title: Isolation, Solubility, and Characterization of D-Mannitol Esters of 4-Methoxybenzeneboronic Acid.
abstract: The purpose of this study was to determine the aqueous solubility of a model phenyl boronic acid, 4-methoxybenzeneboronic acid, as a function of pH both in the absence and in the presence of varying D-mannitol concentration. Solid isolated D-mannitol esters were characterized by differential scanning calorimetry, thermogravimetric analysis, powder X-ray diffraction, and single-crystal X-ray studies, and the boronic acid-to-D-mannitol ratio was quantified by HPLC. Hydrolysis of the monoester was studied using UV spectral differences between the monoester and the parent boronic acid. Two D-mannitol esters of 4-methoxybenzeneboronic acid were isolated. The triboronate ester was very insoluble whereas a symmetrical monoboronate monohydrate was also less soluble than the parent. Both esters were crystalline. The monoboronate monohydrate was, however, more soluble than the parent at alkaline pH values due to its lower pKa value (6.53) compared to the parent acid (9.41). Hydrolysis of the monoboronate was extremely fast when even small amount of water was added to dry acetonitrile solutions of the ester. The hydrolysis was buffer concentration dependent and apparent pH sensitive with hydrolysis accelerated by acid. Implications affecting the formulation of future boronic acid drugs are discussed.
authors.affiliation: Department of Pharmaceutical Chemistry, The University of Kansas, Lawrence, Kansas 66047.
authors.forename: Antonio
authors.initials: A
authors.lastname: Lopalco
authors.affiliation: Department of Pharmaceutical Chemistry, The University of Kansas, Lawrence, Kansas 66047.
authors.forename: William A
authors.initials: WA
authors.lastname: Marinaro
authors.affiliation: Small-Molecule Crystallography Laboratory, The University of Kansas, Lawrence, Kansas 66045.
authors.forename: Victor W
authors.initials: VW
authors.lastname: Day
authors.affiliation: Department of Pharmaceutical Chemistry, The University of Kansas, Lawrence, Kansas 66047. Electronic address: stella@ku.edu.
authors.forename: Valentino J
authors.initials: VJ
authors.lastname: Stella
mesh_terms: D001897:Boronic Acids / Q000737:chemistry*; D002152:Calorimetry, Differential Scanning; D004952:Esters / Q000737:chemistry*; D006868:Hydrolysis; D008353:Mannitol / Q000737:chemistry*; D008958:Models, Molecular; D033362:Powder Diffraction; D012995:Solubility; D014961:X-Ray Diffraction
chemical_list: D001897:Boronic Acids; D004952:Esters; D008353:Mannitol; C010686:benzeneboronic acid
keywords: X-ray powder diffractometry; boronic acid; calorimetry (DSC); chemical stability; crystallography; esters; hydrolysis; mannitol; solubility; stability