I-131-MIBG therapies

Radioligand therapy in sympathetic-adrenal-medullary axis tumors: state of art and perspectives

The approval in 2017 by the European Medicines Agency (EMA) and in 2018 by the US Food and Drug Administration (FDA) of radioligand therapy (RLT) led to its wide application in therapeutic management of neuroendocrine neoplasms (NENs). However, the indications are currently limited to certain specific histotypes belonging to the broader NEN's family, mainly advanced well-differentiated gastro-entero-pancreatic NENs. As a consequence, several other tumors of the NEN spectrum that can potentially benefit, due to their biological characteristics, from RLT are still ineligible and can be considered "RLT-orphans". Among those, the subgroup of NENs originating from the sympathetic-adrenal-medullary (SAM) axis can be listed. This paper discusses the state of art and perspectives of the theragnostic applications in pheochromocytomas and paragangliomas, considering both the traditional theragnostic model - with radiolabelled metaiodobenzylguanidine (MIBG) - and the innovative one with radiolabeled somatostatin analogs (SSAs), that will hopefully become available for these patients in the near future.

Radionuclides for Targeted Therapy: Physical Properties

A search in PubMed revealed that 72 radionuclides have been considered for molecular or functional targeted radionuclide therapy. As radionuclide therapies increase in number and variations, it is important to understand the role of the radionuclide and the various characteristics that can render it either useful or useless. This review focuses on the physical characteristics of radionuclides that are relevant for radionuclide therapy, such as linear energy transfer, relative biological effectiveness, range, half-life, imaging properties, and radiation protection considerations. All these properties vary considerably between radionuclides and can be optimised for specific targets. Properties that are advantageous for some applications can sometimes be drawbacks for others; for instance, radionuclides that enable easy imaging can introduce more radiation protection concerns than others. Similarly, a long radiation range is beneficial in targets with heterogeneous uptake, but it also increases the radiation dose to tissues surrounding the target, and, hence, a shorter range is likely more beneficial with homogeneous uptake. While one cannot select a collection of characteristics as each radionuclide comes with an unchangeable set, all the 72 radionuclides investigated for therapy-and many more that have not yet been investigated-provide numerous sets to choose between.

Integration of molecular imaging in the personalized approach to neuroendocrine tumors

NETs lesions can be difficult to characterize with conventional anatomic imaging (CT and MRI). Functional imaging techniques, and especially PET imaging, are very useful for detecting small neuroendocrine tumors that would not be seen with other techniques. The role of nuclear medicine in the localization, staging, restaging, treatment and monitoring of neuroendocrine tumors (NETs) has become progressively more relevant due to: the availability of tracers on new targets, tracers for positron emission tomography (PET); the development of cyclotrons and generators that allow this availability; as well as to hybrid systems (SPECT/CT, PET/CT and PET/MRI) that, by joining the functional and anatomical image, improve the quality of the images. Teragnosis, a new emerging therapy, in NET used receptor-mediated or nonreceptor- mediated mechanism to facilitate penetration and high-affinity binding between the radiopharmaceutical and the tumor cell. Teragnosis offers the possibility of personalized targeted radionuclide therapy.

Single Photon Emission Computed Tomography Tracer

Single photon emission computed tomography (SPECT) is the state-of-the-art imaging modality in nuclear medicine despite the fact that only a few new SPECT tracers have become available in the past 20 years. Critical for the future success of SPECT is the design of new and specific tracers for the detection, localization, and staging of a disease and for monitoring therapy. The utility of SPECT imaging to address oncologic questions is dependent on radiotracers that ideally exhibit excellent tissue penetration, high affinity to the tumor-associated target structure, specific uptake and retention in the malignant lesions, and rapid clearance from non-targeted tissues and organs. In general, a target-specific SPECT radiopharmaceutical can be divided into two main parts: a targeting biomolecule (e.g., peptide, antibody fragment) and a γ-radiation-emitting radionuclide (e.g., ^99mTc, ¹²³I). If radiometals are used as the radiation source, a bifunctional chelator is needed to link the radioisotope to the targeting entity. In a rational SPECT tracer design, these single components have to be critically evaluated in order to achieve a balance among the demands for adequate target binding, and a rapid clearance of the radiotracer. The focus of this chapter is to depict recent developments of tumor-targeted SPECT radiotracers for imaging of cancer diseases. Possibilities for optimization of tracer design and potential causes for design failure are discussed and highlighted with selected examples.

Molecular Imaging and Therapy for Neuroendocrine Tumors

OPINION STATEMENT

Neuroendocrine tumors (NETs) are relatively rare, with 12,000-15,000 new cases diagnosed annually in the USA. Although NETs are a diverse group of neoplasms, they share common molecular targets that can be exploited using nuclear medicine techniques for both imaging and therapy. NETs have traditionally been imaged with SPECT imaging using ¹¹¹In-labeled octreotide analogs to detect neoplasms with somatostatin receptors. In addition, certain NETs (pheochromocytomas, paragangliomas, and neuroblastomas) are also effectively imaged using ¹²³I- or ¹³¹I-labeled metaiodobenzylguanidine (MIBG), an analog of guanethidine. More recently, PET imaging with ⁶⁸Ga-labeled somatostatin receptor (SSR) analogs allows neuroendocrine tumors to be imaged with much higher sensitivity. ⁶⁸Ga-DOTATATE was approved as a PET tracer by the FDA in June 2016. In addition to imaging, both MIBG and DOTATATE can be labeled with therapeutic radionuclides to deliver targeted radiation selectively to macroscopic and microscopic tumor sites. The incorporation of the same molecular probe for imaging and therapy provides a radio-theranostic approach to identifying, targeting, and treating tumors. Over the years, several centers have experience treating NETs with high-dose ¹³¹I-MIBG. ¹⁷⁷Lu-DOTATATE was approved by the FDA in 2018 for treatment of gastroenteropancreatic NETs and constitutes a major advancement in the treatment of these diseases. In this paper, we provide an overview of imaging and treating neuroendocrine tumors using MIBG and SSR probes. Although uncommon, neuroendocrine tumors have provided the largest experience for targeted radionuclide imaging and therapy (with the exception of radioiodine treatment for thyroid disease). In addition to benefitting patients with these rare tumors, the knowledge gained provides a blueprint for the development of future paired diagnostic/therapeutic probes for treating other diseases, such as prostate cancer.

MIBG (metaiodobenzylguanidine) theranostics in pediatric and adult malignancies

Metaiodobenzylguanidine, a guanithidine analog, labeled with ¹²³I and ¹³¹I, is used for imaging and therapy of neuroblastomas and various neural crest tumors like paragangliomas, pheochromocytomas, medullary cancer of thyroid and carcinoids since the past three to four decades. In this review article, we shall revisit metaiodobenzylguanidine as a radiopharmaceutical and its various applications in neural crest tumors.

Neuroblastoma

Neuroblastoma is one of the most common solid tumors in children and has a diverse clinical behavior that largely depends on the tumor biology. Neuroblastoma exhibits unique features, such as early age of onset, high frequency of metastatic disease at diagnosis in patients over 1 year of age and the tendency for spontaneous regression of tumors in infants. The high-risk tumors frequently have amplification of the MYCN oncogene as well as segmental chromosome alterations with poor survival. Recent advanced genomic sequencing technology has revealed that mutation of ALK, which is present in ~10% of primary tumors, often causes familial neuroblastoma with germline mutation. However, the frequency of gene mutations is relatively small and other aberrations, such as epigenetic abnormalities, have also been proposed. The risk-stratified therapy was introduced by the Japan Neuroblastoma Study Group (JNBSG), which is now moving to the Neuroblastoma Committee of Japan Children's Cancer Group (JCCG). Several clinical studies have facilitated the reduction of therapy for children with low-risk neuroblastoma disease and the significant improvement of cure rates for patients with intermediate-risk as well as high-risk disease. Therapy for patients with high-risk disease includes intensive induction chemotherapy and myeloablative chemotherapy, followed by the treatment of minimal residual disease using differentiation therapy and immunotherapy. The JCCG aims for better cures and long-term quality of life for children with cancer by facilitating new approaches targeting novel driver proteins, genetic pathways and the tumor microenvironment.

Current Consensus on I-131 MIBG Therapy

Metaiodobenzylguanidine (MIBG) is structurally similar to the neurotransmitter norepinephrine and specifically targets neuroendocrine cells including some neuroendocrine tumors. Iodine-131 (I-131)-labeled MIBG (I-131 MIBG) therapy for neuroendocrine tumors has been performed for more than a quarter-century. The indications of I-131 MIBG therapy include treatment-resistant neuroblastoma (NB), unresectable or metastatic pheochromocytoma (PC) and paraganglioma (PG), unresectable or metastatic carcinoid tumors, and unresectable or metastatic medullary thyroid cancer (MTC). I-131 MIBG therapy is one of the considerable effective treatments in patients with advanced NB, PC, and PG. On the other hand, I-131 MIBG therapy is an alternative method after more effective novel therapies are used such as radiolabeled somatostatin analogs and tyrosine kinase inhibitors in patients with advanced carcinoid tumors and MTC. No-carrier-aided (NCA) I-131 MIBG has more favorable potential compared to the conventional I-131 MIBG. Astatine-211-labeled meta-astatobenzylguanidine (At-211 MABG) has massive potential in patients with neuroendocrine tumors. Further studies about the therapeutic protocols of I-131 MIBG including NCA I-131 MIBG in the clinical setting and At-211 MABG in both the preclinical and clinical settings are needed.

Paradigm shift in theranostics of neuroendocrine tumors: conceptual horizons of nanotechnology in nuclear medicine

We present a comprehensive review of Neuroendocrine Tumors (NET) and the current and developing imaging and therapeutic modalities for NET with emphasis on Nuclear Medicine modalities. Subsequently, nanotechnology and its emerging role in cancer management, especially NET, are discussed. The article is both educative and informative. The objective is to provide an insight into the developments made in nuclear medicine and nanotechnology towards management of NET, individually as well as combined together.

Transcript Analysis for Internal Biodosimetry Using Peripheral Blood from Neuroblastoma Patients Treated with (131)I-mIBG, a Targeted Radionuclide

Calculating internal dose from therapeutic radionuclides currently relies on estimates made from multiple radiation exposure measurements, converted to absorbed dose in specific organs using the Medical Internal Radiation Dose (MIRD) schema. As an alternative biodosimetric approach, we utilized gene expression analysis of whole blood from patients receiving targeted radiotherapy. Collected blood from patients with relapsed or refractory neuroblastoma who received (131)I-labeled metaiodobenzylguanidine ((131)I-mIBG) at the University of California San Francisco (UCSF) was used to compare calculated internal dose with the modulation of chosen gene expression. A total of 40 patients, median age 9 years, had blood drawn at baseline, 72 and 96 h after (131)I-mIBG infusion. Whole-body absorbed dose was calculated for each patient based on the cumulated activity determined from injected mIBG activity and patient-specific time-activity curves combined with (131)I whole-body S factors. We then assessed transcripts that were the most significant for describing the mixed therapeutic treatments over time using real-time polymerase chain reaction (RT-PCR). Modulation was evaluated statistically using multiple regression analysis for data at 0, 72 and 96 h. A total of 10 genes were analyzed across 40 patients: CDKN1A; FDXR; GADD45A; BCLXL; STAT5B; BAX; BCL2; DDB2; XPC; and MDM2. Six genes were significantly modulated upon exposure to (131)I-mIBG at 72 h, as well as at 96 h. Four genes varied significantly with absorbed dose when controlling for time. A gene expression biodosimetry model was developed to predict absorbed dose based on modulation of gene transcripts within whole blood. Three transcripts explained over 98% of the variance in the modulation of gene expression over the 96 h (CDKN1A, BAX and DDB2). To our knowledge, this is a novel study, which uses whole blood collected from patients treated with a radiopharmaceutical, to characterize biomarkers that may be useful for biodosimetry. Our data indicate that transcripts, which have been previously identified as biomarkers of external exposures in ex vivo whole blood and in vivo radiotherapy patients, are also good early indicators of internal exposure. However, for internal sources of radiation, the biokinetics and physical decay of the radionuclide strongly influence the gene expression.

Calibration of Traceable Solid Mock (131)I Phantoms Used in an International SPECT Image Quantification Comparison

The International Atomic Energy Agency (IAEA) has organized an international comparison to assess Single Photon Emission Computed Tomography (SPECT) image quantification capabilities in 12 countries. Iodine-131 was chosen as the radionuclide for the comparison because of its wide use around the world, but for logistical reasons solid (133)Ba sources were used as a long-lived surrogate for (131)I. For this study, we designed a set of solid cylindrical sources so that each site could have a set of phantoms (having nominal volumes of 2 mL, 4 mL, 6 mL, and 23 mL) with traceable activity calibrations so that the results could be properly compared. We also developed a technique using two different detection methods for individually calibrating the sources for (133)Ba activity based on a National standard. This methodology allows for the activity calibration of each (133)Ba source with a standard uncertainty on the activity of 1.4 % for the high-level 2-, 4-, and 6-mL sources and 1.7 % for the lower-level 23 mL cylinders. This level of uncertainty allows for these sources to be used for the intended comparison exercise, as well as in other SPECT image quantification studies.

The necessity of nuclear reactors for targeted radionuclide therapies

Nuclear medicine has been contributing towards personalized therapies. Nuclear reactors are required for the working horses of both diagnosis and treatment, i.e., Tc-99m and I-131. In fact, reactors will remain necessary to fulfill the demand for a variety of radionuclides and are essential in the expanding field of targeted radionuclide therapies for cancer. However, the main reactors involved in the global supply are ageing and expected to shut down before 2025. Therefore, the fields of (nuclear) medicine, nuclear industry and politics share a global responsibility, faced with the task to secure future access to suitable nuclear reactors. At the same time, alternative production routes should be industrialized. For this, a coordinating entity should be put into place.