Dose-response effect of Gelofusine on renal uptake and retention of radiolabelled octreotate in rats with CA20948 tumours

Efficiency of succinylated gelatin and amino acid infusions for kidney uptake reduction of radiolabeled αvβ6-integrin targeting peptides: considerations on clinical safety profiles

PURPOSE

68Ga-Trivehexin is an investigational PET radiopharmaceutical (NCT05799274) targeting αvβ6-integrin for PET imaging of carcinomas. 177Lu-D0301 is a structurally related therapeutic peptide tetramer. However, it showed considerable kidney uptake in rodents, impeding clinical applicability. We therefore evaluated the impact of different kidney protection strategies on the biodistribution of both agents in normal and tumor-bearing mice.

METHODS

Ex-vivo biodistribution of 68Ga-Trivehexin (90 min p.i.) and 177Lu-D0301 (90 min and 24 h p.i.) was determined in healthy C57BL/6N and H2009 (human lung adenocarcinoma) xenografted CB17-SCID mice without and with co-infusion of 100 µL of solutions containing 2.5% arginine + 2.5% lysine (Arg/Lys), 4% succinylated gelatin (gelofusine, gelo), or combinations thereof. Arg/Lys was injected either i.p. 30 min before and after the radiopharmaceutical, or i.v. 2 min before the radiopharmaceutical. Gelo was administered either i.v. 2 min prior activity, or pre-mixed and injected together with the radiopharmaceutical (n = 5 per group). C57BL/6N mice were furthermore imaged by PET (90 min p.i.) and SPECT (24 h p.i.).

RESULTS

Kidney uptake of 68Ga-Trivehexin in C57BL/6N mice was reduced by 15% (Arg/Lys i.p.), 25% (Arg/Lys i.v.), and 70% (gelo i.v.), 90 min p.i., relative to control. 177Lu-D0301 kidney uptake was reduced by 2% (Arg/Lys i.p.), 41% (Arg/Lys i.v.), 61% (gelo i.v.) and 66% (gelo + Arg/Lys i.v.) 24 h p.i., compared to control. Combination of Arg/Lys and gelo provided no substantial benefit. Gelo furthermore reduced kidney uptake of 177Lu-D0301 by 76% (90 min p.i.) and 85% (24 h p.i.) in H2009 bearing SCID mice. Since tumor uptake was not (90 min p.i.) or only slightly reduced (15%, 24 h p.i.), the tumor/kidney ratio was improved by factors of 3.3 (90 min p.i.) and 2.6 (24 h p.i.). Reduction of kidney uptake was demonstrated by SPECT, which also showed that the remaining activity was located in the cortex.

CONCLUSIONS

The kidney uptake of both investigated radiopharmaceuticals was more efficiently reduced by gelofusine (61-85%) than Arg/Lys (25-41%). Gelofusine appears particularly suitable for reducing renal uptake of αvβ6-integrin targeted 177Lu-labeled peptide multimers because its application led to approximately three times higher tumor-to-kidney ratios. Since the incidence of severe adverse events (anaphylaxis) with succinylated gelatin products (reportedly 0.0062-0.038%) is comparable to that of gadolinium-based MRI or iodinated CT contrast agents (0.008% and 0.04%, respectively), clinical use of gelofusine during radioligand therapy appears feasible if similar risk management strategies as for contrast agents are applied.

Towards the Magic Radioactive Bullet: Improving Targeted Radionuclide Therapy by Reducing the Renal Retention of Radioligands

Targeted radionuclide therapy (TRT) is an emerging field and has the potential to become a major pillar in effective cancer treatment. Several pharmaceuticals are already in routine use for treating cancer, and there is still a high potential for new compounds for this application. But, a major issue for many radiolabeled low-to-moderate-molecular-weight molecules is their clearance via the kidneys and their subsequent reuptake. High renal accumulation of radioactive compounds may lead to nephrotoxicity, and therefore, the kidneys are often the dose-limiting organs in TRT with these radioligands. Over the years, different strategies have been developed aiming for reduced kidney retention and enhanced therapeutic efficacy of radioligands. In this review, we will give an overview of the efforts and achievements of the used strategies, with focus on the therapeutic potential of low-to-moderate-molecular-weight molecules. Among the strategies discussed here is coadministration of compounds that compete for binding to the endocytic receptors in the proximal tubuli. In addition, the influence of altering the molecular design of radiolabeled ligands on pharmacokinetics is discussed, which includes changes in their physicochemical properties and implementation of cleavable linkers or albumin-binding moieties. Furthermore, we discuss the influence of chelator and radionuclide choice on reabsorption of radioligands by the kidneys.

Optimizing the Safety and Efficacy of Bio-Radiopharmaceuticals for Cancer Therapy

The precise delivery of cytotoxic radiation to cancer cells through the combination of a specific targeting vector with a radionuclide for targeted radionuclide therapy (TRT) has proven valuable for cancer care. TRT is increasingly being considered a relevant treatment method in fighting micro-metastases in the case of relapsed and disseminated disease. While antibodies were the first vectors applied in TRT, increasing research data has cited antibody fragments and peptides with superior properties and thus a growing interest in application. As further studies are completed and the need for novel radiopharmaceuticals nurtures, rigorous considerations in the design, laboratory analysis, pre-clinical evaluation, and clinical translation must be considered to ensure improved safety and effectiveness. Here, we assess the status and recent development of biological-based radiopharmaceuticals, with a focus on peptides and antibody fragments. Challenges in radiopharmaceutical design range from target selection, vector design, choice of radionuclides and associated radiochemistry. Dosimetry estimation, and the assessment of mechanisms to increase tumor uptake while reducing off-target exposure are discussed.

The Urokinase Receptor (uPAR) as a "Trojan Horse" in Targeted Cancer Therapy: Challenges and Opportunities

Simple Summary

Discovered more than three decades ago, the urokinase-type plasminogen activator receptor (uPAR) has now firmly established itself as a versatile molecular target holding promise for the treatment of aggressive malignancies. The copious abundance of uPAR in virtually all human cancerous tissues versus their healthy counterparts has fostered a gradual shift in the therapeutic landscape targeting this receptor from function inhibition to cytotoxic approaches to selectively eradicate the uPAR-expressing cells by delivering a targeted cytotoxic insult. Multiple avenues are being explored in a preclinical setting, including the more innovative immune- or stroma targeting therapies. This review discusses the current state of these strategies, their potentialities, and challenges, along with future directions in the field of uPAR targeting.

Abstract

One of the largest challenges to the implementation of precision oncology is identifying and validating selective tumor-driving targets to enhance the therapeutic efficacy while limiting off-target toxicity. In this context, the urokinase-type plasminogen activator receptor (uPAR) has progressively emerged as a promising therapeutic target in the management of aggressive malignancies. By focalizing the plasminogen activation cascade and subsequent extracellular proteolysis on the cell surface of migrating cells, uPAR endows malignant cells with a high proteolytic and migratory potential to dissolve the restraining extracellular matrix (ECM) barriers and metastasize to distant sites. uPAR is also assumed to choreograph multiple other neoplastic stages via a complex molecular interplay with distinct cancer-associated signaling pathways. Accordingly, high uPAR expression is observed in virtually all human cancers and is frequently associated with poor patient prognosis and survival. The promising therapeutic potential unveiled by the pleiotropic nature of this receptor has prompted the development of distinct targeted intervention strategies. The present review will focus on recently emerged cytotoxic approaches emphasizing the novel technologies and related limits hindering their application in the clinical setting. Finally, future research directions and emerging opportunities in the field of uPAR targeting are also discussed.

Reducing the renal retention of low- to moderate-molecular-weight radiopharmaceuticals

The field of nuclear imaging and therapy is rapidly progressing with the development of targeted radiopharmaceuticals that show rapid targeting and rapid clearance with minimal background. Unfortunately, they are often reabsorbed in the kidneys, leading to possible nephrotoxicity, limiting the therapeutic dose, and/or reducing imaging quality. The blocking of endocytic receptors has been extensively used as a strategy to reduce kidney radiation. Alternatively, the physicochemical properties of radiotracers can be modulated to either prevent their reuptake or promote the excretion of radiometabolites. Other interesting strategies focus on the insertion of a cleavable linker between the radiolabel and the targeting moiety or pretargeting approaches in which the targeting moiety and radiolabel are administered separately. In the context of this review, we will discuss the latest advances and insights on strategies used to reduce renal retention of low- to moderate-molecular-weight radiopharmaceuticals.

HER2-directed antibodies, affibodies and nanobodies as drug-delivery vehicles in breast cancer with a specific focus on radioimmunotherapy and radioimmunoimaging

PURPOSE

The aim of the present paper is to review the role of HER2 antibodies, affibodies and nanobodies as vehicles for imaging and therapy approaches in breast cancer, including a detailed look at recent clinical data from antibody drug conjugates and nanobodies as well as affibodies that are currently under development.

RESULTS

Clinical and preclinical studies have shown that the use of monoclonal antibodies in molecular imaging is impaired by slow blood clearance, associated with slow and low tumor uptake and with limited tumor penetration potential. Antibody fragments, such as nanobodies, on the other hand, can be radiolabelled with short-lived radioisotopes and provide high-contrast images within a few hours after injection, allowing early diagnosis and reduced radiation exposure of patients. Even in therapy, the small radioactively labeled nanobodies prove to be superior to radioactively labeled monoclonal antibodies due to their higher specificity and their ability to penetrate the tumor.

CONCLUSION

While monoclonal antibodies are well established drug delivery vehicles, the current literature on molecular imaging supports the notion that antibody fragments, such as affibodies or nanobodies, might be superior in this approach.

Comparison of Nausea and Vomiting Associated With Amino Acid Formulations Coinfused With Peptide Receptor Radionuclide Therapy: Commercial Parenteral Nutrition Formulas Versus Compounded Arginine/Lysine

OBJECTIVES

Positively charged amino acids (AA) such as arginine/lysine are coinfused with radiolabeled somatostatin analogs to reduce rates of nephrotoxicity. In the phase 3 NETTER-1 trial, commercial AA formulations were used in association with 177Lu-DOTA-0-Tyr3-Octreotate (DOTATATE). These formulations were also used in an early-access program (EAP) before regulatory approval of 177Lu-DOTATATE. Our program transitioned to compounded l-arginine 2.5%/l-lysine 2.5% in 0.9% NaCl after commercial approval of 177Lu-DOTATATE. We sought to compare rates of nausea/vomiting with arginine/lysine versus commercial parenteral AA formulations.

METHODS

Rates of nausea/vomiting of all 20 EAP patients who received commercial AAs (15% Clinisol) were compared with the first 29 patients to receive 177Lu-DOTATATE after commercial approval and coinfused with arginine/lysine. Other parameters reviewed included infusion rates, need for PRN nausea medications, and other toxicities.

RESULTS

Seventeen percent of patients who received compounded arginine/lysine experienced nausea, compared with 100% of patients in the EAP group (P < 0.0001). Infusion-related reactions occurred in 3% of the arginine/lysine cohort versus 35% in the EAP group. Infusion durations were substantially shorter in the arginine/lysine cohort (reduced by 61%).

CONCLUSIONS

Coinfusions of arginine/lysine with radiolabeled somatostatin analogs result in substantially lower rates of nausea/vomiting compared with commercial AA formulations designed for parenteral nutrition.

The Role of VEGF Receptors as Molecular Target in Nuclear Medicine for Cancer Diagnosis and Combination Therapy

Simple Summary

The rapid development of diagnostic and therapeutic methods of the cancer treatment causes that these diseases are becoming better known and the fight against them is more and more effective. Substantial contribution in this development has nuclear medicine that enables very early cancer diagnosis and early start of the so-called targeted therapy. This therapeutic concept compared to the currently used chemotherapy, causes much fewer undesirable side effects, due to targeting a specific lesion in the body. This review article discusses the possible applications of radionuclide-labelled tracers (peptides, antibodies or synthetic organic molecules) that can visualise cancer cells through pathological blood vessel system in close tumour microenvironment. Hence, at a very early step of oncological disease, targeted therapy can involve in tumour formation and growth.

Abstract

One approach to anticancer treatment is targeted anti-angiogenic therapy (AAT) based on prevention of blood vessel formation around the developing cancer cells. It is known that vascular endothelial growth factor (VEGF) and vascular endothelial growth factor receptors (VEGFRs) play a pivotal role in angiogenesis process; hence, application of angiogenesis inhibitors can be an effective approach in anticancer combination therapeutic strategies. Currently, several types of molecules have been utilised in targeted VEGF/VEGFR anticancer therapy, including human VEGF ligands themselves and their derivatives, anti-VEGF or anti-VEGFR monoclonal antibodies, VEGF binding peptides and small molecular inhibitors of VEGFR tyrosine kinases. These molecules labelled with diagnostic or therapeutic radionuclides can become, respectively, diagnostic or therapeutic receptor radiopharmaceuticals. In targeted anti-angiogenic therapy, diagnostic radioagents play a unique role, allowing the determination of the emerging tumour, to monitor the course of treatment, to predict the treatment outcomes and, first of all, to refer patients for AAT. This review provides an overview of design, synthesis and study of radiolabelled VEGF/VEGFR targeting and imaging agents to date. Additionally, we will briefly discuss their physicochemical properties and possible application in combination targeted radionuclide tumour therapy.

Inter and intra-tumor somatostatin receptor 2 heterogeneity influences peptide receptor radionuclide therapy response

Patients with neuroendocrine tumors (NETs) can be treated with peptide receptor radionuclide therapy (PRRT). Here, the somatostatin analogue octreotate radiolabeled with lutetium-177 is targeted to NET cells by binding to the somatostatin receptor subtype 2 (SST₂). During radioactive decay, DNA damage is induced, leading to NET cell death. Although the therapy proves to be effective, mortality rates remain high. To appropriately select more optimal treatment strategies, it is essential to first better understand the radiobiological responses of tumor cells to PRRT.

Methods: We analyzed PRRT induced radiobiological responses in SST₂ expressing cells and xenografted mice using SPECT/MRI scanning and histological and molecular analyses. We measured [¹⁷⁷Lu]Lu-DOTA-TATE uptake and performed analyses to visualize induction of DNA damage, cell death and other cellular characteristics.

Results: The highest accumulation of radioactivity was measured in the tumor and kidneys. PRRT induced DNA damage signaling and repair in a time-dependent manner. We observed intra-tumor heterogeneity of DNA damage and apoptosis, which was not attributed to proliferation or bioavailability. We found a strong correlation between high DNA damage levels and high SST₂ expression. PRRT elicited a different therapeutic response between models with different SST₂ expression levels. Heterogeneous SST₂ expression levels were also confirmed in patient NETs.

Conclusion: Heterogeneous SST₂ expression levels within NETs cause differentially induced DNA damage levels, influence recurrent tumor phenotypes and impact the therapeutic response in different models and potentially in patients. Our results contribute to a better understanding of PRRT effects, which might impact future therapeutic outcome of NET patients.

68 Ga-labeling of internalizing RGD (iRGD) peptide functionalized with DOTAGA and NODAGA chelators

The dual interaction with integrins and neuropilin-1 receptor is the peculiar feature of iRGD peptide. Hence, in the present study, two iRGD peptide analogs were synthesized with DOTAGA and NODAGA as bifunctional chelator and aminohexanoic acid as a spacer for radiometalation with ⁶⁸ GaCl₃ . Negatively charged ⁶⁸ Ga-DOTAGA-iRGD and neutral ⁶⁸ Ga-NODAGA-iRGD radiotracers were investigated through in vitro cell uptake studies and in vivo biodistribution studies. Significant internalization of radiotracers in murine melanoma B16F10 cells was observed during in vitro studies. During in vivo studies, tumor uptake was higher for neutral ⁶⁸ Ga-NODAGA-iRGD, but ⁶⁸ Ga-DOTAGA-iRGD exhibited better tumor-to-blood ratio due to faster blood clearance. High kidney uptake of the two radiotracers was the limitation, which needs to be resolved through modification either in the peptide backbone or spacer/chelator.

Protective effect of Gelofusine against cRGD-siRNA-induced nephrotoxicity in mice

Based on successful targeting to the αvβ3 integrin of cyclic arginine–glycine–aspartic acid (cRGD), cRGD-conjugated small interfering RNA (siRNA) exhibits tumor targeting and has become a new treatment strategy for solid tumors. However, the nephrotoxicity caused by its renal retention limits its clinical application. Here, we evaluated the protective effect of Gelofusine against cRGD-conjugated siRNA-induced nephrotoxicity in mice. Male Kunming mice (six per group) were either co-injected with Gelofusine and cRGD-siRNA or injected with cRGD-siRNA alone. After administration of these treatments five times, creatinine and blood urea nitrogen (BUN) levels were determined. Hematoxylin–eosin staining (HE staining) and transferase dUTP nick end labeling (TUNEL) analysis were used to compare the difference in renal damage between the groups. Additionally, fluorescence imaging was used to observe the distribution of cRGD-siRNA in vivo. The group co-injected with Gelofusine and cRGD-siRNA displayed lower creatinine and BUN levels than the cRGD-siRNA-alone group and showed less renal damage upon HE staining and TUNEL analysis. Gelofusine decreased the retention time and accelerated the elimination of cRGD-siRNA from the organs, as observed in the fluorescence images. These data indicate that Gelofusine significantly increased the excretion of cRGD-conjugated siRNA and reduced the associated renal damage.

177Lu-DOTATATE Peptide Receptor Radionuclide Therapy in Patients with Borderline Low and Discordant Renal Parameters: Treatment Feasibility Assessment by Sequential Estimation of Triple Parameters and Filtration Fraction

The aim was to assess the effect of standard fixed-dose protocol of ¹⁷⁷Lu-DOTATATE peptide receptor radionuclide therapy (PRRT) in patients with borderline low renal function of one parameter (glomerular filtration rate [GFR], effective renal plasma flow [ERPF] or serum creatinine), that was discordant with the remaining parameters and determine the feasibility of this procedure in this group of patients. Renal toxicity of PRRT is a routine issue or concern for such cases. We compared different renal parameters used for pretherapy assessment in patients with borderline low single parameter at baseline and their potential significance with regards to deterioration of renal function subsequently. A retrospective analysis was performed in patients of metastatic neuroendocrine tumors who received therapeutic ¹⁷⁷Lu-DOTATATE (using standard fixed-dose protocol) and had borderline compromised renal parameter values (either of GFR/ERPF/serum creatinine). Filtration fraction (FF) was also estimated in each case and all renal parameters were correlated using kappa statistics. The characteristics of cases showing progressive worsening of renal function in the follow-ups were also studied. A total of 15 patients (11 males, 4 females; age range: 32–75 years) were selected among a population of 450 patients. The follow-up duration ranged from 10 to 48 months and administered cumulative activity ranged 9.9–31.3 GBq (2–5 cycles). Based on the parameter characteristics, the study population was divided into following four groups: (a) patients with reduced GFR and maintained ERPF and normal serum creatinine (n = 3); (b) patients with reduced ERPF with maintained GFR and borderline elevated/normal serum creatinine (n = 3); (c) patients with both reduced GFR and ERPF and maintained serum creatinine (n = 1); (d) patients with compromised single kidney function (n = 5). A total of four patients were found who had normal baseline renal function values but showed progressive worsening in the subsequent period. There was no significant change in renal parameters during the follow-up in both Groups a and c. Two patients of Group b demonstrated well-maintained other renal parameters, whereas in 1 patient, there was the evidence of renal toxicity with gradual fall of GFR and ERPF and progressive increase in serum creatinine level. In patients with compromised single kidney function at baseline (Group d), there was overall maintained normal renal parameters, whereas 3 of 5 (60%) showed the increase of FF of the affected kidney. Interestingly, a compensatory hyperfunction was noted in the contralateral kidney. PRRT with ¹⁷⁷Lu-DOTATATE is feasible and can be considered in patients with reduced GFR and with maintained ERPF and normal serum creatinine and also in the presence of single compromised parameter if the other two are normal; however, these patients need critical monitoring.

Novel peptide probes to assess the tensional state of fibronectin fibers in cancer

Transformations of extracellular matrix (ECM) accompany pathological tissue changes, yet how cell-ECM crosstalk drives these processes remains unknown as adequate tools to probe forces or mechanical strains in tissues are lacking. Here, we introduce a new nanoprobe to assess the mechanical strain of fibronectin (Fn) fibers in tissue, based on the bacterial Fn-binding peptide FnBPA5. FnBPA5 exhibits nM binding affinity to relaxed, but not stretched Fn fibers and is shown to exhibit strain-sensitive ECM binding in cell culture in a comparison with an established Fn-FRET probe. Staining of tumor tissue cryosections shows large regions of relaxed Fn fibers and injection of radiolabeled ¹¹¹In-FnBPA5 in a prostate cancer mouse model reveals specific accumulation of ¹¹¹In-FnBPA5 in tumor with prolonged retention compared to other organs. The herein presented approach enables to investigate how Fn fiber strain at the tissue level impacts cell signaling and pathological progression in different diseases.

Gelofusine Ameliorates Colistin-Induced Nephrotoxicity

Colistin therapy is used as the last line of defense against life-threatening Gram-negative infections. Nephrotoxicity is the major dose-limiting side effect that impedes optimal dosing of patients. This study aims to examine the nephroprotective effect of the plasma volume expander gelofusine against colistin-induced nephrotoxicity. Renal protection was assessed in mice that were subcutaneously injected with colistin sulfate (14 mg/kg of body weight × 6 doses every 2 h; accumulated dose, 84 mg/kg) and simultaneously injected in the intraperitoneal region with gelofusine (75, 150, 300, or 600 mg/kg × 6). At 2 and 20 h after the last colistin dose, mice were euthanized, and the severity of renal alteration was examined histologically. Histological findings in mice revealed that colistin-induced nephrotoxicity was ameliorated by gelofusine in a dose-dependent manner, whereas significant histological abnormalities were detected in the kidneys of mice in the colistin-only group. The impact of coadministered gelofusine on colistin pharmacokinetics was investigated in rats. Rats were administered a single intravenous dose of gelofusine at 400 mg/kg 15 min prior to the intravenous administration of colistin (1 mg/kg). Gelofusine codosing did not alter the pharmacokinetics of colistin in rats; however, gelofusine did significantly lower the accumulation of colistin in the kidney tissue of mice. This is the first study demonstrating the protective effect of gelofusine against colistin-induced nephrotoxicity. These findings highlight the clinical potential of gelofusine as a safe adjunct for ameliorating the nephrotoxicity and increasing the therapeutic index of polymyxins.

High-throughput high-volume nuclear imaging for preclinical in vivo compound screening§

Background

Preclinical single-photon emission computed tomography (SPECT)/CT imaging studies are hampered by low throughput, hence are found typically within small volume feasibility studies. Here, imaging and image analysis procedures are presented that allow profiling of a large volume of radiolabelled compounds within a reasonably short total study time. Particular emphasis was put on quality control (QC) and on fast and unbiased image analysis.

Methods

2–3 His-tagged proteins were simultaneously radiolabelled by ^99mTc-tricarbonyl methodology and injected intravenously (20 nmol/kg; 100 MBq; n = 3) into patient-derived xenograft (PDX) mouse models. Whole-body SPECT/CT images of 3 mice simultaneously were acquired 1, 4, and 24 h post-injection, extended to 48 h and/or by 0–2 h dynamic SPECT for pre-selected compounds. Organ uptake was quantified by automated multi-atlas and manual segmentations. Data were plotted automatically, quality controlled and stored on a collaborative image management platform. Ex vivo uptake data were collected semi-automatically and analysis performed as for imaging data.

Results

>500 single animal SPECT images were acquired for 25 proteins over 5 weeks, eventually generating >3500 ROI and >1000 items of tissue data. SPECT/CT images clearly visualized uptake in tumour and other tissues even at 48 h post-injection. Intersubject uptake variability was typically 13% (coefficient of variation, COV). Imaging results correlated well with ex vivo data.

Conclusions

The large data set of tumour, background and systemic uptake/clearance data from 75 mice for 25 compounds allows identification of compounds of interest. The number of animals required was reduced considerably by longitudinal imaging compared to dissection experiments. All experimental work and analyses were accomplished within 3 months expected to be compatible with drug development programmes. QC along all workflow steps, blinding of the imaging contract research organization to compound properties and automation provide confidence in the data set. Additional ex vivo data were useful as a control but could be omitted from future studies in the same centre. For even larger compound libraries, radiolabelling could be expedited and the number of imaging time points adapted to increase weekly throughput. Multi-atlas segmentation could be expanded via SPECT/MRI; however, this would require an MRI-compatible mouse hotel. Finally, analysis of nuclear images of radiopharmaceuticals in clinical trials may benefit from the automated analysis procedures developed.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-017-0281-4) contains supplementary material, which is available to authorized users.

Metal-Based PSMA Radioligands

Prostate cancer is one of the most common malignancies for which great progress has been made in identifying appropriate molecular targets that would enable efficient in vivo targeting for imaging and therapy. The type II integral membrane protein, prostate specific membrane antigen (PSMA) is overexpressed on prostate cancer cells in proportion to the stage and grade of the tumor progression, especially in androgen-independent, advanced and metastatic disease, rendering it a promising diagnostic and/or therapeutic target. From the perspective of nuclear medicine, PSMA-based radioligands may significantly impact the management of patients who suffer from prostate cancer. For that purpose, chelating-based PSMA-specific ligands have been labeled with various diagnostic and/or therapeutic radiometals for single-photon-emission tomography (SPECT), positron-emission-tomography (PET), radionuclide targeted therapy as well as intraoperative applications. This review focuses on the development and further applications of metal-based PSMA radioligands.

Co-administration of succinylated gelatine with a (99m)Tc-bombesin analogue, effects on pharmacokinetics and tumor uptake

The bombesin analogue, [(99m)Tc-GGC]-(Ornithine)3-BN(2-14), (99m)Tc-BN-O, targeting gastrin releasing peptide receptors (GRPrs) on the surface of tumors, was pre-clinically investigated as potential imaging agent for single photon emission computed tomography (SPECT). In addition, the improvement of its pharmacokinetic profile (PK) was investigated through the co-administration of a succinylated gelatin plasma expander (Gelofusine), aiming to reduce its kidney accumulation and enhance its tumor-to-normal tissue contrast ratios. Biodistribution data were collected from normal mice and rats, and PC-3 tumor bearing mice, in reference to its PK, metabolism and tumor uptake. Imaging data were also collected from PC-3 tumor bearing mice. Biodistribution and imaging experiments showed that (99m)Tc-BN-O was able to efficiently localize the tumor (5.23 and 7.00% ID/g at 30 and 60min post injection, respectively), while at the same time it was rapidly cleared from the circulation through the kidneys. HPLC analysis of kidney samples, collected at 60min p.i. from normal mice and rats, showed that the majority of radioactivity detected was due to intact peptide i.e. 56% for mice and 73% for rats. Co-administration of (99m)Tc-BN-O with Gelo resulted in the reduction of kidney uptake in both animal models. The integrated area under the curve (AUC30-60 min) from the concentration-time plots of kidneys was decreased in both mice and rats by 25 and 50%, respectively. In PC-3 tumor bearing mice, an increase of tumor uptake (AUCtumor increased by 69%) was also observed with Gelo. An improvement in tumor-to-blood and tumor-to-normal tissue ratios was noted in all cases with the exception of the pancreas, which normally expresses GRPr. The results of this preclinical study may also be extended to other similar peptides, which are utilized in prostate cancer imaging and present similar PK profile.

Evaluation of a Fluorescent and Radiolabeled Hybrid Somatostatin Analog In Vitro and in Mice Bearing H69 Neuroendocrine Xenografts

In the treatment of neuroendocrine tumors (NETs), complete surgical removal of malignancy is generally desirable, because it offers curative results. Preoperative guidance with radiolabeled somatostatin analogs, commonly used for NET diagnosis and preoperative planning, is limited by its low resolution, with the risk that tumor margins and small metastases will be incompletely resected with subsequent recurrence. A single hybrid probe combining radiotracer and optical dye would enable high-resolution optical guidance, also during surgery. In the current study, the hybrid labeled somatostatin analog Cy5-DTPA-Tyr(3)-octreotate (DTPA is diethylene triamine pentaacetic acid) was synthesized and evaluated for its ability to specifically trace NET cells in vitro and in an animal model. The performance of the hybrid tracer was compared with that of octreotate with only radiolabel or only optical label.

METHODS

The binding affinity and internalization capacity of Cy5-DTPA-Tyr(3)-octreotate were assessed in vitro. Biodistribution profiles and both nuclear and optical in vivo imaging of Cy5-(111)In -DTPA-Tyr(3)-octreotate were performed in NET-bearing mice and compared with the performance of (111)In-DTPA-Tyr(3)-octreotate.

RESULTS

In vitro studies showed a low receptor affinity and internalization rate for Cy5-DTPA-Tyr(3)-octreotate. The dissociation constant value was 387.7 ± 97.9 nM for Cy5-DTPA-Tyr(3)-octreotate, whereas it was 120.5 ± 18.1 nM for DTPA-Tyr(3)-octreotate. Similarly, receptor-mediated internalization reduced from 33.76% ± 1.22% applied dose for DTPA-Tyr(3)-octreotate to 1.32% ± 0.02% applied dose for Cy5-DTPA-Tyr(3)-octreotate. In contrast, in vivo and ex vivo studies revealed similar tumor uptake values of Cy5-(111)In-DTPA-Tyr(3)-octreotate and (111)In -DTPA-Tyr(3)-octreotate (6.93 ± 2.08 and 5.16 ± 1.27, respectively). All organs except the kidneys showed low background radioactivity, with especially low activities in the liver, and high tumor-to-tissue ratios were achieved-both favorable for the tracer's toxicity profile. Hybrid imaging in mice confirmed that the nuclear and fluorescence signals colocalized.

CONCLUSION

The correlation between findings with the optical and the nuclear probes underlines the potential of combining SPECT imaging with fluorescence guidance and shows the promise of this novel hybrid peptide for preoperative and intraoperative imaging of NET.

Influence of tumour size on the efficacy of targeted alpha therapy with (213)Bi-[DOTA(0),Tyr(3)]-octreotate

Background

Targeted alpha therapy has been postulated to have great potential for the treatment of small clusters of tumour cells as well as small metastases. ²¹³Bismuth, an α-emitter with a half-life of 46 min, has shown to be effective in preclinical as well as in clinical applications. In this study, we evaluated whether ²¹³Bi-[DOTA⁰, Tyr³]-octreotate (²¹³Bi-DOTATATE), a ²¹³Bi-labelled somatostatin analogue with high affinity for somatostatin receptor subtype 2 (SSTR₂), is suitable for the treatment of larger neuroendocrine tumours overexpressing SSTR₂ in comparison to its effectiveness for smaller tumours. We performed a preclinical targeted radionuclide therapy study with ²¹³Bi-DOTATATE in animals bearing tumours of different sizes (50 and 200 mm³) using two tumour models: H69 (human small cell lung carcinoma) and CA20948 (rat pancreatic tumour).

Methods

Pharmacokinetics was determined for calculation of dosimetry in organs and tumours. H69- or CA20948-xenografted mice with tumour volumes of approximately 120 mm³ were euthanized at 10, 30, 60 and 120 min post injection of a single dose of ²¹³Bi-DOTATATE (1.5–4.8 MBq). To investigate the therapeutic efficacy of ²¹³Bi-DOTATATE, xenografted H69 and CA20948 tumour-bearing mice with tumour sizes of 50 and 200 mm³ were administered daily with a therapeutic dose of ²¹³Bi-DOTATATE (0.3 nmol, 2–4 MBq) for three consecutive days. The animals were followed for 90 days after treatment. At day 90, mice were injected with 25 MBq ^99mTc-DMSA and imaged by SPECT/CT to investigate possible renal dysfunction due to ²¹³Bi-DOTATATE treatment.

Results

Higher tumour uptakes were found in CA20948 tumour-bearing animals compared to those in H69 tumour-bearing mice with the highest tumour uptake of 19.6 ± 6.6 %IA/g in CA20948 tumour-bearing animals, while for H69 tumour-bearing mice, the highest tumour uptake was found to be 9.8 ± 2.4 %IA/g. Nevertheless, as the anti-tumour effect was more pronounced in H69 tumour-bearing mice, the survival rate was higher. Furthermore, in the small tumour groups, no regrowth of tumour was found in two H69 tumour-bearing mice and in one of the CA20948 tumour-bearing mice. No renal dysfunction was observed in ²¹³Bi-DOTATATE-treated mice after the doses were applied.

Conclusions

²¹³Bi-DOTATATE demonstrated a great therapeutic effect in both small and larger tumour lesions. Higher probability for stable disease was found in animals with small tumours. ²¹³Bi-DOTATATE was effective in different neuroendocrine (H69 and CA20948) tumour models with overexpression of SSTR₂ in mice.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-016-0162-2) contains supplementary material, which is available to authorized users.

177Lu-labeled carbon nanospheres: a new entry in the field of targeted radionanomedicine

¹⁷⁷Lu-labeled carbon nanospheres loaded with cRGDfK peptide have been developed as radionanoprobes with favorable pharmacokinetics for integrin α_vβ₃-mediated active targeting.

Diabody Pretargeting with Click Chemistry In Vivo

Radioimmunotherapy and nuclear imaging (immuno-PET/SPECT) of cancer with radiometal-labeled antibody fragments or peptides is hampered by low tumor-to-kidney ratios because of high renal radiometal retention. Therefore, we developed and evaluated a pretargeting strategy using click chemistry in vivo to reduce kidney uptake and avoid unwanted radiation toxicity. We focused on the bioorthogonal reaction between a trans-cyclooctene (TCO)-functionalized TAG72 targeting diabody, AVP04-07, and a low-molecular-weight radiolabeled tetrazine probe that was previously shown to have low kidney retention and relatively fast renal clearance.

METHODS

AVP04-07 diabodies were functionalized with TCO tags, and in vitro immunoreactivity toward bovine submaxillary mucin and tetrazine reactivity were assessed. Next, pretargeting biodistribution studies were performed in LS174T tumor-bearing mice with AVP04-07-TCO(n) (where n indicates the number of TCO groups per diabody) and radiolabeled tetrazine to optimize the TCO modification grade (0, 1.8, or 4.7 TCO groups per diabody) and the (177)Lu-tetrazine dose (0.1, 1.0, or 10 Eq with respect to the diabody). Radiolabeled tetrazine was injected at 47 h after diabody injection, and mice were euthanized 3 h later. A pretargeting SPECT/CT study with (111)In-tetrazine was performed with the optimized conditions.

RESULTS

Immunoreactivity for native AVP04-07 was similar to that for TCO-functionalized AVP04-07, and the latter reacted efficiently with radiolabeled tetrazine in vitro. The combination of the pretargeting component AVP04-07 functionalized with 4.7 TCO groups and 1 Eq of (177)Lu-tetrazine with respect to the diabody showed the most promising biodistribution. Specifically, high (177)Lu-tetrazine tumor uptake (6.9 percentage injected dose/g) was observed with low renal retention, yielding a tumor-to-kidney ratio of 5.7. SPECT/CT imaging confirmed the predominant accumulation of radiolabeled tetrazine in the tumor and low nontumor retention.

CONCLUSION

Pretargeting provides an alternative radioimmunotherapy and nuclear imaging strategy by overcoming the high renal retention of low-molecular-weight radiometal tumor-homing agents through the separate administration of a tumor-homing agent and a radioactive probe with fast clearance.

Peptide receptor radionuclide therapy (PRRT) of neuroendocrine tumors: current state and future perspectives

The heterogeneous nature of neuroendocrine tumors, their indolent course and lack of therapeutic options for the management of advanced cases – together form the Achilles heel for oncologists. Somatostatin receptor-specific imaging with Ga-68 labeled peptides has provided an opportunity for management of advanced cancers with their therapeutic radionuclide counterparts (Lu-177/Y-90 labeled peptides). Molecular imaging with positron emission tomography/computed tomography is accepted technique for treatment response assessment, since the radiolabels for imaging and therapy are same, thereby depicting accurate response. We have compiled and reviewed our experience of last 8–10 years in the use of these novel radiolabeled peptides in the treatment of neuroendocrine tumors, focusing on the survival, toxicity profiles and the recent advances and improvements.

Dosimetry of [(177)Lu]-DO3A-VS-Cys(40)-Exendin-4 - impact on the feasibility of insulinoma internal radiotherapy

[(68)Ga]-DO3A-VS-Cys(40)-Exendin-4 has been shown to be a promising imaging candidate for targeting glucagon like peptide-1 receptor (GLP-1R). In the light of radiotheranostics and personalized medicine the (177)Lu-labelled analogue is of paramount interest. In this study we have investigated the organ distribution of [(177)Lu]-DO3A-VS-Cys(40)-Exendin-4 in rat and calculated human dosimetry parameters in order to estimate the maximal acceptable administered radioactivity, and thus potential applicability of [(177)Lu]-DO3A-VS-Cys(40)-Exendin-4 for internal radiotherapy of insulinomas. Nine male and nine female Lewis rats were injected with [(177)Lu]-DO3A-VS-Cys(40)-Exendin-4 for ex vivo organ distribution study at nine time points. The estimation of human organ/total body absorbed and total effective doses was performed using Organ Level Internal Dose Assessment Code software (OLINDA/EXM 1.1). Six more rats (male: n = 3; female: n = 3) were scanned by single photon emission tomography and computed tomography (SPECT-CT). The renal function and potential cell dysfunction were monitored by creatinine ISTAT and glucose levels. The fine uptake structure of kidney and pancreas was investigated by ex vivo autoradiography. Blood clearance and washout from most of the organs was fast. The kidney was the dose-limiting organ with absorbed dose of 5.88 and 6.04 mGy/MBq, respectively for female and male. Pancreatic beta cells demonstrated radioactivity accumulation. Renal function and beta cell function remained unaffected by radiation. The absorbed dose of [(177)Lu]-DO3A-VS-Cys(40)-Exendin-4 to kidneys may limit the clinical application of the agent. However, hypothetically, kidney protection and peptidase inhibition may allow reduction of kidney absorbed dose and amplification of tumour absorbed doses.

Radiolabeled nanobodies as theranostic tools in targeted radionuclide therapy of cancer

INTRODUCTION

The integration of diagnostic testing for the presence of a molecular target is of interest to predict successful targeted radionuclide therapy (TRNT). This so-called 'theranostic' approach aims to improve personalized treatment based on the molecular characteristics of cancer cells. Moreover, it offers new insights in predicting adverse effects and provides appropriate tools to monitor therapy responses. Recent findings using nanobodies emphasize their potential as theranostic tools in cancer treatment. Nanobodies are recombinant, small antigen-binding fragments that are derived from camelid heavy-chain-only antibodies.

AREAS COVERED

We review the current status of theranostic approaches in TRNT, with a focus on antibodies, peptides, scaffold proteins and emerging nanobodies. In recent years, nanobodies have been evaluated intensively for molecular imaging. In addition, novel data on TRNT using radiolabeled nanobodies for carcinomas and multiple myeloma highlight their promising opportunities in cancer treatment.

EXPERT OPINION

We trust that radiolabeled nanobodies will have a future potential as theranostic tools in cancer therapy, both for diagnosis as well as for TRNT.

PET imaging and biodistribution analysis of the effects of succinylated gelatin combined with L-lysine on renal uptake and retention of ⁶⁴Cu-cyclam-RAFT-c(-RGDfK-)₄ in vivo

(64)Cu-cyclam-RAFT-c(-RGDfK-)4, an αVβ3 integrin-targeting tetrameric cyclic RGD peptide probe, is a potential theranostic compound for positron emission tomography (PET) of tumor angiogenesis and for internal radiotherapy owing to the multiple decay modes of (64)Cu. Since kidneys are dose-limiting organs in internal radiotherapy, we aimed to reduce the renal accumulation of (64)Cu-cyclam-RAFT-c(-RGDfK-)4 by co-injection with Gelofusine (GF), a succinylated gelatin solution, and/or L-lysine (Lys), and to explore, for the first time, the related mechanisms using the noninvasive and quantitative PET imaging technology. Biodistribution assays, dynamic and static PET scans, and metabolism studies with radio-thin-layer chromatography (radio-TLC) were performed in healthy or αVβ3-positive tumor-bearing mice. In the results, co-injection with GF markedly reduced the renal uptake and slightly increased the tumor uptake of (64)Cu-cyclam-RAFT-c(-RGDfK-)4. L-Lysine alone had no effect on the probe biodistribution, but the combined use of Lys and GF tended to enhance the effect of GF. Dynamic PET and metabolite analysis by radio-TLC highly revealed that GF blocks the renal reabsorption of (64)Cu-cyclam-RAFT-c(-RGDfK-)4, but does not interfere with its metabolism and excretion. In conclusion, administration of GF and Lys is a useful strategy for kidney protection in (64)Cu-cyclam-RAFT-c(-RGDfK-)4-based internal radiotherapy.

Radionuclide therapy via SSTR: future aspects from experimental animal studies

There is need for better therapeutic options for neuroendocrine tumours. The aim of this review was to summarize results of experimental animal studies and raise ideas for future radionuclide therapy based on high expression of somatostatin (SS) receptors by many neuroendocrine tumours. In summary, one of the major options is individualized treatment for each patient, including choice of SS analogues, radionuclides and treatment schedules. Other options are methods to increase the treatment effect on tumour tissue (increasing tumour uptake and retention by upregulation of receptor expression and avoiding saturation of receptor binding), methods to increase the tumour tissue response (by choice of radionuclides, SS analogues or combined therapies), and methods to reduce side effects (diminished uptake and retention in critical organs and reduced normal tissue response). Furthermore, combination therapy with other radiopharmaceuticals, cytotoxic drugs or radiosensitizers can be considered to enhance the effects of radiolabelled SS analogues.

Treatment of gastroenteropancreatic neuroendocrine tumors with peptide receptor radionuclide therapy

The primary treatment of gastroenteropancreatic neuroendocrine tumors (GEPNETs) is surgery with curative intent or debulking of the tumor mass. In case of metastatic disease, cytoreductive options are limited. A relatively new therapeutic modality, peptide receptor radionuclide therapy (PRRT) with radiolabeled somatostatin analogs, is currently available in a number of mostly European centers. Complete and partial responses obtained after treatment with [90Y-DOTA0,Tyr3]octreotide are in the same range as after treatment with [177Lu-DOTA0,Tyr3]octreotate (i.e. 10-30%). However, significant nephrotoxicity has been observed after treatment with [90Y-DOTA0,Tyr3]octreotide. Options to improve PRRT may include combinations of radioactive labeled somatostatin analogs, intra-arterial administration, and the use of radiosensitizing drugs combined with PRRT. Other therapeutic applications of PRRT may include additional therapy cycles in patients with progressive disease after benefit from initial therapy, PRRT in adjuvant or neoadjuvant setting, or PRRT combined with new targeted therapies, such as sunitinib or everolimus. Randomized clinical trials comparing PRRT with other treatment modalities, or comparing various radioactive labeled somatostatin analogs should be undertaken to determine the best treatment options and treatment sequelae for patients with GEPNETs.

Development of 177Lu-nanobodies for radioimmunotherapy of HER2-positive breast cancer: evaluation of different bifunctional chelators

Nanobodies show favourable pharmacokinetic characteristics for tumor targeting, including high tumor-to-background-ratios. Labelled with a therapeutic radionuclide, nanobodies could be used as an adjuvant treatment option for HER2-overexpressing minimal residual disease. The therapeutic radionuclide Lutetium-177 is linked to the nanobody using a bifunctional chelator. The choice of the bifunctional chelator could affect the in vivo behaviour of the radiolabeled nanobody. Consequently, we compared four different bifunctional chelators - p-SCN-Bn-DOTA, DOTA-NHS-ester, CHX-A"-DTPA or 1B4M-DTPA - in order to select the optimal chemical link between Lutetium-177 and a HER2 targeting nanobody. MS results revealed different degrees of chelator-conjugation. High stability in time was observed, together with nanomolar affinities on HER2-expressing tumor cells. Ex vivo biodistributions as well as SPECT/micro-CT analyses showed high activities in tumors expressing medium HER2 levels with low background activity except for the kidneys. The 1B4M-DTPA-coupled conjugate was further evaluated in a high HER2-expressing tumor model. Here, tumor uptake values of 5.99 ± 0.63, 5.12 ± 0.17, 2.83 ± 0.36 and 2.47 ± 0.38 %IA/g were obtained at 1, 3, 24 and 48h p.i., which coincided with exceptionally low background values, except for the kidneys, and unprecedented tumor-to-background ratios. No specific binding was observed in a HER2-negative model. In conclusion, the in-house developed anti-HER2 nanobody 2Rs15dHIS can be successfully labeled with (177) Lu using different bifunctional chelators. Both macrocyclic and acyclic chelators show high stability in time. High specific tumor uptake combined with the lowest background uptake was measured using the 1B4M-DTPA-based conjugate.

Positron emission tomography imaging of tumor angiogenesis and monitoring of antiangiogenic efficacy using the novel tetrameric peptide probe 64Cu-cyclam-RAFT-c(-RGDfK-)4

⁶⁴Cu-cyclam-RAFT-c(-RGDfK-)₄ is a novel multimeric positron emission tomography (PET) probe for α_Vβ₃ integrin imaging. Its uptake and α_Vβ₃ expression in tumors showed a linear correlation. Since α_Vβ₃ integrin is strongly expressed on activated endothelial cells during angiogenesis, we aimed to determine whether ⁶⁴Cu-cyclam-RAFT-c(-RGDfK-)₄ PET can be used to image tumor angiogenesis and monitor the antiangiogenic effect of a novel multi-targeted tyrosine kinase inhibitor, TSU-68. Athymic nude mice bearing human hepatocellular carcinoma HuH-7 xenografts, which expressed negligible α_Vβ₃ levels on the tumor cells, received intraperitoneal injections of TSU-68 or the vehicle for 14 days. Antiangiogenic effects were determined at the end of therapy in terms of ⁶⁴Cu-cyclam-RAFT-c(-RGDfK-)₄ uptake evaluated using PET, biodistribution assay, and autoradiography, and they were compared with microvessel density (MVD) determined by CD31 immunostaining. ⁶⁴Cu-cyclam-RAFT-c(-RGDfK-)₄ PET enabled clear tumor visualization by targeting the vasculature, and the biodistribution assay indicated high tumor-to-blood and tumor-to-muscle ratios of 31.6 ± 6.3 and 6.7 ± 1.1, respectively, 3 h after probe injection. TSU-68 significantly slowed tumor growth and reduced MVD; these findings were consistent with a significant reduction in the tumor ⁶⁴Cu-cyclam-RAFT-c(-RGDfK-)₄ uptake. Moreover, a linear correlation was observed between tumor MVD and the corresponding standardized uptake value (SUV) (r = 0.829, P = 0.011 for SUV_mean; r = 0.776, P = 0.024 for SUV_max) determined by quantitative PET. Autoradiography and immunostaining showed that the distribution of intratumoral radioactivity and tumor vasculature corresponded. We concluded that ⁶⁴Cu-cyclam-RAFT-c(-RGDfK-)₄ PET can be used for in vivo angiogenesis imaging and monitoring of tumor response to antiangiogenic therapy.

Reduction of renal uptake of 111In-DOTA-labeled and A700-labeled RAFT-RGD during integrin αvβ3 targeting using single photon emission computed tomography and optical imaging

Integrin α(v)β(3) expression is upregulated during tumor growth and invasion in newly formed endothelial cells in tumor neovasculature and in some tumor cells. A tetrameric RGD-based peptide, regioselectively addressable functionalized template-(cyclo-[RGDfK])4 (RAFT-RGD), specifically targets integrin α(v)β(3) in vitro and in vivo. When labeled with indium-111, the RAFT-RGD is partially reabsorbed and trapped in the kidneys, limiting its use for further internal targeted radiotherapy and imaging investigations. We studied the effect of Gelofusine on RAFT-RGD renal retention in tumor-bearing mice. Mice were imaged using single photon emission computed tomography and optical imaging 1 and 24 h following tracer injection. Distribution of RAFT-RGD was further investigated by tissue removal and direct counting of the tracer. Kidney sections were analyzed by confocal microscopy. Gelofusine significantly induced a >50% reduction of the renal reabsorption of (111)In-DOTA-RAFT-RGD and A700-RAFT-RGD, without affecting tumor uptake. Injection of Gelofusine significantly reduced the renal retention of labeled RAFT-RGD, while increasing the tumor over healthy tissue ratio. These results will lead to the development of future therapeutic approaches.

Kidney protection during peptide receptor radionuclide therapy with somatostatin analogues

This review focuses on the present status of kidney protection during peptide receptor radionuclide therapy (PRRT) using radiolabelled somatostatin analogues. This treatment modality for somatostatin receptor-positive tumours is limited by renal reabsorption and retention of radiolabelled peptides resulting in dose-limiting high kidney radiation doses. Radiation nephropathy has been described in several patients. Studies on the mechanism and localization demonstrate that renal uptake of radiolabelled somatostatin analogues largely depends on the megalin/cubulin system in the proximal tubule cells. Thus methods are needed that interfere with this reabsorption pathway to achieve kidney protection. Such methods include coadministration of basic amino acids, the bovine gelatin-containing solution Gelofusine or albumin fragments. Amino acids are already commonly used in the clinical setting during PRRT. Other compounds that interfere with renal reabsorption capacity (maleic acid and colchicine) are not suitable for clinical use because of potential toxicity. The safe limit for the renal radiation dose during PRRT is not exactly known. Dosimetry studies applying the principle of the biological equivalent dose (correcting for the effect of dose fractionation) suggest that a dose of about 37 Gy is the threshold for development of kidney toxicity. This threshold is lower when risk factors for development of renal damage exist: age over 60 years, hypertension, diabetes mellitus and previous chemotherapy. A still experimental pathway for kidney protection is mitigation of radiation effects, possibly achievable by cotreatment with amifostine (Ethylol), a radiation protector, or with blockers of the renin-angiotensin-aldosterone system. Future perspectives on improving kidney protection during PRRT include combinations of agents to reduce renal retention of radiolabelled peptides, eventually together with mitigating medicines. Moreover, new somatostatin analogues with lower renal retention may be developed. Furthermore, knowledge on kidney protection from radiolabelled somatostatin analogues may be expanded to other peptides.