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.
The sodium–iodide symporter (NIS) is capable of intracellularly concentrating certain small anions against their electrochemical gradient and is biologically important for accumulating iodide in thyroid follicles for synthesis of thyroid hormones. Several radioactive substrates of human NIS (hNIS; SC5A5) have been used for radionuclide therapy (131I−) and SPECT (131/123I−, 99mTcO4−) and PET (124I−) imaging of thyroid-related disorders (1). Other radioactive substrates currently being evaluated as next-generation radiopharmaceuticals for these purposes include 186/188ReO4− for therapy (2,3) and 18F-BF4− for PET (4,5). 18F-BF4− is the prototype 18F-labeled NIS tracer, offering the advantages of moderate half-life, excellent imaging characteristics associated with a high yield of low-energy positrons, minimal undesirable photon emissions, low absorbed radiation dose (6), and wide availability. With these characteristics, it is expected to offer imaging superior to that of SPECT with 123/131I and PET with 124I. However, among the known substrates of NIS, it has mid-ranking affinity (half-maximal inhibitory concentration [IC50] of 1.2 μM for inhibition of 124I− uptake, in comparison with an IC50 of 0.1 μM for ClO4− in the same assay (7)), and chemical constraints during synthesis lead to low specific activity (4,8). Consequently, alternative 18F-labeled NIS substrates that overcome these limitations are desirable.
A search of the literature for alternative fluorine-containing NIS substrates reveals several that merit further investigation. Both SO3F− and PO2F2− are known to inhibit radioiodide uptake in mouse thyroid (9), yet in quantitative terms their effectiveness as substrates or inhibitors of hNIS is unknown. PF6− is a highly potent inhibitor of rat NIS (IC50, 15 nM (10)). We therefore performed a preliminary comparison of their ability to inhibit hNIS, using uptake in hNIS-expressing cells with 99mTc-pertechnetate as a probe. From this survey we selected SO3F− on the basis of its high affinity and potentially straightforward radiolabeling.
Here we report a comparison of fluorine-containing anions with other known hNIS substrates and, for the first time, a simple method for radiosynthesis of 18F-SO3F− and its biologic evaluation in hNIS-expressing cells and in vivo in mice.
MATERIALS AND METHODS
General
Unless otherwise stated, all chemicals were from Sigma Aldrich. Ammonium difluorophosphate was synthesized as previously described (11) (the nuclear magnetic resonance data are in Supplemental Figs. 1 and 2; supplemental materials are available at http://jnm.snmjournals.org). 18F− was produced as previously described (4). 99mTcO4− was obtained from Guy’s Hospital Radiopharmacy and was used about 4.5 h after elution (24 h between elutions). Animal experiments were performed under a U.K. Home Office license following the guidance of the U.K. Research Councils and the Medical Research Charities on responsibility in the use of animals in bioscience research, as approved by the local institutional ethics committee. Inhibitory potency was assessed by pertechnetate uptake blockade, using 2 different cell lines expressing hNIS: a virally infected breast adenocarcinoma cell line stably expressing hNIS (MTLn3E.∆34 CXCR4-eGFP hNIS-tag-RFP, hereinafter referred to as 3E.Δ-NIS (12)) and a human colon carcinoma cell line transfected to stably express hNIS under selection pressure (HCT116-hNIS-C19 (5)). Parental cell lines served as negative controls. HCT116 was also used to study the cellular uptake of 18F-SO3F− in the absence of hNIS. Ionic volumes were either taken directly from the literature (13) or calculated from crystallographic data using previously described methods (PO2F2− (14); TcO4− (15,16)).
Optimized Radiosynthesis
The following procedure was arrived at after optimization of reaction time, temperature, and conditions. Solutions of K2CO3 (5.2 mg) in H2O (0.4 mL) and of K[2.2.2] (14.2 mg) in MeCN (1.1 mL) were prepared. K2CO3 solution (0.2 mL) was added to the K[2.2.2] solution to form quaternary methyl ammonium eluent. 18F-fluoride was trapped from 18O-H2O on a quaternary methyl ammonium cartridge (preconditioned with NaHCO3 [10 mL] and H2O [10 mL]), eluted with quaternary methyl ammonium eluent (0.9 mL), and dried by azeotropic distillation of MeCN (0.4 mL) under a N2 stream at 110°C for 5 min and then twice further at 95°C. SO3–pyridine complex (5 mg) in MeCN (1 mL) was then added, followed by heating to 80°C for 10 min. The reaction was quenched with H2O (2 mL), and the solution was passed through a neutral alumina cartridge (preconditioned with H2O [20 mL] and air [10 mL]) and a quaternary methyl ammonium cartridge (preconditioned with 1 M NaCl [5 mL] and H2O [10 mL]). The quaternary methyl ammonium was washed with H2O (4 mL), and the product was eluted with 0.9% NaCl (0.4 mL), ready for biologic evaluation. Radio–thin-layer chromatography (Rf values, 0.43 for SO3F− and 0 for F−) was performed using a neutral alumina stationary phase (10 × 80 mm, Polygram ALOX N/UV254; Macherey-Nagel) and a methanol mobile phase on a Mini-Scan scanner (LabLogic) with a β+ probe (B-FC-3600; LabLogic). Radiochemical identity, purity, specific activity, and sulfate concentration were established by ion chromatography (930 Compact IC Flex; Metrohm) with in-line conductimetric and γ-detectors, using a I-524A ion chromatography column (4.6 × 100 mm; Shodex) eluted with aqueous 2.3 mM phthalic acid and 2.3 mM tris(hydroxymethyl)aminomethane (pH 5.0) at a flow rate of 1.5 mL/min and a column temperature of 30°C. The limit of detection was 894 ng/mL for KSO3F. Pyridine was determined by high-performance liquid chromatography (1200 series; Agilent), with the ultraviolet/visible detector adjusted to 210 nm, on a Zorbax 300-SCX column (Agilent) eluted with 0.2 M sodium phosphate, pH 3, at 1.0 mL/min. The K[2.2.2] concentration was determined as described previously (17).
99mTcO4− Uptake Inhibition Assay (HCT116-hNIS-C19 Cells)
HCT116-hNIS-C19 cells seeded in 12-well plates (5 × 105 cells per well) were incubated with 5% CO2 at 37°C for 24 h, washed twice with Hanks balanced salt solution (HBSS), and then incubated for 30 min in HBSS with the chosen inhibitory anion (700 μL of a 1 × 10−2 to 1 × 10−13 M concentration, n = 3 each). 99mTcO4− (0.1 MBq, ∼37 pM) in HBSS (50 μL) was added, followed by 30 min of incubation. The medium was removed, and the cells were washed with HBSS (750 μL) and extracted with 1 M NaOH (750 μL). Bound radioactivity and unbound radioactivity were γ-counted. IC50 values were determined by least-squares fitting of a sigmoidal curve (Prism, version 5.03; GraphPad).
99mTcO4− Uptake Inhibition Assay (3E.Δ-NIS Cells)
3E.Δ-NIS cells seeded in 6-well plates were incubated as above for 12 h, washed with phosphate-buffered saline (1 mL), incubated with phosphate-buffered saline (1 mL) containing the inhibitor (range, 1 × 10−11 to 5 × 10−4 M, n = 3 each) and 99mTcO4− (50 kBq/mL, ∼14 pM) for 30 min, washed twice with phosphate-buffered saline (1.4 mL), trypsinized (900 μL of 250 U.S. Pharmacopeia trypsin units/mg in phosphate-buffered saline) at 37°C for 5 min, resuspended in phosphate-buffered saline (250 μL), and centrifuged. Cell-bound and combined media and washings were γ-counted. Data were fitted to sigmoidal curves using Origin, version 8.0 (Microcal).
Stability Studies
18F-SO3F− was incubated in 0.9% NaCl or 50 mM glycine/HCl buffer (pH 3.0) at 25°C for 4 h and analyzed by ion chromatography and thin-layer chromatography as described above. 18F-SO3F− (∼25 MBq) in 0.9% NaCl (100 μL) was incubated with human serum (1 mL) at 37°C for 4 h. Hourly samples (100 μL) were mixed with ethanol (100 μL) to precipitate proteins and were centrifuged, and the supernatants were analyzed by thin-layer chromatography.
18F-SO3F− Cell-Binding Study
HCT116-hNIS-C19 or HCT116 cells seeded in 6-well plates (1 × 106 cells per well) were incubated as above for 24 h, washed twice with HBSS, and incubated in HBSS (950 μL) with or without NaClO4 (20 μM) for 30 min. 18F-SO3F− (0.1 MBq) in HBSS (50 μL) was then added, followed by incubation for 30 min (in triplicate). The cells were washed with HBSS (1 mL) and extracted with 1 M NaOH (1 mL), and the cell extract and medium/washings were γ-counted. Intracellular-to-extracellular concentration ratios were estimated using a mean cell diameter of 18.4 μm (18).
18F-SO3F− Self-Inhibition
HCT116-hNIS-C19 cells seeded in 12-well plates (5 × 105/well) were incubated as above for 24 h, washed twice with HBSS, and incubated with KSO3F (range, 1 × 10−2 to 10−12 M, n = 3 each) in HBSS (700 μL) for 30 min. 18F-SO3F− (0.1 MBq) in HBSS (50 μL) was then added and incubated for 30 min. The cells were washed and counted, and IC50 was determined as above.
18F-SO3F− Cell Efflux
HCT116-hNIS-C19 cells were prepared as for the self-inhibition experiment above, incubated with 18F-SO3F− (0.1 MBq) in HBSS (750 μL) for 60 min, and washed with cold HBSS (750 μL) before the addition of fresh tracer-free medium. The cells and medium were sampled and γ-counted as described above.
PET Imaging
Female BALB/c mice (4–8 wk old) were given either NaClO4 (250 mg/kg intraperitoneal injection as a competitive substrate for inhibitive effect) in 0.9% NaCl or an equal volume of plain 0.9% NaCl (sham inhibitor). One hour later, 18F-SO3F− (∼5 MBq in 0.9% NaCl, 150 μL) was injected (tail vein) under anesthesia maintained using 1.5%–2% isoflurane gas (Isocare; Animalcare) in O2. The mice were then transferred to the warmed scanner bed while anesthesia was maintained and vital signs monitored. Dynamic PET was acquired for 2 h (starting <1 min after tracer injection) on the nanoScan PET/CT scanner (Mediso) in list mode using a 400- to 600-keV energy window and a coincidence relation of 1:3, followed by CT (55-keV x-rays, 1,000-ms exposure, 360 projections, and 1 pitch). The PET data were processed with Tera-tomo software (supplied with the system), applying corrections for attenuation, scatter, and dead time; rebinned; and reconstructed into a series of 5-min frames for the first 30 min and 30-min frames thereafter. VivoQuant (InviCro) software was used to view and quantify the data. Regions of interest were manually drawn over thyroid, salivary glands, stomach, bladder, left ventricle (for blood), and muscle at a threshold of 10% of the maximum count to define the edges. Time–activity curves were generated and expressed as percentage injected dose (%ID, with whole-body region-of-interest activity, excluding tail, as the injected dose) and %ID per ex vivo weight of organ (%ID/g).
Ex Vivo Biodistribution
At 2.25 h after injection, the scanned mice were sacrificed by cervical dislocation and the tissues were harvested, weighed, and γ-counted (channels 175–220, 1282 Compugamma; LKB). Data are presented as %ID/g (with total activity of all body parts—including carcass and any urine excreted at the time of sacrifice but excluding the tail—as the injected dose). Calculations requiring thyroid weight were conducted using a standard weight of 3.6 mg (4,19).
RESULTS
99mTcO4− Uptake Inhibition Assay
The IC50 for the anions assessed for their ability to block 99mTcO4− or 18F-BF4− uptake via hNIS are detailed in Supplemental Table 1, which also includes comparable literature data (5,7,8,10,20)). The potency was in the order SCN− < PO2F2− < I− < BF4− ≈ SO3F− < ReO4− < TcO4− < PF6−, which roughly parallels the increase in ionic volume (Fig. 1). Literature data on inhibitory potency and affinity of anions are not comprehensive but when available are broadly consistent with our results. Of the fluorine-containing substrates, PF6− (IC50, 21 nM) was the most potent, but SO3F− had potency (IC50, 0.56 μM) comparable to BF4− and better than iodide (IC50, >2.7 mM) and is more likely to be amenable to simple radiosynthesis than BF4− or PF6−. SO3F− was therefore selected for development of a new PET tracer.
Plot of ionic volume against inhibitory potency (logIC50 based on ability to block anion uptake in NIS-expressing cells) for univalent anions examined in this work and other published works (5–7,9,17). Differing methods, cell lines, probes, and counter-ions were used in each study; further details are given in Supplemental Table 1. 1 = CN−; 2 = Br−; 3 = N3−; 4 = NO3−; 5 = SCN−; 6 = I−; 7 = BF4−; 8 = ClO4−; 9 = TcO4−; 10 = ReO4−; 11 = SO3F−; 12 = PO2F2−; 13 = PF6−.
Radiosynthesis of 18F-SO3F−
Reaction of K[2.2.2]/18F-KF with SO3–pyridine complex (Fig. 2) afforded 18F-SO3F−. Varying the reaction conditions (Supplemental Table 2) led to crude radiochemical yields of as high as 65%. Passage through an alumina column removed unreacted 18F−, and the eluted product could be trapped on a quaternary methyl ammonium cartridge, allowing washing to remove pyridine and K[2.2.2], and eluted in 0.9% NaCl (radiochemical yield, 31.6% ± 9.5% [n = 3, decay corrected]; radiochemical purity, 96% ± 1% [always ≥ 95%]). The total synthesis time from end of bombardment was less than 1 h. The identity of the product was confirmed by ion chromatography with coinjection of authentic SO3F− as a reference (Fig. 3). With a starting radioactivity of about 750 MBq, a specific activity of at least 48.5 ± 13.4 GBq/μmol (n = 3) was obtained in a volume of 0.4 mL. The product contained residual pyridine (1.4 ± 1 μg/mL, 0.56 μg total), K[2.2.2] (<6.25 μg/mL, <2.5 μg total), and SO42− (302 ± 26 μg/mL, 120.7 μg total) and had a pH of 7.
Reaction scheme for production of 18F-SO3F− from SO3–pyridine complex.
Chromatograms resulting from ion chromatography analysis of 18F-SO3F−: conductivity trace for no-carrier-added 18F-SO3F− (Α); conductivity trace for coinjection of 18F-SO3F− with 19F-SO3F− (80 μg/mL) (B); radioactivity trace for no-carrier-added 18F-SO3F− (C); radioactivity trace for 18F− in H2O (D). a = Cl−; b = SO42−; c = SO3F−; i.p. = injection peak; s.p. = system peak.
In Vitro Uptake, Efflux, and Self-Inhibition of 18F-SO3F−
Significant uptake of the radiotracer in hNIS-expressing HCT116-C19 cells was observed and was blocked by NaClO4 (Fig. 4). No uptake occurred in the parental cell line HCT116 (which does not express hNIS), with or without NaClO4. Both uptake and efflux from the cells reached equilibrium within 80 min (Supplemental Figs. 3 and 4). Uptake at equilibrium was consistent with an intracellular-to-extracellular 18F-SO3F− concentration ratio of 76:1. Under identical conditions, the ratio for 99mTcO4− was 44:1 and that for 18F-BF4− was 24:1 (calculated from data obtained during this and other studies (7), respectively). Inhibition of 18F-SO3F− uptake in HCT116-C19 cells by KSO3F occurred with an IC50 of 1.6 μM (Fig. 5).
Uptake of 18F-SO3F− in HCT116-C19 (hNIS-expressing) and HCT116 (hNIS-negative) cell lines in presence and absence of NaClO4 (20 μM). Error bars represent SD. Intergroup differences were measured by unpaired t testing. *P < 0.00001.
Inhibition of uptake of 18F-SO3F− by 19F-SO3F− in HCT116-C19 cells. Error bars represent SD. IC50 = 1.6 μM.
Serum Stability
Ion chromatography and thin-layer chromatography of the tracer both as formulated and under acidic conditions (pH 3.0) were unchanged (radiochemical purity > 95%) after 4 h at room temperature. Similarly, the radiochemical purity was more than 95% after 4 h of incubation (37°C) in serum, as assessed by thin-layer chromatography of the supernatant after protein precipitation with ethanol (no significant radioactivity was associated with the protein pellet). These data are summarized in Supplemental Table 3.
PET/CT Imaging
PET/CT of 18F-SO3F− (Fig. 6) revealed prominent uptake in the thyroid, stomach, and salivary glands of normal mice, but this uptake was suppressed in the perchlorate-treated mice. Time–activity curves (%ID/g, Supplemental Figs. 5 and 6; %ID, Supplemental Figs. 7 and 8) over a 2-h imaging period showed thyroid and salivary gland uptake plateauing at around 30–45 min (563 ± 140 and 32 ± 9 %ID/g, respectively, at 30 min) and stomach uptake plateauing at around 90 min (68 ± 21 %ID/g at 90 min). Bone uptake first became detectable at 30 min and increased to 3 ± 1 %ID/g by 60 min.
(A–C) PET/CT maximum-intensity projections of BALB/c mice 25–30 min after injection of 18F-SO3F− without perchlorate (A), 90–120 min after injection of 18F-SO3F− without perchlorate (B), and 25–30 min after injection of 18F-SO3F− in presence of NaClO4 (250 mg/kg) (C). (D) Ex vivo biodistribution data at 2.25 h after injection (n = 3). Error bars represent SD.
Ex Vivo Biodistribution in Mice
At 2.25 h after injection, uptake of 18F-SO3F− consistent with the PET scans was observed in thyroid (144 ± 71 %ID/g), stomach (59 ± 10 %ID/g), and salivary glands (18 ± 4 %ID/g); uptake in these organs was reduced to 4.3 ± 4.6, 3.0 ± 1.7, and 2.6 ± 1.7 %ID/g, respectively, in mice administered NaClO4 (Fig. 6, and shown as SUV in Supplemental Fig. 9). The radioactivity observed in the bladder (15 ± 14 %ID/g) indicates renal excretion. A small amount of uptake in bone (17 ± 3 %ID/g) occurred and was not blocked by perchlorate, suggesting that some defluorination occurs over 2 h that was not seen during incubation in serum.
DISCUSSION
Evaluation of the inhibitory potency of fluorosulfate on hNIS confirmed its previously suggested (9) status as a potent inhibitor/competitive substrate with greater potency than I− and possibly BF4−. The range of IC50 values found for both BF4− (currently undergoing evaluation as a clinical PET tracer for hNIS) and SO3F− is lower than that found for iodide (Fig. 1; Supplemental Table 1). Although inhibitory potency may not be directly indicative of ability to reach a high intracellular–extracellular concentration gradient in hNIS-expressing cells (which is the key requirement of a good PET tracer for hNIS), the data do suggest that 18F-SO3F− would be a high-affinity NIS tracer. Analysis of the ionic volume (13) in relation to the IC50 of several anions against radioiodide (10), 99mTcO4−, or 18F-BF4− uptake suggests that larger anions are more effective inhibitors of hNIS (Fig. 1). Accordingly, PF6− was found to be the most potent inhibitor examined (IC50, 21 nM). However, in contrast to other known fluorine-containing NIS inhibitors, such as PF6− and BF4−, SO3F− has the advantage of bearing a single fluorine atom. Thus, no-carrier-added radiolabeling of SO3F− would yield a specific activity limited only by the 18F source. We therefore chose 18F-SO3F− as a target for development of a new 18F-labeled hNIS tracer.
Our strategy for synthesizing 18F-SO3F− involved using a Lewis acid–base SO3–pyridine adduct, which is readily available in pure form. Radiolabeling proceeded via displacement of the pyridine by 18F− followed by quenching with water to hydrolyze residual starting material to sulfate. Formation of 18F-SO3F−, confirmed by thin-layer chromatography and ion chromatography, was observed under all reaction conditions. More basic conditions with elevated temperature and precursor concentration enhanced incorporation of 18F− into 18F-SO3F−. Further optimization may lead to improved yields, for example, using pyridine alternatives as the Lewis base in the precursor complex.
Purification using sequential alumina and quaternary methyl ammonium cartridges yielded 18F-SO3F− in saline, conveniently suitable for biologic use, with high radiochemical and chemical purity. The residual pyridine and K[2.2.2] in the final product were within the acceptance limits set out in the British Pharmacopoeia. The sulfate concentration resulting from precursor hydrolysis is also safe considering that MgSO4 can be administered in gram quantities intravenously with minimal side effects (21). The specific activity of 18F-SO3F− (≥48.5 GBq/μmol) is significantly higher than that reported for 18F-BF4− (1 (4), 5 (7), or up to 8.8 (22) GBq/μmol), as there is no 19F naturally present in the precursor. The specific activity is therefore limited only by that of the initial 18F− and the purity of the other reagents. The method is amenable to automation, and modification of the radiosynthesis to conform to good manufacturing practices should be straightforward. The high specific activity minimizes the pharmacologic dose administered.
Assessing the biologic activity of 18F-SO3F− in vitro in an hNIS-expressing cell line confirmed specific NIS-mediated uptake that was absent both in hNIS-negative cells and in the presence of competitive inhibition by perchlorate. Uptake and efflux showed kinetics (equilibrium reached in under 80 min) appropriate for in vivo use. The specific activity of the 18F-SO3F− is very high and more than sufficient to realize the potential benefit of the high affinity: assuming an injected activity of about 10 MBq for a PET/CT scan in a mouse with an extracellular fluid volume of 5 mL, the in vivo concentration of 18/19F-SO3F− will be less than 41 nM, well below the concentration at which in vitro inhibition is observed (Fig. 5). In addition to the high affinity of 18F-SO3F− for hNIS demonstrated by the IC50 of 0.56 μM, the plateau intracellular-to-extracellular radioactivity concentration ratio in vitro (76:1) was higher than for either 99mTc-pertechetate (44:1) or 18F-BF4− (24:1), suggesting that—other pharmacokinetic features being similar—a higher target-to-background ratio might be expected in PET images.
PET/CT imaging of 18F-SO3F− in normal mice during the first hour after injection revealed uptake at sites known to express NIS. This signal was abolished by coadministration of NaClO4, showing that the tracer is an excellent substrate for mouse NIS as well as hNIS. Maxima were reached in both thyroid uptake (Supplemental Fig. 5) and thyroid-to-muscle (as a background reference) uptake ratio (Supplemental Fig. 10) after 30 min, confirming this as the ideal imaging time point. Although barely detectable at 30 min (%ID/g < 1% of that in thyroid), there was increasing signal in the bones at later time points, both in PET images (Supplemental Figs. 11–13) and in the ex vivo biodistribution data. Although the in vivo hydrolysis of fluorosulfate to sulfate and fluoride is known (23), it is unlikely to hamper the utility of the tracer in hNIS imaging because uptake at sites of NIS expression maximizes long before bone uptake is significant.
CONCLUSION
Several fluorine-containing anions are potent hNIS inhibitors. Among them, SO3F− is an hNIS inhibitor containing a single fluorine atom and has an inhibitory potency similar to or greater than that of tetrafluoroborate and greater than that of iodide. It is readily synthesized in 18F-radiolabeled form in high yield, high radiochemical purity, and high specific activity. 18F-SO3F− shows NIS-specific uptake in vitro and in vivo and is an excellent candidate for further preclinical and clinical evaluations as an hNIS PET imaging agent for application in thyroid-related disease and hNIS reporter gene imaging. To our knowledge, this work is the first example of imaging in vivo by PET/CT using a tracer with an S–18F bond. The simplicity of 18F-SO3F− synthesis, and its adequate in vivo stability, suggest that further attention should be given to “inorganic” 18F-radiopharmaceutical synthesis (24) whereby atoms other than carbon, such as aluminum, silicon, boron, and now sulfur (25), serve as binding sites for 18F.
DISCLOSURE
This work was supported by an EPSRC Industrial CASE studentship; the Biomedical Research Centre Award to Guy’s & St Thomas’ NHS Foundation Trust in partnership with King’s College London and King’s College Hospital NHS Foundation Trust; an MRC Confidence in Concept Award administered by King’s Health Partners; the Centre of Excellence in Medical Engineering funded by the Wellcome Trust and EPSRC under grant WT088641/Z/09/Z; and the King’s College London and UCL Comprehensive Cancer Imaging Centre funded by the CRUK and EPSRC in association with the MRC and DoH (England). The views expressed are those of the authors and not necessarily those of the NHS, the NIHR, or the DoH. PET scanning equipment was funded by an equipment grant from the Wellcome Trust. Drs. Khoshnevisan and Blower have filed a patent relating to 18F-SO3F−. No other potential conflict of interest relevant to this article was reported.
Footnotes
Published online Aug. 18, 2016.
- © 2017 by the Society of Nuclear Medicine and Molecular Imaging.
REFERENCES
- Received for publication May 16, 2016.
- Accepted for publication July 11, 2016.
