The main objective is to assess the safety and tolerability of budoprutug in adults with ITP. Pharmacokinetics, pharmacodynamics, and preliminary clinical efficacy will also be assessed.

Budoprutug is a humanized, immunoglobulin (Ig) G1 monoclonal antibody that selectively binds to CD19 and is projected to deplete targeted cells through antibody-dependent cellular cytotoxicity. This Phase 1b/2a, open-label, sequential-cohort, dose escalation and expansion study will evaluate the safety, tolerability, PK, PD, and preliminary clinical effectiveness of budoprutug in subjects with ITP. Budoprutug will be administered as two (2) IV infusions 14 days apart in ascending dose cohorts of patients aged 18 years and above with a platelet count < 30,000/µL despite an adequate trial of at least one prior therapeutic attempt.

Inclusion Criteria:

1. Aged 18 years at the time of consent.
2. Platelet count < 30,000/µL despite an adequate trial of at least one prior therapeutic attempt. Platelet counts of < 30,000/µL must be confirmed on 2 occasions at least 5 days apart, but no more than 14 days apart.
3. Partial thromboplastin time < 1.5 x upper limit of normal (ULN), prothrombin time < 1.5 x ULN, total bilirubin < 1.5 x ULN unless due to Gilbert's syndrome, or an international normalized ratio < 1.5 at screening.

Exclusion Criteria:

1. CD19+ B cell count < 80 cells/µL at Screening, or < 40 cells/µL if B-cell depleting therapy was received within 24 weeks to 2 years prior.
2. Diagnosis of paroxysmal nocturnal hemoglobinuria, Evan's Syndrome, or other bleeding disorders affecting safety or data integrity.
3. Prior B-cell depleting therapy (e.g., rituximab) within 24 weeks before first dose or planned during the study.
4. Chronic use of anticoagulants or antiplatelet agents (e.g., aspirin, NSAIDs, thienopyridines) within 14 days before dosing through follow-up. Intermittent NSAID use is allowed.
5. Immunosuppressants (excluding corticosteroids) within 30 days or 5× half-life before Screening; alkylating agents within 180 days.
6. IVIg treatment within 90 days prior to Screening.
7. Active ITP treatment (other than steroids or TPO agonists) within 30 days or 5× half-life before first dose, unless approved by Medical Monitor.
8. Active, chronic, or latent infections including hepatitis B/C or HIV.
9. Active TB or high TB risk.