Phase 1/ Phase 2 Study to Assess Safety and Efficacy of Orally Administered JBI-802 in Subjects With Myeloproliferative Neoplasms (MPN) and Myelodysplastic/Myeloproliferative Neoplasms (MDS/MPN) With Thrombocytosis

Inclusion Criteria: \- Male or female subjects aged ≥18 years at the time of screening visit. For Dose Escalation Phase: Subjects diagnosed with any one of the following: * Subject with diagnosis of Essential Thrombocythemia (ET) per World Health Organization (WHO) diagnostic criteria for myeloproliferative neoplasms . * Subject requires treatment in order to lower platelet count based on subject age over 60 or history of thrombosis. * Subject with Morphologically confirmed diagnosis of MDS/MPN neoplasms, excluding Juvenile Myelomonocytic Leukaemia (JMML), CMML and aCML (Atypical Chronic Myeloid Leukaemia), in accordance with WHO 2016 revised criteria, that is relapsed and/or refractory or intolerant to standard of care and that, in the opinion of the Investigator, subjects who have no available therapies known to provide clinical benefits. * Subject with Myelodysplastic/myeloproliferative neoplasm, (unclassifiable (MDS/MPN-UC) and MDS/MPN-RS-T). For Dose Expansion Phase Subjects diagnosed with any one of the following: * Subject with diagnosis of Essential Thrombocythemia (ET) per WHO diagnostic criteria for myeloproliferative neoplasms which requires treatment in order to lower platelet count based on subject age over 60 or history of thrombosis. * Subject with diagnosis of Polycythemia Vera (PV) per WHO diagnostic criteria that is relapsed and/or refractory or intolerant to standard of care and that, in the opinion of the Investigator, subjects who have no available therapies known to provide clinical benefits Subject with morphologically confirmed diagnosis of pre-fibrotic myelofibrosis (MF) subject in accordance with the WHO 2016 revised criteria, that is relapsed, intolerant, and/or refractory and that, in the opinion of the Investigator, subjects who have no available therapies known to provide clinical benefits. (Refer Appendix II) * MDS/MPN (MDS/MPN-RS-T, MDS/MPN unclassifiable and CMML subjects providing the marrow blast count is ≤5%.). 2. Subject must have disease that failed at least one standard therapy or being intolerant to standard of care. 3. Subject must have discontinued immediate prior therapy at least 1 week (4 weeks for interferon) prior to study drug administration. 4. Subject with screening laboratory values: * Hb ≥ 9 g/dL, if subject is transfused to meet this criterion, transfusion must be completed ≥ 14 days prior to first dose. * Absolute neutrophil count ≥ 1500 × 109/L * Absolute neutrophil count ≥ 1000 × 109/L, if significant marrow infiltration * Platelet count ≥ 450 × 109/L for dose finding * Platelet count ≥ 100 × 109/L for expansion cohort at RP2D, if subject is transfused to meet this criterion, transfusion must be completed ≥14 days prior to first dose * Total bilirubin ≤ 1.5 × ULN. Subjects with Gilbert's syndrome may be enrolled with up to 3.0 × ULN * Aspartate transaminase (AST) and Alanine transaminase (ALT) ≤ 2.5 × ULN (unless liver metastases are present then up to 5 × ULN is allowed) * Calculated creatinine clearance (CrCL) ≥ 30 mL/min (Cockcroft- Gault formula) (Refer Appendix IV) * Prothrombin Time (PT) or Activated Partial Thromboplastin Time (aPTT) ≤ 1.5 × ULN, if subject is not anticoagulated (Note: If subject is on anticoagulants, the subject must be on a stable dose for at least 2 weeks prior to screening) 5. Subject with resolution of any clinically significant toxic effects of prior therapy to Grade 0 or 1 according to the NCI CTCAE, Version 5.0 (exception of alopecia and Grade 2 peripheral neuropathy, chronic Grade 2 endocrinopathies as a result of prior immunotherapy). (Refer Appendix VII) 6. Subject with Eastern Cooperative Oncology Group (ECOG) performance status of ≤ 2. (Refer Appendix III) 7. Subject able to swallow oral medication. 8. Subject who has willing and able to give informed consent and comply with protocol requirements for the duration of the study. 9. Subject who is willing to undergo bone marrow biopsy with aspiration and tissue collection for disease assessment and correlative studies during screening and periodically throughout the study. 10. Subject with willingness to use contraception by a method that is deemed effective by the Investigator by both males and female of childbearing potential (post-menopausal women must have been amenorrhoeal for at least 12 months to be considered of non-childbearing potential (i.e., surgically sterilised \[hysterectomy, bilateral salpingectomy, bilateral oophorectomy at least 6 weeks before the screening visit\] or postmenopausal \[where postmenopausal is defined as no menses for 12 months without an alternative medical cause and a follicle-stimulating hormone \[FSH\] level consistent with postmenopausal status, per local laboratory guidelines\]) and their partners throughout the treatment period and for at least 3 months following the last dose of study drug. Exclusion Criteria: * 1\. Subject who is treated with systemic anticancer therapy or biological therapy or an investigational agent within 2 weeks or 5 half-lives, whichever is shorter, prior to start of study drug treatment. * For MF subject who come off JAK2 antagonists or hydroxyurea, shorter washout is permitted as these subject progress quickly after treatment discontinuation and remain eligible (steroids must be stop at least 7 day before start of study drug treatment) * Subject who is in need of immediate cytoreduction should be excluded 2. Subject who has undergone autologous/allogeneic Haematopoietic Stem Cell Transplantation (HSCT) therapy within 60 days of the first dose of study drug, or subject on immunosuppressive therapy post-HSCT at the time of screening, or currently with clinically significant Graft-Versus- Host Disease (GVHD) as per treating physician (subjects in relapse after allogeneic transplantation must be off treatment with systemic immunosuppressive agents for at least 4 weeks prior to screening. 3\. Subject with major surgery less than or equal to 21 days prior to starting stud