Background: The prognosis of patients with higher-risk (HR) MDS after hypomethylating agent (HMA) failure is poor with a survival of 4-6 months (Garcia-Manero et al, Lancet Oncology 2016). The combination of azacitidine (Aza) with venetoclax (Ven) in frontline HR MDS is very active (Garcia JS et al, ASH 2021). Methods: This is a phase 1 study evaluating patients ≥ 18 years old after HMA failure with adequate organ function and performance status were enrolled. HMA failure was defined as relapse or progression after ≥ 4 cycles of HMA. Patients with prior Ven exposure were excluded. All patients received Aza 75mg/m2 IV/SC on D1-5. A 3+3 Ven dose escalation was utilized; patients at dose level 0 (DL0), 1 (DL1), and 2 (DL2) received 100 mg, 200 mg, and 400 mg respectively on D1-14. The IWG2006 criteria was utilized to assess response. Results: To date, 12 patients have been enrolled (3 in DL0, 3 in DL1, and 6 in DL2) with a median age of 78 years (range: 67-82). The median bone marrow blasts at enrollment was 9% (range: 6 – 17%) with a median ANC, Hgb, and PLT of 1.15 K/µL, 7.5 mg/dL, and 33 K/µL respectively. When stratified based on cytogenetics, 6 (50%) patients had intermediate risk, 2 (17%) had poor risk, and 4 (33%) had very poor risk cytogenetic alterations. When stratified based on IPSS-R criteria, 2 (16%) had intermediate, 5 (42%) had high, and 5 (42%) had very high risk disease. ASXL1 (50%) and TP53 (42%) mutations were most the common mutations on next generation sequencing. The most frequently observed grade 3-4 adverse events(AE) were cytopenias, occurring in all patients. Four (33%) patients had grade 3-4 cytopenias, either thrombocytopenia or neutropenia, that were attributed to the drug combination, with 3 patients consequently requiring dose reductions. At the time of data cut-off, response was seen in 8 patients with an overall response rate of 75%. The median number of cycles to response was 1 (range: 1-4). One patient achieved a CR. Seven patients achieved a marrow response with 2 having neutrophil recovery and 2 with platelet recovery. Three patients achieved transfusion independence. One patient underwent allogeneic stem cell transplantation after receiving 7 cycles. Four patients had no response with 3 of those having TP53 mutations. With a median follow-up of 13.6 months, the mOS is 8.5 months with a 30-day and 60-day mortality of 8% and 17% respectively. Early mortality was related to infection in 1 patient and disease progression in the other. Conclusions: This phase 1 study in patients with relapsed/refractory HR MDS suggests potential benefit with the addition of Ven to HMA with a 75% overall response and a mOS of 8.5 months. TP53 mutations and complex karyotypes still confer poor prognosis despite the addition of Venetoclax. Overall, this combination was well-tolerated with treatment-related AEs primarily consisting of thrombocytopenia and neutropenia. The study continues to accrue. Clinical trial information: NCT04550442.