Novel biomarker and easy to perform ELISA for monitoring complement inhibition in patients with atypical hemolytic uremic syndrome treated with eculizumab.

Atypical hemolytic uremic syndrome (aHUS) is a devastating disease characterized by thrombus formation in the microvasculature and is associated with complement dysregulation. The recommended treatment is eculizumab, an humanized monoclonal antibody, which binds complement protein C5 and thereby preventing the assembly of the terminal complement complex (TCC, soluble C5b-9) and the generation of C5a, an anaphylatoxin. The objective of the study was to identify a reliable biomarker, which estimates the degree of complement inactivation under normal and pathophysiological conditions, such as an infection. Here we report on serial measurements of the soluble form of C5b-9, measured in plasma and the TCC capacity (soluble C5b9 after ex-vivo activation, measured in serum) in seven patients with aHUS who were treated with eculizumab. By measuring the latter we were able to assess the maximum possible soluble C5b-9 production under a potent complement trigger. Patients were followed up over a median duration of 3.8 years either on biweekly (q2w) or a three-weekly (q3w) maintenance therapy interval. As expected, eculizumab treatment resulted in a profound decrease of TCC capacity (median 55, IQR 44–88 AU/ml) compared to baseline (1249, 529–1806 AU/ml, p < 0.0001) and there was no significant difference in TCC capacity measurements on q2w vs. q3w maintenance dosing schedule. However, during q3w maintenance significantly more single measurements of TCC capacity above the lower cut-off level were detected (50% on q3w vs. 5% on q2w maintenance, p < 0.045). Higher TCC capacity levels were associated with lower eculizumab levels. The TCC capacity may represent a novel and easy to perform parameter to determine level of complement inhibition in patients treated with eculizumab, especially because it identifies the residual capacity of inhibition at pathophysiological stages.