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BACKGROUND AND PURPOSEFor antibody therapies against receptor targets, in vivo outcomes can be difficult to predict because of target-mediated clearance or antigen 'sink' effects. The purpose of this work was to engineer an antibody to the GM-CSF receptor a (GM-CSFRa) with pharmacological properties optimized for chronic, s.c. treatment of rheumatoid arthritis (RA) patients. EXPERIMENTAL APPROACHWe used an in silico model of receptor occupancy to guide the target affinity and a combinatorial phage display approach for affinity maturation. Mechanism of action and internalization assays were performed on the optimized antibody in vitro before refining the modelling predictions of the eventual dosing in man. Finally, in vivo pharmacology studies in cynomolgus monkeys were carried out to inform the predictions and support future clinical development. KEY RESULTSAntibody potency was improved 8600-fold, and the target affinity was reached. The refined model predicted pharmacodynamic effects at doses as low as 1 mg kg -1 and a study in cynomolgus monkeys confirmed in vivo efficacy at 1 mg kg -1 dosing. CONCLUSIONS AND IMPLICATIONSThis rational approach to antibody drug discovery enabled the isolation of a potent molecule compatible with chronic, s.c. self-administration by RA patients. We believe this general approach enables the development of optimal biopharmaceuticals.

Objective Amyloid‐beta oligomers (Aßo) trigger the development of Alzheimer's disease (AD) pathophysiology. Cellular prion protein (PrPC) initiates synaptic damage as a high affinity receptor for Aßo. Here, we evaluated the preclinical therapeutic efficacy of a fully human monoclonal antibody against PrPC. This AZ59 antibody selectively targets the Aβo binding site in the amino‐terminal unstructured domain of PrPC to avoid any potential risk of direct toxicity. Methods Potency of AZ59 was evaluated by binding to PrPC, blockade of Aβo interaction and interruption of Aβo signaling. AZ59 was administered to mice by weekly intraperitoneal dosing and brain antibody measured. APP/PS1 transgenic mice were treated with AZ59 and assessed by memory tests, by brain biochemistry and by histochemistry for Aß, gliosis and synaptic density. Results AZ59 binds PrPC with 100 pmol/L affinity and blocks human brain Aßo binding to PrPC, as well as prevents synaptotoxic signaling. Weekly i.p. dosing of 20 mg/kg AZ59 in a murine form achieves trough brain antibody levels greater than 10 nmol/L. Aged symptomatic APP/PS1 transgenic mice treated with AZ59 for 5–7 weeks show a full rescue of behavioral and synaptic loss phenotypes. This recovery occurs without clearance of plaque pathology or elimination of gliosis. AZ59 treatment also normalizes synaptic signaling abnormalities in transgenic brain. These benefits are dose‐dependent and persist for at least 1 month after the last dose. Interpretation Preclinical data demonstrate that systemic AZ59 therapy rescues central synapses and memory function from transgenic Alzheimer's disease pathology, supporting a disease‐modifying therapeutic potential.

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