A Phase 1 Study to Assess STP938 as a Monotherapy in Adults With High Risk Essential Thrombocythaemia

Key Inclusion Criteria:

• Adults aged 18 years or older

• A confirmed diagnosis of ET according to World Health Organisation (WHO) or BritishSociety 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 thecriteria 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 ofessential thrombocythaemia (wash out periods apply).

• Subjects who have received any investigational agents for the treatment of essentialthrombocythaemia within 4 weeks before enrolment or less than 5 half-lives sincecompletion 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 studymedication (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 notwilling to comply with contraceptive requirements.

• Known hypersensitivity to any of the excipients used in the formulation of the studydrug.

• Corrected QT interval >470 msec based on averaged triplicate electrocardiogram (ECG)readings at the Screening Visit using the QT interval corrected for heart rate usingFridericia's method (QTcF).