Predictive markers of complete pathological response to neoadjuvant chemo-immunotherapy in triple negative breast cancer.

e12650 Background: Results from the Keynote 522 clinical trial, which played a pivotal role in evaluating the efficacy of neoadjuvant pembrolizumab in combination with chemotherapy (NAICT), showed a significant improvement in pathological complete response (pCR) rates compared with chemotherapy alone. To date, no significant markers have been identified to predict pathological complete response to NAICT. The primary objective was to identify markers, both molecular and clinical, predictive of pCR to NAICT in a real-world clinical setting. Methods: The retrospective analysis included 38 consecutively treated pts with early stage TNBC who received NAICT including pembrolizumab at the Institute of Oncology, Ljubljana (OIL) between 4 January 2022 and 17 October 2023. Data collected from electronic medical records included tumour size, nodal status, UICC stage, BRCA status, systemic immune-inflammation index (SII) before NAICT and before surgery, and frequency and grade of immune-related adverse events (iAEs) (according to the Common Terminology Criteria for Adverse Events (CTCAE) version 5). In pts who completed surgical treatment, response to treatment was assessed by pathological examination of post-treatment surgical specimens. Descriptive statistical methods were used to summarise and describe the main characteristics. Pearson chi-squared test was used to analyse categorical variables and t-test to evaluate associations between continuous variables. Results: Of the 38 pts, 28 completed surgical treatment and were further analysed. Pathohistological examination of post-treatment surgical specimens revealed pCR in 11 pts (39.3%), with residual disease in 17 pts (60.7%). Of the 17 pts without pCR, 11 pts had RCB score 2 (64.7%) and 5 pts had score 3 (29.4%). The distribution of size, nodal status, UICC stage at diagnosis, BRCA status, frequency of iAEs in correlation with pCR is shown in the Table. A borderline correlation was found between pCR and SII at the start of pembrolizumab treatment (p=0.05). Age, tumour size, nodal status, UICC stage, BRCA status, AEs, SII before surgery were not found to be predictive of pCR. Conclusions: SII at the start of pembrolizumab was found to be a borderline significant marker predictive of pCR. Other characteristics evaluated did not correlate with pCR. [Table: see text]