A multivariable prognostic score to guide systemic therapy in early-stage HER2-positive breast cancer: a retrospective study with an external evaluation

Background: In early-stage HER2-positive breast cancer, escalation or de-escalation of systemic therapy is a controversial topic. As an aid to treatment decisions, we aimed to develop a prognostic assay that integrates multiple data types for predicting survival outcome in patients with newly diagnosed HER2-positive breast cancer. Methods: We derived a combined prognostic model using retrospective clinical-pathological data on stromal tumour-infiltrating lymphocytes, PAM50 subtypes, and expression of 55 genes obtained from patients who participated in the Short-HER phase 3 trial. The trial enrolled patients with newly diagnosed, node-positive, HER2-positive breast cancer or, if node negative, with at least one risk factor (ie, tumour size >2 cm, histological grade 3, lymphovascular invasion, Ki67 >20%, age ≤35 years, or hormone receptor negativity), and randomly assigned them to adjuvant anthracycline plus taxane-based combinations with either 9 weeks or 1 year of trastuzumab. Trastuzumab was administered intravenously every 3 weeks (8 mg/kg loading dose at first cycle, and 6 mg/kg thereafter) for 18 doses or weekly (4 mg/kg loading dose in the first week, and 2 mg/kg thereafter) for 9 weeks, starting concomitantly with the first taxane dose. Median follow-up was 91·4 months (IQR 75·1-105·6). The primary objective of our study was to derive and evaluate a combined prognostic score associated with distant metastasis-free survival (the time between randomisation and distant recurrence or death before recurrence), an exploratory endpoint in Short-HER. Patient samples in the training dataset were split into a training set (n=290) and a testing set (n=145), balancing for event and treatment group. The training set was further stratified into 100 iterations of Monte-Carlo cross validation (MCCV). Cox proportional hazard models were fit to MCCV training samples using Elastic-Net. A maximum of 92 features were assessed. The final prognostic model was evaluated in an independent combined dataset of 267 patients with early-stage HER2-positive breast cancer treated with different neoadjuvant and adjuvant anti-HER2-based combinations and from four other studies (PAMELA, CHER-LOB, Hospital Clinic, and Padova) with disease-free survival outcome data. Findings: From Short-HER, data from 435 (35%) of 1254 patients for tumour size (T1 vs rest), nodal status (N0 vs rest), number of tumour-infiltrating lymphocytes (continuous variable), subtype (HER2-enriched and basal-like vs rest), and 13 genes composed the final model (named HER2DX). HER2DX was significantly associated with distant metastasis-free survival as a continuous variable (p<0·0001). HER2DX median score for quartiles 1-2 was identified as the cutoff to identify low-risk patients; and the score that distinguished quartile 3 from quartile 4 was the cutoff to distinguish medium-risk and high-risk populations. The 5-year distant metastasis-free survival of the low-risk, medium-risk, and high-risk populations were 98·1% (95% CI 96·3-99·9), 88·9% (83·2-95·0), and 73·9% (66·0-82·7), respectively (low-risk vs high-risk hazard ratio [HR] 0·04, 95% CI 0·0-0·1, p<0·0001). In the evaluation cohort, HER2DX was significantly associated with disease-free survival as a continuous variable (HR 2·77, 95% CI 1·4-5·6, p=0·0040) and as group categories (low-risk vs high-risk HR 0·27, 0·1-0·7, p=0·005). 5-year disease-free survival in the HER2DX low-risk group was 93·5% (89·0-98·3%) and in the high-risk group was 81·1% (71·5-92·1). Interpretation: The HER2DX combined prognostic score identifies patients with early-stage, HER2-positive breast cancer who might be candidates for escalated or de-escalated systemic treatment. Future clinical validation of HER2DX seems warranted to establish its use in different scenarios, especially in the neoadjuvant setting.