The Roles of the Polyspecific Organic Cation Transporters in the Disposition of meta-Iodobenzylguanidine and Implications for Toxicities and Drug Interactions

Abstract

meta-Iodobenzylguanidine (mIBG) is a radiopharmaceutical used as diagnostic agent and a targeted radiotherapy for neuroendocrine cancers. 131I-mIBG enters the cancer cells through the human norepinephrine transporter (hNET) where radioactive decay of 131I causes DNA damage, cell death, and tumor necrosis. Despite its selective accumulation in neuroendocrine tumors, mIBG distributes in several normal tissues and leads to tissue-specific radiation toxicities. The overall goal of this dissertation is to understand the mechanisms involved in the pharmacokinetics and tissue distribution of mIBG. The studies were designed to: 1) characterize the uptake kinetics and interactions of the polyspecific organic cation transporters and their role in the disposition of mIBG and 2) elucidate the impact of organic cation transporter 3 (OCT3) on the pharmacokinetics and tissue distribution of mIBG. To characterize the contribution of the human organic cation transporter 1, 2, and 3 (hOCT1, hOCT2, and hOCT3) and the human multidrug toxin and extrusion protein 1 and 2-K (hMATE1 and hMATE2-K), we conducted in vitro uptake and inhibition assays in HEK293 cells transfected with the transporters. We showed that mIBG is efficiently transported by hOCT1-3 and hMATE1/2-K with comparable uptake kinetics to hNET. We further demonstrated that mIBG is transported across a hOCT2h/MATE1 double-transfected MDCK monolayer, suggesting that the hOCT2/hMATE pathway is involved in renal secretion of mIBG. We conducted an inhibition screen of mIBG uptake by anticancer drugs used for neuroblastoma and showed that irinotecan selectively inhibited hOCT1 while crizotinib preferentially inhibited hOCT3. We proposed that the polyspecific organic cation transporters mediate the tissue distribution and elimination of mIBG and these transporters can be targeted to reduce tissue accumulation and toxicity, enhance the tumor-to-tissue uptake ratio, and predict and prevent adverse drug interactions for mIBG. To elucidate the impact of OCT3 on the pharmacokinetics and tissue distribution of mIBG, we conducted an in vivo pharmacokinetic and biodistribution study in a Oct3 knockout mouse model. We first developed and validated a bioanalytical LC-MS/MS method for the quantification of non-radiolabeled mIBG in plasma and tissue samples. We demonstrated that the method was accurate, specific, and was devoid of matrix effects in the different biological matrices. The pharmacokinetic data revealed that intact mIBG accumulates in the thyroid, a site of long-term and severe hypothyroidism in neuroendocrine patients. Deletion of Oct3 did not affect the plasma pharmacokinetics of mIBG. Most importantly, we found that Oct3 is a key facilitator for accumulation of mIBG in the heart. In contrast to the conventional but unproven notion that cardiac mIBG uptake is mediated by the norepinephrine transporter, this study in Oct3 knockout mice unequivocally showed that approximately 83% of mIBG exposure in the heart is mediated by OCT3. OCT3-mediated uptake of mIBG in the heart is likely responsible for the severe cardiotoxicities seen in neuroendocrine cancer patients receiving 131I-mIBG therapy. Besides its use in radiotherapy for neuroendocrine cancers, 123I-mIBG is FDA-approved for cardiac imaging and evaluation of heart failure and ventricular arrhythmias. Thus, our finding that OCT3 is the major determinant for mIBG uptake in the heart has important diagnostic implications for 123I-mIBG imaging of cardiovascular diseases. In addition to the reduced accumulation in the heart, deletion of Oct3 reduced the accumulation of mIBG in the skeletal muscle, salivary glands, and the lung. These data indicate that reducing OCT3 activity can lead to drastic changes to tissue exposure of mIBG and may reduce toxicities of 131I-mIBG therapy. In summary, this dissertation research has greatly contributed to our understanding of the mechanisms involved in the disposition, toxicity, and drug interactions of mIBG. A clinically relevant finding of this research is that OCT3 is a key determinant of tissue accumulation of mIBG in several tissues, especially in the heart. Ultimately, these findings will help introduce clinical strategies in modulating OCT3 activity to improve the diagnosis and therapeutic efficacy of mIBG in neuroendocrine cancers and other diseases.