characterized by ineffective hematopoiesis and multiple cytopenias. Azacitidine (AZA), a hypomethylating agent (HMA), the standard therapy for higher-risk MDS patients (pts), improves hematopoiesis in 50% of MDS pts, with a median response of 14-24 months. Those pts who initially respond to AZA either relapse or progress with bone marrow failure and have a median survival of 4 to 6 months. Both primary and secondary resistance is a significant challenge and results in poor survival. Rigosertib (RIGO), a small molecule Ras mimetic, as a single agent improved hematopoiesis in 15% of MDS pts who had failed a prior HMA. In vitro data of synergy of RIGO combined with AZA that was sequence dependent (Skiddan et al. AACR 2006), led to a Phase I/II study of the combination of RIGO/AZA and demonstrated an overall response rate of 90% in HMA naïve and 54% in HMA failures pts (Navada et al. ASH 2018). Restoration of functional hematopoiesis in response to treatment with AZA when combined with RIGO in pts, who had failed an HMA, is a unique observation in overcoming the HMA clinical resistance phenotype. Elucidating mechanisms leading to restoration of HMA effects on hematopoiesis could have profound clinical benefit. Methods: We investigated the molecular mechanisms in response to AZA and RIGO either alone or in sequential combinations (SC) in vitro. Pathway specific real time PCR (QPCR) for epigenetic modification genes, hematopoiesis signaling genes, interferon signaling genes, MAPK signaling genes and antiviral response genes was performed in MDS-L (AZA sensitive) as well asBW-90 (AZA resistant) cell lines treated with RIGO, AZA and SC. Functional analysis was performed using Ingenuity pathway analysis (IPA) software. Reverse phase protein array (RPPA) was also performed on the MDS-L and BW-90 cell lines treated with AZA and RIGO alone and SCs for the validation. Results: We observed differential expression (DE) pattern of the genes in response to AZA, RIGO and their SCs both in MDS-L and BW-90 cell lines. The functional pathways associated with DE genes predominantly impacts RIG-I like receptor (RLR) signaling (anti-viral defense pathway), T cell exhaustion signaling, Wnt/β-catenin signaling and hematopoiesis pathway in MDS-L cells treated with RIGO/AZA combinations compared to other treatments (Table 1, Fig 1A). However, RIGO alone induces the dysregulation of RIG-I like receptor signaling and T cell exhaustion signaling in BW-90 cells (Table 1, Fig A). Expression of CXCL8 was observed to be elevated in RIGO and SCs by 7-9 fold compared to untreated MDS-L cells while no significant difference was found in response to any treatment in BW-90 cells (Fig 1B). CXCL8 is a RLR signaling responsive gene and is also one of the genes which were observed to be involved in hematopoiesis signaling identified by pathway enrichment analysis. Similar to QPCR data we observed striking differences in MDS-L and BW-90 cells in response to different treatments at protein level and functional pathways by RPPA analysis and IPA, respectively. RIGO is a RAS mimetic and interrupts RAS-RAF binding (Reddy et al ASH 2014), in this study we observed RIGO in combination with AZA inhibits several MAPK signaling pathways including PI3K/AKT/mTOR, ERK/MAPK and p38 MAPK signaling in MDS-L cells. RIGO had been reported to inhibit PI3K/AKT pathway as a single agent (Xu et al. Sci Rep 2014). In addition, Wnt/β-catenin pathway was predicted to be specifically activated in MDS-L cells with SCs. Both QPCR and RPPA results demonstrated activation of Wnt/β-catenin signaling pathway in response to RIGO alone and the combination with AZA. Importantly, expression of two genes Jun (proto oncogene) and CD44 (Wnt target gene) that are associated with the Wnt/β-catenin signaling pathway were upregulated at both mRNA (Fig 1C) and protein level (Fig 1D) which suggests a crucial role of RIGO in wnt signaling. Conclusions: These data demonstrate that RIGO sequenced with AZA upregulates RLR, Wnt/β-catenin and hematopoiesis signaling. Involvement of CXCL8 (RLR responsive gene) and activation of wnt signaling genes suggests their linkage to hematopoiesis. Further studies are underway to determine the effects of these signaling pathways on improving hematopoiesis both in vitro and in vivo in the HMA clinical resistance setting to identify potential therapeutic targets to reverse bone marrow failure in pts with HMA resistance. Disclosures Navada: Onconova Therapeutics Inc: Research Funding. Reddy:Onconova Therapeutics Inc: Equity Ownership, Research Funding. Silverman:Medimmune: Research Funding; Onconova Therapeutics Inc: Patents & Royalties, Research Funding; Celgene: Research Funding.

Abstract: Background: The COVID-19 (SARS-CoV-2) pandemic has affected cancer patients (pts) in a myriad of ways, including diagnostic & treatment delays, scarcity of blood products, and most importantly, higher risks of morbidity and mortality from the viral infection itself. Though COVID-19's effects on many specific cancers has previously been described, there has been little reported on its effects on pts with MDS. Methods: We prospectively reviewed the records of all pts seen in the MDS clinic of a large New York City tertiary academic medical center between March 12 and May 07 2020. A confirmed case of COVID-19 was defined by a positive (+) result on a real-time reverse-transcriptase polymerase chain reaction (PCR) assay of a specimen collected on a nasopharyngeal swab, or detectable COVID-19 antibodies. COVID-19 antibodies were determined via an IgG assay developed at Mount Sinai with the ability to detect antibody titers to a dilution of 1:2880. Initially, only symptomatic pts were screened due to limited testing availability. However, after April 7, all clinic pts were screened by PCR. Results: Among 85 pts seen in the clinic, 23 were found to have COVID-19 (27.1%). The median age of all pts was 72 years (range 20-90); racial breakdown included 58.8% Caucasian, 12.9% Hispanic, 10.6% African-American, 7.1% Asian, and 10.6% other. Of note, 65.2% of COVID-19+ pts were Caucasian, 13.0% Hispanic, and 17.4% other. The most common diagnoses were MDS (n=61), AML (n=10), multiple myeloma (MM) (n=9), large granular lymphocytic leukemia (LGL) (n=6), ALL (n=6), MPN (n=4), NHL (n=3), and CML (n=2). Forty-two of the MDS patients had no other malignancies; co-diagnoses among the remaining MDS patients included MM (n=8), LGL (n=6), & T-cell dyscrasias (n=5). Of the 23 COVID-19+ pts, 11 (47.8%) were hospitalized and 4 (17.4%) were asymptomatic. Common symptoms were URI symptoms (15) and fever (14). ARDS and pneumonia occurred in 4 pts. The most common treatments were hydroxychloroquine (n=4), steroids (n=3), and azithromycin (n=2). Eighteen of the 23 + pts (78.3%) had MDS; the others included pts with MPN, APL, ALL, AML, or NHL. Among the COVID-19+ MDS pts, IPSS-R was very low (n=5), low (n=3), intermediate (n=1), high (n=2), and very high risk (n=5) in evaluable patients. MDS directed treatment included azacitidine (8) (2 with venetoclax), erythropoietin stimulating agents (3), best-supportive care (4), intravenous immunoglobulin (3), and lenalidomide (1). Disease status at diagnosis was stable disease (SD; n=9), hematologic improvement (HI; n=6), progressive disease (PD; n=1), and complete response (CR; n=1). The median number of co-morbidities per pt was 2. Major co-morbidities included cardiac disease (n=5), hypertension (n=5), diabetes (n=5), and dementia (n=2). COVID-19+ pts remained PCR+ for a range of 14-42 days in those with available serial testing (n=8). Of the PCR+ pts, 14 were tested for the presence of COVID-19 antibodies; 12 were +, with a range of titers from & lt;1:80 to 1:2880. Three patients were PCR negative at least once but tested antibody positive. Nine of 23 (39%) COVID-19+ pts died; mortality among MDS patients included 6/18 (33%) COVID-19+ and 6/61 (9.8%) from the overall cohort. Among the 9 who died, the median number of co-morbidities was 3 and the median age was 75 years-old. Disease status in these patients was SD (2), CR (1), HI (1), and PD (1). IPSS-R was very high (n=4), high (n=1), and intermediate (n=1) in the MDS pts who died. Conclusions: This represents the first reported large case series regarding the risks of developing COVID-19 and its effects at an MDS clinic in the initial US epicenter of the pandemic. Overall, 27.1% of the pt population was diagnosed with COVID-19; 39.1% of these pts died, or 10.6% of the overall cohort. A retrospective study across the US reported a 16% mortality rate of COVID-19 in cancer pts (Dr Rivera et al, 2020). The mortality rate reported here is higher, but with a smaller sample size. The clinical significance of persistently + PCR tests up to 6 weeks is unclear. COVID-19 antibodies were found in 85.7% of COVID-19 PCR+ pts tested, showing MDS pts can mount a humoral response. Likely factors contributing to the high mortality of MDS pts were co-morbidities and age. The majority of pts recovered and have resumed MDS directed therapy. Protecting MDS pts from COVID-19 infection must be a primary overall therapeutic approach until there are more effective COVID-19 treatment strategies. Disclosures Navada: Onconova Therapeutics Inc: Research Funding. Silverman:Medimmune: Research Funding; Onconova Therapeutics Inc: Patents & Royalties, Research Funding; Celgene: Research Funding.

Abstract: Background: Myelodysplastic syndrome (MDS) is a clinically heterogenous disease of hematopoietic stem cells (HSC) characterized by ineffective hematopoiesis, uni/multi-lineage dysplasia and a high tendency to transform into acute myeloid leukemia. Aberrant chromosomal and genetic lesions contribute to MDS pathogenesis which has been associated with chronic activation of the innate immune response and a hyperinflammatory microenvironment (Barryero L, et al. Blood, 2018). Dysfunction of Toll like receptors (TLR) and downstream effectors has been associated with the loss of progenitor function and differentiation of bone marrow (BM) cells in MDS patients. Azacitidine (AZA), a hypomethylating agent (HMA), is the mainstay of therapy for patients with higher-risk MDS (Silverman LR, The Myelodysplastic Syndrome in Cancer Medicine, Editors: R.J. Bast, et al. 2017) and carries an overall response rate (ORR) of 50% in patients with significant effects on hematopoiesis, ranging from improvement in a single lineage to complete restoration of blood counts and transfusion independence with survival benefits (Silverman LR, et al., Leukemia, 1993). The response to AZA is not durable and all patients relapse with worsening bone marrow failure. The paradigm of MDS therapy is now shifting to combinatorial drug treatment to overcome single-agent HMA resistance in higher-risk MDS patients. Rigosertib (RIGO), a Ras mimetic which had been shown to interfere with the Ras-Raf binding domain, has limited single-agent activity (ORR 15%) and failed to provide a survival benefit compared to standard of care in MDS patients failing an HMA. RIGO combined with AZA produced an ORR of 90% in HMA naïve patients and 54% in patients who failed HMA (Navada SC, et al. EHA Library 2019). This represents a critical observation in overcoming the epigenetic clinical resistance phenotype. The mechanism is still unclear. Method: We therefore investigated the pathways that are perturbed by AZA and RIGO monotherapy and in combination (RIGO-AZA). We used the MDS-L cell line as a model to limit the heterogeneity observed in MDS patients. The cells were treated with RIGO, AZA, and RIGO-AZA for 48 hrs and further analyzed by qPCR and western blot. Result: We found an increase in H3K9ac protein expression with RIGO and RIGO-AZA; AZA and RIGO alone each had similar effects on H3K4me3, however, its expression was markedly upregulated with RIGO-AZA (Figure A). Effects on H3K36me3 were comparable in all treated cells. We observed marked effects on the repression marks H3K9me3 and H3K27me3 by RIGO and RIGO-AZA combination (Figure A). Furthermore, we studied the expression of bacterial sensing TLRs (1, 2 and 6), viral sensing endosomal TLRs (3 and 9), and cytosolic viral particle sensing receptors like Retinoic acid inducible gene (RIG)-I, Melanoma differentiation-associated protein 5 (MDA5) and Stimulator of interferon genes (STING); their intermediate adaptor molecules Myeloid differentiation factor 88 (MYD88) (for all TLRs except TLR3), mitochondrial antiviral signaling (MAVS) gene (for RIG-I and MDA5); and interferon regulatory factor (IRF)-3 and -7 by qPCR. We observed that AZA, RIGO, and RIGO-AZA significantly inhibit the expression of TLR1, 2 and 6 (Figure B). However, TLR-3, 9, RIG-I, MDA5, STING, MAVS, MYD88, and IRF-3, 7 were significantly inhibited by RIGO and RIGO-AZA (Figure C-E). Conclusion: RIGO has effects on innate immune signaling and histone modification of both activator and repressor marks. Further studies are underway to determine the correlation of the histone modification and innate immune signaling changes, and if these mechanisms contribute to the improvement in hematopoiesis in MDS patients. Figure 1 Figure 1. Disclosures Navada: Janssen Pharmaceuticals, Inc.: Current Employment. Reddy: Onconova Therapeutics, Inc.: Current equity holder in publicly-traded company.

Abstract: Background: Patients with Myelodysplastic Syndrome (MDS) with cytopenias have limited treatment options to improve blood counts apart from immune-modulatory drugs (IMIDS), hypomethylating agents (HMA), cytokines and transfusional support. Immune dysregulation is common in MDS and occurs in up to 35% of pts (LR Silverman, Cancer Med, 2017). Expanded T-cell clonal populations in the peripheral blood and bone marrow in the presence of MDS may play a role in aggravating MDS related cytopenias, increasing the need for transfusions or exacerbating symptoms. Intravenous immunoglobulin (IVIG) may interact with Fc receptors on T cells or have suppressive effects on the concurrent large granular lymphocytic leukemia (LGL) clone. Targeted therapy with IVIG could be a novel strategy to mitigate the cytopenias in MDS and other bone marrow failure pts. Methods: We reviewed in a single-center series 27 consecutive pts who had either MDS, aplastic anemia (AA), or a myeloproliferative neoplasm (MPN) along with an LGL clone seen on TCR-rearrangement studies and/or peripheral blood flow cytometry in the blood and/or bone marrow. Of the 27 pts, 22 had MDS, 2 had MPN, and 1 each with AA, LGL, or Multiple Myeloma. IVIG was initially used as a single agent administered at a dose of 500mg/kg/day over 4 hours once weekly x4 weeks. After "induction", administration was changed to every other week, and then in responding pts, the interval was extended to every 3-4 weeks. In pts with a stable response, IVIG was discontinued and pts observed. After the initial 4 pts, pts were treated concurrently with prednisone 20 mg/d with a taper over 4 weeks, except in pts in which prednisone was a relative contra-indication. Overall hematologic improvement was determined by MDS IWG 2006 criteria, while hemolytic response was determined to be either a complete normalization (CR) or a partial normalization (PR) if there was 〉 50% improvement in deviation from normal LDH, reticulocytes, indirect bilirubin, or haptoglobin values from baseline. Results: All pts received IVIG administered at 500mg/kg, once weekly, to be spaced out maximally as tolerated. 74% (20/27) of pts received steroids. Baseline characteristics of the pt population: median age of 70 years (range 39-87), representing 21 males and 6 females. Of the pts with MDS evaluable for risk stratification by IPSS-R criteria [n=18], 3 were very low, 5 low, 6 intermediate, 2 high, and 2 very high risk categories. By WHO criteria, the greatest representation was 11/20 RCMD, followed by 3/20 with RAEB2. Baseline cytogenetics [n=21] included 3 pts with very poor, 2 poor, 3 intermediate, 11 good, and 2 very good risk profiles. Pts had a range of 0-6 mutations, most common were SETBP1, ASXL1, TP53 (n=4) and SRSF2, U2AF1, PDGFRB (n=3). 24/27 pts had positive TCR beta and 13/27 had positive TCR gamma gene rearrangements. ORR for any cytopenia was 77.8% (21/27), and for any hemolytic response was 76.2% (16/21). ORR for individual cell lines included: 59.2% (16/27) erythroid, 48.1% (13/27) neutrophil, and 33.3% (9/27) platelet responses. For hemolytic responses, CR/PR rate were measured by LDH (29.6%/11.1%), bilirubin (22.2%/14.8%), retic (33.3%/14.8%), and haptoglobin (22.2%/11.1%). For the 14 MDS pts with very low, low, and intermediate IPSS-R risk, ORR was: 64.3% (9/14) any cytopenia, 35.7% (5/14) erythroid, 50.0% (7/14) neutrophil, 14.3% (2/14) platelets. Combined CR+PR rates for hemolysis in the lower risk categories were 42.9% LDH, 21.4% bili, 42.9% retic, 28.6% haptoglobin responses. For the 5 non-MDS pts, ORR for any cytopenia was 100%, and 60% for any hemolytic response. There were no reported significant adverse events with IVIG administration. Conclusions: IVIG can be used as a first line treatment for pts with autoimmune cytopenias associated with LGL clones and MDS (along with other hematological malignancies). Treatment with IVIG yields an ORR of 77.8% in the entire cohort and 64.3% in pts with lower risk MDS, with a reduction in hemolysis as well. This report expands on a previous analysis and demonstrates further evidence of efficacy for the use of IVIG in treating cytopenias exacerbated by LGL T-cell clones. IVIG therapy can permit a delay in the start of an HMA or IMID treatment, lowering transfusion burden, and increasing ANC to help prevent infectious complications. Further evaluation in a larger cohort of pts is warranted. Studies exploring the mechanism of action are underway. Disclosures Navada: Onconova Therapeutics Inc: Research Funding. Silverman:Onconova Therapeutics Inc: Patents & Royalties, Research Funding; Celgene: Research Funding; Medimmune: Research Funding.

Abstract: Abstract 4932 Background: The hypomethylating agent azacitidine (azaC) which can reverse epigenetic silencing, is the first agent demonstrated to alter the natural history of MDS and improve survival in higher-risk patients (Silverman JCO 2002, Fenaux Lancet Oncology 2009). AzaC also produces comparable rates of response in patients with non-proliferative AML and appears to affect survival (Silverman JCO 2006). The response rate to single agent azaC is about 50% with median duration of response about 14 to 22 months. Patients who are refractory to or relapse following first line therapy with azaC have a median survival of 4 to 6 months (Jabbour 2010, Prebet 2011). Options for these patients failing first-line therapy with azaC are limited and there is no standard of care. Investigational therapies are being explored but not always widely available for these patients. Prior studies with azaC explored higher doses but were inconclusive to a dose response effect secondary to the slow time to response with a median of 2 to 3 cycles. In vitro, azaC has a wide range of concentrations that can induce differentiation up to 4 μm. In patients the standard clinical dose achieves a plasma level of 1. 25 μm thus suggesting that higher doses might have a clinical benefit (Marcucci 2005). Methods: Patients who were refractory to or relapsing following treatment with an azaC based regimen for MDS where alternative investigational options were not available and who were not rapidly progressing (i. e rapidly rising WBC or myeloblast %), were treated with higher doses of single agent azaC. The dose was increased by 33% from the baseline prior failing regimen (eg 55 to 75 or 75 to 100 mg/m2). The azaC was administered SC × 7 days q 28 days; response status was assessed after 2 cycles. Patients stable or responding were continued on 28 day cycles. Myeloid cytokine support was utilized for patients with ANC 〈 200 and ESA support for patients who were RBC dependent. Results: As of the data cut for this submission 13 patients are evaluable for toxicity and response. Among the 13 patients the median age was 68 and 11 were male. All had been treated with azaC based regimens before and 10 had responded: 4 CR; 1 PR; 5 HI; 2 NR; 1 unknown. The immediate therapy prior to increased dosing included: azaC + histone deacetylase inhibitor (HDACi) (6); single agent azaC (5); investigational treatment (2). 6 had MDS (int-1 (2); int-2 (3); high (1) and 7 AML (all transformed following MDS all smoldering). Responses among evaluable patients have occurred in 11 of 13 (85%); 1 PR, 7 HI (4 CRm), 1 CRm, (CR+CRi=53%) 2 PRm, 1 SD, 1 NR. Although responses occurred, the abnormal MDS/AML clone persisted, suggesting that the higher dose did not have a cytotoxic effect on the malignant clone. A total of 117 cycles have been administered, range 2 to 17 with a mean 8 cycles. Median time to treatment failure was 11. 6 months and median survival is 17. 5 months. Eight patients have come off treatment for progression (5); relapse (2); co-morbidities (1). No grade 3 or 4 non-hematologic toxicities were observed. Hematologic toxicity was similar to that seen with standard dose azaC. Conclusion: Modification of the dose of azaC in select patients, who lack alternative options including investigational agents or stem cell transplant, may improve blood counts and reduce the bone marrow blasts. The quality of the response to the increased dose after primary failure does not attain the level of the original response in most, however, this approach may increase therapeutic options in a poor risk population. Investigations into the potential mechanisms of action are being explored. Disclosures: Silverman: celgene: Speakers Bureau. Demakos:celgene: Speakers Bureau.

Abstract: Background : The incidence of myelodysplastic syndromes (MDS) - a heterogeneous group of malignant myeloid stem cell disorders - increases with age and commonly affects older people. The prevalence of MDS in the US has been constantly rising as a result of increasing longevity of the overall population. Analyses of healthcare claims data using associated medical claims information (ICD-9-CM diagnosis codes) are a common way to estimate the number patients (pts) receiving care in specific disease states. We examined the total of unique US claims for MDS submitted over a 3-year period and also analyzed the claims according to type of treatment. Methods : We conducted a retrospective cohort study of patients (pts) with an MDS-associated medical claim (ICD-9-CM diagnosis code 238.7x) in the observation (OB) period (calendar years 2009-2011). In each year of the OB period, pts were classified according to type of treatment: watch and wait (ie, receiving no drug therapy) or interventional treatment [ie, supportive care treatment with erythropoietin stimulating agents (ESA) or growth factors (GF) and active drug treatment (ie, the hypomethylating agents (HMA) azacitidine (AZA) and decitabine (DEC) and the immunomodulating agent lenalidomide (REV)] . A subgroup of newly diagnosed MDS pts was also identified in Years 2 and 3, but this group was not included in Year 1 of the OB period (calendar year 2009); this group of new-to-treatment patients had a claim for MDS in Years 2 and 3 of the OB period. MDS incidence rates were then determined within Years 2 and 3 of the OB period for this group. The total number of physicians treating pts coded for MDS was also collected. Results : We identified more than 100,000 unique pts with an MDS-associated claim in each of the 3 years of the OB period. Our calculated incidence of newly treated MDS pts (34,000) in Years 2 and 3 is consistent with recently reported estimates (Cogle et al, Blood 2011; Goldberg et al, J Clin Oncol 2010) but higher than the SEER database. Over the 3-year OB period, the number of diagnosed and treated MDS pts grew year on year and grew at a slower rate than that of the US population. Watch-and-wait is the mainstay treatment for 47% of MDS pts. We found that 6000 pts per month are treated with an HMA by 2100 physicians, or 2.8 pts per physician and 14,000 pts receive therapy for MDS comprised of ESA, GF, AZA, DEC or REV per year. AZA and DEC were the predominant HMA treatments prescribed for higher-risk MDS. Approximately 30% of HMA-treated reached the target number of 6 cycles. HMA therapies were used in 13.1% of pts. A larger percentage of the AZA- and DEC-treated pts (69.1%) stopped therapy before reaching the target number of doses, with approximately 32% of pts who initiated therapy dropping out after the first cycle. Conclusions : The total number of pts coded for MDS-associated ICD-9 billing in the US each year is substantial. The estimated incidence of 34,000 patients per year with MDS in the US in 2010 and 2011 is similar to results found in other epidemiological databases. The majority of pts are not treated with the optimal number of cycles with an HMA. The average physician treats only a limited number of pts with MDS, which may influence treatment decisions. These data suggest a need for further educational efforts to optimize care with additional insight into past population-based estimates of diagnosis and treatment (Cogle et al, Blood 2011; Goldberg et al, J Clin Oncol 2010). The data showing early discontinuation or failure of HMA therapy accentuates the poor prognosis of patients post-HMA, who have a predicted median survival of 4-6 months (Prebet et al, J Clin Oncol 2011). These data may serve to better inform the medical community of the unmet need of pts who are not successfully treated with first-line MDS therapies, underscoring the need for optimization of care and the need for additional agents beyond those currently available. Table HMA Cycle and Administration Metrics per Line of Therapy Treatment Metric Moving Annual Total 2010 2011 2012 Dacogen Median number of cycles 4-day admin schedule 5-day admin schedule 3.00 14.7% 64.0% 4.00 11.4% 67.6% 3.00 12.6% 66.2% Vidaza Median number of cycles 5-day admin schedule 7+ day admin schedule 4.00 47% 20% 4.00 49% 20% 4.00 46% 20% Disclosures Demakos: Onconova Therapeutics, Inc: Consultancy. Silverman:Onconova Therapeutics, Inc: Consultancy. Lawrence:Onconova Therapeutics, Inc. : Employment. McKearn:Onconova Therapeutics, Inc. : Employment. Megaffin:Onconova Therapeutics, Inc. : Employment. Percy:Onconova Therapeutics, Inc. : Employment. Petrone:Onconova Therapeutics, Inc. : Employment, Stock Options Other.

Abstract: Rigosertib is a small molecule anti-cancer agent which inhibits the PI-3K and PLK pathways, promotes G2/M arrest, and selectively induces apoptosis in cancer cells. We have previously reported results of a phase I/II study of rigosertib in patients (pts) with MDS and AML who had relapsed or were refractory to hypomethylating agents (HMA), a population for which there are currently no approved second line therapies. Rigosertib appeared to be well tolerated in this pt population and to have biologic activity with reduction or stabilization of bone marrow (BM) blasts and improvement in the peripheral blood (PB) counts in few treated pts. Reduction in BM blasts by rigosertib was associated with increased survival. In the current analysis, we evaluate pt characteristics that may predict for response. Methods We analyzed the results of a phase I/II study of Rigosertib that was conducted in pts with MDS and AML. In the phase I component, pts were entered in cohorts of escalating doses in a classic 3+3 design ranging from 650 up to 1700 mg/m2/d continuous IV infusion (CIV) administered for 72 to 144 hours. A MTD of 1375 mg/m2 was identified for the phase II component, and subsequent pts were treated with this dose as a CIV for 72 hours. BMs were performed at baseline, week 4, 8, and then q3 months. Results Twenty-two pts with MDS or AML refractory or relapsing to a HMA have been treated with rigosertib. The study cohort comprised pts with a diagnosis of int-2 MDS (2), high risk MDS (6), CMMOL (1), and AML (13 pts all with antecedent MDS). Responses according to IWG 2006 criteria were observed in the BM and PB: marrow CR (4), marrow PR (2). Two pts also had hematologic improvement of the erythroid (1) and platelet (1) lineages. Four pts had stable disease (SD) after treatment but their courses were complicated by infections requiring hospitalization and removal from study. Three pts were deemed to be inevaluable because they received 〈 2 cycles of treatment or did not have a follow-up BM evaluation. Thus, 10/19 evaluable pts (53%) demonstrated either a BM/PB response (6) or SD (4). The median overall survival (OS) of pts with marrow CR+PR (n=6) was 12 months versus 1.8 months for those without a BM response (n=9) (p=0.0159, log-rank test). Age was not a predictor of response. 1 out of 6 responders had a major elimination in the size of the clonal population. Non-responders did not have a change in the magnitude of the clone. Prior response to HMA was not a predictor for response to rigosertib. Those who did have a marrow CR or PR responded early with median time to response of 2-4 cycles. Those with higher blast counts were less likely to respond. At study entry, the median blast percentage of responders was 16% versus 44% for non-responders. Less than 20% blasts at study entry was a positive predictor of response (p=0.047). Of those pts who did not respond or were inevaluable, the majority (75%) had AML, many with a proliferative course. Nine of 19 pts developed cystitis manifested by dysuria and/or hematuria as a side-effect of therapy. Among responding patients, 5 of 6 had cystitis [grade (GR) 1 (2); GR 2 (1); GR 3 (2)] compared with 3 of 9 non-responders [GR 1(1); GR 2 (1); GR 3(1)] (p=0.08). In one responding pt with grade 3 cystitis, a cystoscopy was performed which revealed polypoid inflammatory changes of the mucosa with hemorrhage. Biopsy showed neutrophilic inflammation without malignant cells. Upon resolution of symptoms, treatment was restarted at 50% of the original dose without complications. Pts who developed symptomatic cystitis were treated with sodium bicarbonate with improvement. The relationship between cystitis and response is being investigated. Conclusions Rigosertib appears to have biologic activity with reduction in BM blasts associated with increased survival and improvement in the PB counts in a subset of treated pts. Pts with 〈 20% blasts at study entry had a greater likelihood of response. Pts with proliferative disease with rapidly rising or high wbc did not respond. Age, cytogenetic profiles, and response to prior therapy were not predictors of response. Cystitis may be a response related biomarker and requires further analysis. A phase III multicenter randomized trial is underway to compare rigosertib to best supportive care with a primary endpoint of OS in pts with higher risk MDS who have failed, progressed, or relapsed after treatment with HMA, and can be used to validate the observations reported here in a larger study. Disclosures: Reddy: Onconova: Equity Ownership, Research Funding. Holland:Onconova: Research Funding. Wilhelm:Onconova Therapeutics: Employment, Equity Ownership. Silverman:Onconova: Research Funding.