Summary Lewy Body Dementias (LBDs) are a series of neurodegenerative disorders characterized by accumulation of misfolded α‐synuclein (αS), including Dementia with Lewy Body (DLB), Parkinson’s Disease with Dementia (PDD), and 1/3 of Alzheimer’s Disease (AD). DLB is the 2nd most common dementia after AD accounting for 30% of dementia cases; approximately 30% of AD cases suffer from α‐synucleinopathy resulting in a more rapid and severe cognitive decline than AD alone. In brief, αS pathology is highly associated with AD and related dementias (ADRD). Braak et al. identified that αS pathology spreading as disease progresses from a substantial number of postmortem studies. Experimental studies have further demonstrated that misfolded αS seeds (i.e., brain extracts, recombinant αS preformed fibrils (PFF) are prion-like particles that can exponentially propagate pathology cell-to-cell, tissue-to-tissue, inducing neuroinflammation and neurodegeneration, driving the disease progression and causing dementia. These αS seeds can enter cells and induce endogenous αS monomer misfolding and aggregation, assessed with the immunoreactivity of anti-αS phosphorylated serine 129 (pS129), a widely used pathological marker. The induced intracellular seeds can be released to extracellular spaces, and then the extracellular seeds start a new cycle of cell-to-cell transmission. Because intracellular and extracellular seeds can be easily interconverted, it brings the difficulty to cease cell-to-cell transmission of pathogenic αS. αS pathology and propagation are observed mainly inside neurons, and αS is more abundant in neurons than in glial cells, indicating that the intraneuronal αS significantly drives the pathogenesis. There have been tremendous efforts on the development of antibodies against αS; however, due to the large size and structural complexity, antibodies have limited penetrability through the plasma membrane. αS antibodies do not enter cells and are inaccessible to intracellular αS seeds, but only to extracellular αS seeds. Furthermore, antibodies are usually not functional in the reducing environment of the cytosol due to the reduction of the disulfide bonds, which are critical for the correct folding of antibodies. In addition, antibody treatment could be costly and inconvenient in long-term disease progression. It is hypothesized that blockage of intracellular αS propagation could be effective against cell-to-cell αS transmission, and all these gaps indicate an urgent need to generate a reagent targeting the intracellular pathogenic αS. In this application, we aim to develop novel nanobody-based agents that can targeting these pathogenic αS to block cell-to-cell transmission.