Breast cancer is the most commonly diagnosed cancer and the second leading cause of cancer related death in women. Around 5–10% of cases are metastatic at diagnosis, and close to 30% of patients with early stage disease will relapse with metastatic disease. Anti-estrogen therapy is an important treatment modality for hormone receptor-positive (HR+) metastatic breast cancer (mBCa) as mono- or combination (e.g. with CDK4/6 inhibitors) first-line (1L) therapy. Unfortunately, despite the high rate of clinical benefit in 1L therapy, disease progression still generally occurs and there remains a need for an efficacious 2L therapy. Although anti-estrogens continue to play a role in 2L treatment there is a critical need for targeting additional mechanisms in combination with anti-estrogens. Progesterone is a major mitogen in the adult human mammary epithelium and, in addition to ER, is a key driver of breast cancer cell proliferation. Thus, blocking both ER and PR would likely be an effective approach for inhibition of all hormone driven breast cancer cell proliferation. Onapristone is a unique PR full antagonist that is efficacious as an antitumor agent in multiple preclinical breast cancer models and exhibits additive/synergistic effects with antiestrogens. Here, we examine the effects of onapristone in in-vitro model systems in combination with CDK4/6 inhibitors. Methods: T47D cells were treated with various concentrations of onapristone or palbociclib in media containing 10%FBS.10 days after treatment cell viability was determined using Cell Titer Glo. Cells were also treated with increasing concentrations of palbociclib in the absence or presence of onapristone and analysed for cell proliferation after 10 days and for gene expression after 3 days of treatment.The effects of onapristone were also tested in soft agar anchorage-independent growth assays as well as 3D tumorsphere assays using ER+/PR+ MCF7 and BT474 cells -/+ onapristone for 21-28 days. Formed colonies or spheres were analyzed morphologically using cell stain and by quantifying the total number and size of colonies or spheres formed. Results:Onapristone inhibited T47D cell proliferation in a concentration-dependent manner. Additionally, it sensitized the cells to inhibition by CDK4/6 inhibitors in the preclinical in-vitro models. Onapristone also had a marked effect on downstream target genes in the cell-based models providing a mechanistic basis for the anti-proliferative effects. Onapristone also blocked progesterone-induced breast cancer cell survival in either soft agar or 3D tumorsphere assays with effects comparable to that of anti-estrogen (fulvestrant). Conclusions: Onapristone inhibits T47D proliferation and key target genes involved in cell proliferation. Additionally, onapristone enhances the anti-proliferative effects of palbociclib in-vitro. Previous studies have provided a clear rationale for combining onapristone and anti-estrogens in the clinic for the treatment of breast cancer. Our data extend these studies to provide additional rationale for the combination of onapristone with CDK4/6 inhibitors such as palbociclib as an effective new approach for the treatment of breast cancer. Citation Format: Lala D, Haque T, Feinman H, Wu J, Wang Y, Dwyer A, Truong T, Lange C. The pure progesterone receptor (PR) antagonist onapristone enhances the anti-proliferative effects of CDK4/6 inhibitors in preclinical in-vitro breast cancer models [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P6-20-13.

Docetaxel is the current first line therapy for metastatic castration-resistant prostate cancer (mCRPC), but there is no standard number of docetaxel cycles given to patients. In their post hoc analysis of the Mainsail study, de Morr ee et al. show that the number of docetaxel cycles administered to a patient is a significant factor contributing to overall survival. These findings warrant further investigation into the standardization of the number of docetaxel cycles administered. KEYWORDS Docetaxel; MRPC; prostate cancerProstate cancer accounts for 21% of new cancer diagnoses in the U. S. and is the second leading cause of cancer-related deaths in men.