ABSTRACT Neuroinflammation is a significant driver of pathology in Alzheimer’s disease (AD) and Alzheimer’s disease related dementia (ADRD). Multiple lines of evidence in patients and in animal models of AD/ADRD point to vascular dysregulation and deposits of fibrin (FN) in the brain and cerebral blood vessels as significant sources of this inflammation. In AD/ADRD patients localized BBB breakdown is an early event and increases with disease progression. Vascular breakdown leads to the extravasation of blood proteins, including fibrinogen (FGN), into the brain and the formation of FN clots in brain parenchyma and cerebral vasculature coincident with Aβ plaques and inflammatory immune cells. In the conversion of FGN to FN a cryptic epitope is exposed on the FN chain This sequence (Fibγ377-395, also known as FN P2) is a ligand for the CD11b/CD18 receptor on macrophages and microglia, triggering activation of these innate immune cells and secretion of inflammatory mediators. Genetic or chemical depletion of FGN or genetic substitution of 6 key amino acids within FN P2 in the 5xFAD mouse model of AD considerably reduces their level of inflammatory cytokines and the rate of cognitive decline. A mouse monoclonal antibody (mAb), 5B8, discovered by Dr. Katerina Akassoglou (founder of Therini Bio Inc.) binds this cryptic epitope and recapitulates the activity of the genetic FN P2 deletion. 5B8 is highly selective for FN over FGN and does not interfere with normal coagulation. Therini Bio Inc. has humanized 5B8 and demonstrated that the lead humanized anti-FN P2 mAb, termed THN227, retains the selective binding for FN over FGN and does not interfere with normal FN-mediated functions such as hemostasis. In an in vivo fibrino(gen)-induced encephalomyelitis model of neuroinflammation, intravenous administration of THN227 reduced microglial activation, macrophage infiltration, and oxidative stress. In this proposal the lead humanized anti-FN P2 mAb will be tested in several key preclinical and nonclinical safety studies to ensure that the antibody is safe and tolerable for human clinical trials. Prior to the initiation of the proposed studies, we will have tested THN227 and several back-up humanized or fully human anti-FN P2 mAbs in ex vivo assays of immunogenicity and in rodent PK and dose range finding studies. In this proposal, we will first test whether our lead anti-FN P2 mAb induces microhemorrhages in a transgenic AD mouse model. This is an important safety test, as several anti-Aβ mAbs induced amyloid related imaging abnormalities associated with hemorrhage (ARIA-H) in a subset of AD patients in previous clinical trials. We will also establish the safe dose range in nonhuman primates (NHPs) and assess the safety or repeated short-term and chronic dose regimens. These key safety studies will drive the preclinical development of our novel anti-FN P2 mAb approach, establish important safety limitations, supporting an IND application and informing the ra...