Endocrine therapy is the treatment of choice in hormone receptor-positive breast cancer. However, the effectiveness of anti-hormone drugs, such as tamoxifen, is limited because of the development of resistance, ultimately leading to disease progression and patient mortality. Using in vitro cell models of anti-hormone resistance, we have previously demonstrated that altered growth factor signalling contributes to an endocrine insensitive phenotype. Significantly, our recent studies have revealed that the acquisition of endocrine resistance in breast cancer is accompanied by a greatly enhanced migratory and invasive phenotype. Furthermore, therapeutic intervention using anti-growth factor monotherapies, despite an initial growth suppressive phase, again results in the development of a resistant state and a further augmentation of their invasive phenotype.

Using the dual specific Src/Abl kinase inhibitor, AZD0530, we have highlighted a central role for Src kinase in promoting the invasive phenotype that accompanies both anti-hormone and anti-growth factor resistance. Importantly, the use of Src inhibitors in combination with anti-growth factor therapies appears to be additive, producing a marked inhibitory effect on cell growth, migration and invasion and ultimately prevents the emergence of a resistant phenotype. These observations suggest that the inhibition of Src activity may present a novel therapeutic intervention strategy, particularly when used as an adjuvant in endocrine-resistant breast disease, with the potential to delay or prevent the acquisition of subsequent resistance to anti-growth factor therapies.