QLC5508 vs. Chemotherapy in Pretreated Advanced or Metastatic Esophageal Squamous Cell Carcinoma

Inclusion Criteria: * Voluntarily consent to participate in this study and sign the informed consent form. * Males and females aged ≥18 years old; * Eastern Cooperative Oncology Group Performance Status (ECOG PS) score of 0 or 1 within 7 days before the first dose; * Estimated survival time of more than 3 months. * A serum pregnancy test must be performed within 7 days prior to randomization for premenopausal women of childbearing potential, and the result must be negative and must not be lactating; * All enrolled patients and the partners should take adequate barrier contraception during the entire treatment cycle and for 6 months after the end of treatment. * Capable of understanding trial requirements, willing and able to comply with trial and follow-up procedures. * Toxicity of previous antineoplastic therapy has returned to ≤ grade 1 defined by National Cancer Institute-Common Terminology Criteria for Adverse Events (NCI-CTCAE) version 6.0 (v6.0); * Has histologically or cytologically documented unresectable locally advanced or metastatic esophageal squamous cell carcinoma (ESCC) according to American Joint Committee on Cancer 8th edition staging system on ESCC. * Has at least 1 measurable lesion according to Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1 (v1.1) as assessed by the investigator. * Sufficient bone marrow and organ function. Exclusion Criteria: * Diagnosis of other primary malignancies within 5 years prior to signing the informed co…