Peptide receptor scintigraphy (PRS) and peptide receptor radionuclide therapy (PRRT) using radiolabeled somatostatin receptor (SSTR) agonists are successfully being applied in the clinic for imaging and treatment of neuroendocrine tumors. Contrary to the paradigm that internalization and the resulting accumulation of radiotracers in cells is necessary for efficient tumor targeting, recent studies have demonstrated superiority of radiolabeled SSTR antagonists for imaging purposes, despite little to no internalization in cells. However, studies comparing the therapeutic anti-tumor effects of radiolabeled SSTR agonists versus antagonists are lacking. The aim of this study was to directly compare the therapeutic effect of ¹⁷⁷Lu-DOTA-octreotate, an SSTR agonist, and ¹⁷⁷Lu-DOTA-JR11, an SSTR antagonist. Methods: We analyzed radiotracer uptake (both membrane-bound and internalized fractions) and the produced DNA double strand breaks, by determining the number of p53 binding protein 1 (53BP1) foci, after incubating SSTR2 positive cells with ¹⁷⁷Lu-DTPA, ¹⁷⁷Lu-DOTA-octreotate or ¹⁷⁷Lu-DOTA-JR11. Also, biodistribution studies were performed in tumor xenografted mice to determine the optimal dose for therapy experiments. Afterwards in vivo therapy experiments comparing the effect of ¹⁷⁷Lu-DOTA-octreotate and ¹⁷⁷Lu-DOTA-JR11 were performed in this same animal model. Results: We found a 5 times higher uptake of ¹⁷⁷Lu-DOTA-JR11 compared to that of ¹⁷⁷Lu-DOTA-octreotate. The major part (88±1%) of the antagonist uptake was membrane bound, while 74±3% of the total receptor agonist uptake was internalized. Cells treated with ¹⁷⁷Lu-DOTA-JR11 showed 2 times more 53BP1 foci compared to cells treated with ¹⁷⁷Lu-DOTA-octreotate. Biodistribution studies with ¹⁷⁷Lu-DOTA-JR11 (0.5 μg/30 MBq) resulted in the highest tumor radiation dose of 1.8±0.7 Gy/MBq, 4.4 times higher than the highest tumor radiation dose found for ¹⁷⁷Lu-DOTA-octreotate. In vivo therapy studies with ¹⁷⁷Lu-DOTA-octreotate and ¹⁷⁷Lu-DOTA-JR11 resulted in a tumor growth delay time of 18±5 d and 26±7 d, respectively. Median survival rates were 43.5 d, 61 d and 71 d for the control group, the ¹⁷⁷Lu-DOTA-octreotate and the ¹⁷⁷Lu-DOTA-JR11 treated group, respectively. Conclusion: Based on these results we conclude that the use of radiolabeled SSTR antagonists such as JR11 might enhance PRS and PRRT of neuroendocrine tumors and provide successful imaging and therapeutic strategies for cancer types with relatively low SSTR expression.

Introduction: Since overexpression of the gastrin releasing peptide receptor (GRPR) has been reported on various cancer types, e.g. prostate cancer and breast cancer, targeting this receptor with radioligands might have significant impact on staging and treatment of GRPR-expressing tumors. NeoBOMB1 is a novel DOTA-coupled GRPR antagonist with high affinity for GRPR and excellent in vivo stability. The purpose of this preclinical study was to further explore the use of NeoBOMB1 for theranostic application by determining the biodistribution of ⁶⁸Ga-NeoBOMB1 and ¹⁷⁷Lu-NeoBOMB1. Materials and Methods: PC-3 tumor-xenografted Balb c nu/nu mice were injected with either ~13 MBq/250 pmol ⁶⁸Ga-NeoBOMB1, or a low (~1 MBq/200 pmol) vs. high (~1 MBq/10 pmol) peptide amount of ¹⁷⁷Lu-NeoBOMB1 after which biodistribution and imaging studies were performed. At 6 time points (15, 30, 60, 2 h, 4 h and 6 h for ⁶⁸Ga-NeoBOMB1 and 1, 4, 24, 48, 96 and 168 h for ¹⁷⁷Lu-NeoBOMB1) post injection (p.i.) tumor and organ uptake was determined. To assess receptor-specificity additional groups of animals were co-injected with an excess of unlabeled NeoBOMB1. Results of the biodistribution studies were used to determine pharmacokinetics and dosimetry. Furthermore, positron emission tomography/computed tomography (PET/CT) and single photon emission computed tomography/magnetic resonance imaging (SPECT/MRI) was performed. Results: Injection of ~250 pmol ⁶⁸Ga-NeoBOMB1 resulted in a tumor and pancreas uptake of 12.4±2.3 and 22.7±3.3 %ID/g tissue 120 min p.i., respectively. ¹⁷⁷Lu-NeoBOMB1 biodistribution studies revealed a higher tumor uptake (17.9±3.3 vs. 11.6±1.3 %ID/g tissue 4 h p.i.) and a lower pancreatic uptake (19.8±6.9 vs. 105±13 %ID/g tissue 4 h p.i.) with the higher peptide amount injected, leading to a significant increase in the absorbed dose to the tumor vs. the pancreas (200 pmol: 570 vs. 265 mGy/MBq, 10 pmol: 435 vs. 1393 mGy/MBq). Using this data to predict patient dosimetry we found a kidney, pancreas and liver exposure of 0.10, 0.65 and 0.06 mGy/MBq, respectively. Imaging studies resulted in good visualization of the tumor with both ⁶⁸Ga-NeoBOMB1 and ¹⁷⁷Lu-NeoBOMB1. Conclusion: Our findings indicate that ⁶⁸Ga/¹⁷⁷Lu-NeoBOMB1 is a very promising radiotracer with excellent tumor uptake and favorable pharmacokinetics for imaging and therapy of GRPR-expressing tumors.

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.

Cancer Associated Fibroblasts (CAFs) form a major component of the tumour microenvironment, they have a complex origin and execute diverse functions in tumour development and progression. As such, CAFs constitute an attractive target for novel therapeutic interventions that will aid both diagnosis and treatment of various cancers. There are, however, a few limitations in reaching successful translation of CAF targeted interventions from bench to bedside. Several approaches targeting CAFs have been investigated so far and a few CAF-targeting tracers have successfully been developed and applied. This includes tracers targeting Fibroblast Activation Protein (FAP) on CAFs. A number of FAP-targeting tracers have shown great promise in the clinic. In this review, we summarize our current knowledge of the functional heterogeneity and biology of CAFs in cancer. Moreover, we highlight the latest developments towards theranostic applications that will help tumour characterization, radioligand therapy and staging in cancers with a distinct CAF population.

Targeted therapies, such as radioimmunotherapy (RIT), present a promising treatment option for the eradication of tumor lesions. RIT has shown promising results especially for hematologic malignancies, but the therapeutic efficacy is limited by unfavorable tumor-to-background ratios resulting in high radiotoxicity. Pretargeting strategies can play an important role in addressing the high toxicity profile of RIT. Key to pretargeting is the concept of decoupling the targeting vehicle from the cytotoxic agent and administrating them separately. Studies have shown that this approach has the ability to enhance the therapeutic index as it can reduce side effects caused by off-target irradiation and thereby increase curative effects due to higher tolerated doses. Pretargeted RIT (PRIT) has been explored for imaging and treatment of different cancer types over the years. This review will give an overview of the various targeted therapies in which pretargeting has been applied, discussing PRIT with alpha- and beta-emitters and as part of combination therapy, plus its use in drug delivery systems.

Breast cancer (BC) consists of multiple subtypes defined by various molecular characteristics, for instance, estrogen receptor (ER) expression. Methods for visualizing BC include mammography, MR imaging, ultrasound, and nuclear medicine–based methods such as ^99mTc-sestamibi and ¹⁸F-FDG PET, unfortunately all lacking specificity. Peptide receptor scintigraphy and peptide receptor radionuclide therapy are successfully applied for imaging and therapy of somatostatin receptor–expressing neuroendocrine tumors using somatostatin receptor radioligands. On the basis of a similar rationale, radioligands targeting the gastrin-releasing peptide receptor (GRP-R) might offer a specific method for imaging and therapy of BC. The aim of this study was to explore the application of GRP-R radioligands for imaging and therapy of BC by introducing valid preclinical in vitro and in vivo models. Methods: GRP-R expression of 50 clinical BC specimens and the correlation with ER expression was studied by in vitro autoradiography with the GRP-R agonist ¹¹¹In-AMBA. GRP-R expression was also analyzed in 9 BC cell lines applying ¹¹¹In-AMBA internalization assays and quantitative reverse transcriptase polymerase chain reaction. In vitro cytotoxicity of ¹⁷⁷Lu-AMBA was determined on the GRP-R–expressing BC cell line T47D. SPECT/CT imaging and biodistribution were studied in mice with subcutaneous and orthotopic ER-positive T47D and MCF7 xenografts after injection of the GRP-R antagonist ¹¹¹In-JMV4168. Results: Most of the human BC specimens (96%) and BC cell lines (6/9) were found to express GRP-R. GRP-R tumor expression was positively (P = 0.026, χ²(4) = 12,911) correlated with ER expression in the human BC specimens. Treatment of T47D cells with 10⁻⁷ M/50 MBq of ¹⁷⁷Lu-AMBA resulted in 80% reduction of cells in vitro. Furthermore, subcutaneous and orthotopic tumors from both BC cell lines were successfully visualized in vivo by SPECT/CT using ¹¹¹In-JMV4168; T47D tumors exhibited a higher uptake than MCF7 xenografts. Conclusion: Targeting GRP-R–expressing BC tumors using GRP-R radioligands is promising for nuclear imaging and therapy, especially in ER-positive BC patients.