Amorphous solid-state form of (R)-(+)-7-(3-amino-2,3,4,5,6,7-hexahydro-1H-azepin-1-yl)-1,4-dihydro-4-oxoquinoline-3-carboxylic acid is characterized by at least one of: (a) an X-ray powder diffraction (“XRPD”) spectrum that comprises peaks at 2θ angles of 6.9-7.1, 9.4, 10.6-10.7, and 13.4-13.7°±0.2°, and a diffuse halo pattern at 11-30° and (b) a DSC (differential scanning calorimetry) melting peak at about 267-272° C. The amorphous solid is prepared by rapid precipitation from a saturated or supersaturated solution of besifloxacin free base in a solvent comprising at least benzyl alcohol.
