Organic cation transporter 3 mediates the non-norepinephrine transporter driven uptake of meta-[211At]astato-benzylguanidine

Abstract

Introduction: Meta-[²¹¹At]astato-benzylguanidine ([²¹¹At]MABG) accumulates in pheochromocytoma via norepinephrine transporter (NET) and leads to a strong antitumor effect, but it also distributed in normal tissues non-specifically. Meta-[¹³¹I]iodo-benzylguanidine ([¹³¹I]MIBG), an iodine-labeled analog of [²¹¹At]MABG, is known to be transported by not only NET but also organic cation transporter (OCT). The involvement of OCT in [²¹¹At]MABG uptake is still largely unknown. We investigated the involvement of OCT in the non-NET-driven uptake of [²¹¹At]MABG both in vitro and in vivo.

Methods: [¹²³I]MIBG and [²¹¹At]MABG uptake was investigated in PC-12 (rat pheochromocytoma cell line), NIH/3T3 (mouse fibroblasts cell line), ACHN (human renal cancer cell line), and BxPC-3 (human pancreatic cancer cell line). Herein, we used desipramine and dl-norepinephrine to inhibit NET, and we used steroids (hydrocortisone and prednisolone) to inhibit OCT3. The [²¹¹At]MABG uptake in OCT3-knockdown cells established with OCT3-selective siRNA was also investigated. We investigated the biodistribution of [²¹¹At]MABG in PC-12 tumor-bearing mice after a preloading of phosphate-buffered saline (PBS) or hydrocortisone solution.

Results: The uptake of both [¹²³I]MIBG and [²¹¹At]MABG was significantly inhibited by desipramine in PC-12 cells but not the other cell lines. The expression of OCT3 was relatively higher than those of the other OCT subtypes in ACHN and BxPC-3 cells. The expression of OCTs was not observed in NIH/3T3 cells. The uptake of both [¹²³I]MIBG and [²¹¹At]MABG in ACHN and BxPC-3 cells was significantly inhibited by the steroid treatments. The [²¹¹At]MABG uptake was also reduced in OCT3-knockdown cells (p < 0.001). The radioactivity of [²¹¹At]MABG was significantly reduced in normal tissues by the preloading of hydrocortisone. In contrast, there was an increasing trend of [²¹¹At]MABG uptake in the PC-12 tumors. The tumor-to-normal tissue ratio was significantly increased by the preloading of hydrocortisone compared to that of PBS.

Conclusion: Our results suggest that OCT3 is involved in non-NET-driven [²¹¹At]MABG uptake. The preloading of hydrocortisone selectively reduced [²¹¹At]MABG accumulation in normal organs in vivo. OCT3 inhibition may therefore be beneficial for a reduction of the radiation risk in healthy organs in the treatment of malignant pheochromocytomas.

Keywords: Meta-[(211)At]astato-benzylguanidine; Norepinephrine transporter; Organic cation transporter 3; α-Emitter.