Suppression of breast cancer by small molecules that block the prolactin receptor

Background: Prolactin (PRL) is a protein hormone whose main production site is the pituitary gland. In humans, however, PRL is produced and secreted not only by the pituitary but also by other cells such as adipocytes, lymphocytes and mammary cells. The PRL receptor (PRLR) belongs to the superfamily of non-tyrosine kinase cytokine receptors. Multiple studies have shown that both circulating (endocrine) and locally produced (paracrine or autocrine) PRL increase breast cancer growth and metastasis and confer resistance to chemotherapy.

Objectives: To identify and then characterize small molecules that block the tumorigenic actions of PRL in breast cancer under in vitro and in vivo conditions.

Methods: Three cell-based assays were employed in high throughput screening of 51,000 small molecules. Of these, we have identified two small molecule inhibitors (SMIs), which we named SMI-1 and SMI-6. Various cell types and athymic nude mice with breast cancer xenografts were then used to verify the anti-tumorigenic actions of the SMI’s.

Results: SMI-1 and SMI-6 bound to the PRLR at 1-3 μM affinity. In silico binding simulation identified the sites of their binding to the extracellular domain of the PRLR. Both compounds abrogated PRL-induced breast cancer cell growth and invasion and suppressed malignant lymphocyte proliferation. SMI-6 effectively reduced the viability of several breast cancer cell types while showing minor activity against non-malignant cells. SMI-6 displayed high selectivity when tested against multiple kinases, with no apparent in vitro or in vivo toxicity. In athymic nude mice, SMI-6 rapidly and dramatically suppressed growth of PRL-expressing breast cancer xenografts.

Conclusions: SMI-6 binds to the PRLR and interferes with receptor activation by PRL. At 1μM, SMI-6 antagonizes PRL-induced JAK2/Stat 5 activation and prevents PRL-induced increases in cell proliferation and invasion in many cell lines. SMI-6 shows high selectivity and no cytotoxicity. SMI-6 robustly suppresses tumor growth, much beyond the promotional effects of PRL. These data suggest that SMI-6 has both PRL-dependent and PRL-independent anti-tumorigenic activities. Advantages of SMI-6 are oral deliverability, potential brain penetration to treat metastases, lack of immunogenicity, ease of structural optimization, and relatively low production costs. This report represents a pre-clinical phase of developing a novel anti-cancer agent with the potential to become effective therapeutics in selected patients.

Nira Ben-Jonathan is Emeritus Professor of Cancer Biology, University of Cincinnati, Ohio, USA. She published 180 manuscripts, contributed 12 chapters to textbooks, and wrote a book on Dopamine. She mentored 65 grad students, medical fellows and research scientists. She was awarded the NIH Research Career Development Award, is an Elected Fellow of the AAAS and the Royal Society of Medicine and was Chairman of the Gordon Research Conference on Prolactin. She received the Rieveschl Award for Outstanding Scientific Research, and the Merker Lectureship in Translational Endocrinology. She served as a member on many committees of the NIH, DOD, and the Komen foundation, and chaired five NIH Study Sections.