Physicochemical investigation of the influence of saccharide-based parenteral formulation excipients on l-p-boronphenylalanine solubilisation for boron neutron capture therapy

This paper investigates the physicochemical properties of possible pharmaceu- tical alternatives to L-p-boronphenylalanine (BPA)–fructose intravenous formulation currently employed in boron neutron capture therapy. The physicochemical properties of BPA in the ab- sence and presence of fructose, mannitol, trehalose and hydroxypropyl-$-cyclodextrin (HPCD) was investigated by determination of pKa values, solubility, precipitation and dissolution us- ing a Sirius T3 instrument. Complex formation was also assessed using 10B-Nuclear magnetic resonance (NMR). The results indicate that fructose and mannitol form a complex with BPA through a reversible interaction with the boronic acid group, determined by changes in the pKa of the boronic acid group, the ultraviolet and NMR spectra, and increase in kinetic sol- ubility. Trehalose and HPCD did not undergo this reaction and, consequently, did not affect boronphenylalanaine physicochemical properties. Although mannitol is complexed with BPA in an identical manner to fructose, it is superior because it provides increased kinetic solubil- ity. Replacement of fructose by mannitol in the current clinical BPA formulation is, therefore, feasible with advantages of increased dosing and removal of issues related to fructose intoler- ance and calorific load. Results also indicated that important pharmaceutical parameters are the complex’s solubility and dissociation behaviours rather than, as originally assumed, the complex formation reaction.