SK-129 is a blood-brain barrier-permeable inhibitor of α-synuclein (αS) oligomers with a Kd of 221 nM. SK-129 preferentially binds to neurotoxic αS oligomers over physiological αS monomers, inhibits αS aggregation, blocks the interaction and co-aggregation of αS with tau protein, and prevents the maturation of αS-tau condensates into amyloid aggregates. SK-129 reduces ROS production, rescues dopaminergic neuron degeneration, improves motor function, restores endogenous dopamine synthesis, increases the number of Tyrosine Hydroxylase-positive neurons, prevents brain histopathological changes, alleviates neuroinflammation, and improves survival rates in relevant models. SK-129 can be used in research related to Parkinson's disease (PD) and Lewy body dementia (LBD).