Efficacy and Safety of Gecacitinib Hydrochloride in Prophylaxis Acute Graft-Versus-Host Disease After Allogeneic Hematopoietic Stem Cell Transplantation in Patients With Myelofibrosis

Inclusion Criteria: * Voluntarily sign the informed consent form (ICF), and be aged ≥ 18 years at the time of signing. * Diagnosis of primary myelofibrosis (PMF) according to the 2016 WHO criteria (DIPSS intermediate-2/high risk, MPISS70 high/very high risk, or RR6 high risk), or post-ET/PV secondary myelofibrosis (MYSEC intermediate-2/high risk), with intention to undergo allogeneic hematopoietic stem cell transplantation (allo-HSCT). * Recipient with myelofibrosis scheduled to receive myeloablative conditioning allo-HSCT from a matched unrelated donor, matched sibling donor, or haploidentical donor. * Karnofsky Performance Scale score ≥ 60. * Eastern Cooperative Oncology Group (ECOG) performance status score 0-2. * Male participants must agree to abstinence or use barrier contraception throughout the study. * Female participants of childbearing potential must have a negative pregnancy test and agree to use two effective methods of contraception throughout the study. * Ability to swallow tablets. * Ability to comply with study and follow-up procedures. Exclusion Criteria: * Prior treatment with chimeric antigen receptor T-cell (CAR-T) therapy. * Hepatic insufficiency defined as bilirubin ≥ 2 × upper limit of normal (ULN), except for patients with a history of Gilbert Syndrome. * Renal impairment defined as serum creatinine \> 2 mg/dL. * Cardiac dysfunction defined as left ventricular ejection fraction (LVEF) ≤ 45%. * Pulmonary dysfunction defined as forced expiratory volume in the first second (FEV1) or diffusing capacity for carbon monoxide (DLCO) (corrected for hemoglobin) ≤ 50% of predicted value. * Chronic or active infection requiring systemic therapy during the peri-transplant and post-transplant period. * Diagnosis of another active malignancy within the past 12 months, except for adequately treated non-melanoma skin cancer, adequately treated melanoma of grade 2 or below, or cervical intraepithelial neoplasia. Active malignancy is defined as malignancy undergoing active treatment. * Any significant clinical or laboratory abnormality that may compromise safety evaluation, such as: 1. Uncontrolled diabetes mellitus (fasting blood glucose \> 13.9 mmol/L); 2. Hypertension uncontrolled by two or more antihypertensive agents (systolic blood pressure ≥ 160 mmHg, diastolic blood pressure ≥ 100 mmHg); 3. Peripheral neuropathy of grade ≥ 2 per NCI-CTCAE v5.0. * Pre-transplant diagnosis of gastrointestinal impairment or disease that may affect drug absorption, including ulcerative disease, uncontrolled nausea, vomiting, diarrhea, or history of gastrectomy or intestinal resection. * Cholestatic disease or sinusoidal obstruction syndrome / hepatic veno-occlusive disease at screening (defined as persistent bilirubin abnormality and progressive organ dysfunction not due to graft-versus-host disease). * Active and uncontrolled viral infection at screening, including CMV, EBV, HIV (positive anti-HIV antibody), HBV (positive HBsAg or positive HBV-DNA), HCV (positive anti-HCV antibody or positive HCV-RNA). * Active tuberculosis within 6 months prior to screening. * History of epilepsy or use of psychotropic or sedative medications at screening. * Female patients who are pregnant, breastfeeding, or planning pregnancy, or who cannot use effective contraception throughout the study; male patients who do not use condoms during dosing and for 2 days (approximately 5 half-lives) after the last dose. * History of malignancy other than the transplanted tumor within the previous 5 years. * Use of anticoagulant or antiplatelet agents (except low-molecular-weight heparin). * Presence of other severe diseases that, in the investigator's opinion, may affect patient safety or compliance. * Suspected hypersensitivity to geltrectinib hydrochloride, its analogs, or any of its excipients. * Participation in another clinical trial of an investigational drug or medical device within 4 weeks prior to screening. * Any other condition that, in the investigator's judgment, renders the patient ineligible for the study.