The purpose of this study is to evaluate the safety and efficacy of narsoplimab in pediatric patients with thrombotic microangiopathies (TMA) following hematopoietic stem cell transplant (HSCT).

This is a Phase 2, uncontrolled, single-dosing regimen study in pediatric patients from 28 days to less than 18 years of age with high risk HSCT-TMA. At least 4 patients will be required from each of 3 age cohorts:

28 days to <2 years of age, 2 years to <12 years of age, and 12 years to <18 years of age.

Treatment will be for 8 weeks and patients will be followed for up to 52 weeks.

Inclusion Criteria:

1. Age at least 28 days and less than 18 years prior to informed consent (Visit 0).

2. Have informed consent from at least one parent or legal guardian as required by local law and regulation. Patient informed consent will be required if the patient has reached the local legal age of majority.

3. Assent from patients as required by local law and regulation.

4. Have received an allogeneic hematopoietic stem cell transplant for the treatment of benign or malignant disease.

5. Have a diagnosis of HSCT-TMA defined as meeting both of the following criteria:

   • Platelet count < 50,000/mL or a decrease in platelet count > 50% from the highest value obtained following transplant.
   • Evidence of microangiopathic hemolysis (presence of schistocytes, serum lactate dehydrogenase [LDH] > upper limit of normal ([ULN], or haptoglobin < lower limit of normal [LLN])

6. Have at least one of the following HSCT-TMA high-risk criteria:

   • HSCT-TMA persistence > 2 weeks following modification of calcineurin inhibitors or sirolimus OR

   • Have evidence of high-risk HSCT-TMA defined as at least one of the following:

     • Spot protein/creatinine ratio > 2 mg/mg
     • Serum creatinine > 1.5 x the creatinine level prior to TMA development
     • Biopsy-proven gastrointestinal TMA
     • TMA-related neurological abnormality
     • Pericardial or pleural effusion without alternative explanation
     • Pulmonary hypertension without alternative explanation
     • Have Grade III or Grade IV graft-versus-host disease (GVHD) or, in the opinion of the Investigator, risk for development of Grade III or Grade IV GVHD if immunosuppression were to be modified
     • Have elevated serum C5b-9 (> 244 ng/mL)

7. If sexually active and of childbearing potential (for female pediatric patients, defined as starting at onset of menses), must agree to practice a highly effective method of birth control throughout study drug treatment and for at least 12 weeks after the last dose of study drug, such method of birth control defined as one that results in a low failure rate (i.e., less than 1% per year) when used consistently and correctly, such as implants, injectables, combined oral contraceptives, some intrauterine devices, sexual abstinence (abstinence is acceptable when it is in line with the patient's preferred and usual lifestyle and is defined as complete abstinence of sexual intercourse, not periodic abstinence or withdrawal), or vasectomized partner.

8. Male patients must be willing to avoid fathering children for at least 12 weeks following the last dose of study medication.

Exclusion Criteria:

1. All treatments for HSCT-TMA are allowed except eculizumab, ravulizumab, and defibrotide within 3 months prior to informed consent, unless failure of therapy can be documented.

   a. Patients may not be on eculizumab, ravulizumab, or defibrotide for any indication at screening.

2. Have Shiga toxin-producing Escherichia coli haemolytic uraemic syndrome (STEC-HUS). Test results obtained within 28 days prior to informed consent may be used.

3. Have ADAMTS13 activity < 10%. Test results obtained within 28 days prior to informed consent may be used.

4. Have a severe, uncontrolled systemic bacterial or fungal infection requiring antimicrobial therapy, or a severe uncontrolled viral infection (as determined by the investigator); prophylactic antimicrobial therapy administered as standard of care is allowed.

5. Have malignant hypertension (blood pressure [BP] > 99th percentile plus 5 mmHg with bilateral hemorrhages or "cotton-wool" exudates on fundoscopic examination).

6. Due to conditions other than HSCT-TMA, have a poor prognosis with a life expectancy of less than 3 months in the opinion of the Investigator.

7. If pregnant or lactating.

8. Have received treatment with an investigational drug or device within 4 weeks of entering study.

9. Have abnormal liver function tests defined as alanine aminotransferase (ALT) or aspartate aminotransferase (AST) > 5 times ULN within 28 days prior to informed consent.

10. Have a positive test by antigen or polymerase chain reaction (PCR) for human immunodeficiency virus (HIV), if negative within 28 days prior to informed consent, the test does not need to be repeated.

11. Patient or one or more of the patient's parents or legal guardians are is an employee or an immediate family member of Omeros, the Clinical Research Organization (CRO), an Investigator, or a study staff member.

12. Have a known hypersensitivity to any constituent of the product.

13. Presence of any condition that the Investigator believes would put the patient at risk.