In vitro and in vivo application of radiolabeled gastrin-releasing peptide receptor ligands in breast cancer

Gastrin Releasing Peptide Receptors-targeted PET Diagnostics and Radionuclide Therapy for Prostate Cancer Management: Preclinical and Clinical Developments of the Past 5 Years

Each tumor has its own distinctive molecular identity. Treatment, therefore, should be tailored to this unique cancer phenotype. Theragnostics uses the same compound for targeted imaging and treatment, radiolabeled to an appropriate radionuclide, respectively. Gastrin-releasing peptide receptors (GRPRs) are overexpressed in prostate cancer, and radiolabeled GRPR antagonists have shown high diagnostic performance at staging and biochemical recurrence. Several GRPR-targeting theragnostic compounds have been developed preclinically. Their translation into clinics is underway with 4 clinical trials recruiting participants with GRPR-expressing tumors.

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.

Targeting the Oxytocin Receptor for Breast Cancer Management: A Niche for Peptide Tracers

Breast cancer is a leading cause of death in women, and its management highly depends on early disease diagnosis and monitoring. This remains challenging due to breast cancer's heterogeneity and a scarcity of specific biomarkers that could predict responses to therapy and enable personalized treatment. This Perspective describes the diagnostic landscape for breast cancer management, molecular strategies targeting receptors overexpressed in tumors, the theranostic potential of the oxytocin receptor (OTR) as an emerging breast cancer target, and the development of OTR-specific optical and nuclear tracers to study, visualize, and treat tumors. A special focus is on the chemistry and pharmacology underpinning OTR tracer development, preclinical in vitro and in vivo studies, challenges, and future directions. The use of peptide-based tracers targeting upregulated receptors in cancer is a highly promising strategy complementing current diagnostics and therapies and providing new opportunities to improve cancer management and patient survival.

68 Ga-labeled, imatinib encapsulated, theranostic liposomes: Formulation, characterization, and in vitro evaluation of anticancer activity

Cancer is still a global health problem. Among cancer types, breast cancer is the most frequently diagnosed one, and it causes a high mortality rate if not diagnosed in the early stages. In our study, imatinib encapsulated, nanosized, neutral/cationic liposome formulations were prepared as theranostic agents for breast cancer. After the characterization studies in which all liposomes exhibited proper profile owing to their particle size between 133 and 250 nm, polydispersity index values lower than 0.4, neutral and cationic zeta potential values, and high drug encapsulation efficiency, controlled drug release behaviors with zero-order kinetic were obtained. The higher than 90% radiolabeling efficiency values were obtained thanks to the determination of optimum radiolabeling condition (80°C temperature, 5 mCi radioactivity, and 10 min incubation period). According to the resazurin assay evaluating the cytotoxic profile of liposomes on MCF7 cells, neutral empty liposome was found as biocompatible, while both cationic liposomes (empty and drug-loaded ones) exhibited high nonspecific cytotoxicity at even low drug concentration due to the existence of stearyl amine in the formulations. However, dose-dependent cytotoxic effect and the highest cellular binding capacity were obtained by imatinib loaded neutral liposomes. In conclusion, 68 Ga-radiolabeled, imatinib-loaded, neutral, nanosized liposome formulation is the most promising one as a theranostic agent among all formulations.

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.

The GRPR Antagonist [99mTc]Tc-maSSS-PEG2-RM26 towards Phase I Clinical Trial: Kit Preparation, Characterization and Toxicity

Gastrin-releasing peptide receptors (GRPRs) are overexpressed in the majority of primary prostate tumors and in prostatic lymph node and bone metastases. Several GRPR antagonists were developed for SPECT and PET imaging of prostate cancer. We previously reported a preclinical evaluation of the GRPR antagonist [^99mTc]Tc-maSSS-PEG₂-RM26 (based on [D-Phe⁶, Sta¹³, Leu¹⁴-NH₂]BBN(6-14)) which bound to GRPR with high affinity and had a favorable biodistribution profile in tumor-bearing animal models. In this study, we aimed to prepare and test kits for prospective use in an early-phase clinical study. The kits were prepared to allow for a one-pot single-step radiolabeling with technetium-99m pertechnetate. The kit vials were tested for sterility and labeling efficacy. The radiolabeled by using the kit GRPR antagonist was evaluated in vitro for binding specificity to GRPR on PC-3 cells (GRPR-positive). In vivo, the toxicity of the kit constituents was evaluated in rats. The labeling efficacy of the kits stored at 4 °C was monitored for 18 months. The biological properties of [^99mTc]Tc-maSSS-PEG₂-RM26, which were obtained after this period, were examined both in vitro and in vivo. The one-pot (gluconic acid, ethylenediaminetetraacetic acid, stannous chloride, and maSSS-PEG₂-RM26) single-step radiolabeling with technetium-99m was successful with high radiochemical yields (>97%) and high molar activities (16-24 MBq/nmol). The radiolabeled peptide maintained its binding properties to GRPR. The kit constituents were sterile and non-toxic when tested in living subjects. In conclusion, the prepared kit is considered safe in animal models and can be further evaluated for use in clinics.

Peptide Radioligands in Cancer Theranostics: Agonists and Antagonists

The clinical success of radiolabeled somatostatin analogs in the diagnosis and therapy-"theranostics"-of tumors expressing the somatostatin subtype 2 receptor (SST₂R) 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, SST₂R-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.

Phase I Trial of [99mTc]Tc-maSSS-PEG2-RM26, a Bombesin Analogue Antagonistic to Gastrin-Releasing Peptide Receptors (GRPRs), for SPECT Imaging of GRPR Expression in Malignant Tumors

The gastrin-releasing peptide receptor (GRPR) is overexpressed in prostate cancer (PCa) and in hormone-driven breast cancer (BCa). The aim of this phase I clinical trial was to evaluate safety, biodistribution, and dosimetry after the administration of the recently developed GRPR-targeting antagonistic bombesin analogue [99mTc]Tc-maSSS-PEG2-RM26 in PCa and BCa patients. Planar and whole-body SPECT/CT imaging was performed in six PCa patients and seven BCa patients 2, 4, 6, and 24 h post the intravenous administration of 40 µg of [99mTc]Tc-maSSS-PEG2-RM26 (600–700 MBq). No adverse events or pathological changes were observed. The rapid blood clearance of [99mTc]Tc-maSSS-PEG2-RM26 was observed with predominantly hepatobiliary excretion. The effective doses were 0.0053 ± 0.0007 for male patients and 0.008 ± 0.003 mSv/MBq for female patients. The accumulation of [99mTc]Tc-maSSS-PEG2-RM26 in tumors was observed in four out of six PCa and in seven out of seven BCa patients. In four BCa patients, a high uptake of the agent into the axillary lymph nodes was detected. Immunohistochemistry revealed positive GRPR expression in 60% of primary PCa, 71.4% of BCa tumors, and 50% of examined BCa lymph nodes. In conclusion, a single administration of [99mTc]Tc-maSSS-PEG2-RM26 was safe and well tolerated. [99mTc]Tc-maSSS-PEG2-RM26 SPECT may be useful for tumor detection in PCa and BCa patients, pending further studies.

Correlation of 68Ga-RM2 PET with Postsurgery Histopathology Findings in Patients with Newly Diagnosed Intermediate- or High-Risk Prostate Cancer

⁶⁸Ga-RM2 targets gastrin-releasing peptide receptors (GRPRs), which are overexpressed in prostate cancer (PC). Here, we compared preoperative ⁶⁸Ga-RM2 PET to postsurgery histopathology in patients with newly diagnosed intermediate- or high-risk PC. Methods: Forty-one men, 64.0 ± 6.7 y old, were prospectively enrolled. PET images were acquired 42-72 min (median ± SD, 52.5 ± 6.5 min) after injection of 118.4-247.9 MBq (median ± SD, 138.0 ± 22.2 MBq) of ⁶⁸Ga-RM2. PET findings were compared with preoperative multiparametric MRI (mpMRI) (n = 36) and ⁶⁸Ga-PSMA11 PET (n = 17) and correlated to postprostatectomy whole-mount histopathology (n = 32) and time to biochemical recurrence. Nine participants decided to undergo radiation therapy after study enrollment. Results: All participants had intermediate- (n = 17) or high-risk (n = 24) PC and were scheduled for prostatectomy. Prostate-specific antigen was 8.8 ± 77.4 (range, 2.5-504) and 7.6 ± 5.3 ng/mL (range, 2.5-28.0 ng/mL) when participants who ultimately underwent radiation treatment were excluded. Preoperative ⁶⁸Ga-RM2 PET identified 70 intraprostatic foci of uptake in 40 of 41 patients. Postprostatectomy histopathology was available in 32 patients in which ⁶⁸Ga-RM2 PET identified 50 of 54 intraprostatic lesions (detection rate = 93%). ⁶⁸Ga-RM2 uptake was recorded in 19 nonenlarged pelvic lymph nodes in 6 patients. Pathology confirmed lymph node metastases in 16 lesions, and follow-up imaging confirmed nodal metastases in 2 lesions. ⁶⁸Ga-PSMA11 and ⁶⁸Ga-RM2 PET identified 27 and 26 intraprostatic lesions, respectively, and 5 pelvic lymph nodes each in 17 patients. Concordance between ⁶⁸Ga-RM2 and ⁶⁸Ga-PSMA11 PET was found in 18 prostatic lesions in 11 patients and 4 lymph nodes in 2 patients. Noncongruent findings were observed in 6 patients (intraprostatic lesions in 4 patients and nodal lesions in 2 patients). Sensitivity and accuracy rates for ⁶⁸Ga-RM2 and ⁶⁸Ga-PSMA11 (98% and 89% for ⁶⁸Ga-RM2 and 95% and 89% for ⁶⁸Ga-PSMA11) were higher than those for mpMRI (77% and 77%, respectively). Specificity was highest for mpMRI with 75% followed by ⁶⁸Ga-PSMA11 (67%) and ⁶⁸Ga-RM2 (65%). Conclusion: ⁶⁸Ga-RM2 PET accurately detects intermediate- and high-risk primary PC, with a detection rate of 93%. In addition, ⁶⁸Ga-RM2 PET showed significantly higher specificity and accuracy than mpMRI and a performance similar to ⁶⁸Ga-PSMA11 PET. These findings need to be confirmed in larger studies to identify which patients will benefit from one or the other or both radiopharmaceuticals.

PET Imaging Using Gallium-68 (68Ga) RM2

Molecular imaging is advancing rapidly with promising new molecular targets emerging for theragnostic, ie, imaging and treatment with the same compound, to provide targeted, personalized medicine. Gastrin-releasing peptide receptors (GRPR) are overexpressed in prostate cancer. Gallium-68 (68Ga) RM2 is a GRPR antagonist and shows high sensitivity and specificity for the detection of primary prostate cancer and recurrent disease. However, compared with the widely used 68Ga-PSMA11 and 18F-DCFPyL, a discordance in uptake pattern is seen reflecting the heterogeneity in tumor biology of prostate cancer. In this review, we present the background, current status, and future perspectives of PET imaging using 68Ga-RM2.

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.

Investigation of Tumor Cells and Receptor-Ligand Simulation Models for the Development of PET Imaging Probes Targeting PSMA and GRPR and a Possible Crosstalk between the Two Receptors

Prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) have both been used in nuclear medicine as targets for molecular imaging and therapy of prostate (PCa) and breast cancer (BCa). Three bioconjugate probes, the PSMA specific: [⁶⁸Ga]Ga-1, ((HBED-CC)-Ahx-Lys-NH-CO-NH Glu or PSMA-11), the GRPR specific: [⁶⁸Ga]Ga-2, ((HBED-CC)-4-amino-1-carboxymethyl piperidine-[D-Phe⁶, Sta¹³]BN(6-14), a bombesin (BN) analogue), and 3 (the BN analogue: 4-amino-1-carboxymethyl piperidine-[(R)-Phe⁶, Sta¹³]BN(6-14) connected with the fluorescent dye, BDP-FL), were synthesized and tested in vitro with PCa and BCa cell lines, more specifically, with PCa cells, PC-3 and LNCaP, with BCa cells, T47D, MDA-MB-231, and with the in-house created PSMA-overexpressing PC-3^(PSMA), T47D^(PSMA), and MDA-MB-231^(PSMA). In addition, biomolecular simulations were conducted on the association of 1 and 2 with PSMA and GRPR. The PSMA overexpression resulted in an increase of cell-bound radioligand [⁶⁸Ga]Ga-1 (PSMA) for PCa and BCa cells and also of [⁶⁸Ga]Ga-2 (GRPR), especially in those cell lines already expressing GRPR. The results were confirmed by fluorescence-activated cell sorting with a PE-labeled PSMA-specific antibody and the fluorescence tracer 3. The docking calculations and molecular dynamics simulations showed how 1 enters the PSMA funnel region and how pharmacophore Glu-urea-Lys interacts with the arginine patch, the S1', and S1 subpockets by forming hydrogen and van der Waals bonds. The chelating moiety of 1, that is, HBED-CC, forms additional stabilizing hydrogen bonding and van der Waals interactions in the arene-binding site. Ligand 2 is diving into the GRPR transmembrane (TM) helical cavity, thereby forming hydrogen bonds through its amidated end, water-mediated hydrogen bonds, and π-π interactions. Our results provide valuable information regarding the molecular mechanisms involved in the interactions of 1 and 2 with PSMA and GRPR, which might be useful for the diagnostic imaging and therapy of PCa and BCa.

Gastrin-Releasing Peptide Receptor Antagonist [68Ga]RM2 PET/CT for Staging of Pre-Treated, Metastasized Breast Cancer

BACKGROUND

Positron emission tomography (PET)/computed tomography (CT) using the gastrin-releasing peptide receptor antagonist [⁶⁸Ga]RM2 has shown to be a promising imaging method for primary breast cancer (BC) with positive estrogen receptor (ER) status. This study assessed tumor visualization by [⁶⁸Ga]RM2 PET/CT in patients with pre-treated ER-positive BC and suspected metastases.

METHODS

This retrospective pilot study included eight female patients with initial ER-positive, pre-treated BC who underwent [⁶⁸Ga]RM2 PET/CT. Most of these patients (seven out of eight; 88%) were still being treated with or had received endocrine therapy. [⁶⁸Ga]RM2 PET/CTs were visually analyzed by two nuclear medicine specialists in consensus. Tumor manifestations were rated qualitatively (i.e., RM2-positive or RM2-negative) and quantitatively using the maximum standardized uptake value (SUVmax). SUVmax values were compared between the two subgroups (RM2-positive vs. RM2-negative).

RESULTS

Strong RM2 binding was found in all metastatic lesions of six patients (75%), whereas tracer uptake in all metastases of two patients (25%) was rated negative. Mean SUVmax of RM2-positive metastases with the highest SUVmax per patient (in lymph node and bone metastases; 15.8 ± 15.1 range: 3.7-47.8) was higher than mean SUVmax of the RM2-negative metastases with the highest SUVmax per patient (in bone metastases; 1.6 ± 0.1, range 1.5-1.7).

CONCLUSIONS

Our data suggest that RM2 binding is maintained in the majority of patients with advanced disease stage of pre-treated ER-positive BC. Thus, [⁶⁸Ga]RM2 PET/CT could support treatment decision in these patients, radiotherapy planning in oligometastatic patients or selection of patients for RM2 radioligand therapy. Further studies with larger patient cohorts are warranted to confirm these findings.

Radiolabeled Bombesin Analogs

Simple Summary

Recent medical advancements have strived for a personalized medicine approach to patients, aimed at optimizing therapy outcomes with minimum toxicity. In this respect, nuclear medicine methodologies have been playing increasingly important roles. For example, the overexpression of peptide receptors, such as the gastrin-releasing peptide receptor (GRPR), on tumor cells as opposed to their lack of expression in healthy surrounding tissues can be elegantly exploited with the aid of “smart” peptide carriers, such as the analogs of the amphibian 14-peptide bombesin (BBN). These molecules can bring clinically attractive radionuclides to malignant lesions in prostate, breast, and other human cancers, sparing healthy tissues. Depending upon the radionuclide in question, diagnostic imaging with single-photon emission computed tomography (SPECT) or positron emission tomography (PET) has been pursued, identifying patients who are eligible for peptide radionuclide receptor therapy (PRRT) in an integrated “theranostic” approach. In the present review, we (i) discuss the major steps taken in the development of anti-GRPR theranostic radioligands, with a focus on those selected for clinical testing; (ii) comment on the present status in this field of research; and (iii) reflect on the current limitations as well as on new opportunities for their broader and more successful clinical applications.

Abstract

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.

Breast Cancer Metastasis Mimicking Meningioma in 68Ga-DOTATOC PET/CT

ABSTRACT

68Ga-DOTATOC PET/CT is a reliable imaging modality in the diagnosis and therapy planning of symptomatic meningiomas. We present a case of a 74-year-old woman where a supposed SSTR-positive sphenoid wing meningioma turned out to be a breast cancer metastasis. Our case shows that dural metastases from breast cancer might represent a clinical pitfall in 68Ga-DOTATOC PET/CT.

[99mTc]Tc-DB15 in GRPR-Targeted Tumor Imaging with SPECT: From Preclinical Evaluation to the First Clinical Outcomes

Diagnostic imaging and radionuclide therapy of prostate (PC) and breast cancer (BC) using radiolabeled gastrin-releasing peptide receptor (GRPR)-antagonists represents a promising approach. We herein propose the GRPR-antagonist based radiotracer [^99mTc]Tc-DB15 ([^99mTc]Tc-N₄-AMA-DGA-_DPhe⁶,Sar¹¹,LeuNHEt¹³]BBN(6-13); N₄: 6-carboxy-1,4,8,11-tetraazaundecane, AMA: aminomethyl-aniline, DGA: diglycolic acid) as a new diagnostic tool for GRPR-positive tumors applying SPECT/CT. The uptake of [^99mTc]Tc-DB15 was tested in vitro in mammary (T-47D) and prostate cancer (PC-3) cells and in vivo in T-47D or PC-3 xenograft-bearing mice as well as in BC patients. DB15 showed high GRPR-affinity (IC₅₀ = 0.37 ± 0.03 nM) and [^99mTc]Tc-DB15 strongly bound to the cell-membrane of T-47D and PC-3 cells, according to a radiolabeled antagonist profile. In mice, the radiotracer showed high and prolonged GRPR-specific uptake in PC-3 (e.g., 25.56 ± 2.78 %IA/g vs. 0.72 ± 0.12 %IA/g in block; 4 h pi) and T-47D (e.g., 15.82 ± 3.20 %IA/g vs. 3.82 ± 0.30 %IA/g in block; 4 h pi) tumors, while rapidly clearing from background. In patients with advanced BC, the tracer could reveal several bone and soft tissue metastases on SPECT/CT. The attractive pharmacokinetic profile of [^99mTc]DB15 in mice and its capability to target GRPR-positive BC lesions in patients highlight its prospects for a broader clinical use, an option currently being explored by ongoing clinical studies.

Preclinical Advances in Theranostics for the Different Molecular Subtypes of Breast Cancer

Breast cancer is the most common cancer in women worldwide. The heterogeneity of breast cancer and drug resistance to therapies make the diagnosis and treatment difficult. Molecular imaging methods with positron emission tomography (PET) and single-photon emission tomography (SPECT) provide useful tools to diagnose, predict, and monitor the response of therapy, contributing to precision medicine for breast cancer patients. Recently, many efforts have been made to find new targets for breast cancer therapy to overcome resistance to standard of care treatments, giving rise to new therapeutic agents to offer more options for patients with breast cancer. The combination of diagnostic and therapeutic strategies forms the foundation of theranostics. Some of these theranostic agents exhibit high potential to be translated to clinic. In this review, we highlight the most recent advances in theranostics of the different molecular subtypes of breast cancer in preclinical studies.

Theranostic Advances in Breast Cancer in Nuclear Medicine

The implication of 'theranostic' refers to targeting an identical receptor for diagnostic and therapeutic purposes, by the same radioligand, simultaneously or separately. In regard to extensive efforts, many considerable theranostic tracers have been developed in recent years. Emerging evidence strongly demonstrates the tendency of nuclear medicine towards therapies based on a diagnosis. This review is focused on the examples of targeted radiopharmaceuticals for the imaging and therapy of breast cancer.

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.

The association of 18F-fluorodeoxyglucose PET/computed tomography parameters with tissue gastrin-releasing peptide receptor and integrin αvβ3 receptor levels in patients with breast cancer

OBJECTIVE

Gastrin-releasing peptide receptor (GRPR) and integrin αvβ3 receptors are significantly associated with primary breast cancer, neovascular endothelial, and metastatic tumor cells. We aimed to evaluate GRPR and integrin αvβ3 receptor staining, F-FDG uptake patterns and possible prognostic factors in breast cancer.

METHODS

Ninety lesions of 87 subjects diagnosed with breast cancer were included in this prospective study. The sections were stained with GRPR and integrin αvβ3. Subjects were divided into four molecular subgroups: luminal A, luminal B, triple negative and HER2. PET/CT imaging was performed on all subjects. The groups were compared in terms of GRPR and integrin αvβ3 staining properties, possible prognostic factors and mean SUVmax values.

RESULTS

Increased F-FDG uptake was significantly associated with estrogen receptor and progesterone receptor negativity. Molecular subtypes were significantly associated with mean integrin scores (P = 0.030), while histopathological subtypes were significantly associated with mean GRPR scores (P = 0.029). Increased integrin αvβ3 expression is significantly associated with ER and PR negativity. Additionally, GRPR score was significantly correlated with estrogen receptor and progesterone receptor expression scores and a negative statistically significant correlation was detected between integrin and progesterone receptor scores. Mean primary lesion SUVmax had a statistically significant positive correlation with integrin αvβ3 score.

CONCLUSION

GRPR and integrin αvβ3 expression results are complementary to F-FDG PET/CT findings, and are also significantly correlated with hormone receptors associated with aggressive subtypes. These results may pave the way for GRPR and integrin αvβ3 targeted imaging with Ga-labeled molecules and systemic radionuclide treatment with Lu-labeled compounds.

A comparative PET imaging study of 44gSc- and 68Ga-labeled bombesin antagonist BBN2 derivatives in breast and prostate cancer models

INTRODUCTION

Radiolabeled peptides play a central role in nuclear medicine as radiotheranostics for targeted imaging and therapy of cancer. We have recently proposed the use of metabolically stabilized GRPR antagonist BBN2 for radiolabeling with 18F and 68Ga and subsequent PET imaging of GRPRs in prostate cancer. The present work studied the impact of 44gSc- and 68Ga-labeled DOTA complexes attached to GRPR antagonist BBN2 on the in vitro GRPR binding affinity, and their biodistribution and tumor uptake profiles in MCF7 breast and PC3 prostate cancer models.

METHODS

DOTA-Ava-BBN2 was radiolabeled with radiometals 68Ga and 44gSc. Gastrin-releasing peptide receptor (GRPR) affinities of peptides were assessed in PC3 prostate cancer cells. GRPR expression profiles were studied in human breast cancer tissue samples and MCF7 breast cancer cells. PET imaging of 68Ga- and 44gSc-labeled peptides was performed in MCF7 and PC3 xenografts as breast and prostate cancer models.

RESULTS

Radiopeptides [68Ga]Ga-DOTA-Ava-BBN2 and [44gSc]Sc-DOTA-Ava BBN2 were prepared in radiochemical yields of 70-80% (decay-corrected), respectively. High binding affinities were found for both peptides (IC50 = 15 nM (natGa) and 5 nM (natSc)). Gene expression microarray analysis revealed high GRPR mRNA expression levels in estrogen receptor (ER)-positive breast cancer, which was further confirmed with Western blot and immunohistochemistry. However, PET imaging showed only low tumor uptake of both radiotracers in MCF7 xenografts ([68Ga]Ga-DOTA-BBN2 (SUV60min 0.27 ± 0.06); [44gSc]Sc-DOTA-BBN2 (SUV60min 0.20 ± 0.03)). In contrast, high tumor uptake and retention were found for both radiopeptides in PC3 tumors ([68Ga]Ga-DOTA-BBN2 (SUV60min 0.46 ± 0.07); [44gSc]Sc-DOTA-BBN2 (SUV60min 0.51 ± 0.11)).

CONCLUSIONS

Comparison of 68Ga- and 44gSc-labeled DOTA-Ava-BBN2 peptides revealed slight but noticeable differences of the radiometal with an impact on the in vitro GRPR receptor binding properties in PC3 cells. No differences were found in their in vivo biodistribution profiles in MCF7 and PC3 xenografts. Radiopeptides [68Ga]Ga-DOTA-Ava-BBN2 and [44gSc]Sc-DOTA-Ava-BBN2 displayed comparable tumor uptake and retention profiles with rapid blood and renal clearance profiles in both tumor models.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

The favorable PET imaging performance of [44gSc]Sc-DOTA-Ava-BBN2 in prostate cancer should warrant the development of an [43Sc]Sc-DOTA-Ava-BBN2 analog for clinical translation which comes with a main γ-line of much lower energy and intensity compared to 44gSc.

Association between gastrin-releasing peptide receptor expression as assessed with [68Ga]Ga-RM2 PET/CT and histopathological tumor regression after neoadjuvant chemotherapy in primary breast cancer

INTRODUCTION

The gastrin-releasing peptide receptor is overexpressed in breast cancer (BC) tissue and can be visualized by positron emission tomography (PET) using the GRPR antagonist [⁶⁸Ga]Ga-RM2. This study assessed tumor binding of RM2 before and after neoadjuvant chemotherapy (NAC) in primary BC with reference to residual tumor size in the resected specimen.

MATERIALS AND METHODS

In this retrospective study, five female patients with biopsy-confirmed estrogen receptor (ER)-positive primary BC (one with bilateral tumors) underwent [⁶⁸Ga]Ga-RM2 PET/CT before and after NAC. PET/CT was acquired 1 h after injection of 143-224 MBq [⁶⁸Ga]Ga-RM2. Time from pre-NAC PET to beginning of NAC was 23 ± 4.9 days, from end of NAC to post-NAC PET 18.7 ± 6.3 days, and from post-NAC PET to surgery 9.5 ± 10.8 days. In vivo tumor uptake of [⁶⁸Ga]Ga-RM2 was assessed before and after NAC and correlated with histopathological response.

RESULTS

All tumors (6/6) showed strongly increased [⁶⁸Ga]Ga-RM2 uptake compared to normal breast tissue on pre-NAC PET (mean SUVmax 13.2 ± 7.3; mean SUVpeak 9.4 ± 4.4). [⁶⁸Ga]Ga-RM2 uptake was significantly reduced on post-NAC PET in all primary tumors (mean SUVmax 2.3 ± 0.8, -79 ± 11%; p = 0.0125; mean SUVpeak 1.6 ± 0.4, -79 ± 10%; p = 0.0096). Residual tumor size in resected specimens correlated well with SUVmax (r = 0.91, p = 0.0057) and SUVpeak (r = 0.88, p = 0.0196) on [⁶⁸Ga]Ga-RM2 PET/CT after NAC.

CONCLUSION AND IMPLICATIONS FOR PATIENT CARE

In this pilot study, residual uptake of [⁶⁸Ga]Ga-RM2 in ER-positive primary BC correlated well with residual vital tumor size after NAC. This suggests that [⁶⁸Ga]Ga-RM2 PET/CT merits further investigation for response assessment to NAC in patients with ER-positive BC.

Downregulation of GRK5 hampers the migration of breast cancer cells

Sunitinib is a multispecific kinase inhibitor and one of its targets is the kinase GRK5, which is regulating a multitude of G protein-coupled receptors (GPCRs). In this study we demonstrate that a decreased GRK5 expression induced by knock-down experiments or sunitinib treatment hampers the migration of cancer cell lines. A proteomic analysis revealed many pathways related to cell migration which were down regulated upon the GRK5 knock-down. Furthermore, we found in MDA-MB-231 breast cancer cells that the inhibition of migration is mediated by the GPCR gastrin releasing peptide receptor (GRPR) leading to a reduced expression of migration regulating downstream targets like CDC42 and ROCK1. An in silico Kaplan Meier analysis revealed that GRK5 and GRPR overexpression reduces the distant metastasis free survival in triple-negative breast cancer (TNBC) patients. Thus, we suggest a novel anti-migratory effect of impaired GRK5 expression which induces a negative feedback loop on GRPR signalling.

Recent Advances in Nuclear Imaging of Receptor Expression to Guide Targeted Therapies in Breast Cancer

Breast cancer remains the most frequent cancer in women with different patterns of disease progression and response to treatments. The identification of specific biomarkers for different breast cancer subtypes has allowed the development of novel targeting agents for imaging and therapy. To date, patient management depends on immunohistochemistry analysis of receptor status on bioptic samples. This approach is too invasive, and in some cases, not entirely representative of the disease. Nuclear imaging using receptor tracers may provide whole-body information and detect any changes of receptor expression during disease progression. Therefore, imaging is useful to guide clinicians to select the best treatments for each patient and to evaluate early response thus reducing unnecessary therapies. In this review, we focused on the development of novel tracers that are ongoing in preclinical and/or clinical studies as promising tools to lead treatment decisions for breast cancer management.

[68Ga]-Dota Peptide PET/CT in Neuroendocrine Tumors: Main Clinical Applications

Objective:

Neuroendocrine Neoplasms (NENs) are generally defined as rare and heterogeneous tumors. The gastrointestinal system is the most frequent site of NENs localization, however they can be found in other anatomical regions, such as pancreas, lungs, ovaries, thyroid, pituitary, and adrenal glands. Neuroendocrine neoplasms have significant clinical manifestations depending on the production of active peptide.

Methods:

Imaging modalities play a fundamental role in initial diagnosis as well as in staging and treatment monitoring of NENs, in particular they vastly enhance the understanding of the physiopathology and diagnosis of NENs through the use of somatostatin analogue tracers labeled with appropriate radioisotopes. Additionally, the use of somatostatin analogues provides the ability to in-vivo measure the expression of somatostatin receptors on NEN cells, a process that might have important therapeutic implications.

Results:

A large body of evidences showed improved accuracy of molecular imaging based on PET/CT radiotracer with SST analogues (e.g. [68Ga]-DOTA peptide) for the detection of NEN lesions in comparison to morphological imaging modalities. So far, the role of imaging technologies in assessing treatment response is still under debate.

Conclusion:

This review offers the systems of classification and grading of NENs and summarizes the more useful recommendations based on data recently published for the management of patients with NENs, with special focus on the role of imaging modalities based on SST targeting with PET / CT radiotracers.

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.

Fluorescent, Plasmonic, and Radiotherapeutic Properties of the 177Lu-Dendrimer-AuNP-Folate-Bombesin Nanoprobe Located Inside Cancer Cells

The integration of fluorescence and plasmonic properties into one molecule is of importance in developing multifunctional imaging and therapy nanoprobes. The aim of this research was to evaluate the fluorescent properties and the plasmonic–photothermal, therapeutic, and radiotherapeutic potential of ¹⁷⁷Lu–dendrimer conjugated to folate and bombesin with gold nanoparticles in the dendritic cavity (¹⁷⁷Lu–DenAuNP–folate–bombesin) when it is internalized in T47D breast cancer cells. The intense near-Infrared (NIR) fluorescence emitted at 825 nm from the conjugate inside cells corroborated the usefulness of DenAuNP–folate–bombesin for optical imaging. After laser irradiation, the presence of the nanosystem in cells caused a significant increase in the temperature of the medium (46.8°C, compared to 39.1°C without DenAuNP–folate–bombesin, P < 0.05), resulting in a significant decrease in cell viability (down to 16.51% ± 1.52%) due to the ¹⁷⁷Lu–DenAuNP–folate–bombesin plasmonic properties. After treatment with ¹⁷⁷Lu–DenAuNP–folate–bombesin, the T47D cell viability decreased 90% because of the radiation-absorbed dose (63.16 ± 4.20 Gy) delivered inside the cells. The ¹⁷⁷Lu–DenAuNP–folate–bombesin nanoprobe internalized in cancer cells exhibited properties suitable for optical imaging, plasmonic–photothermal therapy, and targeted radiotherapy.

New Gastrin Releasing Peptide Receptor-Directed [99mTc]Demobesin 1 Mimics: Synthesis and Comparative Evaluation

We have previously reported on the gastrin releasing peptide receptor (GRPR) antagonist [^99mTc]1, ([^99mTc]demobesin 1, ^99mTc-[N₄'-diglycolate-dPhe⁶,Leu-NHEt¹³]BBN(6-13)). [^99mTc]1 has shown superior biological profile compared to analogous agonist-based ^99mTc-radioligands. We herein present a small library of [^99mTc]1 mimics generated after structural modifications in (a) the linker ([^99mTc]2, [^99mTc]3, [^99mTc]4), (b) the peptide chain ([^99mTc]5, [^99mTc]6), and (c) the C-terminus ([^99mTc]7 or [^99mTc]8). The effects of above modifications on the biological properties of analogs were studied in PC-3 cells and tumor-bearing SCID mice. All analogs showed subnanomolar affinity for the human GRPR, while most receptor-affine 4 and 8 behaved as potent GRPR antagonists in a functional internalization assay. In mice bearing PC-3 tumors, [^99mTc]1-[^99mTc]6 exhibited GRPR-specific tumor uptake, rapidly clearing from normal tissues. [^99mTc]4 displayed the highest tumor uptake (28.8 ± 4.1%ID/g at 1 h pi), which remained high even after 24 h pi (16.3 ± 1.8%ID/g), well surpassing that of [^99mTc]1 (5.4 ± 0.7%ID/g at 24 h pi).

Monomeric and Dimeric 68Ga-Labeled Bombesin Analogues for Positron Emission Tomography (PET) Imaging of Tumors Expressing Gastrin-Releasing Peptide Receptors (GRPrs)

The GRPr, highly expressed in prostate PCa and breast cancer BCa, is a promising target for the development of new PET radiotracers. The chelator HBED-CC ( N, N'-bis[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine- N, N'-diacetic acid) was coupled to the bombesin peptides: HBED-C-BN(2-14) 1, HBED-CC-PEG₂-[d-Tyr⁶,β-Ala¹¹,Thi¹³,Nle¹⁴]-BN(6-14) 2, HBED-CC-Y-[d-Phe⁶,Sta¹³,Leu¹⁴]-BN(6-14) (Y = 4-amino-1-carboxymethylpiperidine) 3, and HBED-CC-{PEG₂-Y-[d-Phe⁶,Sta¹³,Leu¹⁴]-BN(6-14)}₂ 4 (homodimer). Compounds 1-4 presented high binding affinities for GRPr (T47D, 0.56-3.51 nM; PC-3, 2.12-4.68 nM). In PC-3 and T47D cells, agonists [⁶⁸Ga]1 and [⁶⁸Ga]2 were mainly internalized while antagonists [⁶⁸Ga]3 and [⁶⁸Ga]4 were surface bound. Cell-related radioactivity reached a maximum after 45 min, while tracer levels followed GRPr expression (PC-3 > T47D > LNCaP > MDA-MB-231). [⁶⁸Ga]4 showed the highest cell-bound radioactivity (PC-3 and T47D). In vivo, tumor (PC-3) targeting for [⁶⁸Ga]3 and [⁶⁸Ga]4 increased over time, with dynamic μPET showing clearer tumors images at later time points. [⁶⁸Ga]3 and [⁶⁸Ga]4 can be considered suitable PET tracers for imaging PCa and BCa expressing GRPr.

Multimodal molecular 3D imaging for the tumoral volumetric distribution assessment of folate-based biosensors

The aim of this study was to characterize the in vivo volumetric distribution of three folate-based biosensors by different imaging modalities (X-ray, fluorescence, Cerenkov luminescence, and radioisotopic imaging) through the development of a tridimensional image reconstruction algorithm. The preclinical and multimodal Xtreme imaging system, with a Multimodal Animal Rotation System (MARS), was used to acquire bidimensional images, which were processed to obtain the tridimensional reconstruction. Images of mice at different times (biosensor distribution) were simultaneously obtained from the four imaging modalities. The filtered back projection and inverse Radon transformation were used as main image-processing techniques. The algorithm developed in Matlab was able to calculate the volumetric profiles of ^99mTc-Folate-Bombesin (radioisotopic image), ¹⁷⁷Lu-Folate-Bombesin (Cerenkov image), and FolateRSense™ 680 (fluorescence image) in tumors and kidneys of mice, and no significant differences were detected in the volumetric quantifications among measurement techniques. The imaging tridimensional reconstruction algorithm can be easily extrapolated to different 2D acquisition-type images. This characteristic flexibility of the algorithm developed in this study is a remarkable advantage in comparison to similar reconstruction methods.

NeoBOMB1, a GRPR-Antagonist for Breast Cancer Theragnostics: First Results of a Preclinical Study with [67Ga]NeoBOMB1 in T-47D Cells and Tumor-Bearing Mice

BACKGROUND

The GRPR-antagonist-based radioligands [^67/68Ga/¹¹¹In/¹⁷⁷Lu]NeoBOMB1 have shown excellent theragnostic profiles in preclinical prostate cancer models, while [⁶⁸Ga]NeoBOMB1 effectively visualized prostate cancer lesions in patients. We were further interested to explore the theragnostic potential of NeoBOMB1 in GRPR-positive mammary carcinoma, by first studying [⁶⁷Ga]NeoBOMB1 in breast cancer models; Methods: We investigated the profile of [⁶⁷Ga]NeoBOMB1, a [⁶⁸Ga]NeoBOMB1 surrogate, in GRPR-expressing T-47D cells and animal models; Results: NeoBOMB1 (IC₅₀s of 2.2 ± 0.2 nM) and [^natGa]NeoBOMB1 (IC₅₀s of 2.5 ± 0.2 nM) exhibited high affinity for the GRPR. At 37 °C [⁶⁷Ga]NeoBOMB1 strongly bound to the T-47D cell-membrane (45.8 ± 0.4% at 2 h), internalizing poorly, as was expected for a radioantagonist. [⁶⁷Ga]NeoBOMB1 was detected >90% intact in peripheral mouse blood at 30 min pi. In mice bearing T-47D xenografts, [⁶⁷Ga]NeoBOMB1 specifically localized in the tumor (8.68 ± 2.9% ID/g vs. 0.6 ± 0.1% ID/g during GRPR-blockade at 4 h pi). The unfavorably high pancreatic uptake could be considerably reduced (206.29 ± 17.35% ID/g to 42.46 ± 1.31% ID/g at 4 h pi) by increasing the NeoBOMB1 dose from 10 pmol to 200 pmol, whereas tumor uptake remained unaffected. Notably, tumor values did not decline from 1 to 24 h pi; Conclusions: [⁶⁷Ga]NeoBOMB1 can successfully target GRPR-positive breast cancer in animals with excellent prospects for clinical translation.

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.

Improved radiopharmaceutical based on 99mTc-Bombesin-folate for breast tumour imaging

BACKGROUND

Clinical studies in women using technetium-99m (Tc)-Bombesin have shown successful radionuclide imaging of breast tumours overexpressing gastrin-releasing peptide receptors (GRPRs). Recent studies have demonstrated that most breast tumours overexpress folate receptors (FRα).

AIM

The aim of this work was to synthesize the Lys(α,γ-Folate)-Lys(Tc-EDDA/HYNIC)-Bombesin (1-14) conjugate (Tc-Bombesin-Folate), as well as to assess the in-vitro and in-vivo potential of the radiopharmaceutical to target FRα and GRPR.

METHODS

LysLys(HYNIC)-Bombesin (1-14) was conjugated to folic acid and the product was purified by size-exclusion high-performance liquid chromatography. Ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry were used for chemical characterization. Tc labelling was performed using ethylenediamine-N,N'-diacetic acid/tricine as coligands. In-vitro binding studies were carried out in T47D breast cancer cells (positive for FRα and GRPR). Biodistribution studies and micro-single-photon emission computed tomography/computed tomography imaging were carried out on athymic mice with T47D-induced tumours.

RESULTS

High-performance liquid chromatography analyses indicated that the radioconjugate was obtained with high radiochemical purity (96±2.1%). In-vitro and in-vivo results showed significant uptake of the radiopharmaceutical in T47D cells and tumours (5.43% ID/g), which was significantly inhibited by preincubation with cold folic acid or cold Bombesin.

CONCLUSION

The Tc-Bombesin-folate heterobivalent radiopharmaceutical significantly enhances in-vivo tumour uptake because of the concomitant interaction with FRα and GRPR.

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.

Bombesin related peptides/receptors and their promising therapeutic roles in cancer imaging, targeting and treatment

INTRODUCTION

Despite remarkable advances in tumor treatment, many patients still die from common tumors (breast, prostate, lung, CNS, colon, and pancreas), and thus, new approaches are needed. Many of these tumors synthesize bombesin (Bn)-related peptides and over-express their receptors (BnRs), hence functioning as autocrine-growth-factors. Recent studies support the conclusion that Bn-peptides/BnRs are well-positioned for numerous novel antitumor treatments, including interrupting autocrine-growth and the use of over-expressed receptors for imaging and targeting cytotoxic-compounds, either by direct-coupling or combined with nanoparticle-technology.

AREAS COVERED

The unique ability of common neoplasms to synthesize, secrete, and show a growth/proliferative/differentiating response due to BnR over-expression, is reviewed, both in general and with regard to the most frequently investigated neoplasms (breast, prostate, lung, and CNS). Particular attention is paid to advances in the recent years. Also considered are the possible therapeutic approaches to the growth/differentiation effect of Bn-peptides, as well as the therapeutic implication of the frequent BnR over-expression for tumor-imaging and/or targeted-delivery.

EXPERT OPINION

Given that Bn-related-peptides/BnRs are so frequently ectopically-expressed by common tumors, which are often malignant and become refractory to conventional treatments, therapeutic interventions using novel approaches to Bn-peptides and receptors are being explored. Of particular interest is the potential of reproducing with BnRs in common tumors the recent success of utilizing overexpression of somatostatin-receptors by neuroendocrine-tumors to provide the most sensitive imaging methods and targeted delivery of cytotoxic-compounds.

Imaging heterogeneity of peptide delivery and binding in solid tumors using SPECT imaging and MRI

Background

As model system, a solid-tumor patient-derived xenograft (PDX) model characterized by high peptide receptor expression and histological tissue homogeneity was used to study radiopeptide targeting. In this solid-tumor model, high tumor uptake of targeting peptides was expected. However, in vivo SPECT images showed substantial heterogeneous radioactivity accumulation despite homogenous receptor distribution in the tumor xenografts as assessed by in vitro autoradiography. We hypothesized that delivery of peptide to the tumor cells is dictated by adequate local tumor perfusion. To study this relationship, sequential SPECT/CT and MRI were performed to assess the role of vascular functionality in radiopeptide accumulation.

Methods

High-resolution SPECT and dynamic contrast-enhanced (DCE)-MRI were acquired in six mice bearing PC295 PDX tumors expressing the gastrin-releasing peptide (GRP) receptor. Two hours prior to SPECT imaging, animals received 25 MBq ¹¹¹In(DOTA-(βAla)2-JMV594) (25 pmol). Images were acquired using multipinhole SPECT/CT. Directly after SPECT imaging, MR images were acquired on a 7.0-T dedicated animal scanner. DCE-MR images were quantified using semi-quantitative and quantitative models. The DCE-MR and SPECT images were spatially aligned to compute the correlations between radioactivity and DCE-MRI-derived parameters over the tumor.

Results

Whereas histology, in vitro autoradiography, and multiple-weighted MRI scans all showed homogenous tissue characteristics, both SPECT and DCE-MRI showed heterogeneous distribution patterns throughout the tumor. The average Spearman’s correlation coefficient between SPECT and DCE-MRI ranged from 0.57 to 0.63 for the “exchange-related” DCE-MRI perfusion parameters.

Conclusions

A positive correlation was shown between exchange-related DCE-MRI perfusion parameters and the amount of radioactivity accumulated as measured by SPECT, demonstrating that vascular function was an important aspect of radiopeptide distribution in solid tumors. The combined use of SPECT and MRI added crucial information on the perfusion efficiency versus radiopeptide uptake in solid tumors and showed that functional tumor characteristics varied locally even when the tissue appeared homogenous on current standard assessment techniques.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-016-0160-4) contains supplementary material, which is available to authorized users.

In Vivo Stabilization of a Gastrin-Releasing Peptide Receptor Antagonist Enhances PET Imaging and Radionuclide Therapy of Prostate Cancer in Preclinical Studies

A single tool for early detection, accurate staging, and personalized treatment of prostate cancer (PCa) would be a major breakthrough in the field of PCa. Gastrin-releasing peptide receptor (GRPR) targeting peptides are promising probes for a theranostic approach for PCa overexpressing GRPR. However, the successful application of small peptides in a theranostic approach is often hampered by their fast in vivo degradation by proteolytic enzymes, such as neutral endopeptidase (NEP). Here we show for the first time that co-injection of a NEP inhibitor (phosphoramidon (PA)) can lead to an impressive enhancement of diagnostic sensitivity and therapeutic efficacy of the theranostic (68)Ga-/(177)Lu-JMV4168 GRPR-antagonist. Co-injection of PA (300 µg) led to stabilization of (177)Lu-JMV4168 in murine peripheral blood. In PC-3 tumor-bearing mice, PA co-injection led to a two-fold increase in tumor uptake of (68)Ga-/(177)Lu-JMV4168, 1 h after injection. In positron emission tomography (PET) imaging with (68)Ga-JMV4168, PA co-injection substantially enhanced PC-3 tumor signal intensity. Radionuclide therapy with (177)Lu-JMV4168 resulted in significant regression of PC-3 tumor size. Radionuclide therapy efficacy was confirmed by production of DNA double strand breaks, decreased cell proliferation and increased apoptosis. Increased survival rates were observed in mice treated with (177)Lu-JMV4168 plus PA as compared to those without PA. This data shows that co-injection of the enzyme inhibitor PA greatly enhances the theranostic potential of GRPR-radioantagonists for future application in PCa patients.

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.

Synthesis and evaluation of Lys¹(α,γ-Folate)Lys³(¹⁷⁷Lu-DOTA)-Bombesin(1-14) as a potential theranostic radiopharmaceutical for breast cancer

The aim of this work was to synthesize Lys(1)(α,γ-Folate)-Lys(3)((177)Lu-DOTA)-Bombesin (1-14) ((177)Lu-Folate-BN), as well as to assess its potential for molecular imaging and targeted radiotherapy of breast tumors expressing folate receptors (FR) and gastrin-releasing peptide receptors (GRPR). Radiation absorbed doses of (177)Lu-Folate-BN (74 MBq, i.v.) estimated in athymic mice with T47D-induced breast tumors (positive to FR and GRPR), showed tumor doses of 23.9±2.1 Gy. T47D-tumors were clearly visible (Micro-SPECT/CT images). (177)Lu-Folate-BN demonstrated properties suitable as a theranostic radiopharmaceutical.

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.