A Phase 2 Study to Assess STP938 (Dencatistat) as a Monotherapy in Adults With High Risk Essential Thrombocythaemia - The VECTRA Trial

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).