Development and Application of an LC–MS Method for the Pharmacokinetics and Pharmacodynamics of Oral Therapeutic Efficacy of Nalfurafine

ABSTRACT Multiple sclerosis (MS) is a chronic autoimmune disease characterized by central nervous system demyelination, leading to sensory and motor dysfunction. Previous studies have demonstrated that the kappa opioid receptor agonist nalfurafine (Nalf) reduces disease severity and promotes remyelination in the experimental autoimmune encephalomyelitis (EAE) model of MS. However, the pharmacokinetics of Nalf and its optimal administration route for clinical translation remain unexplored. To investigate Nalf's pharmacokinetics and identify an effective oral delivery strategy, we successfully optimized a liquid chromatography–mass spectrometry (LC–MS) method for detecting Nalf in plasma and brain tissues of mice treated via intraperitoneal (ip) or oral administration. Nalf exhibited similar pharmacokinetic profiles following both ip and oral administration, with the oral route achieving almost 70% bioavailability and a longer elimination half‐life compared to ip. Nalf effectively reached the brain and significantly reduced a range of disease parameters in EAE in a dose‐dependent manner. Our findings demonstrate that oral administration of Nalf is pharmacokinetically favourable and effectively reduces disease disability and promotes remyelination in the EAE model. This study supports the clinical development of oral Nalf as a potential treatment for MS, offering an effective and noninvasive alternative to injections.