Purpose: The therascreenPIK3CAmutation assay and the alpha-specific PI3K inhibitor alpelisib are FDA-approved for identifying and treating patients with advancedPIK3CA-mutated (PIK3CAmut) breast cancer (BC). However, it is currently unknown to what extend this assay detects mostPIK3CAmutations in BC. This information is critical as patients and clinicians are using this and other genomic assays to indicate alpelisib.Methods: Data from 6338 patients with BC was explored across 10 publicly available studies. The primary objective was to evaluate the proportion and distribution ofPIK3CAmutations in BC. Secondary objectives were (1) to evaluate in silico the spectrum ofPIK3CAmutations in BC that would be captured by the therascreen panel; (2) to evaluate the proportion and distribution ofPIK3CAmutations in hormone receptor-positive/HER2-negative (HR+/HER2−), HER2+, and triple-negative BC (TNBC); and (3) to explore the identification ofPIK3CAmutations in a cohort of 48 HR+/HER2− advanced BC patients by the Guardant B360 circulating tumor DNA (ctDNA) assay.Results: Patients withPIK3CAmut tumors represented 35.7% (2261/6338). FivePIK3CAmutations comprised 73% of allPIK3CAmutations: H1047R (35%), E545K (17%), E542K (11%), N345K (6%), and H1047L (4%). Therascreen gene list would capture 72% of allPIK3CAmutations and 80% of patients with a knownPIK3CAmut BC. Among patients with doublePIK3CAmut tumors (12% of allPIK3CAmut), the therascreen panel would capture 78% as harboring 1 singlePIK3CAmutation, 17% asPIK3CAmut undetected, and 5% asPIK3CAdouble-mut.PIK3CAmutation rates were lower in TNBC (16%) compared to HR+/HER2 (42%) and HER2+ (31%) BC; however, the distribution of the 4 mainPIK3CAmutations across subtypes was similar. Finally, 28% ofPIK3CAmutations identified in ctDNA in 48 patients with advanced HR+/HER2− BC were not part of the therascreen panel.Conclusion: PIK3CAmutations in BC are heterogenous and ~ 20% of patients with a knownPIK3CAmutation, and 95% with a known doublePIK3CAmut tumor, would not be captured by the therascreen panel. Finally, the clinical utility ofPIK3CAmutations not present in the therascreen companion diagnostic assay or identified by other sequencing-based assays needs further investigation.

Identification of HER2-positive breast cancers with high anti-HER2 sensitivity could help de-escalate chemotherapy. Here, we tested a clinically applicable RNA-based assay that combines ERBB2 and the HER2-enriched (HER2-E) intrinsic subtype in HER2-positive disease treated with dual HER2-blockade without chemotherapy. A research-based PAM50 assay was applied in 422 HER2-positive tumors from five II–III clinical trials (SOLTI-PAMELA, TBCRC023, TBCRC006, PER-ELISA, EGF104090). In SOLTI-PAMELA, TBCRC023, TBCRC006, and PER-ELISA, all patients had early disease and were treated with neoadjuvant lapatinib or pertuzumab plus trastuzumab for 12–24 weeks. Primary outcome was pathological complete response (pCR). In EGF104900, 296 women with advanced disease were randomized to receive either lapatinib alone or lapatinib plus trastuzumab. Progression-free survival (PFS), overall response rate (ORR), and overall survival (OS) were evaluated. A total of 305 patients with early and 117 patients with advanced HER2-positive disease were analyzed. In early disease, HER2-E represented 83.8% and 44.7% of ERBB2-high and ERBB2-low tumors, respectively. Following lapatinib and trastuzumab, the HER2-E and ERBB2 (HER2-E/ERBB2)-high group showed a higher pCR rate compared to the rest (44.5%, 95% confidence interval [CI] = 35.4% to 53.9% vs 11.6%, 95% CI = 6.9% to 18.0%; adjusted odds ratio [OR] = 6.05, 95% CI = 3.10 to 11.80, P < .001). Similar findings were observed with neoadjuvant trastuzumab and pertuzumab (pCR rate of 66.7% in HER2-E/ERBB2-high, 95% CI = 22.3% to 95.7% vs 14.7% in others, 95% CI = 4.9% to 31.1%; adjusted OR = 11.60, 95% CI = 1.66 to 81.10, P = .01). In the advanced setting, the HER2-E/ERBB2-high group was independently associated with longer PFS (hazard ratio [HR] = 0.52, 95% CI = 0.35 to 0.79, P < .001); higher ORR (16.3%, 95% CI = 8.9% to 26.2% vs 3.7%, 95% CI = 0.8% to 10.3%, P = .02); and longer OS (HR = 0.66, 95% CI = 0.44 to 0.97, P = .01). Combining HER2-E subtype and ERBB2 mRNA into a single assay identifies tumors with high responsiveness to HER2-targeted therapy. This biomarker could help de-escalate chemotherapy in approximately 40% of patients with HER2-positive breast cancer.

PURPOSE: The prognostic and predictive value of intrinsic subtypes in hormone receptor–positive, human epidermal growth factor receptor 2–negative advanced breast cancer treated with endocrine therapy and ribociclib (RIB) is currently unknown. We evaluated the association of intrinsic subtypes with progression-free survival (PFS) in the MONALEESA trials. METHODS: A retrospective and exploratory PAM50-based analysis of tumor samples from the phase III MONALEESA-2, MONALEESA-3, and MONALEESA-7 trials was undertaken. The prognostic relationship of PAM50-based subtypes with PFS and risk of disease progression by subtype and treatment were evaluated using a multivariable Cox proportional hazards model, adjusting for age, prior chemotherapy, performance status, visceral disease, bone-only metastases, histological grade, number of metastatic sites, prior endocrine therapy, and de novo metastatic disease. RESULTS: Overall, 1,153 tumors from the RIB (n = 667) and placebo (n = 486) cohorts were robustly profiled. Subtype distribution was luminal A (LumA), 46.8%; luminal B (LumB), 24.0%; normal-like, 14.1%; HER2-enriched (HER2E), 12.6%; and basal-like, 2.4% and was generally consistent across treatment arms and trials. The associations between subtypes and PFS were statistically significant in both arms ( P < .001). The risks of disease progression for LumB, HER2E, and basal-like subtypes were 1.44, 2.32, and 3.87 times higher compared with those for LumA, respectively. All subtypes except basal-like demonstrated significant PFS benefit with RIB. HER2E (hazard ratio [HR], 0.40; P < .0001), LumB (HR, 0.52; P < .0001), LumA (HR, 0.63; P = .0007), and normal-like (HR, 0.47; P = .0005) subtypes derived benefit from RIB. Patients with basal-like subtype (n = 28) did not derive benefit from RIB (HR, 1.14; P = .78). CONCLUSION: In this retrospective exploratory analysis of hormone receptor–positive and human epidermal growth factor receptor 2–negative advanced breast cancer, each intrinsic subtype exhibited a consistent PFS benefit with RIB, except for basal-like.

Gene expression profiling has had a considerable impact on our understanding ofbreastcancer biology. Duringthelast decade, 4 intrinsic molecular subtypes of breast cancer (Luminal A, Luminal B, HER2-enriched [HER2-E] and Basal-like) have been identified and intensively studied. In this article, we review and discuss the clinical implications of the 2 non-luminal subtypes (i.e. HER2-E and Basal-like) identified within hormone receptor (HR)-positive disease. After reviewing 32 studies for a total of 13,091 samples, ∼8% and ∼ 15% of early and metastatic HR+/HER2-negative breast cancer, respectively, were found to be non-luminal. Clinically, HR+/HER2-negative/non-luminal subtypes have been associated with estrogen independence, chemo-sensitivity, resistance to CDK4/6 inhibition and poor outcome. Interestingly, EGFR/HER2 tyrosine kinase inhibition might be of value in the HR+/HER2-negative/HER2-E subtype. Finally, the HER2-E subtype within HR+/HER2 + disease represents ∼ 30% and has been associated with anti-HER2 sensitivity, chemo-sensitivity and resistance to CDK4/6 inhibition. In the upcoming years, retrospective and prospective clinical trials evaluating both biomarkers should lead to improvements in patient outcomes.