Background and PurposeIntracranial aneurysm (IA), common in the general public, causes lethal subarachnoid haemorrhage on rupture. It is, therefore, of utmost importance to prevent the IA from rupturing. However, there is currently no medical treatment. Recent studies suggest that IA is the result of chronic inflammation in the arterial wall caused by endothelial dysfunction and infiltrating macrophages. The sphingosine-1-phosphate receptor type 1 (S1P(1) receptor) is present on the endothelium and promotes its barrier function. Here we have tested the potential of an S1P(1) agonist, ASP4058, to prevent IA in an animal model.Experimental ApproachThe effects of a selective S1P(1) agonist, ASP4058, on endothelial permeability and migration of macrophages across an endothelial cell monolayer were tested in vitro using a Transwell system, and its effects on the size of IAs were evaluated in a rat model of IA.Key ResultsS1P(1) receptor was expressed in endothelial cells of human IA lesions and control arterial walls. ASP4058 significantly reduced FITC-dextran leakage through an endothelial monolayer and suppressed the migration of macrophages across the monolayer in vitro. Oral administration of ASP4058 reduced the vascular permeability, macrophage infiltration and size of the IAs by acting as an S1P(1) agonist in the rat model. This effect was mimicked by another two structurally-unrelated S1P(1) agonists.Conclusion and ImplicationsA selective S1P(1) agonist is a strong drug candidate for IA treatment as it promotes the endothelial cell barrier and suppresses the trans-endothelial migration of macrophages in IA lesions.