Biperiden-d 5 (KL 373-d 5 ) is deuterium labeled Biperiden. Biperiden (KL 373) is a non-selective muscarinic receptor antagonist that competitively binds to M1 muscarinic receptors, thereby inhibiting acetylcholine and enhancing dopamine signaling in the central nervous system. Biperiden has the potential for the research of Parkinson's disease and other related psychiatric disorders.