The role of PI3K and ERBB family gene mutations and other abnormalities in resistance to HER2-targeted therapies in HER2-positive breast cancer
Trastuzumab resistance remains a challenge in the treatment of the 15-20% of breast cancers which are HER2-positive (HER2+ BC). Known mechanisms, including the PI3K pathway, account for some but not all cases of trastuzumab resistance.
The novel, pan-class I PI3K inhibitor copanlisib was tested alone and in combination with HER2-targeted therapies in a panel of HER2+BC cell lines, including models of acquired trastuzumab and/or lapatinib resistance. Reverse phase protein array was used to determine the effect of copanlisib on expression and activation of PI3K pathway components. 227 HER2+ BC tumours were assayed for ERBB family mutations using a custom-designed Sequenom panel. Two ERBB4 mutations, S303F and V721I, were created by site-directed mutagenesis and stably expressed in HER2+ BC cell lines to determine their effect on growth, invasiveness, signalling and response to HER2-targeted therapies.
Copanlisib is highly effective in HER2+ BC cell lines, including those with resistance to HER2-targeted therapies. The combination of HER2-targeted therapies and copanlisib is synergistic, and restores sensitivity to trastuzumab and lapatinib in cells with acquired resistance. Novel, somatic ERBB family mutations are present in 7.05% of HER2+BCs. There is a nonsignificant trend towards a survival effect after trastuzumab. The ERBB4 S303F mutation has increased HER4 kinase activity and is lethal to 2/3 cell lines. ERBB4 V721I increases HER4 overexpression, growth rate, and 3D colony formation. Both ERBB4 mutations studied alter sensitivity to copanlisib and the pan-HER inhibitor afatinib. The combination of copanlisib with HER2-targeted therapies is potentially an improved treatment for trastuzumab resistant HER2-positive breast cancer, which, due to our work presented herein, is now being evaluated in a Phase Ib/II clinical trial (Panther, ICORG 15-02). ERBB family mutations are present in 7.05% of HER2-positive breast cancers. ERBB4 S303F and V721I affect the biology of HER2+ BC and are potential predictive biomarker for copanlisib and afatinib.