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.