Mifepristone (RU486): Cell-Permeable Progesterone Receptor Antagonist for Cancer and Reproductive Research

Executive Summary: Mifepristone (RU486) is a high-affinity antagonist of the progesterone receptor (PR), widely utilized in both contraceptive and oncology research contexts (APExBIO). It demonstrates dose-dependent inhibition of ovarian cancer cell growth, with IC50 values of 6.25 μmol/L (SK-OV-3) and 6.91 μmol/L (OV2008) under standard in vitro conditions [Li et al., 2018]. Beyond reproductive modulation, Mifepristone exerts anti-proliferative effects on endometrial, breast, prostate, and gastric adenocarcinoma cells. The compound is highly soluble in DMSO and ethanol at ≥21.48 mg/mL with gentle warming, but insoluble in water. APExBIO’s B1511 formulation ensures batch consistency and storage reliability, supporting reproducible hormone signaling and tumor xenograft studies.

Biological Rationale

Mifepristone (RU486) is a synthetic steroid that binds competitively to the progesterone receptor (PR), displacing endogenous progesterone. Progesterone is critical in regulating the menstrual cycle, maintaining pregnancy, and modulating cell proliferation in hormone-responsive tissues (APExBIO). By antagonizing PR, Mifepristone alters transcriptional programs governing reproductive function and hormone-driven tumor growth. Its cell-permeable structure allows robust intracellular access, making it a preferred tool in both reproductive biology and oncology research (see this mechanistic review; this article extends that analysis by providing updated benchmarks and practical workflow guidance). The ability to modulate PR and, to a lesser extent, glucocorticoid receptor signaling, positions Mifepristone as a strategic probe for dissecting steroid hormone pathways.

Mechanism of Action of Mifepristone (RU486)

Mifepristone acts as a high-affinity, competitive antagonist of the progesterone receptor (PR). It binds to the ligand-binding domain of PR, preventing progesterone-induced gene transcription (Li et al., 2018). This inhibition results in cell cycle arrest, particularly through downregulation of cyclin A (S phase) and cyclin B1 (M phase) expression, leading to reduced proliferation of hormone-dependent cancer cells. Mifepristone also inhibits the progesterone-induced acrosome reaction and sperm hyperactivation by blocking calcium influx in human sperm cells. At higher concentrations, it exhibits antagonistic activity on the glucocorticoid receptor, extending its utility in mechanistic studies of steroid receptor crosstalk. In cell lines such as SK-OV-3 (ovarian cancer), Mifepristone suppresses cell growth in a dose-dependent manner, with reproducible IC50 values in the low micromolar range under standard culture conditions (Li et al., 2018).

Evidence & Benchmarks

• Mifepristone (RU486) inhibits ovarian cancer cell growth with IC50 values of 6.25 μmol/L (SK-OV-3) and 6.91 μmol/L (OV2008) under 24–48 hour in vitro incubation (Li et al., 2018, DOI).
• Reduces uterine fibroid size in preclinical models when administered at 5–10 mg/kg/day over two weeks (Li et al., 2018, DOI).
• Suppresses progesterone-induced acrosome reaction and sperm hyperactivation at concentrations ≥10 μmol/L, as measured by calcium influx assays (Li et al., 2018, DOI).
• Inhibits meningioma cell proliferation both in vitro (IC50: 8–15 μmol/L) and in xenograft models with dose-dependent tumor volume reduction (Li et al., 2018, DOI).
• Demonstrates anti-proliferative effects on endometrial, breast, prostate, and gastric adenocarcinoma cell lines at 5–20 μmol/L under 48-hour exposure conditions (Li et al., 2018, DOI).

For protocol details and troubleshooting, see this protocol guide; this article provides updated benchmarks and clarifies integration into tumor and reproductive workflows.

Applications, Limits & Misconceptions

Mifepristone’s primary research applications include:

• Reproductive biology: Study of ovulation, implantation, and sperm function modulation.
• Oncology: Investigation of hormone-driven tumor growth in ovarian, endometrial, breast, prostate, and meningioma cell lines.
• Cell cycle research: Analysis of cyclin expression and cell cycle arrest in hormone-responsive cells.
• Glucocorticoid receptor signaling: Secondary applications in glucocorticoid antagonism assays.

Despite its versatility, there are critical boundaries:

Common Pitfalls or Misconceptions

1. Mifepristone is not effective against AR−/lo prostate cancer models; its action is limited to PR-positive or PR-driven pathways (Li et al., 2018).
2. It does not induce apoptosis in all cancer cell types; cytostatic rather than cytotoxic effects predominate at standard in vitro doses.
3. Solubility is limited in aqueous buffers; use only DMSO or ethanol with gentle warming for stocks (APExBIO).
4. Long-term storage of solutions is not recommended; stability is best when stored as a solid at -20°C.
5. Off-target effects at high concentrations may confound results in glucocorticoid signaling studies.

This article clarifies the specificity boundaries relative to the broader mechanistic roadmap in this recent review.

Workflow Integration & Parameters

APExBIO’s Mifepristone (RU486) (SKU B1511) is supplied as a solid and should be stored at -20°C. Stock solutions can be prepared in DMSO or ethanol at ≥21.48 mg/mL with gentle warming. For in vitro studies, working concentrations typically range from 1–20 μmol/L. For in vivo tumor xenograft models, standard dosing is 5–10 mg/kg/day by oral gavage or intraperitoneal injection. Solutions in DMSO are stable below -20°C for several months but should not be stored long-term. Product is shipped on blue ice under standard small molecule shipping protocols.

Validated assay systems include:

• Glucocorticoid and progesterone receptor antagonism in T47D (breast cancer) and A549 (lung cancer) cell lines.
• Tumor xenograft growth inhibition in ovarian and meningioma models.

For scenario-driven best practices and troubleshooting, see this laboratory guide; this article updates those recommendations with new IC50 data and storage protocols.

Conclusion & Outlook

Mifepristone (RU486) remains a gold-standard, cell-permeable progesterone receptor antagonist for research in reproductive biology and hormone-driven oncology (APExBIO). Its molecular specificity, high solubility in DMSO/ethanol, and validated performance in PR-positive models ensure reproducible results. Future directions include expanded use in combinatorial therapy screens and integration into next-generation hormone receptor pathway studies. Researchers are advised to consult both product documentation and recent peer-reviewed protocols for optimal assay design.