Inclusion Criteria:
1. Aged 18-75 years, regardless of gender;
2. Diagnosis of primary myelofibrosis (PMF), post-polycythemia vera myelofibrosis (post-PV-MF), or post-essential thrombocythemia myelofibrosis (post-ET-MF) according to the 2022 WHO diagnostic criteria;
3. Meeting the criteria for intermediate-risk or high-risk groups per the DIPSS-plus classification;
4. Scheduled to undergo allogeneic hematopoietic stem cell transplantation (allo-HSCT), including transplants from HLA-matched or mismatched related or unrelated donors;
5. ECOG performance status ≤2 and Karnofsky performance status ≥60%;
6. Capable of understanding and signing the informed consent form, and able to comply with study and follow-up procedures.
Exclusion Criteria:
1. Patients using other JAK inhibitors (except for Gecacitinib) at the time of screening may be enrolled if they switch to Gecacitinib treatment prior to screening.
2. Patients who have previously undergone allogeneic hematopoietic stem cell transplantation or organ transplantation.
3. Disease progression to accelerated or blast phase (peripheral blood or bone marrow blast percentage ≥10% at any time prior to transplantation).
4. Presence of significant medical conditions or marked organ dysfunction that cannot be adequately controlled and may affect the completion of this study:
1. Congestive heart failure classified as New York Heart Association (NYHA) Class III-IV, or documented history of diastolic or systolic dysfunction (e.g., LVEF \<40% measured by echocardiography), or uncontrolled or unstable angina or myocardial infarction.
2. Uncontrolled diabetes (\>250 mg/dL or \>13.9 mmol/L).
3. Hypertension that cannot be reduced to the following range despite combination antihypertensive therapy (systolic blood pressure \<160 mmHg, diastolic blood pressure \<100 mmHg).
4. Peripheral neuropathy (≥ Grade 2 per NCI-CTC AE v5.0 criteria).
5. Serum creatinine \>1.5 × ULN.
6. ALT or AST \>2.5 × ULN, or DBIL or TBIL \>2.0 × ULN.
5. Patients with any bacterial, viral, or fungal infection not adequately controlled.
6. HIV-positive at screening, or active hepatitis B virus infection (HBsAg-positive with HBV-DNA positivity or above the normal reference range), or HCV antibody-positive with HCV-RNA positivity.
7. History of tuberculosis or positive interferon-gamma release assay at screening.
8. Suspected hypersensitivity to Gecacitinib Hydrochloride, drugs of the same class, or any of their excipients.
9. Pregnant or breastfeeding women, or patients unwilling to use effective contraception during Gecacitinib treatment and for one week after the last dose.
10. Patients with any other comorbidities that may interfere with the study or a history of prior malignancies.
11. Patients unable to take oral tablets.
