Etanercept is a TNFα receptor Fc fusion protein used for the treatment of 
rheumatic disease and psoriasis. Physicochemical and functional investigation of 
process fractions during development of the etanercept biosimilar GP2015 
(Erelzi®) revealed a correlation between reduced potency and incorrect disulfide 
bridging between specific cysteines in the receptor domain. This novel 
structure-function relationship was found to be the molecular basis for reduced 
potency in recent Enbrel® batches, which exhibit higher levels of incorrect 
disulfide bridging. Interestingly, incorrect disulfide bridging was found to be 
reversible under serum-like redox conditions, restoring potency to normal 
levels. This redox dependent reversibility suggests that these variants are 
likely not relevant for clinical efficacy once the drug enters the bloodstream. 
Nonetheless, incorrect disulfide bridging in etanercept represents a new quality 
attribute that is critical for biopharmaceutical functionality and should thus 
be carefully monitored and controlled to guarantee patient safety.