Narsoplimab (OMS721) is a fully human monoclonal antibody that binds to and inhibits mannan-binding lectin-associated serine protease 2 (MASP-2). MASP-2 is the key enzyme responsible for activation of the lectin pathway of the complement system. The lectin pathway is activated when plasma is exposed to molecular patterns present on microbes and injured host cell surfaces, initiating a proteolytic cascade that results in the activation of complement components C4, C2, C3 and C5 to yield multiple biologically active products. Inhibiting MASP-2 activity prevents lectin-mediated generation of complement activation products, thereby reducing inflammation and tissue injury.

Narsoplimab is the first lectin pathway-specific complement inhibitor in clinical development. In published clinical studies and case reports, improvement has been observed in narsoplimab-treated patients with hematopoietic stem cell transplant-associated thrombotic microangiopathy, immunoglobulin A (IgA) nephropathy and atypical hemolytic uremic syndrome. Here we describe the development of pharmacodynamic (PD) assays to quantify MASP-2 inhibition ex vivo and their use to study duration of action and to establish the pharmacokinetic (PK)/PD relationship of narsoplimab in monkeys and humans.


PD assays to measure lectin pathway activity, a measure of MASP-2 activity, in minimally diluted (90%) serum from human and monkey were developed using immobilized mannan as a lectin pathway-specific complement activator with C4b as the activation endpoint. ELISA and flow cytometry assay platforms were employed to assess the PD response of monkeys or normal healthy human volunteers administered narsoplimab, respectively. The PD response was used to characterize the time-course and dose-response of lectin pathway inhibition by narsoplimab. Drug concentration data obtained from matched serum samples were used to estimate the ex vivo EC50 of lectin pathway inhibition in monkeys and humans.

Lectin pathway-specific complement activation assays applicable to minimally diluted serum samples were developed by evaluating the time-course of lectin-induced complement activation response under varying reaction temperatures and selecting optimized test conditions within the dynamic range of the assay platform. The test methods were used to characterize monkey and human lectin pathway inhibition profiles of narsoplimab in vitro, and the flow cytometric method was validated for analysis of human serum samples.


The PD assays were used to characterize the PD response of monkeys administered a single intravenous (IV) bolus injection of narsoplimab at 0.05, 0.15, 0.5, 1.5 or 5 mg/kg, and of normal human healthy volunteers administered a single IV infusion of the drug at 0.25, 0.625 or 2 mg/kg, or 6 weekly IV infusions at 2 or 4 mg/kg. A clear, dose-dependent initial PD response was observed in monkeys in the dose range of 0.5 mg/kg to 5 mg/kg, which subsequently declined over time. In humans, a minimal PD response was observed at 0.025 mg/kg. A near maximal initial PD response was seen at 0.675 mg/kg or higher, followed by a decline over time. In the 6-week repeat-dose study, once-weekly dosing of narsoplimab at 2 mg/kg resulted in a mean trough PD response of ~55% lectin pathway inhibition, which increased to an ~80% mean trough response with once-weekly dosing of 4 mg/kg. PK sample analysis revealed a clear PK/PD relationship in monkeys and humans. Modeling of the PK/PD relationship in monkeys indicated an EC50 value of ~7 µg/mL (~ 46 nM), which is comparable to the IC50 value of lectin pathway inhibition in vitro (33 nM). In humans, modeling of the PK/PD relationship data indicated an EC50 value of ~500 ng/mL (3.3 nM), again very similar to the IC50 value of lectin pathway inhibition in vitro (3.4 nM).


We successfully developed PD assays to monitor blockade of lectin pathway activation in monkeys and humans administered narsoplimab. Using a validated PD assay, we demonstrate that a high level of lectin pathway blockade can be maintained in normal human healthy volunteers by once-weekly administration of narsoplimab at 4 mg/kg IV. We also demonstrate a consistent PK/PD relationship across the study population, indicating that narsoplimab concentration ranges can be used to target the desired level of lectin pathway inhibition.


Freeman:Omeros Corporation: Current Employment. Cummings:Omeros Corporation: Current Employment. Chuidian:Omeros Corporation: Current Employment. Dudler:Omeros Corporation: Current Employment.

Author notes


Asterisk with author names denotes non-ASH members.

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