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

January 27, 2026 updated by: Step Pharma, SAS

An Open-Label, Phase 1b Study to Evaluate Safety, Tolerability & Preliminary Activity of the CTPS1 Inhibitor STP938 in Adult Subjects With High Risk Essential Thrombocythaemia Who Are Resistant to or Intolerant of Hydroxycarbamide Therapy

The goal of this clinical trial is to learn if the drug STP938 works to treat adults with high risk essential thrombocythaemia (ET) who are resistant to, or intolerant of, hydroxycarbamide (also known as hydroxyurea) therapy. The trial will also learn about the safety of STP938. The main questions the trial aims to answer are:

  • Does STP938 control platelet counts
  • Does STP938 control platelet counts without inducing unwanted side effects

Participants will:

  • Take STP938 every day for up to 12 months.
  • Visit the clinic once every week for the first month, then every 2 weeks for checkups and tests.
  • Complete a questionnaire about symptoms once a month.

Study Overview

Status

Recruiting

Conditions

Intervention / Treatment

Detailed Description

The aim of the study is to assess a new drug called STP938 for the treatment of essential thrombocythaemia (ET). The study with assess how effective STP938 in treating ET, and also assess any side effects of taking the drug. The study will enrol individuals with high risk ET who require treatment to lower their platelet count. Individuals enrolling on the study will have already tried treatment with hydroxycarbamide (also known as hydroxyurea) but are in need of a different treatment as hydroxycarbamide either did not control the platelet count or produced unwanted side effects.

STP938 is a new class of drug that inhibits the enzyme cytidine triphosphate synthase 1 (CTPS1). Inhibition of CTPS1 is a novel way of lowering the platelet count. This study is a phase 1b, open-label, multicentre trial. Participants will receive STP938 capsules every day, in cycles of 28 days, for approximately 12 months. Participants may continue to receive study drug for a longer period, so long as it is controlling the platelet count and not causing side effects. During the study, participants will visit the study site about 26 times (2 times per cycle) over an estimated 12 months. Once the treatment is complete, safety follow-up visit(s) will occur to make sure the participant is not experiencing any adverse effects. The following study procedures will be performed: (a) physical examinations (b) ECGs (c) blood tests, (d) urine tests (e) CT/MRI scans (f) bone marrow biopsies (g) drug administration (h) study drug blood level tests and (i) gene testing.

Study Type

Interventional

Enrollment (Estimated)

20

Phase

  • Phase 1

Contacts and Locations

This section provides the contact details for those conducting the study, and information on where this study is being conducted.

Study Contact

Study Contact Backup

Study Locations

  • France
      • Brest, France
        • Recruiting
        • Chu Brest
      • Marseille, France
        • Recruiting
        • Institut Paoli-Calmettes
      • Nantes, France
        • Recruiting
        • Chu Nantes
      • Nice, France
        • Recruiting
        • CHU Nice
      • Nîmes, France
        • Recruiting
        • Chu Nimes
      • Paris, France
        • Recruiting
        • Hôpital Saint-Louis
      • Villejuif, France
        • Recruiting
        • Gustave Roussy
  • United Kingdom
      • Cardiff, United Kingdom
        • Recruiting
        • University Hospital of Wales
      • London, United Kingdom
        • Recruiting
        • Sarah Cannon Research Institute
      • London, United Kingdom
        • Recruiting
        • Imperial College London / Hammersmith Hospital
      • Oxford, United Kingdom, OX3 7LE
        • Recruiting
        • Cancer and Haematology Centre, Churchill Hospital
      • Southampton, United Kingdom
        • Recruiting
        • University of Southampton Hospital
    • South Yorkshire
      • Sheffield, South Yorkshire, United Kingdom, S10 2JF
        • Not yet recruiting
        • Royal Hallamshire Hospital
        • Contact:

Participation Criteria

Researchers look for people who fit a certain description, called eligibility criteria. Some examples of these criteria are a person's general health condition or prior treatments.

Eligibility Criteria

Ages Eligible for Study

  • Adult
  • Older Adult

Accepts Healthy Volunteers

No

Description

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).

Study Plan

This section provides details of the study plan, including how the study is designed and what the study is measuring.

How is the study designed?

Design Details

  • Primary Purpose: Treatment
  • Allocation: N/A
  • Interventional Model: Single Group Assignment
  • Masking: None (Open Label)

Arms and Interventions

Participant Group / Arm
Intervention / Treatment
Experimental: Phase 1b
Up to 5 dose levels with STP938 administered as oral therapy
Drug: STP938
At enrolment all patients will be assigned to a single dose level of STP938 for 4 weeks. After 4 weeks the dose level may be adjusted as needed by the Investigator.

What is the study measuring?

Primary Outcome Measures

Outcome Measure
Measure Description
Time Frame
Clinical Efficacy
Time Frame: Through study completion, an average of 12 months
Complete and partial response rates per European LeukemiaNet criteria
Through study completion, an average of 12 months
Safety and Tolerability
Time Frame: Through study completion, an average of 12 months
Toxicity profile based on National Cancer Institute Common Terminology Criteria for Adverse Events
Through study completion, an average of 12 months

Secondary Outcome Measures

Outcome Measure
Measure Description
Time Frame
Durability of Response to STP938
Time Frame: Through study completion, an average of 12 months
Time from achieving response to loss of response
Through study completion, an average of 12 months
Impact of STP938 on Disease-Related Complications
Time Frame: Through study completion, an average of 12 months
Prevalence of haemorrhagic complications, thromboembolic events and disease transformation.
Through study completion, an average of 12 months

Other Outcome Measures

Outcome Measure
Measure Description
Time Frame
Patient Reported Outcomes and Symptom Burden
Time Frame: Through study completion, an average of 12 months
Myeloproliferative Neoplasm Symptom Assessment Form (18 item). Minimum value is zero (0); the maximum value is 180. A score of zero equates to absence of symptoms, higher scores equate to worse outcome.
Through study completion, an average of 12 months
Molecular Response
Time Frame: Through study completion, an average of 12 months
Change in Variant Allele Fraction: For known ET-associated mutations.
Through study completion, an average of 12 months
Bone Marrow Histology
Time Frame: Screening and Cycle 12 (each cycle is 28 days)
Changes in Histological Appearances
Screening and Cycle 12 (each cycle is 28 days)
Spleen Volume
Time Frame: Through study completion, an average of 12 months
Change in Spleen Volume: Measured by MRI or CT scan.
Through study completion, an average of 12 months

Collaborators and Investigators

This is where you will find people and organizations involved with this study.

Sponsor

Step Pharma, SAS

Investigators

  • Study Director: Maureen Higgins, Step Pharma

Study record dates

These dates track the progress of study record and summary results submissions to ClinicalTrials.gov. Study records and reported results are reviewed by the National Library of Medicine (NLM) to make sure they meet specific quality control standards before being posted on the public website.

Study Major Dates

Study Start (Actual)

June 18, 2025

Primary Completion (Estimated)

May 31, 2026

Study Completion (Estimated)

December 31, 2027

Study Registration Dates

First Submitted

January 8, 2025

First Submitted That Met QC Criteria

January 14, 2025

First Posted (Actual)

January 22, 2025

Study Record Updates

Last Update Posted (Actual)

January 28, 2026

Last Update Submitted That Met QC Criteria

January 27, 2026

Last Verified

January 1, 2026

More Information

Terms related to this study

Keywords

Additional Relevant MeSH Terms

Other Study ID Numbers

  • STP938-301

Plan for Individual participant data (IPD)

Plan to Share Individual Participant Data (IPD)?

NO

Drug and device information, study documents

Studies a U.S. FDA-regulated drug product

No

Studies a U.S. FDA-regulated device product

No

product manufactured in and exported from the U.S.

No

This information was retrieved directly from the website clinicaltrials.gov without any changes. If you have any requests to change, remove or update your study details, please contact register@clinicaltrials.gov. As soon as a change is implemented on clinicaltrials.gov, this will be updated automatically on our website as well.

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