Characterization of Carboxymaltose Structure and Interactions with the Iron Core in Ferric Carboxymaltose (Injectafer) Using NMR Spectroscopy

Abstract Ferric carboxymaltose (FCM), marketed as Injectafer, is an intravenous iron formulation used to treat iron deficiency. It is composed of polynuclear Fe(III)-oxyhydroxide/oxide cores surrounded by carboxymaltose. Although it has been on the market for over 10 years, the structure of carboxymaltose and its interaction with the iron core are not fully understood. This study used nuclear magnetic resonance (NMR) to investigate both the structure of carboxymaltose and how it interacts with the iron core in Injectafer. Carboxymaltose was isolated from Injectafer and compared with other carboxymaltose samples made from maltodextrin with various dextrose equivalents (DEs). The results showed that carboxymaltose from Injectafer resembles most closely a lab-synthesized carboxymaltose with a DE of 4−7. Carboxymaltotriose was also synthesized to help with assigning the peaks in the NMR spectra of carboxymaltose. NMR was also used to study Injectafer in its as-is and diluted form. Compared to the spectra of diluted samples, peaks of the as-is sample were broadened and downfield-shifted in the 13C NMR spectra of Injectafer, due to the presence of iron and possibly due to interactions of carboxymaltose with the iron core. 1H NMR spectra of significantly diluted Injectafer showed 1H NMR peaks adjacent to the carboxy group only at dilutions of 400× or more, whereas the other peaks were observed at 100×. This suggests that the carboxy group interacts with the iron core and that it separates from the core at higher dilutions. Competitive binding studies using both low- and high-molecular-weight carboxymaltose, along with trimethylsilyl propionate (TSP), were used to characterize the binding affinity of carboxy groups to the iron core, as well as the maximum amount of carboxy groups that can bind to the surface of the iron core in Injectafer.