Disclosed is the polymorphic Form A1 of (S)-1-cyanobutan-2-yl (S)-1-(3-(3-(3-methoxy-4-(oxazol-5-yl)phenyl)ureido)phenyl)ethylcarbamate wherein said polymorph comprises less than about 5 percent by weight of amorphous form and wherein the polymorph has a peak position at about 20.7 degrees 2-theta in an x-ray powder diffraction pattern obtained using Cu K alpha radiation. Also disclosed is the polymorphic Form B2 of (S)-1-cyanobutan-2-yl (S)-1-(3-(3-(3-methoxy-4-(oxazol-5-yl)phenyl)ureido)phenyl)ethylcarbamate, wherein the polymorph has a peak position at about 4.1 degrees 2-theta in an x-ray powder diffraction pattern obtained using Cu K alpha radiation. A process for preparing polymorphic form A1 or B2 is also disclosed. Also disclosed is the use of either polymorphic form A1 or B2 in the manufacture of a medicament for treating IMPDH-mediated diseases in a patient.
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