High expression of somatostatin receptor subtype 2 (sst2) in medulloblastoma: implications for diagnosis and therapy

Somatostatin and Somatostatin Receptors in Tumour Biology

Somatostatin (SST), a growth hormone inhibitory peptide, is expressed in endocrine and non-endocrine tissues, immune cells and the central nervous system (CNS). Post-release from secretory or immune cells, the first most appreciated role that SST exhibits is the antiproliferative effect in target tissue that served as a potential therapeutic intervention in various tumours of different origins. The SST-mediated in vivo and/or in vitro antiproliferative effect in the tumour is considered direct via activation of five different somatostatin receptor subtypes (SSTR1-5), which are well expressed in most tumours and often more than one receptor in a single cell. Second, the indirect effect is associated with the regulation of growth factors. SSTR subtypes are crucial in tumour diagnosis and prognosis. In this review, with the recent development of new SST analogues and receptor-specific agonists with emerging functional consequences of signaling pathways are promising therapeutic avenues in tumours of different origins that are discussed.

Theranostic Nuclear Medicine with Gallium-68, Lutetium-177, Copper-64/67, Actinium-225, and Lead-212/203 Radionuclides

Molecular changes in malignant tissue can lead to an increase in the expression levels of various proteins or receptors that can be used to target the disease. In oncology, diagnostic imaging and radiotherapy of tumors is possible by attaching an appropriate radionuclide to molecules that selectively bind to these target proteins. The term "theranostics" describes the use of a diagnostic tool to predict the efficacy of a therapeutic option. Molecules radiolabeled with γ-emitting or β+-emitting radionuclides can be used for diagnostic imaging using single photon emission computed tomography or positron emission tomography. Radionuclide therapy of disease sites is possible with either α-, β-, or Auger-emitting radionuclides that induce irreversible damage to DNA. This Focus Review centers on the chemistry of theranostic approaches using metal radionuclides for imaging and therapy. The use of tracers that contain β+-emitting gallium-68 and β-emitting lutetium-177 will be discussed in the context of agents in clinical use for the diagnostic imaging and therapy of neuroendocrine tumors and prostate cancer. A particular emphasis is then placed on the chemistry involved in the development of theranostic approaches that use copper-64 for imaging and copper-67 for therapy with functionalized sarcophagine cage amine ligands. Targeted therapy with radionuclides that emit α particles has potential to be of particular use in late-stage disease where there are limited options, and the role of actinium-225 and lead-212 in this area is also discussed. Finally, we highlight the challenges that impede further adoption of radiotheranostic concepts while highlighting exciting opportunities and prospects.

Immunohistochemical assessment and clinical, histopathologic, and molecular correlates of membranous somatostatin type-2A receptor expression in high-risk pediatric central nervous system tumors

INTRODUCTION

177Lu-DOTATATE, a radionuclide therapy that binds somatostatin type-2A receptors (SST2A), has demonstrated efficacy in neuroendocrine tumors and evidence of central nervous system (CNS) penetration, supporting potential expansion within pediatric neuro-oncology. Understanding the prevalence of SST2A expression across pediatric CNS tumors is essential to identify patients who may benefit from somatostatin receptor-targeted therapy and to further elucidate the oncogenic role of SST2A.

METHODS

SST2A immunohistochemistry (IHC) was performed on tumor specimens and interpreted by an experienced pathologist (blinded), utilizing semi-quantitative scoring of membranous expression within viable tumor. Immunoreactive cell percentage was visually scored as 0 (none), 1 (<10%), 2 (10-50%), 3 (51-80%), or 4 (>80%). Staining intensity was scored as 0 (none), 1 (weak), 2 (moderate), or 3 (strong). Combined scores for each specimen were calculated by multiplying percent immunoreactivity and staining intensity values (Range: 0-12).

RESULTS

A total of 120 tumor samples from 114 patients were analyzed. Significant differences in SST2A IHC scores were observed across histopathologic diagnoses, with consistently high scores in medulloblastoma (mean ± SD: 7.5 ± 3.6 [n=38]) and meningioma (5.7 ± 3.4 [n=15]), compared to minimal or absent expression in ATRT (0.3 ± 0.6 [n=3]), ETMR (1.0 ± 0 [n=3]), ependymoma (grades I-III; 0.2 ± 0.7 [n=27]), and high-grade glioma (grades III-IV; 0.4 ± 0.7 [n=23]). Pineoblastoma (3.8 ± 1.5 [n=4]) and other embryonal tumors (2.0 ± 4.0 [n=7]) exhibited intermediate, variable expression. Among medulloblastomas, SST2A IHC scores were higher in non-SHH (8.5 ± 3.1) than SHH (5.0 ± 3.3) molecular subgroups (p=0.033). In a subset of paired primary and recurrent specimens from four patients, SST2A IHC scores remained largely unchanged.

DISCUSSION

High membranous SST2A expression was demonstrated in medulloblastoma, meningioma, and some rarer embryonal tumors with potential diagnostic, biologic, and therapeutic implications. Somatostatin receptor-targeted therapy such as 177Lu-DOTATATE deserves further investigation in these highly SST2A-expressing pediatric CNS tumors.

Medulloblastoma in the Modern Era: Review of Contemporary Trials, Molecular Advances, and Updates in Management

Critical discoveries over the past two decades have transformed our understanding of medulloblastoma from a single entity into a clinically and biologically heterogeneous disease composed of at least four molecularly distinct subgroups with prognostically and therapeutically relevant genomic signatures. Contemporary clinical trials also have provided valuable insight guiding appropriate treatment strategies. Despite therapeutic and biological advances, medulloblastoma patients across the age spectrum experience tumor- and treatment-related morbidity and mortality. Using an updated risk stratification approach integrating both clinical and molecular features, ongoing research seeks to (1) cautiously reduce therapy and mitigate toxicity in low-average risk patients, and (2) thoughtfully intensify treatment with incorporation of novel, biologically guided agents for patients with high-risk disease. Herein, we review important historical and contemporary studies, discuss management updates, and summarize current knowledge of the biological landscape across unique pediatric, infant, young adult, and relapsed medulloblastoma populations.

Theranostic Agents in Musculoskeletal Disorders

Theranostic-based strategies, combining therapeutic and diagnostic properties of a single agent, have gained enormous attention in the past few years. Today, various multifunctional theranostic modalities have been examined, using different bioactive targeting, for the detection, quantifying, and monitoring of therapy response in different pathologies. Herein we review the newly emerging approaches in theranostic nanomedicine for the detection and therapy for musculoskeletal disorders to provide valuable insights for developing more efficient agents for clinical use. Some potential preclinical applications of radionuclide nanotheranostic agents are described in rheumatoid arthritis, osteoarthrosis, multiple myeloma, and neoplastic diseases.

A Critical Review of PET Tracers Used for Brain Tumor Imaging

The brain is a common site for metastases as well as primary tumors. Although evaluation of these malignancies with contrast-enhanced MR imaging defines current clinical practice, ¹⁸F-fluorodeoxyglucose (FDG)-PET has shown considerable utility in this area. In addition, many other tracers targeting various aspects of tumor biology have been developed and tested. This article discusses recent developments in PET imaging and the anticipated role of FDG and other tracers in the assessment of brain tumors.

Adult Medulloblastoma Depicted With 99mTc-HYNIC-[D-Phe1, Tyr3-Octreotide] SPECT/CT

Although medulloblastoma constitutes the second most common malignant brain tumor in children, it encountered very rarely in adults. The incidence of adult medulloblastoma is approximately 0.6 to 1 case per million. It accounts for less than 1% of adult brain tumors and is located most commonly at the posterior cranial fossa. We present an extremely rare case of a 67-year-old man with cerebellar medulloblastoma with residual disease after surgery, chemotherapy, and radiation therapy, depicted with Tc-HYNIC-[D-Phe, Tyr-Octreotide] SPECT/CT. Somatostatin receptor scintigraphy was performed for staging and for the possibility of treatment with somatostatin analogs or peptide radionuclide therapy.

An update on PET-based molecular imaging in neuro-oncology: challenges and implementation for a precision medicine approach in cancer care

PET imaging using novel radiotracers show promises for tumor grading and molecular characterization through visualizing molecular and functional properties of the tumors. Application of PET tracers in brain neoplasm depends on both type of the neoplasm and the research or clinical significance required to be addressed. In clinical neuro-oncology, 18F-FDG is used mainly to differentiate tumor recurrence from radiation-induced necrosis, and novel PET agents show attractive imaging properties. Novel PET tracers can offer biologic information not visible via contrast-enhanced MRI or 18F-FDG PET. This review aims to provide an update on the complementary role of PET imaging in neuro-oncology both in research and clinical settings along with presenting interesting cases in this context.

Tumor Targeting and Pharmacokinetics of a Near-Infrared Fluorescent-Labeled δ-Opioid Receptor Antagonist Agent, Dmt-Tic-Cy5

Fluorescence molecular imaging can be employed for the development of novel cancer targeting agents. Herein, we investigated the pharmacokinetics (PK) and cellular uptake of Dmt-Tic-Cy5, a delta-opioid receptor (δOR) antagonist-fluorescent dye conjugate, as a tumor-targeting molecular imaging agent. δOR expression is observed normally in the CNS, and pathologically in some tumors, including lung liver and breast cancers. In vitro, in vivo, and ex vivo experiments were conducted to image and quantify the fluorescence signal associated with Dmt-Tic-Cy5 over time using in vitro and intravital fluorescence microscopy and small animal fluorescence imaging of tumor-bearing mice. We observed specific retention of Dmt-Tic-Cy5 in tumors with maximum uptake in δOR-expressing positive tumors at 3 h and observable persistence for >96 h; clearance from δOR nonexpressing negative tumors by 6 h; and systemic clearance from normal organs by 24 h. Live-cell and intravital fluorescence microscopy demonstrated that Dmt-Tic-Cy5 had sustained cell-surface binding lasting at least 24 h with gradual internalization over the initial 6 h following administration. Dmt-Tic-Cy5 is a δOR-targeted agent that exhibits long-lasting and specific signal in δOR-expressing tumors, is rapidly cleared from systemic circulation, and is not retained in non-δOR-expressing tissues. Hence, Dmt-Tic-Cy5 has potential as a fluorescent tumor imaging agent.

The role of the ubiquitin proteasome system in cerebellar development and medulloblastoma

Cerebellar granule cells precursors are derived from the upper rhombic lip and migrate tangentially independent of glia along the subpial stream pathway to form the external germinal zone. Postnatally, granule cells migrate from the external germinal zone radially through the Purkinje cell layer, guided by Bergmann glia fibers, to the internal granular cell layer.

Medulloblastomas (MBs) are the most common malignant childhood brain tumor. Many of these tumors develop from precursor cells of the embryonic rhombic lips. Four main groups of MB are recognized. The WNT group of MBs arise primarily from the lower rhombic lip and embryonic brainstem. The SHH group of MBs originate from cerebellar granule cell precursors in the external germinal zone of the embryonic cerebellum. The cellular origins of type 3 and type 4 MBs are not clear.

Several ubiquitin ligases are revealed to be significant factors in development of the cerebellum as well as in the initiation and maintenance of MBs. Proteasome dysfunction at a critical stage of development may be a major factor in determining whether progenitor cells which are destined to become granule cells differentiate normally or become MB cells. We propose the hypothesis that proteasomal activity is essential to regulate the critical transition between proliferating granule cells and differentiated granule cells and that proteasome dysfunction may lead to MB. Proteasome dysfunction could also account for various mutations in MBs resulting from deficiencies in DNA checkpoint and repair mechanisms prior to development of MBs.

Data showing a role for the ubiquitin ligases β-TrCP, FBW7, Huwe1, and SKP2 in MBs suggest the possibility of a classification of MBs based on the expression (over expression or under expression) of specific ubiquitin ligases which function as oncogenes, tumor suppressors or cell cycle regulators.

Somatostatin receptor based imaging and radionuclide therapy

Somatostatin (SST) receptors (SSTRs) belong to the typical 7-transmembrane domain family of G-protein-coupled receptors. Five distinct subtypes (termed SSTR1-5) have been identified, with SSTR2 showing the highest affinity for natural SST and synthetic SST analogs. Most neuroendocrine tumors (NETs) have high expression levels of SSTRs, which opens the possibility for tumor imaging and therapy with radiolabeled SST analogs. A number of tracers have been developed for the diagnosis, staging, and treatment of NETs with impressive results, which facilitates the applications of human SSTR subtype 2 (hSSTr2) reporter gene based imaging and therapy in SSTR negative or weakly positive tumors to provide a novel approach for the management of tumors. The hSSTr2 gene can act as not only a reporter gene for in vivo imaging, but also a therapeutic gene for local radionuclide therapy. Even a second therapeutic gene can be transfected into the same tumor cells together with hSSTr2 reporter gene to obtain a synergistic therapeutic effect. However, additional preclinical and especially translational and clinical researches are needed to confirm the value of hSSTr2 reporter gene based imaging and therapy in tumors.

Nanoparticle ligand presentation for targeting solid tumors

Among the many scientific advances to come from the study of nanoscience, the development of ligand-targeted nanoparticles to eliminate solid tumors is predicted to have a major impact on human health. There are many reports describing novel designs and testing of targeted nanoparticles to treat cancer. While the principles of the technology are well demonstrated in controlled lab experiments, there are still many hurdles to overcome for the science to mature into truly efficacious targeted nanoparticles that join the arsenal of agents currently used to treat cancer in humans. One of these hurdles is overcoming unwanted biodistribution to the liver while maximizing delivery to the tumor. This almost certainly requires advances in both nanoparticle stealth technology and targeting. Currently, it continues to be a challenge to control the loading of ligands onto polyethylene glycol (PEG) to achieve maximal targeting. Nanoparticle cellular uptake and subcellular targeting of genes and siRNA also remain a challenge. This review examines the types of ligands that have been most often used to target nanoparticles to solid tumors. As the science matures over the coming decade, careful control over ligand presentation on nanoparticles of precise size, shape, and charge will likely play a major role in achieving success.

Somatostatin receptor-based PET/CT of intracranial tumors: a potential area of application for 68 Ga-DOTA peptides?

OBJECTIVE

Similar to neuroendocrine tumors (NETs) at other sites, a wide array of intracranial tumors also express somatostatin receptors (SSTRs). This expression can be exploited for both imaging and therapy. The introduction of (68)Ga-labeled tetraazacyclododecanetetraacetic acid (DOTA)-peptide PET/CT has given new dimension to SSTR-based imaging because of its improved sensitivity and excellent spatial resolution.

CONCLUSION

However, in contrast to gastropancreatic and bronchopulmonary NETs, limited literature is available regarding the use of (68)Ga-DOTA-peptide PET/CT in intracranial tumors. Here, we briefly review the available literature and highlight the potential role that (68)Ga-DOTA-peptide PET/CT can play in the management of intracranial tumors.

G-protein coupled receptor expression patterns delineate medulloblastoma subgroups

Background

Medulloblastoma is the most common malignant brain tumor in children. Genetic profiling has identified four principle tumor subgroups; each subgroup is characterized by different initiating mutations, genetic and clinical profiles, and prognoses. The two most well-defined subgroups are caused by overactive signaling in the WNT and SHH mitogenic pathways; less is understood about Groups 3 and 4 medulloblastoma. Identification of tumor subgroup using molecular classification is set to become an important component of medulloblastoma diagnosis and staging, and will likely guide therapeutic options. However, thus far, few druggable targets have emerged. G-protein coupled receptors (GPCRs) possess characteristics that make them ideal targets for molecular imaging and therapeutics; drugs targeting GPCRs account for 30-40% of all current pharmaceuticals. While expression patterns of many proteins in human medulloblastoma subgroups have been discerned, the expression pattern of GPCRs in medulloblastoma has not been investigated. We hypothesized that analysis of GPCR expression would identify clear subsets of medulloblastoma and suggest distinct GPCRs that might serve as molecular targets for both imaging and therapy.

Results

Our study found that medulloblastoma tumors fall into distinct clusters based solely on GPCR expression patterns. Normal cerebellum clustered separately from the tumor samples. Further, two of the tumor clusters correspond with high fidelity to the WNT and SHH subgroups of medulloblastoma. Distinct over-expressed GPCRs emerge; for example, LGR5 and GPR64 are significantly and uniquely over-expressed in the WNT subgroup of tumors, while PTGER4 is over-expressed in the SHH subgroup. Uniquely under-expressed GPCRs were also observed. Our key findings were independently validated using a large international dataset.

Conclusions

Our results identify GPCRs with potential to act as imaging and therapeutic targets. Elucidating tumorigenic pathways is a secondary benefit to identifying differential GPCR expression patterns in medulloblastoma tumors.

Somatostatin receptor subtype 2 (sst₂) is a potential prognostic marker and a therapeutic target in medulloblastoma

INTRODUCTION

Neuroectodermal tumors in general demonstrate high and dense expression of the somatostatin receptor subtype 2 (sst₂). It controls proliferation of both normal and neoplastic cells. sst₂ has thus been suggested as a therapeutic target and prognostic marker for certain malignancies.

METHODS

To assess global expression patterns of sst 2 mRNA, we evaluated normal (n = 353) and tumor tissues (n = 340) derived from previously published gene expression profiling studies. These analyses demonstrated specific upregulation of sst 2 mRNA in medulloblastoma (p < 0.001). sst₂ protein was investigated by immunohistochemistry in two independent cohorts.

RESULTS

Correlation of sst₂ protein expression with clinicopathological variables revealed significantly higher levels in medulloblastoma (p < 0.05) compared with CNS-PNET, ependymoma, or pilocytic astrocytoma. The non-SHH medulloblastoma subgroup tumors showed particularly high expression of sst₂, when compared to other tumors and normal tissues. Furthermore, we detected a significant survival benefit in children with tumors exhibiting high sst₂ expression (p = 0.02) in this screening set. A similar trend was observed in a validation cohort including 240 independent medulloblastoma samples.

CONCLUSION

sst₂ is highly expressed in medulloblastoma and deserves further evaluation in the setting of prospective trials, given its potential utility as a prognostic marker and a therapeutic target.

Phase I trial of 90Y-DOTATOC therapy in children and young adults with refractory solid tumors that express somatostatin receptors

The purpose of this study was to conduct a phase I trial of (90)Y-DOTATOC to determine the dose-toxicity profile in children and young adults with somatostatin receptor-positive tumors.

METHODS

A 3 × 3 design was used to determine the highest tolerable dose of (90)Y-DOTATOC, with administered activities of 1.11, 1.48, and 1.85 GBq/m(2)/cycle given in 3 cycles at 6-wk intervals. An amino acid infusion was coadministered with the radiopharmaceutical for renal protection. Eligibility criteria included an age of 2-25 y, progressive disease, a positive lesion on (111)In-diethylenetriaminepentaacetic acid-D-Phe(1)-octreotide scanning, a glomerular filtration rate of 80 mL/min/1.73 m(2) or more, bone marrow cellularity of 40% or more or stored autologous hematopoietic stem cells, 60% or more on the Lansky Play Scale, and informed consent.

RESULTS

Seventeen subjects (age, 2-24 y) received at least 1 dose of (90)Y-DOTATOC; diagnoses included neuroblastoma, embryonal and astrocytic brain tumors, paraganglioma, multiple endocrine neoplasia IIB, and neuroendocrine tumors. No dose-limiting toxicities and no individual dose reductions due to renal or hematologic toxicity were noted. No complete responses were observed; 2 subjects experienced partial response, 5 had minor responses, 6 experienced stable disease, 2 had progressive disease, and 2 withdrew.

CONCLUSION

Peptide receptor radionuclide therapy with (90)Y-DOTATOC is safe in children and young adults and demonstrated a 12% partial response plus 29% minor response rate in patients with somatostatin receptor-positive tumors. No dose-limiting toxicities were observed. The recommended phase II dosing is 3 cycles of 1.85 GBq/m(2)/dose of (90)Y-DOTATOC coadministered with amino acids.

Detection of amino-terminal extracellular domain of somatostatin receptor 2 by specific monoclonal antibodies and quantification of receptor density in medulloblastoma

Somatostatin receptor 2 (SSTR2) is expressed by most medulloblastomas (MEDs). We isolated monoclonal antibodies (MAbs) to the 12-mer (33)QTEPYYDLTSNA(44), which resides in the extracellular domain of the SSTR2 amino terminus, screened the peptide-bound MAbs by fluorescence microassay on D341 and D283 MED cells, and demonstrated homogeneous cell-surface binding, indicating that all cells expressed cell surface-detectable epitopes. Five radiolabeled MAbs were tested for immunoreactive fraction (IRF), affinity (KA) (Scatchard analysis vs. D341 MED cells), and internalization by MED cells. One IgG(3) MAb exhibited a 50-100% IRF, but low KA. Four IgG(2a) MAbs had 46-94% IRFs and modest KAs versus intact cells (0.21-1.2 x 10(8) M(-1)). Following binding of radiolabeled MAbs to D341 MED at 4 degrees C, no significant internalization was observed, which is consistent with results obtained in the absence of ligand. However, all MAbs exhibited long-term association with the cells; binding at 37 degrees C after 2 h was 65-66%, and after 24 h, 52-64%. In tests with MAbs C10 and H5, the number of cell surface receptors per cell, estimated by Scatchard and quantitative FACS analyses, was 3.9 x 10(4) for the "glial" phenotype DAOY MED cell line and 0.6-8.8 x 10(5) for four neuronal phenotype MED cell lines. Our results indicate a potential immunotherapeutic application for these MAbs.

[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.

111In-pentetreotide scintigraphy in medulloblastoma: a comparison with magnetic resonance imaging

Medulloblastoma (MB) is a primitive neuroectodermal tumour constituting a grade IV brain malignancy. Early and correct detection of recurrence or metastasis is desirable for follow-up of patients in this entity. Frequent expression of somatostatin receptors by MB lesions facilitates functional tumour imaging by somatostatin receptor scintigraphy (SRS). To investigate the value of SRS in the follow-up of MB, the results of ten consecutive patients (seven children and three adults) undergoing additional imaging with 111In-pentetreotide were reviewed. Four, 24 and 48 h p.i. planar and whole body images as well as a SPECT study at 4 h p.i. were acquired after intravenous injection of 109 +/- 35 MBq 111In-pentetreotide (Octreoscan). SRS yielded 11 positive and ten negative imaging results, compared to 17 positive and four negative in magnetic resonance imaging (MRI). The lesion-by-lesion analysis with a total of 44 lesions revealed a sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of 42%, 83%, 94%, 18% for SRS and 89.5%, 50%, 92%, 43% for MRI. Based on a per-patient analysis, considering the patient as to be either tumour-free or tumour-positive by one imaging modality, the following values for sensitivity, specificity, PPV and NPV were obtained: 61%, 100%, 100%, 30% for SRS and 94%, 67%, 94%, 67% for MRI. MRI remains the first step imaging technique in medulloblastoma patients before and after surgery and during the follow-up providing the highest sensitivity. However, to improve specificity and contribute to correct diagnosis in MB 111In-pentetreotide scintigraphy should be considered as a confirmatory second step imaging tool, especially in case of equivocal MRI results. Moreover, a positive SRS scan might serve as a reference before and after somatostatin receptor targeted radiotherapy.

Somatostatin receptor scintigraphy in surveillance of pediatric brain malignancies

BACKGROUND

Somatostatin receptor scintigraphy (SRS) has been successfully used in imaging PBTs and, as a functional imaging modality, may be better able to differentiate tumor from scar/necrosis. This retrospective study evaluates the role of SRS in post-treatment surveillance of PBTs.

PROCEDURE

Twenty children (age range: 7 months to 24 years, mean: 9 years) with known brain malignancies underwent serial SRS and MRI. The sensitivity and specificity of SRS and MRI were compared for surveillance scanning using patient outcome as the reference standard. Somatostatin receptors (sstrs) expression was determined by immunohistochemistry (IHC) of available tumor specimens.

RESULTS

SRS was true positive (TP) in 15 of 16 patients with proven disease found post-resection (n = 5) or during follow-up (n = 11). In contrast, MRI was positive in 12 of these 16 patients and equivocal in another two patients where it could not distinguish between radiation necrosis and tumor recurrence. The two patients with false negative (FN) MRI and proven disease were positive by SRS. SRS was negative in all four patients with no evidence of disease by follow-up (mean follow-up = 58 months). The only patient with a FN SRS (TP by MRI) was one without IHC evidence of sstrs. SRS was TP in 7/7 tumors with IHC documented sstrs.

CONCLUSIONS

SRS is a useful adjunct to MRI for post-treatment surveillance of sstr-positive PBTs, particularly when MRI is equivocal.

Single-Photon Emission Computed Tomography/Computed Tomography in Brain Tumors

Anatomic imaging procedures (computed tomography [CT] and magnetic resonance imaging [MRI]) have become essential tools for brain tumor assessment. Functional images (positron emission tomography [PET] and single-photon emission computed tomography [SPECT]) can provide additional information useful during the diagnostic workup to determine the degree of malignancy and as a substitute or guide for biopsy. After surgery and/or radiotherapy, nuclear medicine examinations are essential to assess persistence of tumor, to differentiate recurrence from radiation necrosis and gliosis, and to monitor the disease. The combination of functional images with anatomic ones is of the utmost importance for a full evaluation of these patients, which can be obtained by means of imaging fusion. Despite the fast-growing diffusion of PET, in most cases of brain tumors, SPECT studies are adequate and provide results that parallel those obtained with PET. The main limitation of SPECT imaging with brain tumor-seeking radiopharmaceuticals is the lack of precise anatomic details; this drawback is overcome by the fusion with morphological studies that provide an anatomic map to scintigraphic data. In the past, software-based fusion of independently performed SPECT and CT or MRI demonstrated usefulness for brain tumor assessment, but this process is often time consuming and not practical for everyday nuclear medicine studies. The recent development of dual-modality integrated imaging systems, which allow the acquisition of SPECT and CT images in the same scanning session, and their co-registration by means of the hardware, has facilitated this process. In SPECT studies of brain tumors with various radiopharmaceuticals, fused images are helpful in providing the precise localization of neoplastic lesions, and in excluding the disease in sites of physiologic tracer uptake. This information is useful for optimizing diagnosis, therapy monitoring, and radiotherapy treatment planning, with a positive impact on patient management.

Recent advances in embryonal tumours of the central nervous system

INTRODUCTION

Embryonal tumours of the central nervous system (CNS) are the commonest malignant paediatric brain tumours. This group includes medulloblastomas, supratentorial primitive neuroectodermal tumours, atypical teratoid/rhabdoid tumours, ependymoblastomas, and medulloepitheliomas. Earlier, all these tumours were grouped under a broad category of primitive neuroectodermal tumours (PNETs). However, the current WHO classification (2000) separates them into individual types based on significant progress in the understanding of their distinctive clinical, pathological, molecular genetic, histogenetic, and behavioural characteristics. Furthermore, advances in histopathology and molecular genetics have shown great promise for refining risk assessment in these tumours, especially medulloblastomas, thus providing a more accurate basis for tailoring therapies to individual patients. Correlation of histological changes with genetic events has also led to a new model of medulloblastoma tumorigenesis.

REVIEW

This review presents an updated comparative profile of these tumours, highlighting the clinical and biological relevance of the recent advances.

Three-year recurrence-free survival in a patient with recurrent medulloblastoma after resection, high-dose chemotherapy, and intrathecal Yttrium-90-labeled DOTA0-D-Phe1-Tyr3-octreotide radiopeptide brachytherapy

BACKGROUND

Most medulloblastomas express high levels of somatostatin type 2 receptors (sst2). DOTA0-D-Phe1-Tyr3-octreotide (DOTATOC) specifically binds sst2 in the low nanomolar range. The cytotoxic effect is mediated by the chelated, beta-emitting, metallic radionuclide Yttrium 90 (90Y). The authors applied this innovative treatment option in a boy age 8 years who presented with a recurrent medulloblastoma of the cauda equina: a prognostically poor condition. Targeted radiotherapy was administered to treat minimal sst2-expressing tumor remnants, which persisted despite conventional and high-dose chemotherapy and intercurrent resection of the lesion.

METHODS

A medulloblastoma arising from the floor of the fourth ventricle had been removed surgically; then, the patient was treated with standard adjuvant chemotherapy and craniospinal irradiation according to the prospective HIT '91 protocol. Complete remission was achieved for 20 months, when a drop metastasis of the cauda equina manifested with sensorimotor lumbosacral deficits and urinary incontinence. After four cycles of neoadjuvant chemotherapy (which consisted of combined ifosfamide, carboplatinum and etoposide), two cycles of high-dose chemotherapy and autologous stem cell transplantation were performed; in between, the responding residual tumor within the lumbosacral nerve fibers was microscopically removed. Thereafter, an Indium-111-DOTATOC test injection indicated sst2-expressing tumor remnants within the cauda equina. Consequently, 4 cycles of [90Y]-DOTATOC (4x562.5 megabecquerels) were injected directly into the cerebrospinal fluid in monthly intervals.

RESULTS

The consolidating intrathecal brachytherapy using [90Y]-DOTATOC was tolerated well. A complete remission was achieved for a 3-year period. The only remaining deficit was urinary incontinence.

CONCLUSIONS

Intrathecal administration of targeted radiopeptide brachytherapy in combination with conventional and high-dose chemotherapy and surgical removal represents a promising new option to treat recurrent medulloblastoma and should be explored further.

Radioiodine and 211At-labeled guanidinomethyl halobenzoyl octreotate conjugates: potential peptide radiotherapeutics for somatostatin receptor-positive cancers

Derivatives of the somatostatin analogues octreotide and octreotate labeled with radioiosotopes are used in the diagnosis and therapy of somatostatin receptor (SSTR)-positive tumors. A method has been devised to synthesize {N-(4-guanidinomethyl-3-iodobenzoyl)-Phe1-octreotate (GMIBO). Receptor binding assay and scatchard analysis yielded a Kd of 4.83 +/- 0.19 nM for this peptide. Derivatives of this peptide labeled with radioiodine ([*I]GMIBO) and the alpha-particle-emitting radiohalogen 211At N-(3-[211At]astato-4-guanidinomethylbenzoyl)-Phe1-octreotate; [211At]AGMBO} were prepared in a single step from a tin precursor in radiochemical yields of 30-35% and 15-20%, respectively. Paired-label internalization assays performed with the SSTR-positive D341 Med human medulloblastoma cell line demonstrated that [125I]GMIBO and [211At]AGMBO were specifically internalized 20-40% more than Nalpha-(1-deoxy-D-fructosyl)-[131I]I-Tyr3-octreotate ([131I]I-Glu-TOCA), the radioiodinated octreotide derivative previously shown to exhibit maximum internalization in this cell line. Uptake of [131I]GMIBO in D341 Med subcutaneous xenografts in a murine model (8.34 +/- 1.82 versus 8.10 +/- 2.23% ID/g at 1h) and SSTR-expressing normal tissues was comparable to that of [125I]I-Glu-TOCA and was shown to be specific. However, the uptake of [131I]GMIBO also was substantially higher in liver (16.9 +/- 3.15 versus 1.39 +/- 0.45% ID/g at 1 h) and in kidneys (44.33 +/- 6.47 versus 3.44 +/- 0.68% ID/g at 1h) compared to that of [125I]I-Glu-TOCA. These data suggest that these novel peptide conjugates retain their specificity for SSTR both in vitro and in vivo; however, because of their higher accumulation in normal tissues they would be best applied in settings amenable to loco-regional administration such as medulloblastoma neoplastic meningitis.

Receptor-Mediated Tumor Targeting with Radiopeptides. Part 1. General Concepts and Methods: Applications to Somatostatin Receptor-Expressing Tumors

Radiolabeled peptides have become important tools in nuclear oncology, both as diagnostics and more recently also as therapeutics. They represent a distinct sector of the molecular targeting approach, which in many areas of therapy will implement the old "magic bullet" concept by specifically directing the therapeutic agent to the site of action. In this three-part review, we present a comprehensive overview of the literature on receptor-mediated tumor targeting with the different radiopeptides currently studied. Part I summarizes the general concepts and methods of targeting, the selection of radioisotopes, chelators, and the criteria of peptide ligand development. Then, the >400 studies on the application to somatostatin/somatostatin-release inhibiting factor receptor-mediated tumor localization and treatment will be reviewed, demonstrating that peptide radiopharmaceuticals have gained an important position in clinical medicine.

Somatostatin receptor subtype 2 is expressed by supratentorial primitive neuroectodermal tumors of childhood and can be targeted for somatostatin receptor imaging

PURPOSE

Although gliomas predominate among central nervous system (CNS) neoplasms in adulthood, embryonal tumors are the most common malignant brain tumors in children. Despite novel treatment approaches, including improved radiotherapy and high-dose chemotherapy, survival rates remain unsatisfactory. The timely diagnosis of residual or recurrent embryonal CNS tumors and thus the earliest possible time point for intervention is often hampered by inaccuracies of conventional imaging techniques. Novel and refined imaging methodologies are urgently needed.

EXPERIMENTAL DESIGN

We have previously demonstrated the use of somatostatin receptor imaging (SRI) in the diagnosis of recurrent and residual medulloblastomas. Here, we evaluated somatostatin receptor type 2 (sst(2)) expression using an antibody in an array of CNS tumors of childhood. Eight high-grade gliomas, 4 atypical teratoid/rhabdoid tumors, 7 supratentorial primitive neuroectodermal tumors (stPNET), 1 medulloepithelioma (ME), and 8 ependymomas were screened. Tumors positive in vitro were additionally analyzed in vivo using SRI.

RESULTS

Abundant expression of somatostatin receptor type 2 in stPNET, a ME, and ependymomas warranted in vivo imaging of 7 stPNET, 1 rhabdomyosarcoma, 3 ependymomas, 1 ME, and 1 glioblastoma. Although SRI was positive in 6/7 stPNET, 1 rhabdomyosarcoma, and 1 ME, none of the ependymomas nor the glioblastoma could be imaged using SRI. In selected cases SRI was more sensitive in the detection of relapse than conventional imaging by magnetic resonance imaging and computed tomography.

CONCLUSIONS

SRI should be considered in the evaluation of residual or recurrent embryonal CNS tumors, especially stPNET. The strengths of SRI lie in the differentiation of reactive tissue changes versus residual or recurrent tumor, the detection of small lesions, and possibly in the distinction of stPNET from gliomas.

Peripheral primitive neuroectodermal tumor of the stomach in a 14-year-old boy: a case report

Although PNETs are rare malignancies, they should be considered in the differential diagnosis of submucosal gastric tumours in adolescents with clinical alarm symptoms.NET was confirmed by detection of the characteristic EWS/FLI-1 fusion gene, resulting in a reciprocal translocation t(11;22)(q24;q12). Three distinct liver metastases were detected by CT, MRI, and PET. The tumour failed to respond to neoadjuvant polychemotherapy with vincristine, etoposide, doxorubicin, and ifosfamide. Subtotal gastrectomy was performed and, surprisingly, we found diffuse metastatic infiltration of the liver that had not been detected by preoperative staging. Due to the diffuse metastatic disease the young patient's prognosis has to be considered very poor. Because of the tumour's intense expression of CD117 (c-kit), the patient is now treated with the tyrosine kinase inhibitor imatinib (STI571).

Peptide receptors as molecular targets for cancer diagnosis and therapy

During the past decade, proof of the principle that peptide receptors can be used successfully for in vivo targeting of human cancers has been provided. The molecular basis for targeting rests on the in vitro observation that peptide receptors can be expressed in large quantities in certain tumors. The clinical impact is at the diagnostic level: in vivo receptor scintigraphy uses radiolabeled peptides for the localization of tumors and their metastases. It is also at the therapeutic level: peptide receptor radiotherapy of tumors emerges as a serious treatment option. Peptides linked to cytotoxic agents are also considered for therapeutic applications. The use of nonradiolabeled, noncytotoxic peptide analogs for long-term antiproliferative treatment of tumors appears promising for only a few tumor types, whereas the symptomatic treatment of neuroendocrine tumors by somatostatin analogs is clearly successful. The present review summarizes and critically evaluates the in vitro data on peptide and peptide receptor expression in human cancers. These data are considered to be the molecular basis for peptide receptor targeting of tumors. The paradigmatic peptide somatostatin and its receptors are extensively reviewed in the light of in vivo targeting of neuroendocrine tumors. The role of the more recently described targeting peptides vasoactive intestinal peptide, gastrin-releasing peptide, and cholecystokinin/gastrin is discussed. Other emerging and promising peptides and their respective receptors, including neurotensin, substance P, and neuropeptide Y, are introduced. This information relates to established and potential clinical applications in oncology.

Comparison of somatostatin receptor expression in human gliomas and medulloblastomas

The expression of the five somatostatin receptor subtypes, sst1-5 was compared on tissue containing glial tumours (glioblastomas or oligodendrogliomas), medulloblastomas, and on normal human cortex. By semiquantitative reverse transcription coupled to polymerase chain reaction, the receptor expression profiles were high in cortex and in tissue containing oligodendrogliomas. It was moderate in medulloblastomas. Tissue containing glioblastomas displayed lower expression of somatostatin receptor subtypes, sst1 and sst3 being mostly expressed. By 125I-Tyr0DTrp8 somatostatin-14 or 125I-Leu8DTrp22 Tyr25 somatostatin-28 autoradiography combined with synaptophysin immunohistochemistry, it was possible to differentiate between isolated tumoral cell component infiltrating the cerebral parenchyma (cortex or white matter) and tumoral tissue (without residual parenchyma) in glioblastomas or oligodendrogliomas. Glial tumoral tissue per se presented few somatostatin receptors. By contrast, medulloblastoma tumoral cells exhibited numerous octreotide sensitive somatostatin receptors. sst2 immunocytochemistry demonstrated immunostaining of neuronal cells and neuropile; sst2 and sst3 immunostaining was identified on glioblastoma proliferating vessels endothelial cells and on medulloblastomas tumoral cells. Faint sst2 immunostaining among glial tumoral cells was due to microglia, while glioma cells did not significantly stain. In summary, medulloblastoma tumoral cells express sst2/sst3 receptors at a high level while glioma cells do not. In gliomas, sst expression is restricted to endothelial cells on proliferating vessels (displaying both sst2 and sst3 receptors), including parenchyma and reactive microglia (only sst2). The differential expression of sst2/sst3 receptors on gliomas and medulloblastomas has implications for the therapy of these tumours.

Neuroendocrine tumors in the brain

Somatostatin and other neuropeptides are expressed in tumors originating from neuronal precursors and paraganglia, namely medulloblastoma, central Primitive Neuro-Ectodermal Tumors (cPNETs), neurocytoma, gangliocytoma. olfactory neuroblastoma, paraganglioma. In medulloblastoma, the most common malignant tumor in childhood, there is an extensive expression of somatostatin in addition to somatostatin receptors (SSTR) type 2. Although density of SSTR-2 and intensity of expression of somatostatin genes have no prognostic significance in medulloblastoma. their presence may bring along important information on oncogenesis and relate medulloblastoma to cPNETs. Radio-labeled octreotide scintigraphy may be useful in the follow-up of these patients. allowing differentiation between scar and tumoral tissue. Moreover, on the basis of octreotide-induced inhibition of cell proliferation in medulloblastoma, a trial with octreotide in patients with recurrent or high-risk tumor is warranted. Meningiomas and low-grade astrocytic gliomas, even if not displaying a clear neuroendocrine phenotype, have high levels of SSTR-2. In meningiomas, SSTRs-scintigraphy is not part of the routine pre-operative assessment; moreover, a therapeutic trial with somatostatin-analogues in patients with recurrent or inoperable meningiomas should be carried-out with great caution, because somatostatin and octreotide slightly increase cell proliferation in cultured meningiomatous cells. Low-grade gliomas (WHO grade 2), and a smaller fraction of anaplastic astrocytomas, express SSTR-2, while glioblastomas usually do not. Unfortunately, radiolabeled-octreotide scintigraphy is not useful in the differential diagnosis of gliomas, because the results are altered by the disruption of the blood brain barrier (BBB); in addition, radionuclide-labeled somatostatin analogues are not useful in the therapy of low-grade gliomas, because the intact BBB prevents them from reaching the target SSTR-2. Recently, a pilot study in gliomas, has proposed the use of a radio-labeled somatostostatin analogue with a loco-regional approach in order to overcome the intact BBB.

Review of the molecular genetics and chemotherapeutic treatment of adult and paediatric medulloblastoma

Medulloblastoma is the most common primary brain tumour in children and accounts for 25% of newly diagnosed cases. Recent advances in treatment have extended 5-year survival rates from 3 - > 70% during the past 50 years. These improvements in survival have resulted from a multi-modality approach that includes surgical resection, posterior fossa and craniospinal irradiation and chemotherapy for selected, high-risk patients. The literature regarding chemotherapy of adult and paediatric patients is reviewed in-depth. The most active agents include cisplatin, CCNU, cyclophosphamide, vincristine and carboplatin. Although patients are living longer with their disease, neurocognitive function and quality of life are often impaired following radiation therapy (RT) to the developing brain. To safely allow reductions in the dose of RT, the specificity and efficacy of chemotherapy must be improved. Recent advances in the molecular genetics of medulloblastoma transformation (e.g., myc, PTCH ) are reviewed and discussed. A thorough understanding of these pathways will be critical for the development of more specific, novel drugs. Further clinical trials will be needed to evaluate the activity of these new drugs and determine their role in the treatment plan of patients with medulloblastoma.

Differential expression of somatostatin receptors in ependymoma: implications for diagnosis

Somatostatin receptors (SSts) have been found in a variety of brain tumors, e.g., meningiomas, medulloblastomas and astrocytomas. Our aim was to investigate their expression in ependymomas. Using RT-PCR, expression of mRNA for the different SSt subtypes was analyzed and quantified in 28 ependymomas and correlated with different variables (age, tumor location, histological grade, recurrence and survival). In addition, in 8 cases, protein expression was studied in vitro, using immunohistochemistry, and in vivo, by somatostatin scintigraphy. mRNAs for all 5 subtypes were variably expressed in each ependymoma. The Southern blotting signal obtained after SSt(1) and SSt(2) amplification was higher than that for the other receptor subtypes. No significant correlation was seen between the level of SSt(1) and SSt(2) mRNA expression and age, location, histological grading, recurrence or survival. In the 8 cases, SSt(1) staining was negative in 3 and low in 5. Staining for SSt(2A) was positive but low in every specimen analyzed. SSt(1) and SSt(2) immunoreactivity was seen only in the cytoplasm of tumoral cells. Somatostatin scintigraphy showed clear uptake, which agreed with MRI data in the majority of cases. However, no correlation was seen between tracer uptake intensity and histological grade, SSt(1) and SSt(2) mRNA expression or immunostaining intensity. This evidence for the expression of SSt(2) receptors in ependymomas opens interesting prospects for their follow-up.

Differential expression of somatostatin receptors in medulloblastoma

OBJECT

Somatostatin receptors have been found on a variety of tumours like neuroendocrine breast or brain tumours. Their detection opens new diagnostic and therapeutic paths. The aim of this work was to investigate their expression in medulloblastomas.

METHODS

Using both techniques, reverse transcriptase-polymerase chain reaction and immunohistochemistry, we analysed mRNA of different subtypes of somatostatin receptors in 15 medulloblastomas and the localisation of the subtype SSTR2 receptor at the cellular level in 13 medulloblastomas. All five subtypes mRNA were variably expressed in each medulloblastoma. The signal obtained after Southern blotting for SSTR2 receptor amplification was the highest as compared to the signal obtained for the other receptor subtypes. Immunostaining for SSTR2A receptor was present in every tumour specimen and was specifically located to the cellular membrane of neoplastic cells. No staining was identified at the level of peritumoral veins.

CONCLUSION

The evidence of predominant expression of SSTR2 receptors in medulloblastomas opens interesting prospects for their diagnosis and therapy.

Radioiodination and astatination of octreotide by conjugation labeling

Octreotide was coupled to 3-iodobenzoyl and 3-iodonicotinoyl moieties to obtain [N-(3-iodobenzoyl)-D-Phe(1)]octreotide (IBO) and [N-(3-iodonicotinoyl)-D-Phe(1)]octreotide (INO), respectively. The IC(50) values for the binding of IBO and INO to CA20948 rat pancreatic tumor membranes were 0.90 and 0.13 nM, respectively, compared with 0.35 nM for octreotide itself. Starting from N-succinimidyl 3-[(131)I]iodobenzoate and N-succinimidyl 5-[(131)I]iodopyridine-3- carboxylate, [(131)I]IBO and [(131)I]INO were prepared in overall radiochemical yields of 35%-50%. Likewise, ¿N-(3-[(211)At]astatobenzoyl)-D-Phe(1)¿octreotide ([(211)At]ABO) was prepared in similar yield from N-succinimidyl 3-[(211)At]astatobenzoate. In vitro assays with AR42J rat pancreatic tumor cells demonstrated a higher retention of cell-internalized radioiodine activity for [(131)I]INO compared with [(125)I]IBO. Tissue distribution studies with both conjugates revealed low levels of activity in the thyroid suggesting that dehalogenation of these peptides was minimal.

Medulloblastoma

The utilization of multi-modal therapy in the treatment of medulloblastoma has improved survival rates and overall outcome. Recent large clinical trials have supported the use of radiation and chemotherapy as adjuvant treatment. Treatment advances have been made despite a poor understanding of the biological underpinnings of medulloblastoma. Current laboratory investigations are shedding light on the oncogenesis of medulloblastoma and may lead to improved treatments.