<p>Human papillomavirus (HPV)-negative head and neck squamous cell carcinoma (HNSCC) is characterised by hyperactivation of the cyclin-dependent kinase 4/6 (CDK4/6) pathway. As immunotherapy has become the first-line treatment for HNSCC, resistance to anti-programmed death-1 (PD-1) agents has emerged as a pivotal challenge. This prospective, single arm, phase II study (NCT05721443) evaluated the efficacy and safety of dalpiciclib, a CDK4/6 inhibitor, combined with cetuximab in patients with anti-PD-1-resistant, HPV-negative recurrent and/or metastatic HNSCC. Patients diagnosed with p16-negative R/M HNSCC resistant to first-line anti-PD-1 therapy without prior cetuximab treatment were enroled. Patients received oral dalpiciclib (150 mg daily on days 1-21 of each 28-day cycle) and intravenous cetuximab (400 mg/m² on day 1 of cycle 1, followed by 250 mg/m² weekly in each cycle). The primary endpoint was objective response rate (ORR), secondary endpoints were overall survival, progression-free survival, duration of response, and safety. Between March 2023 and November 2024, a total of 28 patients were enroled. The ORR was 67.9% (19/28; 95% confidence interval [CI], 49.0%-82.0%), which met our primary endpoint. With a median follow-up of 9.2 months (interquartile range [IQR], 5.98-14.12), the median progression-free survival was 7.0 months (95% CI, 4.13- not reached [NR]), and the median overall survival was 17.0 months (95% CI, 10.75-23.25). Treatment-related adverse events (TRAEs) occurred in all patients, predominantly grade 1-2. Grade 3 TRAEs included neutrophil count decreased (9/28, 32.1%) and white blood cell count decreased (9/28, 32.1%). No grade 4 or 5 TRAEs were observed. Dalpiciclib combined with cetuximab was well-tolerated and showed promising efficacy in patients with anti-PD-1-resistant, HPV-negative recurrent and/or metastatic HNSCC.</p>