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Abstract
Introduction: 70% of patients with advanced breast cancer suffer from painful and incurable skeletal metastases. Metastasis is a complex process orchestrated through changes in both the primary tumour and secondary disease site. Hypoxia and activation of the phosphoinositide 3-kinase (PI3K) pathway contribute to an aggressive phenotype in primary breast tumours. Furthermore, both contribute to the bone niche through impact on cellular phenotype and differentiation status by promoting extracellular matrix and bone remodelling and an immunosuppressive milieu. PI3K is an upstream regulator of hypoxia-inducible factor-1 (a key regulator of hypoxia-mediated gene expression), and a recognised therapeutic target for cancer.
Methods: Primary murine bone marrow cells treated with M-CSF and RANKL for a week before co-culture with 4T1 breast cancer cells were used as in vitro model of metastasis to assess the effect of hypoxia and PI3K inhibitor GDC-0941 on osteoclast differentiation. Three in vivomodels were used to study breast tumour progression, breast-to-bone metastasis and PI3K inhibition as an anti-metastatic treatment: orthotopic (mammary fat pad and intra-femoral to model primary and established metastatic disease) and systemic (intracaudal [IC] to model initial bone colonisation). The 4T1 model used was tagged with luciferase, allowing use of bioluminescent imaging to track metastases formation. Flow cytometry was used to detect changes in immune cell populations of the bone niche.
Results:Osteoclast differentiation and function was inhibited by GDC-0941. 4T1 cells increased osteoclastogenesis and to a greater extent when 4T1 were primed in hypoxia. GDC-0941 reduced growth of 4T1 tumours in the mammary fat pad and femur. GDC-0941 administration prior to IC inoculation reduced 4T1 bone colonisation and growth, an effect that was further enhanced when GDC-0941 treatment was maintained post-inoculation. GDC-0941 reduced the myeloid cell population in the bone marrow of tumour-bearing mice, with co-incidental recruitment and expansion of cytotoxic CD8 T cells.
Conclusion:PI3K inhibition has potential as an anti-metastatic treatment in breast cancer. Modifying the bone niche is likely a contributing factor, but the mechanisms of action need to be further validated.
