Abstract P3-08-04: Transient state change, but not permanent subtype change, after HER2-targeted therapy for HER2-positive breast cancer

Background: In CALGB 40601 (Alliance, NCT00770809), a neoadjuvant phase III trial of paclitaxel and trastuzumab with or without lapatinib for 12 weeks for patients with HER2-positive breast cancer, 33% of pretreatment tumors were Luminal A subtype, however, 69% of post-treatment samples with residual disease were Luminal A subtype. In addition, 71% of Luminal B (12/17) and 67% of HER2-Enriched (6/9) tumors changed into Luminal A, while 80% of Luminal A (20/24) remained Luminal A (Carey et al. J Clin Oncol. 2016). It is not known whether this shift to Luminal A was transient or permanent. Methods: We selected matched pairs of pre- and post-treatment 40601 samples with tumor purity >10% based upon DNA analyses to ensure all samples contained tumor. PAM50 intrinsic subtyping was applied to the 40601 samples gene expression data using a two-step normalization process based on The Genome Cancer Atlas, and PAM50 training set. In addition, a HER2-enriched expression subtype patient-derived xenograft (PDX) tumor called WHIM35, was studied and was either untreated (n=10), or treated with lapatinib at 220 mg/kg for 1 week (wk) (n=5), for 2 wks (n=8), or for 3 wks (n=4). We also treated WHIM35 tumors with lapatinib for 2 wks (on) and then removed laptinib for 1 wk (off) (n=6), or for 2 wks on and 2-4 wk off (n=6), and finally for 3 wks on, and 1 wk off (n=3). PAM50 intrinsic subtyping was applied to the PDX gene expression data and subtype assessed as well as a genomic-based proliferation score. ANOVA p-values were calculated by comparing median values across all gene signature or correlation scores. Results: We found 10 pairs of 40601 samples that kept their tumor purity values, however, their subtype changed to Luminal A after treatment (i.e., in the residual disease), and in these cases no minor tumor subclone became a dominant clone in the post treatment sample. Pretreatment subtypes were 6 Luminal B, 3 Luminal A, and 1 HER2-enriched. The tumor purity values did not change after the treatments, but correlation to Luminal A was significantly higher (p=0.01), while correlation to HER2-enriched (p=0.004) and proliferation signature scores (p=0.003) were significantly lower in the post-treatment samples. Among the WHIM35 PDX tumors, one sample changed its subtype from HER2-enriched to Luminal A after the lapatinib treatment and the rest remained HER2-enriched, suggesting environmental differences between patient samples and the PDX model. However, correlation to Luminal A was significantly higher in all lapatinib treated WHIM35 samples (p=8.3e-12), and notably went back to the initial low levels just one week after removing lapatinib. Likewise, correlation to HER2-enriched (p=1.2e-10) and proliferation signature scores (p=6.2e-12) also got lower while treated with lapatinib, but went back to the initial levels after cessation of treatment. Conclusions: Our findings suggest that the apparent subtype change during HER2-targeting therapy is not permanent, but is more likely a transient state change from a HER2-enriched subtype into a more Luminal A-like state. When we plan additional treatment strategies using residual disease phenotypes, it may not be clear what is the true subtype of the sample due to this inherent plasticity. Citation Format: Tanioka M, Parker JS, Henry LN, Tolaney S, Dang C, Krop IE, Harris L, Polley M, Berry DA, Winer EP, Carey LA, Perou CM. Transient state change, but not permanent subtype change, after HER2-targeted therapy for HER2-positive breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-08-04.