Adding the selective BCL-2 inhibitor venetoclax to reduced intensity conditioning (RIC) chemotherapy (fludarabine and busulfan, FluBu2) may enhance anti-leukemic cytotoxicity and thereby reduce the risk of post-transplant relapse. This phase 1 study investigated the recommended phase 2 (RP2D) of venetoclax, a BCL-2 selective inhibitor, when added to FluBu2 in adult patients with high risk acute myeloid leukemia (AML), myelodysplastic syndromes (MDS), and MDS/myeloproliferative neoplasms (MPN) undergoing transplant. Patients received dose-escalated venetoclax (200-400 mg daily starting day -8 for 6-7 doses) in combination with fludarabine 30 mg/m2/day for four doses and busulfan 0.8 mg/kg twice daily for eight doses on day -5 to -2 (FluBu2). Transplant related-toxicity was evaluated from the first venetoclax dose on day -8 to +28. Twenty-two patients were treated. At study entry, 5 MDS and MDS/MPN patients had 5-10% marrow blasts and 18/22 (82%) had a persistent detectable mutation. Grade 3 adverse events included mucositis, diarrhea and liver transaminitis (N=3 each). Neutrophil/platelet recovery and acute/chronic GVHD rates were similar to standard FluBu2. No DLTs were observed. The RP2D of venetoclax was 400 mg daily for 7 doses. With a median follow-up of 14.7 months (8.6-24.8 months), median overall survival was not reached, and progression free survival was 12.2 months (95% CI: 6.0 months, not estimable). In high risk AML, MDS, and MDS/MPN patients, adding venetoclax to FluBu2 was feasible and safe. To further address relapse risk, assessment of maintenance therapy after venetoclax plus FluBu2 transplant is on-going. This study was registered at clinicaltrials.gov as #NCT03613532.
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