PROJECT SUMMARY/ABSTRACT The proposed multi-site clinical trial will fill a critical unmet need for therapeutic development in semantic variant primary progressive aphasia (svPPA), a syndrome that is over 85% predictive of frontotemporal lobar degeneration with mislocalization of TDP-43 on autopsy (FTLD-TDP). We will thereby address the larger critical unmet need for therapeutic development in the greater spectrum of TDP-43 pathology, which is present in up to 20% of suspected Alzheimer's disease and over half of frontotemporal dementia. We hypothesize that microglial myeloperoxidase (MPO), an enzyme responsible for generation of microglial oxidative species, is a key therapeutic target in the pathogenesis of FTLD with TDP-4 mislocalization. We will therefore conduct a phase 1 randomized, double-blind, placebo-controlled, sequential cohort, dose-ranging, tolerability, and preliminary pharmacodynamic study of two doses of Verdiperstat (BHV-3241), an orally administered MPO inhibitor, in patients with svPPA. As the first multi-site clinical trial focused on svPPA, this project will establish the crucial organizational infrastructure to conduct future multicenter trials in this cohort, while leveraging existing clinical expertise and patient recruitment infrastructure from the large NIH-funded ARTFL-LEFFTDS Longitudinal Frontotemporal Lobar Degeneration (ALLFTD) multisite clinical research consortium. In order to establish the appropriateness of follow-up phase 2 trials, we will determine the safety and tolerability (Aim 1) and the pharmacokinetics (in blood and CSF) (Aim 2) of 300mg (low dose) and 600mg (high dose) BHV-324, orally administered twice daily (BID) for 24 weeks in patients with svPPA due to underlying FTLD-TDP. N=55 Participants will be randomized overall to placebo (N=15), low dose (N=10), and high dose (N=30) BHV-3241 respectively. Given the lack of established pharmacodynamic measures of drug efficacy in FTLD, we will also explore the effects of BHV-3241 on peripheral MPO activity, fluid and imaging biomarkers of neurodegeneration, unbiased CSF proteomics, and clinical status in FTLD-TDP (Aim 3). Our proposed discovery and longitudinal characterization of candidate measures of svPPA biological and clinical severity will provide essential information, informing the design of future efficacy trials exploring BHV-3241 and other potential therapies in svPPA and the larger spectrum of FTLD-TDP.