Study to Evaluate Safety, Pharmacokinetic and Pharmacodynamic Dose Escalation and Expansion Study of PXS-5505 in Patients With Primary, Post-polycythemia Vera or Post-essential Thrombocythemia Myelofibrosis

Inclusion Criteria: * Have a pathologically confirmed established diagnosis of primary myelofibrosis or post-essential thrombocythemia/polycythemia vera myelofibrosis as per the World Health Organization 2016 diagnostic criteria (must include at least Grade 2 marrow fibrosis) * Patients who are not eligible for stem cell transplantation * a) Dose escalation / Cohort expansion phase only: Patients not currently on ruxolitinib or fedratinib (where available) treatment due to ineligibility, or previously treated patients who have been discontinued for at least 2 weeks prior to first dose of study drug due to any of the following criteria: * Ineligible: Platelets \<50 x 10\^9/L * Intolerant: Development of red blood cell transfusion dependence of at least two units/month for 2 months OR ≥Grade 3 adverse events of thrombocytopenia, anemia, hematoma, and/or hemorrhage while on treatment with ruxolitinib or fedratinib for at least 28 days * Refractory: \< 10% spleen volume reduction by MRI or CT, or \< 30% decrease from baseline in spleen volume by palpation after at least 3 months treatment with ruxolitinib or fedratinib * Relapsed: Regrowth to \< 10% spleen volume reduction by MRI or CT, or \< 30% decrease from baseline in spleen volume by palpation, following an initial response to ruxolitinib or fedratinib and after at least 3 months treatment * b) Add-on phase only: Are being treated with ruxolitinib for at least 12 weeks prior to first administration of study treatme…