Abstract

Heart failure (HF) is a chronic disease affecting 1%–2% of the global population.¹²³I-labeled meta-iodobenzylguanidine (mIBG) is US Food and Drug Administration-approved for cardiac imaging and prognosis risk assessment in patients with HF. As a norepinephrine analog, mIBG is believed to be transported into adrenergic nerve terminals by the neuronal norepinephrine transporter (NET) and hence image sympathetic innervation of the myocardium. We previously showed that mIBG is an excellent substrate of organic cation transporter 3 (OCT3), an extraneuronal transporter expressed in cardiomyocytes. Here, we evaluated the in vivo impact of Oct3 on mIBG disposition and tissue distribution using Oct3 knockout mice. Oct3^+/+ and Oct3^−/− mice were administered with mIBG intravenously, and mIBG plasma pharmacokinetics and tissue exposures were determined. In Oct3^+/+ mice, mIBG exhibited extensive accumulation in multiple tissues (heart, salivary gland, liver, and adrenal gland). No difference was observed in overall plasma exposure between Oct3^+/+ and Oct3^−/− mice. Strikingly, cardiac mIBG was depleted in Oct3^−/− mice, resulting in 83% reduction in overall cardiac exposure (AUC_{0–24 h}: 12.7 vs. 2.1 μg × h/g). mIBG tissue exposure (AUC_{0–24 h}) was also reduced by 66%, 36%, and 31% in skeletal muscle, salivary gland, and lung, respectively, in Oct3^−/− mice. Our data demonstrated that Oct3 is the primary transporter responsible for cardiac mIBG uptake in vivo and suggested that cardiac mIBG imaging mainly measures OCT3 activity in cardiomyocytes but not NET-mediated uptake in adrenergic nerve endings. Our findings challenge the current paradigm in interpreting cardiac mIBG imaging results and suggest OCT3 as a potential genetic risk marker for HF prognosis.