DSP 5336

Enzomenib (DSP-5336) is Sumitomo Pharma America's menin inhibitor in key Phase 2 for relapsed or refractory acute leukemia. It targets the same protein-protein interaction that revumenib (Revuforj, Syndax) hit when FDA approved it in November 2024 for KMT2A-rearranged acute leukemia and again in October 2025 for NPM1-mutant AML, and that ziftomenib (Kura Oncology) is chasing in NPM1-mutant AML [4][5][7]. Enzomenib enters a validated but suddenly crowded category. Sumitomo's differentiation pitch is now visible in data: at ASH 2025, enzomenib at the recommended Phase 2 dose of 300 mg twice daily showed a complete remission (CR plus CRh) rate of 40% in 15 menin-inhibitor-naive KMT2A-rearranged patients and 44% in 25 NPM1-mutant patients, with QT prolongation (a measurable lengthening of the heart's electrical cycle that at high severity can cause fatal arrhythmia) in only 10% of patients and zero Grade 3 or higher events [8]. Those numbers are favorable against revumenib's FDA-reviewed CR+CRh of 21% and overall response rate of 63.2% in the KMT2A-rearranged AUGMENT-101 cohort, and revumenib carries a boxed warning for differentiation syndrome and QT [4][5]. FDA granted enzomenib Orphan Drug Designation in June 2022 and Fast Track Designation in June 2024 for r/r AML with KMT2A rearrangement or NPM1 mutation [9]. key Phase 2 monotherapy interim analysis enrollment was completed in June 2026, with results expected by end of 2026 and a potential US/Japan submission in FY2027 [10]. Why it matters for the science: menin inhibitors are the first practical targeted class for KMT2A-rearranged leukemia, a population with grim outcomes and almost no targeted options before 2024 [1][2]. Why it matters for the money: enzomenib's early efficacy plus a cleaner cardiac profile could let it land as a credible second-in-class option rather than an also-ran.

Enzomenib is a novel small molecule, not approved for any indication. The development program now has two active arms: the original Horizen-1 Phase 1/2 dose-escalation/expansion study (NCT04988555) [3] and a key Phase 2 monotherapy study in r/r acute leukemia with KMT2A rearrangement, the latter announced in June 2026 as having completed enrollment of the pre-specified interim analysis cohort [10]. FDA granted Orphan Drug Designation for AML in June 2022 and Fast Track Designation for r/r AML with KMT2A rearrangement or NPM1 mutation in June 2024 [9]. Japan's PMDA granted Orphan Drug Designation in September 2024 [9]. At ASH 2025 (data cutoff October 4, 2025; n=116 acute leukemia patients, 93% AML, median 2 prior regimens), Sumitomo reported the strongest dataset to date: at the recommended Phase 2 dose of 300 mg twice daily in menin-inhibitor-naive KMT2A-rearranged patients (n=15), ORR was 73.3% and CR+CRh was 40%, with median duration of CR+CRh of 12.5 months. In NPM1-mutant patients dosed at 200 mg twice daily or higher (n=25), ORR was 52% and CR+CRh was 44%. Across the entire dose-escalation range (40 to 400 mg twice daily), there were zero dose-limiting toxicities, zero treatment-related deaths, QT prolongation in 10% (4 patients) with no Grade 3 or higher events, and differentiation syndrome in 12.9% with no deaths or treatment discontinuations [8]. An enzomenib + venetoclax + azacitidine combination cohort (n=40) reported ORR 85% and composite CR 62% in the 13 patients naive to both venetoclax and menin inhibitors, with no DLTs [8]. Sumitomo Pharma America is the same group behind Orgovyx (relugolix) and the now-genericized Latuda franchise and badly needs a new oncology revenue line; enzomenib is now the lead asset in their reshuffled portfolio after the Myovant integration. The interim analysis from the key Phase 2 monotherapy study is the next major catalyst, expected by end of 2026, with potential US/Japan submission targeted for FY2027 if the primary endpoint is met [10].

Roughly 5 to 10% of acute leukemias carry a chromosomal rearrangement of the KMT2A gene (older name: MLL), which fuses the front half of KMT2A to one of many partner genes. The resulting fusion protein still latches onto leukemia-driving genes (HOXA9, MEIS1) and forces them on, locking blood cells in an immature, fast-dividing state. To do this, the fusion needs a partner protein called menin, which acts as scaffolding that holds the fusion at its target genes [1]. Block the menin-MLL interaction with a small molecule, the fusion falls off the DNA, the leukemia program shuts down, and the cells either mature into normal blood cells or die. The same logic applies to NPM1-mutant AML, where mutant NPM1 also depends on the menin-MLL axis to keep HOX genes turned on. NPM1 mutations show up in roughly 30% of adult AML, so the addressable population for menin inhibitors is much larger than KMT2A alone [2]. The mechanism is validated, not theoretical. Revumenib (Syndax) got FDA approval in November 2024 for r/r KMT2A-rearranged acute leukemia based on a CR+CRh rate of 21% (22/104 patients, 95% CI 13.8-30.3%) and an overall response rate of 63.2% in the AUGMENT-101 trial [4][5]. A second FDA approval followed in October 2025 for r/r NPM1-mutant AML with CR+CRh of 23.1% [5]. Ziftomenib (Kura) has reported similar single-agent activity in NPM1-mutant AML [2][7]. Enzomenib is the same biology with a different molecule, which removes target-validation risk almost entirely and turns the question into one of differentiation. Early head-to-head numbers from ASH 2025 suggest enzomenib's CR+CRh in KMT2A-rearranged disease (40% at RP2D, n=15) is competitive with or better than revumenib's 21%, although the enzomenib sample is small and cross-trial comparison is hazardous [8].

Horizen-1 (NCT04988555) is a Phase 1/2 open-label, dose-escalation and expansion study with a 606-patient target across multiple acute leukemia subtypes [3]. The Phase 1 primary endpoint is incidence of adverse events and serious adverse events, which is standard for dose-finding. Eligible patients have relapsed or refractory AML, acute lymphoblastic leukemia, or mixed-phenotype acute leukemia, with priority on KMT2A-rearranged and NPM1-mutant subsets where the mechanism predicts activity. The recommended Phase 2 dose has been established at 300 mg twice daily based on the ASH 2025 readout [8]. Trial is single-arm with no comparator, which is appropriate for a Phase 1 in a population with essentially no standard of care. A separate key Phase 2 monotherapy study in r/r KMT2A-rearranged acute leukemia is now running; the pre-specified interim analysis cohort completed enrollment in June 2026 and the interim analysis is expected to read out by end of 2026 [10]. Endpoint readouts that matter most for investors: CR+CRh rate (complete remission plus complete remission with partial hematologic recovery, a regulatory composite that captures patients who achieve marrow clearance but have not fully recovered blood counts), overall response rate, duration of response, and rates of differentiation syndrome and QT prolongation. QT prolongation refers to a measurable lengthening of the heart's electrical recovery interval that at high severity can trigger torsades de pointes, a life-threatening arrhythmia; this is why revumenib carries a boxed warning [4][5]. Enzomenib's ASH 2025 numbers on these safety endpoints (QT prolongation 10%, no Grade 3+; differentiation syndrome 12.9%, no deaths) are the basis for the differentiation pitch [8]. The 606-patient Horizen-1 target plus a separate key monotherapy study together suggest Sumitomo plans to support an accelerated approval filing off expansion and key data combined, similar to the path Syndax used for revumenib. The single-arm design means cross-trial comparisons against AUGMENT-101 will dominate the commercial narrative until a head-to-head or combination trial reads out.

Our model estimates a 28% chance this drug is eventually approved. It starts from the historical base rate for Phase 2 drugs in this area (about 21%), then adjusts using ten facts about the trial and sponsor. What moves the number most: it is helped by an unusually multi-arm design (14 arms), larger-than-typical enrollment for this phase, and a non-randomized design; it is held back by the sponsor's thin or weak approval record. The other facts land near average for this stage, so they leave the estimate roughly where the base rate put it.

Class safety risks are well-characterized from revumenib's label, which carries a boxed warning for differentiation syndrome (a sometimes fatal inflammatory reaction when leukemia cells mature quickly and release inflammatory mediators) and signals for QT prolongation [4][5]. Enzomenib's ASH 2025 numbers are favorable on both axes: QT prolongation in only 10% with no Grade 3+ events, differentiation syndrome at 12.9% with no deaths or discontinuations [8]. These need to hold up in the larger key Phase 2 cohort. Efficacy risk is mainly regression to the mean in the larger key monotherapy cohort: the 40% CR+CRh and 73.3% ORR at RP2D were based on only 15 KMT2A-rearranged patients, and small-cohort response rates routinely shrink when expanded. The ALL extension is a stretch, since most KMT2A-rearranged ALL cases are infant or pediatric and have different biology than adult AML. Competitive risk remains a major factor. Revumenib is approved for both KMT2A-r (November 2024) and NPM1m AML (October 2025), Kura's ziftomenib has its own key program in NPM1m [7], and Johnson & Johnson and Daiichi Sankyo have menin inhibitors in earlier development [2]. Sumitomo entered second or third, not first, but the early efficacy/safety differentiation gives them a path that did not look open a year ago. Execution risk: Sumitomo Pharma's parent company (Tokyo-listed) has faced revenue pressure following the loss of US exclusivity on Latuda, and the US subsidiary has had layoffs and portfolio cuts post-Myovant integration. The key Phase 2 study is the largest enrollment commitment in their heme onc portfolio, and resourcing risk is real if the parent's financial situation tightens further. Commercial risk: even with approval, payer pressure on $30k-plus per month oral oncolytics and a multi-drug class will compress pricing, and prescribers will default to the asset with the most mature data and the simplest safety story unless enzomenib's QT advantage shows up in real-world cardiology consults.

Worth watching, and the differentiation story is partially answered. ASH 2025 data showed enzomenib at RP2D delivering CR+CRh of 40% in KMT2A-rearranged patients (n=15) and 44% in NPM1-mutant patients (n=25), with QT prolongation in 10% and zero Grade 3+ events, and differentiation syndrome in 12.9% with no deaths [8]. Those numbers compare favorably to revumenib's FDA-reviewed CR+CRh of 21% in KMT2A-r and the QT-prolongation boxed warning, although small enzomenib cohort sizes mean cross-trial comparison should be held loosely. The next data point that matters: the key Phase 2 monotherapy interim analysis, expected by end of 2026 [10], in a larger KMT2A-rearranged cohort with the same endpoints. If the CR+CRh holds in the high 20s to 30s range with the clean QT profile intact, Sumitomo has a credible accelerated approval submission for FY2027. Watch the combination arm with venetoclax and azacitidine (ASH 2025 reported ORR 85% in 13 venetoclax/menin-naive patients, no DLTs) [8], which is where the menin class probably ends up commercially in newly diagnosed AML. Watch Sumitomo Pharma's parent-level earnings for resourcing signals or partnership/licensing language around DSP-5336. The signal that would matter most: Sumitomo seeking a US co-promotion partner, since that would tell you internal modeling sees a credible commercial path that needs more sales muscle than the US subsidiary currently has, or moving the asset into a randomized combination trial against 7+3 induction (the standard intensive AML chemotherapy backbone of 7 days continuous cytarabine plus 3 days of an anthracycline) plus venetoclax in newly diagnosed NPM1-mutant AML.