# C-POST (cemiplimab after surgery and radiotherapy for high-risk skin squamous cell carcinoma)

Source: https://onco.cc/trials/c-post/  
OnCo record `c-post` (Trial). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

This is the first trial to show that a drug given after surgery and radiotherapy stops the most dangerous skin squamous cancers coming back. Two years on, 87 out of 100 treated were still free of the cancer, against 64 out of 100 given a dummy.

## Summary

Almost everyone with a cutaneous squamous cell carcinoma is cured by an operation. The group C-POST was built for is the small minority who are not: those with extracapsular nodal spread, three or more involved nodes, bone invasion, in-transit metastases or perineural invasion, who have already had surgery and radiotherapy and whose remaining risk is high. Until 2025 they were simply watched.

Four hundred and fifteen patients were randomised to cemiplimab (209) or placebo (206), given at 350 mg every three weeks for twelve weeks and then 700 mg every six weeks for up to a further 36 weeks, 48 weeks in all. At a median follow-up of 24 months, disease-free survival strongly favoured cemiplimab: 24 events against 65, a hazard ratio for recurrence or death of 0.32 (95 per cent confidence interval 0.20 to 0.51, p<0.001). Estimated 24-month disease-free survival was 87.1 per cent (80.3 to 91.6) against 64.1 per cent (55.9 to 71.1). Both components moved: locoregional recurrence in 9 against 40 patients (hazard ratio 0.20, 0.09 to 0.40) and distant recurrence in 10 against 26 (hazard ratio 0.35, 0.17 to 0.72).

The cost is real but modest against that effect. Grade 3 or higher adverse events occurred in 23.9 per cent of the cemiplimab group against 14.2 per cent of the placebo group, and 9.8 per cent stopped because of adverse events against 1.5 per cent.

Overall survival was not the primary endpoint and is not yet mature. The comparison worth holding beside this trial is TROG 05.01, whose radiotherapy-alone arm gave 83 per cent freedom from locoregional relapse at five years in a partly overlapping population: adjuvant immunotherapy is being added on top of local treatment that already works well, which is why the absolute gain, 23 percentage points at two years, is larger than most adjuvant trials in any cancer.

## Fields

- Kind: Trial
- Status: positive
- Last checked: 2026-09-25
- Also known as: C-POST; Cemiplimab POST-operative trial
- Registry id: NCT03969004
- Phase: 3
- Setting: Local or regional cutaneous squamous cell carcinoma after surgical resection and postoperative radiotherapy, at high risk of recurrence through nodal features (extracapsular extension with a node of 20 mm or more, or at least three involved nodes) or non-nodal features (in-transit metastases, a T4 lesion with bone invasion, perineural invasion, or a locally recurrent tumour with at least one other risk feature): cemiplimab against placebo for up to 48 weeks, with disease-free survival as the primary endpoint
- Sponsor: Regeneron Pharmaceuticals and Sanofi
- Enrolled: 415
- Result: Disease-free survival at 24 months 87.1 per cent with adjuvant cemiplimab against 64.1 per cent with placebo (hazard ratio 0.32, 95 per cent confidence interval 0.20 to 0.51, p<0.001), with grade 3 or higher adverse events in 23.9 against 14.2 per cent.
- Outcomes: Disease-free survival at 24 months: Adjuvant cemiplimab 87.1% vs Placebo 64.1%, HR 0.32; Freedom from locoregional recurrence: Adjuvant cemiplimab 9 events vs Placebo 40 events, HR 0.2; Freedom from distant recurrence: Adjuvant cemiplimab 10 events vs Placebo 26 events, HR 0.35; Grade 3 or higher adverse events: Adjuvant cemiplimab 23.9% vs Placebo 14.2%
- Replication: The first randomised adjuvant trial in this disease. STAMP, testing adjuvant pembrolizumab in Merkel cell carcinoma, and ADMEC-O, testing adjuvant nivolumab, are the nearest parallels in a different keratinocyte-adjacent tumour.

## Sources

- ClinicalTrials.gov NCT03969004: https://clinicaltrials.gov/study/NCT03969004
- Primary report (New England Journal of Medicine 2025): https://doi.org/10.1056/NEJMoa2502449
- Primary report on PubMed: https://pubmed.ncbi.nlm.nih.gov/40454639/

## Connected records

- cancers: [Advanced cutaneous squamous cell carcinoma](https://onco.cc/cancers/advanced-cutaneous-scc/), [Cutaneous squamous cell carcinoma](https://onco.cc/cancers/cutaneous-scc/), [Skin cancer (all types)](https://onco.cc/cancers/skin-cancer/)
- technologies: [Immune checkpoint inhibitors](https://onco.cc/technologies/checkpoint-inhibitor/)
- targets: [PD-1](https://onco.cc/targets/pd1/)
- drugs: [Cemiplimab](https://onco.cc/drugs/cemiplimab/)
- companies: [Regeneron](https://onco.cc/companies/regeneron/), [Sanofi](https://onco.cc/companies/sanofi/)
- terms: [Immune-related adverse events (irAEs)](https://onco.cc/terms/irae/), [Perineural invasion (PNI)](https://onco.cc/terms/perineural-invasion/), [Radiotherapy for skin cancer](https://onco.cc/terms/radiotherapy-for-skin-cancer/), [Tumour mutational burden (TMB)](https://onco.cc/terms/tmb/)
- trials: [CK-301-101](https://onco.cc/trials/ck-301-101/), [EMPOWER-CSCC-1](https://onco.cc/trials/empower-cscc-1/), [KEYNOTE-629](https://onco.cc/trials/keynote-629/), [Neoadjuvant cemiplimab for resectable stage II to IV skin squamous cell carcinoma](https://onco.cc/trials/neoadjuvant-cemiplimab-cscc/), [TROG 05.01 (chemotherapy added to radiotherapy after surgery for high-risk skin squamous cell carcinoma)](https://onco.cc/trials/trog-05-01/)
- key papers: [C-POST: adjuvant cemiplimab versus placebo in high-risk cutaneous squamous cell carcinoma](https://onco.cc/key-papers/paper-c-post-adjuvant-cemiplimab-nejm-2025/)

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