Of the nearly 1.4 million new cases of breast cancer diagnosed each year, a large proportion is characterized as hormone receptor negative, lacking estrogen receptors (ER) and/or progesterone receptors (PR). Patients with receptor-negative tumors do not respond to current steroid hormone based therapies and generally have significantly higher risk of recurrence and mortality compared to patients with tumors that are ER- and/or PR-positive. Previous in vitro studies had shown that the progesterone metabolites, 5-dihydroprogesterone (5P) and 3-dihydroprogesterone (3HP), respectively, exhibit pro-cancer and anti-cancer effects on receptor-negative human breast cell lines. Here in vivo studies were conducted to investigate the ability of 5P and 3HP to control initiation, growth and regression of ER/PR-negative human breast cell tumors.

ER/PR-negative human breast cells (MDA-MB-231) were implanted into mammary fat pads of immunosuppressed mice and the effects of 5P and 3HP treatments on tumor initiation, growth, suppression/regression and histopathology were assessed in five separate experiments. Specific radioimmunoassays and gas chromatography-mass spectrometry were used to measure 5P, 3HP and progesterone in mouse serum and tumors.

Onset and growth of ER/PR-negative human breast cell tumors were significantly stimulated by 5P and inhibited by 3HP. When both hormones were applied simultaneously, the stimulatory effects of 5P were abrogated by the inhibitory effects of 3HP and vice versa. Treatment with 3HP subsequent to 5P-induced tumor initiation resulted in suppression of further tumorigenesis and regression of existing tumors. The levels of 5P in tumors, regardless of treatment, were about 10-fold higher than the levels of 3HP and the 5P:3HP ratios were about 5-fold higher than in serum, indicating significant changes in endogenous synthesis of these hormones in tumorous breast tissues.

The studies showed that estrogen/progesterone insensitive breast tumors are sensitive to, and controlled by, the progesterone metabolites 5P and 3HP. Tumorigenesis of ER/PR-negative breast cells is significantly enhanced by 5P and suppressed by 3HP, the outcome depending upon the relative concentrations of these two hormones in the microenvironment in the breast regions. The findings show that the production of 5P greatly exceeds that of 3HP in ER/PR-negative tumors and that treatment with 3HP can effectively block tumorigenesis and regress existing tumors. The results provide the first hormonal theory to explain tumorigenesis of ER/PR-negative breast tissues and support the hypothesis that a high 3HP-to-5P concentration ratio in the microenvironment may foster normalcy in non-cancerous breast regions. The findings suggest new diagnostics based on the relative levels of these hormones and new approaches to prevention and treatment of breast cancers based on regulating the levels and action mechanisms of anti- and pro-cancer progesterone metabolites.