Molecular subtype has been suggested as a superior classification to determine treatment strategy for breast cancer patients. Neoadjuvant chemosensitivity and endocrine sensitivity rates are increasingly accepted as surrogates for efficacy, especially if substantial impact can be demonstrated. In this present study when using BluePrint and Mammaprint for defining molecular subtypes, 22 % (94/426) of breast cancer patients are classified in a different subgroup compared with conventional assessment. Treatment was at the discretion of the physician adhering to NCCN-approved or other established, peer-reviewed regimens and is mostly in line with conventional assessment. This reclassification of patients leads to an improved distribution of response rates in the different subgroups of patients: a lower pCR rate for BPLuminal patients compared with IHC/FISH-defined conventional luminal patients, with more responsive patients reassigned to the HER2 and Basal categories.
BluePrint/MammaPrint subtypes have previously been compared to quality-controlled, centrally assessed IHC/FISH subtypes in the first 621 patients of the MINDACT trial.4 This analysis showed that 58 % of IHC/FISH HER2+ patients were classified as BluePrint HER2 which is almost identical to the 57 % in the present analysis. This indicates that 42–43 % of conventional HER2+ patients are classified differently by BluePrint molecular subtyping. Conventionally classified TN patients using central IHC/FISH pathology were Basal by BluePrint in 98 % of cases, which is similar to the 96 % in this study. BluePrint almost always reconfirms the basal phenotype of TN patients. As for conventional luminal patients (central pathology determined HR+/HER2−) 96 % also were classified as BluePrint Luminal, 14 % lower in the current study (82 %). The latter higher discordance rate between conventionally classified luminal and BluePrint Luminal patients might be related to the variability in IHC ER and/or PR assessment at local institutions. Accurate test performance is crucial, yet there is evidence of wide variability in test performance and inaccurate results (falsely negative or falsely positive) of up to 20 %.5
The observed difference in clinically assessed subgroups of early stage breast cancer patients compared with molecular sub classification of patients has been reported also by others, for instance for subtyping with the intrinsic molecular signature6 and the PAM50 signature.7,8 And even though some of the discordance could potentially be ascribed to technical issues such as test performance and the fact that assessments from different tumour areas are being compared, the discordance also seems to indicate a ‘true’ difference in assigning patients by these 2 types of assessments. Molecular classification is designed such that it captures the true biologic profile regulated by ER/PR/HER and it measures these pathways by measuring a larger number of related genes. The BluePrint 80-gene classifier identifies “functional” molecular subtype based on intact molecular pathways associated with concordant mRNA and protein expression (1).
Clinical Impact
The two largest groups of reassigned patients with potential clinical implications are those conventional HER2+ patients, who are not classified as HER2 by BluePrint, and the conventional luminal (HR+/HER2−) patients who are reclassified by BluePrint to Basal. The BluePrint HER2 group of patients show a significantly higher pCR rate than that for patients classified as HER2+ by IHC/FISH, with less responsive patients reassigned to the BluePrint Luminal category. All of these reassigned patients come from the HR+ subset of conventional HER2+ patients.
Several studies have suggested that pathologic complete response was not particularly prognostic for ER+, HER2+ breast cancers, suggesting the possibility that a subset of HER2+, ER+ breast cancers are driven primarily by ER, and biologically behave more like HER2−, ER+ breast cancers. Identification of this subset of HER2+ breast cancers is essential to avoid overtreatment of patients with small HER2+, ER+ breast cancers, who may be optimally treated with endocrine therapy alone, or in combination with a HER2-directed agent, thereby avoiding the use of chemotherapy.9 In our study, 75 patients are IHC/FISH HER2+/HR+ of whom 36 (48 %) are BluePrint Luminal with a significantly lower pCR rate (3 %) to NCT/trastuzumab versus the 33 IHC/FISH HER2+/HR+ patients (44 %) who are BluePrint HER2 (pCR = 45 %). Therefore with BluePrint functional subtype IHC/FISH HER2+/HR+ patients are subdivided into BPLuminal with a poor response to NCT/trastuzumab and BPHER2 with a good response to NCT/trastuzumab.
An additional 17 (14 %) of 123 conventional HER2 patients were reclassified BPBasal. This finding is likely to become more important as biological subsets within the TN/Basal subtype are delineated.
Many of the conventional luminal patients were reclassified as Basal by BluePrint, enlarging the Basal category while the pCR rate was maintained. This group of conventional luminal patients are reported to have low expression levels of ER and PR and also has been identified by other methods of molecular classification 4,10. For this group of patients conventionally identified as endocrine responsive who are reclassified to the Basal subgroup, it makes sense to consider neoadjuvant chemotherapy.
We have adhered to the recently more stringent definition of pCR: ypT0/is ypN0 as suggested by Cortazar et al. 11. As suggested in this large pooled data analysis, our results can be used to compare response rates as a measure of outcome on a patient level. The overall pCR rate in our study (25 %) is comparable to the overall pCR rate in the Cortazar pooled analyses (22 %). IHC/FISH HR+/HER2− had a pCR rate of 8 % (grade 1 and 2) and 16 % (grade 3) in the pooled analyses, which is similar to the IHC/FISH HR+/HER2− pCR rate of 10 % in our study. Response rates for IHC/FISH HER2+/HR+ (31 vs. 27 %), HER+/HR− (50 vs. 56 %), and TN (34 vs. 37 %) also were comparable. In the pooled analyses of Cortazar et al. 11. The correlation between pCR rate and long-term outcome was strongest for HER2+/HR− patients and TN patients. A limitation of our study is that long-term outcome data are not yet available.
In a retrospective pooled analysis of MammaPrint and BluePrint in patients enrolled in four neoadjuvant chemotherapy trials pCR rate correlated with DMFS in BluePrint HER2 and Basal patients and not in BluePrint Luminal patients.3 The current study confirms the unique identification of a group of patients classified as Luminal A using Molecular Subtyping with MammaPrint and BluePrint who have an extremely low pCR rate (2 %) and who have previously been shown to have excellent survival.
These findings confirm the more accurate identification of molecular subgroups for treatment decision by the 80-gene BluePrint functional subtype classifier, which therefore may serve as a better guide for neoadjuvant treatment than standard, local IHC/FISH assay. Approximately one in five conventional “luminal” patients are reclassified as BPBasal and approximately half of conventional HER2+ HR+ patients are reclassified as BPLuminal.