Stratification of responders towards eculizumab using a structural epitope mapping strategy

Area of Science:

• Immunology and Pharmacology
• Structural Biology

Background:

• Eculizumab is a C5-binding antibody used for paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS).
• Precise classification of eculizumab-responsive patients is needed for safe and cost-effective treatment.
• Knowledge of the drug's binding site on its target is crucial for patient stratification.

Purpose of the Study:

• To identify the precise binding site (epitope) of eculizumab on complement component 5 (C5).
• To enable precise stratification of patients for eculizumab therapy.
• To explore alternative complement inhibitors for non-responsive patients.

Main Methods:

• Structural epitope mapping using bacterial surface display.
• Flow cytometric sorting for identifying binding interactions.
• Validation of identified epitope through haemolytic activity testing.

Main Results:

• Six essential residues for eculizumab binding to C5 were identified.
• The epitope co-localizes with crystallographically determined contact areas and includes mutations found in non-responders.
• Residue W917, unique to human C5, explains eculizumab's lack of cross-reactivity with other primates.
• OmCI demonstrated anti-haemolytic function against C5 mutations within the eculizumab epitope, suggesting it as an alternative therapy.

Conclusions:

• The identified C5 epitope is critical for eculizumab binding and can be used for patient stratification.
• This approach facilitates precision medicine in treating C5-mediated disorders.
• OmCI represents a potential therapeutic alternative for eculizumab non-responders.