Targeting of cholecystokinin-B/gastrin receptors in vivo: preclinical and initial clinical evaluation of the diagnostic and therapeutic potential of radiolabelled gastrin

Radiolabeled peptides and their expanding role in clinical imaging and targeted cancer therapy

There is an expanding body of evidence showing that synthetic peptides in combination with radioactive isotopes can be utilized for medical purposes. This area is of particular interest in oncology where applications in diagnosis and therapy are at different stages of development. We review the contributions in this area by the group originally founded by Carlo Pedone in Naples many years ago. We highlight the work of this group in the context of other developments in this area, focusing on three biologically relevant receptor systems: somatostatin, gastrin-releasing peptide, and cholecystokinin-2/gastrin receptors. We focus on key milestones, state of the art, and challenges in this area of research as well as the current and future outlook for expanding clinical applications.

Peptide-Drug Conjugates: Design, Chemistry, and Drug Delivery System as a Novel Cancer Theranostic

The emergence of peptide-drug conjugates (PDCs) that utilize target-oriented peptide moieties as carriers of cytotoxic payloads, interconnected with various cleavable/noncleavable linkers, resulted in the key-foundation of the new era of targeted therapeutics. They are capable of retaining the integrity of conjugates in the blood circulatory system as well as releasing the drugs at the tumor microenvironment. Other valuable advantages are specificity and selectivity toward targeted-receptors, higher penetration ability, and drug-loading capacity, making them a suitable candidate to play their vital role as promising carrier agents. In this review, we summarized the types of cell-targeting (CTPs) and cell-penetrating peptides (CPPs) that have broad applications in the advancement of targeted drug-delivery systems (DDS). Moreover, the techniques to overcome the limitations of peptide-chemistry for their extensive implementation to construct the PDCs. Besides this, the diversified breakthrough of linker chemistry, and ample knowledge of various cytotoxic payloads used in PDCs in recent years, as well as the mechanism of action of PDCs was critically discussed. The principal aim is to provide scattered and diversified knowledge in one place and to help researchers understand the pinching knots in the science of PDC development, also their progression toward a bright future for PDCs as novel theranostics in clinical practice.

Development of Highly Potent Clinical Candidates for Theranostic Applications against Cholecystokinin-2 Receptor Positive Cancers

Peptide receptor radionuclide therapy (PRRT) is a promising form of systemic radiation therapy designed to eradicate cancer. Cholecystokinin-2 receptor (CCK2R) is an important molecular target that is highly expressed in a range of cancers. This study describes the synthesis and in vivo characterization of a novel series of 177Lu-labeled peptides ([177Lu]Lu-2b-4b) in comparison with the reference CCK2R-targeting peptide CP04 ([177Lu]Lu-1b). [177Lu]Lu-1b-4b showed high chemical purity (HPLC ≥ 94%), low Log D7.4 (-4.09 to -4.55) with strong binding affinity to CCK2R (KD 0.097-1.61 nM), and relatively high protein binding (55.6-80.2%) and internalization (40-67%). Biodistribution studies of the novel 177Lu-labeled peptides in tumors (AR42J and A431-CCK2R) showed uptake one- to eight-fold greater than the reference compound CP04 at 1, 24, and 48 h. Rapid clearance and high tumor uptake and retention were established for [177Lu]Lu-2b-4b, making these compounds excellent candidates for theranostic applications against CCK2R-expressing tumors.

Peptide Radioligands in Cancer Theranostics: Agonists and Antagonists

The clinical success of radiolabeled somatostatin analogs in the diagnosis and therapy-"theranostics"-of tumors expressing the somatostatin subtype 2 receptor (SST₂R) has paved the way for the development of a broader panel of peptide radioligands targeting different human tumors. This approach relies on the overexpression of other receptor-targets in different cancer types. In recent years, a shift in paradigm from internalizing agonists to antagonists has occurred. Thus, SST₂R-antagonist radioligands were first shown to accumulate more efficiently in tumor lesions and clear faster from the background in animal models and patients. The switch to receptor antagonists was soon adopted in the field of radiolabeled bombesin (BBN). Unlike the stable cyclic octapeptides used in the case of somatostatin, BBN-like peptides are linear, fast to biodegradable and elicit adverse effects in the body. Thus, the advent of BBN-like antagonists provided an elegant way to obtain effective and safe radiotheranostics. Likewise, the pursuit of gastrin and exendin antagonist-based radioligands is advancing with exciting new outcomes on the horizon. In the present review, we discuss these developments with a focus on clinical results, commenting on challenges and opportunities for personalized treatment of cancer patients by means of state-of-the-art antagonist-based radiopharmaceuticals.

Peptide-based diagnostic and therapeutic agents: Where we are and where we are heading?

Peptides are increasingly present in all branches of medicine as innovative drugs, imaging agents, theragnostic, and constituent moieties of other sophisticated drugs such as peptide-drug conjugates. Due to new developments in chemical synthesis strategies, computational biology, recombinant technology, and chemical biology, peptide drug development has made a great progress in the last decade. Numerous natural peptides and peptide mimics have been obtained and studied, covering multiple therapeutic areas. Even though peptides have been investigated across the wide therapeutic spectrum, oncology, metabolism, and endocrinology are the most frequent medical indications of them. This review summarizes the current use of and the emerging new opportunities of peptides for diagnosis and treatment of various diseases.

Effects of Side Chain and Peptide Bond Modifications on the Targeting Properties of Stabilized Minigastrin Analogs

Different attempts have been made in the past two decades to develop radiolabeled peptide conjugates with enhanced pharmacokinetic properties in order to improve the application for tumor imaging and peptide receptor radionuclide therapy (PRRT), which targets the cholecystokinin-2 receptor (CCK2R). In this paper, the influence of different side chain and peptide bond modifications has been explored for the minigastrin analog DOTA-DGlu-Ala-Tyr-Gly-Trp-(N-Me)Nle-Asp-1Nal-NH2 (DOTA-MGS5). Based on this lead structure, five new derivatives were synthesized for radiolabeling with trivalent radiometals. Different chemical and biological properties of the new derivatives were analyzed. Receptor interaction of the peptide derivatives and cell internalization of the radiolabeled peptides were studied in A431-CCK2R cells. The stability of the radiolabeled peptides in vivo was investigated using BALB/c mice. Tumor targeting of all 111In-labeled peptide conjugates, and of a selected compound radiolabeled with gallium-68 and lutetium-177, was evaluated in BALB/c nude mice xenografted with A431-CCK2R and A431-mock cells. All 111In-labeled conjugates, except [111In]In-DOTA-[Phe8]MGS5, showed a high resistance against enzymatic degradation. A high receptor affinity with IC50 values in the low nanomolar range was confirmed for most of the peptide derivatives. The specific cell internalization over time was 35.3-47.3% for all radiopeptides 4 h after incubation. Only [111In]In-DOTA-MGS5[NHCH3] exhibited a lower cell internalization of 6.6 ± 2.8%. An overall improved resistance against enzymatic degradation was confirmed in vivo. Of the radiopeptides studied, [111In]In-DOTA-[(N-Me)1Nal8]MGS5 showed the most promising targeting properties, with significantly increased accumulation of radioactivity in A431-CCK2R xenografts (48.1 ± 9.2% IA/g) and reduced accumulation of radioactivity in stomach (4.2 ± 0.5% IA/g). However, in comparison with DOTA-MGS5, a higher influence on the targeting properties was observed for the change of radiometal, resulting in a tumor uptake of 15.67 ± 2.21% IA/g for [68Ga]Ga-DOTA-[(N-Me)1Nal8]MGS5 and 35.13 ± 6.32% IA/g for [177Lu]Lu-DOTA-[(N-Me)1Nal8]MGS5.

Automated Synthesis of 68Ga-Labeled DOTA-MGS8 and Preclinical Characterization of Cholecystokinin-2 Receptor Targeting

The new minigastrin analog DOTA-MGS8 targeting the cholecystokinin-2 receptor (CCK2R) used in this study displays the combination of two site-specific modifications within the C-terminal receptor binding sequence together with an additional N-terminal amino acid substitution preventing fast metabolic degradation. Within this study, the preparation of ⁶⁸Ga-labeled DOTA-MGS8 was validated using an automated synthesis module, describing the specifications and analytical methods for quality control for possible clinical use. In addition, preclinical studies were carried out to characterize the targeting potential. [⁶⁸Ga]Ga-DOTA-MGS8 showed a high receptor-specific cell internalization into AR42J rat pancreatic cells (~40%) with physiological expression of rat CCK2R as well as A431-CCK2R cells transfected to stably express human CCK2R (~47%). A favorable biodistribution profile was observed in BALB/c nude mice xenografted with A431-CCK2R cells and mock-transfected A431 cells as control. The high tumor uptake of ~27% IA/g together with low background activity and limited uptake in non-target tissue confirms the potential for high-sensitivity positron emission tomography of stabilized MG analogs in patients with MTC and other CCK2R-related malignancies.

Update on Preclinical Development and Clinical Translation of Cholecystokinin-2 Receptor Targeting Radiopharmaceuticals

Simple Summary

Peptide analogs, derived from the natural peptide hormone gastrin, are promising candidates for improving the visualization and treatment of tumors. Gastrin specifically binds to the cholecystokinin-2 receptor, a G-protein-coupled receptor expressed on the cell surface of different tumors. This enables specific targeting of tumor cells using gastrin analogs, labeled with radioisotopes. The receptor is expressed at high incidence in medullary thyroid carcinoma, a rare form of thyroid cancer lacking effective treatments at an advanced stage. Different radiolabeled gastrin analogs as well as nonpeptidic compounds targeting CCK2R have been developed. Specific modifications have been introduced in order to safely deliver the radiation to the tumor site. In this review, recent strategies applied to improve the targeting properties are described. These developments enabled the introduction of new radiolabeled peptide analogs for imaging and therapy in cancer patients. In addition to highlighting the current clinical trials, the perspectives for future applications are given.

Abstract

The cholecystokinin-2 receptor (CCK2R) has been a target of interest for molecular imaging and targeted radionuclide therapy for two decades. However, so far CCK2R targeted imaging and therapy has not been introduced in clinical practice. Within this review the recent radiopharmaceutical development of CCK2R targeting compounds and the ongoing clinical trials are presented. Currently, new gastrin derivatives as well as nonpeptidic substances are being developed to improve the properties for clinical use. A team of specialists from the field of radiopharmacy and nuclear medicine reviewed the available literature and summarized their own experiences in the development and clinical testing of CCK2R targeting radiopharmaceuticals. The recent clinical trials with novel radiolabeled minigastrin analogs demonstrate the potential for both applications, imaging as well as targeted radiotherapy, and reinforce the clinical applicability within a theranostic concept. The intense efforts in optimizing CCK2R targeting radiopharmaceuticals has led to new substances for clinical use, as shown in first imaging studies in patients with advanced medullary thyroid cancer. The first clinical results suggest that the wider clinical implication of CCK2R-targeted radiopharmaceuticals is reasonable.

Cholecystokinin-2 Receptor Targeting with Radiolabeled Peptides: Current Status and Future Directions

A wide variety of radiolabeled peptide analogs for specific targeting of cholecystokinin- 2 receptors (CCK2R) has been developed in the last decades. Peptide probes based on the natural ligands Minigastrin (MG) and Cholecystokinin (CCK) have a high potential for molecular imaging and targeted radiotherapy of different human tumors, such as Medullary Thyroid Carcinoma (MTC) and Small Cell Lung Cancer (SCLC). MG analogs with high persistent uptake in CCK2R expressing tumors have been preferably used for the development of radiolabeled peptide analogs. The clinical translation of CCK2R targeting has been prevented due to high kidney uptake or low metabolic stability of the different radiopeptides developed. Great efforts in radiopharmaceutical development have been undertaken to overcome these limitations. Various modifications in the linear peptide sequence of MG have been introduced mainly with the aim to reduce kidney retention. Furthermore, improved tumor uptake could be obtained by in situ stabilization of the radiopeptide against enzymatic degradation through coinjection of peptidase inhibitors. Recent developments focusing on the stabilization of the Cterminal receptor binding sequence (Trp-Met-Asp-Phe-NH2) have led to new radiolabeled MG analogs with highly improved tumor uptake and tumor-to-kidney ratio. In this review, all the different aspects in the radiopharmaceutical development of CCK2R targeting peptide probes are covered, giving also an overview on the clinical investigations performed so far. The recent development of radiolabeled MG analogs, which are highly stabilized against enzymatic degradation in vivo, promises to have a high impact on the clinical management of patients with CCK2R expressing tumors in the near future.

The emerging role of cell surface receptor and protein binding radiopharmaceuticals in cancer diagnostics and therapy

Targeting specific cell membrane markers for both diagnostic imaging and radionuclide therapy is a rapidly evolving field in cancer research. Some of these applications have now found a role in routine clinical practice and have been shown to have a significant impact on patient management. Several molecular targets are being investigated in ongoing clinical trials and show promise for future implementation. Advancements in molecular biology have facilitated the identification of new cancer-specific targets for radiopharmaceutical development.

Stabilization Strategies for Linear Minigastrin Analogues: Further Improvements via the Inclusion of Proline into the Peptide Sequence

Minigastrin (MG) analogues, known for their high potential to target cholecystokinin-2 receptor (CCK2R) expressing tumors, have limited clinical applicability due to low enzymatic stability. By introducing site-specific substitutions within the C-terminal receptor-binding sequence, reduced metabolization and improved tumor targeting can be achieved. In this work, the influence of additional modification within the N-terminal sequence has been explored. Three novel 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-conjugated CCK2R ligands with proline substitution at different positions were synthesized. Substitution did not affect CCK2R affinity, and the conjugates labeled with indium-111 and lutetium-177 showed a high enzymatic stability in different incubation media as well as in vivo (57-79% intact radiopeptide in blood of BALB/c mice at 1 h p.i.) combined with enhanced tumor uptake (29-46% IA/g at 4 h in xenografted BALB/c nude mice). The inclusion of Pro contributes significantly to the development of CCK2R ligands with optimal targeting properties for application in targeted radiotherapy.

Evaluation of Actinium-225 Labeled Minigastrin Analogue [225Ac]Ac-DOTA-PP-F11N for Targeted Alpha Particle Therapy

The overexpression of cholecystokinin B receptor (CCKBR) in human cancers led to the development of radiolabeled minigastrin analogues for targeted radionuclide therapy, which aims to deliver cytotoxic radiation specifically to cancer cells. Alpha emitters (e.g., actinium-225) possess high potency in cancer cell-killing and hold promise for the treatment of malignant tumors. In these preclinical studies, we developed and evaluated CCKBR-targeted alpha particle therapy. The cellular uptake and cytotoxic effect of actinium-225 labeled and HPLC-purified minigastrin analogue [²²⁵Ac]Ac-PP-F11N were characterized in the human squamous cancer A431 cells transfected with CCKBR. Nude mice bearing A431/CCKBR tumors were used for biodistribution and therapy studies followed by histological analysis and SPECT/CT imaging. In vitro, [²²⁵Ac]Ac-PP-F11N showed CCKBR-specific and efficient internalization rate and potent cytotoxicity. The biodistribution studies of [²²⁵Ac]Ac-PP-F11N revealed CCKBR-specific uptake in tumors, whereas the therapeutic studies demonstrated dose-dependent inhibition of tumor growth and extended mean survival time, without apparent toxicity. The histological analysis of kidney and stomach indicated no severe adverse effects after [²²⁵Ac]Ac-PP-F11N administration. The post-therapy SPECT-CT images with [¹¹¹In]In-PP-F11N confirmed no CCKBR-positive tumor left in the mice with complete remission. In conclusion, our study demonstrates therapeutic efficacy of [²²⁵Ac]Ac-PP-F11N without acute radiotoxicity in CCKBR-positive cancer model.

Initial In Vitro and In Vivo Evaluation of a Novel CCK2R Targeting Peptide Analog Labeled with Lutetium-177

Targeting of cholecystokinin-2 receptor (CCK2R) expressing tumors using radiolabeled minigastrin (MG) analogs is hampered by rapid digestion of the linear peptide in vivo. In this study, a new MG analog stabilized against enzymatic degradation was investigated in preclinical studies to characterize the metabolites formed in vivo. The new MG analog DOTA-DGlu-Pro-Tyr-Gly-Trp-(N-Me)Nle-Asp-1Nal-NH₂ comprising site-specific amino acid substitutions in position 2, 6 and 8 and different possible metabolites thereof were synthesized. The receptor interaction of the peptide and selected metabolites was evaluated in a CCK2R-expressing cell line. The enzymatic stability of the ¹⁷⁷Lu-labeled peptide analog was evaluated in vitro in different media as well as in BALB/c mice up to 1 h after injection and the metabolites were identified based on radio-HPLC analysis. The new radiopeptide showed a highly increased stability in vivo with >56% intact radiopeptide in the blood of BALB/c mice 1 h after injection. High CCK2R affinity and cell uptake was confirmed only for the intact peptide, whereas enzymatic cleavage within the receptor specific C-terminal amino acid sequence resulted in complete loss of affinity and cell uptake. A favorable biodistribution profile was observed in BALB/c mice with low background activity, preferential renal excretion and prolonged uptake in CCK2R-expressing tissues. The novel stabilized MG analog shows high potential for diagnostic and therapeutic use. The radiometabolites characterized give new insights into the enzymatic degradation in vivo.

Triazolo-Peptidomimetics: Novel Radiolabeled Minigastrin Analogs for Improved Tumor Targeting

MG11 is a truncated analog of minigastrin, a peptide with high affinity and specificity toward the cholecystokinin-2 receptor (CCK2R), which is overexpressed by different tumors. Thus, radiolabeled MG11 derivatives have great potential for use in cancer diagnosis and therapy. A drawback of MG11 is its fast degradation by proteases, leading to moderate tumor uptake in vivo. We introduced 1,4-disubstituted 1,2,3-triazoles as metabolically stable bioisosteres to replace labile amide bonds of the peptide. The "triazole scan" yielded peptidomimetics with improved resistance to enzymatic degradation and/or enhanced affinity toward the CCK2R. Remarkably, our lead compound achieved a 10-fold increase in receptor affinity, resulting in a 2.6-fold improved tumor uptake in vivo. Modeling of the ligand-CCK2R complex suggests that an additional cation-π interaction of the aromatic triazole moiety with the Arg³⁵⁶ residue of the receptor is accountable for these observations. We show for the first time that the amide-to-triazole substitution strategy offers new opportunities in drug development that go beyond the metabolic stabilization of bioactive peptides.

Gastrin, Cholecystokinin, Signaling, and Biological Activities in Cellular Processes

The structurally-related peptides, gastrin and cholecystokinin (CCK), were originally discovered as humoral stimulants of gastric acid secretion and pancreatic enzyme release, respectively. With the aid of methodological advances in biochemistry, immunochemistry, and molecular biology in the past several decades, our concept of gastrin and CCK as simple gastrointestinal hormones has changed considerably. Extensive in vitro and in vivo studies have shown that gastrin and CCK play important roles in several cellular processes including maintenance of gastric mucosa and pancreatic islet integrity, neurogenesis, and neoplastic transformation. Indeed, gastrin and CCK, as well as their receptors, are expressed in a variety of tumor cell lines, animal models, and human samples, and might contribute to certain carcinogenesis. In this review, we will briefly introduce the gastrin and CCK system and highlight the effects of gastrin and CCK in the regulation of cell proliferation and apoptosis in both normal and abnormal conditions. The potential imaging and therapeutic use of these peptides and their derivatives are also summarized.

Molecular imaging probes derived from natural peptides

Covering: up to the end of 2015.Peptides are naturally occurring compounds that play an important role in all living systems and are responsible for a range of essential functions. Peptide receptors have been implicated in disease states such as oncology, metabolic disorders and cardiovascular disease. Therefore, natural peptides have been exploited as diagnostic and therapeutic agents due to the unique target specificity for their endogenous receptors. This review discusses a variety of natural peptides highlighting their discovery, endogenous receptors, as well as their derivatization to create molecular imaging agents, with an emphasis on the design of radiolabelled peptides. This review also highlights methods for discovering new and novel peptides when knowledge of specific targets and endogenous ligands are not available.

Current Status of Radiopharmaceuticals for the Theranostics of Neuroendocrine Neoplasms

Nuclear medicine plays a pivotal role in the management of patients affected by neuroendocrine neoplasms (NENs). Radiolabeled somatostatin receptor analogs are by far the most advanced radiopharmaceuticals for diagnosis and therapy (radiotheranostics) of NENs. Their clinical success emerged receptor-targeted radiolabeled peptides as an important class of radiopharmaceuticals and it paved the way for the investigation of other radioligand-receptor systems. Besides the somatostatin receptors (sstr), other receptors have also been linked to NENs and quite a number of potential radiolabeled peptides have been derived from them. The Glucagon-Like Peptide-1 Receptor (GLP-1R) is highly expressed in benign insulinomas, the Cholecystokinin 2 (CCK2)/Gastrin receptor is expressed in different NENs, in particular medullary thyroid cancer, and the Glucose-dependent Insulinotropic Polypeptide (GIP) receptor was found to be expressed in gastrointestinal and bronchial NENs, where interestingly, it is present in most of the sstr-negative and GLP-1R-negative NENs. Also in the field of sstr targeting new discoveries brought into light an alternative approach with the use of radiolabeled somatostatin receptor antagonists, instead of the clinically used agonists. The purpose of this review is to present the current status and the most innovative strategies for the diagnosis and treatment (theranostics) of neuroendocrine neoplasms using a cadre of radiolabeled regulatory peptides targeting their receptors.

Improving the In Vivo Profile of Minigastrin Radiotracers: A Comparative Study Involving the Neutral Endopeptidase Inhibitor Phosphoramidon

Minigastrin radiotracers, such as [(111)In-DOTA]MG0 ([(111)In-DOTA-DGlu(1)]minigastrin), have been considered for diagnostic imaging and radionuclide therapy of CCK2R-positive human tumors, such as medullary thyroid carcinoma. However, the high kidney retention assigned to the pentaGlu(2-6) repeat in the peptide sequence has compromised their clinical applicability. On the other hand, truncated des(Glu)(2-6)-analogs, such as [(111)In-DOTA]MG11 ([(111)In-DOTA-DGlu(10),desGlu(2-6)]minigastrin), despite their low renal uptake, show poor bioavailability and tumor targeting. [(111)In]CP04 ([(111)In-DOTA-DGlu(1-6)]minigastrin) acquired by Glu(2-6)/DGlu(2-6) substitution showed promising tumor-to-kidney ratios in rodents. In the present study, we compare the biological profiles of [(111)In]CP04, [(111)In-DOTA]MG11, and [(111)In-DOTA]MG0 during in situ neutral endopeptidase (NEP) inhibition, recently shown to improve the bioavailability of several peptide radiotracers. After coinjection of the NEP inhibitor, phosphoramidon (PA), the stability of [(111)In]CP04 and [(111)In-DOTA]MG0 in peripheral mouse blood increased, with an exceptional >14-fold improvement monitored for [(111)In-DOTA]MG11. In line with these findings, PA treatment increased the uptake of [(111)In]CP04 (8.5 ± 0.4%ID/g to 16.0 ± 2.3%ID/g) and [(111)In-DOTA]MG0 (11.9 ± 2.2%ID/g to 17.2 ± 0.9%ID/g) in A431-CCK2R(+) tumors at 4 hours postinjection, whereas the respective increase for [(111)In-DOTA]MG11 was >6-fold (2.5 ± 0.9%ID/g to 15.1 ± 1.7%ID/g). Interestingly, kidney uptake remained lowest for [(111)In-DOTA]MG11, but unfavorably increased by PA treatment for [(111)In-DOTA]MG0. Thus, overall, the most favorable in vivo profile was displayed by [(111)In-DOTA]MG11 during NEP inhibition, highlighting the need to validate this promising concept in the clinic.

Assessment of cholecystokinin 2 receptor (CCK2R) in neoplastic tissue

The expression of cholecystokinin 2 receptor (CCK2R, CCKBR or gastrin receptor) has been reported on a diverse range of cancers such as colorectal, liver, lung, pancreatic, ovarian, stomach, thyroid and numerous neuroendocrine/carcinoid tumors. Some cancers of the colorectum, lung, pancreas and thyroid have been shown to overexpress CCK2R in relation to normal matched tissues of the same organ. This reported overexpression has led to the development of a number of CCK2R-ligand targeted imaging and therapeutic agents. However, no comprehensive study comparing the expression of CCK2R in multiple cancers to multiple normal tissues has been performed. Herein, we report the immunohistochemical analysis of cancer samples from gastrointestinal stromal tumor (GIST), hepatocellular carcinoma (HCC), non-small cell lung cancer (NSCLC), pancreatic adenocarcinoma, and thyroid cancer against multiple normal tissue samples from esophagus, liver, lung, pancreas, stomach, spleen and thyroid. These results show that CCK2R expression is present in nearly all cancer and normal samples tested and that none of the cancer samples had expression that was statistically greater than that of all of the normal samples.

(99m)Tc-labeled gastrins of varying peptide chain length: Distinct impact of NEP/ACE-inhibition on stability and tumor uptake in mice

INTRODUCTION

In situ inhibition of neutral endopeptidase (NEP) has been recently shown to impressively increase the bioavailability and tumor uptake of biodegradable gastrin radioligands. Furthermore, angiotensin converting enzyme (ACE) has been previously shown to cleave gastrin analogs in vitro. In the present study, we have assessed the effects induced by single or dual NEP/ACE-inhibition on the pharmacokinetic profile of three (99m)Tc-labeled gastrins of varying peptide chain length: [(99m)Tc]SG6 ([(99m)Tc-N4-Gln(1)]gastrin(1-17)), [(99m)Tc]DG2 ([(99m)Tc-N4-Gly(4),DGlu(5)]gastrin(4-17)) and [(99m)Tc]DG4 ([(99m)Tc-N4-DGlu(10)]gastrin(10-17)).

METHODS

Mouse blood samples were collected 5min after injection of each of [(99m)Tc]SG6/DG2/DG4 together with: a) vehicle, b) the NEP-inhibitor phosphoramidon (PA), c) the ACE-inhibitor lisinopril (Lis), or d) PA plus Lis and were analyzed by RP-HPLC for radiometabolite detection. Biodistribution was studied in SCID mice bearing A431-CCK2R(+/-) xenografts at 4h postinjection (pi). [(99m)Tc]SG6 or [(99m)Tc]DG4 was coinjected with either vehicle or the above described NEP/ACE-inhibitor regimens; for [(99m)Tc]DG2 control and PA animal groups were only included.

RESULTS

Treatment of mice with PA induced significant stabilization of (99m)Tc-radiotracers in peripheral blood, while treatment with Lis or Lis+PA affected the stability of des(Glu)5 [(99m)Tc]DG4 only. In line with these findings, PA coinjection led to notable amplification of tumor uptake of radiopeptides compared to controls (P<0.01). Only [(99m)Tc]DG4 profited by single Lis (2.06±0.39%ID/g vs 0.99±0.13%ID/g in controls) or combined Lis+PA coinjection (8.91±1.61%ID/g vs 4.89±1.33%ID/g in PA-group). Furthermore, kidney uptake remained favourably low and unaffected by PA and/or Lis coinjection only in the case of [(99m)Tc]DG4 (<1.9%ID/g) resulting in the most optimal tumor-to-kidney ratios.

CONCLUSIONS

In situ NEP/ACE-inhibition diversely affected the in vivo profile of (99m)Tc-radioligands based on different-length gastrins. Truncated [(99m)Tc]DG4 exhibited overall the most attractive profile during combined NEP/ACE-inhibition in mouse models, providing new opportunities for CCK2R-expressing tumor imaging in man with SPECT.

In vivo inhibition of neutral endopeptidase enhances the diagnostic potential of truncated gastrin (111)In-radioligands

INTRODUCTION

Radiolabeled gastrin analogs represent attractive candidates for diagnosis and therapy of cholecystokinin subtype-2 receptor (CCK2R)-expressing tumors. Radiolabeled des(Glu)5-gastrins show favorably low renal accumulation, but localize poorly in CCK2R-positive lesions. We introduce herein three truncated [DOTA-DGlu(10)]gastrin(10-17) analogs, with oxidation-susceptible Met(15) replaced by: (1), (2), or (3), and study the profile of [(111)In]1/2/3 during in vivo inhibition of neutral endopeptidase (NEP) in comparison to the non-truncated [ ([(111)In]4) reference.

METHODS

Blood samples collected from mice 5 min postinjection (pi) of [(111)In]1/2/3/4 without or with phosphoramidon (PA) coinjection were analyzed by RP-HPLC. Biodistribution was conducted in SCID mice bearing A431-CCK2R(+) or AR42J xenografts 4h after administration of [(111)In]1/2/3/4 without or with PA coinjection.

RESULTS

Firstly, we observed remarkable increases in the amount of radiopeptides detected intact in the blood of PA-treated mice at 5 min pi compared to controls. Secondly, we noted impressive enhancement of [(111)In]1/2/3 localization in AR42J and A431-CCK2R(+) tumors in mice after PA coinjection. Specifically, the uptake of [(111)In]1 at 4h pi increased from 2.6 ± 0.3%ID/g to 13.3 ± 3.5%ID/g in the AR42J tumors and from 4.3 ± 0.6%ID/g to 20.4 ± 3.6%ID/g in the A431-CCK2R(+) xenografts, with comparable improvements noted for [(111)In]2 and [(111)In]3 as well. Thirdly, renal uptake remained favorably low and unaffected by PA (<2.5%ID/g). Conversely, although the stability and tumor targeting of [(111)In]4 improved, its high renal uptake (>85%ID/g) increased even further by PA (>140%ID/g).

CONCLUSIONS

In situ inhibition of NEP represents a promising new tool to enhance the diagnostic efficacy of biodegradable gastrin radioligands in the visualization of CCK2R-positive lesions in man.

EWS/FLI1 Target Genes and Therapeutic Opportunities in Ewing Sarcoma

Ewing sarcoma is an aggressive bone malignancy that affect children and young adults. Ewing sarcoma is the second most common primary bone malignancy in pediatric patients. Although significant progress has been made in the treatment of Ewing sarcoma since it was first described in the 1920s, in the last decade survival rates have remained unacceptably invariable, thus pointing to the need for new approaches centered in the molecular basis of the disease. Ewing sarcoma driving mutation, EWS–FLI1, which results from a chromosomal translocation, encodes an aberrant transcription factor. Since its first characterization in 1990s, many molecular targets have been described to be regulated by this chimeric transcription factor. Their contribution to orchestrate Ewing sarcoma phenotype has been reported over the last decades. In this work, we will focus on the description of a selection of EWS/FLI1 targets, their functional role, and their potential clinical relevance. We will also discuss their role in other types of cancer as well as the need for further studies to be performed in order to achieve a broader understanding of their particular contribution to Ewing sarcoma development.

Evaluation of a nonpeptidic ligand for imaging of cholecystokinin 2 receptor-expressing cancers

Tumor-specific targeting ligands were recently exploited to deliver both imaging and therapeutic agents selectively to cancer tissues in vivo. Because the cholecystokinin 2 receptor (CCK2R) is overexpressed in various human cancers (e.g., lung, medullary thyroid, pancreatic, colon, and gastrointestinal stromal tumors) but displays limited expression in normal tissues, natural ligands of CCK2R were recently explored for use in the imaging of CCK2R-expressing cancers. Unfortunately, the results from these studies revealed not only that the peptidic CCK2R ligands were unstable in vivo but also that the ligands that mediated good uptake by tumor tissues also promoted a high level of retention of the radioimaging agent in the kidneys, probably because of capture of the conjugates by peptide-scavenging receptors. In an effort to reduce the normal organ retention of CCK2R-targeted drugs, we synthesized a nonpeptidic ligand of CCK2R and examined its specificity for CCK2R both in vitro and in vivo.

METHODS

Nonpeptidic agonists and antagonists of CCK2R described in the literature were evaluated for their affinities and specificities for CCK2R. Z-360, a benzodiazepine-derived CCK2R antagonist with subnanomolar affinity, was selected for complexation to (99m)Tc via multiple spacers. After synthesis and purification, 4 complexes with different physicochemical properties were evaluated for binding to CCK2R-transfected HEK 293 cells. The best conjugate, termed CRL-3-(99m)Tc, was injected into mice bearing CCK2R tumor xenografts and examined by γ scintigraphy and SPECT/CT. The uptake of the conjugate in various organs was also quantified by tissue resection and γ counting.

RESULTS

CRL-3-(99m)Tc was shown to bind with low nanomolar affinity to CCK2R in vitro and was localized to tumor tissues in athymic nu/nu mice implanted with CCK2R-expressing tumors. At 4 h after injection, tumor uptake was measured at 12.0 ± 2.0 percentage injected dose per gram of tissue.

CONCLUSION

Because the uptake of CRL-3-(99m)Tc by nonmalignant tissues was negligible and retention in the kidneys was only transient, we suggest that CRL-3-(99m)Tc may be a useful radioimaging agent for the detection, sizing, and monitoring of CCK2R-expressing tumors.

Influence of d-glutamine and d-glutamic acid sequences in optical peptide probes targeted against the cholecystokinin-2/gastrin-receptor on binding affinity, specificity and pharmacokinetic properties

Background

Image-based diagnosis of tumours can be advanced and improved by targeted strategies addressing malignant molecular structures. A promising molecular target is the cholecystokinin-2-receptor (CCK2R) which can be targeted by high-affinity peptides called minigastrins. Here we present how the imaging properties of minigastrins tagged with near-infrared fluorescence (NIRF) dyes can be modulated by the introduction of different spacer sequences. We identify interactions of different probe variants with regard to target affinity, specificity and pharmacokinetic properties to optimize early detection of CCK2R-expressing tumours under clinical conditions.

Methods

Two minigastrin probes with the same near-infrared hemicyanine fluorescence dye (DY-754) for signalling and the same CCK2R-binding peptide A-Y-G-W-M/Nle-N-F-amide but different spacers were designed as follows: ‘QE’ with three alternating d-glutamines and d-glutamic acids and ‘bivQ’ with two minigastrins, each preceded by three d-glutamines. They were tested for affinity and specificity in vitro on CCK2R-expressing and CCK2R-non-expressing cells. In vivo imaging was conducted with subcutaneous tumour-bearing nude mice after i.v. probe injection (54 to 108 nmol/kg) and under competitive conditions with non-fluorescent minigastrin (n = 5/group). We also assessed probe biodistribution as well as NIRF distribution in tumour sections.

Results

Both probes showed high affinity and specificity to CCK2R-expressing cells in vitro. In vivo tumour-to-background contrasts (tumour/background ratios (TBRs) of around 6) enabled identification of CCK2R-expressing tumours by both probes with low accumulation in CCK2R-negative tumours (TBR of around 2). Specificity of the in vivo accumulation, revealed by competition, was higher for QE. Besides renal retention, probe uptake into organs was very low.

Conclusion

The properties of optical minigastrin probes can be specifically modified by the introduction of spacer sequences. A spacer of six hydrophilic amino acids increases affinity. A mix of d-glutamic and d-glutamine acids increased target-to-background contrast. Multimerization could not increase affinity but supposedly lowered stability. The probe QE is a promising candidate for clinical evaluation in terms of diagnosis of CCK2R-expressing tumours.

Distribution, elimination, and renal handling of (99m)technetium-Demogastrin 1

Radiolabeled cholecystokinin/gastrin (CCK) receptor-targeting peptides are promising compounds for radiodiagnosis and radiotherapy of certain malignancies. This study evaluated the pharmacokinetic profile of a CCK-2 receptor-specific peptide, Demogastrin 1, labeled with technetium-99m ((99m)Tc-Demogastrin 1), in rats. To investigate the fate of (99m)Tc-Demogastrin 1 in the rat, biodistribution and elimination studies in vivo were performed, and elimination parameters in perfused rat liver and kidney were determined. Biodistribution studies showed that (99m)Tc-Demogastrin 1 was rapidly cleared from the blood and most organs. A significant amount of radioactivity was detected in the CCK-2 receptor-rich organs, such as the stomach. Low radioactivity was found in the CCK-1 receptor-rich organs. Radioactivity in bowels and stomach declined relatively slowly. High and long-term retention of radioactivity in the kidneys was observed. Elimination of (99m)Tc-Demogastrin 1 via the bile was negligible. A high and rapid renal excretion was observed in elimination experiments in vivo. In the perfused kidney, glomerular filtration was found to be the main renal excretion mechanism of (99m)Tc-Demogastrin 1. Demogastrin 1 was distributed preferentially to the organs expressing CCK-2 receptors. The decisive elimination route of (99m)Tc-Demogastrin 1 in rats was urinary excretion. A high and prolonged renal retention may limit potential clinical use of the compound.

Radiolabelled peptides for oncological diagnosis

Radiolabelled receptor-binding peptides targeting receptors (over)expressed on tumour cells are widely under investigation for tumour diagnosis and therapy. The concept of using radiolabelled receptor-binding peptides to target receptor-expressing tissues in vivo has stimulated a large body of research in nuclear medicine. The ¹¹¹In-labelled somatostatin analogue octreotide (OctreoScan™) is the most successful radiopeptide for tumour imaging, and was the first to be approved for diagnostic use. Based on the success of these studies, other receptor-targeting peptides such as cholecystokinin/gastrin analogues, glucagon-like peptide-1, bombesin (BN), chemokine receptor CXCR4 targeting peptides, and RGD peptides are currently under development or undergoing clinical trials. In this review, we discuss some of these peptides and their analogues, with regard to their potential for radionuclide imaging of tumours.

Peptide-modified liposomes for selective targeting of bombesin receptors overexpressed by cancer cells: a potential theranostic agent

Objectives

Drug delivery systems consisting of liposomes displaying a cell surface receptor-targeting peptide are being developed to specifically deliver chemotherapeutic drugs to tumors overexpressing a target receptor. This study addresses novel liposome composition approaches to specifically target tissues overexpressing bombesin (BN) receptors.

Methods

A new amphiphilic peptide derivative (MonY-BN) containing the BN(7–14) peptide, the DTPA (diethylenetriaminepentaacetate) chelating agent, a hydrophobic moiety with two C₁₈ alkyl chains, and polyethylene glycol spacers, has been synthesized by solid-phase methods. Liposomes have been generated by co-aggregation of MonY-BN with 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC). The structural and biological properties of these new target-selective drug-delivery systems have been characterized.

Results

Liposomes with a DSPC/MonY-BN (97/3 molar ratio) composition showed a diameter of 145.5 ± 31.5 nm and a polydispersity index of 0.20 ± 0.05. High doxorubicin (Dox) loading was obtained with the remote pH gradient method using citrate as the inner buffer. Specific binding to PC-3 cells of DSPC/MonY-BN liposomes was obtained (2.7% ± 0.3%, at 37°C), compared with peptide-free DSPC liposomes (1.4% ± 0.2% at 37°C). Incubation of cells with DSPC/ MonY-BN/Dox showed significantly lower cell survival compared with DSPC/Dox-treated cells, in the presence of 100 ng/mL and 300 ng/mL drug amounts, in cytotoxicity experiments. Intravenous treatment of PC-3 xenograft-bearing mice with DSPC/MonY-BN/Dox at 10 mg/kg Dox dose produced higher tumour growth inhibition (60%) compared with nonspecific DSPC/ Dox liposomes (36%) relative to control animals.

Conclusion

The structural and loading properties of DSPC/MonY-BN liposomes along with the observed in-vitro and in-vivo activity are encouraging for further development of this approach for target-specific cancer chemotherapy.

PET/CT in Neuroendocrine Tumors: Evaluation of Receptor Status and Metabolism

In-111 Octreoscan is considered the gold standard for imaging of neuroendocrine tumors (NET). However, in the absence of morphologic imaging correlation, the exact localisation of the tumor is often difficult. Also the sensitivity of PET imaging is more than Gamma camera (SPECT) imaging. Ga-68 labelled somatostatin analogs (SMS-R) are interesting radiopharmaceuticals for PET receptor imaging of NET. Some other radiopharmaceuticals e.g. F-18 DOPA can also be used to assess metabolism and functional status of NET. The importance of these radiopharmaceuticals, especially SMS-R increases in the absence of any specific biochemical marker or clinical parameter for follow-up of patients after therapy (eg peptide receptor radionuclide therapy, surgery, chemoembolisation, etc). New criteria based on molecular, metabolic and morphologic imaging needs to be developed for correct assessment of response to therapy for these slow-growing, solid tumors.

Influence of biological assay conditions on stability assessment of radiometal-labelled peptides exemplified using a 177Lu-DOTA-minigastrin derivative

INTRODUCTION

Lack of correlation between in vitro and in vivo stability is a general problem for the development of radiopeptides especially in the case of minigastrin derivatives for therapeutic applications. In this study, we compared the influence of experimental conditions on radiopeptide stability results in vitro using a model Minigastrin (MG) analogue labelled with Lu-177. Additionally, we attempted to characterize the main serum enzymatic cleavage sites by matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) analysis.

METHODS

In vitro stability of a DOTA-minigastrin derivative ((177)Lu-DOTA-His-His-Glu-Ala-Tyr-Gly-Trp-NIe-Asp-Phe-NH(2)) was tested in serum, rat tissue homogenates and two different standardised enzymatic mixtures. Quantification of the metabolised radiopeptides at different time intervals was performed using reversed-phase high-performance liquid chromatography (RP-HPLC). Metabolites were characterised by MALDI-TOF-MS. Urine was collected after 15 min p.i. into the mice and compared with in vitro metabolites by RP-HPLC.

RESULTS

Faster degradation of the radiopeptide was found in blood in comparison with plasma and serum incubation and in components from rats faster than from human origin. Fast degradation was observed in kidney and liver homogenates as well as in standardised enzymatic mixtures, also revealing variations in the metabolic profile. In urine, no intact peptide was detected already 5 min post injection. MALDI-TOF-MS revealed major cleavage sites at the carboxy terminus of the peptide.

CONCLUSION

Very variable results may be found when different kind of incubation media for testing radiopeptide stabilities is used. Serum incubation studies may overestimate stability; therefore, results should be interpreted with care and combined with alternative in vitro and in vivo investigations.

Comparison of biological stability and metabolism of CCK2 receptor targeting peptides, a collaborative project under COST BM0607

PURPOSE

Stability of radiolabelled cholecystokinin 2 (CCK2) receptor targeting peptides has been a major limitation in the use of such radiopharmaceuticals especially for targeted radionuclide therapy applications, e.g. for treatment of medullary thyroid carcinoma (MTC). The purpose of this study was to compare the in vitro stability of a series of peptides binding to the CCK2 receptor [selected as part of the COST Action on Targeted Radionuclide Therapy (BM0607)] and to identify major cleavage sites.

METHODS

Twelve different 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (DOTA)-minigastrin/CCK conjugates were provided within an European COST Action (BM0607) by different laboratories and radiolabelled with (177)Lu. Their in vitro stabilities were tested in fresh human serum. Radiochemical yields (RCY) and intact radioligands for half-life calculations were determined by radio-HPLC. Matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) analysis of metabolites was performed to identify cleavage products using conjugates labelled with excess stable (nat)Lu, incubated in serum at 37°C. Urine metabolite analysis after injection in normal mice was performed by radio-HPLC analysis.

RESULTS

Variable stability in human serum was found for the different peptides with calculated half-lives between 4.5 ± 0.1 h and 198 ± 0.1 h (n = 2). In urine of normal mice only metabolised peptide fragments were detected even at short times after injection for all peptides. MALDI-TOF MS revealed a major cleavage site of all minigastrin derivatives between Asp and Phe-NH(2) at the C-terminal end.

CONCLUSION

Development of CCK2 receptor ligands especially for therapeutic purposes in patients with MTC or small cell lung cancer (SCLC) is still ongoing in different laboratories. This comparative study provided valuable insight into the importance of biological stability especially in the context of other results of this comparative trial within the COST Action BM0607.

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.

Radiolabeled CCK/gastrin peptides for imaging and therapy of CCK2 receptor-expressing tumors

Cholecystokinin (CCK) receptors are overexpressed in numerous human cancers, like medullary thyroid carcinomas, small cell lung cancers and stromal ovarian cancers. The specific receptor-binding property of the endogenous ligands for these receptors can be exploited by labeling peptides with a radionuclide and using these as carriers to guide the radioactivity to the tissues that express the receptors. In this way, tumors can be visualized using positron emission tomography and single photon emission computed tomography imaging. A variety of radiolabeled CCK/gastrin-related peptides has been synthesized and characterized for imaging. All peptides have the C-terminal CCK receptor-binding tetrapeptide sequence Trp-Met-Asp-Phe-NH₂ in common or derivatives thereof. This review focuses on the development and application of radiolabeled CCK/gastrin peptides for radionuclide imaging and radionuclide therapy of tumors expressing CCK receptors. We discuss both preclinical studies as well as clinical studies with CCK and gastrin peptides.

Medullary thyroid carcinoma and biomarkers: past, present and future

The clinical management of patients with persistent or recurrent medullary thyroid carcinoma (MTC) is still under debate, because these patients either have a long-term survival, due to an indolent course of the disease, or develop rapidly progressing disease leading to death from distant metastases. At this moment, it cannot be predicted what will happen within most individual cases. Biomarkers, indicators which can be measured objectively, can be helpful in MTC diagnosis, molecular imaging and treatment, and/or identification of MTC progression. Several MTC biomarkers are already implemented in the daily management of MTC patients. More research is being aimed at the improvement of molecular imaging techniques and the development of molecular systemic therapies. Recent discoveries, like the prognostic value of plasma calcitonin and carcino-embryonic antigen doubling-time and the presence of somatic RET mutations in MTC tissue, may be useful tools in clinical decision making in the future. In this review, we provide an overview of different MTC biomarkers and their applications in the clinical management of MTC patients.

18F-dihydroxyphenylalanine PET in patients with biochemical evidence of medullary thyroid cancer: relation to tumor differentiation

Curative treatment for recurrent medullary thyroid cancer (MTC), diagnosed by rising serum calcitonin, is surgery, but tumor localization is difficult. Therefore, the value of 18F-dihydroxyphenylalanine PET (18F-DOPA PET), 18F-FDG PET, (99m)Tc-V-di-mercaptosulfuricacid (DMSA-V) scintigraphy, and MRI or CT was studied.

METHODS

Twenty-one patients with biochemical recurrent or residual MTC underwent 18F-DOPA PET, 18F-FDG PET, DMSA-V scintigraphy, and MRI or CT. Patient- and lesion-based sensitivities were calculated using a composite reference consisting of all imaging modalities.

RESULTS

In 76% of all patients with MTC, one or more imaging modalities was positive for MTC lesions. In 6 of 8 patients with a calcitonin level of <500 ng/L, imaging results were negative. In 15 patients with positive imaging results, 18F-DOPA PET detected 13 (sensitivity, 62%; with 4.6 lesions per patient [lpp]). Morphologic imaging (n = 19) was positive in 7 (sensitivity, 37%; 4.7 lpp), DMSA-V (n = 18) in 5 (sensitivity, 28%; 1.1 lpp), and 18F-FDG PET (n = 17) in 4 (sensitivity, 24%; 1.6 lpp). In a lesion-based analysis, 18F-DOPA PET detected 95 of 134 lesions (sensitivity, 71%), morphologic imaging detected 80 of 126 (sensitivity, 64%), DMSA-V detected 20 of 108 (sensitivity, 19%), and 18F-FDG PET detected 48 of 102 (sensitivity, 30%). In 2 of 3 patients with a calcitonin/carcinoembryonic antigen (CEA) doubling time of < or =12 mo, 18F-FDG PET performed better than 18FDOPA PET; in the third patient, 18F-FDG PET was not performed.

CONCLUSION

MTC lesions are best detectable when serum calcitonin was >500 ng/L. 18F-DOPA PET is superior to 18F-FDG PET, DMSA-V, and morphologic imaging. With short calcitonin doubling times (< or =12 mo), 18F-FDG PET may be superior.

Synthesis and biological evaluation of a novel pentagastrin-toxin conjugate designed for a targeted prodrug mono-therapy of cancer

A novel carbamate prodrug 2 containing a pentagastrin moiety was synthesized. 2 was designed as a detoxified analogue of the highly cytotoxic natural antibiotic duocarmycin SA (1) for the use in a targeted prodrug monotherapy of cancers expressing cholecystokinin (CCK-B)/gastrin receptors. The synthesis of prodrug 2 was performed using a palladium-catalyzed carbonylation of bromide 6, followed by a radical cyclisation to give the pharmacophoric unit 10, coupling of 10 to the DNA-binding subunit 15 and transformation of the resulting seco-drug 3b into the carbamate 2 via addition of a pentagastrin moiety.