Differences in biodistribution between 99mTc-depreotide, 111In-DTPA-octreotide, and 177Lu-DOTA-Tyr3-octreotate in a small cell lung cancer animal model

Comparison of 177Lu-octreotate and 177Lu-octreotide for treatment in human neuroblastoma-bearing mice

Background

Patients with high-risk neuroblastoma (NB) have a 5-year event-free survival of less than 50 %, and novel and improved treatment options are needed. Radiolabeled somatostatin analogs (SSTAs) could be a treatment option. The aims of this work were to compare the biodistribution and the therapeutic effects of 177Lu-octreotate and 177Lu-octreotide in mice bearing the human CLB-BAR NB cell line, and to evaluate their regulatory effects on apoptosis-related genes.

Methods

The biodistribution of 177Lu-octreotide in mice bearing CLB-BAR tumors was studied at 1, 24, and 168 h after administration, and the absorbed dose was estimated to tumor and normal tissues. Further, animals were administered different amounts of 177Lu-octreotate or 177Lu-octreotide. Tumor volume was measured over time and compared to a control group given saline. RNA was extracted from tumors, and the expression of 84 selected genes involved in apoptosis was quantified with qPCR.

Results

The activity concentration was generally lower in most tissues for 177Lu-octreotide compared to 177Lu-octreotate. Mean absorbed dose per administered activity to tumor after injection of 1.5 MBq and 15 MBq was 0.74 and 0.03 Gy/MBq for 177Lu-octreotide and 2.9 and 0.45 Gy/MBq for 177Lu-octreotate, respectively. 177Lu-octreotide treatment resulted in statistically significant differences compared to controls. Fractionated administration led to a higher survival fraction than after a single administration. The pro-apoptotic genes TNSFS8, TNSFS10, and TRADD were regulated after administration with 177Lu-octreotate. Treatment with 177Lu-octreotide yielded regulation of the pro-apoptotic genes CASP5 and TRADD, and of the anti-apoptotic gene IL10 as well as the apoptosis-related gene TNF.

Conclusion

177Lu-octreotide gave somewhat better anti-tumor effects than 177Lu-octreotate. The similar effect observed in the treated groups with 177Lu-octreotate suggests saturation of the somatostatin receptors. Pronounced anti-tumor effects following fractionated administration merited receptor saturation as an explanation. The gene expression analyses suggest apoptosis activation through the extrinsic pathway for both radiopharmaceuticals.

Evaluation of an Affibody-Based Binder for Imaging of Immune Check-Point Molecule B7-H3

Radionuclide molecular imaging could provide an accurate assessment of the expression of molecular targets in disseminated cancers enabling stratification of patients for specific therapies. B7-H3 (CD276) is a transmembrane protein belonging to the B7 superfamily. This protein is overexpressed in different types of human malignancies and such upregulation is generally associated with a poor clinical prognosis. In this study, targeting properties of an Affibody-based probe, AC12, containing a -GGGC amino acid sequence as a chelator (designated as AC12-GGGC) labelled with technetium-99m (99mTc) were evaluated for imaging of B7-H3-expressing tumours. AC12-GGGC was efficiently labelled with 99mTc. [99mTc]Tc-AC12-GGGC bound specifically to B7-H3 expressing cells in vitro with affinities in nanomolar range. In mice bearing B7-H3-expressing xenografts, [99mTc]Tc-AC12-GGGC showed tumour uptake of 2.1 ± 0.5 %ID/g at 2 h after injection. Its clearance from blood, normal organs and tissues was very rapid. This new targeting agent, [99mTc]Tc-AC12-GGGC, provided high tumour-to-blood ratio already at 2 h (8.2 ± 1.9), which increased to 11.0 ± 0.5 at 4 h after injection. Significantly (p < 0.05) higher tumour-to-liver and higher tumour-to-bone ratios at 2 h in comparison with 4 h after injection were observed. Thus, [99mTc]Tc-AC12-GGGC could be a promising candidate for further development.

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.

Imaging Biomarkers in Lung Cancer with 68Ga-DOTATATE, 18F-Fluoride, and 18F-FDG PET/CT Scans and the Theranostics Paradigm

Lung cancer is the number 1 cause of cancer deaths in the United States. The prognosis is quite grim with the exception of stage 1. When faced with several failed therapeutic regimens and rapid progression of the disease, considering alternative therapies such as radiopharmaceutical therapies may be an option. We describe the case of a 36-y-old man with lung adenocarcinoma who had imaging molecular characterization of his disease with ¹⁸F-FDG, ⁶⁸Ga-DOTATATE, and ¹⁸F-fluoride PET/CT scans that were able to shed some light on molecular characterization of his disease and serve as a guide to potential targeted or personalized radiopharmaceutical therapeutic options.

Anti-HER2 monoclonal antibody based-radioimmunoconjugates: Assessment of the chelating agent influence

In the present work, the radioimmunoconjugates ¹¹¹In-DTPA-trastuzumab and ¹⁷⁷Lu-DOTA-trastuzumab were evaluated regarding the influence of the chelating agents on the physical-chemical parameters and human epidermal growth factor receptor 2 (HER2) tumor cell binding. Data showed that both chelating agents, at predetermined molar ratios (antibody:chelator - 1:10 and 1:20), did not influence the immunoconjugates integrity, the radiolabeling process and the radiolabeled antibodies stability. However, differences were observed in the lipophilic feature between DOTA and DTPA radioimmunoconjugates and in the specific binding to SK-BR-3 tumor cells (HER2 positive). Therefore, this study showed the importance of assessing the influence of chelating agents and their molar ratios in the development process of radioimmunoconjugates.

Development of Novel 111-In-Labelled DOTA Urotensin II Analogues for Targeting the UT Receptor Overexpressed in Solid Tumours

Overexpression of G protein-coupled receptors (GPCRs) in tumours is widely used to develop GPCR-targeting radioligands for solid tumour imaging in the context of diagnosis and even treatment. The human vasoactive neuropeptide urotensin II (hUII), which shares structural analogies with somatostatin, interacts with a single high affinity GPCR named UT. High expression of UT has been reported in several types of human solid tumours from lung, gut, prostate, or breast, suggesting that UT is a valuable novel target to design radiolabelled hUII analogues for cancer diagnosis. In this study, two original urotensinergic analogues were first conjugated to a DOTA chelator via an aminohexanoic acid (Ahx) hydrocarbon linker and then -hUII and DOTA-urantide, complexed to the radioactive metal indium isotope to successfully lead to radiolabelled DOTA-Ahx-hUII and DOTA-Ahx-urantide. The 111In-DOTA-hUII in human plasma revealed that only 30% of the radioligand was degraded after a 3-h period. DOTA-hUII and DOTA-urantide exhibited similar binding affinities as native peptides and relayed calcium mobilization in HEK293 cells expressing recombinant human UT. DOTA-hUII, not DOTA-urantide, was able to promote UT internalization in UT-expressing HEK293 cells, thus indicating that radiolabelled 111In-DOTA-hUII would allow sufficient retention of radioactivity within tumour cells or radiolabelled DOTA-urantide may lead to a persistent binding on UT at the plasma membrane. The potential of these radioligands as candidates to target UT was investigated in adenocarcinoma. We showed that hUII stimulated the migration and proliferation of both human lung A549 and colorectal DLD-1 adenocarcinoma cell lines endogenously expressing UT. In vivo intravenous injection of 111In-DOTA-hUII in C57BL/6 mice revealed modest organ signals, with important retention in kidney. 111In-DOTA-hUII or 111In-DOTA-urantide were also injected in nude mice bearing heterotopic xenografts of lung A549 cells or colorectal DLD-1 cells both expressing UT. The observed significant renal uptake and low tumour/muscle ratio (around 2.5) suggest fast tracer clearance from the organism. Together, DOTA-hUII and DOTA-urantide were successfully radiolabelled with 111Indium, the first one functioning as a UT agonist and the second one as a UT-biased ligand/antagonist. To allow tumour-specific targeting and prolong body distribution in preclinical models bearing some solid tumours, these radiolabelled urotensinergic analogues should be optimized for being used as potential molecular tools for diagnosis imaging or even treatment tools.

Effect of a radiolabel biochemical nature on tumor-targeting properties of EpCAM-binding engineered scaffold protein DARPin Ec1

Radionuclide-based imaging of molecular therapeutic targets might facilitate stratifying patients for specific biotherapeutics. New type of imaging probes, based on designed ankyrin repeat proteins (DARPins), have demonstrated excellent contrast of imaging of human epidermal growth factor type 2 (HER2) expression in preclinical models. We hypothesized that labeling approaches, which result in lipophilic radiometabolites (non-residualizing labels), would provide the best imaging contrast for DARPins that internalize slowly after binding to cancer cells. The hypothesis was tested using DARPin Ec1 that binds to epithelial cell adhesion molecule (EpCAM). EpCAM is a promising therapeutic target. Ec1 was labeled with ¹²⁵I using two methods to obtain the non-residualizing labels, while residualizing labels were obtained by labeling it with ^99mTc. All labeled Ec1 variants preserved target specificity and picomolar binding affinity to EpCAM-expressing pancreatic adenocarcinoma BxPC-3 cells. In murine models, all the variants provided similar tumor uptake. However, ¹²⁵I-PIB-H₆-Ec1 had noticeably lower retention in normal tissues, which provided appreciably higher tumor-to-organ ratios. Furthermore, ¹²⁵I-PIB-H₆-Ec1 demonstrated the highest imaging contrast in preclinical models than any other EpCAM-imaging agent tested so far. In conclusion, DARPin Ec1 in combination with a non-residualizing label is a promising probe for imaging EpCAM expression a few hours after injection.

Specific Targeting of Somatostatin Receptor Subtype-2 for Fluorescence-Guided Surgery

PURPOSE

Clinically available intraoperative imaging tools to assist surgeons in identifying occult lesions are limited and partially responsible for the high rate of disease recurrence in patients with neuroendocrine tumors (NET). Using the established clinical efficacy of radiolabeled somatostatin analogs as a model, we demonstrate the ability of a fluorescent somatostatin analog to selectively target tumors that overexpress somatostatin receptor subtype-2 (SSTR2) and demonstrate utility for fluorescence-guided surgery (FGS).

EXPERIMENTAL DESIGN

A multimodality chelator (MMC) was used as a "radioactive linker" to synthesize the fluorescently labeled somatostatin analog, ^67/68Ga-MMC(IR800)-TOC. In vivo studies were performed to determine the pharmacokinetic profile, optimal imaging time point, and specificity for SSTR2-expressing tissues. Meso- and microscopic imaging of resected tissues and frozen sections were also performed to further assess specific binding, and binding to human NETs was examined using surgical biospecimens from patients with pancreatic NETs.

RESULTS

Direct labeling with ⁶⁷Ga/⁶⁸Ga provided quantitative biodistribution analysis that was in agreement with fluorescence data. Receptor-mediated uptake was observed in vivo and ex vivo at the macro-, meso-, and microscopic scales. Surgical biospecimens from patients with pancreatic NETs also displayed receptor-specific agent binding, allowing clear delineation of tumor boundaries that matched pathology findings.

CONCLUSIONS

The radioactive utility of the MMC allowed us to validate the binding properties of a novel FGS agent that could have a broad impact on cancer outcomes by equipping surgeons with real-time intraoperative imaging capabilities.

Could Somatostatin Enhance the Outcomes of Chemotherapeutic Treatment in SCLC?

PURPOSE

Somatostatin is a peptide with a potent and broad antisecretory action, which makes it an invaluable drug target for the pharmacological management of pituitary adenomas and neuroendocrine tumors. Furthermore, somatostatin (SST) receptors (SSTR1, 2A and B, 3, 4 and 5) belong to the G protein coupled receptor family and are overexpressed in tumors. Since, human small-cell lung cancer overexpresses somatostatin receptors (STTR), they could be legitimate targets for treating SCLC.The aim of this study was the evaluation of cytotoxicity of somatostatin in combination with several anticancer drugs in HTB-175 cell line (Small Cell Lung Cancer Cell line that expresses neuron specific enolase).

METHODS

Docetaxel, Paclitaxel, Carboplatin, Cisplatin, Etoposide, Gemzar, Navelbine, Fluorouracil, Farmorubicin are the chemotherapeutic drugs that we used for the combination before and after adding somatostatin in SCLC cell culture. HTB-175 cell line was purchased from ATCC LGC Standards.At indicated time-point, 48h after the combination, cell viability and apoptosis were measured with Annexin V staining by flow cytometry.

RESULTS

Flow cytometry showed that Docetaxel, Paclitaxel, Gemzar and Cisplatin induced apoptosis more when they were added before somatostatin, whereas etoposide induced apoptosis more after somatostatin treatment. Navelbine alone or in combination with somatostatin showed no differences in apoptosis. Farmorubicin showed equal toxicity in all combinations. Fluorouracil and Carboplatin induced apoptosis more when added alone in HTB-175 cell line. However, increased apoptosis was also observed when Carboplatin was administered before somatostatin in higher concentrations.

CONCLUSION

Our results indicated that depending on the drug, somatostatin treatment before or after chemotherapeutic drugs increased apoptosis in small cell lung cancer cells. We suggest that long acting somatostatin analogues could be used as additive and maintenance therapy in combination to antineoplastic agents in SCLC patients.

[99mTc-octreotide receptor scintigraphy in NCI-H446 small cell lung cancer nude mice model]

BACKGROUND

For highly aggressive small cell lung cancer (SCLC), early diagnosis is important for its prognosis, but the current inspection methods are more limited, with poor specificity of the traditional imaging methods, and the high cost of PET/CT, difficult to popularization and application. SCLC is kind of neuroendocrine tumors, high expression of somatostatin receptors, which is the cornerstone of its early molecular imaging diagnosis. The aim of this study is to observe the biodistribution and metabolism of 99mTc-octreotide in normal and the human SCLC bearing nude mice.

METHODS

Dynamic and static scintigraphy at 0.5 h, 2 h, 3 h, 4 h were performed in both normal and tumor bearing nude mice after intravenous injection of 99mTc-octreotide. The technique of drawing region of interest (ROI) was used to obtain the averaged pixel counts and the activity-time (A-T) curve of brain, heart, lung, liver, kidney, tumor, respectively.

RESULTS

① The biodistribution study in normal nude mice showed highest uptake in kidney and liver, lower in lung and heart, lowest in brain. Most 99mTc-octreotide was excreted via kidney. ② All tumors were displayed clearly at 3 h postinjection of 99mTc-octreotide. The averaged T/N ratio at 0.5 h, 2 h, 3 h, 4 h postinjection of 99mTc-octreotide was 1.163 ± 0.03, 2.08 ± 0.12, 3.03 ± 0.23, 2.689 ± 0.31, respectively (F=51.69, P<0.000,1). The radioactivity of tumor was lower than liver, and similar with the lung. The curve of tumor showed a radioactivity peak at 2 min-3 min postinjection.

CONCLUSIONS

99mTc-octreotide receptor imaging on nude mice bearing SCLC shares high positive rate, especially at 3 h postinjection.

The diversity of (68)Ga-based imaging agents

Development of new radiopharmaceuticals and their availability are crucial factors influencing the expansion of clinical nuclear medicine. The number of new (68)Ga-based imaging agents for positron emission tomography (PET) is increasing greatly. (68)Ga has been used for labeling of a broad range of molecules (small organic molecules, peptides, proteins, and oligonucleotides) as well as particles, thus demonstrating its potential to become a PET analog of the legendary generator-produced gamma-emitting (99m)Tc but with added value of higher sensitivity and resolution as well as quantitation and dynamic scanning. Further, the availability of technology for GMP-compliant automated tracer production can facilitate the introduction of new radiopharmaceuticals and enable standardized, harmonized multicenter studies to be conducted for regulatory approval. This chapter presents some examples of tracers for targeted, pretargeted, and nontargeted imaging with emphasis on the potential of (68)Ga to facilitate clinically practical PET development and to promote the PET technique worldwide for earlier and better diagnostics, and personalized medicine with the ultimate objective of improved therapeutic outcome.

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.

Radiation induces up-regulation of somatostatin receptors 1, 2, and 5 in small cell lung cancer in vitro also at low absorbed doses

BACKGROUND

Radiation can be used to up-regulate the expression of the somatostatin receptor (sstr) subtype 2 in small cell lung cancer (SCLC) cells at absorbed doses of 2-8 Gy. Increased sstr expression results in increased binding of radiolabeled somatostatin analogs to the tumor cell, which enhances the efficacy of systemic radionuclide therapy. The aim of this study was to determine if lower absorbed doses could up-regulate sstr2 expression, and possibly influence other sstr subtypes.

METHODS

Human H69 SCLC cells were irradiated with an absorbed dose of 0.12-6.0 Gy and the sstr mRNA expression 3 days after irradiation was measured by quantitative real-time polymerase chain reaction for sstr1-5. At the same time point was the binding of [(177)Lu]-DOTA(0)-Tyr(3)-octreotate to the cells measured after irradiation to an absorbed dose of 0.12-2.0 Gy and compared to the binding to nonirradiated cells.

RESULTS

mRNA expression of sstr1, sstr2, and sstr5 was increased by a factor of 1.5-2 in cells irradiated with absorbed doses≥4 Gy and the binding of [(177)Lu]-DOTA(0)-Tyr(3)-octreotate was, accordingly, 2-3 times higher to irradiated cells for all absorbed doses, except 0.25 Gy.

CONCLUSION

The binding of [(177)Lu]-DOTA(0)-Tyr(3)-octreotate was increased after radiation exposure. This increase was observed at low absorbed doses in parallel with up-regulation of sstr1, sstr2, and sstr5 mRNA.

Binding of TS1, an anti-keratin 8 antibody, in small-cell lung cancer after 177Lu-DOTA-Tyr3-octreotate treatment: a histological study in xenografted mice

Background

Small-cell lung carcinoma (SCLC) is an aggressive malignancy characterised by an early relapse, a tendency towards drug resistance, and a high incidence of metastasis. SCLC cells are of neuroendocrine origin and express high levels of somatostatin receptors; therefore, future treatment might involve targeting tumours with radiolabelled somatostatin analogues. This therapy induces abundant necrotic patches that contain exposed keratins; thus, keratin 8, which is one of the most abundant cytoskeletal proteins may represent an interesting secondary target for SCLC. This study aimed to investigate the effects of¹⁷⁷Lu-DOTA-Tyr³-octerotate and the binding of the monoclonal anti-keratin 8 antibody, TS1, in vitro in treated SCLC- and midgut-xenografted mouse models.

Methods

NCI-H69- and GOT1-xenotransplanted mice were treated with three doses of 30 MBq¹⁷⁷Lu-DOTA-Tyr³-octreotate administered 24 h apart. Mice xenotransplanted with NCI-H69 were sacrificed 1, 5, 12, 20 and 150 days post-injection or when the tumour had regrown to its original size. GOT1-xenotransplanted mice were sacrificed 3 days post-injection. Immunohistochemistry was performed to evaluate TS1 staining in tumours and in seven human biopsies of primary SCLC from pulmonary bronchi. Central cell density and nucleus size were determined in NCI-H69 sections.

Results

Twelve days after¹⁷⁷Lu-DOTA-Tyr³-octerotate treatment, the SCLC xenograft response was extensive. Twenty days after treatment, one of three analysed tumours displayed complete remission. The other two tumours showed 1/4 the cell density of untreated controls and cell nuclei were about three times larger than those of untreated controls. At 150 days after treatment, one of four mice exhibited complete remission. Treated tumours displayed increased TS1 antibody accumulation and high TS1 binding in necrotic patches. All seven human SCLC biopsies displayed necrotic areas with TS1 staining.

Conclusions

Radiation treatment with three injections of 30 MBq¹⁷⁷Lu-DOTA-Tyr³-octreotate had pronounced effects on tumour cell density and cell nuclei, which indicated mitotic catastrophe. Despite these anti-tumour effects, two of three SCLC tumours recurred. Further studies should investigate the nature of tumour cell survival and develop more effective treatments. High TS1 accumulation in tumour sections in vitro after¹⁷⁷Lu-DOTA-Tyr³-octerotate treatment indicated that TS1 might represent a promising secondary therapeutic strategy.

Biodistribution and clearance of small molecule hapten chelates for pretargeted radioimmunotherapy

PURPOSE

The favorable pharmacokinetics and clinical safety profile of metal-chelated 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) suggests that it might be an ideal hapten for pretargeted radioimmunotherapy. In an effort to minimize hapten retention in normal tissues and determine the effect of various chemical adducts on in vivo properties, a series of DOTA-based derivatives were evaluated.

PROCEDURES

Biodistribution and whole-body clearance were evaluated for (177)Lu-labeled DOTA, DOTA-biotin, a di-DOTA peptide, and DOTA-aminobenzene in normal CD1 mice. Kidney, liver, and bone marrow doses were estimated using standard Medical Internal Radiation Dose methodology.

RESULTS

All haptens demonstrated similar low tissue and whole-body retention, with 2-4% of the injected dose remaining in mice 4 h postinjection. The kidney is predicted to be dose limiting for all (177)Lu-labeled haptens tested with an estimated kidney dose of approximately 0.1 mGy/MBq.

CONCLUSIONS

We present here a group of DOTA-based haptens that exhibit rapid clearance and exceptionally low whole-body retention 4 h postinjection. Aminobenzene, tyrosine-lysine, and biotin groups have minimal effects on the blood clearance and biodistribution of (177)Lu-DOTA.

[Lu]-DOTA-Tyr-octreotate: A Potential Targeted Radiotherapeutic for the Treatment of Medulloblastoma

Medulloblastoma, the most common pediatric brain tumor, is difficult to treat because conventional therapeutic approaches result in significant toxicity to normal central nervous system tissues, compromising quality of life. Given the fact that medulloblastomas express the somatostatin subtype 2 receptor, [(177)Lu-DOTA(0),Tyr(3)]octreotate ([(177)Lu]DOTA-TATE) could be a potentially useful targeted radiotherapeutic for the treatment of this malignancy. The current study was undertaken to evaluate this possibility in preclinical models of D341 MED human medulloblastoma by comparing the properties of [(177)Lu]DOTA-TATE to those of glucose-[(125)I-Tyr(3)]-octreotate ([(125)I]Gluc-TOCA), a radiopeptide previously shown to target this cell line. In vitro assays indicated that both labeled peptides exhibited similar cell-associated and internalized radioactivity after a 30-min incubation at 37°C; however, at the end of the 4 h incubation period, the internalized radioactivity for [(177)Lu]DOTA-TATE (6.22 " 0.75%) was nearly twice that for [(125)I]Gluc-TOCA (3.16 " 0.27%), with similar differences seen in total cell-associated radioactivity levels. Consistent with the results from the internalization assays, results from paired-label tissue distribution studies in athymic mice with subcutaneous D341 MED medulloblastoma xenografts showed a similar degree of tumor accumulation for [(177)Lu]DOTA-TATE and [(125)I]Gluc-TOCA at early time points but by 24 h, a more than 5-fold advantage was observed for the (177)Lu-labeled peptide. Tumor-to-normal tissue ratios generally were more favorable for [(177)Lu]DOTA-TATE at all time points, due in part to its lower accumulation in normal tissues except kidneys. Taken together, these results suggest that [(177)Lu]DOTA-TATE warrants further investigation as a targeted radiotherapeutic for medulloblastoma treatment.

Effects of linker variation on the in vitro and in vivo characteristics of an 111In-labeled RGD peptide

INTRODUCTION

Due to the selective expression of the alpha(v)beta3 integrin in tumors, radiolabeled arginine-glycine-aspartic acid (RGD) peptides are attractive candidates for tumor targeting. Minor modifications of these peptides could have a major impact on in vivo characteristics. In this study, we systematically investigated the effects of linker modification between two cyclic RGD sequences and DOTA (1,4,7,10-tetraazadodecane-N,N',N",N'''-tetraacetic acid) on the in vitro and in vivo characteristics of the tracer.

METHODS

A dimeric RGD peptide was synthesized and conjugated either directly with DOTA or via different linkers: PEG4 (polyethylene glycol), glutamic acid or lysine. The RGD peptides were radiolabeled with 111In, and their in vitro and in vivo alpha(v)beta3-binding characteristics were determined.

RESULTS

LogP values varied between -2.82+/-0.06 and -3.95+/-0.33. The IC50 values for DOTA-E-[c(RGDfK)]2, DOTA-PEG4-E-[c(RGDfK)]2, DOTA-E-E-[c(RGDfK)]2 and DOTA-K-E-[c(RGDfK)]2 were comparable. Two hours after injection, the tumor uptakes of the 111In-labeled compounds were not significantly different. The kidney accumulation of [111In]-DOTA-K-E-[c(RGDfK)]2 [4.05+/-0.20% of the injected dose per gram (ID/g)] was significantly higher as compared with that of [111In]-DOTA-E-[c(RGDfK)]2 (2.63+/-0.19% ID/g; P<.05) as well as that of [111In]-DOTA-E-E-[c(RGDfK)]2 (2.16+/-0.21% ID/g; P<.01). The liver uptake of [111In]-DOTA-E-E-[c(RGDfK)]2 (2.12+/-0.09% ID/g) was significantly higher as compared with that of [111In]-DOTA-E-[c(RGDfK)]2 (1.64+/-0.1% ID/g; P<.05) as well as that of [111In]-DOTA-K-E-[c(RGDfK)]2 (1.52+/-0.04% ID/g; P<.01).

CONCLUSIONS

Linker variation did not affect affinity for alpha(v)beta3 and tumor uptake. Insertion of lysine caused enhanced kidney retention; that of glutamic acid also resulted in enhanced retention in the kidneys. PEG4 appeared to be the most suitable linker as compared with glutamic acid and lysine because it has the highest tumor-to-blood ratio and the lowest uptake in the kidney and liver.

Comparison of [(177)Lu-DOTA(0),Tyr(3)]octreotate and [(177)Lu-DOTA(0),Tyr(3)]octreotide: which peptide is preferable for PRRT?

PURPOSE

Patients with somatostatin receptor subtype 2-positive metastasised neuroendocrine tumours can be treated with [(177)Lu-DOTA(0),Tyr(3)]octreotate. Some use octreotide as the peptide for peptide receptor radionuclide therapy (PRRT). We compared in seven patients [(177)Lu-DOTA(0),Tyr(3)]octreotide ((177)Lu-DOTATOC) and [(177)Lu-DOTA(0),Tyr(3)]octreotate ((177)Lu-DOTATATE), to see which peptide should be preferred for PRRT with (177)Lu.

METHODS

In the same patients, 3,700 MBq (177)Lu-DOTATOC and 3,700 MBq (177)Lu-DOTATATE was administered in separate therapy sessions. Amino acids were co-administered. Whole-body scanning was performed on days 1, 4 and 7 post therapy. Blood and urine samples were collected. We calculated residence times for tumours, spleen and kidneys.

RESULTS

All patients had longer residence times in spleen, kidneys and tumours after use of (177)Lu-DOTATATE (p=0.016 in each case). Comparing (177)Lu-DOTATATE with (177)Lu-DOTATOC, the mean residence time ratio was 2.1 for tumour, 1.5 for spleen and 1.4 for kidneys. Dose-limiting factors for PRRT are bone marrow and/or kidney dose. Although the residence time for kidneys was longer when using (177)Lu-DOTATATE, the mean administered dose to tumours would still be advantageous by a factor of 1.5, assuming a fixed maximum kidney dose is reached. Plasma radioactivity after (177)Lu-DOTATATE was comparable to that after (177)Lu-DOTATOC. Urinary excretion of radioactivity was comparable during the first 6 h; thereafter there was a significant advantage for (177)Lu-DOTATOC.

CONCLUSION

(177)Lu-DOTATATE had a longer tumour residence time than (177)Lu-DOTATOC. Despite a longer residence time in kidneys after (177)Lu-DOTATATE, tumour dose will always be higher. Therefore, we conclude that the better peptide for PRRT is octreotate.