A Prospective, Single-Arm, Single-Center Phase II Study Evaluating the Efficacy and Safety of Chidamide Combined With PD-L1 Inhibitor, Carboplatin, and Etoposide as First-Line Treatment in Patients With Extensive-Stage Small-Cell Lung Cancer (ES-SCLC)

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.