A Safety and Tolerability Study of Jaktinib

Inclusion Criteria: * Diagnosis of myelofibrosis (primary, post-polycythemia vera, or post-essential thrombocythemia). * Relapsed/refractory to a marketed (FDA approved) JAK inhibitor. * At least 18 years of age. * ECOG PS 0, 1, or 2. * Expected life expectancy is greater than 24 weeks. Exclusion Criteria: * Any chemotherapy, immunomodulatory therapy, immunosuppressive therapy, corticosteroids, or growth factor treatment within 14 days prior to initiation of study drug. * Major surgery or radiation therapy within 28 days prior to initiation of study drug. * With suspected allergies to jaktinib or its excipient. * Another clinical trial of a new drug or medical instrument within 3 months before screening. * Females who are pregnant, currently breastfeeding, planning to become pregnant. * Unable to adopt effective contraceptive methods during the study.