Regeneron cemiplimab shows 37% OS benefit in adjuvant CSCC trial
Regeneron Pharmaceuticals has reported longer-term data from its Phase 3 C-POST trial of cemiplimab (Libtayo) as adjuvant treatment for high-risk cutaneous squamous cell carcinoma (CSCC), showing a 37% reduction in the risk of death at a median follow-up of 44 months. The results, presented as a late-breaking rapid oral at the European Society for Medical Oncology Congress 2026, extend a trend observed at earlier readouts and were accompanied by a new exploratory analysis using circulating tumour DNA.
The hazard ratio for overall survival now stands at 0.63 (95% CI: 0.36–1.11), compared with 0.78 at 30 months and 0.86 at 24 months. Confidence intervals remain wide, reflecting the relatively small number of events, but the directional improvement is notable given that 88% of placebo patients who experienced disease recurrence subsequently crossed over to receive cemiplimab. In trials with high crossover rates, detecting an OS signal typically requires a large and durable treatment effect; the emerging benefit here therefore adds weight to the adjuvant strategy.
ctDNA subgroup and disease-free survival
An exploratory analysis drawing on baseline circulating tumour DNA data from 191 patients found that adjuvant cemiplimab improved disease-free survival irrespective of minimal residual disease status at baseline. Patients who were MRD negative at baseline, and therefore at lower baseline recurrence risk, still derived benefit from cemiplimab over placebo (HR: 0.45; 95% CI: 0.22–0.95). The MRD-positive subgroup showed an even sharper DFS advantage (HR: 0.24; 95% CI: 0.07–0.86), though the very wide confidence interval in this smaller group warrants caution.
Lead investigator Danny Rischin, head of head and neck cancer and cutaneous SCC at the Peter MacCallum Cancer Centre in Melbourne, described the accumulating dataset as "compelling evidence of the long-term, durable benefit of adjuvant treatment with Libtayo in high-risk CSCC," adding that the data further support cemiplimab as a standard of care in this setting.
Safety remained broadly consistent with the established cemiplimab profile. Grade 3 or worse treatment-emergent adverse events occurred in 26% of cemiplimab patients versus 17% on placebo; treatment discontinuation due to adverse events was 10% versus 3%. Deaths attributed to treatment-emergent adverse events occurred in 2% of patients in each arm.
Regulatory and competitive context
Cemiplimab received US FDA approval for the adjuvant CSCC indication in October 2025, followed by European Commission approval in November 2025, making it the first and only immunotherapy cleared for this use. The C-POST data were originally published in the New England Journal of Medicine alongside their presentation at ASCO 2025.
The adjuvant PD-1 inhibitor space in skin cancer is becoming more contested. Several other checkpoint inhibitors are investigating perioperative strategies in squamous and basal cell carcinomas, and Regeneron's own pipeline includes bispecific and costimulatory antibody combinations that could eventually complement or compete with single-agent cemiplimab. For CSCC specifically, the relatively rare incidence of the high-risk, post-operative subpopulation limits commercial scale, but the favourable regulatory precedent and maturing survival data position cemiplimab firmly in treatment guidelines.
Investors and oncologists will look to longer follow-up for OS confidence interval narrowing and for real-world evidence from post-approval registries, which will be critical to formulary decisions and guideline entrenchment across markets outside the US and EU.