The invention relates to a solid form, namely crystalline Form A, of Compound 1, (1) and discloses the process for making said solid form of Compound 1. Also disclosed are further solid forms of Compound 1, including its hydrate and amorphous form. The present invention further relates to a pharmaceutical composition comprising crystalline Form A of Compound 1, and methods of using said form and pharmaceutical composition in the treatment or prevention of Alzheimer' s disease or cerebral amyloid angiopathy.