Provided herein are polymorphs and crystalline forms of 2-fluoro-5-((5S,5aS,6S,9R)-1-fluoro-12-(((2R,7aS)-2-fluorotetrahydro-1H-pyrrolizin-7a(5H)-yl)methoxy)-5-methyl-5a,6,7,8,9,10-hexahydro-5H-4-oxa-3,10a,11,13,14-pentaaza-6,9-methanonaphtho[1,8-ab]heptalen-2-yl)-3-methyl-4-(
trifluoromethyl)
aniline (Compound of Formula (I)), pharmaceutical compositions thereof, and methods of their use, for example, in the treatment of
cancer. Also provided herein are synthetic processes for the manufacture of a compound of Formula (I) (i.e., 2-fluoro-5-((5S,5aS,6S,9R)-1-fluoro-12-(((2R,7aS)-2-fluorotetrahydro-1H-pyrrolizin-7a(5H)-yl)methoxy)-5-methyl-5a,6,7,8,9,10-hexahydro-5H-4-oxa-3,10a,11,13,14-pentaaza-6,9-methanonaphtho[1,8-ab]heptalen-2-yl)-3-methyl-4-(
trifluoromethyl)
aniline). Also provided are compounds useful in its synthesis (e.g., those of Formulae (A), (B), (By), and (H)) and synthetic processes directed thereto.