Amifostine protects rat kidneys during peptide receptor radionuclide therapy with [177Lu-DOTA0,Tyr3]octreotate

Application of Cleavable Linkers to Improve Therapeutic Index of Radioligand Therapies

Radioligand therapy (RLT) is an emergent drug class for cancer treatment. The dose administered to cancer patients is constrained by the radiation exposure to normal tissues to maintain an appropriate therapeutic index. When a radiopharmaceutical or its radiometabolite is retained in the kidneys, radiation dose deposition in the kidneys can become a dose-limiting factor. A good exemplar is [¹⁷⁷Lu]Lu-DOTATATE, where patients receive a co-infusion of basic amino acids for nephroprotection. Besides peptides, there are other classes of targeting vectors like antibody fragments, antibody mimetics, peptidomimetics, and small molecules that clear through the renal pathway. In this review, we will review established and emerging strategies that can be used to mitigate radiation-induced nephrotoxicity, with a focus on the development and incorporation of cleavable linkers for radiopharmaceutical designs. Finally, we offer our perspectives on cleavable linkers for RLT, highlighting future areas of research that will help advance the technology.

Overcoming nephrotoxicity in peptide receptor radionuclide therapy using [177Lu]Lu-DOTA-TATE for the treatment of neuroendocrine tumours

Peptide receptor radionuclide therapy (PRRT) is used for the treatment of patients with unresectable or metastasized somatostatin receptor type 2 (SSTR2)-expressing gastroenteropancreatic neuroendocrine tumours (GEP-NETs). The radiolabelled somatostatin analogue [177Lu]Lu-DOTA-TATE delivers its radiation dose to SSTR2-overexpressing tumour cells, resulting in selective cell killing during radioactive decay. While tumour control can be achieved in many patients, complete remissions remain rare, causing the majority of patients to relapse after a certain period of time. This raises the question whether the currently fixed treatment regime (4 × 7.4 GBq) leaves room for dose escalation as a means of improving therapy efficacy. The kidneys have shown to play an important role in defining a patient's tolerability to PRRT. As a consequence of the proximal tubular reabsorption of [177Lu]Lu-DOTA-TATE, via the endocytic megalin/cubilin receptor complex, the radionuclides are retained in the renal interstitium. This results in extended retention of radioactivity in the kidneys, generating a risk for the development of radiation nephropathy. In addition, a decreased kidney function has shown to be associated with a prolonged circulation of [177Lu]Lu-DOTA-TATE, causing increased irradiation to the bone marrow. This can on its turn lead to myelosuppression and haematological toxicity, owing to the marked radio sensitivity of the rapidly proliferating cells in the bone marrow. In contrast to external beam radiotherapy (EBRT), the exact absorbed dose limits for these critical organs (kidneys and bone marrow) in PRRT with [177Lu]Lu-DOTA-TATE are still unclear. Better insights into these uncertainties, can help in optimizing PRRT to reach its maximum therapeutic potential, while avoiding severe adverse events, like nephropathy and hematologic toxicities. In this review we focus on the nephrotoxic effects of PRRT with [177Lu]Lu-DOTA-TATE for the treatment of GEP-NETs. If the absorbed dose to the kidneys can be lowered, higher activities can be administered, enlarging the therapeutic window for PRRT. Therefore, we evaluated the renal protective potential of current and promising future strategies and discuss the importance of (renal) dosimetry in PRRT.

Evaluation of the PSMA-Binding Ligand 212Pb-NG001 in Multicellular Tumour Spheroid and Mouse Models of Prostate Cancer

Radioligand therapy targeting the prostate-specific membrane antigen (PSMA) is rapidly evolving as a promising treatment for metastatic castration-resistant prostate cancer. The PSMA-targeting ligand p-SCN-Bn-TCMC-PSMA (NG001) labelled with ²¹²Pb efficiently targets PSMA-positive cells in vitro and in vivo. The aim of this preclinical study was to evaluate the therapeutic potential of ²¹²Pb-NG001 in multicellular tumour spheroid and mouse models of prostate cancer. The cytotoxic effect of ²¹²Pb-NG001 was tested in human prostate C4-2 spheroids. Biodistribution at various time points and therapeutic effects of different activities of the radioligand were investigated in male athymic nude mice bearing C4-2 tumours, while long-term toxicity was studied in immunocompetent BALB/c mice. The radioligand induced a selective cytotoxic effect in spheroids at activity concentrations of 3–10 kBq/mL. In mice, the radioligand accumulated rapidly in tumours and was retained over 24 h, while it rapidly cleared from nontargeted tissues. Treatment with 0.25, 0.30 or 0.40 MBq of ²¹²Pb-NG001 significantly inhibited tumour growth and improved median survival with therapeutic indexes of 1.5, 2.3 and 2.7, respectively. In BALB/c mice, no signs of long-term radiation toxicity were observed at activities of 0.05 and 0.33 MBq. The obtained results warrant clinical studies to evaluate the biodistribution, therapeutic efficacy and toxicity of ²¹²Pb-NG001.

Toxicity of a combined therapy using the mTOR-inhibitor everolimus and PRRT with [177Lu]Lu-DOTA-TATE in Lewis rats

PURPOSE

Peptide receptor radionuclide therapy (PRRT) with [177Lu]Lu-DOTA0,TYR3-octreotate ([177Lu]Lu-DOTA-TATE) and the mechanistic target of rapamycin (mTOR) inhibitor everolimus are both approved for the treatment of neuroendocrine tumours (NET). However, tumour progression is still frequent, and treatment strategies need further improvement. One possible approach could be to combine different therapy options. In this study, we investigated the toxicity of a combined treatment with everolimus and [177Lu]Lu-DOTA-TATE in female Lewis rats.

METHODS

Animals received 200 MBq of [177Lu]Lu-DOTA-TATE once and/or 5 mg/kg body weight everolimus or placebo weekly for 16 weeks and were divided into four groups (group 1, placebo; group 2, everolimus; group 3, placebo + [177Lu]Lu-DOTA-TATE; group 4, everolimus + [177Lu]Lu-DOTA-TATE). Blood levels of creatinine and blood urea nitrogen (BUN) were assessed weekly to monitor nephrotoxicity, and a full blood count was performed at the time of euthanasia to monitor myelotoxicity. Additionally, renal function was analysed by sequential [99mTc]Tc-mercaptoacetyltriglycine ([99mTc]Tc-MAG3) scintigraphies. Histopathological examination was performed in all the kidneys using a standardized renal damage score (RDS).

RESULTS

Rats receiving everolimus showed a significantly lower increase in creatinine levels than those receiving placebo. Everolimus therapy reduced white blood count significantly, which was not observed for [177Lu]Lu-DOTA-TATE. Functional renal scintigraphies using [99mTc]Tc-MAG3 showed a compromised initial tracer uptake after PRRT and slower but still preserved excretion after everolimus. Histology showed no significant RDS differences between groups.

CONCLUSION

Renal scintigraphy is a highly sensitive tool for the detection of renal function impairment after a combination of everolimus and PRRT. Additional treatment with everolimus does not increase renal and haematological toxicity of PRRT with [177Lu]Lu-DOTA-TATE.

Effects of l-Carnitine on the Follicle-Stimulating Hormone, Luteinizing Hormone, Testosterone, and Testicular Tissue Oxidative Stress Levels in Streptozotocin-Induced Diabetic Rats

The present study was designed to investigate the antioxidant property of l-carnitine (LC) on serum levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH), testosterone (TH) and testis oxidative stress in streptozotocin (STZ)-induced diabetic rats. The rats were divided into the following groups: group I, control; group II, LC 100 mg/kg/d; group III, diabetic; and groups IV to VI, diabetic rats treated with 50, 100, and 200 mg/kg/d of LC, respectively. Daily injections were given intraperitoneally for 7 weeks. At the end of experimental period, after sacrificing the rats, FSH, LH, TH, total antioxidant capacity (TAC), malondialdehyde (MDA), glutathione (GSH), catalase (CAT), mitochondrial function (MTT), protein carbonyl (PC), and reactive oxygen species (ROS) levels were measured. STZ caused an elevation of MDA, ROS, and PC (P < .001) with reduction of GSH, CAT, TAC, and MTT (P < .001) in the serum levels. Group VI had significantly increased FSH, LH, and TH levels versus the untreated diabetic group (P < .001). Although groups V and VI significantly decreased MDA (P < .001), PC (P < .01), and ROS (P < .01) compared with the untreated diabetic group; only in group VI, the activity of GSH (P < .001), CAT (P < .01), TAC (P < .001), and MTT (P < .001) significantly increased. The results of the present study suggest that LC decreased diabetes-induced oxidative stress complications and also improved serum level of FSH, LH, and TH by reducing levels of lipid peroxidation and increasing antioxidant enzymes.

Theranostics of Neuroendocrine Tumors

Somatostatin receptor positron emission tomography/computed tomography using ⁶⁸Ga-labeled somatostatin analogs is the mainstay for the evaluation of receptor status in neuroendocrine tumors (NETs). This translates towards better therapy options, with increasing evidence of peptide receptor radionuclide therapy (PRRT) as the treatment of choice for advanced or progressive NETs. There are benefits in progression-free and overall survival as well as a significant improvement in clinical condition. In patients with progressive NETs, fractionated, personalized PRRT results in good therapeutic responses with no significant severe hematological and/or renal toxicity, thus improving quality of life.

A Short-Term Biological Indicator for Long-Term Kidney Damage after Radionuclide Therapy in Mice

Folate receptor (FR)-targeted radionuclide therapy using folate radioconjugates is of interest due to the expression of the FR in a variety of tumor types. The high renal accumulation of radiofolates presents, however, a risk of radionephropathy. A potential option to address this challenge would be to use radioprotectants, such as amifostine. Methods for early detection of kidney damage that—in this case—cannot be predicted based on dose estimations, would facilitate the development of novel therapies. The aim of this study was, therefore, to assess potentially changing levels of plasma and urine biomarkers and to determine DNA damage at an early stage after radiofolate application. The identification of an early indicator for renal damage in mice would be useful since histological changes become apparent only several months after treatment. Mice were injected with different quantities of ¹⁷⁷Lu-folate (10 MBq, 20 MBq and 30 MBq), resulting in mean absorbed kidney doses of ~23 Gy, ~46 Gy and ~69 Gy, respectively, followed by euthanasia two weeks (>85% of the mean renal radiation dose absorbed) or three months later. Whereas all investigated biomarkers remained unchanged, the number of γ-H2AX-positive nuclei in the renal cortex showed an evident dose-dependent increase as compared to control values two weeks after treatment. Comparison with the extent of kidney injury determined by histological changes five to eight months after administration of the same ¹⁷⁷Lu-folate activities suggested that the quantitative assessment of double-strand breaks can be used as a biological indicator for long-term radiation effects in the kidneys. This method may, thus, enable faster assessment of radiopharmaceuticals and protective measures by preventing logistically challenging long-term investigations to detect kidney damage.

Renal Function Assessment During Peptide Receptor Radionuclide Therapy

Theranostics labeled with Y-90 or Lu-177 are highly efficient therapeutic approaches for the systemic treatment of various cancers including neuroendocrine tumors and prostate cancer. Peptide receptor radionuclide therapy (PRRT) has been used for many years for metastatic or inoperable neuroendocrine tumors. However, renal and hematopoietic toxicities are the major limitations for this therapeutic approach. Kidneys have been considered as the "critical organ" because of the predominant glomerular filtration, tubular reabsorption by the proximal tubules, and interstitial retention of the tracers. Severe nephrotoxity, which has been classified as grade 4-5 based on the "Common Terminology Criteria on Adverse Events," was reported in the range from 0%-14%. There are several risk factors for renal toxicity; patient-related risk factors include older age, preexisting renal disease, hypertension, diabetes mellitus, previous nephrotoxic chemotherapy, metastatic lesions close to renal parenchyma, and single kidney. There are also treatment-related issues, such as choice of radionuclide, cumulative radiation dose to kidneys, renal radiation dose per cycle, activity administered, number of cycles, and time interval between cycles. In the literature, nephrotoxicity caused by PRRT was documented using different criteria and renal function tests, from serum creatinine level to more accurate and sophisticated methods. Generally, serum creatinine level was used as a measure of kidney function. Glomerular filtration rate (GFR) estimation based on serum creatinine was preferred by several authors. Most commonly used formulas for estimation of GFR are "Modifications of Diet in Renal Disease" (MDRD) equation and "Cockcroft-Gault" formulas. However, more precise methods than creatinine or creatinine clearance are recommended to assess renal function, such as GFR measurements using Tc-99m-diethylenetriaminepentaacetic acid (DTPA), Cr-51-ethylenediaminetetraacetic acid (EDTA), or measurement of Tc-99m-MAG3 clearance, particularly in patients with preexisting risk factors for long-term nephrotoxicity. Proximal tubular reabsorption and interstitial retention of tracers result in excessive renal irradiation. Coinfusion of positively charged amino acids, such as l-lysine and l-arginine, is recommended to decrease the renal retention of the tracers by inhibiting the proximal tubular reabsorption. Furthermore, nephrotoxicity may be reduced by dose fractionation. Patient-specific dosimetric studies showed that renal biological effective dose of <0Gy was safe for patients without any risk factors. A renal threshold value <28Gy was recommended for patients with risk factors. Despite kidney protection, renal function impairment can occur after PRRT, especially in patients with risk factors and high single or cumulative renal absorbed dose. Therefore, patient-specific dosimetry may be helpful in minimizing the renal absorbed dose while maximizing the tumor dose. In addition, close and accurate renal function monitoring using more precise methods, rather than plasma creatinine levels, is essential to diagnose the early renal functional changes and to follow-up the renal function during the treatment.

Nephroprotective effects of enalapril after [177Lu]-DOTATATE therapy using serial renal scintigraphies in a murine model of radiation-induced nephropathy

Background

Radiation-induced nephropathy is still dose limiting in radionuclide therapy of neuroendocrine tumors. We investigated the nephroprotective potential of the angiotensine converting enzyme inhibiting drug enalpril after [177Lu]-DOTATATE therapy in a murine model of radiation-induced nephropathy by renal scintigraphy.

At first, the appropriate therapy activity to induce nephropathy was identified. Baseline scintigraphy (n = 12) entailed 12-min dynamic acquisitions after injection of 25 MBq [99mTc]-MAG3, which was followed by radionuclide therapy at four escalating activities of [177Lu]-DOTATATE: group (Gp) 1: 10 MBq; Gp 2: 20 MBq; Gp 3: 40 MBq; Gp 4: 65 MBq. Follow-up [99mTc]-MAG3 scintigraphy was carried out at days 9, 23, 44, and 65. The treatment activity for the intervention arm was selected on the basis of histological examination and declining renal function. In the second part, daily administration by gavage of 10 mg/kg/d enalapril or water (control group) was initiated on the day of radionuclide therapy. Follow-up scintigraphy was carried out at days 9, 23, 44, 65, and 86. We also created a non-therapy control group to detect therapy-independent changes of renal function over time. For all scintigraphies, mean renogram curves were analyzed and the “fractional uptake rate” (FUR; %I.D./min ± SEM) of the tracer by the kidneys was calculated as an index of renal clearance.

Results

At day 65 of follow-up, no significant change in the FUR relative to baseline (11.0 ± 0.3) was evident in radionuclide therapy groups 1 (11.2 ± 0.5) and 2 (10.1 ± 0.6), but FUR was significantly reduced in groups 3 (8.93 ± 0.6, p < 0.05) and 4 (6.0 ± 0.8, p < 0.01); we chose 40 MBq [177Lu]-DOTATATE (Gp 3) for the intervention study. Here, at the last day of follow-up (day 86), FUR was unaltered in enalapril-treated mice (11.8 ± 0.5) relative to the baseline group (12.4 ± 0.3) and non-therapy group (11.9 ± 0.8), whereas FUR in the control group had undergone a significant decline (9.3 ± 0.5; p < 0.01). Histological examination revealed prevention of kidney damage by enalapril treatment.

Conclusions

Treatment with enalapril is effective for nephroprotection during radionuclide therapy with [177Lu]-DOTATATE in mice. Although these results are only limitedly transferable to human studies, enalapril might serve as a promising drug in the mitigation of nephropathy following treatment with [177Lu]-DOTATATE.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-016-0219-2) contains supplementary material, which is available to authorized users.

Peptides in receptor-mediated radiotherapy: from design to the clinical application in cancers

Short peptides can show high affinity for specific receptors overexpressed on tumor cells. Some of these are already used in cancerology as diagnostic tools and others are in clinical trials for therapeutic applications. Therefore, peptides exhibit great potential as a diagnostic tool but also as an alternative or an additional antitumoral approach upon the covalent attachment of a therapeutic moiety such as a radionuclide or a cytotoxic drug. The chemistry offers flexibility to graft onto the targeting-peptide either fluorine or iodine directly, or metallic radionuclides through appropriate chelating agent. Since short peptides are straightforward to synthesize, there is an opportunity to further improve existing peptides or to design new ones for clinical applications. However, several considerations have to be taken into account to optimize the recognition properties of the targeting-peptide to its receptor, to improve its stability in the biological fluids and its residence in the body, or to increase its overall therapeutic effect. In this review, we highlight the different aspects which need to be considered for the development of an efficient peptide receptor-mediated radionuclide therapy in different neoplasms.

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.

Radiolabeled peptides: valuable tools for the detection and treatment of cancer

Human cancer cells overexpress many peptide receptors as molecular targets. Radiolabeled peptides that bind with high affinity and specificity to the receptors on tumor cells hold great potential for both diagnostic imaging and targeted radionuclide therapy. The advantage of solid-phase peptide synthesis, the availability of different chelating agents and prosthetic groups and bioconjugation techniques permit the facile preparation of a wide variety of peptide-based targeting molecules with diverse biological and tumor targeting properties. Some of these peptides, including somatostatin, bombesin, vasoactive intestinal peptide, gastrin, neurotensin, exendin and RGD are currently under investigation. It is anticipated that in the near future many of these peptides may find applications in nuclear oncology. This article presents recent developments in the field of small peptides, and their applications in the diagnosis and treatment of cancer.

Reduction of renal uptake of radiolabeled octreotate by amifostine coadministration

Megalin-mediated renal retention of radiolabeled somatostatin analogs may lead to nephrotoxicity during peptide receptor radionuclide therapy (PRRT). The cytoprotective agent amifostine protected rats from long-term nephrotoxicity after PRRT with (177)Lu-DOTA,Tyr(3)-octreotate. This study describes the direct effect of amifostine on kidney and tumor uptake of (111)In-DOTA,Tyr(3)-octreotate.

METHODS

In vivo biodistribution studies were performed using CA20948 tumor-bearing rats, with or without amifostine coadministration, via several routes. In vitro uptake was studied in somatostatin receptor-expressing CA20948 and megalin or cubilin receptor-expressing BN-16 cells, in the absence or presence of amifostine or its active metabolite WR-1065.

RESULTS

Coadministration of amifostine decreased renal uptake of radiolabeled octreotate in vivo, whereas tumor uptake was not affected. In agreement, amifostine and WR-1065 coincubation reduced uptake in BN-16 but not in CA20948 cells.

CONCLUSION

Amifostine may provide renal protection during PRRT using somatostatin analogs, both by mitigation of radiation damage and the currently observed reduction of absorbed kidney radiation dose.

Nephrotoxicity profiles and threshold dose values for [177Lu]-DOTATATE in nude mice

INTRODUCTION

In peptide receptor radionuclide therapy for neuroendocrine tumors the main dose-limiting tissue is found in the kidneys because of tubular reabsorption and retention of radioactivity. The aim of this study was to quantify late effects in renal cortex of nude mice exposed to high amounts of [(177)Lu]-DOTA-Tyr(3)-octreotate ([(177)Lu]-DOTATATE), and to determine whether a threshold dose value exists for these findings.

METHODS

Nude mice were exposed to 90, 120 or 150 MBq of [(177)Lu]-DOTATATE. Renal toxicity was evaluated up to 6 months after injection. Blood samples were collected to examine renal functional markers, and after sacrifice at 6 months changes in renal morphology were explored. Tissue damage was estimated by quantifying the relative area of the different subunits in the renal cortex using point counting. Additional morphological signs of radiation damage were also noted. The absorbed doses to the kidneys were estimated by previously determined kidney pharmacokinetics and Monte Carlo simulations for different assumptions regarding the activity distribution.

RESULTS

Increased serum creatinine and urea values indicated long-term renal toxicity. The tissue area occupied by proximal tubules decreased with increasing doses of [(177)Lu]-DOTATATE, whereas the other subunits in cortex slightly increased. The mean absorbed dose in the renal cortex for [(177)Lu]-DOTATATE was estimated to be 35-58 Gy for the different groups of animals. A dose-response relationship was observed for proximal tubular damage, and a threshold dose value of 24 Gy (BED 37 Gy) was determined.

CONCLUSIONS

Selective morphological changes in kidney cortex of nude mice were quantified and appeared in a dose dependent manner after injection of high amounts of [(177)Lu]-DOTATATE.

Targeted radiotherapy with radiolabeled somatostatin analogs

Targeted radiopeptide therapy with (90)Yttrium- or (177)Lutetium-labeled somatostatin analogs has been proven to improve significantly quality of life and survival in patients suffering from metastatic or unresectable neuroendocrine tumors (NETs). Roughly 25% of patients achieve partial remission; progression-free survival is estimated to be 30 to 40 months. A wide range of protocols using different somatostatin analogs, isotopes, injected activity per cycle of administration, and number of cycles are reported. More patient-based therapy protocols are under development, taking into consideration the complexity of NET cell biology, dosimetric issues, and the availability of different radiolabeled analogs. This article reviews the effectiveness and safety of the different protocols and discusses several clinical algorithms used in an attempt to optimize targeted radiopeptide therapy.

177Lu-immunotherapy of experimental peritoneal carcinomatosis shows comparable effectiveness to 213Bi-immunotherapy, but causes toxicity not observed with 213Bi

PURPOSE

(213)Bi-d9MAb-immunoconjugates targeting gastric cancer cells have effectively cured peritoneal carcinomatosis in a nude mouse model following intraperitoneal injection. Because the β-emitter (177)Lu has proven to be beneficial in targeted therapy, (177)Lu-d9MAb was investigated in this study in order to compare its therapeutic efficacy and toxicity with those of (213)Bi-d9MAb.

METHODS

Nude mice were inoculated intraperitoneally with HSC45-M2 gastric cancer cells expressing d9-E-cadherin and were treated intraperitoneally 1 or 8 days later with different activities of specific (177)Lu-d9MAb immunoconjugates targeting d9-E-cadherin or with nonspecific (177)Lu-d8MAb. Therapeutic efficacy was evaluated by monitoring survival for up to 250 days. For evaluation of toxicity, both biodistribution of (177)Lu-d9MAb and blood cell counts were determined at different time points and organs were examined histopathologically.

RESULTS

Treatment with (177)Lu-immunoconjugates (1.85, 7.4, 14.8 MBq) significantly prolonged survival. As expected, treatment on day 1 after tumour cell inoculation was more effective than treatment on day 8, and specific (177)Lu-d9MAb conjugates were superior to nonspecific (177)Lu-d8MAb. Treatment with 7.4 MBq of (177)Lu-d9MAb was most successful, with 90% of the animals surviving longer than 250 days. However, treatment with therapeutically effective activities of (177)Lu-d9MAb was not free of toxic side effects. In some animals lymphoblastic lymphoma, proliferative glomerulonephritis and hepatocarcinoma were seen but were not observed after treatment with (213)Bi-d9MAb at comparable therapeutic efficacy.

CONCLUSION

The therapeutic efficacy of (177)Lu-d9MAb conjugates in peritoneal carcinomatosis is impaired by toxic side effects. Because previous therapy with (213)Bi-d9MAb revealed comparable therapeutic efficacy without toxicity it should be preferred for the treatment of peritoneal carcinomatosis.

Preclinical and clinical studies of peptide receptor radionuclide therapy

In the 1980s, the (111)In-labeled somatostatin analog OctreoScan (Covidien, Hazelwood, MO) was developed for imaging of somatostatin receptor subtype 2 (sst(2)) overexpressing tumors. On the basis of this success, peptide receptor radionuclide therapy (PRRT) was developed using similar somatostatin analogs with different therapeutic radionuclides. Clinical application of PRRT demonstrated impressive results on tumor response, overall survival, and quality of life in patients with gastroenteropancreatic neuroendocrine tumors. The peptides 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), Tyr(3)-octreotate (DOTATATE) and DOTA, Tyr(3)-octreotide (DOTATOC) (brand name Onalta), predominantly targeting sst(2), have been granted Orphan Drug status by the European Medicines Agency and the US Food and Drug Administration for application in PRRT. Besides somatostatin receptor-targeting peptides, multiple other radiopeptide analogs were developed targeting several other receptors overexpressed on various tumors. Some of these peptide analogs, including cholecystokinin, gastrin, gastrin-releasing peptide, arginine-glycine-aspartate (RGD)-peptides, and glucagon-like peptide 1 analogs appeared very promising in preclinical and clinical imaging and PRRT studies. Although the success of PRRT with radiolabeled somatostatin analogs has been established, there is still room for improvement. The therapeutic window of PRRT could be enlarged by the use of new and improved targeting compounds, of which new antagonists with excellent tumor to background ratios are very promising. Furthermore, locoregional administration, improved healthy tissue protection, and combination treatment can be applied to increase the effectiveness of PRRT. Combination treatment might include cocktails of different peptide analogs of different therapeutic radionuclides and of radiolabeled peptides with chemotherapeutic or radiosensitizing agents. This review summarizes results of PRRT and describes clinical and preclinical studies regarding PRRT optimizing strategies.

Recombinant bispecific monoclonal antibodies prepared by the dock-and-lock strategy for pretargeted radioimmunotherapy

The selective delivery of therapeutic radionuclides is a promising approach for treating cancer. Antibody-targeted radionuclides are of particular interest, with 2 products approved for the treatment of certain forms of non-Hodgkin lymphoma. However, for many other cancers, radioimmunotherapy has been ineffective, being limited by prolonged exposure to the highly radiosensitive bone marrow. An alternative approach, known as pretargeting, separates radionuclide from the antibody, allowing the radiation to be delivered on a small molecule that can quickly and efficiently migrate into the tumor, and then rapidly clear from the body with minimal retention in tissues. Several pretargeting methods have been developed that differ in the way they selectively capture the radionuclide. This review focuses on the development of a novel form of bispecific monoclonal antibody (bsMAb) pretargeting that uses a unique radiolabeled hapten-peptide system that can be modified to bind numerous therapeutic and imaging radionuclides. Together with a specialized recombinant humanized bsMAb prepared with by a technique known as the Dock-and-Lock method, this pretargeting procedure has been examined in many different animal models, showing a high level of sensitivity and specificity for localizing tumors, and improved efficacy with less hematologic toxicity associated with directly radiolabeled IgG. The bsMAb is a tri-Fab structure, having 2 binding arms for the tumor antigen and 1 capable of binding a hapten-peptide. Preclinical studies were preformed to support the clinical use of a bsMAb and a hapten-peptide bearing a single DOTA moiety (IMP-288). A phase 0 trial found an (131)I-tri-Fab bsMAb, TF2, that targets carcinoembryonic antigen was stable in vivo, quickly clears from the blood, and localizes known tumors. The first-in-patient pretargeting experience with the (111)In-IMP-288 also observed rapid clearance and low tissue (kidney) retention, as well as localization of tumors, providing initial promising evidence for developing these materials for radioimmunotherapy.

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.

Tumor imaging and therapy using radiolabeled somatostatin analogues

Molecular imaging plays an essential role in balancing the clinical benefits and risks of radionuclide-based cancer therapy. To effectively treat individual patients, careful assessment of biodistribution, dosimetry, and toxicity is essential. In this Account, we describe advances that combine features of molecular imaging and radionuclide therapy to provide new avenues toward individualized cancer treatment. Selective receptor-targeting radiopeptides have emerged as an important class of radiopharmaceuticals for molecular imaging and therapy of tumors that overexpress peptide receptors on the cell membrane. After such peptides labeled with gamma-emitting radionuclides bind to their receptors, they allow clinicians to visualize receptor-expressing tumors non-invasively. Peptides labeled with beta-particle emitters could also eradicate receptor-expressing tumors. The somatostatin receptors, which are overexpressed in a majority of neuroendocrine tumors, represent the first and best example of targets for radiopeptide-based imaging and radionuclide therapy. The somatostatin analogue (111)In-octreotide permits the localization and staging of neuroendocrine tumors that express the appropriate somatostatin receptors. Newer modified somatostatin analogues, including Tyr(3)-octreotide and Tyr(3)-octreotate, are successfully being used for tumor imaging and radionuclide therapy. Because there are few effective therapies for patients with inoperable or metastasized neuroendocrine tumors, this therapy is a promising novel treatment option for these patients. Peptide receptor imaging and radionuclide therapy can be combined in a single probe, called a "theranostic". To select patients who are likely to benefit from this type of intervention, we first use a peptide analogue labeled with a diagnostic radionuclide to obtain a scan. Selected patients will be treated using the same or a similar peptide analogue labeled with a therapeutic radionuclide. The development of such theranostics could greatly advance the development of personalized treatments. Apart from patient selection for radionuclide therapy, other imaging applications of targeted radiopeptides include localization of primary tumors, detection of metastatic disease (staging/restaging), dosimetry (prediction of response and radiotoxicity), monitoring effects of surgery, radio(nuclide)therapy or chemotherapy, and detection of progression of disease or relapse (follow up). For further evaluation of tumor receptor expression and to increase the value of cancer targeting using radiopeptides, researchers have introduced and evaluated different radiolabeled analogues of other peptide families, such as cholecystokinin (CCK), gastrin, bombesin, substance P, vasoactive intestinal peptide (VIP), and neuropeptide (NP)-Y analogues. We expect improvements in the development of new peptide analogues: such advances could reduce side effects and allow for the use of combination therapy (for example, combining radiopeptide analogues with chemotherapeutics).

Peptide-receptor radionuclide therapy for endocrine tumors

Peptide-receptor radionuclide therapy (PRRT) with radiolabeled somatostatin analogs is a promising option for the treatment of somatostatin-receptor-positive endocrine tumors. Treatment with somatostatin analogs labeled with (111)In, (90)Y or (177)Lu can result in symptomatic improvement, although tumor remission is seldom achieved with (111)In-labeled analogs. In this Review, the findings of several studies on the use of PRRT for endocrine tumors are evaluated. Large variation in the antitumor effects of (90)Y-octreotide was reported between studies: an objective response (> or =50% tumor regression) was achieved in 9-33% of patients. After treatment with (177)Lu-octreotate, an objective response was achieved in 29% of patients and a minor response (25-50% tumor regression) was achieved in 16% of patients; stable disease was present in 35% of patients. Treatment with (177)Lu-octreotate resulted in a survival benefit of several years and markedly improved quality of life. Serious, delayed adverse effects were rare after PRRT. Although randomized, clinical trials have not yet been performed, data on the use of PRRT compare favorably with those from other treatment approaches, such as chemotherapy. If these results can be replicated in large, controlled trials, PRRT might become the preferred option in patients with metastatic or inoperable gastroenteropancreatic neuroendocrine tumors.

[(90)Yttrium-DOTA]-TOC response is associated with survival benefit in iodine-refractory thyroid cancer: long-term results of a phase 2 clinical trial

BACKGROUND

The authors aimed to explore the efficacy of (90)Yttrium-1,4,7,10-tetra-azacyclododecane N,N',N'',N'''-tetraacetic acid [(90)Y-DOTA]-Tyr(3)-octreotide (TOC) in advanced iodine-refractory thyroid cancer.

METHODS

In a phase 2 trial, the authors investigated biochemical response (assessed by serum thyroglobulin levels), survival, and the long-term safety profile of systemic [(90)Y-DOTA]-TOC treatment in metastasized iodine-refractory thyroid cancer. Adverse events were assessed according to the National Cancer Institute criteria. Survival analyses were performed by using multiple regression models.

RESULTS

A total of 24 patients were enrolled. A median cumulative activity of 13.0 GBq (range, 1.7-30.3 GBq) was administered. Response was found in 7 (29.2%) patients. Eight (33.3%) patients developed hematologic toxicity grade 1-3, and 4 (16.7%) patients developed renal toxicity grade 1-4. The median survival was 33.4 months (range, 3.6-126.8 months) from time of diagnosis and 16.8 months (range, 1.8-99.1 months) from time of first [(90)Y-DOTA]-TOC treatment. Response to treatment was associated with longer survival from time of diagnosis (hazard ratio [HR], 0.17; 95% confidence interval [CI], 0.03-0.92; P = .04) and from time of first [(90)Y-DOTA]-TOC therapy (HR, 0.20; 95% CI, 0.04-0.94; P = .04). The visual grade of scintigraphic tumor uptake was not associated with treatment response (odds ratio [OR], 0.98; 95% CI, 0.26-3.14; P = 1.00).

CONCLUSIONS

Response to [(90)Y-DOTA]-TOC in metastasized iodine-refractory thyroid cancer was associated with longer survival. Upcoming trials should aim to increase the number of treatment cycles.

Renal uptake and metabolism of radiopharmaceuticals derived from peptides and proteins

Radiolabeled anti-CD20 antibodies have demonstrated impressive efficacy in the treatment of relapsed non-Hodgkin lymphoma. This encourages the treatment of solid tumor with radiolabeled antibody fragments and peptides. However, both preclinical and clinical studies revealed that persistent localization of radioactivity in the kidney constitutes a major obstacle that compromises therapeutic efficacy. Recent extensive studies show that long residence times of radiolabeled end products from lysosomes are responsible for the renal radioactivity levels. Recent studies have also elucidated the involvement of megalin-cubilin in renal tubular reabsorption of radiolabeled antibody fragments and peptides. In light of these findings, efforts are being made to block tubular reabsorption of radiolabeled antibody fragments and peptides by competitive inhibitors, charge modification, and PEGylation. An interposition of an enzyme-cleavable linkage between antibody fragments and radiolabels would constitute an alternative approach to reduce renal radioactivity levels. Recent findings of these studies will be described.

Targeted nanomaterials for radiotherapy

Nanomaterials have garnered increasing interest recently as potential therapeutic drug-delivery vehicles. Among the existing nanomaterials are the pure carbon-based particles, such as fullerenes and nanotubes, various organic dendrimers, liposomes and other polymeric compounds. These vehicles have been decorated with a wide spectrum of target-reactive ligands, such as antibodies and peptides, which interact with cell-surface tumor antigens or vascular epitopes. Once targeted, these new nanomaterials can then deliver radioisotopes or isotope generators to the cancer cells. Here, we will review some of the more common nanomaterials under investigation and their current and future applications as drug-delivery scaffolds with particular emphasis on targeted cancer radiotherapy.

Neuroendocrine tumors. Peptide receptor radionuclide therapy

Peptide receptor radionuclide therapy with radiolabelled somatostatin analogues is an emerging and convincing treatment modality for patients with unresectable, somatostatin-receptor-positive neuroendocrine tumours. Using radiolabelled somatostatin analogues for imaging became the gold standard for staging of neuroendocrine tumours. The somatostatin receptor is strongly over-expressed in most tumours, resulting in high tumour-to-background ratios. Consequently, the next step was to try to treat these patients by increasing the radioactivity of the administered radiolabelled somatostatin analogue in an attempt to bring about tumour cure. Many patients have been treated successfully with this approach, roughly 25% of them achieving objective tumour shrinkage >50%. Serious side-effects have been rare. This article reviews the effectiveness and safety of the different radiolabelled somatostatin analogues used. Furthermore, clinical issues--including indication and timing of therapy--are discussed. Finally, important directions for future research are mentioned to illustrate new strategies for increasing therapy efficacy.