This study is a prospective, single-arm, phase II clinical trial involving patients with advanced HNSCC receiving immunotherapy plus platinum-based dual-drug chemotherapy. It aims to evaluate the myeloprotective efficacy of administering trilaciclib prior to immunotherapy and platinum-based chemotherapy. The objective is to reduce the incidence of chemotherapy-induced myelosuppression (CIM) through pre-chemotherapy myeloprotection, thereby enabling patients to receive chemotherapy on schedule and at full dose. This approach is intended to ensure the efficacy of the chemotherapy regimen and ultimately achieve survival benefits for the patients.

Inclusion Criteria:

• 1.Age ≥18 and ≤75 years, male or female. 2.Histologically or cytologically confirmed diagnosis of head and neck squamous cell carcinoma (HNSCC).

  3.Recurrent and/or metastatic HNSCC not suitable for locoregional therapy. Patients with recurrent-only disease (without metastasis) must have previously received radiotherapy (either as adjuvant therapy after surgery or as treatment for locally advanced SCCHN) as "locoregional therapy," and radiotherapy must have been completed more than 6 months prior to screening imaging.

  4.At least one measurable lesion per RECIST 1.1 criteria. 5.Laboratory tests meeting the following criteria:

  1. Hemoglobin ≥ 100 g/L (female) / 110 g/L (male)

  2. Absolute neutrophil count ≥ 2.0 × 10⁹/L

  3. Platelet count ≥ 100 × 10⁹/L

  4. Serum creatinine ≤ 15 mg/L or creatinine clearance (CrCl) ≥ 60 mL/min (calculated by Cockcroft-Gault formula)

  5. Total bilirubin ≤ 1.5 × upper limit of normal (ULN)

  6. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 3 × ULN, or ≤ 5 × ULN in patients with liver metastases

  7. Albumin ≥ 30 g/L 6.ECOG Performance Status score of 0 or 1. 7.Expected survival time ≥ 3 months. 8.No plans for conception or breastfeeding from 2 weeks before the start of study treatment until 3 months after the end of the study.

     9.Ability to understand and willingness to sign the informed consent form.

     Exclusion Criteria:

• 1.Diagnosis of a malignancy other than HNSCC within 5 years before the first dose (except for curatively treated basal cell carcinoma of the skin, squamous cell carcinoma of the skin, and/or carcinoma in situ that has undergone radical resection).

  2.Uncontrolled ischemic heart disease or clinically significant congestive heart failure (NYHA Class III or IV).

  3.History of stroke or major cerebrovascular event within 6 months prior to enrollment.

  4.QTcF interval >480 msec at screening, or >500 msec for patients with a ventricular pacemaker.

  5.Prior hematopoietic stem cell or bone marrow transplantation. 6.Known hypersensitivity to the study drug or any of its components. 7.Any other condition for which the investigator deems the subject unsuitable for participation in this study.