Background: Previous studies suggest that within the triple positive HR+HER2+ subtype, patients classified as BluePrint (BP) Luminal subtype are more responsive to pertuzumab and trastuzumab (P/H) as opposed to trastuzumab (H) alone. In the I-SPY2 TRIAL, HER2-targeted treatment arms include H, P/H, neratinib (N), and T-DM1/Pertuzumab (T-DM1/P); and patients were classified by BP molecular subtyping in addition to conventional receptors. We evaluated BP subtype as a predictor of response in HR+HER2+ patients and assessed pathway differences between BP molecular subtypes.
Methods: 125 HR+HER2+ patients (N: 42; P/H: 29, T-DM1/P: 35; H: 19) with pre-treatment Agilent microarrays and BP subtype assignments were considered. We assess association between BP subtypes and pCR using Fisher's exact test. To identify genes associated with BP Luminal vs. BP HER2 subtype, we (1) apply a Wilcoxon rank sum test and (2) fit a logistic model, with the Benjamini-Hochberg (BH) multiple testing correction (BH p<0.05 from both tests). We then perform pathway enrichment analysis using DAVID. Our study is exploratory and does not adjust for multiplicities of other biomarkers in the trial outside this study.
Results: Of the 125 HR+HER2+ patients, 71 were BP HER2-type and 50 were BP Luminal-type. The distribution of pCR rates in BP Luminal/ HER2 subtypes are as follows:
In a whole transcriptome analysis, 1725 genes were differentially expressed. By DAVID enrichment analysis, the most significantly enriched pathways were related to immune function, with the BP HER2 subgroup showing higher expression.
Conclusion: Our analysis suggests that HR+HER2+ BP Luminal subtype is associated with lower response rates to HER2-targeted agents, including P/H, and may need an alternative strategy. BP HER2 subtype appears associated with higher expression of immune-related genes, relative to BP Luminal; and suggests that immune signaling may contribute to HER2-targeted therapy sensitivity.