Zoledronic acid Impurity B (inner salt)

Zoledronic acid Impurity B (inner salt) is a zwitterionic bisphosphonate derivative characterized by a central amino group flanked by two phosphorylated methylene moieties and a terminal carboxylate group. Its inner salt configuration arises from proton transfer between the amino and carboxylate functionalities, creating a charge-balanced species. Structurally, it diverges from the parent drug by lacking one bisphosphonate group, resulting in altered hydrolytic stability. This impurity forms via partial hydrolysis of zoledronic acid under acidic conditions. It serves as an HPLC reference standard for quantifying degradation pathways in pharmaceutical quality control.