Effective ‘ Activated PI 3 K δ Syndrome ’-targeted therapy with the PI 3 K δ inhibitor leniolisib Running head : Effective therapy of APDS by PI 3 K δ inhibitor leniolisib

Pathogenic gain-of-function variants in the genes encoding PI3Kδ (phosphoinositide 3-kinase delta) lead to accumulation of transitional B cells and senescent T cells, lymphadenopathy and immune-deficiency (Activated PI3Kδ Syndrome, APDS). Knowing the genetic etiology of APDS afforded us the opportunity to explore PI3Kδ inhibition as a precision-medicine therapy. Here, we report in vitro and in vivo effects of inhibiting PI3Kδ in APDS. Treatment with leniolisib (CDZ173), a selective PI3Kδ inhibitor, caused a dose-dependent suppression of PI3Kδ pathway hyperactivation (measured as phosphorylation of AKT/S6) in cell lines ectopically expressing APDScausative p110δ variants and in T cell blasts derived from patients. A clinical trial with six APDS patients was conducted as a 12-week, open-label, multi-site, within-subject dose-escalation study of oral leniolisib to assess safety, pharmacokinetics and effects on lymphoproliferation and immune dysregulation. Oral leniolisib led to a dosedependent reduction in PI3K/AKT pathway activity assessed ex vivo and improved immune dysregulation. We observed normalization of circulating transitional and naïve B cells, reduction in PD-1+CD4+ and senescent CD57+CD4T cells, and decreases in elevated serum IgM and inflammatory markers including IFNγ, TNF, CXCL13 and CXCL10 with leniolisib therapy. After 12 weeks of treatment, all patients showed amelioration of lymphoproliferation with lymph node sizes and spleen volumes reduced by 39% (mean, range 26-57%) and 40% (mean, range: 13-65%), respectively. Thus, leniolisib was well tolerated and improved laboratory and clinical parameters in APDS, supporting the specific inhibition of PI3Kδ as a promising new targeted therapy in APDS and other diseases characterized by overactivation of the PI3Kδ pathway. The study is registered at ClinicalTrials.gov, number NCT02435173.