Selective hepatic arterial infusion of In-111-DTPA-Phe1-octreotide in neuroendocrine liver metastases

Synthesis and Preclinical Evaluation of PSMA-Targeted 111In-Radioconjugates Containing a Mitochondria-Tropic Triphenylphosphonium Carrier

Nuclear DNA is the canonical target for biological damage induced by Auger electrons (AE) in the context of targeted radionuclide therapy (TRT) of cancer, but other subcellular components might also be relevant for this purpose, such as the energized mitochondria of tumor cells. Having this in mind, we have synthesized novel DOTA-based chelators carrying a prostate-specific membrane antigen (PSMA) inhibitor and a triphenyl phosphonium (TPP) group that were used to obtain dual-targeted 111In-radioconjugates ([111In]In-TPP-DOTAGA-PSMA and [111In]In-TPP-DOTAGA-G3-PSMA), aiming to promote a selective uptake of an AE-emitter radiometal (111In) by PSMA+ prostate cancer (PCa) cells and an enhanced accumulation in the mitochondria. These dual-targeted 111In-radiocomplexes are highly stable under physiological conditions and in cell culture media. The complexes showed relatively similar binding affinities toward the PSMA compared to the reference tracer [111In]In-PSMA-617, in line with their high cellular uptake and internalization in PSMA+ PCa cells. The complexes compromised cell survival in a dose-dependent manner and in the case of [111In]In-TPP-DOTAGA-G3-PSMA to a higher extent than observed for the single-targeted congener [111In]In-PSMA-617. μSPECT imaging studies in PSMA+ PCa xenografts showed that the TPP pharmacophore did not interfere with the excellent in vivo tumor uptake of the "golden standard" [111In]In-PSMA-617, although it led to a higher kidney retention. Such kidney retention does not necessarily compromise their usefulness as radiotherapeutics due to the short tissue range of the Auger/conversion electrons emitted by 111In. Overall, our results provide valuable insights into the potential use of mitochondrial targeting by PSMA-based radiocomplexes for efficient use of AE-emitting radionuclides in TRT, giving impetus to extend the studies to other AE-emitting trivalent radiometals (e.g., 161Tb or 165Er) and to further optimize the designed dual-targeting constructs.

Treatment of Neuroendocrine Neoplasms with Radiolabeled Peptides-Where Are We Now

Peptide receptor radionuclide therapy (PRRT) has been one of the most successful and exciting examples of theranostics in nuclear medicine in recent decades and is now firmly embedded in many treatment algorithms for unresectable or metastatic neuroendocrine neoplasms (NENs) worldwide. It is widely considered to be an effective treatment for well- or moderately differentiated neoplasms, which express high levels of somatostatin receptors that can be selectively targeted. This review article outlines the scientific basis of PRRT in treatment of NENs and describes its discovery dating back to the early 1990s. Early treatments utilizing Indium-111, a γ-emitter, showed promise in reduction in tumor size and improvement in biochemistry, but were also met with high radiation doses and myelotoxic and nephrotoxic effects. Subsequently, stable conjugation of DOTA-peptides with β-emitting radionuclides, such as Yttrium-90 and Lutetium-177, served as a breakthrough for PRRT and studies highlighted their potential in eliciting progression-free survival and quality of life benefits. This article will also elaborate on the key trials which paved the way for its approval and will discuss therapeutic considerations, such as patient selection and administration technique, to optimize its use.

90Y/177Lu-DOTATOC: From Preclinical Studies to Application in Humans

The PRRT (Peptide Receptor Radionuclide Therapy) is a promising modality treatment for patients with inoperable or metastatic neuroendocrine tumors (NETs). Progression-free survival (PFS) and overall survival (OS) of these patients are favorably comparable with standard therapies. The protagonist in this type of therapy is a somatostatin-modified peptide fragment ([Tyr3] octreotide), equipped with a specific chelating system (DOTA) capable of creating a stable bond with β-emitting radionuclides, such as yttrium-90 and lutetium-177. In this review, covering twenty five years of literature, we describe the characteristics and performances of the two most used therapeutic radiopharmaceuticals for the NETs radio-treatment: [⁹⁰Y]Y-DOTATOC and [¹⁷⁷Lu]Lu-DOTATOC taking this opportunity to retrace the most significant results that have determined their success, promoting them from preclinical studies to application in humans.

Auger: The future of precision medicine

First reported by Lise Meitner in 1922 and independently by Pierre Auger in 1923, the Auger effect has been explored as a potential source for targeted radiotherapy. The Auger effect is based on the emission of a low energy electron (typically <25 keV) from an atom post electron capture (EC), internal conversion (IC), or incident X-rays excitation. This phenomenon can cause the emission of a primary electron and multiple electron tracks typically in the nearest proximity of the emission site (2-500 nm). The short range of the emitted Auger cascade results in medium/high levels of linear energy transfer (4-26 keV/μm) exerted on the surrounding tissue. This property makes Auger emitters the ideal candidates for delivering high levels of targeted radiation to a specific target with dimensions comparable to, for example, the DNA. By using a targeting vector such as a small molecule, peptide or antibody, one has the potential of delivering high levels of radiation to tumor specific biomarkers while circumventing off-site toxicity in healthy cells; a challenge which is harder to overcome when using other, longer range sources of radiation such as beta and alpha emitting radionuclides. Several reviews on Auger emitters have been published over the years with two recent examples. For these reviews and others, we support their analysis and therefore to avoid simple repetition, this commentary will seek to address additional aspects and viewpoints. Specifically, we will focus on those most promising preclinical and clinical studies using small molecules, peptides, antibodies and how these studies may serve as a template for future studies.

Intra-arterial versus standard intravenous administration of lutetium-177-DOTA-octreotate in patients with NET liver metastases: study protocol for a multicenter, randomized controlled trial (LUTIA trial)

BACKGROUND

Lutetium-177-DOTA-octreotate (¹⁷⁷Lu-DOTATATE) significantly increases survival and response rates in patients with grade I and grade II neuroendocrine tumors (NETs). However, survival and response rates are significantly lower in patients with bulky liver metastases. Increasing the tumor-absorbed dose in liver metastases may improve response to ¹⁷⁷Lu-DOTATATE. The LUTIA (Lutetium Intra-Arterial) study aims to increase the tumor-absorbed dose in liver metastases by intra-arterial (IA) administration of ¹⁷⁷Lu-DOTATATE, compared to conventional intravenous (IV) administration.

METHODS

A multicenter, within-patient randomized controlled trial (RCT) in 26 patients with progressive, liver-dominant, unresectable grade I or grade II NET will be conducted. Patients with bilobar bulky disease will be randomly allocated to receive IA treatment into either the left or the right hepatic artery. Using this approach, one liver lobe will be treated intra-arterially (first-pass effect), while the contralateral lobe will receive an intravenous treatment as a second-pass effect. The primary endpoint of this study is the difference in tumor-to-non-tumor ratio of ¹⁷⁷Lu-DOTATATE uptake between the two liver lobes on post-treatment SPECT/CT (IA versus IV). Secondary endpoints include absorbed dose in both liver lobes, tumor response, dose-response relationship, toxicity, uptake in extrahepatic lesions, and renal uptake.

DISCUSSION

This multicenter, within-patient RCT will investigate whether IA administration of ¹⁷⁷Lu-DOTATATE results in a higher activity concentration in liver metastases compared to IV administration.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT03590119. Registered on 17 July 2018.

Delivery systems exploiting natural cell transport processes of macromolecules for intracellular targeting of Auger electron emitters

The presence of Auger electrons (AE) among the decay products of a number of radionuclides makes these radionuclides an attractive means for treating cancer because these short-range electrons can cause significant damage in the immediate vicinity of the decomposition site. Moreover, the extreme locality of the effect provides a potential for selective eradication of cancer cells with minimal damage to adjacent normal cells provided that the delivery of the AE emitter to the most vulnerable parts of the cell can be achieved. Few cellular compartments have been regarded as the desired target site for AE emitters, with the cell nucleus generally recognized as the preferred site for AE decay due to the extreme sensitivity of nuclear DNA to direct damage by radiation of high linear energy transfer. Thus, the advantages of AE emitters for cancer therapy are most likely to be realized by their selective delivery into the nucleus of the malignant cells. To achieve this goal, delivery systems must combine a challenging complex of properties that not only provide cancer cell preferential recognition but also cell entry followed by transport into the cell nucleus. A promising strategy for achieving this is the recruitment of natural cell transport processes of macromolecules, involved in each of the aforementioned steps. To date, a number of constructs exploiting intracellular transport systems have been proposed for AE emitter delivery to the nucleus of a targeted cell. An example of such a multifunctional vehicle that provides smart step-by-step delivery is the so-called modular nanotransporter, which accomplishes selective recognition, binding, internalization, and endosomal escape followed by nuclear import of the delivered radionuclide. The current review will focus on delivery systems utilizing various intracellular transport pathways and their combinations in order to provide efficient targeting of AE to the cancer cell nucleus.

Auger electrons for cancer therapy - a review

Background

Auger electrons (AEs) are very low energy electrons that are emitted by radionuclides that decay by electron capture (e.g. ¹¹¹In, ⁶⁷Ga, ^99mTc, ^195mPt, ¹²⁵I and ¹²³I). This energy is deposited over nanometre-micrometre distances, resulting in high linear energy transfer (LET) that is potent for causing lethal damage in cancer cells. Thus, AE-emitting radiotherapeutic agents have great potential for treatment of cancer. In this review, we describe the radiobiological properties of AEs, their radiation dosimetry, radiolabelling methods, and preclinical and clinical studies that have been performed to investigate AEs for cancer treatment.

Results

AEs are most lethal to cancer cells when emitted near the cell nucleus and especially when incorporated into DNA (e.g. ¹²⁵I-IUdR). AEs cause DNA damage both directly and indirectly via water radiolysis. AEs can also kill targeted cancer cells by damaging the cell membrane, and kill non-targeted cells through a cross-dose or bystander effect. The radiation dosimetry of AEs considers both organ doses and cellular doses. The Medical Internal Radiation Dose (MIRD) schema may be applied. Radiolabelling methods for complexing AE-emitters to biomolecules (antibodies and peptides) and nanoparticles include radioiodination (¹²⁵I and ¹²³I) or radiometal chelation (¹¹¹In, ⁶⁷Ga, ^99mTc). Cancer cells exposed in vitro to AE-emitting radiotherapeutic agents exhibit decreased clonogenic survival correlated at least in part with unrepaired DNA double-strand breaks (DSBs) detected by immunofluorescence for γH2AX, and chromosomal aberrations. Preclinical studies of AE-emitting radiotherapeutic agents have shown strong tumour growth inhibition in vivo in tumour xenograft mouse models. Minimal normal tissue toxicity was found due to the restricted toxicity of AEs mostly on tumour cells targeted by the radiotherapeutic agents. Clinical studies of AEs for cancer treatment have been limited but some encouraging results were obtained in early studies using ¹¹¹In-DTPA-octreotide and ¹²⁵I-IUdR, in which tumour remissions were achieved in several patients at administered amounts that caused low normal tissue toxicity, as well as promising improvements in the survival of glioblastoma patients with ¹²⁵I-mAb 425, with minimal normal tissue toxicity.

Conclusions

Proof-of-principle for AE radiotherapy of cancer has been shown preclinically, and clinically in a limited number of studies. The recent introduction of many biologically-targeted therapies for cancer creates new opportunities to design novel AE-emitting agents for cancer treatment. Pierre Auger did not conceive of the application of AEs for targeted cancer treatment, but this is a tremendously exciting future that we and many other scientists in this field envision.

Antitumor Activity of Auger Electron Emitter 111In Delivered by Modular Nanotransporter for Treatment of Bladder Cancer With EGFR Overexpression

Gamma-ray emitting ¹¹¹In, which is extensively used for imaging, is also a source of short-range Auger electrons (AE). While exhibiting negligible effect outside cells, these AE become highly toxic near DNA within the cell nucleus. Therefore, these radionuclides can be used as a therapeutic anticancer agent if delivered precisely into the nuclei of tumor target cells. Modular nanotransporters (MNTs) designed to provide receptor-targeted delivery of short-range therapeutic cargoes into the nuclei of target cells are perspective candidates for specific intracellular delivery of AE emitters. The objective of this study was to evaluate the in vitro and in vivo efficacy of ¹¹¹In attached MNTs to kill human bladder cancer cells overexpressing epidermal growth factor receptor (EGFR). The cytotoxicity of ¹¹¹In delivered by the EGFR-targeted MNT (¹¹¹In-MNT) was greatly enhanced on EJ-, HT-1376-, and 5637-expressing EGFR bladder cancer cell lines compared with ¹¹¹In non-targeted control. In vivo microSPECT/CT imaging and antitumor efficacy studies revealed prolonged intratumoral retention of ¹¹¹In-MNT with t½ = 4.1 ± 0.5 days as well as significant dose-dependent tumor growth delay (up to 90% growth inhibition) after local infusion of ¹¹¹In-MNT in EJ xenograft-bearing mice.

Theranostics of Neuroendocrine Tumors

Somatostatin receptor positron emission tomography/computed tomography using ⁶⁸Ga-labeled somatostatin analogs is the mainstay for the evaluation of receptor status in neuroendocrine tumors (NETs). This translates towards better therapy options, with increasing evidence of peptide receptor radionuclide therapy (PRRT) as the treatment of choice for advanced or progressive NETs. There are benefits in progression-free and overall survival as well as a significant improvement in clinical condition. In patients with progressive NETs, fractionated, personalized PRRT results in good therapeutic responses with no significant severe hematological and/or renal toxicity, thus improving quality of life.

Targeted Therapy for Medullary Thyroid Cancer: A Review

Medullary thyroid cancers (MTCs) constitute between 2 and 5% of all thyroid cancers. The 10-year overall survival (OS) rate of patients with localized disease is around 95% while that of patients with regional stage disease is about 75%. Only 20% of patients with distant metastases at diagnosis survive 10 years which is significantly lower than for differentiated thyroid cancers. Cases with regional metastases at presentation have high recurrence rates. Adjuvant external radiation confers local control but not improved OS. The management of residual, recurrent, or metastatic disease till a few years ago was re-surgery with local measures such as radiation. Chemotherapy was used with marginal benefit. The development of targeted therapy has brought in a major advantage in management of such patients. Two drugs—vandetanib and cabozantinib—have been approved for use in progressive or metastatic MTC. In addition, several drugs acting on other steps of the molecular pathway are being investigated with promising results. Targeted radionuclide therapy also provides an effective treatment option with good quality of life. This review covers the rationale of targeted therapy for MTC, present treatment options, drugs and methods under investigation, as well as an outline of the adverse effects and their management.

Merkel Cell Carcinoma Therapeutic Update

OPINION STATEMENT

Merkel cell carcinoma (MCC) is a rare and aggressive neuroendocrine tumor of the skin. Early-stage disease can be cured with surgical resection and radiotherapy (RT). Sentinel lymph node biopsy (SLNB) is an important staging tool, as a microscopic MCC is frequently identified. Adjuvant RT to the primary excision site and regional lymph node bed may improve locoregional control. However, newer studies confirm that patients with biopsy-negative sentinel lymph nodes may not benefit from regional RT. Advanced MCC currently lacks a highly effective treatment as responses to chemotherapy are not durable. Recent work suggests that immunotherapy targeting the programmed cell death receptor 1/programmed cell death ligand 1 (PD-1/PD-L1) checkpoint holds great promise in treating advanced MCC and may provide durable responses in a portion of patients. At the same time, high-throughput sequencing studies have demonstrated significant differences in the mutational profiles of tumors with and without the Merkel cell polyomavirus (MCV). An important secondary endpoint in the ongoing immunotherapy trials for MCC will be determining if there is a response difference between the virus-positive MCC tumors that typically lack a large mutational burden and the virus-negative tumors that have a large number of somatic mutations and predicted tumor neoantigens. Interestingly, sequencing studies have failed to identify a highly recurrent activated driver pathway in the majority of MCC tumors. This may explain why targeted therapies can demonstrate exceptional responses in case reports but fail when treating all comers with MCC. Ultimately, a precision medicine approach may be more appropriate for treating MCC, where identified driver mutations are used to direct targeted therapies. At a minimum, stratifying patients in future clinical trials based on tumor viral status should be considered as virus-negative tumors are more likely to harbor activating driver mutations.

Peptide Receptor Radionuclide Therapy in the Treatment of Neuroendocrine Tumors

Peptide receptor radionuclide therapy (PRRT) is a promising new treatment modality for inoperable or metastasized gastroenteropancreatic neuroendocrine tumors patients. Most studies report objective response rates in 15% to 35% of patients. Progression-free (PFS) and overall survival (OS) compare favorably with that for somatostatin analogues, chemotherapy, or newer, "targeted" therapies. Prospective, randomized data regarding the potential PFS and OS benefit of PRRT compared with standard therapies is anticipated.

Localized Irradiation of Cell Membrane by Auger Electrons Is Cytotoxic Through Oxidative Stress-Mediated Nontargeted Effects

AIMS

We investigated whether radiation-induced nontargeted effects are involved in the cytotoxic effects of anticell surface monoclonal antibodies labeled with Auger electron emitters, such as iodine 125 (monoclonal antibodies labeled with (125)I [(125)I-mAbs]).

RESULTS

We showed that the cytotoxicity of (125)I-mAbs targeting the cell membrane of p53(+/+) HCT116 colon cancer cells is mainly due to nontargeted effects. Targeted and nontargeted cytotoxicities were inhibited in vitro following lipid raft disruption with Methyl-β-cyclodextrin (MBCD) or filipin or use of radical oxygen species scavengers. (125)I-mAb efficacy was associated with acid sphingomyelinase activation and modulated through activation of the AKT, extracellular signal-related kinase ½ (ERK1/2), p38 kinase, c-Jun N-terminal kinase (JNK) signaling pathways, and also of phospholipase C-γ (PLC-γ), proline-rich tyrosine kinase 2 (PYK-2), and paxillin, involved in Ca(2+) fluxes. Moreover, the nontargeted response induced by directing 5-[(125)I]iodo-2'-deoxyuridine to the nucleus was comparable to that of (125)I-mAb against cell surface receptors. In vivo, we found that the statistical significance of tumor growth delay induced by (125)I-mAb was removed after MBCD treatment and observed oxidative DNA damage beyond the expected Auger electron range. These results suggest the involvement of nontargeted effects in vivo also.

INNOVATION

Low-energy Auger electrons, such as those emitted by (125)I, have a short tissue range and are usually targeted to the nucleus to maximize their cytotoxicity. In this study, we show that targeting the cancer cell surface with (125)I-mAbs produces a lipid raft-mediated nontargeted response that compensates for the inferior efficacy of non-nuclear targeting.

CONCLUSION

Our findings describe the mechanisms involved in the efficacy of (125)I-mAbs targeting the cancer cell surface. Antioxid. Redox Signal. 25, 467-484.

Peptide receptor radionuclide therapy: focus on bronchial neuroendocrine tumors

Well-differentiated bronchial neuroendocrine tumors (B-NETs) are rare. They represent 1-5 % of all lung cancers. The incidence of these neoplasms has risen over the past 30 years and, especially for advanced or metastatic disease, management is complex and requires a multidisciplinary approach. Treatment with somatostatin analogs (SSAs) is the most important first-line therapy, in particular in well-differentiated NETs with high somatostatin type receptor (SSTR) expression. In these tumors, the role of mammalian target of rapamycin (m-TOR) inhibitors and the potential utility of other target therapies remain unclear while chemotherapy represents the gold standard treatment only for aggressive forms with low SSTR expression. Peptide receptor radionuclide therapy (PRRT) is an emerging treatment modality for advanced NETs. There are many cumulative evidences about the effectiveness and tolerability of this therapeutic approach, especially in gastro-entero-pancreatic (GEP)-NETs. For B-NETs, scientific research is moving more slowly. Here, we performed a review in order to evaluate the efficacy and toxicity of PRRT with a focus on patients with inoperable or metastatic well-differentiated B-NETs.

Comparison of 111In-[DTPA0]Octreotide Versus Non Carrier Added 177Lu- [DOTA0,Tyr3]-Octreotate Efficacy in Patients With GEP-NET Treated Intra-arterially for Liver Metastases

AIM

In patients with progressive, metastatic neuroendocrine tumors (NET), intra-arterial radionuclide infusions with high activities of In-[DTPA]-octreotide and more recently with non-carrier added (nca) Lu-[DOTA,Tyr]-octreotate have been performed with encouraging results. However, the affinity profiles (IC50) of these radiopeptides for human sst2 receptors are markedly different (In-[DTPA]-octreotide, 22 ± 3.6 nM and nca Lu-[DOTA,Tyr]-octreotate, 1.5 ± 4.0 nM). The total administered activity is determined by organ dose limits (kidneys and bone marrow), and our aim therefore was to compare and evaluate the therapeutic efficacy of both radiopeptides in metastatic NETs.

METHODS

Thirty patients with gastroenteropancreatic (GEP) somatostatin-positive NETs with liver metastases confirmed on biopsy and In-pentetreotide scan were included. They were treated with In-[DTPA]-octreotide (n = 17) or nca Lu-[DOTA,Tyr]-octreotate (n = 13). Blood samples were collected 2, 4, 8, and 24 hours postadministration to calculate residence time in blood and in red marrow. The maximum percentage uptake in organs and tumors was estimated by region of interest analysis, and tumor dosimetry calculations were performed using OLINDA/EXM/ 1.0 software.

RESULTS

ncaLu-[DOTA,Tyr3]-octreotate blood radioactivity, expressed as a percentage of the injected dose, was significantly lower than In-[DTPA]-octreotide (P < 0.05), as clearly depicted from the time-activity curves; the background-corrected tumor uptake was significantly higher than In-[DTPA]-octreotide but without any significant difference in other organs (spleen, kidneys, and liver).

CONCLUSIONS

Using Lu-[DOTA,Tyr]-octreotate, a 3-fold higher absorbed dose to tumor tissue was achieved compared with In-[DTPA] octreotide. Residence time of nca Lu-[DOTA,Tyr]-octreotate results in a significantly higher absorbed dose to bone marrow compared with In-[DTPA]-octreotide. However, a drawback of In-[DTPA]-octreotide therapy is that the number of administrations would need to be almost doubled to achieve an equal therapeutic outcome as compared with Lu-[DOTA,Tyr]-octreotate.

The quest to exploit the Auger effect in cancer radiotherapy - a reflective review

To identify the emergence of the recognition of the potential of the Auger effect for clinical application, and after tracing the salient milestones towards that goal, to evaluate the status quo and future prospects. It was not until 40 years after the discovery of Auger electrons, that the availability of radioactive DNA precursors enabled the biological power, and the clinical potential, of the Auger effect to be appreciated. Important milestones on the path to clinical translation have been identified and reached, but hurdles remain. Nevertheless the potential is still evident, and there is reasonable optimism that the goal of clinical translation is achievable.

Peptide receptor radionuclide therapy of neuroendocrine tumours

In the past decades, the number of neuroendocrine tumours that are detected is increasing. A relative new and promising therapy for patients with metastasised or inoperable disease is peptide receptor radionuclide therapy (PRRT). This therapy involves an infusion of somatostatin analogues linked to radionuclides like Yttrium-90 or Lutetium-177. Objective response rates are reported in 15-35%. Response rates may vary between type of tumour and radionuclide. Besides the objective response rate, overall survival and progression free survival increase significantly. Also, the quality of life improves as well. Serious side-affects are rare. PRRT is usually well tolerated, also in patients with extensive metastasised disease. Recent studies combined PRRT with other types of therapies. Unfortunately no randomised trials comparing these strategies are available. In the future, more research is needed to evaluate the best therapy combinations or sequence of therapies.

SPECT- and PET-based patient-tailored treatment in neuroendocrine tumors: a comprehensive multidisciplinary team approach

The overexpression of somatostatin receptors on the tumor cell surface of neuroendocrine tumors (NETs) detected by multimodal functional imaging modalities such as SPECT and PET tracers constitutes a therapeutic option using targeting radiolabeled compounds. We will introduce the theranostic concept in general, explain in more detail its development in NETs, and discuss available SPECT and PET tracers regarding their potential for diagnostic imaging, visualization of target expression, and treatment tailoring. Moreover, we will discuss the currently available peptide receptor radionuclide therapy principles and compare them to previously published studies. Finally, we will discuss which new concepts will most likely influence the theranostic treatment approach in NETs in the future.

GEPNETs update: Radionuclide therapy in neuroendocrine tumors

Peptide receptor radionuclide therapy (PRRT) is a promising new treatment modality for inoperable or metastasized gastroenteropancreatic neuroendocrine tumors (GEPNETs) patients. Most studies report objective response rates in 15-35% of patients. Also, outcome in terms of progression free survival (PFS) and overall survival compares very favorably with that for somatostatin analogs, chemotherapy, or new, 'targeted' therapies. They also compare favorably to PFS data for liver-directed therapies. Two decades after the introduction of PRRT, there is a growing need for randomized controlled trials comparing PRRT to 'standard' treatment, that is treatment with agents that have proven benefit when tested in randomized trials. Combining PRRT with liver-directed therapies or with targeted therapies could improve treatment results. The question to be answered, however, is whether a combination of therapies performed within a limited time-span from one another results in a better PFS than a strategy in which other therapies are reserved until after (renewed) tumor progression. Randomized clinical trials comparing PRRT with other treatment modalities should be undertaken to determine the best treatment options and treatment sequelae for patients with GEPNETs.

Current status of Interventional Radiology in the management of Gastro-Entero-Pancreatic Neuroendocrine Tumours (GEP-NETs)

Within the group of Gastro-Entero-Pancreatic Neuroendocrine tumours (GEP-NETs), several heterogeneous malignancies are included with a variety of clinical manifestations and imaging characteristics. Often these cases are inoperable and minimal invasive treatment offered by image-guided procedures appears to be the only option. Interventional radiology offers a valid solution in the management of primary and metastatic GEP-NETs. The purpose of this review article is to describe the current status of the role of Interventional Radiology in the management of GEP-NETs.

Intra-arterial liver-directed therapies for neuroendocrine hepatic metastases

Hepatic metastases, which are frequently seen in patients with neuroendocrine tumors (NETs), have a major adverse impact on the patient's quality of life and survival. Surgery is the treatment of choice for hepatic metastases but is possible in only a small percentage of patients. Systemic chemotherapy yields disappointing results. Somatostatin analogs are effective in controlling symptoms in many of these patients; however, the disease can become refractory to treatment. Transcatheter intra-arterial liver-directed therapies, such as hepatic artery embolization, chemoembolization, and radioembolization are frequently used in patients with NETs metastatic to the liver, especially in patients with refractory, unresectable, or recurrent disease. These treatments are effective in palliating the hormonal symptoms as well as achieving objective tumor responses. This review focuses on the technique, safety, and clinical efficacy of hepatic artery embolization, chemoembolization, and radioembolization in patients with metastatic NETs.

Peptide receptor radionuclide therapy using radiolabeled somatostatin analogs: focus on future developments

Peptide receptor radionuclide therapy (PRRT) has been shown to be an effective treatment for neuroendocrine tumors (NETs) if curative surgery is not an option. A majority of NETs abundantly express somatostatin receptors. Consequently, following administration of somatostatin (SST) analogs labeled with γ-emitting radionuclides, these tumors can be imaged for diagnosis, staging or follow-up purposes. Furthermore, when β-emitting radionuclides are used, radiolabeled peptides (radiopeptides) can also be used for the treatment for NET patients. Even though excellent results have been achieved with PRRT, complete responses are still rare, which means that there is room for improvement. In this review, we highlight some of the directions currently under investigation in pilot clinical studies or in preclinical development to achieve this goal. Although randomized clinical trials are still lacking, early studies have shown that tumor response might be improved by application of other radionuclides, such as α-emitters or radionuclide combinations, or by adjustment of radiopeptide administration routes. Individualized dosimetry and better insight into tumor and normal organ radiation doses may allow adjustment of the amount of administered activity per cycle or the number of treatment cycles, resulting in more personalized treatment schedules. Other options include the application of novel (radiolabeled) SST analogs with improved tumor uptake and radionuclide retention time, or a combination of PRRT with other systemic therapies, such as chemotherapy or treatment with radio sensitizers. Though promising directions appear to bring improvements of PRRT within reach, additional research (including randomized clinical trials) is needed to achieve such improvements.

Requisites for successful theranostics with radionuclide-based reporter gene imaging

Radionuclide-based theranostic strategy has been widely used in diagnosis and treatment of patients with hyperthyroidism or differentiated thyroid cancer for a long time, and sodium iodide symporter gene is the radionuclide-based reporter gene used in theranostics. Theranostics, which is a promising approach, offering the ideal combination of accurate diagnosis and successful therapy in various clinical fields, is expected to become a key area of personalized medicine. Rapid advancements in biotechnologies using theranostic reporter genes and theranostic radiochemistry have led to development of the concept of theranostics using radionuclide-based imaging reporter genes; the theranostic approach is almost ready for application in a limited arena of clinics. In order to fulfill both the diagnostic and therapeutic purposes, theranostics with radionuclide-based imaging reporter requires use of successful combinations of various components, such as radionuclide-based reporter genes, promoters/enhancers that regulate expression of reporter genes, delivery vectors/vehicles, imaging or therapeutic probes and prodrugs, transductional and transcriptional targeting strategies, transgene amplification systems, etc. In this review, overview and recent updates on theranostics using radionuclide-based imaging reporter genes will be discussed.

Treatment of gastroenteropancreatic neuroendocrine tumors with peptide receptor radionuclide therapy

The primary treatment of gastroenteropancreatic neuroendocrine tumors (GEPNETs) is surgery with curative intent or debulking of the tumor mass. In case of metastatic disease, cytoreductive options are limited. A relatively new therapeutic modality, peptide receptor radionuclide therapy (PRRT) with radiolabeled somatostatin analogs, is currently available in a number of mostly European centers. Complete and partial responses obtained after treatment with [90Y-DOTA0,Tyr3]octreotide are in the same range as after treatment with [177Lu-DOTA0,Tyr3]octreotate (i.e. 10-30%). However, significant nephrotoxicity has been observed after treatment with [90Y-DOTA0,Tyr3]octreotide. Options to improve PRRT may include combinations of radioactive labeled somatostatin analogs, intra-arterial administration, and the use of radiosensitizing drugs combined with PRRT. Other therapeutic applications of PRRT may include additional therapy cycles in patients with progressive disease after benefit from initial therapy, PRRT in adjuvant or neoadjuvant setting, or PRRT combined with new targeted therapies, such as sunitinib or everolimus. Randomized clinical trials comparing PRRT with other treatment modalities, or comparing various radioactive labeled somatostatin analogs should be undertaken to determine the best treatment options and treatment sequelae for patients with GEPNETs.

Peptide receptor radionuclide therapy (PRRT) for GEP-NETs

Peptide receptor radionuclide therapy (PRRT) with radiolabelled somatostatin analogues plays an increasing role in the treatment of patients with inoperable or metastasised gatroenteropancreatic neuroendocrine tumours (GEP-NETs). (90)Y-DOTATOC and (177)Lu-DOTATATE are the most used radiopeptides for PRRT with comparable tumour response rates (about 15-35%). The side effects of this therapy are few and mild. However, amino acids should be used for kidney protection, especially during infusion of (90)Y-DOTATOC. Options to improve PRRT may include combinations of radioactive labelled somatostatin analogues and the use of radiosensitising drugs combined with PRRT. Other therapeutic applications of PRRT may include intra-arterial administration, neo-adjuvant treatment and additional PRRT cycles in patients with progressive disease, who have benefited from initial therapy. Considering the mild side-effects, PRRT may well become the first-line therapy in patients with metastasised or inoperable GEP-NETs if more widespread use of PRRT can be accomplished.

Radioembolization and chemoembolization for unresectable neuroendocrine liver metastases - a systematic review

This review examines the clinical efficacy and safety of the use of hepatic arterial chemoembolization, bland embolization and radioembolization in the treatment of unresectable neuroendocrine tumor liver metastases (NETLM). Response to treatment, survival outcome and toxicity were examined in this review of 37 studies comprising 1575 patients. These therapies are safe and effective in the treatment of NETLM. Prospective clinical trials to compare the relative efficacy and toxicity are warranted.

¹¹¹In-DTPA⁰-octreotide (Octreoscan), ¹³¹I-MIBG and other agents for radionuclide therapy of NETs

This paper is a critical review of the literature on NET radionuclide therapy with (111)In-DTPA(0)-octreotide (Octreoscan) and (131)I-MIBG, focusing on efficacy and toxicity. Some potential future applications and new candidate therapeutic agents are also mentioned. Octreoscan has been a pioneering agent for somatostatin receptor radionuclide therapy. It has achieved symptomatic responses and disease stabilization, but it is now outperformed by the corresponding β-emitter agents (177)Lu-DOTATATE and (90)Y-DOTATOC. (131)I-MIBG is the radionuclide therapy of choice for inoperable or metastatic phaeochromocytomas/paragangliomas, which avidly concentrate this tracer via the noradrenaline transporter. Symptomatic, biochemical and tumour morphological response rates of 50-89%, 45-74% and 27-47%, respectively, have been reported. (131)I-MIBG is a second-line radiopharmaceutical for treatment of enterochromaffin carcinoids, mainly offering the benefit of amelioration of hormone-induced symptoms. High specific activity, non-carrier-added (131)I-MIBG and meta-astato((211)At)-benzylguanidine (MABG) are tracers with potential for enhanced therapeutic efficacy, yet their integration into clinical practice awaits further exploration. Amongst other promising agents, radiolabelled exendin analogues show potential for imaging and possibly therapy of insulinomas, while preclinical studies are currently evaluating DOTA peptides targeting the CCK-2/gastrin receptors that are overexpressed by medullary thyroid carcinoma cells.

Lutetium-labelled peptides for therapy of neuroendocrine tumours

Treatment with radiolabelled somatostatin analogues is a promising new tool in the management of patients with inoperable or metastasized neuroendocrine tumours. Symptomatic improvement may occur with ¹⁷⁷Lu-labelled somatostatin analogues that have been used for peptide receptor radionuclide therapy (PRRT). The results obtained with ¹⁷⁷Lu-[DOTA⁰,Tyr³]octreotate (DOTATATE) are very encouraging in terms of tumour regression. Dosimetry studies with ¹⁷⁷Lu-DOTATATE as well as the limited side effects with additional cycles of ¹⁷⁷Lu-DOTATATE suggest that more cycles of ¹⁷⁷Lu-DOTATATE can be safely given. Also, if kidney-protective agents are used, the side effects of this therapy are few and mild and less than those from the use of ⁹⁰Y-[DOTA⁰,Tyr³]octreotide (DOTATOC). Besides objective tumour responses, the median progression-free survival is more than 40 months. The patients' self-assessed quality of life increases significantly after treatment with ¹⁷⁷Lu-DOTATATE. Lastly, compared to historical controls, there is a benefit in overall survival of several years from the time of diagnosis in patients treated with ¹⁷⁷Lu-DOTATATE. These findings compare favourably with the limited number of alternative therapeutic approaches. If more widespread use of PRRT can be guaranteed, such therapy may well become the therapy of first choice in patients with metastasized or inoperable neuroendocrine tumours.

Neuroendocrine tumors: a focus on liver metastatic lesions

Transhepatic radionuclide infusion has been introduced as a new treatment approach for unresectable liver neuroendocrine metastatic lesions with the prerequisite of a positive In-111 Pentetreotide (Octreoscan). Patients with multiple liver neuroendocrine metastases can be locally treated after selective hepatic artery catheterization and infusion of radiolabeled somatostatin analogs, and in case of extra-hepatic secondary spread, after simple i.v. application. According to the world wide references, the average dose per session to each patient is 6.3 ± 0.3 GBq (∼160-180 mCi) of In-111-DTPA-Phe1-Pentetreotide, 10- to 12-fold in total, administered monthly or of 4.1 ± 0.2 GBq (∼105-116 mCi) of Y-90 DOTA TOC, threefold in total, or of 7.0 ± 0.4 GBq (∼178-200 mCi) of Lu-177 DOTA TATE, fourfold to sixfold in total (the choice of which being based on the tumor size, assessed by CT or MRI). Follow-up at monthly intervals has to be performed by means of ultrasonography (US). Treatment response has to be assessed according to the WHO criteria (RECIST or SWOG).

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.

Somatostatin receptor-targeted radionuclide therapy in patients with gastroenteropancreatic neuroendocrine tumors

Treatment with radiolabeled somatostatin analogs is a promising tool in the management of patients with inoperable or metastasized neuroendocrine tumors. Symptomatic improvement may occur with all (111)Indium-, (90)Yttrium-, or (177)Lutetium-labeled somatostatin analogs used for peptide receptor radionuclide therapy. If kidney protective agents are used, the side-effects are few and mild, and the median duration of the therapy response is 30 and 40 months, respectively. Overall survival is several years from diagnosis. These data compare favorably with the limited number of alternative treatments. If more widespread use of PRRT can be guaranteed, such therapy may become the therapy of first choice.

Multimodal management of neuroendocrine liver metastases

BACKGROUND

The incidence of neuroendocrine tumours (NET) has increased over the past three decades. Hepatic metastases which occur in up to 75% of NET patients significantly worsen their prognosis. New imaging techniques with increasing sensitivity enabling tumour detection at an early stage have been developed. The treatment encompasses a panel of surgical and non-surgical modalities.

METHODS

This article reviews the published literature related to management of hepatic neuroendocrine metastases.

RESULTS

Abdominal computer tomography, magnetic resonance tomography and somatostatin receptor scintigraphy are widely accepted imaging modalities. Hepatic resection is the only potentially curative treatment. Liver transplantation is justified in highly selected patients. Liver-directed interventional techniques and locally ablative measures offer effective palliation. Promising novel therapeutic options offering targeted approaches are under evaluation.

CONCLUSIONS

The treatment of neuroendocrine liver metastases still needs to be standardized. Management in centres of expertise should be strongly encouraged in order to enable a multidisciplinary approach and personalized treatment. Development of molecular prognostic factors to select treatment according to patient risk should be attempted.

Preclinical and clinical studies of peptide receptor radionuclide therapy

In the 1980s, the (111)In-labeled somatostatin analog OctreoScan (Covidien, Hazelwood, MO) was developed for imaging of somatostatin receptor subtype 2 (sst(2)) overexpressing tumors. On the basis of this success, peptide receptor radionuclide therapy (PRRT) was developed using similar somatostatin analogs with different therapeutic radionuclides. Clinical application of PRRT demonstrated impressive results on tumor response, overall survival, and quality of life in patients with gastroenteropancreatic neuroendocrine tumors. The peptides 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), Tyr(3)-octreotate (DOTATATE) and DOTA, Tyr(3)-octreotide (DOTATOC) (brand name Onalta), predominantly targeting sst(2), have been granted Orphan Drug status by the European Medicines Agency and the US Food and Drug Administration for application in PRRT. Besides somatostatin receptor-targeting peptides, multiple other radiopeptide analogs were developed targeting several other receptors overexpressed on various tumors. Some of these peptide analogs, including cholecystokinin, gastrin, gastrin-releasing peptide, arginine-glycine-aspartate (RGD)-peptides, and glucagon-like peptide 1 analogs appeared very promising in preclinical and clinical imaging and PRRT studies. Although the success of PRRT with radiolabeled somatostatin analogs has been established, there is still room for improvement. The therapeutic window of PRRT could be enlarged by the use of new and improved targeting compounds, of which new antagonists with excellent tumor to background ratios are very promising. Furthermore, locoregional administration, improved healthy tissue protection, and combination treatment can be applied to increase the effectiveness of PRRT. Combination treatment might include cocktails of different peptide analogs of different therapeutic radionuclides and of radiolabeled peptides with chemotherapeutic or radiosensitizing agents. This review summarizes results of PRRT and describes clinical and preclinical studies regarding PRRT optimizing strategies.

Transarterial liver-directed therapies of neuroendocrine hepatic metastases

Neuroendocrine tumors (NETs) comprise a diverse group of slowly growing tumors with an indolent course, characterized by the capacity to synthesize and secrete polypeptide products that are hormonally active. Presence of liver metastases results in significant debilitating hormonal symptoms, and is associated with poor prognosis. Systemic chemotherapy has limited success in the management of patients with NET hepatic metastases. Although somatostatin analogs are effective in controlling symptoms in many of these patients, the disease can become refractory to treatment. For these reasons, interventional radiologic techniques for liver-directed therapy have become an important treatment option in patients with metastatic NETs. Transcatheter arterial procedures such as transarterial embolization (TAE), transarterial chemoembolization (TACE), and selective internal radiation therapy (SIRT) have been shown to reduce hormone levels, palliate symptoms, and reduce the tumor burden in many patients with unresectable and symptomatic NET hepatic metastases. This article summarizes the most recent information on arterial-based liver-directed therapies in the treatment of metastatic NETs.