18F-Fluorosulfate for PET Imaging of the Sodium-Iodide Symporter: Synthesis and Biologic Evaluation In Vitro and In Vivo

Abstract

Anion transport by the human sodium-iodide symporter (hNIS) is an established target for molecular imaging and radionuclide therapy. Current radiotracers for PET of hNIS expression are limited to ¹²⁴I^- and ¹⁸F-BF₄^- We sought new ¹⁸F-labeled hNIS substrates offering higher specific activity, higher affinity, and simpler radiochemical synthesis than ¹⁸F-BF₄^- METHODS: The ability of a range of anions, some containing fluorine, to block ^99mTcO₄^- uptake in hNIS-expressing cells was measured. SO₃F^- emerged as a promising candidate. ¹⁸F-SO₃F^- was synthesized by reaction of ¹⁸F^- with SO₃-pyridine complex in MeCN and purified using alumina and quaternary methyl ammonium solid-phase extraction cartridges. Chemical and radiochemical purity and serum stability were determined by radiochromatography. Radiotracer uptake and efflux in hNIS-transduced HCT116-C19 cells and the hNIS-negative parent cell line were evaluated in vitro in the presence and absence of a known competitive inhibitor (NaClO₄). PET/CT imaging and ex vivo biodistribution measurement were conducted on BALB/c mice, with and without NaClO₄ inhibition.

Results: Fluorosulfate was identified as a potent inhibitor of ^99mTcO₄^- uptake via hNIS in vitro (half-maximal inhibitory concentration, 0.55-0.56 μM (in comparison with 0.29-4.5 μM for BF₄^-, 0.07 μM for TcO₄^-, and 2.7-4.7 μM for I^-). Radiolabeling to produce ¹⁸F-SO₃F^- was simple and afforded high radiochemical purity suitable for biologic evaluation (radiochemical purity > 95%, decay-corrected radiochemical yield = 31.6%, specific activity ≥ 48.5 GBq/μmol). Specific, blockable hNIS-mediated uptake in HCT116-C19 cells was observed in vitro, and PET/CT imaging of normal mice showed uptake in thyroid, salivary glands (percentage injected dose/g at 30 min, 563 ± 140 and 32 ± 9, respectively), and stomach (percentage injected dose/g at 90 min, 68 ± 21).

Conclusion: Fluorosulfate is a high-affinity hNIS substrate. ¹⁸F-SO₃F^- is easily synthesized in high yield and very high specific activity and is a promising candidate for preclinical and clinical PET imaging of hNIS expression and thyroid-related disease; it is the first example of in vivo PET imaging with a tracer containing an S-¹⁸F bond.

Keywords: 18F; PET; fluorosulfate; human sodium/iodide symporter (SC5A5); thyroid.