Distinct In Vitro Binding Profile of the Somatostatin Receptor Subtype 2 Antagonist [177Lu]Lu-OPS201 Compared to the Agonist [177Lu]Lu-DOTA-TATE

Evolution of dosimetric parameters through PRRT and potential impact on clinical practice: data from the prospective phase II LUMEN study

BACKGROUND

Peptide receptor radionuclide therapy (PRRT) with [177Lu]Lu-DOTA-TATE has emerged as a promising treatment for gastroenteropancreatic neuroendocrine tumours (GEP-NETs). Its treatment protocol is currently standardised for all patients, resulting in different patient outcomes. This study investigates the variability of tumours and organs-at-risk (kidneys and red marrow) dosimetric parameters across treatment cycles in patients with pancreatic and intestinal NETs. Data from 37 patients enrolled in a prospective phase II study (LuMEn) were analysed. Treatment consisted of four cycles of [177Lu]Lu-DOTA-TATE administered 8-12 weeks apart. Three-time-point SPECT/CT imaging was performed after each treatment cycle, and dosimetry of tumours and organs-at-risk (kidneys and red marrow) was conducted following the medical internal radiation dose formalism. Coefficients of variation (CoV) assessed the variability of absorbed doses, activity concentrations on day 1, and effective half-lives. Linear mixed effect models (SAS software) were used to investigate the evolution of the dosimetric parameters over cycles, discerning between different primary NET types and grades of tumours.

RESULTS

There is an important variability in absorbed doses and activity concentrations among patients, particularly in tumours (CoV: ~50%). Tumour absorbed doses and activity concentrations decreased over treatment cycles in pancreatic NETs, although at a limited rate (~-13%/cycle). An opposite trend was observed for the kidneys ( ~ + 8%/cycle). Effective half-lives remained relatively constant across cycles for both organs-at-risk and tumours. The primary NET type significantly influenced effective half-lives in tumours, shorter in pancreatic NETs than intestinal NETs (77 h vs. 107 h, p < 0.0001). No significant effect of the grade was observed on either of the variables investigated.

CONCLUSIONS

Our study revealed considerable variations in tumour absorbed doses among patients with NETs treated with a standardized protocol. These findings confirm the need for personalized dosimetry approaches in PRRT, considering patient and tumour characteristics.

TRIAL REGISTRATION

EudraCT Number: 2012-003666-41.

CLINICALTRIALS

gov identifier: NCT01842165. Registered 25 April 2013, https://clinicaltrials.gov/ct2/show/NCT01842165 .

Copper-61 is an advantageous alternative to gallium-68 for PET imaging of somatostatin receptor-expressing tumors: a head-to-head comparative preclinical study

Background

Gallium-68 positron emission tomography (68Ga-PET) with the two registered somatostatin analogs, [68Ga]Ga-DOTA-Tyr3-octreotide ([68Ga]Ga-DOTA-TOC) and [68Ga]Ga-DOTA-Tyr3-octreotate ([68Ga]Ga-DOTA-TATE), where DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, is routinely used for imaging of somatostatin receptor (SST)-expressing tumors. We investigated copper-61 (61Cu) as an alternative radiometal for PET imaging of SST-expressing tumors. Compared to gallium-68, copper-61 (t1/2 = 3.33 h, E β + max = 1.22 MeV) can be produced on a large scale, enables late time point imaging, and has the therapeutic twin copper-67. Herein, DOTA-TOC and 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA)-TOC were labeled with copper-61 and compared with the clinically used [68Ga]Ga-DOTA-TOC.

Methods

[61Cu]CuCl2 was produced from an irradiated natural nickel target. DOTA-TOC and NODAGA-TOC were labeled with [61Cu]CuCl2 in ammonium acetate buffer so to achieve a reaction pH of 5-6 and a temperature of 95°C for DOTA-TOC or room temperature for NODAGA-TOC. The radioligands were evaluated head-to-head in vitro using human embryonic kidney (HEK)-SST2 cells (affinity, binding sites, cellular uptake, and efflux) and in vivo using HEK-SST2 xenografts [PET/computed tomography (CT) imaging, biodistribution, and pharmacokinetics] and compared with [68Ga]Ga-DOTA-TOC, which was prepared using a standard procedure. Dosimetry estimates were made for [61Cu]Cu-NODAGA-TOC.

Results

[61Cu]Cu-DOTA-TOC and [61Cu]Cu-NODAGA-TOC were prepared at an apparent molar activity of 25 MBq/nmol with radiochemical purities of ≥96% and ≥98%, respectively. In vitro, both presented a sub-nanomolar affinity for SST2 (IC50 = 0.23 and 0.34 nM, respectively). They were almost entirely internalized upon binding to SST2-expressing cells and had similar efflux rates at 37°C. In vivo, [61Cu]Cu-DOTA-TOC and [61Cu]Cu-NODAGA-TOC showed the same accumulation in SST2-expressing tumors. However, PET/CT images and biodistribution analyses clearly showed an unfavorable biodistribution for [61Cu]Cu-DOTA-TOC, characterized by accumulation in the liver and the abdomen. [61Cu]Cu-NODAGA-TOC displayed favorable biodistribution, comparable with [68Ga]Ga-DOTA-TOC at 1 h post-injection (p.i.). Notwithstanding, [61Cu]Cu-NODAGA-TOC showed advantages at 4 h p.i., due to the tumor retention and improved tumor-to-non-tumor ratios. The effective dose (2.41 × 10-3 mSv/MBq) of [61Cu]Cu-NODAGA-TOC, but also the dose to the other organs and the kidneys (9.65 × 10-2 mGy/MBq), suggested a favorable safety profile.

Conclusion

Somatostatin receptor 61Cu-PET imaging not only matches the performance of 68Ga-PET at 1 h p.i. but has advantages in late-time imaging at 4 h p.i., as it provides improved tumor-to-non-tumor ratios. [61Cu]Cu-NODAGA-TOC is superior to [61Cu]Cu-DOTA-TOC in vivo. The use of the chelator NODAGA allows quantitative labeling with copper-61 at room temperature and enables the straightforward use of a kit formulation for simple manufacturing in medical centers.

The heterobivalent (SSTR2/albumin) radioligand [67Cu]Cu-NODAGA-cLAB4-TATE enables efficient somatostatin receptor radionuclide theranostics

Somatostatin type 2 receptor (SSTR2) radionuclide therapy using β- particle-emitting radioligands has entered clinical practice for the treatment of neuroendocrine neoplasms (NENs). Despite the initial success of [177Lu]Lu‑DOTA-TATE, theranostic SSTR2 radioligands require improved pharmacokinetics and enhanced compatibility with alternative radionuclides. Consequently, this study evaluates the pharmacokinetic effects of the albumin-binding domain cLAB4 on theranostic performance of copper‑67-labeled NODAGA-TATE variants in an SSTR2-positive mouse pheochromocytoma (MPC) model. Methods: Binding, uptake, and release of radioligands as well as growth-inhibiting effects were characterized in cells grown as monolayers and spheroids. Tissue pharmacokinetics, absorbed tumor doses, and projected human organ doses were determined from quantitative SPECT imaging in a subcutaneous tumor allograft mouse model. Treatment effects on tumor growth, leukocyte numbers, and renal albumin excretion were assessed. Results: Both copper‑64- and copper‑67-labeled versions of NODAGA-TATE and NODAGA-cLAB4‑TATE showed similar SSTR2 binding affinity, but faster release from tumor cells compared to the clinical reference [177Lu]Lu‑DOTA-TATE. The bifunctional SSTR2/albumin-binding radioligand [67Cu]Cu‑NODAGA-cLAB4‑TATE showed both an improved uptake and prolonged residence time in tumors resulting in equivalent treatment efficacy to [177Lu]Lu‑DOTA-TATE. Absorbed doses were well tolerated in terms of leukocyte counts and kidney function. Conclusion: This preclinical study demonstrates therapeutic efficacy of [67Cu]Cu‑NODAGA-cLAB4‑TATE in SSTR2-positive tumors. As an intrinsic radionuclide theranostic agent, the radioligand provides stable radiocopper complexes and high sensitivity in SPECT imaging for prospective determination and monitoring of therapeutic doses in vivo. Beyond that, copper‑64- and copper‑61-labeled versions offer possibilities for pre- and post-therapeutic PET. Therefore, NODAGA-cLAB4-TATE has the potential to advance clinical use of radiocopper in SSTR2-targeted cancer theranostics.

Current clinical application of lutetium‑177 in solid tumors (Review)

Radionuclide-based therapy represents a novel treatment regimen for tumors. Among these therapies, lutetium-177 (177Lu) has gained significant attention due to its stability and safety, as well as its ability to emit both γ and β rays, allowing for both imaging with single photon emission computed tomography and tumor treatment. As a result, 177Lu can be used for both diagnosis and treatment for diseases such as prostatic and gastric cancer. Therefore, based on the available data, the present review provides a brief overview of the clinical applications of 177Lu-targeted radionuclide therapy in metastatic prostate cancer, neuroendocrine tumors and other types of solid tumors, and highlights the current therapeutic effect, reduction in damage to normal tissues and future research directions, including the development of new nuclides and the application of more nuclides in different tumors. In the future, such treatments could be used in more tumors.

Radiolabeled Somatostatin Receptor Antagonist Versus Agonist for Peptide Receptor Radionuclide Therapy in Patients with Therapy-Resistant Meningioma: PROMENADE Phase 0 Study

Our primary aim was to compare the therapeutic index (tumor-to-bone marrow and tumor-to-kidney absorbed-dose ratios) of the new radiolabeled somatostatin receptor antagonist [177Lu]Lu-DOTA-JR11 with the established radiolabeled somatostatin receptor agonist [177Lu]Lu-DOTATOC in the same patients with progressive, standard therapy-refractory meningioma. Methods: In this prospective, single-center, open-label phase 0 study (NCT04997317), 6 consecutive patients were included: 3 men and 3 women (mean age, 63.5 y). Patients received 6.9-7.3 GBq (standard injected radioactivity) of [177Lu]Lu-DOTATOC followed by 3.3-4.9 GBq (2 GBq/m2 × body surface area) of [177Lu]Lu-DOTA-JR11 at an interval of 10 ± 1 wk. In total, 1 [177Lu]Lu-DOTATOC and 2-3 [177Lu]Lu-DOTA-JR11 treatment cycles were performed. Quantitative SPECT/CT was done at approximately 24, 48, and 168 h after injection of both radiopharmaceuticals to calculate meningioma and organ absorbed doses as well as tumor-to-organ absorbed-dose ratios (3-dimensional segmentation approach for meningioma, kidneys, liver, bone marrow, and spleen). Results: The median of the meningioma absorbed dose of 1 treatment cycle was 3.4 Gy (range, 0.8-10.2 Gy) for [177Lu]Lu-DOTATOC and 11.5 Gy (range, 4.7-22.7 Gy) for [177Lu]Lu-DOTA-JR11. The median bone marrow and kidney absorbed doses after 1 treatment cycle were 0.11 Gy (range, 0.05-0.17 Gy) and 2.7 Gy (range, 1.3-5.3 Gy) for [177Lu]Lu-DOTATOC and 0.29 Gy (range, 0.16-0.39 Gy) and 3.3 Gy (range, 1.6-5.9 Gy) for [177Lu]Lu-DOTA-JR11, resulting in a 1.4 (range, 0.9-1.9) times higher median tumor-to-bone marrow absorbed-dose ratio and a 2.9 (range, 2.0-4.8) times higher median tumor-to-kidney absorbed-dose ratio with [177Lu]Lu-DOTA-JR11. According to the Common Terminology Criteria for Adverse Events version 5.0, 2 patients developed reversible grade 2 lymphopenia after 1 cycle of [177Lu]Lu-DOTATOC. Afterward, 2 patients developed reversible grade 3 lymphopenia and 1 patient developed reversible grade 3 lymphopenia and neutropenia after 2-3 cycles of [177Lu]Lu-DOTA-JR11. No grade 4 or 5 adverse events were observed at 15 mo or more after the start of therapy. The disease control rate was 83% (95% CI, 53%-100%) at 12 mo or more after inclusion. Conclusion: Treatment with 1 cycle of [177Lu]Lu-DOTA-JR11 showed 2.2-5.7 times higher meningioma absorbed doses and a favorable therapeutic index compared with [177Lu]Lu-DOTATOC after injection of 1.4-2.1 times lower activities. The first efficacy results demonstrated a high disease control rate with an acceptable safety profile in the standard therapy for refractory meningioma patients. Therefore, larger studies with [177Lu]Lu-DOTA-JR11 are warranted in meningioma patients.

Comparative Saturation Binding Analysis of 64Cu-Labeled Somatostatin Analogues Using Cell Homogenates and Intact Cells

The development of novel ligands for G-protein-coupled receptors (GPCRs) typically entails the characterization of their binding affinity, which is often performed with radioligands in a competition or saturation binding assay format. Since GPCRs are transmembrane proteins, receptor samples for binding assays are prepared from tissue sections, cell membranes, cell homogenates, or intact cells. As part of our investigations on modulating the pharmacokinetics of radiolabeled peptides for improved theranostic targeting of neuroendocrine tumors with a high abundance of the somatostatin receptor sub-type 2 (SST₂), we characterized a series of ⁶⁴Cu-labeled [Tyr³]octreotate (TATE) derivatives in vitro in saturation binding assays. Herein, we report on the SST₂ binding parameters measured toward intact mouse pheochromocytoma cells and corresponding cell homogenates and discuss the observed differences taking the physiology of SST₂ and GPCRs in general into account. Furthermore, we point out method-specific advantages and limitations.

Development of the 99mTc-Labelled SST2 Antagonist TECANT-1 for a First-in-Man Multicentre Clinical Study

Broad availability and cost-effectiveness of 99Mo/99mTc generators worldwide support the use, and thus the development, of novel 99mTc-labelled radiopharmaceuticals. In recent years, preclinical and clinical developments for neuroendocrine neoplasms patient management focused on somatostatin receptor subtype 2 (SST2) antagonists, mainly due to their superiority in SST2-tumour targeting and improved diagnostic sensitivity over agonists. The goal of this work was to provide a reliable method for facile preparation of a 99mTc-labelled SST2 antagonist, [99mTc]Tc-TECANT-1, in a hospital radiopharmacy setting, suitable for a multi-centre clinical trial. To ensure successful and reproducible on-site preparation of the radiopharmaceutical for human use shortly before administration, a freeze-dried three-vial kit was developed. The final composition of the kit was established based on the radiolabelling results obtained during the optimisation process, in which variables such as precursor content, pH and buffer, as well as kit formulations, were tested. Finally, the prepared GMP-grade batches met all predefined specification parameters together with long-term kit stability and stability of the product [99mTc]Tc-TECANT-1. Furthermore, the selected precursor content complies with micro-dosing, based on an extended single-dose toxicity study, where histopathology NOEL was established at 0.5 mg/kg BW, being more than 1000 times higher than the planned human dose of 20 µg. In conclusion, [99mTc]Tc-TECANT-1 is suitable to be advanced into a first-in-human clinical trial.

Alpha-peptide receptor radionuclide therapy using actinium-225 labeled somatostatin receptor agonists and antagonists

Peptide receptor radionuclide therapy (PRRT) has over the last two decades emerged as a very promising approach to treat neuroendocrine tumors (NETs) with rapidly expanding clinical applications. By chelating a radiometal to a somatostatin receptor (SSTR) ligand, radiation can be delivered to cancer cells with high precision. Unlike conventional external beam radiotherapy, PRRT utilizes primarily β or α radiation derived from nuclear decay, which causes damage to cancer cells in the immediate proximity by irreversible direct or indirect ionization of the cells' DNA, which induces apoptosis. In addition, to avoid damage to surrounding normal cells, PRRT privileges the use of radionuclides that have little penetrating and more energetic (and thus more ionizing) radiations. To date, the most frequently radioisotopes are β- emitters, particularly Yttrium-90 (90Y) and Lutetium-177 (177Lu), labeled SSTR agonists. Current development of SSTR-targeting is triggering the shift from using SSTR agonists to antagonists for PRRT. Furthermore, targeted α-particle therapy (TAT), has attracted special attention for the treatment of tumors and offers an improved therapeutic option for patients resistant to conventional treatments or even beta-irradiation treatment. Due to its short range and high linear energy transfer (LET), α-particles significantly damage the targeted cancer cells while causing minimal cytotoxicity toward surrounding normal tissue. Actinium-225 (225Ac) has been developed into potent targeting drug constructs including somatostatin-receptor-based radiopharmaceuticals and is in early clinical use against multiple neuroendocrine tumor types. In this article, we give a review of preclinical and clinical applications of 225Ac-PRRT in NETs, discuss the strengths and challenges of 225Ac complexes being used in PRRT; and envision the prospect of 225Ac-PRRT as a future alternative in the treatment of NETs.

Comparison of the Anti-Tumour Activity of the Somatostatin Receptor (SST) Antagonist [177Lu]Lu-Satoreotide Tetraxetan and the Agonist [177Lu]Lu-DOTA-TATE in Mice Bearing AR42J SST2-Positive Tumours

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 [177Lu]Lu-satoreotide tetraxetan with the agonist [177Lu]Lu-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, [177Lu]Lu-satoreotide tetraxetan recognised twice as many SST2 binding sites as [177Lu]Lu-DOTA-TATE. In mice treated once a week for four consecutive weeks, [177Lu]Lu-satoreotide tetraxetan (15 MBq) revealed a significantly greater median time taken to reach a tumour volume of 850 mm3 (68 days) compared to [177Lu]Lu-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 [177Lu]Lu-satoreotide tetraxetan (15 MBq). At the end of the study, complete tumour senescence was noted in 20% of animals treated with [177Lu]Lu-satoreotide tetraxetan, in 13% of those treated with [177Lu]Lu-DOTA-TATE at 30 MBq, and in none of those treated with [177Lu]Lu-DOTA-TATE at 15 MBq. In conclusion, repeated administrations of [177Lu]Lu-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 [177Lu]Lu-DOTA-TATE.

Theranostics bei gut bis mäßig differenzierten GEP-NEN

Neuroendokrine Neoplasien (NEN) sind seltene, heterogene und typischerweise langsam wachsende Tumoren. Die häufigsten Lokalisationen finden sich im gastro-entero-pankreatischen System (GEP-NEN). NENs werden nach proliferativer Aktivität (Ki-67-Index) eingeteilt (G1–3). Gut differenzierte Tumoren exprimieren dabei typischerweise Somatostatinrezeptoren (SSTR), die als Zielstruktur in der nuklearmedizinischen Theranostik dienen. Bei diesem Prinzip kann nach einer diagnostischen molekularen Bildgebung, meist mittels Positronenemissionstomografie/Computertomografie (PET/CT), eine individuell zugeschnittene Peptidradiorezeptortherapie (PRRT) mit einem β-Strahler-markierten Radiopharmakon erfolgen. In Metaanalysen zeigte die Diagnostik mittels SSTR-gerichteter PET/CT eine Sensitivität von 93% und eine Spezifität von 96%. Die SSTR-gerichtete Diagnostik kann auch zur radioaktiven Markierung von Tumoren verwendet werden, um eine zielgerichtete Chirurgie zu ermöglichen. Die Indikation zur Einleitung einer PRRT soll stets in einer interdisziplinären Tumorkonferenz getroffen werden. Ein Tumorprogress unter der vorangegangenen Therapie sollte dokumentiert sein. Die Therapie wird intravenös und insgesamt 4-mal in 8-wöchigem Abstand in spezialisierten nuklearmedizinischen Zentren verabreicht. Die Wirksamkeit der PRRT wurde in der NETTER-1-Studie prospektiv untersucht und konnte eine signifikante Verbesserung des progressionsfreien Überlebens (primärer Endpunkt) zeigen. Ausgehend von diesen Studienergebnissen steht mit Lutathera (177Lu-DOTATATE) inzwischen ein in Deutschland zugelassenes Radiopharmazeutikum zu Behandlung von nicht resektablen oder metastasierten bzw. progredienten, gut differenzierten (G1 und G2), SSTR-positiven GEP-NEN zur Verfügung.

Radiolabeled Somatostatin Analogs-A Continuously Evolving Class of Radiopharmaceuticals

Somatostatin receptors (SSTs) are recognized as favorable molecular targets in neuroendocrine tumors (NETs) and neuroendocrine neoplasms (NENs), with subtype 2 (SST₂) being the predominantly and most frequently expressed. PET/CT imaging with ⁶⁸Ga-labeled SST agonists, e.g., ⁶⁸Ga-DOTA-TOC (SomaKit TOC^®) or ⁶⁸Ga-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 ¹⁷⁷Lu-DOTA-TOC or ¹⁷⁷Lu-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 SST₂ 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 ²²⁵Ac, or β^- and Auger electrons, such as ¹⁶¹Tb, 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.