ESMO Congress 2026

Within the breast cancer immunotherapy landscape, the evidence base for the PD-1 inhibitor pembrolizumab plus chemotherapy currently rests in triple-negative breast cancer (TNBC): this combination has received FDA approval for both PD-L1 positive metastatic and early-stage TNBC ^[1], and PD-1/PD-L1 checkpoint inhibitor plus chemotherapy has demonstrated efficacy in both early and advanced TNBC trials, leading to approvals ^[2]. In the large HR+/HER2− population, endocrine therapy (ET) remains the cornerstone, and CDK4/6 inhibitors plus ET have become the standard of care in advanced HR+ breast cancer, but resistance inevitably occurs, leaving a clear unmet need in later lines ^[5]; whether checkpoint inhibitors can be extended to HR+/HER2− disease remains an area of active investigation ^[1][2].

KEYNOTE-B49, reported at ESMO26, directly tests this question: in patients with PD-L1 CPS ≥1, HR+/HER2− metastatic breast cancer previously treated with ET, pembrolizumab plus chemotherapy was compared with chemotherapy alone. The numbers from the floor: PFS of 6.7 vs 6.0 months (HR 0.74), and OS of 21.0 vs 22.2 months (HR 1.04), meaning no overall survival advantage and a numerically lower value; in the CPS ≥10 population the PFS HR was 0.59, suggesting greater benefit in the enriched subgroup, and the posting expert noted he looks forward to the subgroup analyses presented at ESMO. The same expert commented that in the ADC era, this modest PFS benefit without an OS advantage has little clinical relevance to him, and that evaluating immunotherapy in combination with ADCs or after ADC treatment may be more rational; separately, CancerNetwork listed KEYNOTE-B49 among 6 breast cancer trials to watch at this ESMO, alongside novel ADCs.

Relative to prior knowledge, KEYNOTE-B49 moves the evidence for checkpoint inhibitors in HR+/HER2− breast cancer one step forward: against a backdrop where approvals and major positive data have centered on TNBC and HR+ disease has only been exploratory ^[1][2], it delivers a randomized readout of adding pembrolizumab to chemotherapy in this population, and the lower PFS HR at CPS ≥10 is directionally consistent with the established PD-L1-selected approval logic for pembrolizumab ^[1]. The poster's proposal of IO+ADC combinations or sequencing after ADC also echoes the background literature, where ADCs are FDA approved for TNBC and HER2+ disease and are being studied in combination with checkpoint inhibitors ^[1].

As for limitations, the available information comes only from a conference floor post; safety data, prior CDK4/6 inhibitor exposure, P values, and statistical design details are undisclosed; OS numerically disfavored the combination arm (21.0 vs 22.2 months); full results and subgroup analyses should await formal publication, and this interpretation does not constitute investment or clinical advice.