Radionuclide therapy via SSTR: future aspects from experimental animal studies

Co-administration with A1M does not influence apoptotic response of 177Lu-octreotate in GOT1 neuroendocrine tumors

Recombinant α₁-microglobulin (A1M) is a proposed radioprotector during ¹⁷⁷Lu-octreotate therapy of neuroendocrine tumors (NETs). To ensure a maintained therapeutic effect, we previously demonstrated that A1M does not affect the ¹⁷⁷Lu-octreotate induced decrease in GOT1 tumor volume. However, the underlying biological events of these findings are still unknown. The aim of this work was to examine the regulation of apoptosis-related genes in GOT1 tumors short-time after i.v. administration of ¹⁷⁷Lu-octreotate with and without A1M or A1M alone. Human GOT1 tumor-bearing mice received 30 MBq ¹⁷⁷Lu-octreotate or 5 mg/kg A1M or co-treatment with both. Animals were sacrificed after 1 or 7 days. Gene expression analysis of apoptosis-related genes in GOT1 tissue was performed with RT-PCR. In general, similar expression patterns of pro- and anti-apoptotic genes were found after ¹⁷⁷Lu-octreotate exposure with or without co-administration of A1M. The highest regulated genes in both irradiated groups compared to untreated controls were FAS and TNFSFRS10B. Administration of A1M alone only resulted in significantly regulated genes after 7 days. Co-administration of A1M did not negatively affect the transcriptional apoptotic response of ¹⁷⁷Lu-octreotate in GOT1 tumors.

Hyperfractionated Treatment with 177Lu-Octreotate Increases Tumor Response in Human Small-Intestine Neuroendocrine GOT1 Tumor Model

Simple Summary

Neuroendocrine tumors are slow growing and initially associated with vague symptoms and, therefore, often spread in the patient’s body at diagnosis, leading to a poor prognosis without means of curation through surgery. Although tumor-targeting treatments exist and are used in clinics, they are not fully optimized. The aim of this study was to test different dosages and time intervals of the radioactive pharmaceutical ¹⁷⁷Lu-octreotate. We found that dividing a dosage into several portions and administering it at short time intervals resulted in a stronger tumor reduction and/or prolonged time for regrowth in mice than if given as a single dose. The biggest differences were seen in the lower dosage levels of the study. The findings indicate that there is clear room for improvements in the treatment of neuroendocrine tumors with ¹⁷⁷Lu-octreotate.

Abstract

Radionuclide treatment of patients with neuroendocrine tumors has advanced in the last decades with favorable results using ¹⁷⁷Lu-octreotate. However, the gap between the high cure rate in animal studies vs. patient studies indicates a potential to increase the curation of patients. The aim of this study was to investigate the tumor response for different fractionation schemes with ¹⁷⁷Lu-octreotate. BALB/c mice bearing a human small-intestine neuroendocrine GOT1 tumor were either mock treated with saline or injected intravenously with a total of 30–120 MBq of ¹⁷⁷Lu-octreotate: 1 × 30, 2 × 15, 1 × 60, 2 × 30, 1 × 120, 2 × 60, or 3 × 40 MBq. The tumor volume was measured twice per week until the end of the experiment. The mean tumor volume for mice that received 2 × 15 = 30 and 1 × 30 MBq ¹⁷⁷Lu-octreotate was reduced by 61% and 52%, respectively. The mean tumor volume was reduced by 91% and 44% for mice that received 2 × 30 = 60 and 1 × 60 MBq ¹⁷⁷Lu-octreotate, respectively. After 120 MBq ¹⁷⁷Lu-octreotate, given as 1–3 fractions, the mean tumor volume was reduced by 91–97%. Multiple fractions resulted in delayed regrowth and prolonged overall survival by 20–25% for the 120 MBq groups and by 45% for lower total activities, relative to one fraction. The results indicate that fractionation and hyperfractionation of ¹⁷⁷Lu-octreotate are beneficial for tumor reduction and prolongs the time to regrowth.

Enhancing the anti-tumour activity of 177Lu-DOTA-octreotate radionuclide therapy in somatostatin receptor-2 expressing tumour models by targeting PARP

Peptide receptor radionuclide therapy (PRRT) is an important treatment option for patients with somatostatin receptor-2 (SSTR2)-expressing neuroendocrine tumour (NET) though tumour regression occurs in only a minority of patients. Therefore, novel PRRT regimens with improved therapeutic activity are needed. Radiation induced DNA damage repair is an attractive therapeutic target to increase PRRT efficacy and consequently, we have characterised a panel of preclinical models for their SSTR2 expression, in vivo growth properties and response to 177Lu-DOTA-octreotate (LuTate) PRRT to identify models with features suitable for evaluating novel therapeutic combinations. In vitro studies using the SSTR2 expressing AR42J model demonstrate that the combination of LuTate and the small molecule Poly(ADP-ribose) polymerase-1 (PARP) inhibitor, talazoparib led to increased DNA double strand breaks, as assessed by γ-H2AX foci formation, as compared to LuTate alone. Furthermore, using the AR42J tumour model in vivo we demonstrate that the combination of LuTate and talazoparib significantly improved the anti-tumour efficacy of LuTate alone. These findings support the clinical evaluation of the combination of LuTate and PARP inhibition in SSTR2-expressing NET.

Increased therapeutic effect on medullary thyroid cancer using a combination of radiation and tyrosine kinase inhibitors

Since patients with medullary thyroid cancer (MTC) often have metastatic disease at the time of diagnosis, the development of efficient systemic treatment options for MTC is important. Vandetanib and cabozantinib are two tyrosine kinase inhibitors (TKIs) that were recently approved by FDA and EMA for systemic treatment of metastatic MTC. Additionally, since MTC is of a neuroendocrine tumour type, treatment with radiolabelled somatostatin analogues (e.g. ¹⁷⁷Lu-octreotate) is a valid option for patients with MTC. The aim of this study was to investigate the potentially increased therapeutic effect of combining radiation therapy with these TKIs for treatment of MTC in a mouse model. Nude mice carrying patient-derived MTC tumours (GOT2) were treated with external beam radiotherapy (EBRT) and/or one of the two TKIs vandetanib or cabozantinib. The tumour volume was determined and compared with that of mock-treated controls. The treatment doses were chosen to give a moderate effect as monotherapy to be able to detect any increased therapeutic effect from the combination therapy. At the end of follow-up, tumours were processed for immunohistochemical (IHC) analyses. The animals in the combination therapy groups showed the largest reduction in tumour volume and the longest time to tumour progression. Two weeks after start of treatment, the tumour volume for these mice was reduced by about 70–75% compared with controls. Furthermore, also EBRT and TKI monotherapy resulted in a clear anti-tumour effect with a reduced tumour growth compared with controls. The results show that an increased therapeutic effect could be achieved when irradiation is combined with TKIs for treatment of MTC. Future studies should evaluate the potential of using ¹⁷⁷Lu-octreotate therapy in combination with TKIs in patients.

Gemcitabine potentiates the anti-tumour effect of radiation on medullary thyroid cancer

Patients with medullary thyroid cancer (MTC) are often diagnosed with spread tumour disease and the development of better systemic treatment options for these patients is important. Treatment with the radiolabelled somatostatin analogue ¹⁷⁷Lu-octreotate is already a promising option but can be optimised. For example, combination treatment with another substance could increase the effect on tumour tissue. Gemcitabine is a nucleoside analogue that has been shown to sensitise tumour cells to radiation. The aim of this study was to investigate potentially additive or synergistic effects of combining radiation with gemcitabine for treatment of MTC. Nude mice transplanted with patient-derived MTC tumours (GOT2) were divided into groups and treated with radiation and/or gemcitabine. Radiation treatment was given as ¹⁷⁷Lu-octreotate or external beam radiotherapy (EBRT). The volume of treated and untreated tumours was followed. The absorbed dose and amount of gemcitabine were chosen to give moderate tumour volume reduction when given as monotherapy to enable detection of increased effects from combination treatment. After follow-up, the mice were killed and tumours were immunohistochemically (IHC) analysed. Overall, the animals that received a combination of EBRT and gemcitabine showed the largest reduction in tumour volume. Monotherapy with EBRT or gemcitabine also resulted in a clear detrimental effect on tumour volume, while the animals that received ¹⁷⁷Lu-octreotate monotherapy showed similar response as the untreated animals. The GOT2 tumour was confirmed in the IHC analyses by markers for MTC. The IHC analyses also revealed that the proliferative activity of tumour cells was similar in all tumours, but indicated that fibrotic tissue was more common after EBRT and/or gemcitabine treatment. The results indicate that an additive, or even synergistic, effect may be achieved by combining radiation with gemcitabine for treatment of MTC. Future studies should be performed to evaluate the full potential of combining ¹⁷⁷Lu-octreotate with gemcitabine in patients.

Local treatment of liver metastases by administration of 177Lu-octreotate via isolated hepatic perfusion - A preclinical simulation of a novel treatment strategy

INTRODUCTION

Systemic ¹⁷⁷Lu-octreotate treatment for metastatic neuroendocrine tumours is restricted by organs at risk. By administering ¹⁷⁷Lu-octreotate during isolated hepatic perfusion (IHP), the uptake in organs at risk might be strongly reduced. The aim of this study was to investigate the feasibility to use the combination of IHP and radionuclide therapy.

METHODS

To simulate IHP, the liver of a pig was prepared for ex vivo perfusion. Blood containing 490 MBq ¹⁷⁷Lu-octreotate was circulated through the liver for 60 min, after which the liver was rinsed. After IHP, the liver was examined by SPECT/CT. Lastly, an intraoperative gamma detector (IGD) was used to determine ¹⁷⁷Lu activity concentration in the liver and results were compared with the activity concentration in corresponding liver biopsies.

RESULTS

Detector measurements over the liver during the IHP showed a fast increase with a maximum after approximately 10-15 min. After IHP, about 75% of the ¹⁷⁷Lu in the liver could be washed out. The SPECT/CT images revealed a relatively inhomogeneous distribution. Nevertheless, the IGD values of ¹⁷⁷Lu activity concentration showed acceptable agreement with the biopsy values.

CONCLUSIONS

Our results in pig show that it could be feasible to treat patients with liver metastases from NETs with ¹⁷⁷Lu-octreotate via IHP ¹⁷⁷. However, an inhomogeneous distribution of ¹⁷⁷Lu-octreotate in normal liver tissue is expected, and in order to determine the activity concentration with satisfactory accuracy using an IGD, measurements need to be performed at several positions over the liver.

Recombinant α1-Microglobulin Is a Potential Kidney Protector in 177Lu-Octreotate Treatment of Neuroendocrine Tumors

Treatment of neuroendocrine tumors with ¹⁷⁷Lu-octreotate results in prolonged survival and improved quality of life for the patient. However, the treatment is today limited by side effects on kidney and bone marrow, and complete tumor remission is rarely seen. A possible way to minimize dose-limiting toxicity and to optimize this treatment method is to use radioprotectors in conjunction with radiotherapy. A recombinant form of α₁-microglobulin (rA1M) was recently shown to preserve kidney structure and function after ¹⁷⁷Lu-octreotate injection in mice and was suggested as a radioprotector in peptide receptor radionuclide therapy. The aims of this work were to investigate the influence of rA1M on the in vivo biokinetics of ¹⁷⁷Lu-octreotate, with a focus on tumor tissue, and to study the impact of rA1M on the therapeutic response in tumor tissue subjected to ¹⁷⁷Lu-octreotate treatment. Methods: The biodistribution of ¹⁷⁷Lu-octreotate was examined in BALB/c nude mice with GOT2 tumors 1-168 h after injection with either ¹⁷⁷Lu-octreotate or coadministration of ¹⁷⁷Lu-octreotate and rA1M. The effects of rA1M on the tumor response after ¹⁷⁷Lu-octreotate treatment were studied in BALB/c nude mice with GOT1 tumors. Three groups of mice were administered rA1M, ¹⁷⁷Lu-octreotate, or both. Another group served as untreated controls. Tumor volume was measured to follow the treatment effects. Results: No statistically significant difference in biodistribution of ¹⁷⁷Lu was observed between the groups receiving ¹⁷⁷Lu-octreotate or coinjection of ¹⁷⁷Lu-octreotate and rA1M. The therapy study showed a decrease in mean tumor volume during the first 2 wk for both the ¹⁷⁷Lu-octreotate group and the coadministration group, followed by tumor regrowth. No statistically significant difference between the groups was found. Conclusion: rA1M did not negatively impact absorbed dose to tumor or therapeutic response in combination with ¹⁷⁷Lu-octreotate and may be a promising kidney protector during ¹⁷⁷Lu-octreotate treatment of patients with neuroendocrine tumors.

Transcriptional effects of 177Lu-octreotate therapy using a priming treatment schedule on GOT1 tumor in nude mice

BACKGROUND

¹⁷⁷Lu-octreotate is used for therapy of somatostatin receptor expressing neuroendocrine tumors with promising results, although complete tumor remission is rarely seen. Previous studies on nude mice bearing the human small intestine neuroendocrine tumor, GOT1, have shown that a priming injection of ¹⁷⁷Lu-octreotate 24 h before the main injection of ¹⁷⁷Lu-octreotate resulted in higher ¹⁷⁷Lu concentration in tumor, resulting in increased absorbed dose, volume reduction, and time to regrowth. To our knowledge, the cellular effects of a priming treatment schedule have not yet been studied. The aim of this study was to identify transcriptional changes contributing to the enhanced therapeutic response of GOT1 tumors in nude mice to ¹⁷⁷Lu-octreotate therapy with priming, compared with non-curative monotherapy.

RESULTS

RNA microarray analysis was performed on tumor samples from GOT1-bearing BALB/c nude mice treated with a 5 MBq priming injection of ¹⁷⁷Lu-octreotate followed by a second injection of 10 MBq of ¹⁷⁷Lu-octreotate after 24 h and killed after 1, 3, 7, and 41 days after the last injection. Administered activity amounts were chosen to be non-curative, in order to facilitate the study of tumor regression and regrowth. Differentially regulated transcripts (RNA samples from treated vs. untreated animals) were identified (change ≥ 1.5-fold; adjusted p value < 0.01) using Nexus Expression 3.0. Analysis of the biological effects of transcriptional regulation was performed using the Gene Ontology database and Ingenuity Pathway Analysis. Transcriptional analysis of the tumors revealed two stages of pathway regulation for the priming schedule (up to 1 week and around 1 month) which differed distinctly from cellular responses observed after monotherapy. Induction of cell cycle arrest and apoptotic pathways (intrinsic and extrinsic) was found at early time points after treatment start, while downregulation of pro-proliferative genes were found at a late time point.

CONCLUSIONS

The present study indicates increased cellular stress responses in the tumors treated with a priming treatment schedule compared with those seen after conventional ¹⁷⁷Lu-octreotate monotherapy, resulting in a more profound initiation of cell cycle arrest followed by apoptosis, as well as effects on PI3K/AKT-signaling and unfolded protein response.

Time-dependent transcriptional response of GOT1 human small intestine neuroendocrine tumor after 177Lu[Lu]-octreotate therapy

INTRODUCTION

Patients with neuroendocrine tumors expressing somatostatin receptors are often treated with ¹⁷⁷Lu[Lu]-octreotate. Despite being highly effective in animal models, ¹⁷⁷Lu[Lu]-octreotate-based therapies in the clinical setting can be optimized further. The aims of the study were to identify and elucidate possible optimization venues for ¹⁷⁷Lu[Lu]-octreotate tumor therapy by characterizing transcriptional responses in the GOT1 small intestine neuroendocrine tumor model in nude mice.

METHODS

GOT1-bearing female BALB/c nude mice were intravenously injected with 15 MBq ¹⁷⁷Lu[Lu]-octreotate (non-curative amount) or mock-treated with saline solution. Animals were killed 1, 3, 7 or 41 d after injection. Total RNA was extracted from the tumor samples and profiled using Illumina microarray expression analysis. Differentially expressed genes were identified (treated vs. control) and pathway analysis was performed.

RESULTS

Distribution of differentially expressed transcripts indicated a time-dependent treatment response in GOT1 tumors after ¹⁷⁷Lu[Lu]-octreotate administration. Regulation of CDKN1A, BCAT1 and PAM at 1 d after injection was compatible with growth arrest as the initial response to treatment. Upregulation of APOE and BAX at 3 d, and ADORA2A, BNIP3, BNIP3L and HSPB1 at 41 d after injection suggests first activation and then inhibition of the intrinsic apoptotic pathway during tumor regression and regrowth, respectively.

CONCLUSION

Transcriptional analysis showed radiation-induced apoptosis as an early response after ¹⁷⁷Lu[Lu]-octreotate administration, followed by pro-survival transcriptional changes in the tumor during the regrowth phase. Time-dependent changes in cell cycle and apoptosis-related processes suggest different time points after radionuclide therapy when tumor cells may be more susceptible to additional treatment, highlighting the importance of timing when administering multiple therapeutic agents.

The neuroendocrine phenotype, genomic profile and therapeutic sensitivity of GEPNET cell lines

Experimental models of neuroendocrine tumour disease are scarce, and no comprehensive characterisation of existing gastroenteropancreatic neuroendocrine tumour (GEPNET) cell lines has been reported. In this study, we aimed to define the molecular characteristics and therapeutic sensitivity of these cell lines. We therefore performed immunophenotyping, copy number profiling, whole-exome sequencing and a large-scale inhibitor screening of seven GEPNET cell lines. Four cell lines, GOT1, P-STS, BON-1 and QGP-1, displayed a neuroendocrine phenotype while three others, KRJ-I, L-STS and H-STS, did not. Instead, these three cell lines were identified as lymphoblastoid. Characterisation of remaining authentic GEPNET cell lines by copy number profiling showed that GOT1, among other chromosomal alterations, harboured losses on chromosome 18 encompassing the SMAD4 gene, while P-STS had a loss on 11q. BON-1 had a homozygous loss of CDKN2A and CDKN2B, and QGP-1 harboured amplifications of MDM2 and HMGA2 Whole-exome sequencing revealed both disease-characteristic mutations (e.g. ATRX mutation in QGP-1) and, for patient tumours, rare genetic events (e.g. TP53 mutation in P-STS, BON-1 and QGP-1). A large-scale inhibitor screening showed that cell lines from pancreatic NETs to a greater extent, when compared to small intestinal NETs, were sensitive to inhibitors of MEK. Similarly, neuroendocrine NET cells originating from the small intestine were considerably more sensitive to a group of HDAC inhibitors. Taken together, our results provide a comprehensive characterisation of GEPNET cell lines, demonstrate their relevance as neuroendocrine tumour models and explore their therapeutic sensitivity to a broad range of inhibitors.

Hedgehog inhibitor sonidegib potentiates 177Lu-octreotate therapy of GOT1 human small intestine neuroendocrine tumors in nude mice

Background

¹⁷⁷Lu-octreotate can be used to treat somatostatin receptor expressing neuroendocrine tumors. It is highly effective in animal models, but clinical studies have so far only demonstrated low cure rates. Hedgehog inhibitors have shown therapeutic effect as monotherapy in neuroendocrine tumor model systems and might be one option to enhance the efficacy of ¹⁷⁷Lu-octreotate therapy. The aim of this study was to determine the therapeutic effect of combination therapy using ¹⁷⁷Lu-octreotate and the Hedgehog signaling pathway inhibitor sonidegib.

Methods

GOT1-bearing BALB/c nude mice were treated with either sonidegib (80 mg/kg twice a week via oral gavage), a single injection of 30 MBq ¹⁷⁷Lu-octreotate i.v., or a combination of both. Untreated animals served as controls. Tumor size was measured twice-weekly using calipers. The animals were killed 41 d after injection followed by excision of the tumors. Total RNA was extracted from each tumor sample and then subjected to gene expression analysis. Gene expression patterns were compared with those of untreated controls using Nexus Expression 3.0, IPA and Gene Ontology terms. Western blot was carried out on total protein extracted from the tumor samples to analyze activation-states of the Hh and PI3K/AKT/mTOR pathways.

Results

Sonidegib monotherapy resulted in inhibition of tumor growth, while a significant reduction in mean tumor volume was observed after ¹⁷⁷Lu-octreotate monotherapy and combination therapy. Time to progression was prolonged in the combination therapy group compared with ¹⁷⁷Lu-octreotate monotherapy. Gene expression analysis revealed a more pronounced response following combination therapy compared with both monotherapies, regarding the number of regulated genes and biological processes. Several cancer-related signaling pathways (i.e. Wnt/β-catenin, PI3K/AKT/mTOR, G-protein coupled receptor, and Notch) were affected by the combination therapy, but not by either monotherapy. Protein expression analysis revealed an activation of the Hh- and PI3K/AKT/mTOR pathways in tumors exposed to ¹⁷⁷Lu-octreotate monotherapy and combination therapy.

Conclusions

A comparative analysis of the different treatment groups showed that combination therapy using sonidegib and ¹⁷⁷Lu-octreotate could be beneficial to patients with neuroendocrine tumors. Gene expression analysis revealed a functional interaction between sonidegib and ¹⁷⁷Lu-octreotate, i.e. several cancer-related signaling pathways were modulated that were not affected by either monotherapy. Protein expression analysis indicated a possible PI3K/AKT/mTOR-dependent activation of the Hh pathway, independent of SMO.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-017-3524-x) contains supplementary material, which is available to authorized users.

Evaluation of two intraoperative gamma detectors for assessment of 177Lu activity concentration in vivo

BACKGROUND

Patients with somatostatin receptor-expressing neuroendocrine tumours can be treated with intravenously administered ¹⁷⁷Lu-octreotate. Few patients are cured with the present protocol due to the current dose limitation of normal organs at risk, such as the kidneys. By locally administering ¹⁷⁷Lu-octreotate to the liver for the purpose of treating liver metastases, a substantially reduced absorbed dose to organs at risk could be achieved. The development of such a technique requires the capability of measuring the ¹⁷⁷Lu activity concentration in tissues in vivo. The aim of this study was to evaluate different performance parameters of two commercially available intraoperative gamma detectors in order to investigate whether intraoperative gamma detector measurements could be used to determine ¹⁷⁷Lu activity concentration in vivo.

RESULTS

Measurements were made using different sources containing ¹⁷⁷Lu. Response linearity, sensitivity, spatial resolution and its depth dependence, organ thickness dependence of the measured count rate and tumour detectability were assessed for two intraoperative gamma detectors. The two detectors (a scintillation and a semiconductor detector) showed differences in technical performance. For example, the sensitivity was higher for the scintillation detector, while the spatial resolution was better for the semiconductor detector. Regarding organ thickness dependence and tumour detectability, similar results were obtained for both detectors, and even relatively small simulated tumours of low tumour-to-background activity concentration ratios could be detected.

CONCLUSIONS

Acceptable results were obtained for both detectors, although the semiconductor detector proved more advantageous for our purpose. The measurements demonstrated factors that must be corrected for, such as organ thickness or dead-time effects. Altogether, intraoperative gamma detector measurements could be used to determine ¹⁷⁷Lu activity concentration in vivo.

Priming increases the anti-tumor effect and therapeutic window of 177Lu-octreotate in nude mice bearing human small intestine neuroendocrine tumor GOT1

Background

¹⁷⁷Lu-[DOTA⁰, Tyr³]-octreotate (¹⁷⁷Lu-octreotate) is used for treatment of patients with somatostatin receptor (SSTR) expressing neuroendocrine tumors. However, complete tumor remission is rarely seen, and optimization of treatment protocols is needed. In vitro studies have shown that irradiation can up-regulate the expression of SSTR1, 2 and 5, and increase ¹⁷⁷Lu-octreotate uptake.

The aim of the present study was to examine the anti-tumor effect of a ¹⁷⁷Lu-octreotate priming dose followed 24 h later by a second injection of ¹⁷⁷Lu-octreotate compared to a single administration of ¹⁷⁷Lu-octreotate, performed on the human small intestine neuroendocrine tumor cell line, GOT1, transplanted to nude mice.

Results

Priming resulted in a 1.9 times higher mean absorbed dose to the tumor tissue per administered activity, together with a reduced mean absorbed dose for kidneys. Priming gave the best overall anti-tumor effects. Magnetic resonance imaging showed no statistically significant difference in tumor response between treatment with and without priming. Gene expression analysis demonstrated effects on cell cycle regulation. Biological processes associated with apoptotic cell death were highly affected in the biodistribution and dosimetry study, via differential regulation of, e.g., APOE, BAX, CDKN1A, and GADD45A.

Conclusions

Priming had the best overall anti-tumor effects and also resulted in an increased therapeutic window. Results indicate that potential biomarkers for tumor regrowth may be found in the p53 or JNK signaling pathways. Priming administration is an interesting optimization strategy for ¹⁷⁷Lu-octreotate therapy of neuroendocrine tumors, and further studies should be performed to determine the mechanisms responsible for the reported effects.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-016-0247-y) contains supplementary material, which is available to authorized users.

Physiologically based pharmacokinetic modeling of (18)F-SiFAlin-Asp3-PEG1-TATE in AR42J tumor bearing mice

PURPOSE

Peptide receptor radionuclide therapy (PRRT) is commonly performed in the treatment of neuroendocrine tumors (NET), where somatostatin analogs (DOTATATE) are radiolabeled with (90)Y, (68)Ga or (111)In for pre-therapeutic and therapeutic purposes. Quantitative evaluation of the biokinetic data can be performed by using physiologically based pharmacokinetic (PBPK) models. Knowledge about the biodistribution in a pre-clinical setting would allow optimizing the translation from bench to bedside. The aim of this study was to develop a PBPK model to describe the biodistribution of a novel sst2-targeting radiotracer.

METHODS

Biokinetic data of six mice after injection of (18)F-SiFAlin-Asp3-PEG1-TATE were investigated using two PBPK models. The PBPK models describe the biodistribution of the tracer in the tumor, kidneys, liver, remainder and whole body via blood flow to these organs via absorption, distribution, metabolism and excretion. A recently published sst2 PBPK model for humans (model 1) was used to describe the data. Physiological information in this model was adapted to that of a mouse. Model 1 was further modified by implementing receptor-mediated endocytosis (model 2). Model parameters were fitted to the biokinetic data of each mouse. Model selection was performed by calculating Akaike weights wi using the corrected Akaike Information Criterion (AICc).

RESULTS

The implementation of receptor-mediated endocytosis considerably improved the description of the biodistribution (Akaike weights w1=0% and w2=100% for model 1 and 2, respectively). The resulting time-integrated activity coefficients determined by model 2 were for tumor (0.05 ± 0.02) h, kidneys (0.11 ± 0.01) h and liver (0.02 ± 0.01) h.

CONCLUSION

Simply downscaling a human PBPK model does not allow for an accurate description of (18)F-SiFAlin-Asp3-PEG1-TATE in mice. Biokinetics of this tracer can be accurately and adequately described using a physiologically based pharmacokinetic model including receptor-mediated endocytosis. Thus, an optimized translation from bench to bedside is possible.

Potential Biomarkers for Radiation-Induced Renal Toxicity following 177Lu-Octreotate Administration in Mice

The kidneys are one of the main dose-limiting organs in peptide receptor radionuclide therapy and due to large inter-individual variations in renal toxicity, biomarkers are urgently needed in order to optimize therapy and reduce renal tissue damage. The aim of this study was to investigate the transcriptional, functional, and morphological effects on renal tissue after ¹⁷⁷Lu-octreotate administration in normal mice, and to identify biomarkers for radiation induced renal toxicity.

The amount of injected 177Lu-octreotate strongly influences biodistribution and dosimetry in C57BL/6N mice

BACKGROUND

(177)Lu-octreotate therapy has proven to give favorable results after treatment of patients with neuroendocrine tumors. Much focus has been on the binding and uptake of (177)Lu-octreotate in tumor tissue, but biodistribution properties in normal tissues is still not fully understood, and the effect of receptor saturation may be important. The aim of this study was to investigate the influence of the amount of (177)Lu-octreotate on the biodistribution of (177)Lu-octreotate in normal tissues in mice.

MATERIAL AND METHODS

C57BL/6N female mice were intravenously injected with 0.1-150 MBq (177)Lu-octreotate (0.039 μg peptide/MBq). The mice were killed 0.25 h to 14 days after injection by cardiac puncture under anesthesia. Activity concentration was determined in blood, bone marrow, kidneys, liver, lungs, pancreas, and spleen, and mean absorbed doses were calculated.

RESULTS

The activity concentration varied with time and amount of injected activity. At 4-8 h after injection, a local maximum in activity concentration was found for liver, lungs, pancreas, and spleen. With the exception for the lower injected activities (0.1-1 MBq), the overall highest uptake was found in the kidneys (%IA/g). Large variations were found and the activity concentration in kidneys was 11-23%IA/g at 4 h, and 0.22-1.9%IA/g at 7 days after injection. Furthermore, a clear reduction in activity concentration with increased injected activity was observed for lungs, pancreas and spleen.

CONCLUSION

The activity concentration in all tissues investigated was strongly influenced by the amount of (177)Lu-octreotate injected. Large differences in mean absorbed dose per unit injected activity were found between low (0.1-1 MBq, 0.0039-0.039 μg) and moderate amounts (5-45 MBq, 0.2-1.8 μg). Furthermore, the results clearly showed the need for better ways to estimate absorbed dose to bone marrow other than methods based on a single blood sample analysis. Since the absorbed dose to critical organs will limit the amount of (177)Lu-octreotate administered, these findings must be taken into consideration when optimizing this type of therapy.

Distinct microRNA expression profiles in mouse renal cortical tissue after 177Lu-octreotate administration

Aim

The aim of this study was to investigate the variation of the miRNA expression levels in normal renal cortical tissue after ¹⁷⁷Lu-octreotate administration, a radiopharmaceutical used for treatment of neuroendocrine cancers.

Methods

Female BALB/c nude mice were i.v. injected with 1.3, 3.6, 14, 45, or 140 MBq ¹⁷⁷Lu-octreotate, while control animals received saline. The animals were killed at 24 h after injection and total RNA, including miRNA, was extracted from the renal cortical tissue and hybridized to the Mouse miRNA Oligo chip 4plex to identify differentially regulated miRNAs between exposed and control samples.

Results

In total, 57 specific miRNAs were differentially regulated in the exposed renal cortical tissues with 1, 29, 21, 27, and 31 miRNAs identified per dose-level (0.13, 0.34, 1.3, 4.3, and 13 Gy, respectively). No miRNAs were commonly regulated at all dose levels. miR-194, miR-107, miR-3090, and miR-3077 were commonly regulated at 0.34, 1.3, 4.3, and 13 Gy. Strong effects on cellular mechanisms ranging from immune response to p53 signaling and cancer-related pathways were observed at the highest absorbed dose. Thirty-nine of the 57 differentially regulated miRNAs identified in the present study have previously been associated with response to ionizing radiation, indicating common radiation responsive pathways.

Conclusion

In conclusion, the ¹⁷⁷Lu-octreotate associated miRNA signatures were generally dose-specific, thereby illustrating transcriptional regulation of radiation responsive miRNAs. Taken together, these results imply the importance of miRNAs in early immunological responses in the kidneys following ¹⁷⁷Lu-octreotate administration.

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

Time- and dose rate-related effects of internal (177)Lu exposure on gene expression in mouse kidney tissue

INTRODUCTION

The kidneys are the dose-limiting organs in some radionuclide therapy regimens. However, the biological impact of internal exposure from radionuclides is still not fully understood. The aim of this study was to examine the effects of dose rate and time after i.v. injection of (177)LuCl3 on changes in transcriptional patterns in mouse kidney tissue.

METHODS

To investigate the effect of dose rate, female Balb/c nude mice were i.v. injected with 11, 5.6, 1.6, 0.8, 0.30, and 0 MBq of (177)LuCl3, and killed at 3, 6, 24, 48, 168, and 24 hours after injection, respectively. Furthermore, the effect of time after onset of exposure was analysed using mice injected with 0.26, 2.4, and 8.2 MBq of (177)LuCl3, and killed at 45, 90, and 140 days after injection. Global transcription patterns of irradiated kidney cortex and medulla were assessed and enriched biological processes were determined from the regulated gene sets using Gene Ontology terms.

RESULTS

The average dose rates investigated were 1.6, 0.84, 0.23, 0.11 and 0.028 mGy/min, with an absorbed dose of 0.3 Gy. At 45, 90 and 140 days, the absorbed doses were estimated to 0.3, 3, and 10 Gy. In general, the number of differentially regulated transcripts increased with time after injection, and decreased with absorbed dose for both kidney cortex and medulla. Differentially regulated transcripts were predominantly involved in metabolic and stress response-related processes dependent on dose rate, as well as transcripts associated with metabolic and cellular integrity at later time points.

CONCLUSION

The observed transcriptional response in kidney tissue was diverse due to difference in absorbed dose, dose rate and time after exposure. Nevertheless, several transcripts were significantly regulated in all groups despite differences in exposure parameters, which may indicate potential biomarkers for exposure of kidney tissue.

Transcriptional response of kidney tissue after 177Lu-octreotate administration in mice

INTRODUCTION

The kidneys are one of the main dose limiting organs in (177)Lu-octreotate therapy of neuroendocrine tumors. Therefore, biomarkers for radiation damage would be of great importance in this type of therapy. The purpose of this study was to investigate the absorbed dose dependency on early transcriptional changes in the kidneys from (177)Lu-octreotate exposure.

METHODS

Female Balb/c nude mice were i.v. injected with 1.3, 3.6, 14, 45 or 140 MBq (177)Lu-octreotate. The animals were killed 24 h after injection followed by excision of the kidneys. The absorbed dose to the kidneys ranged between 0.13 and 13 Gy. Total RNA was extracted from separated renal tissue samples, and applied to Illumina MouseRef-8 Whole-Genome Expression Beadchips to identify regulated transcripts after irradiation. Nexus Expression 2.0 and Gene Ontology terms were used for data processing and to determine affected biological processes.

RESULTS

Distinct transcriptional responses were observed following (177)Lu-octreotate administration. A higher number of differentially expressed transcripts were observed in the kidney medulla (480) compared to cortex (281). In addition, 39 transcripts were regulated at all absorbed dose levels in the medulla, compared to 32 in the cortex. Three biological processes in the cortex and five in the medulla were also shared by all absorbed dose levels. Strong association to metabolism was found among the affected processes in both tissues. Furthermore, an association with cellular and developmental processes was prominent in kidney medulla, while transport and immune response were prominent in kidney cortex.

CONCLUSION

Specific biological and dose-dependent responses were observed in both tissues. The number of affected transcripts and biological processes revealed distinct response differences between the absorbed doses delivered to the tissues.