Entinostat

Entinostat (research code MS-275) is Syndax Pharmaceuticals' oral class I selective HDAC inhibitor, a small molecule in clinical development since the early 2000s that pulls acetyl chemical tags off histones to reawaken silenced genes in cancer cells. Its Phase 3 registration study in hormone receptor-positive metastatic breast cancer (HR+, meaning tumors that use estrogen to grow) that also lacks HER2 overexpression (HER2-, a separate growth-driver protein), E2112 (NCT02115282, run by ECOG-ACRIN with 608 patients randomized to exemestane, an aromatase inhibitor that shuts down estrogen synthesis, plus entinostat or exemestane plus placebo), read out negative at San Antonio Breast Cancer Symposium in December 2020 and was formally published in Journal of Clinical Oncology in 2021 for both progression-free and overall survival [1][5]. Syndax has since redirected its commercial focus to revumenib (Revuforj, menin inhibitor approved November 15, 2024) and axatilimab (Niktimvo, chronic GvHD approved August 14, 2024), with entinostat remaining in a handful of investigator-initiated Phase 1/2 combination studies with checkpoint inhibitors and cytokines, none of them powered for registration [6][7]. The phase 3 status label attached to this node reflects the trial's active_not_recruiting bookkeeping for long-term follow-up, not an ongoing efficacy readout.

Not a novel compound. Entinostat originated as MS-275 at Mitsui Pharmaceuticals in Japan in the late 1990s, was later held by Schering AG / Bayer, and was licensed to Syndax more than 15 years ago (the Schering / Bayer internal development code sometimes cited as BAY 86-5274 in secondary sources is not consistently confirmed in the primary literature, so treat that code with caution). FDA granted breakthrough therapy designation in 2013 for HR+/HER2- metastatic breast cancer based on the Phase 2 ENCORE 301 study, which showed roughly a 4.3 vs 2.3 month progression-free survival benefit and a 28.1 vs 19.8 month overall survival signal when added to exemestane [8]. That designation was effectively voided in practical terms by the E2112 Phase 3 miss [5], though the breakthrough designation itself remains listed on the FDA record. What remains active are academic combination studies: NCT02115282 (E2112, active for follow-up but data already reported), NCT03179930 (MSKCC pembrolizumab combination in relapsed lymphoma), NCT02453620 (NCI nivolumab/ipilimumab in HER2-negative breast), NCT04708470 (NCI triple immunotherapy in HPV-associated malignancies), and NCT01038778 (aldesleukin in kidney cancer). None carry a defined regulatory endpoint. Syndax's 2025 10-K and 2026 10-Q disclosures center commercial narrative and revenue reporting on revumenib and axatilimab; entinostat is not the subject of a Syndax-sponsored registrational study, and no publicly disclosed regulatory catalyst exists for 2026 or 2027 [4]. On exclusivity: the original entinostat composition-of-matter patents from the 1990s Mitsui era have long since expired, and formulation and combination-use patents that extended some coverage into the mid-2020s provide only narrow protection. Any future approval would face near-term generic exposure, which further weakens the commercial thesis.

HDACs are enzymes that pull acetyl chemical tags off histones, the spool-shaped proteins that DNA wraps around. When those tags come off, DNA winds tighter and the genes in that region get switched off. Cancer cells exploit this system to silence tumor suppressor genes and differentiation programs that would otherwise slow them down. HDAC inhibitors block the enzymes, so acetyl tags accumulate and silenced genes come back on. Entinostat selectively hits class I HDACs (HDAC1, 2, 3), the subset most involved in transcriptional repression during tumor development, with roughly nanomolar potency against HDAC1 and HDAC3 and negligible activity against HDAC8 or class II enzymes [9]. In HR+ breast cancer, the biological logic was that HDAC inhibition would reverse acquired resistance to hormone therapy by resensitizing tumor cells to estrogen receptor blockade. In immuno-oncology combinations the logic shifts: HDAC inhibition depletes myeloid-derived suppressor cells (MDSCs, immune cells tumors recruit to shield themselves) and increases tumor antigen presentation, both of which should make checkpoint inhibitors work harder. The target class is validated in a narrow sense. Five HDAC inhibitors are approved worldwide (vorinostat, romidepsin, belinostat, panobinostat, chidamide), but every one of them is in a hematologic cancer indication. No HDAC inhibitor has ever been approved for a solid tumor. That is the honest ceiling for this class.

NCT02115282 is E2112, an ECOG-ACRIN cooperative group Phase 3 in postmenopausal women with HR+/HER2- advanced breast cancer whose disease progressed on a non-steroidal aromatase inhibitor (a drug that blocks estrogen production in postmenopausal women) [1]. Randomization was 1:1 to exemestane 25 mg daily plus either entinostat 5 mg weekly or matching placebo, with dual primary endpoints of progression-free survival and overall survival. Enrollment finished at 608 patients between March 2014 and October 2018. The design was defensible for the era it was conceived in (pre-CDK4/6 dominance), and the Phase 2 ENCORE 301 signal it was built to confirm looked real at the time [8]. E2112 did not replicate that signal. Reported at SABCS 2020 and published by Connolly et al. in Journal of Clinical Oncology in 2021: median progression-free survival was 3.3 months for the entinostat arm versus 3.1 months for placebo (hazard ratio 0.87, not statistically significant), and median overall survival was 23.4 vs 21.7 months (hazard ratio 0.99, p=0.94) [5]. Prespecified correlative work showed that entinostat did deliver its intended pharmacodynamic effect (a roughly 1.5-fold increase in protein lysine acetylation in peripheral blood mononuclear cells versus no change in the placebo arm), so the negative outcome is not attributable to drug failing to hit the target. It is a genuine efficacy miss. Candidate exploratory biomarkers of progression-free survival benefit (lysine acetylation in CD3+ cells, plasma IP-10, dendritic cell CD86, CD4+ HLA-DR and ICOS) were examined but did not identify a subgroup with a compelling benefit that would support renewed development. The trial's active_not_recruiting listing reflects continued long-term follow-up, not an unread primary analysis. The other listed active studies are much smaller Phase 1/2 combination trials at NCI and MSKCC (NCT02453620 n=57, NCT03179930 n=47, NCT01038778 n=47, NCT04708470 n=55), none powered for or designed as registration studies. A recent meta-analysis of exemestane plus entinostat across randomized trials confirmed the class-average lack of benefit [2].

Our model estimates a 16% chance this drug is eventually approved. It starts from the historical base rate for Phase 3 drugs in this area (about 48%), then adjusts using ten facts about the trial and sponsor. What moves the number most: it is held back by the sponsor's thin or weak approval record, weak or limited earlier-phase results, a randomized design, and a comparator/control arm. The other facts land near average for this stage, so they leave the estimate roughly where the base rate put it.

Efficacy risk is the largest one, and it has already partially materialized. HDAC inhibitors have produced zero solid tumor approvals despite two decades of clinical development, and the mechanism tends to deliver modest, biomarker-agnostic tumor cytostasis that washes out in randomized trials when the control arm is competent. E2112 is the case study [5]. Safety is manageable but not clean. Entinostat causes dose-related neutropenia, thrombocytopenia, fatigue, and GI toxicity, all of which limit combination dosing with chemotherapy or myelosuppressive checkpoint regimens. A 2026 review catalogued cytopenia and hepatotoxicity signals across the HDAC class in pediatric populations [10]. QT prolongation has dogged panobinostat and romidepsin and remains a monitored risk for entinostat combinations. Execution risk: the drug now sits largely in cooperative group and single-center academic trials, not commercial-sponsor trials, which slows enrollment and dilutes protocol quality. Commercial risk: even if a combination study reads out positive, entinostat would enter an HR+ breast market dominated by CDK4/6 inhibitors and oral SERDs (selective estrogen receptor degraders, a newer class of pills like elacestrant that both block and break down the estrogen receptor), and immuno-oncology combinations face abemaciclib and ribociclib as the benchmarks. Approved HDAC inhibitors in small hematologic indications have generated modest revenue (romidepsin and belinostat both under $100M annually before generic entry), so payer enthusiasm and pricing power for a solid-tumor HDAC entrant would be limited even in a positive scenario. Layered on top: composition-of-matter exclusivity for entinostat has effectively lapsed, so any approval would face rapid generic pressure and depress the risk-adjusted commercial upside further.

Noise, with a small chance of a signal from one of the combination studies. Syndax's growth story now sits on revumenib (Revuforj, approved November 15, 2024 for KMT2A-rearranged acute leukemia in adults and pediatrics one year and older, with an expanded NPM1-mutated AML approval in October 2025) and axatilimab (Niktimvo, approved August 14, 2024 for chronic GvHD after two prior lines of therapy), and neither depends on entinostat [6][7]. Entinostat is a legacy asset with a failed Phase 3, kept alive by investigator-initiated combination work. The specific data point that would flip this to a signal: a Phase 2 combination with a checkpoint inhibitor reporting an objective response rate meaningfully above what the checkpoint achieves alone in a defined tumor type, ideally with a biomarker readthrough (MDSC depletion, tumor-infiltrating lymphocyte increase) that mechanistically explains the effect. Neither the pancreatic nor the lymphoma combinations have delivered that yet [3]. Worth checking back after any Syndax Q3/Q4 2026 earnings call where entinostat is mentioned by name (relative silence across recent calls is itself informative), or if the 2025 meta-analysis of exemestane plus entinostat sparks renewed subgroup work on E2112 archival samples [2]. Otherwise this is a watch-only asset, not a near-term catalyst. Not investment advice.