Background: Lintuzumab-Ac225 is a humanized CD33 antibody conjugated to an alpha emitting isotope, Ac225, delivering high energy radiation to CD33 expressing AML blasts. Preclinical studies indicate Lintuzumab-Ac225 also depletes the anti-apoptotic protein MCL-1, which is a known resistance factor in BCL2-inhibitor (venetoclax (Ven)) exposed patients (pts). We performed a Phase I study where Lintuzumab-Ac225 was administered after salvage chemotherapy, and this study demonstrated safety and encouraging response rates in R/R AML. Herein, we report results of high-risk patients who enrolled to this study. Methods: R/R AML pts deemed fit, and with CD33 expression on >25% of blasts were enrolled onto a Phase I study, Lintuzumab-Ac225 in Combination with Cladribine + Cytarabine + Filgrastim + Mitoxantrone (CLAG-M). Induction consisted of G-CSF, 300mcg/d on D1-6, cladribine 5mg/m2 on D2-6, cytarabine 2g/m2 on D2-6, and mitoxantrone 10mg/m2 on D2-4. Lintuzumab-Ac225 was administered once on D8 ±1 day. The trial enrolled to 4 cohorts, with Lintuzumab-Ac225 at doses ranging from 0.25 uCi/kg to 1.0 uCi/kg. We defined high-risk pts as those who met ELN 2019 criteria for adverse risk disease, or pts who received a Ven combination prior to trial enrollment. Response is descriptively reported, and KM estimator was used to report overall survival (OS). Results: 19 pts on trial met high-risk criteria. Of these, 16 pts had ELN adverse risk disease, a TP53 mutation was detected in 75% (n=12) of pts. Median age was 65 yrs, pts received a median 2 lines of prior therapy, including prior allo-HCT in 58% (n=11), and prior Ven combination in 68% (n=13) of pts. In high-risk pts, 42% (n=8) of pts achieved CRc (CR+CRi), 16% (n=3) achieved MLFS, for an overall response rate (ORR) of 58%. Among evaluable pts achieving CRc, 71% achieved flow MRD negativity. Median OS was 7.4 months. Table reports results in select subgroups. Prior to treatment initiation, 7 pts were deemed eligible for first HCT. 4 (57%) were successfully able to bridge to HCT. Median survival among transplanted pts was 28 months. Post-HCT, one pt died due to GVHD, one due to relapse. Conclusions: CLAG-M in combination with Lintuzumab-Ac225 yielded significantly better clinical outcomes in high-risk populations, particularly in pts previously treated with Ven combinations (median OS 13.8 mo.) and in pts with TP53 mutations (median OS 7.3 mo.), for which recent studies have reported a survival of 4 months or less. The results support further advance in late-stage clinical development of Lintuzumab-Ac225 in combination with CLAG-M in R/R AML, especially in high-risk pts. Clinical trial information: NCT03441048.