Safety, treatment delivery, and pathological response before and after pembrolizumab implementation for early...

Pembrolizumab has become standard neoadjuvant systemic therapy (NST) for early estrogen receptor (ER)-negative, human epidermal growth factor receptor 2 (HER2)-normal early breast cancer (eBC). Electronic health record (EHR) data may support real-world assessment of its implementation in routine care.

Using centralized EHR data from eastern Denmark, we identified adults with ER-negative, HER2-normal eBC receiving NST. Patients were grouped by treatment start before (April 2021-June 2022; comparator) or after (April 2023-June 2024; study) pembrolizumab implementation. We compared treatment exposure, adverse events (AEs) from laboratory data, clinical notes, and AE-indicating medications, and pathological complete response (pCR) from pathology records. Follow-up ended 200 days after surgery, progression, or death. Manual validation included 50 patients.

Among 210 included patients, 109 were in the study population and 101 in the comparator population. Baseline characteristics were broadly similar. Pembrolizumab was received by 77% versus 0% and carboplatin omission was less frequent after implementation (7% versus 47%). During NST, grade 3/4 AEs were observed in 77 study patients (71%) and 66 comparator patients (65%). Suspected grade 3/4 autoimmune disorders during NST occurred in 18 pembrolizumab-treated study patients (21%) and in no comparator patients, and no grade 5 AEs were identified. pCR was achieved in 74 study patients (68%) versus 49 comparator patients (49%). Manual validation showed high structured-data completeness after resolution of a small number of data processing errors but also identified missing data.

After introduction of pembrolizumab into NST, patients with early ER-negative, HER2-normal breast cancer (BC) had a higher pCR proportion, alongside a higher toxicity.