Mifepristone (RU486): Progesterone Receptor Antagonist for Advanced Cancer and Reproductive Research

Executive Summary: Mifepristone (RU486) is a cell-permeable progesterone receptor antagonist with well-characterized anti-proliferative effects on multiple cancer cell lines (https://www.apexbt.com/mifepristone.html). It shows dose-dependent inhibition of ovarian cancer cells, with IC₅₀ values of 6.25 μmol/L (SK-OV-3) and 6.91 μmol/L (OV2008) under standard culture conditions. The compound also reduces uterine fibroid size and suppresses meningioma growth both in vitro and in vivo. Mifepristone modulates human sperm function by inhibiting progesterone-induced acrosome reaction and hyperactivation. It is provided by APExBIO as SKU B1511, supporting reproducible workflows in hormone receptor and oncology research (Li et al., 2018, DOI).

Biological Rationale

Mifepristone (RU486) targets the progesterone receptor (PR), a nuclear hormone receptor central to reproductive processes and implicated in hormone-dependent cancers. PR antagonism disrupts progesterone-driven gene transcription, affecting cell proliferation, differentiation, and apoptosis in target tissues. PR signaling cross-talks with glucocorticoid and androgen receptor pathways, relevant in gynecologic and hormone-sensitive cancers (Li et al., 2018, DOI). Ovarian, endometrial, and some breast cancers display PR expression; antagonists like Mifepristone are used as research tools to dissect these pathways. In reproductive biology, PR blockade impacts ovulation, implantation, and sperm function, offering mechanistic insight for contraceptive and fertility studies.

Mechanism of Action of Mifepristone (RU486)

Mifepristone is a competitive antagonist of the progesterone receptor. It binds the ligand-binding domain of PR with high affinity, displacing endogenous progesterone and inhibiting receptor activation. This prevents PR-mediated transcription of genes involved in cell cycle progression and survival. Mifepristone also exhibits partial antagonism of the glucocorticoid receptor (GR), influencing cellular responses to glucocorticoids. In cancer models, Mifepristone downregulates cyclin A and cyclin B1 expression, resulting in S-phase and M-phase cell cycle arrest. In sperm, it inhibits progesterone-induced acrosome reaction, hyperactivation, and intracellular calcium influx. These effects are dose-dependent and observable in vitro at micromolar concentrations (B1511 datasheet, APExBIO).

Evidence & Benchmarks

• Mifepristone dose-dependently inhibits ovarian cancer cell growth, with IC₅₀ values of 6.25 μmol/L (SK-OV-3) and 6.91 μmol/L (OV2008) after 48 hours incubation in DMEM/F12 medium (B1511 datasheet, APExBIO).
• Reduces uterine fibroid size and meningioma growth in preclinical xenograft models (Zhou et al., 2022, DOI).
• Suppresses proliferation in endometrial, breast, prostate, and gastric adenocarcinoma cell lines, via PR antagonism and cell cycle blockade (Li et al., 2018, DOI).
• Inhibits progesterone-induced human sperm acrosome reaction and hyperactivation at ≥10 μM in capacitation buffer, 37°C, 1–2 hours (Mahony et al., 1991, PubMed).
• Demonstrates stable solubility at ≥21.48 mg/mL in DMSO and ethanol with gentle warming; insoluble in water (B1511 datasheet, APExBIO).
• Modulates cyclin A and B1 expression, inducing G1/S and G2/M cell cycle arrest in ovarian cancer cells (Wang et al., 2011, DOI).
• Glucocorticoid receptor antagonism validated in T47D and A549 cell lines, as measured by dexamethasone-induced reporter assays (B1511 datasheet, APExBIO).
• Xenograft tumor models demonstrate dose-dependent tumor growth inhibition with Mifepristone at 10–50 mg/kg/day, administered orally (Li et al., 2018, DOI).

This article extends the workflow and mechanistic guidance found in "Mifepristone (RU486): Robust Solutions for Cell Viability..." by providing quantitative IC₅₀ benchmarks and highlighting applications in hormone-driven cancer models.

Compared to "Harnessing Mifepristone (RU486) for Next-Generation Hormo...", this review incorporates recent findings on AR heterogeneity and cross-talk with PR signaling, clarifying translational implications for combinatorial therapy.

Applications, Limits & Misconceptions

Mifepristone (RU486) is indicated for research in the following domains:

• Hormone-dependent cancer biology (ovarian, endometrial, breast, prostate).
• Reproductive biology—contraceptive mechanism, sperm function, endometrial remodeling.
• Cell cycle and apoptosis assays in PR-expressing lines.
• Glucocorticoid and progesterone receptor cross-antagonism studies.

Common Pitfalls or Misconceptions

• Not suitable for direct androgen receptor antagonism: While Mifepristone affects PR and GR, it does not directly inhibit androgen receptor signaling; use specific AR antagonists for prostate cancer AR+ models (Li et al., 2018, DOI).
• Low water solubility limits aqueous applications: Mifepristone is insoluble in water and requires DMSO or ethanol for cell culture use; improper dissolution can confound results.
• Not validated for in vivo contraceptive efficacy in all species: Efficacy and safety profiles are species-specific and not interchangeable.
• Long-term storage of stock solutions not recommended: Degradation may occur; prepare fresh aliquots as needed for reproducibility (B1511 datasheet, APExBIO).
• Does not induce apoptosis in PR-negative cell lines: Mechanism is receptor-dependent; negative controls are essential.

This analysis clarifies and updates the mechanistic focus in "Mifepristone (RU486): Unlocking Precision in Progesterone..." by specifying conditions where Mifepristone activity is limited by receptor status or solubility.

Workflow Integration & Parameters

Mifepristone (RU486), available as a solid from APExBIO (SKU B1511), is dissolved in DMSO or ethanol at ≥21.48 mg/mL with gentle warming. For cell-based assays, working dilutions are typically 1–50 μM, added to pre-warmed media. Vehicle controls (DMSO ≤0.1%) are required. Stock solutions should be aliquoted and stored at ≤–20°C for short-term use. For hormone receptor antagonism assays, T47D (PR+) and A549 (GR+) cell lines are recommended. In xenograft tumor models, oral dosing regimens of 10–50 mg/kg/day have shown tumor growth inhibition. Shipping is on blue ice, and all handling should observe appropriate chemical safety protocols.

Conclusion & Outlook

Mifepristone (RU486) is a validated, versatile research tool for dissecting progesterone receptor signaling in cancer and reproductive biology. Its potent, cell-permeable antagonism enables precise modulation of cell cycle and hormone-driven processes. While not suitable for direct AR antagonism, its role in combinatorial studies and cross-pathway analysis is expanding (Li et al., 2018, DOI). APExBIO provides high-purity Mifepristone supporting reproducible workflows. Ongoing research will further clarify its translational potential in targeted therapy and fertility models.