Kinetics of perrhenate uptake and comparative biodistribution of perrhenate, pertechnetate, and iodide by NaI symporter-expressing tissues in vivo

Pertechnetate (as ^99mTcO₄^-), ¹²³I^-, and ¹³¹I^- have a long and successful history of use in the diagnosis and therapy of thyroid cancer, with uptake into thyroid tissue mediated by the sodiumiodide symporter (NIS). NIS has also emerged as a potential target for radiotherapy of nonthyroid malignancies that express the endogenous or transfected symporter. Perrhenates (as ¹⁸⁸ReO₄^- and ¹⁸⁶ReO₄ ^-) are promising therapeutic substrates of NIS, although less is known about their behavior in vivo. In this study, we endeavored to characterize the biologic behavior of perrhenate, especially in relation to iodide and pertechnetate, to better explore its possible therapeutic role. Methods: We describe the simultaneous biodistribution and uptake in vivo of iodide, pertechnetate, and perrhenate in groups of healthy CD1 mice, either with or without coadministration of perchlorate (ClO₄^-), a potent NIS inhibitor. Animals administered single radiopharmaceuticals were imaged as a means of illustrating these findings. Kinetic properties of perrhenate were compared with those of iodide in a stably transfected NIS-bearing Madin-Darby canine kidney (MDCK) cell line. Results: Biodistributions of iodide, pertechnetate, and perrhenate in live mice were remarkably similar. Activity in salivary gland and stomach was severalfold greater than in blood, remained elevated over the initial 2 h, and subsequently washed out. A similar pattern characterized pertechnetate and perrhenate uptake by the thyroid, in which the 2-h concentration was slightly more elevated than at the 20-min time point. However, uptake subsequently decreased by 19 h. In contrast, iodide continued to increase through the 19-h time point, presumably as a result of organification. The addition of perchlorate sharply decreased uptake of all 3 radiopharmaceuticals by the stomach, salivary glands, and thyroid and resulted in their rapid clearance, paralleling blood-pool clearance. In tissues that do not express NIS (liver, muscle, spleen), uptake of all 3 radiopharmaceuticals was low and rapidly decreased over time, paralleling blood-pool clearance. Similar findings were seen in kidney, where only minimal amounts of NIS are expressed in tubular cells. In stably transfected MDCK cells, steady-state accumulation of iodide was approximately 4-fold higher than that of perrhenate at 30 min. No active transport was demonstrated in nontransfected MDCK cell lines or after perchlorate administration. Uptake values measured at different concentrations of substrate demonstrated saturation kinetics. Apparent maximal velocity values for perrhenate and iodide were 25.6 ± 1.4 and 106 ± 3.2 pmol/μg, respectively, and corresponding affinity constant values were 4.06 ± 0.87 and 24.6 ± 1.81 ± μmol/L. Conclusion: Perrhenate is avidly taken up by NIS in a manner similar to iodide and pertechnetate in vivo, with the exception of organification of iodide by the thyroid. By more fully appreciating the behavior of perrhenate, especially in relation to iodide and pertechnetate, we can better realize its potential role in the diagnosis and therapy of NIS-bearing tissues.