Abstract P3-07-03: PIK3CA mutations predict resistance to trastuzumab/pertuzumab and nab-paclitaxel in primary HER2-positive breast cancer – Massive parallel sequencing analysis of 293 pretherapeutic core biopsies of the GeparSepto study

Abstract

Abstract Background: Phosphatidylinositol 3-kinase mutations (PIK3CA) are common in breast cancer (BC). Mutations are predominantly found in hot-spots located in the helical and kinase domains (exons 9 and 20). We recently demonstrated that PIK3CA mutations predict lower pathological complete response (pCR) to double blockade with trastuzumab/lapatinib in HER2+ve primary BC. Methods: We evaluated PIK3CA mutations in 293/403 HER2+ve tumors of participants of the neoadjuvant GeparSepto (G7) study (Untch et al. SABCS 2014). The G7 study investigated the effect of exchanging paclitaxel for nab-paclitaxel prior to EC. All patients received trastuzumab and pertuzumab. The G7 study showed a significantly higher pCR rate in patients receiving nab-paclitaxel. HER2, hormone receptors (HR), Ki67 and tumor infiltrating lymphocytes (TILs) were centrally assessed prior to randomization. PIK3CA mutations in exons 9 and 20 were evaluated in formalin-fixed, paraffin embedded core biopsies taken before therapy using deep targeted massive parallel sequencing with a minimum coverage of 500 and a mean coverage of 6520 and 6346 per amplicon, (exon9 and exon 20). Only non-synonymous mutations in the coding region that were called at variant allele frequency ≥10% were taken into consideration. Only cases with a tumor cell content of ≥20% were included. Results: In the G7 study, 396 patients with HER2+ve BC have been randomized from 06/2012 to 01/2014 and started treatment. From these 293 could be sequenced. Median age in the analyzed cohort was 49 years (range 22-75); most tumors were cT1-2 (89.9%); cN0 (54.4%); ductal invasive (88.7%), grade 3 (53.9%), HR+ve (69.6%), Ki67>20% (69.3%), LPBC-negative (83.2%). Overall, 22.2% of the tumors were found to have a PIK3CA mutation, 20.1% in HR+ve and 27.0% in HR-ve. Overall, the pCR rate was significantly lower in the PIK3CA mutant tumors compared to the wild type (wt) group (47.7% vs. 66.7%; p=0.009). This effect was seen both in the HR+ve (43.9% vs. 61.3%; p=0.052) and the HR-ve population (54.2% vs. 80.0%; p=0.029). There was also a significant difference in pCR according to PIK3CA mutation status dependant on the taxane. In the nab-paclitaxel group, pCR rates were significantly lower in patients with PIK3CA mutations compared to those without PIK3CA mutations (38.7% vs. 72.0%; p=0.001), whereas in the paclitaxel group, there was no significant difference between patients with and without a PIK3CA mutation (55.9% vs. 60.9%; p=0.690). The respective interaction could be demonstrated in univariate (p=0.039) as well as multivariate regression analysis (p=0.010) after adjusting for known baseline factors. Conclusion: Patients with PIK3CA mutant HER2+ve BC have a significantly lower pCR rate compared to patients with wt tumors. In contrast to the results with double anti-HER2 blockade consisting of trastuzumab/lapatinib, the effect was evident irrespective of the HR status. In addition, PIK3CA mutation status was significantly associated with higher pCR following nab-paclitaxel. The project has partly been funded within the EU-FP7 project RESPONSIFY No 278659 and the German Cancer Consortium (DKTK). Citation Format: Loibl S, Budczies J, Weichert W, Furlanetto J, Stenzinger A, Pfarr N, von Minckwitz G, Jackisch C, Schneeweiss A, Fasching P, Schmatloch S, Aktas B, Nekljudova V, Weber K, Untch M, Denkert C. PIK3CA mutations predict resistance to trastuzumab/pertuzumab and nab-paclitaxel in primary HER2-positive breast cancer – Massive parallel sequencing analysis of 293 pretherapeutic core biopsies of the GeparSepto study. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P3-07-03.