Study to Evaluate Rilzabrutinib in Adults and Adolescents With Persistent or Chronic Immune Thrombocytopenia (ITP)

Inclusion Criteria: 1. Male and female with primary ITP with duration of \>6 months in pediatric participants aged 12 to \<18 years (pediatric participants aged 10 to \<12 years will be enrolled in the EU \[EEA countries\] only) and duration of \>3 months in ages 18 years and above 2. Patients who had a response (achievement of platelet count ≥50,000/µL) to IVIg/anti-D or CSs that was not sustained and who have documented intolerance, insufficient response or any contra-indication to any appropriate courses of standard of care ITP therapy 3. An average of 2 platelet counts at least 5 days apart of \<30,000/µL during the Screening period and no single platelet count \>35,000/µL, within 14 days prior to the first dose of study drug. \- Pediatric patients must additionally be determined to need treatment for ITP as per clinical assessment by the Investigator. 4. Adequate hematologic, hepatic, and renal function (absolute neutrophil count ≥1.5 X 10\^9/L, AST/ALT ≤1.5 x upper limit of normal \[ULN\], albumin ≥3 g/dL, total bilirubin ≤1.5 x ULN \[unless the patient has documented Gilbert syndrome\], glomerular filtration rate \>50 \[Cockcroft and Gault method for adult and Bedside Schwartz Equation for Pediatric participants\]) 5. Hemoglobin \>9 g/dL within 1 week prior to Study Day 1 6. All contraceptive use by men and women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies 7. Patients must be able to provide written informed consent or informed assent with corresponding informed consent obtained from the patient's guardian and agree to the schedule of assessments Exclusion Criteria: 1. Patients with secondary ITP 2. Pregnant or lactating women 3. History (within 5 years of Study Day 1) or current, active malignancy requiring or likely to require chemotherapeutic or surgical treatment during the study, with the exception of non melanoma skin cancer 4. Transfusion with blood, blood products, plasmapheresis, or use of any other rescue medications with intent to increase platelet count within 14 days before Study Day 1 5. Change in CS and/or TPO-RA dose within 14 days prior to Study Day 1 (more than 10% variation from current doses) 6. Immunosuppressant drugs other than CSs within 5 times the elimination half-life of the drug or 14 days of Study Day 1, whichever is longer 7. Treatment with rituximab or splenectomy within the 3 months prior to Study Day 1 \- Patients treated with rituximab will have normal B-cell counts prior to enrollment 8. Had received any investigational drug within the 30 days before receiving the first dose of study medication, or at least 5 times elimination half-life of the drug (whichever is longer); patient should not be using an investigational device at the time of dosing * Patients who previously received treatment with Bruton's Tyrosine Kinase (BTK) inhibitors (except rilzabrutinib) within 30 days before the first dose of study drug are not eligible * Patients who previously received rilzabrutinib at any time are not eligible 9. History of solid organ transplant 10. Myelodysplastic syndrome 11. Live vaccine within 28 days prior to Study Day 1 or plan to receive one during the study 12. Planned surgery in the time frame of the dosing period