Radiolabeled peptides can deliver radiation selectively to tumors via targeting peptide receptors that are overexpressed on the surface of cancer cells. The radiation is used either for detection ...(imaging) or for destruction (therapy) of these tumors. The Division of Radiopharmaceutical
Chemistry at the University Hospital Basel has conducted pioneering work on the development of peptide-based radiopharmaceuticals. Our research covers the entire spectrum of such developments, from bench-to-bedside, and it is illustrated in this article by selective cases.
Somatostatin receptors (SSTs) are recognized as favorable molecular targets in neuroendocrine tumors (NETs) and neuroendocrine neoplasms (NENs), with subtype 2 (SST
) being the predominantly and most ...frequently expressed. PET/CT imaging with
Ga-labeled SST agonists, e.g.,
Ga-DOTA-TOC (SomaKit TOC
) or
Ga-DOTA-TATE (NETSPOT
), plays an important role in staging and restaging these tumors and can identify patients who qualify and would potentially benefit from peptide receptor radionuclide therapy (PRRT) with the therapeutic counterparts
Lu-DOTA-TOC or
Lu-DOTA-TATE (Lutathera
). This is an important feature of SST targeting, as it allows a personalized treatment approach (theranostic approach). Today, new developments hold promise for enhancing diagnostic accuracy and therapeutic efficacy. Among them, the use of SST
antagonists, such as JR11 and LM3, has shown certain advantages in improving image sensitivity and tumor radiation dose, and there is evidence that they may find application in other oncological indications beyond NETs and NENs. In addition, PRRT performed with more cytotoxic α-emitters, such as
Ac, or β
and Auger electrons, such as
Tb, presents higher efficacy. It remains to be seen if any of these new developments will overpower the established radiolabeled SST analogs and PRRT with β
-emitters.
Treatment of neuroendocrine tumours with the radiolabelled somatostatin receptor subtype 2 (SST
) peptide agonist
LuLu-DOTA-TATE is effective and well-established. Recent studies suggest improved ...therapeutic efficacy using the SST
peptide antagonist
LuLu-OPS201. However, little is known about the cellular mechanisms that lead to the observed differences. In the present in vitro study, we compared kinetic binding, saturation binding, competition binding, cellular uptake and release of
LuLu-OPS201 versus
LuLu-DOTA-TATE using HEK cells stably transfected with the human SST
. While
LuLu-OPS201 and
LuLu-DOTA-TATE exhibited comparable affinity (K
, 0.15 ± 0.003 and 0.08 ± 0.02 nM, respectively),
LuLu-OPS201 recognized four times more binding sites than
LuLu-DOTA-TATE. Competition assays demonstrated that a high concentration of the agonist displaced only 30% of
LuLu-OPS201 bound to HEK-SST
cell membranes; an indication that the antagonist binds to additional sites that are not recognized by the agonist.
LuLu-OPS201 showed faster association and slower dissociation than
LuLu-DOTA-TATE. Whereas most of
LuLu-OPS201 remained at the cell surface,
LuLu-DOTA-TATE was almost completely internalised inside the cell. The present data identified distinct differences between
LuLu-OPS201 and
LuLu-DOTA-TATE regarding the recognition of receptor binding sites (higher for
LuLu-OPS201) and their kinetics (faster association and slower dissociation of
LuLu-OPS201) that explain, to a great extent, the improved therapeutic efficacy of
LuLu-OPS201 compared to
LuLu-DOTA-TATE.
The clinical success of radiolabeled somatostatin analogs in the diagnosis and therapy-"theranostics"-of tumors expressing the somatostatin subtype 2 receptor (SST
R) has paved the way for the ...development of a broader panel of peptide radioligands targeting different human tumors. This approach relies on the overexpression of other receptor-targets in different cancer types. In recent years, a shift in paradigm from internalizing agonists to antagonists has occurred. Thus, SST
R-antagonist radioligands were first shown to accumulate more efficiently in tumor lesions and clear faster from the background in animal models and patients. The switch to receptor antagonists was soon adopted in the field of radiolabeled bombesin (BBN). Unlike the stable cyclic octapeptides used in the case of somatostatin, BBN-like peptides are linear, fast to biodegradable and elicit adverse effects in the body. Thus, the advent of BBN-like antagonists provided an elegant way to obtain effective and safe radiotheranostics. Likewise, the pursuit of gastrin and exendin antagonist-based radioligands is advancing with exciting new outcomes on the horizon. In the present review, we discuss these developments with a focus on clinical results, commenting on challenges and opportunities for personalized treatment of cancer patients by means of state-of-the-art antagonist-based radiopharmaceuticals.
The gastrin-releasing peptide receptor (GRPR) is expressed in high numbers in a variety of human tumors, including the frequently occurring prostate and breast cancers, and therefore provides the ...rationale for directing diagnostic or therapeutic radionuclides on cancer lesions after administration of anti-GRPR peptide analogs. This concept has been initially explored with analogs of the frog 14-peptide bombesin, suitably modified at the N-terminus with a number of radiometal chelates. Radiotracers that were selected for clinical testing revealed inherent problems associated with these GRPR agonists, related to low metabolic stability, unfavorable abdominal accumulation, and adverse effects. A shift toward GRPR antagonists soon followed, with safer analogs becoming available, whereby, metabolic stability and background clearance issues were gradually improved. Clinical testing of three main major antagonist types led to promising outcomes, but at the same time brought to light several limitations of this concept, partly related to the variation of GRPR expression levels across cancer types, stages, previous treatments, and other factors. Currently, these parameters are being rigorously addressed by cell biologists, chemists, nuclear medicine physicians, and other discipline practitioners in a common effort to make available more effective and safe state-of-the-art molecular tools to combat GRPR-positive tumors. In the present review, we present the background, current status, and future perspectives of this endeavor.
Strong overexpression of glucagonlike peptide-1 (GLP-1) receptors in human insulinoma provides an attractive target for imaging. The first clinical trials demonstrated that GLP-1 receptor SPECT/CT ...using Lys(40)(Ahx 6-aminohexanoic acid-DOTA-(111)In)NH(2)-exendin-4 can localize hardly detectable insulinomas. However, Lys(40)(Ahx-DOTA-(111)In)NH(2)-exendin-4 imaging has drawbacks related to the use of (111)In in that it is costly and carries a relatively high radiation burden for the patient. The aim of this study was the preclinical evaluation of Lys(40)(Ahx-DOTA-(68)Ga)NH(2)-exendin-4 for PET/CT and Lys(40)(Ahx-hydrazinonicotinamide HYNIC-(99m)Tc)NH(2)-exendin-4 for SPECT/CT.
Internalization, biodistribution, dosimetry, and imaging studies were performed in the Rip1Tag2 mouse model of pancreatic beta-cell carcinogenesis and compared with our gold standard Lys(40)(Ahx-DOTA-(111)In)NH(2)-exendin-4. Poly-glutamic acid and Gelofusine, a gelatin-based plasma expander, were used for renal uptake reduction studies.
The tumor uptake of Lys(40)(Ahx-DOTA-(68)Ga)NH(2)-exendin-4 was 205 +/- 59 percentage injected activity per gram of tissue at 4 h. Other GLP-1 receptor-positive organs showed more than 4.8 times lower radioactivity uptake. Lys(40)(Ahx-HYNIC-(99m)Tc/ethylenediaminediacetic acid EDDA)NH(2)-exendin-4, compared with its (111)In- and (68)Ga-labeled sister compounds, showed significantly less tumor and organ uptake. The significantly lower tumor and organ uptake of Lys(40)(Ahx-HYNIC-(99m)Tc/EDDA)NH(2)-exendin-4 did not result in inferior tumor-to-organ ratios or reduced image quality. All radiopeptides tested showed a high tumor-to-background ratio, resulting in the visualization of small tumors (maximum diameter between 1.0 and 3.2 mm) by SPECT and PET. The only exception was the kidneys, which also showed high uptake. This uptake could be reduced by 49%-78% using poly-glutamic acid, Gelofusine, or a combination of the 2. The estimated effective radiation dose was 3.7 muSv/MBq for Lys(40)(Ahx-HYNIC-(99m)Tc/EDDA)NH(2)-exendin-4, which was 8 times less than that for Lys(40)(Ahx-DOTA-(68)Ga)NH(2)-exendin-4 and 43 times less than that for Lys(40)(Ahx-DOTA-(111)In)NH(2)-exendin-4.
These promising pharmacokinetic and imaging data show that Lys(40)(Ahx-DOTA-(68)Ga)NH(2)-exendin-4 and Lys(40)(Ahx-HYNIC-(99m)Tc/EDDA)NH(2)-exendin-4 are suitable candidates for clinical GLP-1 receptor imaging studies.
Targeted radionuclide therapy of somatostatin receptor (SST)-expressing tumors is only partially addressed by the established somatostatin analogs having an affinity for the SST subtype 2 (SST2). ...Aiming to target a broader spectrum of tumors, we evaluated the bis-iodo-substituted somatostatin analog ST8950 ((4-amino-3-iodo)-d-Phe-cCys-(3-iodo)-Tyr-d-Trp-Lys-Val-Cys-Thr-NH2), having subnanomolar affinity for SST2 and SST5, labeled with 177LuLu3+ via the chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). Human Embryonic Kidney (HEK) cells stably transfected with the human SST2 (HEK-SST2) and SST5 (HEK-SST5) were used for in vitro and in vivo evaluation on a dual SST2- and SST5-expressing xenografted mouse model. natLu-DOTA-ST8950 showed nanomolar affinity for both subtypes (IC50 (95% confidence interval): 0.37 (0.22–0.65) nM for SST2 and 3.4 (2.3–5.2) for SST5). The biodistribution of 177LuLu-DOTA-ST8950 was influenced by the injected mass, with 100 pmol demonstrating lower background activity than 10 pmol. 177LuLu-DOTA-ST8950 reached its maximal uptake on SST2- and SST5-tumors at 1 h p.i. (14.17 ± 1.78 and 1.78 ± 0.35%IA/g, respectively), remaining unchanged 4 h p.i., with a mean residence time of 8.6 and 0.79 h, respectively. Overall, 177LuLu-DOTA-ST8950 targets SST2-, SST5-expressing tumors in vivo to a lower extent, and has an effective dose similar to clinically used radiolabeled somatostatin analogs. Its main drawbacks are the low uptake in SST5-tumors and the persistent kidney uptake.
Limited experiments have compared the treatment effects of repetitive cycles of radiolabelled somatostatin (SST) analogues. In vitro and in vivo experiments were conducted in an AR42J cancer cell ...model, comparing the antagonist 177LuLu-satoreotide tetraxetan with the agonist 177LuLu-DOTA-TATE in terms of their binding properties, biodistribution, anti-tumour activity and toxicity. Histopathological and immunohistochemical examinations were performed at different timepoints. In the in vitro assays, 177LuLu-satoreotide tetraxetan recognised twice as many SST2 binding sites as 177LuLu-DOTA-TATE. In mice treated once a week for four consecutive weeks, 177LuLu-satoreotide tetraxetan (15 MBq) revealed a significantly greater median time taken to reach a tumour volume of 850 mm3 (68 days) compared to 177LuLu-DOTA-TATE at 15 MBq (43 days) or 30 MBq (48 days). This was associated with a higher tumour uptake, enhanced DNA damage and no or mild effects on body weight, haematological toxicity, or renal toxicity with 177LuLu-satoreotide tetraxetan (15 MBq). At the end of the study, complete tumour senescence was noted in 20% of animals treated with 177LuLu-satoreotide tetraxetan, in 13% of those treated with 177LuLu-DOTA-TATE at 30 MBq, and in none of those treated with 177LuLu-DOTA-TATE at 15 MBq. In conclusion, repeated administrations of 177LuLu-satoreotide tetraxetan were able to potentiate peptide receptor radionuclide therapy with a higher tumour uptake, longer median survival, and enhanced DNA damage, with a favourable efficacy/safety profile compared to 177LuLu-DOTA-TATE.
The glucagon-like peptide-1 (GLP-1) receptors are important biomarkers for imaging pancreatic β-cell mass and detection of benign insulinomas. Using GLP-1 receptor antagonists, we aimed to eliminate ...the insulin-related side effects reported for all GLP-1 receptor agonists. Additionally, using a nonresidualizing tracer, (125)I-Bolton-Hunter-Exendin(9-39)NH2 ((125)I-BH-Ex(9-39)NH2), we aimed to reduce the high kidney uptake, enabling a better detection of insulinomas in the tail and head of the pancreas.
The affinity and biodistribution of Ex(9-39)NH2-based antagonists, modified with DOTA or NODAGA chelators at positions Lys(27) and Lys(40) and labeled with (68)Ga and (125)I-BH-Ex(9-39)NH2, were compared with the reference GLP-1 receptor agonist Nle(14),Lys(40)(Ahx-DOTA-(68)Ga)NH2Ex-4. The inhibitory concentration of 50% (IC50) values were determined using autoradiography on human tissues with (125)I-GLP-1(7-36)NH2 as a radioligand. Pharmacokinetics and PET imaging were studied in nude mice bearing rat Ins-1E tumors.
Conjugation of DOTA and NODAGA chelators at positions Lys(27) and Lys(40) of Ex(9-39)NH2 resulted in a distinct loss of affinity toward GLP-1 receptor in vitro. Among the studied antagonists, Lys(40)(NODAGA-(nat)Ga)NH2Ex(9-39) showed the lowest IC50 value (46.7 ± 16.3 nM). The reference agonist Nle(14),Lys(40)(Ahx-DOTA)NH2Ex-4 demonstrated the highest affinity (IC50 = 0.9 ± 0.3 nM). Biodistribution of Nle(14),Lys(40)(Ahx-DOTA-(68)Ga)NH2Ex-4 at 1 h after injection demonstrated 40.2 ± 8.2 percentage injected activity per gram (%IA/g) uptake in Ins-1E tumor, 12.5 ± 2.2 %IA/g in the pancreas, and 235.8 ± 17.0 %IA/g in the kidney, with tumor-to-blood and tumor-to-kidney ratios of 100.52 and 0.17, respectively. Biodistribution of Lys(40)(NODAGA-(68)Ga)NH2Ex(9-39) showed only 2.2 ± 0.2 %IA/g uptake in Ins-1E tumor, 1.0 ± 0.1 %IA/g in the pancreas, and 78.4 ± 8.5 %IA/g in the kidney at 1 h after injection, with tumor-to-blood and tumor-to-kidney ratios of 7.33 and 0.03, respectively. In contrast, (125)I-BH-Ex(9-39)NH2 showed tumor uptake (42.5 ± 8.1 %IA/g) comparable to the agonist and 28.8 ± 5.1 %IA/g in the pancreas at 1 h after injection. As we hypothesized, the kidney uptake of (125)I-BH-Ex(9-39)NH2 was low, only 12.1 ± 1.4 %IA/g at 1 h after injection. The tumor-to-kidney ratio of (125)I-BH-Ex(9-39)NH2 was improved 20-fold.
Our results suggest that iodinated Ex(9-39)NH2 may be a promising tracer for imaging GLP-1 receptor expression in vivo. Because of the 20-fold improved tumor-to-kidney ratio (125)I-BH-Ex(9-39)NH2 may offer higher sensitivity in the detection of insulinomas and imaging of β-cell mass in diabetic patients. Further studies with (124)I-BH-Ex(9-39)NH2 are warranted.
Background
Somatostatin receptor (SST) targeting, specifically of the subtype 2 (SST2), with radiolabeled somatostatin analogs, is established for imaging and treatment of neuroendocrine tumors. ...Owing to the concomitant and heterogeneous expression of several subtypes on the same tumor, analogs targeting more subtypes than SST2 potentially target a broader spectrum of tumors and/or increase the uptake of a given tumor. The analog ST8950 ((4-amino-3-iodo)-
d
-Phe-cCys-(3-iodo)-Tyr-
d
-Trp-Lys-Val-Cys-Thr-NH
2
), bearing 2 iodo-amino acids, exhibits sub-nanomolar affinity to SST2 and SST5. We report herein the development and preclinical evaluation of DOTA-ST8950 labeled with
68
Ga, for imaging SST2- and SST5-expressing tumors. Comparative in vitro and in vivo studies were performed with the de-iodinated DOTA-ST8951 ((4-amino)-
d
-Phe-cCys-Tyr-
d
-Trp-Lys-Val-Cys-Thr-NH
2
) and with the reference compounds DOTA-TATE (SST2 selective) and DOTA-NOC (for SST2 and SST5).
Results
Compared with
nat
Ga-DOTA-NOC,
nat
Ga-DOTA-ST8950 exhibited higher affinity to SST2 and SST5 (IC
50
(95%CI), nM = 0.32 (0.20–0.50) and 1.9 (1.1–3.1) vs 0.70 (0.50-0.96) and 3.4 (1.8-6.2), respectively), while
nat
Ga-DOTA-ST8951 lost affinity for both subtypes.
nat
Ga-DOTA-ST8950 had the same potency for inducing SST2-mediated cAMP accumulation as
nat
Ga-DOTA-TATE and slightly better than
nat
Ga-DOTA-NOC (EC
50
, nM = 0.46 (0.23–0.92) vs 0.47 (0.15–1.5) vs 0.59 (0.18–1.9), respectively).
67
GaGa-DOTA-ST8950 had a similar internalization rate as
67
GaGa-DOTA-NOC in SST2-expressing cells (12.4 ± 1.6% vs 16.6 ± 2.2%, at 4 h,
p
= 0.0586). In vivo,
68
GaGa-DOTA-ST8950 showed high and specific accumulation in SST2- and SST5-expressing tumors, comparable with
68
GaGa-DOTA-NOC (26 ± 8 vs 30 ± 8 %IA/g,
p
= 0.4630 for SST2 and 15 ± 6 vs 12 ± 5 %IA/g,
p
= 0.3282, for SST5, 1 h p.i.) and accumulation in the SST-positive tissues, the kidneys and the liver. PET/CT images of
68
GaGa-DOTA-ST8950, performed in a dual HEK-SST2 and HEK-SST5 tumor xenografted model, clearly visualized both tumors and illustrated high tumor-to-background contrast.
Conclusions
68
GaGa-DOTA-ST8950 reveals its potential for PET imaging SST2- and SST5-expressing tumors. It compares favorably with the clinically used
68
GaGa-DOTA-NOC in terms of tumor uptake; however, its uptake in the liver remains a challenge for clinical translation. In addition, this study reveals the essential role of the iodo-substitutions in positions 1 and 3 of
68
GaGa-DOTA-ST8950 for maintaining affinity to SST2 and SST5, as the de-iodinated
68
GaGa-DOTA-ST8951 lost affinity for both receptor subtypes.
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