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Onapristone


Drug Description

Onapristone is a progesterone receptor antagonist that has demonstrated anti-tumor activity in preclinical and clinical studies of hormone-dependent tumors. Its mechanism of action is thought to be a direct result of binding to the progesterone receptor and preventing the progesterone receptor from binding to DNA, thereby substantially reducing or eliminating progesterone-receptor-induced transcription resulting in death or differentiation of the malignant cells.

Development History

Onapristone was originally developed by Schering AG which evaluated its effects on hormone-dependent mammary tumors in a variety of animal models. Early stage clinical studies conducted in Germany and the United Kingdom in post-menopausal women with breast cancer showed encouraging anti-tumor activity but also transient elevated liver function test results. Invivis Pharmaceuticals is conducting additional research to potentially identify patients who would benefit most from onapristone and has entered into a licensing agreement with Arno Therapeutics in 2012 for further development.

Clinical Findings

Onapristone has been evaluated in clinical studies as both first- and second-line treatment for primary and metastatic breast cancer. In clinical studies, onapristone has demonstrated a 56% objective response rate as a first-line endocrine treatment of breast cancer and a 49% clinical benefit rate in patients with breast cancer with tamoxifen-resistant disease. Future studies of onapristone may be conducted in combination with other endocrine agents to improve previous results observed in patients with breast cancer.

Current Development Plan

Arno plans to develop this compound for the treatment of a variety of tumor types, including breast and endometrial cancer. The company is preparing to conduct preclinical toxicology studies and has begun work on a predictive diagnostic biomarker to determine which patients would benefit most from onapristone therapy. Following completion of preclinical studies and development of a companion diagnostic, we intend to file an IND with the FDA to begin a dose-escalation Phase I study of onapristone in patients with endometrial and breast cancer.

Select Bibliography

  • J.F.R. Robertson et al., Onapristone, a Progesterone Receptor Antagonist, as First-Line Therapy in Primary Breast Cancer, European Journal of Cancer, Vol. 35, No. 2, pp. 214-218, 1999.
  • J.G.M. Klijn et al., Progesterone antagonists and progesterone receptor modulators in the treatment of breast cancer, Steroids, v. 65, pp. 825-830, 2000.
  • W. Jonatt et al., The clinical efficacy of progesterone antagonists in breast cancer, in Endocrine Therapy of Breast Cancer, pp. 117-124.


Arno Therapeutics, Inc.
200 Route 31 North
Suite 104
Flemington, NJ 08822

(862) 703-7170 Main
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info@arnothera.com