The goal of this clinical trial is to demonstrate GL-2045 safety and efficacy proof of concept by demonstrating intravenous immunoglobulin (IVig)-like platelet responses in adult patients with Immune Thrombocytopenia (ITP).

This is a Phase 1b, open-label, multiple subcutaneous (SC) injection dose study in participants with ITP. The specific aims of this study are to determine:

• The safety of the study drug when given to participants with ITP
• The effect of the study drug on safety blood tests and blood platelet counts
• How the study drug affects certain responses in the body such as severity of bleeding, levels of fatigue and health-related quality of life
• How much of the study drug gets into the bloodstream
• The body's immune response to the study drug

This information, together with pharmacokinetic (PK) data, will help to establish doses and dosing regimens suitable for use in future studies. The effects of GL-2045 on multiple biomarkers will also be investigated.

Key Inclusion Criteria:

1. Male or female between 18 and 80 years of age with ITP or certain limited ITP conditions.
2. Females must not be pregnant or lactating. Males and females of childbearing potential must agree to use contraception.
3. A platelet count of 10 to 49 × 10^9/L at baseline.
4. Study subjects must have prior response to corticosteroids or to intravenous immunoglobin (IVIg) documented by a consultant hematologist. Prior response to thrombopoietin receptor agonists is insufficient for eligibility.
5. The subject may be on a stable dose of corticosteroids for the 3 months prior to study entry, limited to a daily dose of prednisone 10 mg or equivalent. No reductions or increases of steroids are allowed during the study.

Key Exclusion Criteria:

1. Any serious adverse event (SAE) with prior IVIg dosing.
2. Prior splenectomy within 1 year of randomization or planned splenectomy during the study period.
3. Participants with grade 2 bleeding by World Health Organization (WHO) bleeding criteria in the 8 weeks prior to treatment.
4. Abnormal organ function including liver and kidney.
5. Any previous or current treatments prohibited by the protocol.
6. Treatment within the last 4 weeks or intention to treat during the study with a thrombopoietin receptor agonist including romiplostim (Nplate®), eltrombopag (Revolade®), or avatrombopag (Doptelet®).