23P Longitudinal evaluation of PAM50 molecular subtypes in patients receiving neoadjuvant HER2-targeted therapy in the PREDIX HER2 trial

All four intrinsic subtypes (Luminal A, Luminal B, HER2-enriched HER2-E and Basal-like) are identified in clinically HER2-positive breast cancer (HER2+ BC). In this study, we evaluated the distribution, longitudinal changes, and prognostic implications of PAM50 subtypes in patients receiving neoadjuvant HER2-targeted therapy in the randomized phase II PREDIX HER2 trial. RNA sequencing was performed on 195 baseline (of 197 randomized patients), 168 on-treatment and 127 surgical fresh-frozen tissue specimens from early HER2+ BC patients treated with docetaxel, trastuzumab and pertuzumab (THP) or trastuzumab emtansine (T-DM1) within the PREDIX HER2 trial. PAM50 subtypes were calculated by the sspbc. subtype-PAM50 model. Baseline PAM50 subtype correlations with pathologic complete response (pCR) and event-free survival (EFS) were analyzed using multivariate logistic and Cox regression, respectively. The shift in subtype from baseline to on-treatment is described among patients with paired samples available. PAM50 intrinsic subtypes at baseline, on-treatment, and post-surgical stages were calculated for all patients sequenced across the three time points, while paired samples for baseline and on-treatment were available for 167 patients. At baseline, 108 (55%) patients were classified as HER2-E, 36 (18%) as luminal A, 35 (18%) as luminal B and 16 (8%) as basal-like. Baseline HER2-E subtype was significantly associated with pCR (ORadj=4.05; 95% CI, 2.02-8.38; P <0.001) and EFS (HRadj=0.17; 95% CI, 0.06-0.55; P =0.003). Among patients with paired baseline and on-treatment samples, 71/93 baseline HER2-E patients switched to a non-HER2-E subtype (McNemar's P<0.001). A 50% agreement of PAM50 subtypes was observed in 14 patients with two available baseline biopsies. Molecular subtypes are prognostic for response to neoadjuvant HER2-targeted treatment and for long-term outcomes. Subtypes exhibit significant intratumoral and temporal heterogeneity. Ongoing analyses investigate the prognostic implications of subtype switch.