Key Inclusion Criteria:

• Adults aged 18 years or older
• A confirmed diagnosis of ET according to World Health Organisation (WHO) or British Society for Haematology (BSH) criteria.
• Meeting criteria for high risk ET.
• Raised platelet count (>600 x109/L) requiring cytoreductive therapy.
• Neutrophil count >1.0 x109/L.
• Either intolerant of or resistant to hydroxycarbamide having met any one of the criteria at any point in their disease whilst receiving hydroxycarbamide.
• Adequate hepatic function.
• Adequate blood coagulation parameters.
• Eastern Cooperative Oncology Group (ECOG) status of 0-2.

Key Exclusion Criteria:

• Subjects currently or recently receiving cytoreductive therapy for the treatment of essential thrombocythaemia (wash out periods apply).
• Subjects who have received any investigational agents for the treatment of essential thrombocythaemia within 4 weeks before enrolment or less than 5 half-lives since completion of prior therapy, whichever is shorter.
• Proven or suspected transformation to polycythaemia vera, myelofibrosis, myelodysplasia, acute myeloid leukaemia or another myeloid neoplasm.
• Known malabsorption syndrome or other condition that may impair absorption of study medication (e.g. gastrectomy).
• Previous splenectomy.
• Any uncontrolled intercurrent illness.
• Immunocompromised subjects with increased risk of opportunistic infections.
• Known active or chronic hepatitis B or active hepatitis C virus (HCV) infection.
• Pregnant females, breastfeeding females, and women of child bearing age or males not willing to comply with contraceptive requirements.
• Known hypersensitivity to any of the excipients used in the formulation of the study drug.
• Corrected QT interval >470 msec based on averaged triplicate electrocardiogram (ECG) readings at the Screening Visit using the QT interval corrected for heart rate using Fridericia's method (QTcF).