Intravenous iron therapy with ferric carboxymaltose results in a rapid and sustained rise in myocardial iron content through a non-canonical pathway: a translational study

Abstract

Background and Aims: Intravenous iron therapies contain iron-carbohydrate complexes, designed to ensure iron becomes bioavailable via the intermediary of spleen and liver reticuloendothelial macrophages. How other tissues obtain and handle this iron remains unknown. This study addresses this question in the context of the heart. Methods: A prospective observational study was conducted in 12 patients receiving ferric carboxymaltose (FCM) for iron deficiency. Myocardial, spleen and liver magnetic resonance relaxation times, and plasma iron markers were collected longitudinally. To examine the handling of iron taken up by the myocardium, intracellular labile iron pool (LIP) was imaged in FCM-treated mice and cells. Results: In patients, myocardial relaxation time T1 dropped maximally 3hrs post FCM, remaining low 42 days later, while splenic T1 dropped maximally at 14 days, recovering by 42 days. In plasma, non-transferrin bound iron (NTBI) peaked at 3hrs, while ferritin peaked at 14 days. Changes in liver T1 diverged amongst patients. In mice, myocardial LIP rose 1h and remained elevated 42 days after FCM. In cardiomyocytes, FCM exposure raised LIP rapidly. This was prevented by inhibitors of NTBI transporters T-type and L-Type calcium channels and divalent metal transporter 1. Conclusions: Intravenous iron therapy with FCM delivers iron to the myocardium rapidly through NTBI transporters, independently of reticuloendothelial macrophages. This iron remains labile for weeks, reflecting the myocardium's limited iron storage capacity. These findings challenge current notions of how the heart obtains iron from these therapies and highlight the potential for long-term dosing to cause cumulative iron build-up in the heart.