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

Executive Summary: Mifepristone (RU486) is a high-affinity antagonist of the progesterone receptor, widely employed in preclinical models of cancer and reproductive biology. It inhibits progesterone-mediated signaling, leading to cell cycle arrest and apoptosis in hormone-responsive cancer cell lines (Li et al., 2018). In ovarian cancer cells, Mifepristone achieves dose-dependent growth inhibition with IC₅₀ values of 6.25 μmol/L (SK-OV-3) and 6.91 μmol/L (OV2008) (APExBIO product page). The compound also reduces uterine fibroid size and impedes meningioma growth, both in vitro and in vivo. Its solubility profile (≥21.48 mg/mL in DMSO/ethanol) and well-documented storage requirements enable robust integration into diverse experimental workflows.

Biological Rationale

Mifepristone (RU486) is a synthetic steroid designed to antagonize the progesterone receptor (PR). PR is a nuclear hormone receptor critical for reproductive tissue development, menstrual regulation, and pregnancy maintenance. In oncology, PR signaling modulates cell proliferation and survival in multiple tumor types, including ovarian, breast, prostate, and endometrial cancers (Li et al., 2018). Dysregulation of PR pathways contributes to tumor growth and therapy resistance. By targeting PR, Mifepristone disrupts downstream gene expression, impairs cell cycle progression, and sensitizes cells to cytotoxic agents. In reproductive biology, Mifepristone acts as a contraceptive by blocking progesterone-dependent endometrial changes essential for embryo implantation.

Mechanism of Action of Mifepristone (RU486)

Mifepristone exerts its effects by competitively binding to the ligand-binding domain of the progesterone receptor, preventing natural ligand (progesterone) association. This antagonism blocks PR-mediated transcriptional activation, arrests the cell cycle at G1/S or G2/M, and induces apoptosis in sensitive cell lines (APExBIO). The compound also demonstrates glucocorticoid receptor antagonist activity, albeit with lower potency compared to PR antagonism. In ovarian cancer models, Mifepristone downregulates cyclin A (S phase) and cyclin B1 (M phase), which are essential for cell cycle progression. It inhibits progesterone-induced acrosome reaction and hyperactivation in human sperm, impacting fertility at multiple stages. Mifepristone's cell-permeability facilitates rapid intracellular receptor engagement, making it suitable for both in vitro and in vivo studies.

Evidence & Benchmarks

• Mifepristone inhibits ovarian cancer cell growth with IC₅₀ values of 6.25 μmol/L (SK-OV-3) and 6.91 μmol/L (OV2008) under standard culture conditions (37°C, 5% CO₂, 48 h exposure) (APExBIO).
• Reduces uterine fibroid size and suppresses meningioma cell proliferation in both in vitro and animal models (APExBIO).
• Decreases cyclin A and cyclin B1 expression in ovarian cancer cells, leading to cell cycle arrest (S and M phase, respectively) (APExBIO).
• Blocks progesterone-induced acrosome reaction and intracellular Ca²⁺ elevation in human sperm (APExBIO).
• Demonstrates anti-proliferative effects in breast, prostate, and gastric adenocarcinoma cell lines (Li et al., 2018).
• Supplied at ≥98% purity, soluble in DMSO and ethanol (≥21.48 mg/mL, 20–25°C with gentle warming), and stable for several months at -20°C as solid or DMSO stock (APExBIO).

Applications, Limits & Misconceptions

Mifepristone (RU486) is validated for use in studies of:

• Ovarian, breast, prostate, and endometrial cancer cell growth inhibition.
• Hormone receptor signaling pathway interrogation (progesterone and glucocorticoid receptors).
• Evaluation of contraceptive mechanisms and sperm function modulation.
• Animal xenograft models to monitor tumor growth suppression.

Compared to "Mifepristone (RU486): Advanced Mechanisms and Emerging On...", which discusses novel mechanistic insights, this article provides a structured, benchmark-driven overview for experimentalists seeking standardized results.

For protocol guidance, see "Mifepristone (RU486): Unlocking Precision in Progesterone..."; the current article updates those workflows with recent evidence and quantitative benchmarks.

To contextualize APExBIO's Mifepristone within hormone signaling research, "Harnessing Mifepristone (RU486) for Next-Generation Hormo..." explores translational opportunities, whereas this article focuses on product-specific evidence and use-case boundaries.

Common Pitfalls or Misconceptions

• Water Solubility: Mifepristone is insoluble in water; DMSO or ethanol must be used for stock preparation (≥21.48 mg/mL, gentle warming).
• Glucocorticoid Receptor Selectivity: Although Mifepristone antagonizes glucocorticoid receptors, its affinity is lower than for PR; it is not a selective GR inhibitor.
• Long-term Solution Stability: Stock solutions in DMSO are stable at -20°C for months, but prolonged storage of working solutions is discouraged due to degradation risk.
• In Vivo Dosage Extrapolation: In vitro IC₅₀ values do not directly translate to animal or clinical doses; in vivo efficacy and toxicity must be independently validated.
• Target Specificity: Off-target effects may occur at high concentrations; always verify receptor expression in the model system.

Workflow Integration & Parameters

APExBIO's Mifepristone (RU486, B1511) is supplied as a solid, with recommended storage at -20°C. For experimental use, dissolve in DMSO or ethanol to create stock solutions (≥21.48 mg/mL) with gentle warming (20–25°C). Avoid water as a solvent. Working concentrations typically range from 1–10 μmol/L for cell-based assays. For sperm function assays, optimal concentrations and timing depend on the specific endpoint measured. In animal studies, tumor xenograft models utilize dose titration, with efficacy and toxicity monitored per standard protocols. Shipping uses blue ice to maintain compound integrity.

For detailed workflow illustrations, see the updated protocols in this internal guide, which this article extends by integrating new quantitative benchmarks and product specifications.

Conclusion & Outlook

Mifepristone (RU486) is a validated, cell-permeable progesterone receptor antagonist for cancer and reproductive biology research. Its well-defined mechanism, robust in vitro and in vivo benchmarks, and standardized handling protocols support its use in high-precision experimental workflows. For additional mechanistic context, see this review, which complements the current article's focus on product-specific evidence. APExBIO provides high-quality Mifepristone (B1511) for academic and translational research. Future directions include exploring combinatorial regimens in hormone receptor–positive cancer models and further dissecting PR/AR signaling interplay in therapy resistance (Li et al., 2018).