Phase i trial of palbociclib, a selective cyclin dependent kinase 4/6 inhibitor, in combination with cetuximab in patients with recurrent/metastatic head and neck squamous cell carcinoma

Objectives To test the safety of the CDK4/6 inhibitor palbociclib with cetuximab in patients with recurrent/metastatic head and neck squamous cell carcinoma (HNSCC). Materials and Methods A phase I trial using 3+3 design was performed to determine the dose limiting toxicity (DLT) and maximum tolerated dose (MTD) of palbociclib with standard dose weekly cetuximab. Palbociclib was administered orally days 1-21 every 28 days: dose level 1 (100 mg/d) and 2 (125 mg/d; approved monotherapy dose). Pharmacokinetic assessments were performed on cycle 2, day 15. Cyclin D1, p16^INK4a, and Rb protein expression were measured on pre-treatment tumor. Tumor response was assessed using RECIST1.1. Results Nine patients (five p16^INK4a negative; four positive) were enrolled across dose levels 1 (n = 3) and 2 (n = 6) and none experienced a DLT. A MTD of palbociclib was not reached. Myelosuppression was the most common adverse event. Six of nine patients had cetuximab-resistant and 4/9 had platin-resistant disease. Disease control (DC) occurred in 89%, including partial response (PR) in two (22%) and stable disease in six (67%) patients. PRs occurred in p16^INK4a negative HNSCC. Five patients (56%) had measurable decreases in tumor target lesions. In cetuximab-resistant HNSCC, best tumor response was PR in 1 and DC in 5 and median TTP was 112 days (range: 28-168). In platin-resistant HNSCC, best tumor response: PR in 1, DC in 3 and median TTP was 112 days (range: 28-112). The C_max and AUC_0-24h appeared comparable in patients receiving 125 vs 100 mg dose of palbociclib. Conclusion This trial, the first to evaluate a CDK4/6 inhibitor in HNSCC, determined that palbociclib 125 mg/day on days 1-21 every 28 days with cetuximab was safe. Tumor responses were observed, even in cetuximab- or platin-resistant disease.