173 - C-Post Study of Adjuvant Cemiplimab for High-Risk Cutaneous Squamous Cell Carcinoma: Impact of Adjuvant Radiotherapy on Disease-Free Survival

10:51am - 10:54am ET

Room 257

Presenter(s)

Gerard Adams, MB, ChB, FRANZCR - Genesis Care Bundaberg, Bundaberg, QLD

S. V. Porceddu¹, F. Day², D. Brungs³, H. Christie⁴, J. E. Jackson⁵, B. N. Stein⁶, Y. B. Su⁷, G. Adams⁸, S. E. Bowyer⁹, N. Yamazaki¹⁰, P. Bossi^11,12, A. Challapalli¹³, R. Ladwa¹⁴, A. Hauschild¹⁵, J. Pouliot¹⁶, F. Seebach¹⁶, S. Y. Yoo¹⁶, P. Goncalves¹⁶, M. G. Fury¹⁶, and D. Rischin¹⁷; ¹Department of Radiation Oncology, Peter MacCallum Cancer Centre, Melbourne, VIC, Australia, ²Department of Medical Oncology, Calvary Mater Newcastle, Waratah, NSW, Australia, ³Illawarra Cancer Care Centre, Wollongong Hospital, Wollongong, NSW, Australia, ⁴Cancer Care Centre Hervey Bay, Urraween, QLD, Australia, ⁵Radiation Oncology Centers, Gold Coast, QLD, Australia, ⁶Adelaide Cancer Centre, Adelaide, SA, Australia, ⁷Head and Neck Medical Oncology, Nebraska Cancer Specialists, Omaha, NE, ⁸Radiation Oncology, Genesis Cancer Care, Bundaberg, QLD, Australia, ⁹Department of Medical Oncology, Sir Charles Gairdner Hospital, Perth, Western Australia, Australia, ¹⁰National Cancer Center Hospital, Tokyo, Japan, ¹¹IRCCS Humanitas Research Hospital, Milan, Italy, ¹²Department of Biomedical Sciences, Humanitas University, Milan, Italy, ¹³Bristol Cancer Institute, University Hospitals Bristol & Weston NHS Foundation Trust, Bristol, United Kingdom, ¹⁴Princess Alexandra Hospital, Woolloongabba, QLD, Australia, ¹⁵Department of Dermatology, University Hospital (UKSH), Kiel, Germany, ¹⁶Regeneron Pharmaceuticals, Inc., Tarrytown, NY, ¹⁷Department of Medical Oncology, Peter MacCallum Cancer Centre, Melbourne, VIC, Australia

Purpose/Objective(s): The C-POST trial previously demonstrated superior disease-free survival (DFS) for adjuvant cemiplimab (cemi) vs placebo (pbo) (HR, 0.32; P<0.0001) in patients (pts) with high-risk cutaneous squamous cell carcinoma (CSCC) after surgery and radiotherapy (RT), and a safety profile consistent with the known safety of cemi in advanced solid malignancies. With approximately 30 months of follow-up, we present DFS subgroup analyses based on timing and regimen of adjuvant RT to the high-risk tumor. The goal of this post hoc exploratory analysis is to better understand the impact of adjuvant RT on DFS benefit with cemi.

Materials/Methods: Pts with local/locoregional CSCC who underwent surgery and postoperative RT and had a high risk of recurrence based on nodal and/or non-nodal criteria were randomized 1:1 to cemi or pbo Q3W for 12 wk, then Q6W for 36 wk. Randomization occurred within 2-11 wk after completion of RT. DFS subgroup analyses were performed per total adjuvant RT dose (<60 Gy vs =60 Gy), the technique used for RT delivery (intensity-modulated RT [IMRT] vs 3D conventional RT or others), and the interval between completion of adjuvant RT and study randomization (2-6 wk vs >6 wk). Data cutoff was April 7, 2025.

Results: Among 415 pts (209/206 cemi/pbo) randomized, the DFS HR was 0.35 (95% CI, 0.23-0.55) at a median follow-up of 30 months. Most pts received a total adjuvant RT dose of =60 Gy (77%). The most common delivery technique was IMRT (74%). DFS was improved with adjuvant cemi vs pbo across all subgroups irrespective of interval between completion of RT and study randomization, dose, or modality (Table). While not statistically significant, more favorable HRs were observed with =60 Gy vs <60 Gy (HR, 0.31 vs 0.65) and IMRT vs 3D conventional RT or others (HR, 0.30 vs 0.57). The interval between adjuvant RT completion and study randomization had no impact on DFS.

Conclusion: In this phase 3 study, adjuvant cemi demonstrated a DFS benefit vs pbo across all adjuvant RT subgroups assessed. (NCT03969004)

Abstract 173 – Table 1: DFS with cemi vs pboH: Analyses per Adjuvant RT

	DFS events in cemi arm, n/N (%)	DFS events in pbo arm, n/N (%)	DFS HR (95% CI)^a
All participants	29/209 (13.9)	68/206 (33.0)	0.35 (0.23-0.55)
Total adjuvant RT dose
<60 Gy	10/47 (21.3)	16/46 (34.8)	0.65 (0.29-1.47)
=60 Gy	19/160 (11.9)	52/159 (32.7)	0.31 (0.18-0.54)
Technique used for RT
IMRT	18/159 (11.3)	49/149 (32.9)	0.30 (0.18-0.52)
3D conventional RT/other	11/49 (22.4)	19/56 (33.9)	0.57 (0.26-1.25)
Interval between RT completion and study randomization
2-6 wk	17/117 (14.5)	37/112 (33.0)	0.39 (0.22-0.70)
>6 wk	12/91 (13.2)	31/93 (33.3)	0.36 (0.19-0.71)
^aBased on high-risk tumor (nodal vs non-nodal) and geographical region (North America vs Australia/New Zealand vs rest of world) per stratified proportional hazards model (cemi vs pbo).