A polymorph of a mesylate salt of Compound A: selected from Form A, Form B, Form C, Form D, Form E, Form F, Form G, Form H, Form I, Form J, and Form K, wherein: the Form A is characterized by having X-ray powder diffraction peaks at approximately 9.4 and 23.0 °2Θ using Cu Kα radiation, or at 9.1 and 22.8 °2Θ using Cu Kα radiation, the Form B is characterized by having X-ray powder diffraction peaks at approximately 6.2 and 14.3 °2Θ using Cu Kα radiation, or at 6.0 and 14.6 °2Θ using Cu Kα radiation, the Form C is characterized by having X-ray powder diffraction peaks at approximately 20.3 and 22.8 °2Θ using Cu Kα radiation, or at 20.1 and 22.6 °2Θ using Cu Kα radiation, the Form D is characterized by having X-ray powder diffraction peaks at approximately 14.5 and 23.0 °2Θ using Cu Kα radiation, the Form E is characterized by having X-ray powder diffraction peaks at approximately 20.9 and 21.9 °2Θ using Cu Kα radiation, the Form F is characterized by having X-ray powder diffraction peaks at approximately 16.7 and 17.0 °2Θ using Cu Kα radiation, the Form G is characterized by having X-ray powder diffraction peaks at approximately 5.8 and 22.1 °2Θ using Cu Kα radiation, the Form H is characterized by having X-ray powder diffraction peaks at approximately 10.9 and 22.8 °2Θ using Cu Kα radiation, the Form I is characterized by having X-ray powder diffraction peaks at approximately 5.2 and 10.5 °2Θ using Cu Kα radiation, the Form J is characterized by having X-ray powder diffraction peaks at approximately 17.0 and 22.8 °2Θ using Cu Kα radiation, and the Form K is characterized by having X-ray powder diffraction peaks at approximately 9.2 and 10.0 °2Θ using Cu Kα radiation; wherein "approximately" refers to the recited X-ray powder diffraction peak ± 0.3 °2Θ. the Form A is characterized by having X-ray powder diffraction peaks at approximately 9.4 and 23.0 °2Θ using Cu Kα radiation, or at 9.1 and 22.8 °2Θ using Cu Kα radiation, the Form B is characterized by having X-ray powder diffraction peaks at approximately 6.2 and 14.3 °2Θ using Cu Kα radiation, or at 6.0 and 14.6 °2Θ using Cu Kα radiation, the Form C is characterized by having X-ray powder diffraction peaks at approximately 20.3 and 22.8 °2Θ using Cu Kα radiation, or at 20.1 and 22.6 °2Θ using Cu Kα radiation, the Form D is characterized by having X-ray powder diffraction peaks at approximately 14.5 and 23.0 °2Θ using Cu Kα radiation, the Form E is characterized by having X-ray powder diffraction peaks at approximately 20.9 and 21.9 °2Θ using Cu Kα radiation, the Form F is characterized by having X-ray powder diffraction peaks at approximately 16.7 and 17.0 °2Θ using Cu Kα radiation, the Form G is characterized by having X-ray powder diffraction peaks at approximately 5.8 and 22.1 °2Θ using Cu Kα radiation, the Form H is characterized by having X-ray powder diffraction peaks at approximately 10.9 and 22.8 °2Θ using Cu Kα radiation, the Form I is characterized by having X-ray powder diffraction peaks at approximately 5.2 and 10.5 °2Θ using Cu Kα radiation, the Form J is characterized by having X-ray powder diffraction peaks at approximately 17.0 and 22.8 °2Θ using Cu Kα radiation, and the Form K is characterized by having X-ray powder diffraction peaks at approximately 9.2 and 10.0 °2Θ using Cu Kα radiation; wherein "approximately" refers to the recited X-ray powder diffraction peak ± 0.3 °2Θ.