The impact of eculizumab on routine complement assays

Area of Science:

• Immunology
• Pharmacology
• Clinical Chemistry

Background:

• Eculizumab (ECU) inhibits complement C5 cleavage, preventing C5a formation and membrane attack complex-mediated cell lysis.
• ECU's presence in circulation interferes with routine complement assays used for treatment monitoring.

Purpose of the Study:

• To evaluate the impact of eculizumab (ECU) on various complement assays.
• To determine the effective concentration of ECU for complement inhibition in vitro and in patients.
• To identify reliable complement assays for monitoring ECU pharmacodynamics.

Main Methods:

• Spiked serum and plasma samples with varying ECU concentrations (25–600 μg/mL) were analyzed.
• Seventy-one patient samples with known ECU levels were assessed.
• Assays included total complement (CH50), alternative pathway (AH50), C5 function, C5 concentration, and soluble membrane attack complex (sMAC).
• ECU concentrations were quantified using mass spectrometry.

Main Results:

• ECU demonstrated dose-dependent complement inhibition in spiked samples.
• Complete inhibition of AH50 (<10%) was observed at 100 μg/mL ECU.
• C5 concentrations remained stable, while sMAC levels decreased with ECU presence.
• Patient samples with ECU >100 μg/mL showed significantly reduced C5 function (<29 U/mL).

Conclusions:

• Eculizumab effectively blocks complement pathways, as evidenced by CH50, AH50, and C5 function assays.
• CH50, AH50, and C5 function assays can serve as pharmacodynamic markers for eculizumab therapy.
• Laboratory testing, combined with ECU concentration, can aid in tailoring patient treatment.