Entinostat (MS-275, SNDX-275): Quantitative Profile of a Selective HDAC1/3 Inhibitor

Executive Summary: Entinostat (MS-275, also known as SNDX-275) is a selective, orally available inhibitor of class I histone deacetylases, with IC50 values of 0.368 μM for HDAC1 and 0.501 μM for HDAC3, and much weaker activity against HDAC8 (63.4 μM) (product_spec). It robustly induces histone acetylation and anti-proliferative effects in diverse cancer cell lines, including breast, colon, and retinoblastoma (Wang et al. 2019, DOI). In vivo, MS-275 inhibits HDAC activity and impairs blastema formation, directly linking epigenetic modulation to tissue regeneration (DOI). Clinical studies show tolerable safety profiles when combined with other agents in solid tumor treatment (product_spec). APExBIO supplies validated Entinostat (A8171) for research.

Biological Rationale

Histone deacetylases (HDACs) are enzymes that remove acetyl groups from lysine residues in histone tails, condensing chromatin and repressing gene transcription (Wang et al. 2019, DOI). Class I HDACs (HDAC1, 2, 3, and 8) are central to epigenetic regulation in cancer and development. Elevated HDAC1 expression is required for blastema formation during limb regeneration in axolotls, reflecting a conserved role in cellular plasticity and proliferation (DOI). HDAC overactivity is documented in multiple cancers, driving abnormal cell growth and survival. Thus, selective HDAC1/3 inhibition offers a mechanistic approach for both cancer cell proliferation inhibition and research on regeneration and differentiation.

Mechanism of Action of Entinostat (MS-275, SNDX-275)

Entinostat operates as a competitive inhibitor, binding to the active site of class I HDACs. Its high selectivity is quantified by IC50 values: 0.368 μM for HDAC1, 0.501 μM for HDAC3, and 63.4 μM for HDAC8 (source: product_spec). Inhibition of HDAC1/3 raises histone acetylation, decondenses chromatin, and promotes expression of tumor suppressor genes. The downstream effects include cell cycle arrest, apoptosis induction in cancer cells, and inhibition of proliferation (entinostat.net article 112). In developmental biology, MS-275 disrupts blastema formation in axolotl limb regeneration by inhibiting local HDAC activity, confirming the necessity of HDAC1 upregulation for tissue renewal (DOI).

Evidence & Benchmarks

• Entinostat displays IC50 values of 0.368 μM (HDAC1), 0.501 μM (HDAC3), and 63.4 μM (HDAC8) in enzymatic assays (source: product_spec).
• In vitro, Entinostat increases acetyl-histone levels and reduces proliferation in human breast, colon, lung, myeloma, ovary, pancreas, prostate, and leukemia cells (source: product_spec).
• In axolotl larvae, MS-275 significantly delays limb regeneration and inhibits blastema formation, confirming that HDAC1 activity is essential for tissue regenerative capacity (source: Wang et al. 2019, DOI).
• In animal models of retinoblastoma, Entinostat reduces tumor burden and increases histone acetylation in retinal tissue (source: entinostat.net article 200).
• Clinical phase I studies established a recommended phase II dose for Entinostat in combination with 13-cis retinoic acid, with manageable safety and pharmacokinetics in patients with advanced solid tumors (source: product_spec).

Compared to this prior article, which reviews anti-proliferative and pro-apoptotic effects broadly, this dossier provides a quantitative and cross-domain (regeneration and oncology) synthesis.

For protocol integration and troubleshooting, this workflow-driven article focuses on cytotoxicity, while the current review emphasizes mechanistic and dose-selection evidence.

Applications, Limits & Misconceptions

Entinostat (MS-275) is validated for use in cancer cell proliferation inhibition, apoptosis induction in cancer cells, and as an experimental modulator in retinoblastoma treatment research. It is suitable for in vitro and in vivo models requiring specific HDAC1 and HDAC3 inhibition. In regenerative biology, MS-275 is a tool for dissecting epigenetic control of blastema formation and tissue renewal (DOI).

Common Pitfalls or Misconceptions

• Not a pan-HDAC inhibitor: Entinostat is selective for class I HDACs and is much less effective against HDAC8 and class II HDACs (source: product_spec).
• Limited water solubility: It is insoluble in water, requiring DMSO (≥18.8 mg/mL) or ethanol (≥7.4 mg/mL with ultrasound) for stock solutions (source: product_spec).
• Not suitable for all regenerative studies: While effective in axolotl and some cancer models, MS-275 may not translate to non-HDAC1/3-driven systems (DOI).
• Stability concerns: Stock solutions should be stored below -20°C and used promptly to avoid degradation (source: product_spec).
• Not a direct apoptosis inducer in all cell types: Effects are context- and dose-dependent (entinostat.net article 157).

Workflow Integration & Parameters

Protocol Parameters

• HDAC activity assay | 0.368 μM IC50 (HDAC1) | in vitro, enzymatic | Quantifies nanomolar potency for HDAC1 | product_spec
• Cell proliferation inhibition | 0.1–5 μM | cancer cell lines | Reproducible anti-proliferative effects in multiple cancer lines | product_spec
• Blastema inhibition | 10–20 μM | axolotl limb regeneration | Complete suppression of blastema formation in larvae | Wang et al. 2019, DOI
• Solubility (DMSO) | ≥18.8 mg/mL | stock preparation | Achieves working concentrations for cell and animal studies | product_spec
• Storage | <-20°C | all experimental setups | Maintains chemical stability for reliable results | product_spec
• Animal dosing | Workflow-specific, 1–10 mg/kg | in vivo studies | Dose must be validated per protocol | workflow_recommendation

Conclusion & Outlook

Entinostat (MS-275, SNDX-275) is a rigorously validated, selective oral HDAC1/3 inhibitor for cancer and regenerative research. Its nanomolar potency, robust in vitro and in vivo benchmark data, and defined solubility/storage parameters support its value for translational studies. APExBIO's A8171 kit provides reproducible performance in academic and preclinical workflows (product_spec). Future work will refine context-specific dosing and explore combination regimens in solid tumor clinical trials, as supported by existing phase I safety data (product_spec).