The physicochemical properties of A-75998, a synthetic antagonist of luteinizing hormone releasing hormone with potential for treatment of hormone-sensitive cancers and endometriosis, are described. An accelerated solution stability study indicated that the compound is relatively stable and showed a U-shaped pH-rate profile, with maximum stability between pH 4.5 and 6.5. The acid dissociation behavior of A-75998 was examined by UV-visible spectrophotometry at 25 "C in a series of buffers ranging from pH 1 to 13. The data were fit to a model in which the dissociations of all four ionizable groups contributed to changes in the absorbance. The estimated macroscopic acid dissociation constants were ppl = 3.230 k 0.022, pp2 = 4.885 k 0.030, pp3 = 9.871 k 0.022, and pp4 = 11.026 f 0.157. The corresponding microscopic dissociation constants were pkl = 3.24 (nicotinyl), pk2 = 4.88 (pyridyl), p k ~ = 9.91 (tyrosyl), and p k = 10.99 (isopropyllysyl). The apparent noctanollwater partition coefficients were measured from pH 2 to 13, and the profile was consistent with the expected aciddissociation behavior. While appearing fairly water-soluble at pH < 5, dynamic light scattering of A-75998 in pH 4.5 buffer indicated the formation of aggregates of nonuniform size distribution. A-75998 exhibited reverse or thermal gelation; sodium chloride exacerbates this gel formation and self-association. Surface activity was pH-dependent, but no evidence was found for micelle formation. Based on the results, development of a parenteral formulation of A-75998 appears feasible, provided that aggregation can be minimized.