Research ArticleTheranostics

Radiolabeled (4-Fluoro-3-Iodobenzyl)Guanidine Improves Imaging and Targeted Radionuclide Therapy of Norepinephrine Transporter–Expressing Tumors

Aiko Yamaguchi, Hirofumi Hanaoka, Tetsuya Higuchi and Yoshito Tsushima
Journal of Nuclear Medicine May 2018, 59 (5) 815-821; DOI: https://doi.org/10.2967/jnumed.117.201525

Abstract

18F- or 131I-labeled (4-fluoro-3-iodobenzyl)guanidine (FIBG) has been a promising yet unattainable derivative of radioiodine-labeled meta-iodobenzylguanidine (MIBG), because of the complex radiofluorination method. In this study, we proposed a 2-step radiosynthetic method to obtain 18F-FIBG and evaluated the diagnostic and therapeutic potential of 18F-FIBG and 131I-FIBG in a pheochromocytoma model (PC-12). Methods: 18F-FIBG was prepared from a (mesityl)(aryl)iodonium salt precursor in the presence of a copper catalyst. Biodistribution studies, PET imaging, and a therapeutic study were performed on the PC-12 xenograft mice with either 18F- or 131I-FIBG. The association between the therapeutic effect and the tumor uptake of pretherapy 18F-FIBG PET was also evaluated. Results: The copper-mediated radiofluorination method readily yielded 18F-FIBG, as well as its regioisomer, 18F-IFBG. The isolated 18F-FIBG showed a higher accumulation in the PC-12 xenograft tumor than in any other tissue. The high tumor uptake of 18F-FIBG allowed clear tumor visualization in the PET images as early as 1 h after injection, with an excellent tumor-to-background ratio. A biodistribution study with 131I-FIBG revealed its higher and prolonged retention in the tumor in comparison with 125I-MIBG. As a result, a therapeutic dose of 131I-FIBG delayed tumor growth significantly more than did 131I-MIBG. The tumor uptake of 18F-FIBG was proportional to the therapeutic effect of 131I-FIBG. Conclusion: These results suggest the potential usefulness of FIBG as a diagnostic and therapeutic agent for the management of norepinephrine transporter (NET)–expressing tumors.

Radioiodine-labeled meta-iodobenzylguanidine (MIBG) plays a role in the diagnosis and treatment of neuroblastomas and malignant pheochromocytomas (1–3). 123I-MIBG is preferred for diagnostic imaging, whereas 131I-MIBG therapy has produced a noticeable response rate: most patients with malignant pheochromocytomas achieve disease stabilization, and approximately 30% of relapsed or refractory patients with neuroblastomas achieve a complete or partial response to therapy. Despite this clinical usefulness, both of these radioiodine-labeled MIBGs have their limitations (1,2,4), and attempts have therefore been made to develop alternative MIBG derivatives (5–8).

Among the MIBG derivatives developed thus far, a promising pair of candidates is the combination of 18F- and 131I-labeled (4-fluoro-3-iodobenzyl)guanidine (FIBG) (Fig. 1A) (8). Although both MIBGs and FIBGs can offer integrated diagnosis and therapy by the structurally identical agents, FIBGs may provide additional value because they allow radiolabeling with 18F, the most frequently used positron-emitter isotope. The diagnostic superiority of a PET probe over a single-photon probe for image-guided therapy is exemplified by the success of radiolabeled peptides targeting somatostatin receptors (9). The MIBG derivatives evaluated in this study, 18F- and 131I-FIBG, showed basic properties comparable or superior to MIBG in cellular and normal-mouse studies (10,11). However, likely because of the complex radiosynthesis of 18F-FIBG (8), further properties, such as tumor uptake and the therapeutic effectiveness of 131I-FIBG, remain unknown.

FIGURE 1.
FIGURE 1.

(A) Chemical structures of radiolabeled MIBG and FIBG. (B) Synthesis of 6a and 6b. (C) Radiosynthesis of 8a and 8b. Boc = tert-butoxycarbonyl; (Boc)2O = di-tert-butyl dicarbonate; BPO = benzoyl peroxide; DAMP = 4-(dimethylamino)pyridine; DMF = N,N-dimethylformamide; NBS = N-bromosuccinimide; TEA = triethylamine; TFA = trifluoroacetic acid; THF = tetrahydrofuran; TMSOAc = trimethylsilyl acetate; TMSOTf = trimethylsilyl triflate.

Owing to recent developments in transition-metal–mediated radiofluorination methods for arenes, simple preparations of various 18F-fluoroarenes have become possible, including probes that were previously difficult to develop (12). Given the applicability of these techniques to the radiosynthesis of another MIBG analog, meta-18F-fluorobenzylguanidine (13–15), we hypothesized that the synthesis of 18F-FIBG could be simplified by adapting one of these methods.

In this study, we developed a 2-step 18F-FIBG preparation method using the copper-mediated fluorination of mesityl(aryl)iodonium salt, a technique developed by Ichiishi et al. (16). The usefulness of 18F-FIBG for the detection and 131I-FIBG for the treatment of norepinephrine transporter (NET)–expressing tumors was evaluated using pheochromocytoma xenograft mice.

MATERIALS AND METHODS

Unless otherwise stated, reagents and solvents were commercially available and used without further purification. Reagents were purchased from Sigma-Aldrich, TCI, or Wako. No-carrier-added radioiodinated MIBG derivatives were prepared as previously described (17).

Synthesis of Iodonium Salt Precursors (Compounds 6a and 6b)

The mesityl-iodonium salt precursors were prepared according to published methods (15,16) with slight modifications (Fig. 1B). A cold fluorination reaction was performed as previously described (Supplemental Fig. 1A; supplemental materials are available at http://jnm.snmjournals.org) (16).

Synthesis of 18F-FIBG and 18F-IFBG (Compounds 8a and 8b)

18F-fluoride was produced via the 18O(p,n)18F nuclear reaction using a cyclotron at Gunma University Hospital. The 18F labeling of the intermediate compounds (7a and 7b) was performed as previously described (16) with subtle modifications (Fig. 1C). A 2-mL volume of water and a grain of Chelex 100 chelating resin (Bio-Rad) were then added to the reaction vial to trap the copper ion. The mixture was diluted with an additional 10 mL of water and subjected onto a C18 Sep-Pak cartridge (Waters Corp.) via a filtration unit. Unreacted 18F was washed with an additional 10 mL of water, and the column was dried with N2. The mixture (7a and 7b) was eluted with 1.5 mL of CH3CN into a glass tube. The elution was evaporated with a stream of N2 at 60°C, and then the deprotection reaction was performed with trifluoroacetic acid at 75°C. After 15 min, the solvent was evaporated with a stream of N2. The resulting regioisomer mixture of 18F-FIBG and 18F-IFBG (8a and 8b) was diluted with approximately 1 mL of 25/75 MeOH/H2O, and the regioisomers were separated using semipreparative high-performance liquid chromatography (HPLC). The radiochemical yield (RCY) of the intermediate compound (7a and 7b) was determined by dividing the integrated area under the objective compound spot by the total integrated area of the thin-layer chromatography plate (Supplemental Fig. 2).

Cellular Studies for MIBG Derivatives

The rat pheochromocytoma cell line PC-12 was obtained from the American Type Culture Collection. The cellular uptake studies for 18F-FIBG and 18F-IFBG were performed as previously described (10). The cellular uptake and release profiles for 131I-MIBG and 131I-FIBG in a 3-dimensional cell culture model, and the cellular therapeutic study for 131I-FIBG and 131I-MIBG, were performed as previously described (11) with subtle modifications. For a 3-dimensional culture model, the PC-12 cells were seeded into 96-well Nunclon Sphera microplates (Thermo Fisher Scientific, 1.0 × 105 cells per well) 24 h before the experiments.

Preparation of PC-12–Bearing Mice

All experimental protocols were approved by the Laboratory Animal Care and Use Committee of Gunma University. Five- to 7-wk-old female BALB/c nu/nu mice (Japan SLC) were inoculated with PC-12 cells (5 × 106 cells per mouse) in the flank. Approximately 3 wk after inoculation, the experiments described below were performed.

Tissue Distribution in Mice

Two studies were conducted on PC-12–bearing mice. First, a biodistribution study was performed in a paired-label format by coinjecting the mice with 50 kBq of 18F-labeled and 5 kBq of 125I-labeled MIBG derivatives (18F-FIBG + 125I-FIBG, 18F-IFBG + 125I-IFBG, or the regioisomer mixture + 125I-MIBG; 4‒5 mice per group) at 1 h after injection. Next, a biodistribution study was also performed in a paired-label format by coinjecting the mice with 5 kBq each of 125I-MIBG and 131I-FIBG at 1, 3, 6, 24, and 48 h (4‒5 mice per group), as well as at 5 d, after injection (3 mice per group). In both studies, the mice were humanely killed by decapitation. The tissues of interest were isolated and weighed. A γ-counter (ARC7001; Hitachi Aloka Medical) was used to determine 18F, 125I, or 131I activity. Another biodistribution study was performed on normal mice with the injection of 50 kBq each of 125I-FIBG or 125I-MIBG, and the absorbed radiation dose in humans was estimated using OLINDA/EXM, version 1.1 (Vanderbilt University).

PET Imaging and Image Analysis

The PC-12–bearing mice were intravenously administered 1–2 MBq of 18F-FIBG, 18F-IFBG, or the regioisomer mixture. The mice were anesthetized using isoflurane inhalation, and PET scans were performed 1 and 2 h after administration of the MIBG derivative using a small-animal PET scanner (Inveon; Siemens) with 10-min emission scanning. For 18F-FDG PET imaging, the PC-12–bearing mice were intravenously administered 5 MBq of 18F-FDG after a 12-h fast, on the next day of 18F-FIBG PET imaging. PET imaging was performed as described above. Semiquantitative analysis was performed for each identified tumor using AMIDE software (Stanford University). Volumes of interest were drawn manually to trace the contours of the tumor without correcting for partial-volume effects. The background region of interest was placed in the muscles surrounding the shoulder. The mean tumor uptake was divided by the mean background uptake to calculate the tumor-to-background ratio. SUVmax was determined using an Inveon Research Workplace workstation (Siemens).

Therapeutic Study In Vivo

When the tumors were fully established (0.16 ± 0.08 cm3), the animals were randomly assigned to groups. There was no significant difference in average tumor size between groups. The mice were intravenously administered 131I-MIBG (10 MBq) or 131I-FIBG (1, 3, 5, or 10 MBq). Saline-injected mice were used as controls. At least 5 mice were allotted to each group, with the exception of the 131I-FIBG 1-MBq group, which consisted of only 3 mice. The weight and tumor diameters of the mice were measured regularly. The tumor volumes were determined using the following formula: (length) × (width)2 × 0.5. Each tumor volume was divided by the initial tumor volume to determine the relative tumor volume. To evaluate the relationship between 18F-FIBG uptake and the therapeutic effect of 131I-FIBG, 5 MBq of 131I-FIBG were injected into each animal on the next day of 18F-FDG PET imaging (2 d after the 18F-FIBG PET imaging), and the animals were monitored as described above.

Statistical Analysis

GraphPad Prism was used for the statistical analyses. Data are presented as mean ± SD. Statistical significance was determined on the basis of the Student t test (a 2-sample test or a paired test) or ANOVA with the Tukey post hoc test when appropriate. Simple correlation between variables was analyzed using the Pearson correlation coefficient. Values of P that were less than 0.05 were considered statistically significant.

RESULTS

Synthesis of Iodonium Salt Precursors (Compounds 6a and 6b)

The direct 1-pot oxidation/iodine arylation of 4 with mesytil-BF3K yielded a mixture of p- and m-substituted asymmetric diaryliodonium triflates (5a and 5b), which were difficult to separate by silica gel chromatography. Because attempts to produce isomerically pure 5a failed, a regioisomer mixture was used in the following experiments. An anion metathesis reaction with saturated aqueous LiBF4 produced a free-flowing powder of mesityl(aryl)iodonium tetrafluoroborate as a regioisomer mixture (6a and 6b), with an overall yield of 12.7%. The subsequent 19F nuclear magnetic resonance analysis of the cold fluorination reaction product revealed that the mixture readily gave a 3:2 ratio of tetra-Boc–protected FIBG and tetra-Boc–protected IFBG (Supplemental Fig. 1B).

Synthesis of 18F-FIBG and 18F-IFBG (Compounds 8a and 8b)

We then attempted 18F labeling of 6 (step a in Fig. 1C). Although the procedure in the literature (16) provided the intermediate compounds (7a and 7b), the RCY and reproducibility of the reaction were low (13.6% ± 10%, n = 4). By increasing the amount of Cu(MeCN)4OTf to 5 equivalents of the iodonium salt 6, we obtained the highest RCY (63.9%), yet with unsatisfying reproducibility (18.2% ± 19.7%, n = 14). In contrast, when we used an alternative copper catalyst (Cu(OTf)2), a fair RCY was obtained, with moderate reproducibility (19.7% ± 8.0%, n = 13). In the following deprotection reaction (Fig. 1C), we first tried to replicate the previously developed procedure by directly adding 6N HCl to the reaction mixture and incubating it at 120°C (15). However, a large degree of defluorination and the formation of multiple by-products decreased the recovery yield of 8 from 7 to less than 50%. By removing copper and unreacted 18F−, and by reducing the temperature to 70°C, 18F-FIBG and 18F-IFBG (8a and 8b) could be obtained in almost quantitative yield from 7a and 7b. The regioisomer separation was achieved using semipreparative HPLC with high radiochemical purity (>99%, Supplemental Fig. 3). The products 8a and 8b were obtained with an overall RCY of 5.06% ± 1.73% and 2.75% ± 0.95%, respectively (decay-corrected, n = 4).

In Vitro Profiles of Radiolabeled MIBG Derivatives in PC-12 Cells

The accumulation of 18F-FIBG in pheochromocytoma cells was approximately 1.5-fold higher than that of 125I-MIBG (Supplemental Fig. 4A), whereas the accumulation of 18F-IFBG was approximately 0.75-fold lower than that of 125I-MIBG. The uptake of all compounds was decreased by NET inhibitors to comparable levels (Supplemental Fig. 4B). The uptake level of 125I-FIBG was also higher than that of 131I-MIBG in the 3-dimensional cell culture model, and a comparable release profile was found between 131I-FIBG and 131I-MIBG (Supplemental Figs. 5A and 5B, respectively). As shown in Supplemental Figure 5C, 131I-FIBG exhibited greater cytotoxic effects at doses higher than 1 MBq/mL.

Biodistribution of 18F-FIBG and 18F-IFBG in Mice

We next evaluated the biodistribution profiles of 18F-FIBG, 18F-IFBG, and the regioisomer mixture in comparison with 125I-MIBG (Fig. 2) in PC-12–bearing mice 1 h after injection. All fluorinated compounds showed the highest uptake within the tumor, similar to the distribution pattern of MIBG. Tumor uptake of 18F-FIBG was slightly higher than that of 125I-MIBG, although the difference was not statistically significant (41.5 ± 14.5 vs. 35.4 ± 13.3 percentage injected dose [%ID]/g, respectively). In contrast, tumor uptake of 18F-IFBG was approximately half that of 18F-FIBG (18.6 ± 10.4 %ID/g, P < 0.05). There was no significant difference between the tumor uptake of the regioisomer mixture and 125I-MIBG. Comparable biodistribution profiles were found between 18F-FIBG and 125I-FIBG and between 18F-IFBG and 125I-IFBG (Supplemental Fig. 6). The biodistribution profiles of 125I-FIBG and 125I-MIBG up to 7 d are shown in Supplemental Table 1. The calculated absorbed dose of the red marrow, the critical organ for 131I-MIBG, was 8.70 and 5.03 μSv/MBq for 131I-FIBG and 131I-MIBG, respectively. The effective dose equivalent was 125 and 80.9 μSv/MBq for 131I-FIBG and 131I-MIBG, respectively.

FIGURE 2.
FIGURE 2.

Biodistribution profiles of 18F- and 125I-MIBG derivatives in PC-12–bearing mice at 1 h after injection (mean ± SD, 4–5 per group).

PET Imaging

To evaluate the tumor detectability of 18F-FIBG and 18F-IFBG, PET imaging was performed on PC-12–bearing mice (Fig. 3). As early as 1 h after injection, 18F-FIBG, 18F-IFBG, and the regioisomer mixture clearly depicted even small tumors (4 mm). The tumor-to-background ratios of 18F-FIBG, 18F-IFBG, and the regioisomer mixture were 9.92 ± 3.43 (n = 12), 6.95 ± 1.42 (n = 6), and 18.4 (n = 1), respectively. In the 2-h image, 18F-FIBG showed improved tumor-to-background ratios (13.3 ± 4.33, n = 12, P < 0.05; Supplemental Fig. 7).

FIGURE 3.
FIGURE 3.

Representative PET images with 18F-FIBG (A), 18F-IFBG (B), and regioisomer mixture (C) in PC-12–bearing mice at 1 h after injection. Yellow arrows indicate tumor locations, and orange arrow indicates small tumor (4 mm). Bl = bladder; L = liver; S = salivary gland.

Tumor Retention of 125I-FIBG and 131I-MIBG in PC-12–Bearing Mice up to 5 Days

The biodistribution profiles of 125I-FIBG and 131I-MIBG in PC-12–bearing mice up to 5 d after injection are shown in Supplemental Table 2. Overall, 125I-FIBG showed a similar biodistribution profile to 131I-MIBG, although the initial uptake was higher than for 131I-MIBG in almost all organs. A relatively fast clearance of 125I-FIBG from nontarget organs (low NET-expressing) was noted. In contrast, in NET-positive tissues such as the heart, adrenal glands, and tumor, 125I-FIBG retention was longer and higher than 131I-MIBG retention for up to 5 d. In particular, 125I-FIBG accumulation in the tumor was higher than in any other tissue during the entire observation period. The uptake peaked 24 h after injection (83.9 ± 7.37 %ID/g), with 18.4 ± 3.78 %ID/g remaining 5 d later. In contrast, the uptake of 131I-MIBG peaked 3–6 h after injection, with little remaining 5 d later (Fig. 4).

FIGURE 4.
FIGURE 4.

Tumor uptake of 125I-FIBG and 131I-MIBG in PC-12–bearing mice (mean ± SD, 3–5 per group).

Tumor Growth Inhibition Effect of 131I-FIBG

The growth curves of PC-12 xenografts after administration of 131I-FIBG or 131I-MIBG are shown in Figure 5A. At doses higher than 3 MBq, a single treatment with 131I-FIBG suppressed the tumor growth rate as compared with the saline-injected control group (P < 0.0001). Injections of 10 MBq of 131I-MIBG resulted in a 2.5-fold increase in tumor volume (T2.5) 22.6 ± 2.88 d after injection (Supplemental Table 3). The same dose of 131I-FIBG delayed T2.5 until a later time (31.7 ± 2.88 d, P = 0.033). Even at lower doses (3 and 5 MBq), T2.5 was later after administration for 131I-FIBG than for 131I-MIBG (10 MBq), although this difference was not statically significant. No significant difference was observed in the body weight change (Fig. 5B).

FIGURE 5.
FIGURE 5.

(A) Effect of 131I-FIBG and 131I-MIBG on growth of PC-12 tumor–bearing mice. (B) Body weight changes. Data are mean ± SD (at least 5 per group, except 3 in 131I-FIBG 1-MBq group).

Relationship Between 18F-FIBG Uptake and 18F-FDG Uptake and Between 18F-FIBG Uptake and 131I-FIBG Growth Inhibition

Reflecting the fact that xenograft tumors derived from PC-12 show various differentiation states and various uptake levels of MIBG derivatives (18), the PC-12 tumors showed various SUVmax levels in the pretreatment 18F-FIBG PET images. In the linear regression analysis of pretreatment PET imaging, 18F-FIBG uptake and 18F-FDG uptake showed a strong inverse correlation in the PC-12 xenograft tumors (n = 9, Pearson correlation = −0.88, P < 0.05; Fig. 6A). Uptake of 18F-FIBG also showed a strong inverse correlation with the relative volume of the PC-12 xenograft 33 d after treatment with 131I-FIBG (n = 9, Pearson correlation = −0.76, P < 0.05; Fig. 6B). Figures 6C and 6D show representative PET images with high 18F-FIBG uptake and low 18F-FDG uptake and with low 18F-FIBG uptake and high 18F-FDG uptake, respectively. In addition to the contrasting levels in these tumors, 18F-FIBG and 18F-FDG showed visibly different intratumoral distributions. Change in tumor size between the 2 scans was negligible.

FIGURE 6.
FIGURE 6.

(A and B) Relationships between 18F-FIBG uptake and 18F-FDG uptake (A) and between 18F-FIBG uptake and relative tumor volume on day 33 after administration of 5 MBq of 131I-FIBG (B). (C and D) Representative pretreatment 18F-FIBG (left) and 18F-FDG (right) PET images of mice with high (C) or low (D) 18F-FIBG uptake. Arrows indicate tumor locations. Bl = bladder; L = liver; S = salivary gland.

DISCUSSION

This study established a 2-step radiofluorination method to produce 18F-FIBG, a promising diagnostic agent for NET-expressing tumors. Despite the structural similarities, the additional iodine in 18F-FIBG made it challenging to apply the radiofluorination methods used for the production of meta-18F-fluorobenzylguanidine (13–15). Indeed, the additional iodine affected all precursor preparation, radiofluorination, and deprotection steps. First, among the various methods tested (19–21), the reaction developed by Qin et al. (22) was solely amenable to producing a single iodine-substituted precursor 6, because the 18F-fluoride labile leaving groups are introduced via a substitution reaction with iodine. Second, 5 equivalents of Cu(OTf)2 were required to produce radiofluorinated intermediate compound 7, presumably because of the presence of multiple nitrogen atoms in the guanidine group and the additional iodine, both of which are also able to interact with copper. Finally, in the next deprotection step, a moderate temperature (70°C) turned out to be effective in maintaining the recovery yield of 8 better than the initial harsh conditions by preventing temperature-dependent deiodination (8). The following HPLC regioisomer separation finally produced isomerically pure 18F-FIBG (8a) and 18F-IFBG (8b).

The subsequent in vitro and in vivo studies revealed the tumor imaging potential of the isolated 18F-FIBG. At 1 h after injection, 18F-FIBG showed a biodistribution profile similar to that of 125I-MIBG, enabling clear visualization of the tumor in the PET images. Considering the clinical situation, a 24-h uptake period is required for single-photon imaging of 123I-MIBG, because of the slow clearance of 123I-MIBG and the resolution issue. The similarly high initial background uptake of 18F-FIBG may also be problematic. However, because our 2-h PET image showed an improved tumor-to-background ratio due to clearance from nontarget organs and persistent uptake within the tumor, the late-phase image may improve the detectability of 18F-FIBG. In addition, the advantage of PET probes over single-photon probes in terms of detectability has been well documented elsewhere (9). The first-in-human trial of meta-18F-fluorobenzylguanidine indeed demonstrated that it can depict the tumor as early as 1 h after injection, despite its decreased tumor uptake versus *I-MIBG (1). In further studies, the tumor-imaging capability of 18F-FIBG will be worth investigating in more clinically relevant models with various NET expression levels.

To evaluate whether we could use the regioisomer mixture without needing cumbersome HPLC separation, we also compared the properties of 18F-IFBG. Both in vitro and in vivo studies showed its reduced uptake in the PC-12 cells, though 18F-IFBG and the regioisomer mixture also clearly depicted the PC-12 tumor in the PET images. The distribution patterns of 18F-FIBG and 18F-IFBG were comparable to their radioiodine-labeled counterparts, indicating that their radioactivity represents biodistribution of the intact compounds. The NET-positive cells take up MIBG derivatives by either NET-mediated specific uptake or passive diffusion (23). Because all NET inhibitors reduced uptake of each probe to a similar extent, and FIBG and IFBG should have comparable lipophilicities, the difference in their uptake may be attributed to their affinity for NET. These results suggest that the positions of iodine and fluorine in the aromatic ring affect the affinity for NET, in addition to the lipophilicity of the compound (24), and thus the regioisomer separation is recommended.

The high and prolonged uptake of FIBG within the tumor shown in the long-term in vivo biodistribution studies suggests the possibility that 131I-FIBG improves therapeutic efficacy through an extended effective half-life in the tumor, along with the long physical half-life of 131I. Given that FIBG and MIBG showed comparable release profiles in vitro, various factors, such as reuptake in the surrounding cells, lipophilicity, and the deiodination tolerance of the compound, must have contributed differently in the biologic situation and caused prolonged retention of FIBG in the xenograft tumor.

As expected, 131I-FIBG demonstrated a better therapeutic effect than 131I-MIBG both in vitro and in vivo. The therapeutic efficacy of 131I-MIBG is limited because myelosuppression restricts the administration of an adequate tumoricidal dose to patients, though numerous clinical studies suggest a dose-dependent response rate for 131I-MIBG therapy (25). The estimated absorbed radiation dose in the red marrow was approximately 1.7-fold higher for 131I-FIBG because it has a relatively higher normal-organ uptake than that of 131I-MIBG; therefore, the maximum tolerated dose of 131I-FIBG should be smaller than that of 131I-MIBG. However, because the 131I-FIBG 3-MBq group showed a T2.5 comparable to that of the 131I-MIBG 10-MBq group, its high and prolonged tumor uptake would allow higher and more selective radiation dose exposure in the tumor. Further evaluation of the maximum tolerated dose and cumulative dose estimate in various tumor conditions is necessary to maximize the therapeutic efficacy of targeted radiation therapy with 131I-FIBG and consolidate its usefulness in comparison with 131I-MIBG.

The correlation between the SUVmax of 18F-FIBG PET and the therapeutic effect of 131I-FIBG suggests the usefulness of pretherapy 18F-FIBG PET for selecting patients suitable for 131I-FIBG therapy, in addition to its potential usefulness for detecting lesions and monitoring therapy. Radiation dose estimation methods for 123I-MIBG have not been established, primarily because of the semiquantitative nature and resolution issues, as well as the heterogeneous expression of NET at multiple sites (25). Although the applicability of a short-lived PET tracer such as 18F for estimating therapeutic absorbed doses requires further investigations, 2 retrospective studies exhibited an association between early lesion uptake and absorbed dose after PET scans with 68Ga-labeled somatostatin analogs (26,27). To evaluate whether the pretherapy 18F-FIBG uptake is associated with absorbed radiation dose in the tumor may be the objective of future research.

Of note, our data showed a strong inverse correlation between uptake levels of an MIBG analog and 18F-FDG, confirming several clinical studies that observed differential uptake patterns between MIBG and 18F-FDG (3,28). This finding is likely due to the individual microenvironmental differences in the implanted area, such as accessibility to a large blood vessel. The poorly differentiated tumor sites are known to either decrease MIBG uptake in NET-expressing tumors or increase 18F-FDG uptake in various types of cancer (25,29,30). These results suggest that 18F-FIBG, together with 18F-FDG, could be used to develop an optimum combination therapy for 131I-FIBG treatment (4) or therapeutic monitoring.

There were several limitations to our study. First, although we were able to simplify the radiolabeling method, the yield was still low and the procedures were rather complicated for clinical use, especially as regioisomer separation was required. Further studies are needed to optimize the radiosynthetic procedure, including selective synthesis of isomerically pure 18F-FIBG and the development of an automated method. Second, the data need deliberate interpretation as we used only one xenograft model with exceptionally high NET expression levels, whereas clinical NET-expressing tumors express heterogeneous phenotypes. Nevertheless, the promising properties shown in this study suggest that FIBG warrants further evaluation in more clinically relevant models such as various neuroblastoma xenografts and their metastatic models.

CONCLUSION

We found that copper-mediated radiofluorination successfully produces 18F-FIBG in 2 steps plus HPLC purification. Excellent tumor detectability and uptake comparable to MIBG were proven with 18F-FIBG. Moreover, 131I-FIBG showed a greater therapeutic effect in malignant pheochromocytomas than did 131I-MIBG. These results support the potential usefulness of FIBG as a diagnostic and therapeutic agent for the management of NET-expressing tumors.

DISCLOSURE

This work was supported by JSPS KAKENHI grant 26860981 to Aiko Yamaguchi. No other potential conflict of interest relevant to this article was reported.

Acknowledgments

We thank Naoko Ichiishi (Department of Chemistry, University of Michigan) and Peter J. H. Scott (Department of Radiology and Interdepartmental Program in Medicinal Chemistry, University of Michigan) for providing expert technical advice on 18F-labeling, and we thank Takashi Ogasawara (Cyclotron Facility, Gunma University Hospital) for producing 18F-fluoride and 18F-FDG.

Footnotes

  • Published online Dec. 7, 2017.

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  • Received for publication August 31, 2017.
  • Accepted for publication November 16, 2017.
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