Long-term toxicity of [(177)Lu-DOTA (0),Tyr (3)]octreotate in rats

225Ac-Labeled Somatostatin Analogs in the Management of Neuroendocrine Tumors: From Radiochemistry to Clinic

The widespread use of peptide receptor radionuclide therapy (PRRT) represents a major therapeutic breakthrough in nuclear medicine, particularly since the introduction of ¹⁷⁷Lu-radiolabeled somatostatin analogs. These radiopharmaceuticals have especially improved progression-free survival and quality of life in patients with inoperable metastatic gastroenteropancreatic neuroendocrine tumors expressing somatostatin receptors. In the case of aggressive or resistant disease, the use of somatostatin derivatives radiolabeled with an alpha-emitter could provide a promising alternative. Among the currently available alpha-emitting radioelements, actinium-225 has emerged as the most suitable candidate, especially regarding its physical and radiochemical properties. Nevertheless, preclinical and clinical studies on these radiopharmaceuticals are still few and heterogeneous, despite the growing momentum for their future use on a larger scale. In this context, this report provides a comprehensive and extensive overview of the development of ²²⁵Ac-labeled somatostatin analogs; particular emphasis is placed on the challenges associated with the production of ²²⁵Ac, its physical and radiochemical properties, as well as the place of ²²⁵Ac-DOTATOC and ²²⁵Ac-DOTATATE in the management of patients with advanced metastatic neuroendocrine tumors.

Evaluation of the Efficacy of a Combined Treatment Using the mTOR-Inhibitor Everolimus and [177Lu]Lu-DOTA-TATE in Nude CD1 Mice with SSTR-Expressing Pancreatic AR42J Xenograft Tumors

Therapy options for advanced pancreatic neuroendocrine tumors (pNETs) include the mTOR inhibitor everolimus and peptide receptor radionuclide therapy (PRRT) with [177Lu]Lu-DOTA-TATE, however further optimization in the therapeutic landscape is required as response rates are still low. In this study, we investigated the synergistic and potentially enhanced efficacy of a combined treatment with everolimus and [177Lu]Lu-DOTA-TATE in a mouse model. Baseline [68Ga]Ga-DOTA-TATE PET scans were obtained five days after athymic CD1 mice were inoculated with AR42J tumor cells, before separating the animals into four groups. Group 1 received a placebo, group 2 everolimus, group 3 a placebo and PRRT, and group 4 everolimus and PRRT. The treatment response was monitored by manually measuring the tumor volumes (manual tumor volume, MTV) and conducting sequential [68Ga]Ga-DOTA-TATE PET scans at one, two, and four weeks after treatment induction. The biological tumor volume (BTV) was derived from PET scans using threshold-based volume of interest (VOI) measurements. Tracer uptake was measured semi-quantitatively as a tumor to background ratio (TBR). Mice were euthanized due to excessive tumor growth according to the ethics protocol; blood samples were drawn for the preparation of full blood counts and kidneys were obtained for histological analysis. For the histological assessment, a standardized score (renal damage score, RDS) was used. Full blood counts showed significantly increased numbers of neutrophils and lymphocytes in the groups receiving PRRT. All other parameters did not differ relevantly. In the histological analysis, groups receiving PRRT had a significantly higher RDS, whereas everolimus only tended to cause an increase in the RDS. Mice in groups 1 and 2 had to be euthanized due to excessive tumor growth two weeks after the start of the therapy, whereas follow-up in groups 3 and 4 comprised four weeks. PRRT significantly inhibited tumor growth; the administration of everolimus did not induce an additional effect. A good correlation existed between MTV and BTV. PRRT significantly reduced the TBR. [68Ga]Ga-DOTA-TATE PET is suitable for monitoring tumor growth in the applied model. The high efficacy of [177Lu]Lu-DOTA-TATE is not enhanced by the combination with everolimus.

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.

The radiosensitizer Onalespib increases complete remission in 177Lu-DOTATATE-treated mice bearing neuroendocrine tumor xenografts

PURPOSE

177Lu-DOTATATE targeting the somatostatin receptor (SSTR) is utilized for treatment of neuroendocrine tumors (NETs). Onalespib, a heat shock protein 90 (HSP90) inhibitor, has demonstrated radiosensitizing properties and may thus enhance the effect of 177Lu-DOTATATE. Consequently, the aim of this study was to assess the potential of Onalespib in combination with 177Lu-DOTATATE in vivo and to examine the toxicity profiles of the treatments.

METHODS

177Lu-DOTATATE selectivity and distribution in NET xenografts were studied using biodistribution and autoradiography. Therapeutic effects of Onalespib in combination with 177Lu-DOTATATE were studied in NET xenografts. Histological analyses were used to assess molecular effects from treatment and to establish toxicity profiles.

RESULTS

Biodistribution and autoradiography confirmed the SSTR-selective tumor uptake of 177Lu-DOTATATE, which was unaffected by Onalespib treatment. Immunohistochemistry verified molecular responses to Onalespib therapy in the tumors. While Onalespib and 177Lu-DOTATATE monotherapies resulted in a 10% and 33% delay in tumor doubling time compared with control, the combination treatment resulted in a 73% delayed tumor doubling time. Moreover, combination treatment increased complete remissions threefold from 177Lu-DOTATATE monotherapy, resulting in 29% complete remissions. In addition, histological analyses demonstrated radiation-induced glomerular injury in the 177Lu-DOTATATE monotherapy group. The damage was decreased tenfold in the combination group, potentially due to Onalespib-induced HSP70 upregulation in the kidneys.

CONCLUSION

Treatment with Onalespib potentiated 177Lu-DOTATATE therapy of NET xenografts with a favorable toxicity profile. Utilizing Onalespib's radiosensitizing properties with 177Lu-DOTATATE may lead to better therapeutic results in the future and may reduce unwanted side effects in dose-limiting organs.

Radioactive Main Group and Rare Earth Metals for Imaging and Therapy

Radiometals possess an exceptional breadth of decay properties and have been applied to medicine with great success for several decades. The majority of current clinical use involves diagnostic procedures, which use either positron-emission tomography (PET) or single-photon imaging to detect anatomic abnormalities that are difficult to visualize using conventional imaging techniques (e.g., MRI and X-ray). The potential of therapeutic radiometals has more recently been realized and relies on ionizing radiation to induce irreversible DNA damage, resulting in cell death. In both cases, radiopharmaceutical development has been largely geared toward the field of oncology; thus, selective tumor targeting is often essential for efficacious drug use. To this end, the rational design of four-component radiopharmaceuticals has become popularized. This Review introduces fundamental concepts of drug design and applications, with particular emphasis on bifunctional chelators (BFCs), which ensure secure consolidation of the radiometal and targeting vector and are integral for optimal drug performance. Also presented are detailed accounts of production, chelation chemistry, and biological use of selected main group and rare earth radiometals.

Proof of Therapeutic Efficacy of a 177Lu-Labeled Neurotensin Receptor 1 Antagonist in a Colon Carcinoma Xenograft Model

Increased expression of neurotensin receptor 1 (NTR1) has been shown in a large number of tumor entities such as pancreatic or colon carcinoma. Hence, this receptor is a promising target for diagnostic imaging and radioligand therapy. Using the favorable biodistribution data of the NTR1-targeting agent ¹¹¹In-3BP-227, we investigated the therapeutic effect of its ¹⁷⁷Lu-labeled analog on the tumor growth of NTR1-positive HT29 colon carcinoma xenografts. Methods: 3BP-227 was labeled with ¹⁷⁷Lu. To assess its biodistribution properties, SPECT and CT scans of HT29-xenografted nude mice injected with ¹⁷⁷Lu-3BP-227 were acquired, and ex vivo tissue activity was determined. To evaluate therapeutic efficacy, 2 groups of mice received the radiopharmaceutical in a median dose of either 165 MBq (129-232 MBq, n = 10) or 110 MBq (82-116 MBq, n = 10), whereas control mice were injected with vehicle (n = 10). Tumor sizes and body weights were monitored for up to 49 d. Renal function and histologic morphology were evaluated. Results: Whole-body SPECT/CT images allowed clear tumor visualization with low background activity and high tumor-to-kidney and -liver ratios. Ex vivo biodistribution data confirmed high and persistent uptake of ¹⁷⁷Lu-3BP-227 in HT29 tumors (19.0 ± 3.6 vs. 2.7 ± 1.6 percentage injected dose per gram at 3 and 69 h after injection, respectively). The application of ¹⁷⁷Lu-3BP-227 resulted in a distinct delay of tumor growth. Median tumor doubling time for controls was 5.5 d (interquartile range [IQR], 2.8-7.0), compared with 17.5 d (IQR, 5.5-22.5 d) for the 110-MBq and 41.0 d (IQR, 27.5-55.0) for the 165-MBg group. Compared with controls, median relative tumor volume at day 23 after injection was reduced by 55% (P = 0.034) in the 110-MBq and by 88% (P < 0.01) in the 165-MBq group. Renal histology and clinical chemistry results did not differ between radiotherapy groups and controls, suggesting absence of therapy-induced acute renal damage. Conclusion: These data demonstrate that the novel NTR1-targeting theranostic agent 3BP-227 is an effective and promising candidate for radioligand therapy, with a favorable preliminary safety profile and high potential for clinical translation.

α- Versus β-Emitting Radionuclides for Pretargeted Radioimmunotherapy of Carcinoembryonic Antigen-Expressing Human Colon Cancer Xenografts

Pretargeted radioimmunotherapy (PRIT) with the β-emitting radionuclide ¹⁷⁷Lu is an attractive approach to treat carcinoembryonic antigen (CEA)-expressing tumors. The therapeutic efficacy of PRIT might be improved using α-emitting radionuclides such as ²¹³Bi. Herein, we report and compare the tumor-targeting properties and therapeutic efficacy of ²¹³Bi and ¹⁷⁷Lu for PRIT of CEA-expressing xenografts, using the bispecific monoclonal antibody TF2 (anti-CEA × anti-histamine-succinyl-glycine [HSG]) and the di-HSG-DOTA peptide IMP288. Methods: The in vitro binding characteristics of ²¹³Bi-IMP288 were compared with those of ¹⁷⁷Lu-IMP288. Tumor targeting of ²¹³Bi-IMP288 and ¹⁷⁷Lu-IMP288 was studied in mice bearing subcutaneous LS174T tumors that were pretargeted with TF2. Finally, the effect of ²¹³Bi-IMP288 (6, 12, or 17 MBq) and ¹⁷⁷Lu-IMP288 (60 MBq) on tumor growth and survival was assessed. Toxicity was determined by monitoring body weight, analyzing blood samples for hematologic and renal toxicity (hemoglobin, leukocytes, platelets, creatinine), and immunohistochemical analysis of the kidneys. Results: The in vitro binding characteristics of ²¹³Bi-IMP288 (dissociation constant, 0.45 ± 0.20 nM) to TF2-pretargeted LS174T cells were similar to those of ¹⁷⁷Lu-IMP288 (dissociation constant, 0.53 ± 0.12 nM). In vivo accumulation of ²¹³Bi-IMP288 in LS174T tumors was observed as early as 15 min after injection (9.2 ± 2.0 percentage injected dose [%ID]/g). ²¹³Bi-IMP288 cleared rapidly from the circulation; at 30 min after injection, the blood levels were 0.44 ± 0.28 %ID/g. Uptake in normal tissues was low, except for the kidneys, where uptake was 1.8 ± 1.1 %ID/g at 30 min after injection. The biodistribution of ²¹³Bi-IMP288 was comparable to that of ¹⁷⁷Lu-IMP288. Mice treated with a single dose of ²¹³Bi-IMP288 or ¹⁷⁷Lu-IMP288 showed significant inhibition of tumor growth. Median survival for the groups treated with phosphate-buffered saline, 6 MBq ²¹³Bi-IMP288, 12 MBq ²¹³Bi-IMP288, and 60 MBq ¹⁷⁷Lu-IMP288 was 22, 31, 45, and 42 d, respectively. Mice receiving 17 MBq ²¹³Bi-IMP288 showed significant weight loss, resulting in a median survival of only 24 d. No changes in hemoglobin, platelets, or leukocytes were observed in the treatment groups. However, immunohistochemical analysis of the kidneys of mice treated with 17 or 12 MBq ²¹³Bi-IMP288 showed signs of tubular damage, indicating nephrotoxicity. Conclusion: To our knowledge, this study shows for the first time that PRIT with TF2 and ²¹³Bi-IMP288 is feasible and at least as effective as ¹⁷⁷Lu-IMP288. However, at higher doses, kidney toxicity was observed. Future studies are warranted to determine the optimal dosing schedule to improve therapeutic efficacy while reducing renal toxicity.

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.

Folate receptor-targeted radionuclide therapy: preclinical investigation of anti-tumor effects and potential radionephropathy

INTRODUCTION

Application of therapeutic folate radioconjugates is a promising option for the treatment of folate receptor (FR)-positive tumors, although high uptake of radiofolates in the kidneys remains a critical issue. Recently, it was shown that enhancing the blood circulation of radiofolates results in increased tumor uptake and reduced retention of radioactivity in the kidneys. In this study, we investigated and compared the anti-tumor effects and potential long-term damage to the kidneys after application of an albumin-binding ((177)Lu-cm09), and a conventional ((177)Lu-EC0800) folate radioconjugate.

METHODS

In vivo studies were performed with KB tumor-bearing nude mice. (177)Lu-EC0800 and (177)Lu-cm09 were applied at variable quantities (10-30 MBq/mouse), and the tumor growth was monitored over time. Mice without tumors were injected with the same radiofolates and investigated over eight months by determination of creatinine and blood urea nitrogen plasma levels and by measuring renal uptake of (99m)Tc-DMSA using SPECT. At the study end, the morphological changes were examined on renal tissue sections using variable staining methods.

RESULTS

Compared to untreated controls, dose-dependent tumor growth inhibition and prolonged survival was observed in all treated mice. In line with the resulting absorbed dose, the treatment was more effective with (177)Lu-cm09 than with (177)Lu-EC0800, enabling complete tumor remission after application of ≥20MBq (≥28Gy). Application of radiofolates with an absorbed renal dose ≥23 Gy showed increased levels of renal plasma parameters and reduced renal uptake of (99m)Tc-DSMA. Morphological changes observed on tissue sections confirmed radionephropathy of variable stages.

CONCLUSIONS

(177)Lu-cm09 showed more favorable anti-tumor effects and significantly less damage to the kidneys compared to (177)Lu-EC0800 as was expected based on improved tumor-to-kidney ratios. It was demonstrated that enhancing the blood circulation time of radiofolates was favorable regarding the risk-benefit profile of a therapeutic application. These results hold promise for future translation of the albumin-binder concept to the clinics, potentially enabling FR-targeted radionuclide therapy in patients.

Evaluation of retinol binding protein 4 and carbamoylated haemoglobin as potential renal toxicity biomarkers in adult mice treated with (177)Lu-octreotate

Background

The kidneys are regarded as one of the main dose-limiting organs in the treatment of neuroendocrine tumours with ¹⁷⁷Lu-[DOTA⁰, Tyr³]-octreotate (¹⁷⁷Lu-octreotate), despite the successful use of kidney uptake blocking agents such as lysine and arginine. To avoid renal toxicity but still give each patient as high amount of ¹⁷⁷Lu-octreotate as possible, there is a need for methods/biomarkers that indicate renal injury in an early stage of the treatment. The aim of this study was to investigate the potential of using urinary retinol binding protein 4 (RBP4) and carbamoylated haemoglobin (Hb) in blood as biomarkers of nephrotoxic effects on adult mice after ¹⁷⁷Lu-octreotate treatment.

Methods

Adult BALB/c nude mice were injected with 60 MBq or 120 MBq of ¹⁷⁷Lu-octreotate or with saline (control). Urine was collected before injection and concentrations of urinary RBP4 and creatinine were determined 14 to 90 days after injection Blood samples were collected after 90 days, and carbamoylated N-terminal valine in Hb, formed from urea, was measured as valine hydantoin (VH) after detachment from Hb.

Results

The RBP4 values increased with administered activity and time. For the 60 and 120 MBq groups, statistically significantly higher RBP4 levels (p <0.05) were found at day 60 and 90 compared to baseline, also at day 30 for 120 MBq group. For VH, the mean values were similar for the 60 MBq and control groups, while a small increase was observed for the 120 MBq group; but there were no statistically significant differences between any of the groups (p >0.05). No morphological changes in the kidney tissue were found.

Conclusions

Urinary RBP4 is a promising new biomarker for radiation-induced renal toxicity. For the conditions used in this experiment, carbamoylated Hb (from urea) measured as VH may not be a sufficiently sensitive biomarker to be used for renal toxicity.

Trial registration

ID 326-2008

Therapeutic application of CCK2R-targeting PP-F11: influence of particle range, activity and peptide amount

BACKGROUND

Targeted radionuclide therapy with high-energy beta-emitters is generally considered suboptimal to cure small tumours (<300 mg). Tumour targeting of the CCK2 receptor-binding minigastrin analogue PP-F11 was determined in a tumour-bearing mouse model at increasing peptide amounts. The optimal therapy was analysed for PP-F11 labelled with (90)Y, (177)Lu or (213)Bi, accounting for the radionuclide specific activities (SAs), the tumour absorbed doses and tumour (radio) biology.

METHODS

Tumour uptake of (111)In-PP-F11 was determined in nude mice bearing CCK2 receptor-transfected A431 xenografts at 1 and 4 h post-injection for escalating peptide masses of 0.03 to 15 nmol/mouse. The absorbed tumour dose was estimated, assuming comparable biodistributions of the (90)Y, (177)Lu or (213)Bi radiolabelled peptides. The linear-quadratic (LQ) model was used to calculate the tumour control probabilities (TCP) as a function of tumour mass and growth.

RESULTS

Practically achievable maximum SAs for PP-F11 labelled with (90)Y and (177)Lu were 400 MBq (90)Y/nmol and 120 MBq(177)Lu/nmol. Both the large elution volume from the 220 MBq (225)Ac generator used and reaction kinetics diminished the maximum achieved (213)Bi SA in practice: 40 MBq (213)Bi/nmol. Tumour uptakes decreased rapidly with increasing peptide amounts, following a logarithmic curve with ED50 = 0.5 nmol. At 0.03 nmol peptide, the (300 mg) tumour dose was 9 Gy after 12 MBq (90)Y-PP-F11, and for (111)In and (177)Lu, this was 1 Gy. A curative dose of 60 Gy could be achieved with a single administration of 111 MBq (90)Y labelled to 0.28 nmol PP-F11 or with 4 × 17 MBq (213)Bi (0.41 nmol) when its α-radiation relative biological effectiveness (RBE) was assumed to be 3.4. Repeated dosing is preferable to avoid complete tumour receptor saturation. Tumours larger than 200 mg are curable with (90)Y-PP-F11; the other radionuclides perform better in smaller tumours. Furthermore, (177)Lu is not optimal for curing fast-growing tumours.

CONCLUSIONS

Receptor saturation, specific radiopharmaceutical activities and absorbed doses in the tumour together favour therapy with the CCK2 receptor-binding peptide PP-F11 labelled with (90)Y, despite its longer β-particle range in tissue, certainly for tumours larger than 300 mg. The predicted TCPs are of theoretical nature and need to be compared with the outcome of targeted radionuclide experiments.

Accurate assessment of long-term nephrotoxicity after peptide receptor radionuclide therapy with (177)Lu-octreotate

PURPOSE

Renal radiation during peptide receptor radionuclide therapy (PRRT) may result in glomerular damage, a potential reduction of glomerular filtration rate (GFR) and ultimately lead to renal failure. While reported PRRT nephrotoxicity is limited to data derived from serum creatinine-allowing only approximate estimates of GFR-the aim of this study is to accurately determine PRRT-induced long-term changes of renal function and associated risk factors according to state-of-the-art GFR measurement.

METHODS

Nephrotoxicity was analysed using (99m)Tc-diethylenetriaminepentaacetic acid (DTPA) clearance data of 74 consecutive patients with gastroenteropancreatic neuroendocrine tumours (GEP NET) undergoing PRRT with (177)Lu-octreotate. The mean follow-up period was 21 months (range 12-50) with a median of five GFR measurements per patient. The change of GFR was analysed by linear curve fit. Potential risk factors including diabetes mellitus, arterial hypertension, previous chemotherapy, renal impairment at baseline and cumulative administered activity were analysed regarding potential impact on renal function loss. In addition, Common Terminology Criteria for Adverse Events (CTCAE) v3.0 were used to compare nephrotoxicity determined by (99m)Tc-DTPA clearance versus serum creatinine.

RESULTS

The alteration in GFR differed widely among the patients (mean -2.1 ± 13.1 ml/min/m(2) per year, relative yearly reduction -1.8 ± 18.9%). Fifteen patients (21%) experienced a mild (2-10 ml/min/m(2) per year) and 16 patients (22%) a significant (>10 ml/min/m(2) per year) decline of GFR following PRRT. However, 11 patients (15%) showed an increase of >10 ml/min/m(2) per year. Relevant nephrotoxicity according to CTCAE (grade ≥3) was observed in one patient (1.3%) with arterial hypertension and history of chemotherapy. Nephrotoxicity according to serum creatinine was discordant to that defined by GFR in 15% of the assessments and led to underestimation in 12% of patients. None of the investigated factors including cumulative administered activity contributed to the decline of renal function.

CONCLUSION

Serious nephrotoxicity after PRRT with (177)Lu-octreotate is rare (1.3%). However, slight renal impairment (GFR loss >2 ml/min/m(2) per year) can frequently (43%) be detected by (99m)Tc-DTPA clearance assessments. Cumulative administered activity of (177)Lu-octreotate is not a major determinant of renal impairment in our study.

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.

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.

Repeated cycles of peptide receptor radionuclide therapy (PRRT)--results and side-effects of the radioisotope 90Y-DOTA TATE, 177Lu-DOTA TATE or 90Y/177Lu-DOTA TATE therapy in patients with disseminated NET

PURPOSE

PRRT is a known tool in the management of patients with disseminated and inoperable NETs. The aim of study was to assess the effectiveness of the repeated cycles of PRRT in patients with disseminated and inoperable NETs.

MATERIAL AND METHODS

Eighty nine patients were included in the PRRT. Among them 16 patients (18%) were qualified for a repeated PRRT cycle due to progression of the disease. In one of the patients qualified for the repeated cycle, PRRT was used as neoadjuvant therapy. The results and side-effects of the repeated cycles of PRRT were analyzed.

RESULTS

Disease stabilization was observed in 10 patients 6 months after the repeated PRRT cycle and in 5 patients after 12 and 18 months. Ten of the patients who had received repeated PRRT cycles died. In the case of neoadjuvant therapy, further reduction of the tumor size was observed, enabling qualification for surgery. Clinically significant reduction in the mean values of morphological parameters was not observed. Only after 12 and 18 months the mean values of creatinine levels were higher than the normal range (only in 2 patients).

CONCLUSIONS

The repeated cycles of PRRT did not cause a clinically significant increase of the toxicity of PRRT. The changes in kidney and blood morphology parameters were transient. The repeated cycles of PRRT enabled stabilization of the disease.

Pretargeted 177Lu radioimmunotherapy of carcinoembryonic antigen-expressing human colonic tumors in mice

Pretargeted radioimmunotherapy (PRIT) with bispecific antibodies in combination with a radiolabeled peptide reduces the radiation dose to normal tissues, especially the bone marrow. In this study, the optimization, therapeutic efficacy, and toxicity of PRIT of colon cancer with a (177)Lu-labeled peptide was determined in mice with carcinoembryonic antigen (CEA)-expressing human tumors.

METHODS

To obtain the optimal therapeutic efficacy, several strategies were evaluated to increase the total amount of radioactivity targeted to subcutaneous LS174T colon cancer tumors in BALB/c nude mice. First, the maximum amount of bispecific anti-CEA and antihapten antibody TF2 and the peptide IMP288 that could be targeted was determined. Second, the tumor targeting of repeated administrations of radiolabeled IMP288 was investigated. Mice received 1 TF2 injection, followed by multiple IMP288 injections (3-h interval) or multiple cycles, with each IMP288 administration preceded by a new TF2 injection (72-h interval). PRIT was administered at maximum doses of TF2 and (177)Lu-labeled IMP288 in groups of 9 mice with subcutaneous LS174T tumors. Mice received 1, 2, or 3 successive cycles of treatment (26 MBq/mouse/cycle) or carrier only. The primary endpoint was survival; secondary endpoints were tumor growth, body weight, bone marrow, and renal toxicity.

RESULTS

The highest amount of radioactivity delivered to a subcutaneous colon tumor was achieved by the administration of 5.0 nmol of TF2 and 0.28 nmol of IMP288 in 3 successive cycles, with each IMP288 preceded by a new TF2 injection (72-h interval). PRIT effectively delayed tumor growth and prolonged survival significantly. Higher activity doses, administered in successive cycles, correlated with longer survival: the median survival of untreated mice was 13 d (range, 6-20 d), whereas that of mice treated with 1, 2, or 3 cycles of PRIT was 24 (range, 24-31 d), 45 (range, 38 ≥ 130 d), and 65 (range, 48 ≥ 130 d) days, respectively. Toxicity was limited: no significant changes in mean body weight were measured. Minimal changes in leukocyte counts were measured at 2 and 3 wk after injection, with full recovery within 7 wk after treatment. Platelet counts were unaffected. Serum creatinine levels were not increased significantly; thus, there was no indication of acute renal toxicity.

CONCLUSION

This study indicates that PRIT in mice is an effective treatment modality against colon cancer, with limited toxicity.

Dynamic and static small-animal SPECT in rats for monitoring renal function after 177Lu-labeled Tyr3-octreotate radionuclide therapy

High kidney radiation doses during clinical peptide receptor radionuclide therapy (PRRT) with β-particle-emitting radiolabeled somatostatin analogs will lead to renal failure several months after treatment, urging the coinfusion of the cationic amino acids lysine and arginine to reduce the renal radiation dose. In rat PRRT studies, renal protection by the coadministration of lysine was confirmed by histologic examination of kidney specimens indicating nephrotoxicity. In the current study, we investigated dedicated small-animal SPECT/CT renal imaging in rats to monitor renal function in vivo during follow-up of PRRT, with and without lysine.

METHODS

The following 3 groups of rats were imaged using a multipinhole SPECT/CT camera: controls (group 1) and rats at more than 90 d after therapy with 460 MBq (15 μg) of (177)Lu-DOTA-Tyr(3)-octreotate without (group 2) or with (group 3) a 400-mg/kg lysine coinjection as kidney protection (n ≥ 6 per group). At 90 and 140 d after therapy, static kidney scintigraphy was performed at 2 h after injection of 25 MBq of (99m)Tc-dimercaptosuccinic acid ((99m)Tc-DMSA). In addition, dynamic dual-isotope renography was performed using 50 MBq of (111)In-diethylenetriaminepentaacetic acid ((111)In-DTPA) and 50 MBq of (99m)Tc-mercaptoacetyltriglycine ((99m)Tc-MAG3) at 100-120 d after therapy.

RESULTS

(111)In-DTPA and (99m)Tc-MAG3 studies revealed a time-activity pattern comparable to those in patients, with a peak at 2-6 min followed by a decline of renal radioactivity. Reduced (111)In-DTPA, (99m)Tc-MAG3, and (99m)Tc-DMSA uptake indicated renal damage in group 2, whereas group 3 showed only a decrease of (99m)Tc-MAG3 peak activity. These results indicating nephrotoxicity in group 2 and renal protection in group 3 correlated with levels of urinary protein and serum creatinine and urea and were confirmed by renal histology.

CONCLUSION

Quantitative dynamic dual-isotope imaging using both (111)In-DTPA and (99m)Tc-MAG3 and static (99m)Tc-DMSA imaging in rats is feasible using small-animal SPECT, enabling longitudinal monitoring of renal function. (99m)Tc-MAG3 renography, especially, appears to be a more sensitive marker of tubular function after PRRT than serum chemistry or (99m)Tc-DMSA scintigraphy.

Nephrotoxicity in mice after repeated imaging using 111In-labeled peptides

We determined the renal radiation dose of a series of (111)In-labeled peptides using animal SPECT. Because the animals' health deteriorated, renal toxicity was assessed.

METHODS

Wild-type and megalin-deficient mice were imaged repeatedly at 3- to 6-wk intervals to quantify renal retention after injection of 40-50 MBq of (111)In-diethylenetriaminepentaacetic acid-labeled peptides (octreotide, exendin, octreotate, neurotensin, and minigastrin analogs), and the absorbed kidney radiation doses were estimated. Body weight, renal function parameters, and renal histology were determined at 16-20 wk after the first scan and compared with those in naive animals.

RESULTS

Because of high renal retention, (111)In-diethylenetriaminepentaacetic acid-exendin-4 scans resulted in a 70-Gy kidney radiation dose in wild-type mice. Megalin-deficient kidneys received 20-40 Gy. The other peptides resulted in much lower renal doses. Kidney function monitoring indicated renal damage in imaged animals.

CONCLUSION

Micro-SPECT enables longitudinal studies in 1 animal. However, long-term nephrotoxic effects may be induced after high renal radiation doses, even with (111)In-labeled radiotracers.

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.

Albumin-derived peptides efficiently reduce renal uptake of radiolabelled peptides

PURPOSE

In peptide-receptor radionuclide therapy (PRRT), the maximum activity dose that can safely be administered is limited by high renal uptake and retention of radiolabelled peptides. The kidney radiation dose can be reduced by coinfusion of agents that competitively inhibit the reabsorption of radiolabelled peptides, such as positively charged amino acids, Gelofusine, or trypsinised albumin. The aim of this study was to identify more specific and potent inhibitors of the kidney reabsorption of radiolabelled peptides, based on albumin.

METHODS

Albumin was fragmented using cyanogen bromide and six albumin-derived peptides with different numbers of electric charges were selected and synthesised. The effect of albumin fragments (FRALB-C) and selected albumin-derived peptides on the internalisation of (111)In-albumin, (111)In-minigastrin, (111)In-exendin and (111)In-octreotide by megalin-expressing cells was assessed. In rats, the effect of Gelofusine and albumin-derived peptides on the renal uptake and biodistribution of (111)In-minigastrin, (111)In-exendin and (111)In-octreotide was determined.

RESULTS

FRALB-C significantly reduced the uptake of all radiolabelled peptides in vitro. The albumin-derived peptides showed different potencies in reducing the uptake of (111)In-albumin, (111)In-exendin and (111)In-minigastrin in vitro. The most efficient albumin-derived peptide (peptide #6), was selected for in vivo testing. In rats, 5 mg of peptide #6 very efficiently inhibited the renal uptake of (111)In-minigastrin, by 88%. Uptake of (111)In-exendin and (111)In-octreotide was reduced by 26 and 33%, respectively.

CONCLUSIONS

The albumin-derived peptide #6 efficiently inhibited the renal reabsorption of (111)In-minigastrin, (111)In-exendin and (111)In-octreotide and is a promising candidate for kidney protection in PRRT.

Expression of somatostatin and somatostatin receptor subtypes 1-5 in human normal and diseased kidney

Somatostatin mediates inhibitory functions through five G protein-coupled somatostatin receptors (sst1-5). We used immunohistochemistry, immunofluorescence, and RT-PCR to determine the presence of somatostatin receptors sst1, sst2A, sst2B, sst3, sst4, and sst5 in normal and IgA nephropathy human kidney. All somatostatin receptors were detected in the thin tubules (distal convoluted tubules and loops of Henle) and thick tubules (proximal convoluted tubules) in the tissue sections from nephrectomy and biopsy samples. Immunopositive sst1 and sst4 staining was more condensed in the cytoplasm of tubular epithelial cells. In normal kidney tissue sections, podocytes and mesangial cells in the glomeruli stained for sst1, sst2B, sst4 and sst5, and stained weakly for sst3. In IgA kidney tissue, the expression of somatostatin receptors was significantly increased with particular immmunopositive staining for sst1, sst2B, sst4, and sst5 within glomeruli. In the epithelial cells, the staining for sst2B and sst4 in proximal tubules and sst1, sst2B, and sst5 in distal tubules was increased. The mRNA expression of sst1-5 was also detected by RT-PCR. Somatostatin and all five receptor subtypes were ubiquitously distributed in normal kidney and IgA nephropathy. The increased expression of somatostatin receptors in IgA nephropathy kidney might be the potential pathogenesis of inflammatory renal disease.

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

PURPOSE

In peptide receptor radionuclide therapy (PRRT) using radiolabelled somatostatin analogues, the kidneys are the major dose-limiting organs, because of tubular reabsorption and retention of radioactivity. Preventing renal uptake or toxicity will allow for higher tumour radiation doses. We tested the cytoprotective drug amifostine, which selectively protects healthy tissue during chemo- and radiotherapy, for its renoprotective capacities after PRRT with high-dose [(177)Lu-DOTA(0),Tyr(3)]octreotate.

METHODS

Male Lewis rats were injected with 278 or 555 MBq [(177)Lu-DOTA(0),Tyr(3)]octreotate to create renal damage and were followed up for 130 days. For renoprotection, rats received either amifostine or co-injection with lysine. Kidneys, blood and urine were collected for toxicity measurements. At 130 days after PRRT, a single-photon emission computed tomography (SPECT) scan was performed to quantify tubular uptake of (99m)Tc-dimercaptosuccinic acid (DMSA), a measure of tubular function.

RESULTS

Treatment with 555 MBq [(177)Lu-DOTA(0),Tyr(3)]octreotate resulted in body weight loss, elevated creatinine and proteinuria. Amifostine and lysine treatment significantly prevented this rise in creatinine and the level of proteinuria, but did not improve the histological damage. In contrast, after 278 MBq [(177)Lu-DOTA(0),Tyr(3)]octreotate, creatinine values were slightly, but not significantly, elevated compared with the control rats. Proteinuria and histological damage were different from controls and were significantly improved by amifostine treatment. Quantification of (99m)Tc-DMSA SPECT scintigrams at 130 days after [(177)Lu-DOTA(0),Tyr(3)]octreotate therapy correlated well with 1/creatinine (r(2)=0.772, p<0.001).

CONCLUSION

Amifostine and lysine effectively decreased functional renal damage caused by high-dose [(177)Lu-DOTA(0),Tyr(3)]octreotate. Besides lysine, amifostine might be used in clinical PRRT as well as to maximise anti-tumour efficacy.