Radioiodinated meta‐iodobenzylguanidine (131I‐mIBG) is utilized as a targeted radiotherapy for neuroendocrine cancers.131I‐mIBG selectively accumulates in the cancer cells through uptake by the norepinephrine transporter. However, whole‐body scintigraphy also shows extensive mIBG uptake and accumulation in several normal tissues (e.g. salivary glands, liver, heart) via mechanisms that are not fully understood. The accumulation of radioactive 131I‐mIBG in these tissues is associated with a range of adverse reactions and tissue damages due to the unnecessary radiation exposure. We hypothesize that the efficacy and safety of 131I‐mIBG can be improved by selectively inhibiting mIBG uptake in healthy tissues without affecting its uptake in tumor cells. The goal of this study is to elucidate the impact of organic cation transporter 3 (OCT3) on the in vivo pharmacokinetics andtissue distribution of mIBG using a mouse model with genetic deletion of Oct3. Additionally, we conducted in vitro studies in HEK293 cell lines stably transfected with mouse Oct3 (mOct3) and human OCT1‐3 (hOCT1‐3) to determine the uptake kinetics of mIBG and dose‐dependent inhibition of OCT1‐3‐mediated uptake of mIBG. The in vitro kinetic analysis revealed similar apparent affinities in the uptake of mIBG by hOCT3 and mOct3 (Km of 14.5 ± 7.1 and 29.4 ± 3.4 µM, respectively), suggesting minimal species difference in mIBG transport by OCT3. Interestingly, hOCT3‐mediated uptake of mIBG was inhibited by crizotinib (IC50 of 2.1 ± 0.3 µM) and crizotinib was 6.7‐fold and 18‐fold more potent towards hOCT3 than for hOCT2 and hOCT1, respectively. For the in vivo pharmacokinetic studies, full concentration‐time profiles were determined to calculate mIBG exposure, defined as area under the concentration‐time curve (AUC), in the plasma and each analyzed tissue. After retro‐orbital injection (10 mg/kg), mIBG exhibited extensive accumulation in all tested tissues with exposures ranging from 4 to 90‐fold higher than the plasma AUC in Oct3+/+ mice. In contrast, deletion of Oct3 significantly reduced mIBG exposure in the salivary glands by approximately 40%. The exposure of mIBG in the skeletal muscle, heart, and the lungs were also significantly reduced in Oct3‐/‐ mice, demonstrating an important role of Oct3 in mediating uptake and accumulation of mIBG in various healthy tissues. On the other hand, the plasma concentration‐time profiles of mIBG were similar between the Oct3+/+ and Oct3‐/‐ mice with no significant changes in systemic AUC and other pharmacokinetic parameters. Additionally, no changes were observed between the two groups in the kidney, the main route of elimination for mIBG, suggesting that Oct3 does not contribute to the renal elimination of mIBG. In summary, we demonstrated that OCT3‐mediated uptake of mIBG can be selectively inhibited and that blocking this pathway leads to decreased mIBG uptake and accumulation in normal tissues. Furthermore, these findings support inhibition of OCT3 as a viable clinical strategy to reduce 131I‐mIBG accumulation ...
Neuroblastoma (NB) is the most common extracranial solid tumor in pediatric cancer. Despite recent improvements, the long‐term survival rate for patients with high‐risk NB is still less than 50%. Radiolabeled meta‐iodobenzylguanidine (mIBG), 123I‐mIBG and 131I‐mIBG, are used clinically for both diagnostic and therapeutic purposes, and several clinical trials are ongoing to investigate the potential of 131I‐mIBG as a frontline therapy for high‐risk NB. Previous studies established that mIBG enters NB cells through the norepinephrine transporter (NET). However, not all NET‐expressing NBs respond to 131I‐mIBG treatment and other mechanisms may be involved in the uptake and retention of mIBG in NB. Here, we investigated the expression and potential influence of additional mIBG transporters in NB. By searching through an NCI database with 156 NB patients, mRNA Seq results showed a high and variable mRNA expression of the plasma membrane monoamine transporter (PMAT) in NB tumor samples. Interestingly, Kaplan–Meier estimation suggested an association of low PMAT expression with poor survival rates in these patients. Using qPCR analysis, we further demonstrated a high expression of PMAT in 32 local NB samples as well as two human NB cell lines SH‐SY5Y and SK‐N‐BE(2). Immunofluorescence imaging of PMAT in NB tumor specimens and cultured cells suggested that unlike its normal expression on plasma membrane, PMAT protein in NB cells is predominantly expressed in intracellular organelles but also with a low expression at the plasma membrane. In HEK293 cells stably expressing human PMAT, mIBG is avidly transported by PMAT with a Km value of 42.0 ± 10.2 μM. In SH‐SY5Y and SK‐N‐BE(2) cells, uptake of mIBG was performed with or without sodium in uptake buffer to distinguish the NET‐mediated uptake. Preliminary data showed that sodium depletion only inhibited 50–60% of mIBG uptake whereas addition of a PMAT inhibitor to sodium‐free buffer completely inhibited mIBG accumulation in both cell lines. In summary, our current study suggests that PMAT is highly expressed in NB tumors and NB‐derived cells lines. PMAT transports mIBG and may be involved in the uptake and retention of mIBG in NB cells, and hence may represent a novel determinant of mIBG tumor targeting and clinical response.Support or Funding InformationThis work is supported by NIH Grants GM066233 and GM066233‐14S1.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.