Cardiac Uptake of the Adrenergic Imaging Agentmeta-Iodobenzylguanidine (mIBG) Is Mediated by Organic Cation Transporter 3 (Oct3)

Heart failure (HF) is a chronic disease affecting 1-2% of the global population. ¹²³I-labeled meta-iodobenzylguanidine (mIBG) is FDA-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, 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 hrs}: 12.7 versus 2.1 µghr/g). mIBG tissue exposure (AUC_{0-24 hrs}) 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. Significance Statement¹²³I-mIBG is used for cardiac imaging and risk assessment in heart failure patients. Contrary to the current belief that mIBG tracks cardiac sympathetic innervation due to its uptake by the neuronal norepinephrine transporter, we have demonstrated that cardiac mIBG uptake is mediated by the extraneuronal transporter Oct3. Our findings warrant a re-evaluation of the scientific rationale behind cardiac mIBG imaging and further suggest OCT3 as a risk factor for disease progression in heart failure patients.