This is a multicenter, open-label, Phase II clinical study to assess the efficacy, safety, tolerability, PK characteristics, and immunogenicity of SKB571 in participants with recurrent or metastatic HNSCC.

Recurrent or metastatic HNSCC with treatment failure after prior first-line PD-1 inhibitor and/or platinum-based chemotherapy, receiving SKB571 monotherapy.

A total of 20-30 participants are planned to be enrolled to receive SKB571 monotherapy. Each dosing cycle is 3 weeks.

Inclusion Criteria:

• Age ≥ 18 and ≤ 75 years at the time of signing the informed consent form.
• Histologically or cytologically confirmed recurrent or metastatic head and neck squamous cell carcinoma (HNSCC) that is not curable by local therapy and not amenable to curative surgery, with the primary tumor located in the oral cavity, oropharynx, hypopharynx, or larynx.
• Subjects with at least one measurable lesion assessed by the investigator according to RECIST v1.1.
• Eastern Cooperative Oncology Group (ECOG) performance status of 0-1 within 7 days before the first dose.
• Participants who have adequate bone marrow, liver, kidney, and coagulation function.
• Male and female participants must agree to use highly effective methods of contraception during the specified period of the study.
• Participants must voluntarily join this study, sign the informed consent form (ICF), and be able to comply with the visits and related procedures specified in the protocol.

Exclusion Criteria:

• History or current metastases to central nervous system.
• Subjects with other malignant tumors within 3 years prior to the first dose.
• Presence of any cardiovascular and cerebrovascular disorders or risk factors.
• Presence of uncontrolled systemic disease.
• Presence of clinically symptomatic or uncontrolled pleural effusion, pericardial effusion, or ascites requiring repeated drainage.
• History of interstitial lung disease (ILD) or non-infectious pneumonitis.
• Presence within 3 months before the first dose of other moderate to severe lung disorders.
• Tumor invasion or compression of surrounding vital organs and major blood vessels.
• Unresolved toxicity from prior anti-tumor therapy.
• Serious infection within 4 weeks before the first dose.
• Presence of active HIV, hepatitis B or hepatitis C or co-infection with HBV and HCV.
• Known active pulmonary tuberculosis.
• Known history of allogeneic organ transplant or hematopoietic stem cell transplant.
• History of allergy to any component of the study drug or severe hypersensitivity to other monoclonal antibodies.
• Participants who have undergone major surgery or had severe trauma within 4 weeks before the first dose, or are expected to require major surgery during the study.
• Participants who have received other investigational drug treatments within 4 weeks before the first dose.
• Prior vaccination with a therapeutic anti-tumor vaccine, or any live vaccine within 4 weeks before the first dose, or planned vaccination with a live vaccine during the study.
• Participants who received systemic corticosteroid therapy with >10 mg/day of prednisone or other immunosuppressive drugs within 2 weeks before the first dose.
• Received strong inhibitors or strong inducers of cytochrome P450 (CYP3A4) or breast cancer resistance protein (BCRP) inhibitors within 2 weeks prior to the first dose or within 5 half-lives of the known drug (whichever is longer).
• Pregnant or breastfeeding women.
• Known history of psychosis or drug abuse that prevents the participant from cooperating with the study.
• Have local or systemic diseases caused by non-malignant tumors, or diseases or symptoms secondary to tumors, which may lead to higher medical risk and/or uncertainty in survival assessment, or may affect protocol compliance.
• Any condition that, in the investigator's opinion, interferes with the evaluation of the investigational product, participant safety, or interpretation of study results, or any other condition that the investigator deems unsuitable for participation in this study.