ODP178 Development and Validation of a Noncompetitive Immunoassay Optimized for the Assessment of Pharmacokinetics of Biologically Active Efruxifermin in Humans

Efruxifermin (EFX) is a novel fusion biotherapeutic consisting of two modified human fibroblast growth factor (FGF21) variants tethered via short linkers to a hIgG1 Fc domain. EFX is under clinical development by Akero Therapeutics, Inc. for treatment of non-alcoholic steatohepatitis (NASH). Fusion to the IgG Fc moiety extends the circulating half-life of EFX, enabling once-weekly subcutaneous administration. In addition, point mutations in the FGF21 moiety of EFX, relative to native human FGF21, protect against degradation of the C-terminus by the endogenous endopeptidase fibroblast activation protein (FAP), and increase affinity for the FGF21 co-receptor b-Klotho, which together enhance in vivo PD activity of EFX. Accordingly, to support clinical PK a sensitive and specific assay is needed for determination of biologically active (i. e., C-terminal intact) EFX serum concentrations that is minimally cross-reactive with endogenous FGF21 or potential inactive metabolites of EFX. Moreover, the assay needs to have reliability with serum samples from NASH patients, in whom hyperlipidemia and insulin resistance are prevalent. To achieve the analytical aims, we undertook development of a noncompetitive immunosorbent assay design, wherein a biotinylated version of a rat mAb directed against EFX's intact C-terminus serves as the capture reagent, while an affinity purified chicken anti-EFX pAb labeled with Sulfo-TAG functions as the detection reagent. This novel ECLIA design permits measurement of an electrochemiluminescence signal for maximizing assay sensitivity. In brief, serum samples are diluted 1: 10 in buffer and added to the MSD plate coated with rat mAb. After incubation and washing, bound EFX is detected using an affinity purified chicken anti-EFX PAB labeled with SULFO-TAG. After washing and addition of read buffer, the signal responses are measured in a SQ120 reader and concentration in test samples is estimated by dose-interpolation using a weighted 5-PL curve fit with an EFX standard curve ranging from 10–2,000 ng/mL. The new assay allows measurement of serum EFX at concentrations as low at 20 ng/mL. Upon pre-study validation, the assay has met analytical performance criteria outlined in BMV regulatory guidances. We conclude the optimized ECLIA will be useful for reliable assessment of EFX PK in patients with NASH. Presentation: No date and time listed