Association of PIK3CA mutation with response (ExRx) to cetuximab (C) in metastatic (met) triple-negative breast cancer (TNBC)

151 Background: 10 to 17% of TNBCs harbor PIK3CA mutations (TCGA Nature 490:61, 2012; Khambata-Ford S. ASCO 2010, abst 1056). Here we report clinical history and molecular findings for a TNBC patient with loss of mutant PIK3CA in a C-refractory metastasis that was present in her primary BC, who has had an ExRx to C. Methods: Following IRB-approved informed consent, targeted NGS was performed on the pt’s FFPE primary TNBC and on a C-refractory recurrent lung metastasis at a CLIA-certified laboratory (Foundation Medicine) to characterize all classes of genomic alterations across 287 cancer-related genes. RPPA was performed at a CLIA-certified laboratory (Theranostics Health) where immunostaining with 24 antibodies was directed against HER1/2/3 pathway proteins and AR. Results: 37 yo woman presented in 2006 with grade 3 primary TNBC, infraclavicular LNs and lung metastases at 33 weeks gestation. There was no response to preop doxorubicin/cyclophosphamide and following delivery of a healthy baby, she was treated with irinotecan, carboplatin, and C (#NCT00248287) and had near complete response (CR) in her lungs and pathologic CR in breast. In 2008, after 20 mos on C, chest CT showed a new lung met which was resected and she remains disease-free on C alone. NGS: Primary BC: PIK3CA C420R, TP53 mutations, MCL1 amplification (amp), and RAD51D germline mutation; C-refractory lung met: TP53, MLL2 mutations, MCL1, MYC, KDM5A, CCNE1 amp and RAD51D germline mutation (loss of PIK3CAmutation confirmed). RPPA: Primary BC: p-AR S650 (3+); p-ERK (2+), and HER1, p-HER1, p-HER3, p-AKT, p-S6, p-4EBP1 (1+); C-refractory lung met: loss of p-ERK; p-HER1, p-4EBP1 (2+), and p-AR, p-AKT, p-mTOR (1+). PTEN: Primary BC: 60% cells positive by IHC MAb 6H2.1 (Cascade Biosciences). Conclusions: The pt’s ExRx to C was dependent on presence of PIK3CA mutation which was lost in the C-refractory lung met. Loss of RAD51D function may also have contributed (Liping L. Ca Res 68:9141, 2008). High p-AR expression did not preclude response to C. Activating PIK3CA mutations induce an EGFR/ERK paracrine signaling axis in TNBCs (Young CD. Mol Cell Proteomics 2015). A prospective trial of EGFR inhibition in PIK3CA-mutant TNBC is warranted.