Radiolabeled Somatostatin Analogs Research Articles

68Ga-DOTATOC Versus 68Ga-DOTATATE PET/CT in Functional Imaging of Neuroendocrine Tumors

Radiolabeled somatostatin analogs represent valuable tools for both in vivo diagnosis and therapy of neuroendocrine tumors (NETs) because of the frequent tumoral overexpression of somatostatin receptors (sst). The 2 compounds most often used in functional imaging with PET are (68)Ga-DOTATATE and (68)Ga-DOTATOC. Both ligands share a quite similar sst binding profile. However, the in vitro affinity of (68)Ga-DOTATATE in binding the sst subtype 2 (sst2) is approximately 10-fold higher than that of (68)Ga-DOTATOC. This difference may affect their efficiency in the detection of NET lesions because it is the sst2 that is predominantly overexpressed in NET. We thus compared the diagnostic value of PET/CT with both radiolabeled somatostatin analogs ((68)Ga-DOTATATE and (68)Ga-DOTATOC) in the same NET patients. Forty patients with metastatic NETs underwent (68)Ga-DOTATOC and (68)Ga-DOTATATE PET/CT as part of the work-up before prospective peptide receptor radionuclide therapy. The performance of both imaging methods was analyzed and compared for the detection of individual lesions per patient and for 8 defined body regions. A region was regarded positive if at least 1 lesion was detected in that region. In addition, radiopeptide uptake in terms of the maximal standardized uptake value (SUVmax) was compared for concordant lesions and renal parenchyma. Seventy-eight regions were found positive with (68)Ga-DOTATATE versus 79 regions with (68)Ga-DOTATOC (not significant). Overall, however, significantly fewer lesions were detected with (68)Ga-DOTATATE than with (68)Ga-DOTATOC (254 vs. 262, P < 0.05). Mean (68)Ga-DOTATATE SUVmax across all lesions was significantly lower than (68)Ga-DOTATOC (16.0 ± 10.8 vs. 20.4 ± 14.7, P < 0.01). Mean SUVmax for renal parenchyma was not significantly different between (68)Ga-DOTATATE and (68)Ga-DOTATOC (12.7 ± 3.0 vs. 13.2 ± 3.3). (68)Ga-DOTATOC and (68)Ga-DOTATATE possess a comparable diagnostic accuracy for the detection of NET lesions, with (68)Ga-DOTATOC having a potential advantage. The approximately 10-fold higher affinity for the sst2 of (68)Ga-DOTATATE does not prove to be clinically relevant. Quite unexpectedly, SUVmax of (68)Ga-DOTATOC scans tended to be higher than their (68)Ga-DOTATATE counterparts.

Radiation Induces Up-Regulation of Somatostatin Receptors 1, 2, and 5 in Small Cell Lung Cancer In Vitro Also at Low Absorbed Doses

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. 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. 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. 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.

Evaluation of copper-64-labeled somatostatin agonists and antagonist in SSTr2-transfected cell lines that are positive and negative for p53: implications for cancer therapy

Radiolabeled somatostatin analogs have become important agents for molecular imaging and targeted radiotherapy of somatostatin receptor-positive tumors. Here we determine the effect of the tumor suppressor protein, p53, on trafficking (64)Cu to tumor cell nuclei from DOTA vs. CB-TE2A-conjugated agonist Y3-TATE and the antagonist (64)Cu-CB-TE2A-sst2-ANT in cell lines that are positive or negative for p53. Receptor binding, internalization, cyclic adenosine monophosphate (cAMP) and nuclear localization studies were performed with the somatostatin receptor subtype 2 (SSTr2) agonists, (64)Cu-CB-TE2A-Y3-TATE and (64)Cu-DOTA-Y3-TATE vs. antagonist, (64)Cu-CB-TE2A-sst2-ANT, in SSTr2-transfected p53 +/+ and -/- HCT116 colorectal carcinoma cells. The antagonist, (64)Cu-CB-TE2A-sst2-ANT, bound 8-9-fold more SSTr2 binding sites than did the (64)Cu-labeled agonists. (64)Cu-CB-TE2A-Y3-TATE was more efficiently internalized than (64)Cu-DOTA-Y3-TATE, while (64)Cu-CB-TE2A-sst2-ANT showed lower yet significant levels of internalization. CB-TE2A-Y3-TATE acted as a full agonist, inhibiting cAMP production, whereas CB-TE2A-sst2-ANT showed no inhibition of cAMP production. The (64)Cu from agonists (64)Cu-DOTA-Y3-TATE and (64)Cu-CB-TE2A-Y3-TATE showed greater nuclear localization at 24 h in p53 +/+ vs. -/- cells; however, there was no difference in the levels of (64)Cu from the antagonist based on p53 status. Surprisingly, the DOTA and CB-TE2A-conjugated agonists showed similar nuclear localization in the p53 +/+ and -/- cells, suggesting no difference in (64)Cu release from these chelators in the HCT116 cell lines. Based on these in vitro data, the agonist (64)Cu-CB-TE2A-Y3-TATE demonstrates the most promise as an agent for targeted radiotherapy in p53 positive, SSTr2-positive tumors.

Nuclear medicine techniques for the imaging and treatment of neuroendocrine tumours

Nuclear medicine plays a pivotal role in the imaging and treatment of neuroendocrine tumours (NETs). Somatostatin receptor scintigraphy (SRS) with [(111)In-DTPA(0)]octreotide has proven its role in the diagnosis and staging of gastroenteropancreatic NETs (GEP-NETs). New techniques in somatostatin receptor imaging include the use of different radiolabelled somatostatin analogues with higher affinity and different affinity profiles to the somatostatin receptor subtypes. Most of these analogues can also be labelled with positron-emitting radionuclides that are being used in positron emission tomography imaging. The latter imaging modality, especially in the combination with computed tomography, is of interest because of encouraging results in terms of improved imaging quality and detection capabilities. Considerable advances have been made in the imaging of NETs, but to find the ideal imaging method with increased sensitivity and better topographic localisation of the primary and metastatic disease remains the ultimate goal of research. This review provides an overview of the currently used imaging modalities and ongoing developments in the imaging of NETs, with the emphasis on nuclear medicine and puts them in perspective of clinical practice. The advantage of SRS over other imaging modalities in GEP-NETs is that it can be used to select patients with sufficient uptake for treatment with radiolabelled somatostatin analogues. Peptide receptor radionuclide therapy (PRRT) is a promising new tool in the management of patients with inoperable or metastasised NETs as it can induce symptomatic improvement with all Indium-111, Yttrium-90 or Lutetium-177-labelled somatostatin analogues. The results that were obtained with [(90)Y-DOTA(0),Tyr(3)]octreotide and [(177)Lu-DOTA(0),Tyr(3)]octreotate are even more encouraging in terms of objective tumour responses with tumour regression and documented prolonged time to progression. In the largest group of patients receiving PRRT, treated with [(177)Lu-DOTA(0),Tyr(3)]octreotate, a survival benefit of several years compared with historical controls has been reported.

Radiolabelling of peptides for PET, SPECT and therapeutic applications using a fully automated disposable cassette system

Radiolabelled somatostatin analogues have found wide clinical use in nuclear medicine for both diagnostic and therapeutic applications. Here, we describe the development of a fully automated synthesis system allowing radiolabelling of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-derivatized peptides with ⁶⁸Ga/¹¹¹In/¹⁷⁷Lu and ⁹⁰Y, meeting radiation safety and pharmaceutical requirements. The system consists of a syringe pump, a holder for insertion of a single use multivalve cassette, a heater and a removable radiation shielding. ⁶⁸Ga labelling was performed in acetate buffer and ¹⁷⁷Lu, ⁹⁰Y and ¹¹¹In labelling in ascorbate buffer, respectively, followed by purification on a C18 cartridge and final sterile filtration. Cross-contamination was prevented by using disposable cassettes and also by ensuring pharmaceutical standards. Radiochemical purity (RCP) was determined by instant thin-layer chromatography on silica gel impregnated glass fibres and reversed-phase high performance liquid chromatography. ⁶⁸Ga-DOTA-peptides were prepared with high RCP (>91%) and radiochemical yields (RCY>80% decay corrected) and ⁶⁸Ge content was less than 0.0001% in all cases. Synthesis time did not exceed 30 min. ¹¹¹In, ¹⁷⁷Lu and ⁹⁰Y labelling of DOTA-peptides resulted again in high yields (approximately 90%) and RCP (approximately 95%) and total synthesis time of less than 45 min. Radiation dose to fingers was considerably reduced when compared with manual labelling procedures. The described system allows fully automated, aseptic preparation of DOTA-peptides radiolabelled with different radionuclides in high radiochemical yields and pharmaceutical quality suitable for clinical application.

Novel Medical Therapies of Recurrent and Metastatic Gastroenteropancreatic Neuroendocrine Tumors

Neuroendocrine tumors (NETs) of the gastrointestinal tract and pancreas are slow-growing but commonly advanced malignancies with increasing incidence and prevalence. While locoregional disease can be effectively managed with resection, treatment of recurrent, progressive or metastatic disease has until recently been limited to palliative embolization and cytoreducitve surgery, with cytotoxic chemotherapeutic agents being the last resort. However, novel molecular targeted therapies inhibiting malignant cell proliferation and neoangiogenesis, as well as new cytotoxic chemotherapy drugs and somatostatin analogues, are all being investigated for their potential use in advanced neuroendocrine tumors. Long-acting release forms of octreotide have been shown to not only improve symptoms in carcinoid syndrome but to also delay progression of gastrointestinal NETs. On the other hand, phase III trials have demonstrated everolimus (with octreotide) and sunitinib to increase progression-free survival in pancreatic NETs. Use of bevacizumab has also shown promise in a phase II study, and results of an ongoing phase III trial comparing it to interferon are eagerly expected. Use of radiolabeled somatostatin analogues is still under investigation, though several phase II studies are encouraging. New cytotoxic agents, most notably temozolomide and capecitabine, are already in use, but their relative effectiveness compared to streptozocin in pancreatic NETs is yet to be determined.

Improved Control of Severe Hypoglycemia in Patients with Malignant Insulinomas by Peptide Receptor Radionuclide Therapy

Insulinomas are relatively rare neuroendocrine tumors of the pancreas. Only 10% are considered malignant. Control of insulin hypersecretion and hypoglycemia in patients with malignant insulinomas may be extremely difficult. Different medications and chemotherapy schedules have been used. Five patients with metastatic insulinomas and severe, poorly controllable, hypoglycemia are described. These patients required continuous glucose infusion to control severe hypoglycemia, which were induced by the high levels of insulin secretion. Conventional medications, such as diazoxide, or streptozotocin-based chemotherapies had been used to control hypoglycemia but were ineffective and/or produced adverse effects. All patients were treated with sc octreotide. Peptide receptor radionuclide therapy with radiolabeled-somatostatin analogs was used. After the start of radiolabeled somatostatin analog therapy, the five patients with metastatic insulinomas had stable disease for a mean period of 27 months. During these months, the patients were without any hypoglycemic episodes. Finally, three of five patients died because of progressive disease. Radiolabeled somatostatin analog therapy can stabilize tumor growth and can be very successful in further controlling severe hypoglycemia in malignant insulinomas. In our series, this eventually resulted in improved survival outside the hospital setting.

Synthesis and molecular study of a new NMB analogue (BBN) for imaging prostate cancer

Radiolabeled somatostatin analogues are important tools for the in vivo localization and targeted radionuclide therapy of somatostatin-receptor-positive tumors. The aim of this study was to evaluate a new somatostatin analogue designed for the labeling with 99mTc: [6-hydrazinopyridine-3-carboxylic acid (HYNIC0), 1-Nal3, Thr8]-octreotide ([HYNIC]-NATE), using ethylenediamine-N,N′-diacetic acid (EDDA) and tricine as coligands.Synthesis was preformed on a solid phase using a standard Fmoc strategy. Labeling with 99mTc was performed at 100°C for 10 min using SnCl2 as a reductant. Radiochemical analysis involved ITLC and high-performance liquid chromatography methods. Peptide conjugate affinity was determined in AR4-2J cell membranes. The internalization and externalization rates were studied in sstr2-expressing AR4-2J cells. Biodistribution of radiopeptide was studied in rats bearing the AR4-2J tumor.Radiolabeling was performed at high specific activities, and radiochemical purity was >95%. Peptide conjugate showed high affinity binding for sstr2. The radioligand showed a moderate and specific internalization into AR4-2J cells (14.13±0.61% at 4 h). In animal biodistribution studies, a receptor-specific uptake of radioactivity was observed in somatostatin-receptor-positive organs. After 4 h, uptake in the AR4-2J tumor was 1.33±0.23%ID/g (percentage of injected dose per gram of tissue).These data show that [99mTc/EDDA/tricine/HYNIC]-NATE is a specific radioligand for the somatostatin-receptor-positive tumors and is a suitable candidate for clinical studies.

New therapeutic options for metastatic malignant insulinomas

Insulinomas are the most common, functioning, pancreatic neuroendocrine tumours. The great majority (>90%) of insulinomas are nonmetastatic at presentation and can be surgically cured. The <10% patients with distant (liver-bone) metastases have a median survival of < 2 years. Everolimus and sunitinib have been recently introduced as targeted therapies for metastatic pancreatic neuroendocrine tumours. An additional advantage of everolimus in the treatment of patients with metastatic insulinomas is its capability to increase blood glucose levels. Peptide receptor radiotherapy using radiolabelled somatostatin analogues has also been shown to be successful in controlling tumour growth of metastatic pancreatic neuroendocrine tumours. In patients with metastatic insulinomas, this therapeutic modality was also effective in controlling hypoglycaemia, even in the presence of tumour regrowth. With the introduction of these new therapeutic modalities, the therapeutic arsenal for the 'tailor-made' approach of patients with metastatic insulinomas is further expanded.

Novel DOTA-Neurotensin Analogues for 111In Scintigraphy and 68Ga PET Imaging of Neurotensin Receptor-Positive Tumors

Overexpression of the high affinity neurotensin receptor 1 (NTSR1), demonstrated in several human cancers, has been proposed as a new marker for human ductal pancreatic carcinoma and as an independent factor for poor prognosis for ductal breast cancer, head and neck squamous cell carcinoma, and non-small cell lung cancer. The aim of the present study was to develop new DOTA-neurotensin analogues for positron emission tomography (PET) imaging with (68)Ga and for targeted radiotherapy with (90)Y or (177)Lu. We synthesized a DOTA-neurotensin analogue series. Two of these peptides bear two sequence modifications for metabolic stability: DOTA-NT-20.3 shares the same peptide sequence as the previously described DTPA-NT-20.3. In the sequence of DOTA-NT-20.4, the Arg(8)-Arg(9) bond was N-methylated instead of the Pro(7)-Arg(8) bond in DOTA-NT-20.3. An additional sequence modification was introduced in DOTA-LB119 to increase stability. A spacer was added between DOTA and the peptide sequence to increase affinity. Binding to HT29 cells, which express NTSR1, in vivo stability, and biodistribution of the various analogues were compared, and the best candidate was used to image tumors of various sizes with the microPET in mice. (111)In-DOTA-NT-20.3, in spite of a relatively high uptake in kidneys, showed specific tumor uptake and elevated tumor to other organ uptake ratios. High contrast images were obtained at early time points after injection that allowed tumor detection at a time interval postinjection appropriate for imaging with the short-lived radionuclide (68)Ga. (111)In-DOTA-NT-20.4 displayed inferior binding to HT29 cells and reduced tumor uptake. (111)In-DOTA-LB119 displayed at early time points a significantly lower renal uptake but also a lower tumor uptake than (111)In-DOTA-NT-20.3, although binding to HT29 cells was similar. (68)Ga-DOTA-NT-20.3 displayed higher tumor uptake than (68)Ga-DOTA-LB119 and allowed the detection of very small tumors by PET. In conclusion, DOTA-NT-20.3 is a promising candidate for (68)Ga-PET imaging of neurotensin receptor-positive tumors. DOTA-NT-20.3 may also be considered for therapy, as the yttrium-labeled peptide has higher affinity than that of the indium-labeled one. A prerequisite for therapeutic application of this neurotensin analogue would be to lower kidney uptake, for example, by infusion of basic amino acids, gelofusin, or albumin fragments, to prevent nephrotoxicity, as with radiolabeled somatostatin analogues.

Synthesis, radiolabelling and biodistribution of HYNIC-Tyr3 octreotide: a somatostatin receptor positive tumour imaging agent

In vivo imaging of tumours using radiolabelled somatostatin (SST) analogues has become an accepted clinical tool in oncology. HYNIC-Tyr3 octreotide and Tyr3 octreotide were synthesized by FMOC solid-phase peptide synthesis using a semi-automated synthesizer. These were analyzed and purified by RP-HPLC, mass spectroscopy, IR spectroscopy, 1H NMR and 13C NMR. The prochelator 6-BOC-HYNIC was also synthesised and characterised indigenously. HYNIC-Tyr3 octreotide was labelled with 99mTc using Tricine and EDDA as coligand by SnCl2 method. Labelling with 99mTc was performed at 100 °C for 15 min and radiochemical analysis by ITLC and HPLC methods. The radiochemical purity of the complex was over 98% and log p value was found to be −1.27 ± 0.12. The stability of radiolabelled peptide complex was checked at 37 °C up to 24 h. Blood clearance and protein-binding study was also performed. In vivo biodistribution studies in rat showed uptake of 99mTc-HYNIC-TOC in kidney than any other organs. The blood clearance was faster with rapid excretion through kidneys and relatively low uptake in liver.

Clinical results of radionuclide therapy of neuroendocrine tumours with 90Y-DOTATATE and tandem 90Y/177Lu-DOTATATE: which is a better therapy option?

PurposePeptide receptor radionuclide therapy (PRRT) using radiolabelled somatostatin analogues is a treatment option for patients with disseminated neuroendocrine tumours (NET). A combination treatment using the high-energy 90Y beta emitter for larger lesions and the lower energy 177Lu for smaller lesions has been postulated in the literature.The aim of the study was to evaluate combined 90Y/177Lu-DOTATATE therapy in comparison to 90Y-DOTATATE alone.MethodsFifty patients with disseminated NET were included in the study prospectively and divided into two groups: group A (n = 25) was treated with 90Y-DOTATATE, whereas group B (n = 25) received the 1:1 90Y/177Lu-DOTATATE. The administered activity was based on 3.7 GBq/m2 body surface area in three to five cycles, with amino acid infusion for nephroprotection.ResultsThe median overall survival time in group A was 26.2 months while in group B median survival was not reached. Overall survival was significantly higher in group B (p = 0.027). Median event-free survival time in group A was 21.4 months and in group B 29.4 months (p > 0.1). At the 12-month follow-up, comparison of group A vs group B showed stable disease (SD) in 13 vs 16 patients, disease regression (RD) in 5 vs 3 patients and disease progression (PD) in 3 vs 4 patients; 4 and 2 patients died, respectively. The 24-month follow-up results were SD in nine vs ten patients, RD in one patient vs none and PD in four patients in both groups; three and four patients died, respectively. Side effects were rare and mild.ConclusionThe results indicate that therapy with tandem radioisotopes (90Y/177Lu-DOTATATE) provides longer overall survival than with a single radioisotope (90Y-DOTATATE) and the safety of both methods is comparable.

Rapid isocratic HPLC investigation of radiochemical purity for 90Y-DOTATATE

DOTA-conjugated peptides, such as [DOTA0, Tyr3]ocreotide (DOTATOC) and [DOTA0, Tyr3]octreotate (DOTATATE) can be labeled with radionuclides such as 90Y, 177Lu and 111In at high specific activities. These radiolabelled somatostatin analogues are used for peptide receptor radionuclide therapy (PRRT). Currently, radio-high performance liquid chromatography (radio-HPLC) and radio-thin layer chromatography (radio-TLC) are the methods of choice for the analysis of the labeled compounds. In literature, radiochemical purity of the radiolabelled DOTATATE was investigated using gradient reversed-phase radio-HPLC. However, these studies indicate long retention time of the radiolabelled compound of 14.52 min. In our study, a new simple and rapid reversed-phase isocratic system enables the radiochemical purity of the radiolabelled DOTATATE within a few minutes.

Advances in the diagnostic imaging of pheochromocytomas

Pheochromocytomas (PCs) and paragangliomas (PGLs) are routinely localized by computed tomography (CT), magnetic resonance imaging (MRI), and metaiodobenzylguanidine (MIBG) scintigraphy. CT can identify tumors with high sensitivity but rather low specificity. MRI has higher sensitivity and specificity than CT and is superior to detect extra-adrenal disease. Radioiodinated MIBG scintigraphy has been used for more than 30 years and is based on MIBG uptake via the norepinephrine transporter on the cell membrane. The technique is very useful for whole-body studies in case of multiple primary tumors or metastases. Tumors with sole production of dopamine usually cannot be visualized with MIBG and may require positron emission tomographic (PET) studies with 18 F-labeled radiotracers. Somatostatin receptor scintigraphy (SRS) using the radiolabeled somatostatin analog octreotide (based on the expression of the somatostatin receptors 2 and 5 by the tumor) can demonstrate PGL or metastases not visualized by MIBG. In this article, we review the use of MIBG scintigraphy to diagnose PC/PGL and compare the sensitivity and specificity with that of CT and MRI. We also describe the recent SRS and PET techniques and review the latest results of clinical studies by comparing these imaging modalities. Future perspectives of functional imaging modalities for PC/PGL are finally presented.

Somatostatin receptor endoradiotherapy of neuroendocrine tumors: experience in Hungarian patients

Beside conventional therapies for the treatment of neuroendocrine tumors, a new therapeutical approach, peptide receptor radionuclide therapy has been developed recently. There are two important features which make this therapy feasible: somatostatin receptors are strongly over-expressed in most neuroendocrine tumors resulting in a high tumor-to-background ratio and internalization of the somatostatin-receptor complex in neuroendocrine cells. Due to these features, neuroendocrine tumors can be treated with radiolabelled somatostatin analogues. For peptide receptor radionuclide therapy, somatostatin analogues are conjugated to a chelator that can bind a radionuclide. The most frequently used radionuclides for neuroendocrine tumor treatment are the β-emitter Yttrium-90 (⁹⁰Y) and the β+γ emitter Lutetium-177 (¹⁷⁷Lu). Candidates for somatostatin receptor endoradiotherapy are patients with progressive, metastatic, somatostatin-receptor positive neuroendocrine tumors. Many patients have been successively treated with this approach: according to international results major remission can be achieved in 25% of the cases. Although this therapy is still unavailable in Hungary, Hungarian patients can be treated with somatostatin receptor endoradiotherapy with financial support from the National Health Fund in a co-operation with the University of Basel since 2005. During the past 5 years, 51 Hungarian patients have been treated with this therapy. This review briefly summarizes the theoretical background, indications, effectiveness and side effects of somatostatin receptor endoradiotherapy and the authors present the first data obtained from Hungarian patients.

Targeted Therapy in Advanced Well-Differentiated Neuroendocrine Tumors

Treatments for advanced neuroendocrine tumors were, until recently, rather limited. Salvage surgery and liver-directed therapy both have relatively limited impact, and systemic cytotoxic chemotherapy has minimal efficacy. In the absence of other effective treatments, somatostatin analogs have been used for years to control disease and neuroendocrine symptoms, without cytotoxic intent. Advances in targeted therapy for neuroendocrine tumors have opened several potentially new treatment paradigms in the management of these otherwise relatively drug-resistant neoplasms. Promising results have emerged from studies evaluating radiolabeled somatostatin analogs and inhibitors of the vascular endothelial growth factor and mammalian target of rapamycin pathways. This article reviews several of the more encouraging developments in this field.

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

BackgroundSmall-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.MethodsNCI-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.ResultsTwelve 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.ConclusionsRadiation 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.

Yttrium-90 DOTATOC therapy in GEP-NET and other SST2 expressing tumors: a selected review

Treatment of somatostatin receptor-positive tumors with radiolabeled somatostatin analog is a promising option. Several phase I and phase II studies done at a few centers around the world reported encouraging results with [⁹⁰Y-DOTA-Tyr³]-octreotide (DOTATOC) and/or [(177)Lu-DOTA-Tyr³-Thr⁸]-octreotate (DOTATATE). The current article is a selective review of patients who were treated mainly with ⁹⁰Y-DOTATOC after failure with conventional therapy. The aim is to provide an updated comprehensive evaluation of the overall effectiveness of ⁹⁰Y-DOTATOC therapy in patients with somatostatin-positive tumors. Review of several studies revealed an objective response rate ranging from 20 to 28% for all neuroendocrine tumors (NET)s. For gastroenteropancreatic-NET (GEP-NET), the response rate was found to be consistently better in the range 28-38%. Overall, the cumulative response rate was found to be 24%. An important issue in peptide receptor radionuclide therapy (PRRT) is the dose-response relationship and finding the correct dose of ⁹⁰Y-DOTATOC that will achieve an optimum tumor kill. Nephrotoxicity was common but could be minimized by taking adequate renal protective measures. In conclusion, PRRT remains a good option in patients with inoperable and/or metastatic NETs particularly of GEP origin. Over a decade of experience with ⁹⁰Y-DOTATOC proves that it is still an effective tool for the treatment of large infiltrative NETs with achievement of objective radiological responses in nearly a quarter and disease stabilization in more than half the patients studied so far.

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. 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. (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. 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.

Radiometallated Peptides Targeting Guanylate Cyclase C and the Urokinase-Type Plasminogen Activator Receptor

Research is currently underway worldwide into the development of receptor-specific radiopharmaceuticals for the imaging and treatment of cancer. The successful clinical development of radiolabeled somatostatin analogs for imaging and treatment of cancers overexpressing somatostatin receptors has catalyzed further preclinical investigation of other radiolabeled peptides for molecular imaging and peptide-receptor radiotherapy, including such well-studied peptide vectors as cholecystokinin, neurotensin, bombesin and RGD peptides. Within this larger context, this article will focus on the current status of two more recent additions to the list of molecular imaging targets - guanylate cyclase C, a specific marker for colorectal cancer, and the urokinase plasminogen activator receptor, a cell-surface receptor overexpressed in diverse cancer types.