(68)Ga-AMBA and (18)F-FDG for preclinical PET imaging of breast cancer: effect of tamoxifen treatment on tracer uptake by tumor

Gastrin-releasing peptide receptor as a theranostic target in breast cancer: a systematic scoping review

The gastrin-releasing peptide receptor (GRPR) is known to be overexpressed in breast cancer, making it a promising target for both imaging and therapy within a theranostic framework. Various radioligands targeting GRPR have undergone investigation in preclinical and clinical studies related to breast cancer. This systematic scoping review aimed to assess the current evidence on GRPR-targeted radioligands for diagnostic and therapeutic applications in breast cancer. The methodology followed the PRISMA-ScR protocol. The literature search was conducted in September 2023 and encompassed MEDLINE, Embase, Cochrane, and Scopus databases. We included original peer-reviewed studies focused on breast cancer patients or in vivo breast cancer models. Two reviewers performed the study selection process independently. Data were extracted, synthesized, and categorized into preclinical and clinical studies, further subdivided based on radioligand properties. A total of 35 original studies were included in the review, with three of them evaluating therapeutic outcomes. The results indicated that GRPR-radioantagonists are superior to GRPR-agonists, exhibiting preferable in vivo stability, rapid, specific tumor targeting, and enhanced retention. Both preclinical and clinical evaluations demonstrated renal excretion and high uptake in normal GRPR-expressing tissue, primarily the pancreas. A significant positive correlation was observed between GRPR and estrogen-receptor expression. In the clinical setting, GRPR-radioligands effectively detected primary tumors and, to a lesser extent, lymph node metastases. Moreover, GRPR-targeted radioantagonists successfully identified distant metastases originating from various sites in advanced metastatic disease, strongly correlated with positive estrogen receptor expression. Preclinical therapeutic evaluation of GRPR-radioligands labeled with lutetium-177 showed promising tumor responses, and none of the studies reported any observed or measured side effects, indicating a safe profile. In conclusion, the evidence presented in this review indicates a preference for GRPR-targeted antagonists over agonists, owing to their superior kinetics and promising diagnostic potential. Clinical assessments suggested diagnostic value for GRPR-targeted theranostics in breast cancer patients, particularly those with high estrogen receptor expression. Nevertheless, in the therapeutic clinical context, paying attention to the radiation dose administered to the pancreas and kidneys is crucial.

Advances of radiolabeled GRPR ligands for PET/CT imaging of cancers

GRPR is a type of seven-transmembrane G-protein coupled receptor that belongs to the bombesin protein receptor family. It is highly expressed in various cancers, including prostate cancer, breast cancer, lung cancer, gastrointestinal cancer, and so on. As a result, molecular imaging studies have been conducted using radiolabeled GRPR ligands for tumor diagnosis, as well as monitoring of recurrence and metastasis. In this paper, we provided a comprehensive overview of relevant literature from the past two decades, with a specific focus on the advancements made in radiolabeled GRPR ligands for imaging prostate cancer and breast cancer.

GRPR-targeting radiotheranostics for breast cancer management

Breast Cancer (BC) is the most common cancer worldwide and, despite the advancements made toward early diagnosis and novel treatments, there is an urgent need to reduce its mortality. The Gastrin-Releasing Peptide Receptor (GRPR) is a promising target for the development of theranostic radioligands for luminal BC with positive estrogen receptor (ER) expression, because GRPR is expressed not only in primary lesions but also in lymph nodes and distant metastasis. In the last decades, several GRPR-targeting molecules have been evaluated both at preclinical and clinical level, however, most of the studies have been focused on prostate cancer (PC). Nonetheless, given the relevance of non-invasive diagnosis and potential treatment of BC through Peptide Receptor Radioligand Therapy (PRRT), this review aims at collecting the available preclinical and clinical data on GRPR-targeting radiopeptides for the imaging and therapy of BC, to better understand the current state-of-the-art and identify future perspectives and possible limitations to their clinical translation. In fact, since luminal-like tumors account for approximately 80% of all BC, many BC patients are likely to benefit from the development of GRPR-radiotheranostics.

GRPr Theranostics: Current Status of Imaging and Therapy using GRPr Targeting Radiopharmaceuticals

Addressing molecular targets, that are overexpressed by various tumor entities, using radiolabeled molecules for a combined diagnostic and therapeutic (theranostic) approach is of increasing interest in oncology. The gastrin-releasing peptide receptor (GRPr), which is part of the bombesin family, has shown to be overexpressed in a variety of tumors, therefore, serving as a promising target for those theranostic applications. A large amount of differently radiolabeled bombesin derivatives addressing the GRPr have been evaluated in the preclinical as well as clinical setting showing fast blood clearance and urinary excretion with selective GRPr-binding. Most of the available studies on GRPr-targeted imaging and therapy have evaluated the theranostic approach in prostate and breast cancer applying bombesin derivatives tagged with the predominantly used theranostic pair of ⁶⁸Ga/¹⁷⁷Lu which is the focus of this review.

Radiolabeled Peptides for SPECT and PET Imaging in the Detection of Breast Cancer: Preclinical and Clinical Perspectives

Breast cancer is the most common cancer in women worldwide. Due to the heterogeneous nature of breast cancer, the optimal treatment and expected response for each patient may not necessarily be universal. Molecular imaging techniques could play an important role in the early detection and targeted therapy evaluation of breast cancer. This review focuses on the development of peptides labeled with SPECT and PET radionuclides for breast cancer imaging. We summarized the current status of radiolabeled peptides for different receptors in breast cancer. The characteristics of radionuclides and major techniques for peptide labeling are also briefly discussed.

A comparative study of peptide-based imaging agents [68Ga]Ga-PSMA-11, [68Ga]Ga-AMBA, [68Ga]Ga-NODAGA-RGD and [68Ga]Ga-DOTA-NT-20.3 in preclinical prostate tumour models

INTRODUCTION

Peptide-based imaging agents targeting prostate-specific membrane antigen (PSMA) have revolutionized the evaluation of biochemical recurrence of prostate cancer (PCa) but lacks sensitivity at very low serum prostate specific antigen (PSA) levels. Once recurrence is suspected, other positron emission tomography (PET) radiotracers could be of interest to discriminate between local and distant relapse. We studied [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG) targeting glucose metabolism, [¹⁸F]fluorocholine ([¹⁸F]FCH) targeting membrane metabolism and peptide-based imaging agents [⁶⁸Ga]Ga-PSMA-11, [⁶⁸Ga]Ga-AMBA, [⁶⁸Ga]Ga-NODAGA-RGD and [⁶⁸Ga]Ga-DOTA-NT-20.3 targeting PSMA, gastrin releasing peptide receptor (GRPr), αvβ3 integrin and neurotensin type 1 receptor (NTSR1) respectively, in different PCa tumour models.

METHODS

Mice were xenografted with 22Rv1, an androgen-receptor (AR)-positive, PCa cell line that expresses PSMA and PC3, an AR-negative one that does not express PSMA. PET imaging using the different radiotracers was performed sequentially and the uptake characteristics compared to one other. NTSR1 and PSMA expression levels were analysed in tumours by immunohistochemistry.

RESULTS

[¹⁸F]FDG displayed low but sufficient uptake to visualize PC3 and 22Rv1 derived tumours. We also observed a low efficacy of [¹⁸F]FCH PET imaging and a low [⁶⁸Ga]Ga-NODAGA-RGD tumour uptake in those tumours. As expected, an elevated tumour uptake was obtained for [⁶⁸Ga]Ga-PSMA-11 in 22Rv1 derived tumour although no uptake was measured in the androgen independent cell line PC3, derived from a bone metastasis of a high-grade PCa. Moreover, in PC3 cell line, we obtained good tumour uptake, high tumour-to-background contrast using [⁶⁸Ga]Ga-AMBA and [⁶⁸Ga]Ga-DOTA-NT-20.3. Immunohistochemistry analysis confirmed high NTSR1 expression in PC3 derived tumours and conversely high PSMA expression in 22Rv1 derived tumours.

CONCLUSION

PET imaging using [⁶⁸Ga]Ga-AMBA and [⁶⁸Ga]Ga-DOTA-NT-20.3 demonstrates that GRPr and NTSR1 could represent viable alternative targets for diagnostic or therapeutic applications in PCa with limited PSMA expression levels. More preclinical and clinical studies will follow to explore this potential.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT

Peptide-based imaging agents targeting PSMA represent a major progress in the evaluation of biochemical recurrence of PCa but sometimes yield false negative results in some lesions. Continuing efforts have thus been made to evaluate other radiotracers. Our preclinical results suggest that [⁶⁸Ga]labelled bombesin and neurotensin analogues could serve as alternative PET radiopharmaceuticals for diagnostic or therapy in cases of PSMA-negative PCa.

GPCR Modulation in Breast Cancer

Breast cancer is the most prevalent cancer found in women living in developed countries. Endocrine therapy is the mainstay of treatment for hormone-responsive breast tumors (about 70% of all breast cancers) and implies the use of selective estrogen receptor modulators and aromatase inhibitors. In contrast, triple-negative breast cancer (TNBC), a highly heterogeneous disease that may account for up to 24% of all newly diagnosed cases, is hormone-independent and characterized by a poor prognosis. As drug resistance is common in all breast cancer subtypes despite the different treatment modalities, novel therapies targeting signaling transduction pathways involved in the processes of breast carcinogenesis, tumor promotion and metastasis have been subject to accurate consideration. G protein-coupled receptors (GPCRs) are the largest family of cell-surface receptors involved in the development and progression of many tumors including breast cancer. Here we discuss data regarding GPCR-mediated signaling, pharmacological properties and biological outputs toward breast cancer tumorigenesis and metastasis. Furthermore, we address several drugs that have shown an unexpected opportunity to interfere with GPCR-based breast tumorigenic signals.

Review: Receptor Targeted Nuclear Imaging of Breast Cancer

Receptor targeted nuclear imaging directed against molecular markers overexpressed on breast cancer (BC) cells offers a sensitive and specific method for BC imaging. Currently, a few targets such as estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), somatostatin receptor (SSTR), and the gastrin releasing peptide receptor (GRPR) are being investigated for this purpose. Expression of these targets is BC subtype dependent and information that can be gained from lesion visualization is dependent on the target; ER-targeting radiotracers, e.g., can be used to monitor response to anti-estrogen treatment. Here we give an overview of the studies currently under investigation for receptor targeted nuclear imaging of BC. Main findings of imaging studies are summarized and (potential) purposes of lesion visualization by targeting these molecular markers are discussed. Since BC is a very heterogeneous disease and molecular target expression can vary per subtype, but also during disease progression or under influence of treatment, radiotracers for selected imaging purposes should be chosen carefully.

Prospects of Targeting the Gastrin Releasing Peptide Receptor and Somatostatin Receptor 2 for Nuclear Imaging and Therapy in Metastatic Breast Cancer

BACKGROUND

The gastrin releasing peptide receptor (GRPR) and the somatostatin receptor 2 (SSTR2) are overexpressed on primary breast cancer (BC), making them ideal candidates for receptor-mediated nuclear imaging and therapy. The aim of this study was to determine whether these receptors are also suitable targets for metastatic BC.

METHODS

mRNA expression of human BC samples were studied by in vitro autoradiography and associated with radioligand binding. Next, GRPR and SSTR2 mRNA levels of 60 paired primary BCs and metastases from different sites were measured by quantitative reverse transcriptase polymerase chain reaction. Receptor mRNA expression levels were associated with clinico-pathological factors and expression levels of primary tumors and corresponding metastases were compared.

RESULTS

Binding of GRPR and SSTR radioligands to tumor tissue correlated significantly with receptor mRNA expression. High GRPR and SSTR2 mRNA levels were associated with estrogen receptor (ESR1)-positive tumors (p<0.001 for both receptors). There was no significant difference in GRPR mRNA expression of primary tumors versus paired metastases. Regarding SSTR2 mRNA expression, there was also no significant difference in the majority of cases, apart from liver and ovarian metastases which showed a significantly lower expression compared to the corresponding primary tumors (p = 0.02 and p = 0.03, respectively).

CONCLUSION

Targeting the GRPR and SSTR2 for nuclear imaging and/or treatment has the potential to improve BC care in primary as well as metastatic disease.

Decoding Intratumoral Heterogeneity of Breast Cancer by Multiparametric In Vivo Imaging: A Translational Study

Differential diagnosis and therapy of heterogeneous breast tumors poses a major clinical challenge. To address the need for a comprehensive, noninvasive strategy to define the molecular and functional profiles of tumors in vivo, we investigated a novel combination of metabolic PET and diffusion-weighted (DW)-MRI in the polyoma virus middle T antigen transgenic mouse model of breast cancer. The implementation of a voxelwise analysis for the clustering of intra- and intertumoral heterogeneity in this model resulted in a multiparametric profile based on [(18)F]Fluorodeoxyglucose ([(18)F]FDG)-PET and DW-MRI, which identified three distinct tumor phenotypes in vivo, including solid acinar, and solid nodular malignancies as well as cystic hyperplasia. To evaluate the feasibility of this approach for clinical use, we examined estrogen receptor-positive and progesterone receptor-positive breast tumors from five patient cases using DW-MRI and [(18)F]FDG-PET in a simultaneous PET/MRI system. The postsurgical in vivo PET/MRI data were correlated to whole-slide histology using the latter traditional diagnostic standard to define phenotype. By this approach, we showed how molecular, structural (microscopic, anatomic), and functional information could be simultaneously obtained noninvasively to identify precancerous and malignant subtypes within heterogeneous tumors. Combined with an automatized analysis, our results suggest that multiparametric molecular and functional imaging may be capable of providing comprehensive tumor profiling for noninvasive cancer diagnostics. Cancer Res; 76(18); 5512-22. ©2016 AACR.

Gastrin-releasing Peptide Receptor Imaging in Breast Cancer Using the Receptor Antagonist (68)Ga-RM2 And PET

Introduction: The gastrin-releasing peptide receptor (GRPR) is overexpressed in breast cancer. The present study evaluates GRPR imaging as a novel imaging modality in breast cancer by employing positron emission tomography (PET) and the GRPR antagonist ⁶⁸Ga-RM2.

Methods: Fifteen female patients with biopsy confirmed primary breast carcinoma (3 bilateral tumors; median clinical stage IIB) underwent ⁶⁸Ga-RM2-PET/CT for pretreatment staging. In vivo tumor uptake of ⁶⁸Ga-RM2 was correlated with estrogen (ER) and progesterone (PR) receptor expression, HER2/neu status and MIB-1 proliferation index in breast core biopsy specimens.

Results: 13/18 tumors demonstrated strongly increased ⁶⁸Ga-RM2 uptake compared to normal breast tissue (defined as PET-positive). All PET-positive primary tumors were ER- and PR-positive (13/13) in contrast to only 1/5 PET-negative tumors. Mean SUV_MAX of ER-positive tumors was 10.6±6.0 compared to 2.3±1.0 in ER-negative tumors (p=0.016). In a multivariate analysis including ER, PR, HER2/neu and MIB-1, only ER expression predicted ⁶⁸Ga-RM2 uptake (model: r²=0.55, p=0.025). Normal breast tissue showed inter- and intraindividually variable, moderate GRPR binding (SUV_MAX 2.3±1.0), while physiological uptake of other organs was considerably less except pancreas. Of note, ⁶⁸Ga-RM2-PET/CT detected internal mammary lymph nodes with high ⁶⁸Ga-RM2 uptake (n=8), a contralateral axillary lymph node metastasis (verified by biopsy) and bone metastases (n=1; not detected by bone scan and CT).

Conclusion: Our study demonstrates that ⁶⁸Ga-RM2-PET/CT is a promising imaging method in ER-positive breast cancer. In vivo GRPR binding assessed by ⁶⁸Ga-RM2-PET/CT correlated with ER expression in primary tumors of untreated patients.

Mouse Models of Breast Cancer: Platforms for Discovering Precision Imaging Diagnostics and Future Cancer Medicine

Representing an enormous health care and socioeconomic challenge, breast cancer is the second most common cancer in the world and the second most common cause of cancer-related death. Although many of the challenges associated with preventing, treating, and ultimately curing breast cancer are addressable in the laboratory, successful translation of groundbreaking research to clinical populations remains an important barrier. Particularly when compared with research on other types of solid tumors, breast cancer research is hampered by a lack of tractable in vivo model systems that accurately recapitulate the relevant clinical features of the disease. A primary objective of this article was to provide a generalizable overview of the types of in vivo model systems, with an emphasis primarily on murine models, that are widely deployed in preclinical breast cancer research. Major opportunities to advance precision cancer medicine facilitated by molecular imaging of preclinical breast cancer models are discussed.

Preclinical and first clinical experience with the gastrin-releasing peptide receptor-antagonist [⁶⁸Ga]SB3 and PET/CT

PURPOSE

Gastrin-releasing peptide receptors (GRPR) represent attractive targets for tumor diagnosis and therapy because of their overexpression in major human cancers. Internalizing GRPR agonists were initially proposed for prolonged lesion retention, but a shift of paradigm to GRPR antagonists has recently been made. Surprisingly, radioantagonists, such as [(99m)Tc]DB1 ((99m)Tc-N4'-DPhe(6),Leu-NHEt(13)]BBN(6-13)), displayed better pharmacokinetics than radioagonists, in addition to their higher inherent biosafety. We introduce here [(68)Ga]SB3, a [(99m)Tc]DB1 mimic-carrying, instead of the (99m)Tc-binding tetraamine, the chelator DOTA for labeling with the PET radiometal (68)Ga.

METHODS

Competition binding assays of SB3 and [(nat)Ga]SB3 were conducted against [(125)I-Tyr(4)]BBN in PC-3 cell membranes. Blood samples collected 5 min postinjection (pi) of the [(67)Ga]SB3 surrogate in mice were analyzed using high-performance liquid chromatography (HPLC) for degradation products. Likewise, biodistribution was performed after injection of [(67)Ga]SB3 (37 kBq, 100 μL, 10 pmol peptide) in severe combined immunodeficiency (SCID) mice bearing PC-3 xenografts. Eventually, [(68)Ga]SB3 (283 ± 91 MBq, 23 ± 7 nmol) was injected into 17 patients with breast (8) and prostate (9) cancer. All patients had disseminated disease and had received previous therapies. PET/CT fusion images were acquired 60-115 min pi.

RESULTS

SB3 and [(nat)Ga]SB3 bound to the human GRPR with high affinity (IC50: 4.6 ± 0.5 nM and 1.5 ± 0.3 nM, respectively). [(67)Ga]SB3 displayed good in vivo stability (>85 % intact at 5 min pi). [(67)Ga]SB3 showed high, GRPR-specific and prolonged retention in PC-3 xenografts (33.1 ± 3.9%ID/g at 1 h pi - 27.0 ± 0.9%ID/g at 24 h pi), but much faster clearance from the GRPR-rich pancreas (≈160%ID/g at 1 h pi to <17%ID/g at 24 h pi) in mice. In patients, [(68)Ga]SB3 elicited no adverse effects and clearly visualized cancer lesions. Thus, 4 out of 8 (50 %) breast cancer and 5 out of 9 (55 %) prostate cancer patients showed pathological uptake on PET/CT with [(68)Ga]SB3.

CONCLUSION

[(67)Ga]SB3 showed excellent pharmacokinetics in PC-3 tumor-bearing mice, while [(68)Ga]SB3 PET/CT visualized lesions in about 50 % of patients with advanced and metastasized prostate and breast cancer. We expect imaging with [(68)Ga]SB3 to be superior in patients with primary breast or prostate cancer.

Clinical Relevance of Targeting the Gastrin-Releasing Peptide Receptor, Somatostatin Receptor 2, or Chemokine C-X-C Motif Receptor 4 in Breast Cancer for Imaging and Therapy

Imaging and therapy using radioligands targeting receptors overexpressed on tumor cells is successfully applied in neuroendocrine tumor patients. Because expression of the gastrin-releasing peptide receptor (GRPR), somatostatin receptor 2 (SSTR2), and chemokine C-X-C motif receptor 4 (CXCR4) has been demonstrated in breast cancer, targeting these receptors using radioligands might offer new imaging and therapeutic opportunities for breast cancer patients. The aim of this study was to correlate messenger RNA (mRNA) expression of GRPR, SSTR2, and CXCR4 with clinicopathologic and biologic factors, and with prognosis and prediction to therapy response, in order to identify specific breast cancer patient groups suited for the application of radioligands targeting these receptors.

METHODS

First, we studied GRPR and SSTR2 expression in 13 clinical breast cancer specimens by in vitro autoradiography and correlated this with corresponding mRNA levels to investigate whether mRNA levels reliably represent cell surface expression. Next, GRPR, SSTR2, and CXCR4 mRNA levels were measured by quantitative reverse transcriptase polymerase chain reaction in 915 primary breast cancer tissues and correlated with known clinicopathologic and biologic factors, disease-free survival, distant metastasis-free survival, and overall survival (DFS, MFS, and OS, respectively). In 224 adjuvant hormonal treatment-naïve estrogen receptor (ER, ESR1)-positive patients who received tamoxifen as first-line therapy for recurrent or metastatic disease, the expression levels of the receptors were correlated with progression-free survival.

RESULTS

Our results showed a significant positive correlation between GRPR and SSTR2 expression analyzed by in vitro autoradiography and by quantitative reverse transcriptase polymerase chain reaction (Spearman's rank correlation coefficient [Rs]=0.94, P<0.001, and Rs=0.73, P=0.0042, respectively). Furthermore, high GRPR and SSTR2 mRNA levels were observed more frequently in ESR1-positive specimens, whereas high CXCR4 expression was associated with ESR1-negative specimens. Also, high mRNA expression of CXCR4 was associated with a prolonged DFS, MFS, and OS (multivariate hazard ratio MFS=0.76 [95% confidence interval, 0.64-0.90], P=0.001), whereas high mRNA levels of GRPR were associated with a prolonged progression-free survival after the start of first-line tamoxifen treatment (multivariate hazard ratio=0.68 [95% confidence interval, 0.48-0.97], P=0.031).

CONCLUSION

Our data indicate that imaging and therapy using GRPR or SSTR2 radioligands might especially be beneficial for ESR1-positive breast cancer and CXCR4 radioligands for ESR1-negative breast cancer.