Population Pharmacokinetics of the Anti-Interferon-Gamma Monoclonal Antibody Emapalumab: An Updated Analysis

Emapalumab is a fully human monoclonal antibody that targets free and receptor-bound interferon-gamma (IFNγ), neutralizing its biological activity. IFNγ levels differ by orders of magnitude between patients with primary hemophagocytic lymphohistiocytosis (HLH) and macrophage activation syndrome (MAS; a form of secondary HLH) in systemic juvenile idiopathic arthritis (sJIA). Therefore, this study aimed to develop a population pharmacokinetic model for emapalumab across a patient population with a wide range of total (free and emapalumab-bound) IFNγ levels using observations from patients with primary HLH or MAS in sJIA in clinical trials. Pharmacokinetic data were pooled (n = 58; 2709 observations) from studies enrolling patients administered emapalumab for primary HLH or MAS in sJIA. Patients with primary HLH were administered emapalumab 1 mg/kg (potentially increasing to 3, 6, and up to 10 mg/kg based on clinical response) every 3 days. Patients with MAS in sJIA were administered emapalumab 6 mg/kg, followed by 3 mg/kg every 3 days until day 15 and twice weekly until day 28. An earlier population PK model was re-parameterized using this data. The final model for emapalumab comprised a 2-compartment model with first-order elimination. Emapalumab clearance remains constant when the total IFNγ concentration (free and emapalumab-bound) is < ~ 10,000 pg/ml but increases proportionally to total IFNγ concentration above this threshold. Emapalumab clearance was estimated to be 0.00218, 0.00308, 0.00623 and 0.01718 l/h at total serum IFNγ concentrations of 103, 104, 105 and 106 pg/ml, respectively, with corresponding terminal half-lives of 19.2, 13.8, 7.18 and 3.12 days for a 1-year-old patient weighing 10 kg with primary HLH. The median terminal half-life for emapalumab in patients with MAS in sJIA was estimated to be 24.0 (range, 6.13–32.4) days, which is similar to observations in healthy volunteers. Emapalumab pharmacokinetics in patients with primary HLH and MAS in sJIA were described by a two-compartment model with fixed allometric exponents and an age-related effect. Differences in total IFNγ levels between patients with primary HLH and MAS may affect emapalumab pharmacokinetics, suggesting that each indication may require different dosing to rapidly control hyperinflammation. Clinicaltrials.gov identifiers: NCT01818492, NCT03311854 and NCT02069899. Patients with a rare condition called hemophagocytic lymphohistiocytosis (HLH) produce excessive amounts of a molecule called interferon-gamma. Excessive interferon-gamma causes extreme (or hyper) inflammation, which can be fatal. A drug called emapalumab can be used to block the action of interferon-gamma. However, we need to understand how the concentration of emapalumab in the blood changes over time to ensure that the correct dose is administered when attempting to control interferon-gamma-driven hyperinflammation in patients with HLH. Because HLH is a rare condition, data from a small number of patients were used to create a mathematical model that predicts emapalumab concentrations in the blood at various times after it is administered. Importantly, the amount of interferon-gamma observed in patients with different types of HLH is highly variable, which can alter how quickly emapalumab is removed from the blood. The higher interferon-gamma levels go above a certain threshold, the faster emapalumab is removed. In particular, interferon-gamma levels generally only exceed this threshold in patients with a familial or genetic form of HLH (primary HLH). Interferon-gamma levels in patients with a type of HLH called macrophage activation syndrome, which can occur in patients with systemic juvenile idiopathic arthritis (sJIA), do not usually cross the threshold associated with faster removal of emapalumab. This means that higher dosing may be required for patients with primary HLH compared with patients who have macrophage activation syndrome in sJIA to expedite control of hyperinflammation because of differences in the rate at which emapalumab is removed from the blood.