This is a 2-part, prospective, open-label, single arm, multicenter study to evaluate the safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PDx), and efficacy of leniolisib in at least 15 pediatric patients (aged 1 to 6 years) with activated phosphoinositide 3-kinase delta (PI3Kδ) syndrome (APDS)

Part I will consist of a 12-week period to assess the safety and efficacy of treatment with leniolisib. Part II will consist of a 1-year, long-term, safety follow-up extension with a possible interim analysis.

The leniolisib doses to be used in study were selected based on safety, tolerability, PK, and PDx data from the adult Phase 2/3 study, as well as PK modeling data. In both parts of the study, leniolisib will be administered orally based on weight.

Inclusion Criteria:

1. Patient is male or female and between the age of 1 to 6 years old at time of the first study procedure.

2. Patient weighs ≥8 and ≤37 kg at baseline.

3. Patient has a confirmed PI3Kδ genetic mutation of either the PIK3CD (APDS1) or PIK3R1 (APDS2) gene.

4. Patient has at least 1 measurable nodal lesion on MRI or low-dose CT within 6 months of screening.

5. Patient has nodal or extranodal lymphoproliferation and clinical findings consistent with APDS (eg, a history of repeated oto-sino-pulmonary infections or organ dysfunction consistent with APDS).

6. Patient has the ability to ingest unaltered study-related medications without difficulty in the investigator's opinion.

7. At screening, vital signs (body temperature, systolic BP, diastolic BP, and pulse rate [PR]) will be assessed in the sitting position after the patient has been at rest for at least 3 minutes. Patient's sitting vital signs should be within the following ranges:

   1. Systolic BP: Less than the 95th percentile adjusted for sex, age, and height percentile. See Section 10.5, Appendix 5 to determine BP percentiles adjusted for sex, age, and height percentile. (National High Blood Pressure Education Program Working Group on High Blood Pressure in Children and Adolescents, 2004). See Section 10.5, Appendix 5 to determine height percentiles.
   2. Diastolic BP: Less than the 95th percentile adjusted for sex, age, and height percentile. See Section 10.5, Appendix 5 to determine BP percentiles adjusted for sex, age, and height percentile. See Section 10.6, Appendix 6 to determine height percentiles.
   3. Pulse rate (Fleming 2011):

   i. Age <2 years: 100 to 190 bpm ii. Age 2 to 6 years: 60 to 140 bpm

8. Institutional review board- or IEC-approved written informed consent or assent and privacy language as per national and local regulations must be obtained from the patient and/or parent or legal guardian prior to any study-related procedures.

9. Patient parent or legal guardian is willing and able to complete the informed consent or assent process and comply with study procedures and visit schedule.

10. Patient parent or legal guardian agrees patient will not participate in any other interventional study while enrolled in this study.

Exclusion Criteria:

1. Patient has previous or concurrent use of immunosuppressive medication such as:

   1. an mTOR inhibitor (eg, sirolimus, rapamycin, everolimus) or a PI3Kδ inhibitor (selective or non-selective PI3K inhibitors) within 6 weeks prior to first dose.

      o Short-term use for up to a total of 5 days is allowed but only up to 1 month prior to enrollment in the study.

   2. B cell depleters (eg, rituximab) within 6 months prior to first dose of study medication.

      o If patient has received prior treatment with a B cell depleter, absolute B lymphocyte counts in the blood must have regained normal values.

   3. Belimumab or cyclophosphamide within 6 months prior to first dose of study medication.

   4. Cyclosporine A, mycophenolate, 6-mercaptopurine, azathioprine, or methotrexate within 3 months prior to first dose of study medication.

   5. Glucocorticoids above a dose equivalent to either ≥2 mg/kg of body weight for weights less than 10 kg or ≥20 mg/day for weights ≥ 10 kg of prednisone or prednisolone or equivalent within 2 weeks prior to first dose of study medication.

   6. Other immunosuppressive medication where effects are expected to persist at start of dosing of study medication.

2. Patient has a history or current diagnosis of ECG abnormalities indicating significant risk of safety for patients participating in the study such as:

   1. History of familial long QT syndrome or known family history of Torsades de Pointes.
   2. Concomitant clinically significant cardiac arrhythmias, eg, sustained ventricular tachycardia, and clinically significant second or third degree atrioventricular block without a pacemaker.
   3. Resting QTc (Fridericia preferred, but Bazett acceptable) >460 msec if the measurement is confirmed with an additional ECG repeated as soon as possible.
   4. Concomitant use of agents known to prolong the QT interval unless it can be permanently discontinued for the duration of the study.

3. Patient is currently using a medication known to be strong inhibitor or moderate or strong inducer of isoenzyme CYP3A (see Table 2), if treatment cannot be discontinued or switched to a different medication prior to starting study treatment.

4. Patient is currently using medications that are metabolized by isoenzyme CYP1A2 and have a narrow therapeutic index (NTI) (drugs whose exposure response indicates that increases in their exposure levels by the concomitant use of potent inhibitors may lead to serious safety concerns [eg, Torsades de Pointes]).

5. Patient had been administered live vaccines (this includes any attenuated live vaccines) starting from 6 weeks before the anticipated first study drug administration, during the study, and up to 7 days after the last dose of leniolisib.

6. Patient has clinically significant abnormalities in hematology or clinical chemistry (blood chemistry or urinalysis) parameters as determined by the investigator or medical monitor.

7. Patient has liver disease or liver injury as indicated by clinically significant abnormal liver function tests (LFTs) (alanine aminotransferase and aspartate aminotransferase >2.5 times upper limit of normal), history of renal injury or renal disease (eg, renal trauma, glomerulonephritis, or one kidney only), or presence of impaired renal function as indicated by a serum creatinine level >1.5 mg/dL (133 μmol/L).

8. Patient has moderate or severe hepatic impairment (Child-Pugh Class B or C).

9. Patient is receiving concurrent treatment with another investigational therapy or use of another investigational therapy less than 4 weeks from the first study procedure.

10. Patient has active hepatitis B (eg, hepatitis B surface antigen reactive) or active hepatitis C (eg, hepatitis C virus RNA [qualitative] is detected) at screening.

11. Patient has human immunodeficiency virus (HIV) infection (HIV 1 or 2) at screening.

12. Patient has a positive COVID-19 result (polymerase chain reaction or antigen) within 1 week prior to first dose. The patient can be rescreened after a subsequent negative result.

13. Patient has a history of malignancy (except lymphoma) within 3 years before the first study procedure or has evidence of residual disease from a previously diagnosed malignancy.

14. Patient has a previous diagnosis of lymphoma that has been treated with chemotherapy, radiotherapy, or transplant within 1 year of the first study procedure or is anticipated to require lymphoma treatment within 6 months of the first study procedure.

15. Patient has a history of uncontrolled diabetes mellitus within 3 months of the first study procedure.

16. Patient has had major surgery requiring hospitalization or radiotherapy within 4 weeks prior to the first study procedure.

17. Patient has uncontrolled chronic or recurrent infectious disease (with the exception of those that are considered to be characteristic of APDS) or evidence of tuberculosis infection as defined by a positive QuantiFERON Gold test at screening. If presence of latent tuberculosis is established, then treatment according to local country guidelines must have been completed before patients can be considered for enrollment.

18. Patient has a known allergy or hypersensitivity to study defined medications or their excipients.

19. Patient has a planned or expected major surgical procedure.

20. Patient or parent or legal guardian is unable or unwilling to comply with study procedures or is unable to travel for repeat visits.

21. Patient or parent or legal guardian is unwilling to keep study results or observations confidential or to refrain from posting confidential study results or observations on social media sites.

22. Patient or parent or legal guardian refuses to sign consent or assent form.

23. Patient has other underlying medical condition that, in the opinion of the investigator, would impair the ability of the patient to receive or tolerate the planned procedures or follow-up.

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