On-Treatment Changes In Tumor-Infiltrating Lymphocytes (Til) During Neoadjuvant Her2 Therapy (Nat) And Clinical Outcome.

574 Background: Higher quantity of pretreatment TIL (PT) is associated with improved pCR and EFS in HER2+ early breast cancer (BC). The value of on-treatment TIL is unknown. Methods: The NeoALTTO trial randomized 455 women with HER2+ BC to 12 weeks NAT with trastuzumab, lapatinib or combination with paclitaxel, followed by FEC after surgery. In the PAMELA trial 151 women received 18 weeks NAT with lapatinib and trastuzumab (±hormonal therapy). TIL were quantified on PT and on-treatment (W2) biopsies using the published method on H&E slides, and tested for associations with pCR (logistic regression), EFS and OS (Cox models) in univariate (UV) and multivariate (MV) analyses. The likelihood ratio test assessed added prognostic value to clinicopathological (CP) variables. pCR associations were validated in PAMELA. We investigated enrichment of immune cell subsets using previously published RNAseq data from NeoALTTO. Results: In NeoALTTO, PT and W2 TIL were evaluable in 277/455 (61%). We defined two groups: immune-poor (L+F) and immune-enriched (II+P), see Table. Immune-enriched (41%; 134) vs poor (59%; 164) patients had significantly higher pCR rates (40% vs 21%; UV OR 2.24; 95%CI 1.31-3.85; P = .003; MV P = .009), and added significant value to CP + PT TIL for prediction of pCR (P = .003). This was further confirmed in PAMELA (N = 94/151) (26% vs 6%; UV P = .021; MV P = .028). In NeoALTTO, the immune-enriched vs poor patients had significantly improved EFS (5 yr est 85% vs 60%; UV HR 0.31; 95%CI 0.18-0.54; P < .001; MV P < .001) and OS (5 yr est 91% vs 77%; UV HR 0.40; 95%CI 0.20-0.82; P = .012; MV P = 0.026), and provided significant added prognostic value beyond CP + pCR + PT TIL (EFS P < .001) In NeoALTTO PT samples, II vs F patients had enrichment of DCs, NKs and CD8+ including tissue resident memory cells (P = .009) suggesting requirement of key immune subsets. Further validation by IHC is ongoing. Conclusions: On-treatment TIL identifies patients more likely to achieve pCR and have improved EFS in early-stage HER2+ BC, beyond CP + PT TIL. This information could aid future trial design. Clinical trial information: NCT00553358, NCT01973660. [Table: see text]