This is a Phase II, single-arm, single-center study evaluating Chidamide combined with a PD-L1 inhibitor, carboplatin, and etoposide as first-line therapy in extensive-stage small-cell lung cancer (ES-SCLC) patients. The primary objective is to assess Progression-Free Survival (PFS) per RECIST v1.1. Secondary objectives include Objective Response Rate (ORR), Disease Control Rate (DCR), Duration of Response (DOR), Overall Survival (OS), and safety. Approximately 36 participants will receive induction therapy (Chidamide + chemotherapy + PD-L1 inhibitor) for 4 cycles, followed by Chidamide maintenance until progression or unacceptable toxicity.

Inclusion Criteria:

• Age ≥18 and ≤75 years.
• Histologically confirmed ES-SCLC, unsuitable for local radical therapy.
• No prior systemic therapy for ES-SCLC.
• ECOG performance status 0-1.
• At least one measurable lesion per RECIST v1.1.
• Expected survival ≥3 months.
• Adequate organ function (hematological, hepatic, renal, cardiac).
• Effective contraception from consent until 180 days after last dose.
• For active HBV infection: HBV DNA <2000 IU/mL within 28 days before treatment and on stable antiviral therapy.
• Recovery from prior therapy toxicities to ≤ Grade 1 (except alopecia).
• Signed informed consent.

Exclusion Criteria:

• Factors significantly affecting oral drug absorption.
• Prior HDAC inhibitor or immune checkpoint inhibitor therapy.
• Known allergy to any study drug component.
• Other malignancy within past 5 years (except certain cured cancers).
• Participation in another clinical trial within 4 weeks.
• Immunodeficiency, HIV positivity, or organ transplant history.
• Uncontrolled cardiovascular disease or QTc >450 ms.
• Pregnancy, lactation, or unwillingness to use effective contraception.
• Other severe comorbid conditions deemed unsafe by investigator.
• History of neurological or psychiatric disorders.
• Any condition making the patient unsuitable per investigator judgment.