Targeting Gastrointestinal Stromal Tumor with 68Ga-Labeled Peptides: An In Vitro Study on Gastrointestinal Stromal Tumor-Cell Lines

Imaging Properties and Tumor Targeting of 68Ga-NeoBOMB1, a Gastrin-Releasing Peptide Receptor Antagonist, in GIST Patients

Background: Gastrin-releasing peptide receptors (GRPRs) are molecular imaging targets in multiple malignancies. Recently, NeoBOMB1, a 68Ga-labelled antagonist to GRPRs, was developed for PET. Here we report the outcome of a Phase I/IIa clinical trial (EudraCT 2016-002053-38) describing diagnostic properties and covariates influencing uptake of 68Ga-NeoBOMB1 in oligometastatic gastrointestinal stromal tumor (GIST) patients. Methods: Nine patients with advanced GIST using PET/CT (computed tomography) were included. After kit-based 68Ga-NeoBOMB1 preparation with a licensed 68Ge/68Ga generator, 3 MBq/kg body weight were injected intravenously. PET/CT included dynamic and static PET scans 5, 12 and 18 min and 1, 2, and 3−4 h post injection (first six patients) and static PET scans 2 and 3−4 h post injection (last three participants). Tumor targeting was assessed on a per-lesion and per-patient basis. Results: Six patients showed visible radiotracer uptake in at least one tumor lesion. Seventeen out of 37 tumor lesions exhibited significant 68Ga-NeoBOMB1 uptake (median SUVmax 11.8 [range 2.8−51.1] 2 h p.i. and 13.2 [range 2.5−53.8] 3−4 h p.i) and improved lesion-to-background contrast over time. Five lesions (13.5%) were identified only by 68Ga-NeoBOMB1-PET, with no correlation on contrast-enhanced CT. Three patients showed no radiotracer accumulation in any lesions. Tracer uptake correlated with male sex (p < 0.0001), higher body mass index (p = 0.007), and non-necrotic lesion appearance (p = 0.018). There was no association with whole-lesion contrast enhancement, hepatic localization, mutational status, or disease duration. Conclusions: 68Ga-NeoBOMB1-PET exhibits variable tumor uptake in advanced-stage GIST patients, correlating with lesion vitality based on CT contrast uptake, opening the possibility of a theragnostic approach in selected cases.

New treatment strategies for advanced-stage gastrointestinal stromal tumours

When gastrointestinal stromal tumour (GIST), the most common form of sarcoma, was first recognized as a distinct pathological entity in the 1990s, patients with advanced-stage disease had a very poor prognosis owing to a lack of effective medical therapies. The discovery of KIT mutations as the first and most prevalent drivers of GIST and the subsequent development of the first KIT tyrosine kinase inhibitor (TKI), imatinib, revolutionized the treatment of patients with this disease. We can now identify the driver mutation in 99% of patients with GIST via molecular diagnostic testing, and therapies have been developed to treat many, but not all, molecular subtypes of the disease. At present, seven drugs are approved by the FDA for the treatment of advanced-stage GIST (imatinib, sunitinib, regorafenib, ripretinib, avapritinib, larotrectinib and entrectinib), all of which are TKIs. Although these agents can be very effective for treating certain GIST subtypes, challenges remain and new therapeutic approaches are needed. In this Review, we discuss the molecular subtypes of GIST and the evolution of current treatments, as well as their therapeutic limitations. We also highlight emerging therapeutic approaches that might overcome clinical challenges through novel strategies predicated on the biological features of the distinct GIST molecular subtypes.

Incidental Uptake of 68Ga-DOTATATE Gastrointestinal Stromal Tumor of Duodenum

ABSTRACT

A 56-year-old man underwent 68Ga-DOTATATE PET/CT with suspicion of a pancreatic neuroendocrine tumor. However, in this imaging, a mass with 68Ga-DOTATATE uptake was observed not in pancreas but in the duodenojejunal junction. Abdominal tomography confirmed that the heterogeneously enhanced mass was originated from duodenum. The patient was operated, and the histopathology result was evaluated as low-risk gastrointestinal stromal tumor. Although there was low or no specific binding to gastrointestinal stromal tumor cells for all 68Ga-labeled DOTA peptides, uptake of 68Ga-DOTATATE was observed incidentally in this case.

Succinate dehydrogenase-deficient gastrointestinal stromal tumor: from diagnostic dilemma to novel personalised therapy in 2 case reports

Succinate dehydrogenase (SDH)-deficient gastrointestinal stromal tumor (GIST) is a unique and distinctive subtype of gastric GIST. The literature on this subtype from developing countries is exceedingly sparse. Patients with SDH-deficient GIST often experience a lack or delay in genomic profiling, despite stereotypical clinicopathologic features, potentially resulting in sub-optimal management. SDH-deficient GISTs are highly syndromic, typically have more indolent behavior, a prognosis not predicted by size and mitotic rate, a tendency to lymph node metastases, and are insensitive to standard tyrosine kinase inhibitors (TKIs). We report two women with SDH-deficient GIST. In the first case, SDH deficiency was identified late due to lack of awareness and poor access to diagnostic facilities. The patient progressed through TKI therapy, but responded to temozolomide, which is under investigation in clinical trials. In the second case, SDH deficiency was identified at diagnosis, and the patient responded well to ¹⁷⁷Lutetium peptide radionuclide receptor therapy (PRRT) after progressing through two lines of TKIs. We aim to highlight the need for more awareness and access to genomic diagnostic facilities for GIST patients, temozolomide as a novel therapy for SDH-deficient GIST, and the potential value of DOTATATE positron emission tomography (PET) and PRRT as a novel imaging modality and therapy for TKI insensitive GIST patients.

In Vivo Biodistribution and Efficacy Evaluation of NeoB, a Radiotracer Targeted to GRPR, in Mice Bearing Gastrointestinal Stromal Tumor

Simple Summary

NeoB is undergoing evaluation as a novel theragnostic agent—that is, that it can be employed either for the diagnosis of tumor expressing gastrin-releasing peptide receptor (GRPR) using nuclear imaging, or for the therapy of such GRPR positive tumors using internal radiotherapy. The switch from diagnosis to therapy simply rely on the choice of the radioisotope that is coupled to NeoB. The aim of our study was to investigate—for the first time—the potency of NeoB for tumor therapy once labeled with the beta- emitter Lu-177. This study has been conducted in mice bearing human Gastrointestinal Stromal Tumors (GIST). [¹⁷⁷Lu]Lu-NeoB was found to accumulate in the tumor, with only minimal retention in off-target organs. Consequently, mice treated with therapeutic doses of [¹⁷⁷Lu]Lu-NeoB (37MBq/week for three weeks) exhibited tumor regression and therefore long term survival in comparison to the control untreated mice.

Abstract

NeoB is a radiotracer targeting the gastrin-releasing peptide receptor (GRPR), a G-protein–coupled receptor expressed in various cancers. The aim of the present study was to evaluate the biodistribution and efficacy of this new therapeutic agent in Gastrointestinal Stromal Tumors (GIST). Eighty-two SCID mice bearing GIST-882 tumors were employed. [¹⁷⁷Lu]Lu-NeoB biodistribution was evaluated up to seven days by organ sampling (200 pmol/0.8 MBq, i.v.). For efficacy evaluation, mice received either saline, 400 pmol or 800 pmol of [¹⁷⁷Lu]Lu-NeoB (37MBq, 1/w, 3 w, i.v.). SPECT/CT imaging was performed at 24 h, and tumor volume was determined up to 100 days. Elevated and specific [¹⁷⁷Lu]Lu-NeoB uptake was found in the GIST tumor, as demonstrated by in vivo competition (19.1 ± 3.9 %ID/g vs. 0.3 ± 0.1 %ID/g at 4h). [¹⁷⁷Lu]Lu-NeoB tumor retention (half-life of 40.2 h) resulted in elevated tumor-to-background ratios. Tumor volumes were significantly reduced in both treated groups (p < 0.01), even leading to complete tumor regression at the 400 pmol dose. [¹⁷⁷Lu]Lu-NeoB exhibited excellent pharmacokinetics with elevated and prolonged tumor uptake and low uptake in non-target organs such as pancreas. The potential of this new theragnostic agent in different indications, including GIST, is under evaluation in the FIH [¹⁷⁷Lu]Lu-NeoB clinical trial.

The role of [68 Ga]Ga-DOTATATE PET/CT in wild-type KIT/PDGFRA gastrointestinal stromal tumours (GIST)

BACKGROUND

[68 Ga]Ga-DOTATATE PET/CT is now recognised as the most sensitive functional imaging modality for the diagnosis of well-differentiated neuroendocrine tumours (NET) and can inform treatment with peptide receptor radionuclide therapy with [177Lu]Lu-DOTATATE. However, somatostatin receptor (SSTR) expression is not unique to NET, and therefore, [68 Ga]Ga-DOTATATE PET/CT may have oncological application in other tumours. Molecular profiling of gastrointestinal stromal tumours that lack activating somatic mutations in KIT or PDGFRA or so-called 'wild-type' GIST (wtGIST) has demonstrated that wtGIST and NET have overlapping molecular features and has encouraged exploration of shared therapeutic targets, due to a lack of effective therapies currently available for metastatic wtGIST.

AIMS

To investigate (i) the diagnostic role of [68 Ga]Ga-DOTATATE PET/CT; and, (ii) to investigate the potential of this imaging modality to guide treatment with [177Lu]Lu-DOTATATE in patients with wtGIST.

METHODS

[68 Ga]Ga-DOTATATE PET/CT was performed on 11 patients with confirmed or metastatic wtGIST and one patient with a history of wtGIST and a mediastinal mass suspicious for metastatic wtGIST, who was subsequently diagnosed with a metachronous mediastinal paraganglioma. Tumour expression of somatostatin receptor subtype 2 (SSTR2) using immunohistochemistry was performed on 54 tumour samples including samples from 8/12 (66.6%) patients who took part in the imaging study and 46 tumour samples from individuals not included in the imaging study.

RESULTS

[68 Ga]Ga-DOTATATE PET/CT imaging was negative, demonstrating that liver metastases had lower uptake than background liver for nine cases (9/12 cases, 75%) and heterogeneous uptake of somatostatin tracer was noted for two cases (16.6%) of wtGIST. However, [68 Ga]Ga-DOTATATE PET/CT demonstrated intense tracer uptake in a synchronous paraganglioma in one case and a metachronous paraganglioma in another case with wtGIST.

CONCLUSIONS

Our data suggest that SSTR2 is not a diagnostic or therapeutic target in wtGIST. [68 Ga]Ga-DOTATATE PET/CT may have specific diagnostic utility in differentiating wtGIST from other primary tumours such as paraganglioma in patients with sporadic and hereditary forms of wtGIST.

MITIGATE-NeoBOMB1, a Phase I/IIa Study to Evaluate Safety, Pharmacokinetics, and Preliminary Imaging of 68Ga-NeoBOMB1, a Gastrin-Releasing Peptide Receptor Antagonist, in GIST Patients

Gastrin-releasing peptide receptors (GRPRs) are potential molecular imaging targets in a variety of tumors. Recently, a ⁶⁸Ga-labeled antagonist to GRPRs, NeoBOMB1, was developed for PET. We report on the outcome of a phase I/IIa clinical trial (EudraCT 2016-002053-38) within the EU-FP7 project Closed-loop Molecular Environment for Minimally Invasive Treatment of Patients with Metastatic Gastrointestinal Stromal Tumors ('MITIGATE') (grant agreement no. 602306) in patients with oligometastatic gastrointestinal stromal tumors (GIST). Methods: The main objectives were evaluation of safety, biodistribution, dosimetry, and preliminary tumor targeting of ⁶⁸Ga-NeoBOMB1 in patients with advanced tyrosine-kinase inhibitors-treated GIST using PET/CT. Six patients with histologically confirmed GIST and unresectable primary lesion or metastases undergoing an extended protocol for detailed pharmacokinetic analysis were included. ⁶⁸Ga-NeoBOMB1 was prepared using a kit procedure with a licensed ⁶⁸Ge/⁶⁸Ga generator. ⁶⁸Ga-NeoBOMB1 (3 MBq/kg of body weight) was injected intravenously, and safety parameters were assessed. PET/CT included dynamic imaging at 5, 11, and 19 min as well as static imaging at 1, 2, and 3-4 h after injection for dosimetry calculations. Venous blood samples and urine were collected for pharmacokinetic analysis. Tumor targeting was assessed on a per-lesion and per-patient basis. Results: ⁶⁸Ga-NeoBOMB1 (50 μg) was prepared with high radiochemical purity (yield > 97%). Patients received 174 ± 28 MBq of the radiotracer, which was well tolerated in all patients over a follow-up period of 4 wk. Dosimetry calculations revealed a mean effective dose of 0.029 ± 0.06 mSv/MBq, with the highest organ dose to the pancreas (0.274 ± 0.099 mSv/MBq). Mean plasma half-life was 27.3 min with primarily renal clearance (mean 25.7% ± 5.4% of injected dose 4 h after injection). Plasma metabolite analyses revealed high stability; metabolites were detected only in the urine. In 3 patients, a significant uptake with increasing maximum SUVs (SUV_max at 2 h after injection: 4.3-25.9) over time was found in tumor lesions. Conclusion: This phase I/IIa study provides safety data for ⁶⁸Ga-NeoBOMB1, a promising radiopharmaceutical for targeting GRPR-expressing tumors. Safety profiles and pharmacokinetics are suitable for PET imaging, and absorbed dose estimates are comparable to those of other ⁶⁸Ga-labeled radiopharmaceuticals used in clinical routine.

Radiometal-Containing Aryl Diazonium Salts for Chemoselective Bioconjugation of Tyrosine Residues

Tyrosine is an attractive target for chemo- and site-selective protein modification. The particular chemical nature of tyrosine residues allows bioconjugation chemistry with reactive aryl diazonium salts via electrophilic aromatic substitution to produce diazo compounds. In this work, we describe the preparation of ⁶⁴Cu- and ⁶⁸Ga-labeled 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA)-diazonium salts as building blocks for azo coupling chemistry with tyrosine and tyrosine-containing peptides and proteins under mild conditions. 2-S-(4-aminobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (p-NH₂-Bn-NOTA) was used to form the corresponding ⁶⁴Cu- and ⁶⁸Ga-labeled complexes, followed by diazotization with NaNO₂ in the presence of HCl. ⁶⁴Cu- and ⁶⁸Ga-labeled NOTA complexes were prepared in high radiochemical yields >80% starting from 20 μg of p-NH₂-Bn-NOTA. Conversion of p-NH₂-Bn-NOTA complexes into diazonium salts followed by azo coupling with l-tyrosine afforded ⁶⁴Cu- and ⁶⁸Ga-labeled tyrosine in radiochemical yields of 80 and 56%, respectively. Azo coupling with tyrosine-containing hexapeptide neurotensin NT(8-13) afforded ⁶⁴Cu- and ⁶⁸Ga-labeled NT(8-13) in radiochemical yields of 45 and 11%, respectively. Azo coupling of ⁶⁴Cu-labeled NOTA-diazonium salt with human serum albumin (HSA) gave ⁶⁴Cu-labeled HSA in radiochemical yields of 20%. The described azo coupling chemistry represents an innovative and versatile bioconjugation strategy for selective targeting of tyrosine residues in peptides and proteins.

Comparative In Vitro Study of 11C-Methionine and 11C-Deuterodeprenyl Uptake in Three Human Glioma Cell Lines

AIM

To compare the uptake of ¹¹C-deuterodeprenyl (¹¹C-DED) and ¹¹C-methionine (¹¹C-MET) in three human glioma cell lines and study the relationship with glial fibrillary acid protein (GFAP) and monoamine oxidase B (MAO B) expression. ¹¹C-DED is used in positron emission tomography imaging as a marker of astrocytosis in various central nervous system pathologies. It binds irreversibly to MAO B, a glial dimeric enzyme with increased activity in some neurological pathologies.

MATERIALS AND METHODS

Binding and internalization studies of ¹¹C-MET and ¹¹C-DED were performed in astrocytoma grade III, glioblastoma grade IV, and radio-resistant glioblastoma grade IV cells. Immunofluorescence was used.

RESULTS

¹¹C-MET specific activity bound to membrane was 9.0%-11.1% and that internalized was 88.9%-91.0%. ¹¹C-DED specific activity bound to membrane was 34.8%-58.0% and that internalized was 38.7%-65.2%. Immunocytochemistry revealed GFAP and MAO B expression.

CONCLUSIONS

The expression of MAO B measured by ¹¹C-DED uptake or immunocytochemistry was not significantly different in grade III or IV cells. The GFAP signal was higher for grade IV compared to grade III. ¹¹C-MET uptake was high in all the tumor cells. ¹¹C-DED is a dopamine analogue and the transport across cell membranes is expected to be mediated by DAT receptors present in astrocytes. Reactive astrocytes surround tumor lesions; so the authors suggest that the ¹¹C-DED uptake might be caused by the reactive astrocytosis and not by MAO B expression in tumor cells.