Claudin 5-binding small molecule transiently opens the blood-brain barrier and safely enhances brain drug delivery

The blood–brain barrier (BBB) protects the brain from harmful substances, but it also limits drug delivery, hindering the treatment of brain diseases. Claudin 5, a tight junction protein in endothelial cells, plays a key role in maintaining the BBB integrity. Although modulation of Claudin 5 is a promising strategy for transiently opening the BBB, the currently available modulators often cause adverse effects due to strong or prolonged activity. Thus, we aimed to develop a moderate Claudin 5 modulator that enables safe and transient opening of the BBB. We identified a Claudin 5-binding small molecule (CL5B), capable of binding Claudin 5 via a high-throughput screening of 9600 compounds using a Claudin 5 antibody and proteoliposomes. CL5B increased endothelial permeability and tracer permeation across the endothelial monolayer by altering the localization of Claudin 5 without affecting its expression. In mice, CL5B transiently opened the BBB for less than 30 min, delivering drugs specifically to the brain. Notably, CL5B-facilitated delivery of methylscopolamine, a drug with limited brain penetration, alleviated seizure symptoms in a mouse epilepsy model. These findings demonstrate that CL5B is a safe BBB modulator that enhances brain drug delivery and holds therapeutic potential for brain diseases.