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 consent form. * Having histologically or cytologically confirmed adenosquamous carcinoma subtype. * Brain metastases (unless asymptomatic and no progression confirmed by imaging ≥4 weeks prior to randomization); * Presence of leptomeningeal metastases or brainstem metastases; * Spinal cord compression (identified via imaging, regardless of symptoms); * Previous or ongoing treatment with topoisomerase I inhibitors * Having previously received B7-H3-targeted therapy. * Being ineligible to any chemotherapies in the control arm due to prior progression or intolerance. * Insufficient washout of prior anticancer therapies prior to randomization. * Having underwent major organ surgery (excluding biopsy) or significant trauma within 4 weeks prior to randomization; * Requiring elective surgery during the study. * Having received live vaccine or live attenuated vaccine within 4 weeks before study randomization. * Having received treatment with systemic corticosteroids (prednisone at \>10 mg/day, or similar drugs at equivalent dose) or other immunosuppressive agents within 14 days prior to randomization; * Moderate to severe pulmonary disease significantly impairing lung function, including idiopathic pulmonary fibrosis, autoimmune/connective tissue disorders with lung involvement, or prior pneumonectomy. * Having a history of interstitial lung disease (ILD)/ non-infectious pneumonitis that required corticosteroids, current ILD/ non-infectious pneumonitis, or suspected ILD/ non-infectious pneumonitis that cannot be ruled out by imaging at screening; * Active tuberculosis; * Autoimmune diseases not in clinical remission, other acquired or congenital immunodeficiency diseases, * A history of allogeneic stem cell, bone marrow, or organ transplantation. * Serious infections (e.g., bacteremia, or severe pneumonia) within 4 weeks prior to randomization; * active infection requiring systemic antibiotic therapy within 1 weeks prior to randomization; * Has positive results in virus serology tests (hepatitis B virus infection participants receiving antiviral treatment other than interferon are allowed to be enrolled); * Uncontrolled or significant cardiovascular disease. * Clinically uncontrolled third-space effusion. * Known hypersensitivity to investigational product components, analogues, or control drugs (e.g., docetaxel, paclitaxel, irinotecan hydrochloride). * Drug abuse; * Any other medical conditions that may interfere with study participation or the results of the clinical study as per the discretion of investigator; * Has alcohol or drug dependence. * Poor compliance as per investigator discretion; * Has a history of other serious systemic diseases.