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

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.

Bispecific GRPR-Antagonistic Anti-PSMA/GRPR Heterodimer for PET and SPECT Diagnostic Imaging of Prostate Cancer

Simultaneous targeting of the prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) could improve the diagnostic accuracy in prostate cancer (PCa). The aim of this study was to develop a PSMA/GRPR-targeting bispecific heterodimer for SPECT and positron emission tomography (PET) diagnostic imaging of PCa. The heterodimer NOTA-DUPA-RM26 was produced by manual solid-phase peptide synthesis. NOTA-DUPA-RM26 was labeled with ¹¹¹In and ⁶⁸Ga, with yields >98%, and demonstrated a high stability and binding specificity to PSMA and GRPR. IC₅₀ values for ^natIn-NOTA-DUPA-RM26 were 4 ± 1 nM towards GRPR and 824 ± 230 nM towards PSMA. An in vivo binding specificity 1 h pi of ¹¹¹In-NOTA-DUPA-RM26 in PC3-PIP-xenografted mice demonstrated partially blockable tumor uptake when co-injected with an excess of PSMA- or GRPR-targeting agents. Simultaneous co-injection of both agents induced pronounced blocking. The biodistribution of ¹¹¹In-NOTA-DUPA-RM26 and ⁶⁸Ga-NOTA-DUPA-RM26 revealed fast activity clearance from the blood and normal organs via the kidneys. Tumor uptake exceeded normal organ uptake for both analogs 1 h pi. ⁶⁸Ga-NOTA-DUPA-RM26 had a significantly lower tumor uptake (8 ± 2%ID/g) compared to ¹¹¹In-NOTA-DUPA-RM26 (12 ± 2%ID/g) 1 h pi. Tumor-to-organ ratios increased 3 h pi, but decreased 24 h pi, for ¹¹¹In-NOTA-DUPA-RM26. MicroPET/CT and microSPECT/CT scans confirmed biodistribution data, suggesting that ⁶⁸Ga-NOTA-DUPA-RM26 and ¹¹¹In-NOTA-DUPA-RM26 are suitable candidates for the imaging of GRPR and PSMA expression in PCa shortly after administration.

68Ga-NOTA-Aca-BBN(7-14) PET imaging of GRPR in children with optic pathway glioma

PURPOSE

Optic pathway glioma (OPG) is a rare neoplasm that arises predominantly during childhood. Its location in a sensitive region involving the optic pathways, onset in young patients and controversial therapy choice make the management of OPG a challenge in paediatric neuro-oncology. In this study we assessed gastrin-releasing peptide receptor (GRPR)-targeted positron emission tomography (PET) imaging in children with OPG, and the application of a PET/MRI imaging-guided surgery navigation platform.

METHODS

Eight children (five boys, mean age 8.81 years, range 5-14 years) with suspicion of optic pathway glioma on MRI were recruited. Written informed consent was obtained from all patients and legal guardians. Brain PET/CT or PET/MRI acquisitions were performed 30 min after intravenous injection of 1.85 MBq/kg body weight of 68Ga-NOTA-Aca-BBN(7-14). Four patients also underwent 18F-FDG brain PET/CT for comparison. All patients underwent surgical resection within 1 week.

RESULTS

All 11 lesions (100%) in the eight patients showed prominent 68Ga-NOTA-Aca-BBN(7-14) uptake with excellent contrast in relation to surrounding normal brain tissue. Tumour-to-background ratios (SUVmax and SUVmean) were significantly higher for 68Ga-NOTA-Aca-BBN(7-14) than for 18F-FDG (28.4 ± 5.59 vs. 0.47 ± 0.11 and 18.3 ± 4.99 vs. 0.35 ± 0.07, respectively). Fusion images for tumour delineation were obtained in all patients using the PET/MRI navigation platform. All lesions were pathologically confirmed as OPGs with positive GRPR expression, and 75% were pilocytic astrocytoma WHO grade I and 25% were diffuse astrocytoma WHO grade II. There was a positive correlation between the SUV of 68Ga-NOTA-Aca-BBN(7-14) and the expression level of GRPR (r2 = 0.56, P < 0.01, for SUVmax; r2 = 0.47, P < 0.05, for SUVmean).

CONCLUSION

This prospective study showed the feasibility of 68Ga-NOTA-Aca-BBN(7-14) PET in children with OPG for tumour detection and localization. 68Ga-NOTA-Aca-BBN(7-14) PET/MRI may be helpful for assisting surgery planning in OPG patients with severe symptoms, GRPR-targeted PET has the potential to provide imaging guidance for further GRPR-targeted therapy in patients with OPG.

Evaluation of neurotensin receptor 1 as potential biomarker for prostate cancer theranostic use

INTRODUCTION

Despite recent developments in the diagnosis and treatment of prostate cancer, the advanced stages still have poor survival rates. This warrants further exploration of related molecular targets for patient screening, detection of metastatic disease, and treatment/treatment monitoring. Recent studies have indicated that neurotensin receptors (NTSRs) and their ligand neurotensin (NTS) critically affect the progression of prostate cancers. In this study, we evaluated the expression of neurotensin receptor1 (NTSR1) in patient tissues and performed NTSR1 PET imaging in a prostate cancer animal model.

METHODS

The NTSR1 expression was evaluated in 97 cases of prostate cancer and 100 cases of benign prostatic hyperplasia (BPH) of clinical patients by immunohistochemistry staining. The expression profile of PSMA and GRPR was also performed for comparison. The mRNA expression of NTSR1 in LnCap and PC-3 cells was measured by PCR. NTSR1 PET, and biodistribution studies were performed in PC-3 xenografts using ¹⁸F-DEG-VS-NT.

RESULTS

NTSR1 showed high or moderate expression in 91.8% of prostate cancer tissue, compared with PSMA (86.7%) and GRPR (65.3%). All examined PSMA-negative tissues showed positive NTSR1 expression, suggesting the potential complementary role of NTSR1 targeted imaging or therapy. Only 8% of BPH shows strong or moderate expression of NTSR1, which is significantly lower than that in prostate cancer (91.8%). PCR results indicated LNCap (an androgen-dependent prostate cancer cell) showed negative NTSR1 expression while PC-3 demonstrated positive expression (an androgen-independent prostate cancer cell), which correlated well with previously reported western blot results. In a preclinical animal model, NTSR1 targeted PET probe ¹⁸F-DEG-VS-NT demonstrated prominent tumor accumulation and low background.

CONCLUSION

We have demonstrated that NTSR1 is a promising molecular marker for prostate cancer based on patient tissue staining. The NTSR targeted probe ¹⁸F-DEG-VS-NT demonstrated high tumor to background contrast in animal models, which could be valuable in selecting patients for therapies targeting NTSR1 as well as monitoring therapeutic efficacy during treatment accordingly.

Synthesis, radiolabeling, and evaluation of gastrin releasing peptide receptor antagonist 68 Ga-HBED-CC-RM26

The acyclic chelator HBED-CC has attained huge clinical significance owing to high thermodynamic and kinetic stability of ⁶⁸ Ga-HBED-CC chelate. It provides an excellent platform for quick preparation of ⁶⁸ Ga-based radiotracers in high yield. Thus, the present study aimed at conjugation of gastrin releasing peptide receptor (GRPr) antagonist, RM26, with HBED-CC chelator for ⁶⁸ Ga-labeling. In vitro and vivo behavior of the peptide tracer, ⁶⁸ Ga-HBED-CC-PEG₂ -RM26, was assessed and compared with ⁶⁸ Ga-NODAGA-PEG₂ -RM26. The peptide tracers, ⁶⁸ Ga-HBED-CC-PEG₂ -RM26 and ⁶⁸ Ga-NODAGA-PEG₂ -RM26, prepared either by wet chemistry or formulated using freeze-dried kits exhibited excellent radiochemical yield and in vitro stability. The two peptide tracers cleared rapidly from the blood. Biodistribution studies in normal mice demonstrated slightly higher or comparable uptake of ⁶⁸ Ga-HBED-CC-PEG₂ -RM26 in GRPr-expressing organs pancreas, stomach, and intestine. The preliminary studies suggest high potential of ⁶⁸ Ga-HBED-CC-PEG₂ -RM26 for further investigation as a GRPr imaging agent and the wide scope of HBED-CC chelator in development of ⁶⁸ Ga-based peptide tracers.

From Bench to Bedside-The Bad Berka Experience With First-in-Human Studies

Precision oncology is being driven by rapid advances in novel diagnostics and therapeutic interventions, with treatments targeted to the needs of individual patients on the basis of genetic, biomarker, phenotypic, or psychosocial characteristics that distinguish a given patient from other patients with similar clinical presentations. Inherent in the theranostics paradigm is the assumption that diagnostic test results can precisely determine whether an individual is likely to benefit from a specific treatment. As part and integral in the current era of precision oncology, theranostics in the context of nuclear medicine aims to identify the appropriate molecular targets in neoplasms (diagnostic tool), so that the optimal ligands and radionuclides (therapeutic tool) with favorable labeling chemistry can be selected for personalized management of a specific disease, taking into consideration the specific patient, and subsequently monitor treatment response. Over the past two decades, the use of gallium-68 labeled peptides for somatostatin receptor (SSTR)-targeted PET/CT (or PET/MRI) imaging followed by lutetium-177 and yttrium-90 labeled SSTR-agonist for peptide receptor radionuclide therapy has demonstrated remarkable success in the management of neuroendocrine neoplasms, and paved the way to other indications of theranostics. Rapid advances are being made in the development of other peptide-based radiopharmaceuticals, small molecular-weight ligands and with newer radioisotopes with more favorable kinetics, potentially useful for theranostics strategies for the clinical application. The present review features the Bad Berka experience with first-in-human studies of new radiopharmaceuticals, for example, prostate-specific membrane antigen ligand, gastrin-releasing peptide receptor, neurotensin receptor 1 ligand, novel SSTR-targeting peptides and nonpeptide, and bone-seeking radiopharmaceuticals. Also new radioisotopes, for example, actinium (²²⁵Ac), copper (⁶⁴Cu), scandium (⁴⁴Sc), and terbium (¹⁵²Tb/¹⁶¹Tb) will be discussed briefly demonstrating the development from basic science to precision oncology in the clinical setting.

Comparative evaluation of the new GRPR-antagonist 111 In-SB9 and 111 In-AMBA in prostate cancer models: Implications of in vivo stability

Gastrin-releasing peptide receptors (GRPRs) are overexpressed in prostate cancer, representing attractive targets for diagnosis and therapy with bombesin (BBN)-like radioligands. GRPR-antagonists have lately attracted much attention owing to inherent biosafety and favorable pharmacokinetics. We herein present the GRPR-antagonist SB9 structurally resembling the known BBN-based agonist AMBA (SB9 = [Leu¹³ NHEt-desMet¹⁴ ]AMBA). The profiles of ¹¹¹ In-SB9 and ¹¹¹ In-AMBA were directly compared in PC-3 cells and tumor-bearing mice. SB9 and AMBA displayed high GRPR affinities. ¹¹¹ In-AMBA strongly internalized in PC-3 cells, while ¹¹¹ In-SB9 remained bound on the cell surface showing a typical GRPR-radioantagonist profile. ¹¹¹ In-SB9 was more stable than ¹¹¹ In-AMBA, but coinjection of the neprilysin (NEP) inhibitor phosphoramidon (PA) stabilized both in vivo. The radioligands displayed high tumor uptake (20.23 ± 3.41 %ID/g and 18.53 ± 1.54 %ID/g, respectively, at 4 hours pi), but ¹¹¹ In-SB9 washed faster from background. PA coinjection led to significant increase of tumor uptake, combined with better clearance for ¹¹¹ In-SB9. In short, this study has revealed superior pharmacokinetics and higher stability for the GRPR-antagonist ¹¹¹ In-SB9 vs the corresponding agonist ¹¹¹ In-AMBA consolidating previous evidence that GRPR antagonists are preferable to agonists for tumor imaging and therapy. It has also demonstrated that further pharmacokinetic improvements were feasible by in situ metabolic radioligand stabilization using PA.

In Vivo Stabilized SB3, an Attractive GRPR Antagonist, for Pre- and Intra-Operative Imaging for Prostate Cancer

Purpose

The gastrin-releasing peptide receptor (GRPR), overexpressed on various tumor types, is an attractive target for receptor-mediated imaging and therapy. Another interesting approach would be the use of GRPR radioligands for pre-operative imaging and subsequent radio-guided surgery, with the goal to improve surgical outcome. GRPR radioligands were successfully implemented in clinical studies, especially Sarabesin 3 (SB3) is an appealing GRPR antagonist with high receptor affinity. Gallium-68 labeled SB3 has good in vivo stability, after labeling with Indium-111; however, the molecule shows poor in vivo stability, which negatively impacts tumor-targeting capacity. A novel approach to increase in vivo stability of radiopeptides is by co-administration of the neutral endopeptidase (NEP) inhibitor, phosphoramidon (PA). We studied in vivo stability and biodistribution of [¹¹¹In]SB3 without/with (−/+) PA in mice. Furthermore, SPECT/MRI on a novel, state-of-the-art platform was performed.

Procedures

GRPR affinity of SB3 was determined on PC295 xenograft sections using [¹²⁵I]Tyr⁴-bombesin with tracer only or with increasing concentrations of SB3. For in vivo stability, mice were injected with 200/2000 pmol [¹¹¹In]SB3 −/+ 300 μg PA. Blood was collected and analyzed. Biodistribution and SPECT/MRI studies were performed at 1, 4, and 24 h postinjection (p.i.) of 2.5 MBq/200 pmol or 25 MBq/200 pmol [¹¹¹In]SB3 −/+ 300 μg PA in PC-3-xenografted mice.

Results

SB3 showed high affinity for GRPR (IC₅₀ 3.5 nM). Co-administration of PA resulted in twice higher intact peptide in vivo vs [¹¹¹In]SB3 alone. Biodistribution studies at 1, 4, and 24 h p.i. show higher tumor uptake values with PA co-administration (19.7 ± 3.5 vs 10.2 ± 1.5, 17.6 ± 5.1 vs 8.3 ± 1.1, 6.5 ± 3.3 vs 3.1 ± 1.9 % ID/g tissue (P < 0.0001)). Tumor imaging with SPECT/MRI clearly improved after co-injection of PA.

Conclusions

Co-administration of PA increased in vivo tumor targeting capacity of [¹¹¹In]SB3, making this an attractive combination for GRPR-targeted tumor imaging.

Neuroendocrine differentiation in castration resistant prostate cancer. Nuclear medicine radiopharmaceuticals and imaging techniques: A narrative review

BACKGROUND

Androgen Deprivation Therapy (ADT) is the primary treatment for patients suffering from relapsing or advanced prostate cancer (PC). Hormone therapy generally guarantees adequate clinical control of the disease for some years, even in those patients affected by widespread skeletal and soft tissue metastases. Despite ADT, however, most patients treated with hormones eventually progress to castration-resistant prostate cancer (CRPC), for which there are no effective treatments. This clinical reality is an open challenge to the oncologist because of those neoplasms which elaborate neuroendocrine differentiation (NED).

METHODS

An online search of current and past literature on NED in CRPC was performed. Relevant articles dealing with the biological and pathological basis of NED, with nuclear medicine imaging in CRPC and somatostatin treatment in NED were analyzed.

EVIDENCE FROM THE LITERATURE

NED may arise in prostate cancer patients in the late stages of ADT. The onset of NED offers prognostic insight because it reflects a dramatic increase in the aggressive nature of the neoplasm. Several genetic, molecular, cytological and immunohistochemical markers are associated with this transformation. Among these, overexpression of somatostatin receptors, seen through Nuclear Medicine testing, is one of the most studied.

CONCLUSIONS

Preliminary studies show that the overexpression of somatostatin receptors related to NED in CRPC may easily be studied in vivo with PET/CT. This finding offers a potentially useful objective for targeted therapy in CRPC. If the overexpression of SSTRs is shown to afflict a significant segment of patients with CRPC, this will open further study of possible therapeutic options based on this marker.

Comparing Gly11/dAla11-Replacement vs. the in-Situ Neprilysin-Inhibition Approach on the Tumor-targeting Efficacy of the 111In-SB3/111In-SB4 Radiotracer Pair

Background: The GRPR-antagonist ⁶⁸Ga-SB3 visualized prostate cancer lesions in animal models and in patients. Switching radiometal from ⁶⁸Ga to ¹¹¹In impaired tumor targeting in mice, but coinjection of the neprilysin (NEP)-inhibitor phosphoramidon (PA) stabilized ¹¹¹In-SB3 in circulation and remarkably increased tumor uptake. We herein report on the biological profile of ¹¹¹In-SB4: ¹¹¹In-[dAla¹¹]SB3. Methods: The biological responses of ¹¹¹In-SB3/SB4 were compared in PC-3 cells and animal models. Results: Gly¹¹/dAla¹¹-replacement deteriorated GRPR-affinity (SB4 IC₅₀: 10.7 ± 0.9 nM vs. SB3 IC₅₀: 4.6 ± 0.3 nM) and uptake in PC-3 cells (¹¹¹In-SB4: 1.3 ± 0.4% vs. ¹¹¹In-SB3 16.2 ± 0.8% at 1 h). ¹¹¹In-SB4 was more stable than ¹¹¹In-SB3, but PA-coinjection stabilized both radiotracers in peripheral mice blood. Unmodified ¹¹¹In-SB3 showed higher uptake in PC-3 xenografts (8.8 ± 3.0%ID/g) vs. ¹¹¹In-SB4 (3.1 ± 1.1%ID/g) at 4 h pi. PA-coinjection improved tumor uptake, with ¹¹¹In-SB3 still showing superior tumor targeting (38.3 ± 7.9%ID/g vs. 7.4 ± 0.3%ID/g for ¹¹¹In-SB4). Conclusions: Replacement of Gly¹¹ by dAla¹¹ improved in vivo stability, however, at the cost of GRPR-affinity and cell uptake, eventually translating into inferior tumor uptake of ¹¹¹In-SB4 vs. unmodified ¹¹¹In-SB3. On the other hand, in-situ NEP-inhibition turned out to be a more efficient and direct strategy to optimize the in vivo profile of ¹¹¹In-SB3, and potentially other peptide radiotracers.

New Targets for PET Molecular Imaging of Prostate Cancer

Prostate cancer (PCa) is the most common cancer in men worldwide, but it exhibits a highly variable biological behavior ranging from indolent to highly aggressive disease. The standard conventional imaging for staging PCa consists of CT, MRI, and bone scans, but this imaging has suboptimal accuracy for extraprostatic tumor detection, particularly in the scenario of early biochemical relapse when the prostate-specific antigen levels are still low indicating a low volume of recurrent disease. This gap between known disease (as indicated by a rising prostate-specific antigen) and the failure to detect it on conventional imaging, has led to the development of novel imaging probes most of which have positron emitting radioactive tags. In the last decade, multiple PET probes have demonstrated promising performance in detecting sites of recurrence and extent of disease in patients with PCa. The landscape of available PET radiotracers is changing rapidly and includes radiolabeled choline, anti1-amino-3-¹⁸F-fluorocyclobutane-1-carboxylic acid (¹⁸F-fluciclovine), bombesin, dihydrotestosterone, and prostate-specific membrane antigen (PSMA) ligands, among others. Of these, radiolabeled PSMA-PET agents have shown the most encouraging results in terms of sensitivity and are likely to become universally available for imaging PCa within a few years Other PET radiotracers such as bombesin-based radiotracers and antagonist of gastrin releasing-peptide receptor (RM2) are emerging as possible alternatives for PCa imaging. This review article discusses the current and near-future of PET molecular imaging probes.

Prostate Cancer Theranostics Targeting Gastrin-Releasing Peptide Receptors

Gastrin-releasing peptide receptors (GRPRs), part of the bombesin (BBN) family, are aberrantly overexpressed in many cancers, including those of the breast, prostate, pancreas, and lung, and therefore present an attractive target for cancer diagnosis and therapy. Different bombesin analogs have been radiolabeled and used for imaging diagnosis, staging, evaluation of biochemical recurrence, and assessment of metastatic disease in patients with prostate cancer. Recently, interest has shifted from BBN-like receptor agonists to antagonists, because the latter does not induce adverse effects and demonstrate superior in vivo pharmacokinetics. We review the preclinical and clinical literatures on the use of GRPRs as targets for imaging and therapy of prostate cancer, with a focus on the newer developments and theranostic potential of GRPR peptides.

Positron Emission Tomography Imaging of the Gastrin-Releasing Peptide Receptor with a Novel Bombesin Analogue

The gastrin-releasing peptide receptor (GRPR), a G protein-coupled receptor, is overexpressed in solid malignancies and particularly in prostate cancer. We synthesized a novel bombesin derivative, [⁶⁸Ga]Ga-ProBOMB1, evaluated its pharmacokinetics and potential to image GRPR expression with positron emission tomography (PET), and compared it with [⁶⁸Ga]Ga-NeoBOMB1. ProBOMB1 (DOTA-pABzA-DIG-d-Phe-Gln-Trp-Ala-Val-Gly-His-Leu-ψ(CH₂N)-Pro-NH₂) was synthesized by solid-phase peptide synthesis. The polyaminocarboxylate chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was coupled to the N-terminal and separated from the GRPR-targeting sequence by a p-aminomethylaniline-diglycolic acid (pABzA-DIG) linker. The binding affinity to GRPR was determined using a cell-based competition assay, whereas the agonist/antagonist property was determined with a calcium efflux assay. ProBOMB1 was radiolabeled with ⁶⁸GaCl₃. PET imaging and biodistribution studies were performed in male immunocompromised mice bearing PC-3 prostate cancer xenografts. Blocking experiments were performed with coinjection of [d-Phe⁶,Leu-NHEt¹³,des-Met¹⁴]bombesin(6-14). Dosimetry calculations were performed with OLINDA software. ProBOMB1 and the nonradioactive Ga-ProBOMB were obtained in 1.1 and 67% yield, respectively. The K _i value of Ga-ProBOMB1 for GRPR was 3.97 ± 0.76 nM. Ga-ProBOMB1 behaved as an antagonist for GRPR. [⁶⁸Ga]Ga-ProBOMB1 was obtained in 48.2 ± 10.9% decay-corrected radiochemical yield with 121 ± 46.9 GBq/μmol molar activity and >95% radiochemical purity. Imaging/biodistribution studies showed that the excretion of [⁶⁸Ga]Ga-ProBOMB1 was primarily through the renal pathway. At 1 h postinjection (p.i.), PC-3 tumor xenografts were clearly delineated in PET images with excellent contrast. The tumor uptake for [⁶⁸Ga]Ga-ProBOMB1 was 8.17 ± 2.57 percent injected dose per gram (% ID/g) and 9.83 ± 1.48% ID/g for [⁶⁸Ga]Ga-NeoBOMB1, based on biodistribution studies at 1 h p.i. This corresponded to tumor-to-blood and tumor-to-muscle uptake ratios of 20.6 ± 6.79 and 106 ± 57.7 for [⁶⁸Ga]Ga-ProBOMB1 and 8.38 ± 0.78 and 39.0 ± 12.6 for [⁶⁸Ga]Ga-NeoBOMB1, respectively. Blockade with [d-Phe⁶,Leu-NHEt¹³,des-Met¹⁴]bombesin(6-14) significantly reduced the average uptake of [⁶⁸Ga]Ga-ProBOMB1 in tumors by 62%. The total absorbed dose was lower for [⁶⁸Ga]Ga-ProBOMB1 in all organs except for bladder compared with [⁶⁸Ga]Ga-NeoBOMB1. Our data suggest that [⁶⁸Ga]Ga-ProBOMB1 is an excellent radiotracer for imaging GRPR expression with PET. [⁶⁸Ga]Ga-ProBOMB1 achieved a similar uptake as [⁶⁸Ga]Ga-NeoBOMB1 in tumors, with enhanced contrast and lower whole-body absorbed dose.

Comparison of the binding of the gastrin-releasing peptide receptor (GRP-R) antagonist 68Ga-RM2 and 18F-FDG in breast cancer samples

The Gastrin-Releasing Peptide Receptor (GRPR) is over-expressed in estrogen receptor (ER) positive breast tumors and related metastatic lymph nodes offering the opportunity of imaging and therapy of luminal tumors. ⁶⁸Ga-RM2 binding and ¹⁸F-FDG binding in tumoral zones were measured and compared using tissue micro-imaging with a beta imager on 14 breast cancer samples (10 primaries and 4 associated metastatic lymph nodes). Results were then assessed against ER expression, progesterone receptor (PR) expression, HER2 over-expression or not and Ki-67 expression. GRPR immunohistochemistry (IHC) was also performed on all samples. We also retrospectively compared ⁶⁸Ga-RM2 and ¹⁸F-FDG bindings to ¹⁸F-FDG SUV_max on the pre-therapeutic PET/CT examination, if available. ⁶⁸Ga-RM2 binding was significantly higher in tumors expressing GRPR on IHC than in GRPR-negative tumors (P = 0.022). In ER⁺ tumors, binding of ⁶⁸Ga-RM2 was significantly higher than ¹⁸F-FDG (P = 0.015). In tumors with low Ki-67, ⁶⁸Ga-RM2 binding was also significantly increased compared to ¹⁸F-FDG (P = 0.029). Overall, the binding of ⁶⁸Ga-RM2 and ¹⁸F-FDG displayed an opposite pattern in tumor samples and ⁶⁸Ga-RM2 binding was significantly higher in tumors that had low ¹⁸F-FDG binding (P = 0.021). This inverse correlation was also documented in the few patients in whom a ¹⁸F-FDG PET/CT examination before surgery was available. Findings from this in vitro study suggest that GRPR targeting can be an alternative to ¹⁸F-FDG imaging in ER⁺ breast tumors. Moreover, because GRPR antagonists can also be labeled with lutetium-177 this opens new avenues for targeted radionuclide therapy in the subset of patients with progressive metastatic disease following conventional treatments.

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.

68Ga-NOTA-RM26 PET/CT in the Evaluation of Breast Cancer: A Pilot Prospective Study

BACKGROUND

This prospective pilot study investigated the value of Ga-NOTA-RM26, an antagonist targeting gastrin-releasing peptide receptor, in evaluation of breast cancer.

METHODS

Thirty-five women in suspicion of breast cancer based on mammography or ultrasonography were recruited with informed consent. They underwent PET/CT scans 30 minutes after intravenous injection of Ga-NOTA-RM26 in a dose of 1.85 MBq (0.05 mCi) per kilogram body weight within 1 week before surgery. The Ga-NOTA-RM26 uptake was correlated with the pathological and immunohistochemical findings.

RESULTS

Ga-NOTA-RM26 positivity was found correlated with estrogen receptor (ER) expression (P = 0.006) and menstrual status (P = 0.019). In 34 patients diagnosed with breast cancer, the SUVmax was found significantly higher in the ER-positive breast cancer (4.97 ± 1.89) than in the ER-negative breast cancer (2.78 ± 0.65, P < 0.001). Ga-NOTA-RM26 was also found accumulated in normal breast tissue, with the SUVmax significantly higher in patients at the secretory phase of menstrual cycle (2.27 ± 0.71) than in those at the nonsecretory phase (1.59 ± 0.49, P = 0.017) and postmenopause (1.43 ± 0.44, P = 0.002). If the secretory phase patients were excluded, the sensitivity, specificity, and accuracy for differentiation of breast cancer from breast tissue increased from 85.3%, 86.8%, and 86.1% to 100%, 90.9%, and 95.5%, respectively.

CONCLUSIONS

This pilot study indicates that the diagnostic accuracy of Ga-NOTA-RM26 PET/CT in breast cancer may correlate with ER expression and menstrual status of the patient. It may be better to avoid performing this examination during the menstrual secretory phase to reduce physiological uptake in normal breast tissue.

PET Using a GRPR Antagonist 68Ga-RM26 in Healthy Volunteers and Prostate Cancer Patients

This study was designed to analyze the safety, biodistribution, and radiation dosimetry of a gastrin-releasing peptide receptor (GRPR) antagonist PET tracer, 68Ga-RM26; to assess its clinical diagnostic value in prostate cancer patients; and to perform a direct comparison between GRPR antagonist 68Ga-RM26 and agonist 68Ga-BBN. Methods: Five healthy volunteers were enrolled to validate the safety of 68Ga-RM26 and calculate dosimetry. A total of 28 patients with prostate cancer (17 newly diagnosed and 11 posttherapy) were recruited and provided written informed consent. All the cancer patients underwent PET/CT at 15-30 min after intravenous injection of 1.85 MBq (0.05 mCi) per kilogram of body weight of 68Ga-RM26. Among them, 22 patients (11 newly diagnosed and 11 posttherapy) underwent 68Ga-BBN PET/CT for comparison within 1 wk. 99mTc-MDP (methylene diphosphonate) bone scans were obtained within 2 wk for comparison. GRPR immunohistochemical staining of tumor samples was performed. Results: The administration of 68Ga-M26 was well tolerated by all subjects, with no adverse symptoms being noticed or reported during the procedure and at 2-wk follow-up. The total effective dose equivalent and effective dose were 0.0912 ± 0.0140 and 0.0657 ± 0.0124 mSv/MBq, respectively. In the 17 patients with newly diagnosed prostate cancer, 68Ga-RM26 PET/CT showed positive prostate-confined findings in 15 tumors with an SUVmax of 6.49 ± 2.37. In the 11 patients who underwent prostatectomy or brachytherapy with or without androgen deprivation therapy, 68Ga-RM26 PET/CT detected 8 metastatic lymph nodes in 3 patients with an SUVmax of 4.28 ± 1.25 and 21 bone lesions in 8 patients with an SUVmax of 3.90 ± 3.07. Compared with 68Ga-RM26 PET/CT, GRPR agonist 68Ga-BBN PET/CT detected fewer primary lesions and lymph node metastases as well as demonstrated lower tracer accumulation. There was a significant positive correlation between SUV derived from 68Ga-RM26 PET and the expression level of GRPR (P < 0.001). Conclusion: This study indicates the safety and significant efficiency of GRPR antagonist 68Ga-RM26. 68Ga-RM26 PET/CT would have remarkable value in detecting both primary prostate cancer and metastasis. 68Ga-RM26 is also expected to be better than GRPR agonist as an imaging marker to evaluate GRPR expression in prostate cancer.

Radiometal-Dependent Biological Profile of the Radiolabeled Gastrin-Releasing Peptide Receptor Antagonist SB3 in Cancer Theranostics: Metabolic and Biodistribution Patterns Defined by Neprilysin

Recent advances in oncology involve the use of diagnostic/therapeutic radionuclide-carrier pairs that target cancer cells, offering exciting opportunities for personalized patient treatment. Theranostic gastrin-releasing peptide receptor (GRPR)-directed radiopeptides have been proposed for the management of GRPR-expressing prostate and breast cancers. We have recently introduced the PET tracer ⁶⁸Ga-SB3 (SB3, DOTA- p-aminomethylaniline-diglycolic acid-DPhe-Gln-Trp-Ala-Val-Gly-His-Leu-NHEt), a receptor-radioantagonist that enables the visualization of GRPR-positive lesions in humans. Aiming to fully assess the theranostic potential of SB3, we herein report on the impact of switching ⁶⁸Ga to ¹¹¹In/¹⁷⁷Lu-label on the biological properties of resulting radiopeptides. Notably, the bioavailability of ¹¹¹In/¹⁷⁷Lu-SB3 in mice drastically deteriorated compared with metabolically robust ⁶⁸Ga-SB3, and as a result led to poorer ¹¹¹In/¹⁷⁷Lu-SB3 uptake in GRPR-positive PC-3 xenografts. The peptide cleavage sites were identified by chromatographic comparison of blood samples from mice intravenously receiving ¹¹¹In/¹⁷⁷Lu-SB3 with each of newly synthesized ¹¹¹In/¹⁷⁷Lu-SB3-fragments. Coinjection of the radioconjugates with the neprilysin (NEP)-inhibitor phosphoramidon led to full stabilization of ¹¹¹In/¹⁷⁷Lu-SB3 in peripheral mouse blood and resulted in markedly enhanced radiolabel uptake in the PC-3 tumors. In conclusion, in situ NEP-inhibition led to indistinguishable ⁶⁸Ga/¹¹¹In/¹⁷⁷Lu-SB3 profiles in mice emphasizing the theranostic prospects of SB3 for clinical use.

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).

Positron Emission Tomography Imaging of Prostate Cancer with Ga-68-Labeled Gastrin-Releasing Peptide Receptor Agonist BBN7-14 and Antagonist RM26

Radiolabeled bombesin (BBN) analogs have long been used for developing gastrin-releasing peptide receptor (GRPR) targeted imaging probes, and tracers with excellent in vivo performance including high tumor uptake, high contrast, and favorable pharmacokinetics are highly desired. In this study, we compared the ⁶⁸Ga-labeled GRPR agonist (Gln–Trp–Ala–Val–Gly–His–Leu–Met–NH₂, BBN_7–14) and antagonist (d-Phe–Gln–Trp–Ala–Val–Gly–His–Sta–Leu–NH₂, RM26) for the positron emission tomography (PET) imaging of prostate cancer. The in vitro stabilities, receptor binding, cell uptake, internalization, and efflux properties of the probes ⁶⁸Ga–1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA)–Aca–BBN_7–14 and ⁶⁸Ga–NOTA–poly(ethylene glycol)₃ (PEG₃)–RM26 were studied in PC-3 cells, and the in vivo GRPR targeting abilities and kinetics were investigated using PC-3 tumor xenografted mice. BBN_7–14, PEG₃-RM26, NOTA–Aca–BBN_7–14, and NOTA–PEG₃–RM26 showed similar binding affinity to GRPR. In PC-3 tumor-bearing mice, the tumor uptake of ⁶⁸Ga–NOTA–PEG₃–RM26 remained at around 3.00 percentage of injected dose per gram of tissue within 1 h after injection, in contrast with ⁶⁸Ga–NOTA–Aca–BBN_7–14, which demonstrated rapid elimination and high background signal. Additionally, the majority of the ⁶⁸Ga–NOTA–PEG₃–RM26 remained intact in mouse serum at 5 min after injection, while almost all of the ⁶⁸Ga–NOTA–Aca–BBN_7–14 was degraded under the same conditions, demonstrating more-favorable in vivo pharmacokinetic properties and metabolic stabilities of the antagonist probe relative to its agonist counterpart. Overall, the antagonistic GRPR targeted probe ⁶⁸Ga–NOTA–PEG₃–RM26 is a more-promising candidate than the agonist ⁶⁸Ga–NOTA–Aca–BBN_7–14 for the PET imaging of prostate cancer patients.

68Ga-BBN-RGD PET/CT for GRPR and Integrin αvβ3 Imaging in Patients with Breast Cancer

Purpose: This study was to assess a gastrin-releasing peptide receptor (GRPR) and integrin αvβ3 dual targeting tracer 68Ga-BBN-RGD for positron emission tomography (PET)/computed tomography (CT) imaging of breast cancer and metastasis. Materials and Methods: Twenty-two female patients were recruited either with suspected breast cancer on screening mammography (n = 16) or underwent breast cancer radical mastectomy (n = 6). All the 22 patients underwent PET/CT at 30-45 min after intravenous injection of 68Ga-BBN-RGD. Eleven out of 22 patients also accepted 68Ga-BBN PET/CT within 2 weeks for comparison. A final diagnosis was made based on the histopathologic examination of surgical excision or biopsy. Results: Both the primary cancer and metastases showed positive 68Ga-BBN-RGD accumulation. The T/B ratios of 68Ga-BBN-RGD accumulation were 2.10 to 9.44 in primary cancer and 1.10 to 3.71 in axillary lymph node metastasis, 3.80 to 10.7 in distant lymph nodes, 2.70 to 5.35 in lung metastasis and 3.17 to 22.8 in bone metastasis, respectively. For primary lesions, the SUVmax from 68Ga-BBN-RGD PET in ER positive group was higher than that in ER negative group (P < 0.01). For both primary and metastatic lesions, SUVmean quantified from 68Ga-BBN-RGD PET correlated well with both GRPR expression and integrin αvβ3 expression. Conclusion: This study demonstrated significant uptake of a new type of dual integrin αvβ3 and GRPR targeting radiotracer in both the primary lesion and the metastases of breast cancer. 68Ga-BBN-RGD PET/CT may be of great value in discerning both primary breast cancers, axillary lymph node metastasis and distant metastases.

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.

Prospective Evaluation of 68Ga-RM2 PET/MRI in Patients with Biochemical Recurrence of Prostate Cancer and Negative Findings on Conventional Imaging

⁶⁸Ga-labeled DOTA-4-amino-1-carboxymethyl-piperidine-d-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH₂ (⁶⁸Ga-RM2) is a synthetic bombesin receptor antagonist that targets gastrin-releasing peptide receptor (GRPr). GRPr proteins are highly overexpressed in several human tumors, including prostate cancer (PCa). We present data from the use of ⁶⁸Ga-RM2 in patients with biochemical recurrence (BCR) of PCa and negative findings on conventional imaging. Methods: We enrolled 32 men with BCR of PCa, who were 59-83 y old (mean ± SD, 68.7 ± 6.4 y). Imaging started at 40-69 min (mean, 50.5 ± 6.8 min) after injection of 133.2-151.7 MBq (mean, 140.6 ± 7.4 MBq) of ⁶⁸Ga-RM2 using a time-of-flight-enabled simultaneous PET/MRI scanner. T1-weighted, T2-weighted, and diffusion-weighted images were acquired. Results: All patients had a rising level of prostate-specific antigen (PSA) (range, 0.3-119.0 ng/mL; mean, 10.1 ± 21.3 ng/mL) and negative findings on conventional imaging (CT or MRI, and a ^99mTc-methylene diphosphonate bone scan) before enrollment. The observed ⁶⁸Ga-RM2 PET detection rate was 71.8%. ⁶⁸Ga-RM2 PET identified recurrent PCa in 23 of the 32 participants, whereas the simultaneous MRI scan identified findings compatible with recurrent PCa in 11 of the 32 patients. PSA velocity was 0.32 ± 0.59 ng/mL/y (range, 0.04-1.9 ng/mL/y) in patients with negative PET findings and 2.51 ± 2.16 ng/mL/y (range, 0.13-8.68 ng/mL/y) in patients with positive PET findings (P = 0.006). Conclusion:⁶⁸Ga-RM2 PET can be used for assessment of GRPr expression in patients with BCR of PCa. High uptake in multiple areas compatible with cancer lesions suggests that ⁶⁸Ga-RM2 is a promising PET radiopharmaceutical for localization of disease in patients with BCR of PCa and negative findings on conventional imaging.

Amide-to-triazole switch vs. in vivo NEP-inhibition approaches to promote radiopeptide targeting of GRPR-positive tumors

INTRODUCTION

Radiolabeled bombesin (BBN)-analogs have been proposed for diagnosis and therapy of gastrin-releasing peptide receptor (GRPR)-expressing tumors, such as prostate, breast and lung cancer. Metabolic stability represents a crucial factor for the success of this approach by ensuring sufficient delivery of circulating radioligand to tumor sites. The amide-to-triazole switch on the backbone of DOTA-PEG₄-[Nle¹⁴]BBN(7-14) (1) was reported to improve the in vitro stability of resulting ¹⁷⁷Lu-radioligands. On the other hand, in-situ inhibition of neutral endopeptidase (NEP) by coinjection of phosphoramidon (PA) was shown to significantly improve the in vivo stability and tumor uptake of biodegradable radiopeptides. We herein compare the impact of the two methods on the bioavailability and localization of ¹⁷⁷Lu-DOTA-PEG₄-[Nle¹⁴]BBN(7-14) analogs in GRPR-positive tumors in mice.

METHODS

The 1,4-disubstituted [1-3]-triazole was used to replace one (2: Gly¹¹-His¹²; 3: Ala⁹-Val¹⁰) or two (4: Ala⁹-Val¹⁰ and Gly¹¹-His¹²) peptide bonds in 1 (reference) and all compounds were labeled with ¹⁷⁷Lu. Each of [¹⁷⁷Lu]1-[¹⁷⁷Lu]4 was injected without (control) or with PA in healthy mice. Blood samples collected 5min post-injection (pi) were analyzed by HPLC. Biodistribution of [¹⁷⁷Lu]1-[¹⁷⁷Lu]4 was conducted in SCID mice bearing human prostate adenocarcinoma PC-3 xenografts at 4h pi. Groups of 4 animals were injected with radioligand, alone (controls), or with coinjection of PA, or of a mixture of PA and excess and [Tyr⁴]BBN to determine GRPR-specificity of uptake (Block).

RESULTS

The in vivo stability of the radioligands was: [¹⁷⁷Lu]1 (25% intact), [¹⁷⁷Lu]2 (45% intact), [¹⁷⁷Lu]3 (30% intact) and [¹⁷⁷Lu]4 (40% intact). By PA-coinjection these values notably increased to 90%-93%. Moreover, treatment with PA induced an impressive and GRPR-specific uptake of all radioligands in the PC-3 xenografts at 4h pi: [¹⁷⁷Lu]1: 4.7±0.4 to 24.8±4.9%ID/g; [¹⁷⁷Lu]2: 8.3±1.2 to 26.0±1.1%ID/g; [¹⁷⁷Lu]3: 6.6±0.4 to 21.3±4.4%ID/g; and [¹⁷⁷Lu]4: 4.8±1.6 to 13.7±3.8%ID/g.

CONCLUSIONS

This study has shown that amide-to-triazole substitutions in ¹⁷⁷Lu-DOTA-PEG₄-[Nle¹⁴]BBN(7-14) induced minor effects on bioavailability and tumor uptake in mice models, whereas in-situ NEP-inhibition(s) by PA impressively improved in vivo profiles.

From Bench to Bed: New Gastrin-Releasing Peptide Receptor-Directed Radioligands and Their Use in Prostate Cancer

Gastrin-releasing peptide receptors (GRPRs) are overexpressed in prostate and breast cancer, and are therefore attractive molecular targets for diagnosis and therapy with radiolabeled GRPR-directed peptide probes. The amphibian tetradecapeptide bombesin or the mammalian gastrin-releasing peptide and neuromedin C have been modified with a variety of chelators. As a result, labeling with radiometals attractive for SPECT or PET imaging and for radionuclide therapy has led to the development of peptide radioligands suitable for in vivo targeting of prostate cancer. A shift of paradigm from internalizing GRPR-agonists to antagonists has occurred owing to the higher biosafety and superior pharmacokinetics of radioantagonists.

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.

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.

Radiolabeling of DOTA-like conjugated peptides with generator-produced (68)Ga and using NaCl-based cationic elution method

Gallium-68 (⁶⁸Ga) is a generator-produced radionuclide with a short half-life (t_½ = 68 min) that is particularly well suited for molecular imaging by positron emission tomography (PET). Methods have been developed to synthesize ⁶⁸Ga-labeled imaging agents possessing certain drawbacks, such as longer synthesis time because of a required final purification step, the use of organic solvents or concentrated hydrochloric acid (HCl). In our manuscript, we provide a detailed protocol for the use of an advantageous sodium chloride (NaCl)-based method for radiolabeling of chelator-modified peptides for molecular imaging. By working in a lead-shielded hot-cell system, ⁶⁸Ga³⁺ of the generator eluate is trapped on a cation exchanger cartridge (100 mg, ∼8 mm long and 5 mm diameter) and then eluted with acidified 5 M NaCl solution directly into a sodium acetate-buffered solution containing a DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) or DOTA-like chelator-modified peptide. The main advantages of this procedure are the high efficiency and the absence of organic solvents. It can be applied to a variety of peptides, which are stable in 1 M NaCl solution at a pH value of 3–4 during reaction. After labeling, neutralization, sterile filtration and quality control (instant thin-layer chromatography (iTLC), HPLC and pH), the radiopharmaceutical can be directly administered to patients, without determination of organic solvents, which reduces the overall synthesis-to-release time. This procedure has been adapted easily to automated synthesis modules, which leads to a rapid preparation of ⁶⁸Ga radiopharmaceuticals (12–16 min).

A study of radiogallium aqueous chemistry: in vitro and in vivo characterisation of (67) Ga-hydrolysed-stannous fluoride particles

The objective of this study was to explore the aqueous chemistry of gallium using (67) Ga-chloride starting material, by radiolabelling hydrolysed(h)-stannous fluoride particles and then characterising the optimal formulation for radiochemical purity (RCP) and radioactive particle size distribution in vitro. The pilot reactions determined stannous fluoride was added to (67) Ga-acetate under nitrogen and then heated at 100 °C for 20 min to achieve ≥95% RCP and (67) Ga-particles were >3 µm in diameter. A high radioactive concentration of (67) Ga-h-SnF2 particles could be prepared similarly in ≥97% RCP with 74% as 3-5 µm and 26% >5 µm in diameter. The latter formulation had larger particles than (99m) Tc-h-SnF2 colloid (96% of 1-3 µm), and it resulted in a rat biodistribution of 41% in the lungs, 41% in the liver plus spleen and 18% in the carcass at 20 min after injection. The carcass activity was attributed to bone marrow and some (67) Ga-transferrin formed in blood. Isolated mixed human leucocytes were radiolabelled with (67) Ga-h-SnF2 particles in 100% efficiency, and the (67) Ga-cells did not release soluble (67) Ga(3+) at room temperature over 3 h. The (67) Ga-h-SnF2 particle formulation could find a use in labelling leucocyte cells for in vivo homing studies when delayed animal imaging is required.