Described herein are solid forms of (5R,6S)-10,11-difluoro-12-((2-fluoro-4-iodophenyl)amino)-5,6-dihydroxy-4,5,6, 7-tetrahydro-1H-spiro[benzo[b][1,5, 4]oxathiazecine-3,1′-cyclopropane]2,2-dioxide in amorphous and crystalline forms, and processes of preparation, and pharmaceutical compositions thereof. Also provided are methods of their use for treating, preventing, or ameliorating a disease or disorder in which the RAS pathway is upregulated such as cancer, RASopathies and laminopathies. Formula (I).