Analytical interference of Burosumab therapy on intact fibroblast growth factor 23 (iFGF23) measurements using an immunoassay: preliminary evaluation

Our study evaluated the possible interference of Burosumab (human recombinant monoclonal antibody directed against N-terminal domain of FGF23) on the immunoassay of intact FGF23 (iFGF23) with the Liaison XL. The analytical method uses three different antibodies, one of which directed against the N-terminal portion of FGF23. The evaluation of the method accuracy involved the fully automated execution of a dilution test on EDTA plasma from 5 subjects who had not received any monoclonal antibody (mAb), 20 EDTA plasma from patients treated with Burosumab, and 2 EDTA plasma from subjects who had not received any mAb in witch an adequate volume of Burosumab had been added in vitro. One sample with specific diluent (LIAISON® FGF 23) with an adequate volume of Burosumab had been added in vitro. The dilution assay provided highly inaccurate iFGF23 results in samples with therapeutic concentrations of Burosumab and in samples with concentrations below the LoQ (6.5 pg/mL). The addition of Burosumab to the diluent did not produce any analytical interference. Dissociation of iFGF23 from the mAb-target complex in diluted sample could explain the loss of accuracy in the iFGF23 immunoassay using the Liaison XL analyzer. Burosumab could be an interferent in immunoassay procedures of iFGF23.