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Abstract Introduction: Primary hemophagocytic lymphohistiocytosis (HLH) is a rare, life-threatening, immune disorder characterized by a hyperinflammatory state in which interferon gamma (IFNγ) is considered a key cytokine. The treatment goal of primary HLH is to stabilize the disease by controlling the associated hyperinflammation to bring patients to allogeneic hematopoietic stem cell transplantation (HSCT), the only curative therapy so far. Current conventional therapy for HLH is based on immunochemotherapies, namely etoposide and glucocorticoids; this treatment, however, is associated with opportunistic infections and severe myelotoxicity. Emapalumab, a fully human, anti-IFNγ monoclonal antibody that neutralizes IFNγ, is the only FDA-approved treatment for primary HLH patients with refractory, recurrent or progressive disease, or intolerance to conventional HLH therapy. In patients with primary HLH, the pharmacokinetics (PK) of emapalumab is highly influenced by body weight, and also by IFNγ production due to target-mediated drug disposition. In the pivotal trial (Locatelli et al NEJM 2020;382:1811-22), treatment of primary HLH patients with emapalumab was associated with a favorable safety and tolerability profile, with no unexpected safety concerns. Objective: To describe prespecified exploratory exposure-safety analyses that were performed on data from patients with primary HLH receiving emapalumab in the pivotal trial. Methods: Data from a multicenter, open-label, pivotal phase 2/3 study (NCT01818492) and its long-term follow-up study (NCT02069899) were included in this analysis. The safety of emapalumab was assessed in 34 patients (27 treatment experienced; 7 treatment naïve) with active primary HLH. Emapalumab was initiated at a dose of 1 mg/kg administered intravenously every 3 days, on a background of dexamethasone 5-10 mg/kg/day. Subsequent doses could be increased to 3, 6 and 10 mg/kg, if required, based on predefined laboratory and clinical response parameters. Treatment duration was up to 8 weeks, with possible shortening to a minimum of 4 weeks, or extension up to time of transplantation if needed. Exploratory graphical analyses were performed to determine the incidence of adverse events (AEs) as a function of the exposure parameters at the time of the AE. The relationship between emapalumab exposure and the incidence of treatment-emergent AEs, serious AEs, severe AEs, and AEs related to infections and infusion-related reactions (IRRs) was explored by logistic regression analyses. Selected parameters of renal (creatine clearance [CRCL]) and liver (total bilirubin [TBIL] and alanine aminotransferase [ALT]) function were explored graphically. Exposure parameters were obtained from the population PK/pharmacodynamic (PD) data file that was used for the population PK and PK/PD analyses. Observed individual concentration-time data were used to derive the individual exposure parameters as a function of time. The analyses considered AEs that emerged after the start of the first infusion until last infusion and prior to initiation of HSCT conditioning. Results: Exploratory graphical exposure-safety analyses did not reveal any apparent relationship between the number of AEs and exposure to emapalumab. Logistic exposure-safety regression analyses using the observed exposure to emapalumab at the time of an AE for patients experiencing an event and the highest observed exposure to emapalumab for those patients experiencing no AE, indicated that the incidence of AEs did not increase as a function of increasing emapalumab concentration. In fact, a statistically significant decrease in the incidence of severe AEs and the incidence of AEs related to IRRs was observed. No multivariate effects were identified in the multivariate regression analyses. No clear trend was observed for TBIL, ALT or CRCL as a function of the duration of emapalumab treatment. The exposure of emapalumab did not appear to influence the levels of TBIL, ALT or CRCL in individual patients during treatment. Conclusion: In this study, the exposure-safety evaluation did not reveal any significant relationships between exposure to emapalumab and observed incidence rates of AEs, serious AEs, infections, or IRRs. These findings support the primary evidence of a favorable benefit-risk profile of emapalumab across the dose range used in this fragile patient population. Disclosures Jordan: Sobi: Consultancy. Locatelli: Amgen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Miltenyi: Speakers Bureau; Medac: Speakers Bureau; Jazz Pharamceutical: Speakers Bureau; Takeda: Speakers Bureau. Jacqmin: Sobi: Consultancy. Laveille: Sobi: Consultancy. Snoeck: Sobi: Consultancy. de Min: Sobi: Consultancy, Ended employment in the past 24 months.