18F-Fluoroethyl triazole-βAG-[(d)-Phe1-c(Cys2-Tyr3-(d)-Trp4-Lys5-Thr6-Cys7)Thr8]

18F-FET-βAG-TOCA

Shan L.

Publication Details

Image

Table

In vitro Rodents

Background

[PubMed]

The 18F-labeled fluoroethyl triazole (FET)-βAG-[(d)-Phe1-c(Cys2-Tyr3-(d)-Trp4-Lys5-Thr6-Cys7)Thr8] (TOCA or Tyr3-octreotide), abbreviated as 18F-FET-βAG-TOCA, is a Tyr3-octreotate analog that was synthesized for positron emission tomography of gastroenteropancreatic neuroendocrine tumors (GEP-NETs) by targeting somatostatin receptors (SSTR, mainly SSTR-2) (1, 2).

GEP-NETs are a class of fairly rare tumors with secretion of various peptides and neuroamines. Although significant progress has been made in the understanding of the molecular events underlying these tumors, it is challenging for the early diagnosis, new target identification, and effective management of these tumors (3, 4). In 1989, Krenning et al. first introduced the use of 123I-labeled Tyr3-octreotide for in vivo imaging of SSTR-expressing GEP-NETs (5). However, this agent exhibited high nonspecific accumulation in the liver and intestine, which can obscure the detection of early-stage GEP-NETs. In 1993, Krenning et al. developed another 111In-labeled somatostatin analog, [111In-DTPA0]octreotide (6), and since then this agent has become the gold standard in nuclear imaging for patients with GEP-NETs (6, 7). An obvious disadvantage for [111In-DTPA0]octreotide is that imaging has to be performed 1–2 days after its injection to ensure sufficient contrast (half-life of 111In, 2.8 days). Recently, studies have been mainly focused on two aspects: searching for new analogs that would have a better affinity profile compared with [111In-DTPA0]octreotide, and developing effective methods for other radionuclide labeling (4, 7, 8).

In general, positron-emitting radionuclide-labeled analogs share excellent imaging quality with better spatial resolution compared to the imaging quality of γ-emitting analogs. One representative agent is [68Ga-DOTA0,Tyr3]octreotide (68Ga-DOTATOC), which is the first 68Ga-labeled somatostatin analog that is studied in patients (9). 18F-Labeled analogs, such as the glycosylated analog Gluc-Lys([18F]FP)-TOCA, which is comparable with 68Ga-DOTATOC in in vivo imaging, have also been studied (10). However, the preparation of Gluc-Lys([18F]FP)-TOCA is time-consuming. Iddon et al. labeled a novel class of alkyne-linked Tyr3-octreotate analogs with a copper-catalyzed azide-alkyne cycloaddition reaction (CuAAC) to form a 1,4-substituted triazole using the reagent [18F]2-fluoroethyl azide (1). The 18F-labeling process is fast and efficient. With this labeling method, investigators have identified two lead alkyne-linked Tyr3-octreotate analogs, G-TOCA and βAG-TOCA (1). Both lead analogs were highly reactive in the CuAAC reaction, showing complete conversion to the [18F]2-fluoroethyl triazole-linked Tyr3-octreotate analogs FET-G-TOCA and FET-βAG-TOCA under mild conditions and with short synthesis times (5 min at 20°C). Leyton et al. further tested these 18F-labeled analogs for their receptor binding and in vivo tumor imaging (2). This chapter summarizes the data obtained with these analogs (1, 2).

Synthesis

[PubMed]

Iddon et al. labeled five Tyr3-octreotate analogs (compound 1b–5b) and one scrambled peptide (compound 6b) with [18/19F]2-fluoroethyl azide using click chemistry (1). Compounds 1b and 2b were the PEGylated forms of 3b and 4b. All radiotracers were successfully prepared with >98% radiochemical purity. The total synthesis time for each compound was ~1.5 h. The 19F-labeled analogs were used for in vitro analysis of SSTR subtype binding. The 18F-labeled analogs were used for other studies. The molecular weights (MW), radiochemical yields (RCY), specific radioactivity (SR) (GBq/µmol), and octanol/PBS partition coefficients (Log D) of the tracers are shown in Table 1 (1, 2).

Table 1: Characteristics of the Tyr3-octreotate analogs

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

Leyton et al. evaluated the binding affinity of 19F-labeled Tyr3-octreotate analogs for SSTR-2 and SSTR-4 in SSTR-2– or SSTR-4–expressing Chem-1 cells with a fluorometric imaging plate reader assay (2) All analogs exhibited agonist activity on SSTR-2, with the scrambled peptide (compound 6b) having expectedly poor affinity (Table 2). The affinity of the ligands (half-maximal effective concentration) ranged from 4 nM to 19 nM (somatostatin, 5.6 nM), with the PEGylated analogs (1b and 2b) showing the lowest affinity to SSTR-2. All analogs except for the scrambled peptide also showed detectable activity against SSTR-4, but the affinity was poor (≥5.4 mM). None of the compounds exhibited detectable activity against SSTR-3 (data not shown).

Table 2: The SSTR-binding affinity of the Tyr3-octreotate analogs

ND, not determined.

Animal Studies

Rodents

[PubMed]

Leyton et al. examined the in vivo plasma stability of 18F-FET-βAG-TOCA at 30 min after tail vein injection (~3.7 MBq (0.1 mCi)) into BALB/c nu/nu mice (2). The radiochromatograms of the mouse plasma showed no metabolites of 18F-FET-βAG-TOCA.

Dynamic PET scans were performed over 60 min after tail vein injection of 3.0–3.7 MBq (0.08–0.10 mCi) of different radiolabeled analogs in mice bearing SSTR-2–expressing AR42J tumors (n = 3–6 mice/agent) (2). The radiotracer uptake for each organ was normalized to injected dose and expressed as percentage of injected activity per milliliter of tissue (% ID/mL) (Table 3). Tissues were collected after the 60-min PET scan, and the radioactivity was accounted and expressed as a percentage injected dose per gram tissue (% ID/g) (Table 3) (2). The results were compared with the data obtained with clinical radiotracer 68Ga-DOTATATE and recently synthesized 18F-AIF-NOTA-OC.

The tumor uptake for all radiotracers was characterized by a rapid increase over the entire scanning period of 60 min. 18F-FET-G-TOCA had the highest tumor uptake, followed by 18F-FET-βAG-TOCA. With respect to tumor uptake, tumor/muscle (T/M) ratio, and tumor/blood (T/B) ratio, these tracers were superior to the clinical radiotracer 68Ga-DOTATATE and comparable to the recently synthesized 18F-AIF-NOTA-OC (statistical data were not reported) (Table 3). PEG linkers conjugated to 18F-FET-G-PEG-TOCA and 18F-FETE-PEG-TOCA reduced tumor uptake but increased T/M and T/B ratios. Nonspecific uptake in the liver was, in general, low (<7% ID/mL), with 18F-FET-βAG-TOCA showing the lowest liver uptake and 18F-FET-G-TOCA and 18F-FETE-PEG-TOCA showing the highest liver uptake. The kinetic profiles of the analogs in the kidney, which also expresses SSTR-2, were different from those in tumors, showing the highest uptake for 18F-FET-βAG-TOCA and 18F-FET-G-TOCA. The mean muscle uptake was <3% ID/mL for all radiotracers. Radiotracer uptake in the bone was low for all analogs, indicating little or no defluorination. In contrast, no radiotracer localization was seen in tumors for the scrambled peptide 18F-FET-βAG-[W-c-(CTFTYC)K]; in this case, tracer localization was seen mainly in the brain, urine, liver, and intestines (data not shown). Uptake in the liver was higher with 18F-FET-βAG-[W-c-(CTFTYC)K] than with 18F-FET-βAG-TOCA.

Table 3: Biodistribution of octreotate analogs

*Data were from PET scans at 60 min after injection.

**Blocking study data for 18F-FET-βAG-TOCA.

Blocking studies were performed for 18F-FET-βAG-TOCA with injection of 10 mg/kg body weight of unlabeled octreotide in mice 10 min before injection of 18F-FET-βAG-TOCA (Table 3) (2). This injected dose of octreotide was ~100-fold higher than the equivalent dose of unlabeled 18F-FET-βAG-TOCA. Pre-injection of octreotide resulted in a two-fold (by direct counting) lower uptake of 18F-FET-βAG-TOCA in AR42J xenografts and higher reductions in T/M and T/B ratios (Table 3). After blocking with unlabeled octreotide, muscle radioactivity increased; kidney radioactivity decreased (early time points only) but subsequently increased; urine radioactivity decreased. The specificity of 18F-FET-βAG-TOCA uptake was also indicated by the low uptake in HCT116 xenografts (low SSTR expression), showing 2.42 ± 0.35% ID/mL in the tumor at 60 min (PET scanning data) and 0.52 ± 0.39, 0.58 ± 0.45, and 1.19 ± 0.89% ID/g in the tumor, muscle, and blood, respectively, after 60 min (ex vivo counting) (2).

Other Non-Primate Mammals

[PubMed]

No references are currently available.

Non-Human Primates

[PubMed]

No references are currently available.

Human Studies

[PubMed]

No references are currently available.

References

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Iddon L. et al. Synthesis and in vitro evaluation of [18F]fluoroethyl triazole labelled [Tyr3]octreotate analogues using click chemistry. Bioorg Med Chem Lett. 2011;21(10):3122–7. [PubMed: 21458258]
2.
Leyton J. et al. Targeting somatostatin receptors: preclinical evaluation of novel 18F-fluoroethyltriazole-Tyr3-octreotate analogs for PET. J Nucl Med. 2011;52(9):1441–8. [PubMed: 21852355]
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6.
Krenning E.P. et al. Somatostatin receptor scintigraphy with [111In-DTPA-D-Phe1]- and [123I-Tyr3]-octreotide: the Rotterdam experience with more than 1000 patients. Eur J Nucl Med. 1993;20(8):716–31. [PubMed: 8404961]
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Evans H.L. et al. Copper-free click--a promising tool for pre-targeted PET imaging. Chem Commun (Camb) 2012;48(7):991–3. [PubMed: 22158912]
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Hofmann M. et al. Biokinetics and imaging with the somatostatin receptor PET radioligand (68)Ga-DOTATOC: preliminary data. Eur J Nucl Med. 2001;28(12):1751–7. [PubMed: 11734911]
10.
Meisetschlager G. et al. Gluc-Lys([18F]FP)-TOCA PET in patients with SSTR-positive tumors: biodistribution and diagnostic evaluation compared with [111In]DTPA-octreotide. J Nucl Med. 2006;47(4):566–73. [PubMed: 16595488]