Blockade of the Neonatal Fc Receptor (FcRn) Represents an Effective Mechanism for the Removal of Pathogenic Autoantibodies in Primary Immune Thrombocytopenia

Introduction: The neonatal Fc receptor, FcRn is ubiquitously expressed, & is responsible for maintaining the half-life of IgG & albumin, by rescuing these proteins from intracellular lysosomal degradation. Blockade of the interaction of FcRn with IgG would be expected to prevent salvage of both non-pathogenic and pathogenic IgG. Patients with primary immune thrombocytopenia (ITP) have autoantibodies against platelet membrane proteins such as the cell adhesion receptor CD61/CD41 (GPIIb/IIIa). Rozanolixizumab (a high affinity IgG₄ monoclonal antibody (mAb) that specifically inhibits IgG binding to FcRn) is currently being evaluated in phase 2 clinical studies in patients with ITP. The aim of the current study was to understand the in vitro properties of rozanolixizumab & to explore the in vivo effects of a surrogate anti-mouse FcRn mAb (murinized "4464") in a mouse model of ITP.

Methods: The affinity of rozanolixizumab for FcRn was tested using surface plasmon resonance technology (Biacore), with the antibody captured by an anti-Fc immobilized on the sensor chip & soluble FcRn in solution, whilst the affinity of fluorescently labelled rozanolixizumab for human FcRn expressed on the surface of Madin-Darby canine kidney (MDCK) cells was measured by flow cytometry. Recycling & transcytosis of human IgG (hIgG) was also assessed in FcRn-transfected MDCK cells. For IgG tracking experiments, endothelial & epithelial cell lines were incubated with an AF647-conjugated IgG for 24 hours in the presence or absence of rozanolixizumab. Prior to imaging, nuclei were labelled using Hoechst at 37°C & cells were then visualized by fluorescence microscopy. Phagocytosis of anti-human MHC I (W6/32)-coated platelets by THP-1 cells was assessed by flow cytometry at 1 hour. The effect of rozanolixizumab on IgG recycling in vivo was assessed in the human FcRn-transgenic (hFcRn-Tg) mouse: hIgG at 500mg/kg was administered IV to mice on day 1 & dosed with antibodies on day 2; serial bleeds were then taken at multiple time points post antibody dosing & levels of hIgG, albumin & rozanolixizumab were quantified using LC-MS/MS. For the induction of ITP in mice, animals received an anti-CD41 antibody (MWReg30), delivered by osmotic minipump (~1µg of antibody delivered every 24 hours) & the anti-mouse FcRn mAb (4464) was administered at 30mg/kg either on days 0, 2 & 4 (prophylactic dosing) or on days 3 & 5 (therapeutic dosing); IVIg (1g/kg) was also evaluated & control mice received either PBS or an isotype-matched control mAb (101.4). Daily blood samples were taken to assess platelet numbers.

Results: Rozanolixizumab has a high affinity for FcRn in a protein-protein (Biacore) assay with a K_D of ~25pM at both pH6.0 & pH7.4; in the cell-based assay, the affinity was 1nM at both pH's. Rozanolixizumab efficiently inhibited the recycling & transcytosis of IgG in human FcRn-transfected MDCK cells (IC₅₀ 0.4nM & 1.1nM, respectively). Confocal microscopy experiments showed that there was an increased accumulation of IgG in cells in the presence of rozanolixizumab, relative to an isotype control mAb, & that the IgG was co-localized in lysosomes where it appeared to be degraded. FcRn has been postulated to play a direct role in phagocytosis, but the phagocytosis of opsonized platelets by THP-1 cells was unaffected by blockade with rozanolixizumab. Treatment with rozanolixizumab resulted in a dose-dependent accelerated clearance of hIgG in the hFcRn-Tg mouse; hIgG levels in all treated groups were significantly lower vs PBS controls from 24 hours post-dosing until the end of the experiment 8 days post dosing (p<0.01). No effect on albumin levels occurred in these mice. In the mouse model of ITP, both prophylactic & therapeutic treatment regimens with 4464 resulted in an increased number of platelets compared to control animals that received either PBS or 101.4. Animals that received 4464 from day 0 had significantly greater platelet numbers than control animals on days 4 & 5 (p<0.05); therapeutic dosing gave a significantly greater platelet number than control animals on day 5 (p<0.05).

Conclusion: Rozanolixizumab is a high affinity mAb that inhibits the IgG recycling function of FcRn in vitro & in vivo . An anti-mouse FcRn mAb demonstrated efficacy in a mouse model of ITP. These data support the evaluation of rozanolixizumab in patients with ITP, & a phase 2 clinical study is currently ongoing in this patient population.