Background
Phase III, double-blind, placebo-controlled RCT. 847 patients with previously untreated unresectable locally recurrent or metastatic TNBC. PD-L1 expression assessed by combined positive score (CPS); CPS ≥10 present in ~45% of patients.
Interventions and follow up
Arm A: Pembrolizumab 200mg q3wk + chemotherapy (investigator's choice: nab-paclitaxel, paclitaxel, or gemcitabine-carboplatin)
Arm B: Placebo + chemotherapy
Primary endpoint: PFS and OS tested hierarchically (CPS ≥10 → CPS ≥1 → ITT)
mFollow up: 44.1mo
Arm B: Placebo + chemotherapy
Primary endpoint: PFS and OS tested hierarchically (CPS ≥10 → CPS ≥1 → ITT)
mFollow up: 44.1mo
Results
PFS (CPS ≥10): 9.7 vs 5.6mo, HR 0.65, P<.001
OS (CPS ≥10): 23.0 vs 16.1mo, HR 0.73, P=.0093
OS (CPS ≥1): 17.6 vs 16.0mo, HR 0.86 — prespecified boundary not met
OS (ITT): 17.2 vs 15.5mo, HR 0.89 — not significant
OS (CPS ≥10): 23.0 vs 16.1mo, HR 0.73, P=.0093
OS (CPS ≥1): 17.6 vs 16.0mo, HR 0.86 — prespecified boundary not met
OS (ITT): 17.2 vs 15.5mo, HR 0.89 — not significant
Adverse events
Overall (grade ≥3): 68.1% vs 66.9% (A vs B)
Immune-mediated (any grade): 26.8% vs 5.5%; hypothyroidism 15.8%
Pulmonary: Pneumonitis grade ≥3 3.3%
Discontinuation due to AEs: 19.3% vs 11.3%
Immune-mediated (any grade): 26.8% vs 5.5%; hypothyroidism 15.8%
Pulmonary: Pneumonitis grade ≥3 3.3%
Discontinuation due to AEs: 19.3% vs 11.3%
Conclusions
Pembrolizumab + chemotherapy significantly improved OS in patients with PD-L1 CPS ≥10 metastatic TNBC, establishing a new standard of care in this biomarker-selected population. No benefit demonstrated in CPS <10.
Key Limitations
The CPS ≥10 cutoff was selected mid-trial after observing IMpassion130 data — a post-hoc amendment raising concern of enrichment bias. PD-L1 assays differ across trials (22C3/CPS here vs SP142/TC+IC in IMpassion130), limiting cross-trial comparisons. No OS benefit in CPS ≥1 or ITT populations. Early relapsers (6–12mo from adjuvant chemo) were eligible — a population typically excluded as taxane-resistant in other mTNBC trials, affecting generalizability.
Clinical Context
OS gain of ~7 months (HR 0.73) in CPS ≥10 is widely considered clinically meaningful. Pembrolizumab + chemo is now standard of care first-line in PD-L1 CPS ≥10 mTNBC, having largely supplanted atezolizumab + nab-paclitaxel (IMpassion130) due to broader FDA approval. ESMO-MCBS score: 4.