Background: Relapsed/refractory (R/R)acute leukemia withalterations in KMT2A (also called MLL1; 9-15% of adult AML, 10% of ALL) or NPM1 (30% of adult AML) are often associated with poor outcomes. Pre-clinical studies demonstrated the relevance of the menin-KMT2A protein-protein interaction in sustaining leukemic cells with KMT2A and NPM1 alterations (Kuhn 2016). JNJ-75276617 is a potent and selective inhibitor of the interaction between the scaffolding protein menin and the methyltransferase KMT2A with preclinical activity in KMT2A-rearranged or NPM1-mutated leukemic cell lines and primary leukemia patient samples in vitro and in vivo (Kwon 2022). We report initial data investigating JNJ-75276617 in adult participants (pts) with R/R acute leukemia harboring KMT2A alterations (rearrangements, amplifications, or partial tandem duplications) or NPM1 mutations. Methods: 75276617ALE1001 (NCT04811560) is an ongoing Phase 1, multicenter, open-label, dose-finding study. Pts in dose escalation receive JNJ-75276617 orally on a 28-day cycle. As of 8 April 2023, multiple dose levels ≥15 mg have been explored on either a daily or twice daily (BID) dosing schedule. AEs were graded by CTCAE v5.0. Responses were investigator-assessed per ELN2017. Preliminary safety, efficacy and PD data are reported herein, with a focused review of the efficacy in higher dose levels with ≥3 pts dosed. Results: Fifty-eight pts received JNJ-75276617. The median age was 63 (range: 19-83) years; 56 pts (97%) had R/R AML and 2 (3%) had R/R ALL. The median number of prior lines of treatment was 2 (range: 1-7), including 10 (17%) pts with a prior allogeneic stem cell transplant. A KMT2A or NPM1 alteration was present in 33 (57%) and 25 (43%) pts, respectively. Thirty (52%) pts experienced ≥1 treatment-related AE (TRAE); most commonly differentiation syndrome (DS) (8 14%). Grade ≥3 TRAEs were observed in 17 (29%) pts; those reported in ≥2 pts were neutropenia (6 10%), anemia and thrombocytopenia (4 7% each), DS (3 5%), and ALT and AST increase (2 3% each). Dose limiting toxicities (DLTs) were observed in 5 (9%) pts, with DS (2 3%) as the only DLT reported in ≥2 pts. In 26 (63%) of the 41 pts with disease evaluation data, there was a reduction in bone marrow (BM) disease burden ( Figure 1). Of these, a ≥50% decrease in BM blasts was observed in 16 (39%) pts. In the highest dose level with ≥3 pts (90 mg BID; n=8), the ORR (≥PR) was 50% (n=4), with all responders ongoing ( Figure 2). These responders (2 NPM1-, 2 KMT2A-altered) achieved CR (1 pt), CRh (1 pt), and CRi (2 pts). In a review of higher dose levels with ≥3 pts (≥45 mg BID; n=20), the ORR was 40% (n=8), with 7 responders ongoing ( Figure 2). These responders (5 NPM1-, 3 KMT2A-altered) achieved CR (3 pts), CRh (1 pt), CRi (3 pts), and PR (1 pt); median (range) time to first response (≥PR) 1.81 mos (1.0-3.3; n=8); time to CR, CRh, or CRi 1.77 mos (1.0-3.3; n=7); and time to CR 2.79 mos (1.8-2.9; n=3). Across all cohorts there were 12 responders, including 1 MRD negative CR. One responder discontinued treatment for allogeneic transplant; however, 8 responders continue on treatment, including 2 pts in cycle 9. Preliminary PD data from unfractionated BM and/or PBMCs in paired samples among responders (n=12) show biologic activity as indicated by reduction in expression (mean fold change from baseline calculated as on-tx-baseline/baseline range) of menin-KMT2A target genes ( MEIS1 -0.42 -1.0-9.0; HOXA9 -0.03 -1.0-21.7; FLT3 18.6 -1.0-425) and induction of genes associated with differentiation ( ITGAM 55.0 -0.93-1467; MNDA 5.9 -1.0-83.5). Compared to baseline, the percentage of KMT2A-altered cells or NPM1 variant allele frequency (VAF) was reduced in responders, with a decrease in KMT2A-altered cells by break-apart FISH probe from 59.2% at baseline to 8.1% post-treatment and in NPM1 VAF using a myeloid gene NGS panel from 13.1% at baseline to 2.8% post-treatment. Conclusions: Dose escalation in 75276617ALE1001 is ongoing with the RP2D(s) yet to be determined. Pts in dose expansion will receive JNJ-75276617 at the identified RP2D(s). Preliminary results of this FIH Phase 1 study demonstrate that JNJ-75276617 monotherapy has an acceptable safety profile, encouraging antileukemic activity, and emerging biologic activity consistent with the proposed mechanism of action in pts with R/R acute leukemia harboring KMT2A or NPM1 alterations.