Selinexor Monotherapy for Cytoreduction in BCR::ABL1-Negative Myeloproliferative Neoplasms

Inclusion Criteria: * Aged 18 to 80 years at the time of informed consent. * Diagnosis of Philadelphia chromosome-negative myeloproliferative neoplasm, including polycythemia vera, essential thrombocythemia, or primary myelofibrosis, according to World Health Organization criteria. * Presence of an indication for cytoreductive therapy, including at least one of the following: * Extreme thrombocytosis, defined as platelet count \>1500 x 10\^9/L. * Progressive leukocytosis, defined as white blood cell count \>25 x 10\^9/L. * Symptomatic splenomegaly documented by imaging. * Severe disease-related symptoms, such as significant weight loss within the past 6 months. * Unwillingness or inability to tolerate standard cytoreductive therapies, such as hydroxyurea or interferon. * Eastern Cooperative Oncology Group performance status of 0 to 2. * Adequate organ function, including renal, hepatic, cardiac, and coagulation function, as determined by laboratory tests and clinical evaluation in the opinion of the investigator. * Adequate baseline hematologic function without recent transfusion or growth factor support. * Ability to comply with study procedures, visits, and assessments. * Ability to understand and willingness to sign a written informed consent form. * For women of childbearing potential, a negative pregnancy test before study entry. * For participants of reproductive potential, agreement to use effective contraception during the study and for at least 120 days after the last dose of study drug. Exclusion Criteria: * Diagnosis of Philadelphia chromosome-positive myeloproliferative neoplasm or chronic myeloid leukemia. * Concurrent acute leukemia or other active hematologic malignancy. * Severe or uncontrolled comorbid condition, including but not limited to significant cardiac or pulmonary disease, decompensated liver disease, or end-stage renal disease. * Active uncontrolled bacterial, viral, or fungal infection. * Active gastrointestinal disorder associated with significant bleeding or impaired drug absorption. * Significant neurologic or psychiatric disorder that may interfere with study participation or compliance. * Known hypersensitivity to selinexor or any of its components. * Receipt of another investigational agent or participation in another interventional clinical trial within 4 weeks before enrollment. * Recent receipt of cytotoxic chemotherapy, radiotherapy, immunotherapy, or other cytoreductive treatment before study entry. * History of substance abuse, alcohol dependence, or illicit drug use that may interfere with adherence to study requirements. * Another active malignancy requiring systemic treatment. * Clinically significant bleeding disorder, severe coagulopathy, or requirement for long-term therapeutic anticoagulation. * Receipt of a live vaccine within 30 days before the first dose of study drug, or planned receipt of a live vaccine during the study. * Known active infection with human immunodeficiency virus or active viral hepatitis. * Estimated life expectancy of less than 12 weeks. * Inability to provide informed consent due to cognitive impairment or severe psychiatric illness. * Any condition that, in the investigator's judgment, would make the participant unsuitable for participation in this study.