Validation of a Novel CHX-A‘ ‘ Derivative Suitable for Peptide Conjugation: Small Animal PET/CT Imaging Using Yttrium-86-CHX-A‘ ‘-Octreotide

Expanding the PET radioisotope universe utilizing solid targets on small medical cyclotrons

Molecular imaging with medical radioisotopes enables the minimally-invasive monitoring of aberrant biochemical, cellular and tissue-level processes in living subjects. The approach requires the administration of radiotracers composed of radioisotopes attached to bioactive molecules, the pairing of which considers several aspects of the radioisotope in addition to the biological behavior of the targeting molecule to which it is attached. With the advent of modern cellular and biochemical techniques, there has been a virtual explosion in potential disease recognition antigens as well as targeting moieties, which has subsequently opened new applications for a host of emerging radioisotopes with well-matched properties. Additionally, the global radioisotope production landscape has changed rapidly, with reactor-based production and its long-defined, large-scale centralized manufacturing and distribution paradigm shifting to include the manufacture and distribution of many radioisotopes via a worldwide fleet of cyclotrons now in operation. Cyclotron-based radioisotope production has become more prevalent given the commercial availability of instruments, coupled with the introduction of new target hardware, process automation and target manufacturing methods. These advances enable sustained, higher-power irradiation of solid targets that allow hospital-based radiopharmacies to produce a suite of radioisotopes that drive research, clinical trials, and ultimately clinical care. Over the years, several different radioisotopes have been investigated and/or selected for radiolabeling due to favorable decay characteristics (i.e. a suitable half-life, high probability of positron decay, etc.), well-elucidated chemistry, and a feasible production framework. However, longer-lived radioisotopes have surged in popularity given recent regulatory approvals and incorporation of radiopharmaceuticals into patient management within the medical community. This review focuses on the applications, nuclear properties, and production and purification methods for some of the most frequently used/emerging positron-emitting, solid-target-produced radioisotopes that can be manufactured using small-to-medium size cyclotrons (≤24 MeV).

86Y PET imaging

Yttrium-86 is a non-standard positron emitter that can provide dosimetry information prior to therapy with yttrium-90 radiopharmaceuticals and be used to follow biochemical processes. In this chapter, we discuss the production, purification and applications of ⁸⁶Y for PET imaging. More specifically, ⁸⁶Y radiolabeling is highlighted and protocols to determine the radiochemical purity of ⁸⁶Y-DOTA and ⁸⁶Y-DTPA are presented.

Expanding PET-applications in life sciences with positron-emitters beyond fluorine-18

Positron-emission-tomography (PET) has become an indispensable diagnostic tool in modern nuclear medicine. Its outstanding molecular imaging features allow repetitive studies on one individual and with high sensitivity, though no interference. Rather few positron-emitters with near favourable physical properties, i.e. carbon-11 and fluorine-18, furnished most studies in the beginning, preferably if covalently bound as isotopic label of small molecules. With the advancement of PET-devices the scope of in vivo research in life sciences and especially that of medical applications expanded, and other than "standard" PET-nuclides received increasing significance, like the radiometals copper-64 and gallium-68. Especially during the last decades, positron-emitters of other chemical elements have gotten into the focus of interest, concomitant with the technical advancements in imaging and radionuclide production. With known nuclear imaging properties and main production methods of emerging positron-emitters their usefulness for medical application is promising and even proven for several ones already. Unfortunate decay properties could be corrected for, and β+-emitters, especially with a longer half-life, provided new possibilities for application where slower processes are of importance. Further on, (bio)chemical features of positron-emitters of other elements, among there many metals, not only expanded the field of classical clinical investigations, but also opened up new fields of application. Appropriately labelled peptides, proteins and nanoparticles lend itself as newer probes for PET-imaging, e.g. in theragnostic or PET/MR hybrid imaging. Furthermore, the potential of non-destructive in-vivo imaging with positron-emission-tomography directs the view on further areas of life sciences. Thus, exploiting the excellent methodology for basic research on molecular biochemical functions and processes is increasingly encouraged as well in areas outside of health, such as plant and environmental sciences.

Overview of Radiolabeled Somatostatin Analogs for Cancer Imaging and Therapy

Identified in 1973, somatostatin (SST) is a cyclic hormone peptide with a short biological half-life. Somatostatin receptors (SSTRs) are widely expressed in the whole body, with five subtypes described. The interaction between SST and its receptors leads to the internalization of the ligand-receptor complex and triggers different cellular signaling pathways. Interestingly, the expression of SSTRs is significantly enhanced in many solid tumors, especially gastro-entero-pancreatic neuroendocrine tumors (GEP-NET). Thus, somatostatin analogs (SSAs) have been developed to improve the stability of the endogenous ligand and so extend its half-life. Radiolabeled analogs have been developed with several radioelements such as indium-111, technetium-99 m, and recently gallium-68, fluorine-18, and copper-64, to visualize the distribution of receptor overexpression in tumors. Internal metabolic radiotherapy is also used as a therapeutic strategy (e.g., using yttrium-90, lutetium-177, and actinium-225). With some radiopharmaceuticals now used in clinical practice, somatostatin analogs developed for imaging and therapy are an example of the concept of personalized medicine with a theranostic approach. Here, we review the development of these analogs, from the well-established and authorized ones to the most recently developed radiotracers, which have better pharmacokinetic properties and demonstrate increased efficacy and safety, as well as the search for new clinical indications.

111In-DTPA-d-Phe-1-Asp0-d-Phe1-octreotide exhibits higher tumor accumulation and lower renal radioactivity than 111In-DTPA-d-Phe1-octreotide

INTRODUCTION

¹¹¹In-DTPA-d-Phe¹-octreotide scintigraphy is an important method of detecting neuroendocrine tumors. We previously reported that a new derivative of ¹¹¹In-DTPA-d-Phe¹-octreotide, ¹¹¹In-DTPA-d-Phe^-1-Asp⁰-d-Phe¹-octreotide, accomplished the reduction of prolonged renal accumulation of radioactivity. The aim of this study was to evaluate the tumor accumulation of ¹¹¹In-DTPA-d-Phe^-1-Asp⁰-d-Phe¹-octreotide in vitro and in vivo by comparing it with ¹¹¹In-DTPA-d-Phe¹-octreotide.

METHODS

The tumor accumulation of this octreotide derivative was determined by measuring its uptake using cultured AR42J cells in vitro and biodistribution studies in vivo. The distribution of the radiotracer and the extent of somatostatin receptor-specific uptake in the tumor were estimated by a counting method using AR42J tumor-bearing mice. The radioactive metabolite species in the tumor and kidney were identified by HPLC analyses at 3 and 24h post-injection of the ¹¹¹In-DTPA-conjugated peptide.

RESULTS

In both cases, in vitro and in vivo, the tumor radioactivity levels of ¹¹¹In-DTPA-d-Phe^-1-Asp⁰-d-Phe¹-octreotide were approximately 2-4 times higher than those of ¹¹¹In-DTPA-d-Phe¹-octreotide. On in vitro cellular uptake inhibition and radioreceptor assay, ¹¹¹In-DTPA-d-Phe^-1-Asp⁰-d-Phe¹-octreotide exhibited a binding affinity to somatostatin receptor highly similar to that of ¹¹¹In-DTPA-d-Phe¹-octreotide. As the additional cellular uptake of ¹¹¹In-DTPA-d-Phe^-1-Asp⁰-d-Phe¹-octreotide was significantly lower at low temperature than at 37°C, it was considered that a cellular uptake pathway is involved in energy-dependent endocytotic processes. In the radiometabolite analysis of ¹¹¹In-DTPA-d-Phe^-1-Asp⁰-d-Phe¹-octreotide, ¹¹¹In-DTPA-d-Phe-Asp-OH was a major metabolite in the tumor at 24h post-injection.

CONCLUSION

¹¹¹In-DTPA-d-Phe^-1-Asp⁰-d-Phe¹-octreotide exhibited higher tumor accumulation and persistence of tumor radioactivity than ¹¹¹In-DTPA-d-Phe¹-octreotide. We reasoned that this higher tumor accumulation would not be based on the receptor affinity but on a receptor-mediated endocytotic process involved in temperature-dependent cellular uptake. The present study demonstrated the great potential of the pharmaceutical development of a new radiolabeled peptide with high tumor accumulation and low renal radioactivity by the chemical modification of ¹¹¹In-DTPA-d-Phe¹-octreotide.

Efficient route to pre-organized and linear polyaminopolycarboxylates: Cy-TTHA, Cy-DTPA and mono/di- reactive, tert-butyl protected TTHA/Cy-TTHA

Pre-organized polyaminopolycarboxylate chelators Cy-TTHA and Cy-DTPA were synthesized via modular five-step syntheses from commercially available starting materials in ~ 62% and 47% overall yields, respectively. Furthermore, strategies are reported for the efficient preparation of mono- and di-reactive, tert-butyl-protected TTHA/Cy-TTHA to selectively functionalize central chelators' carboxylic acids.

Evaluation of (177)Lu-CHX-A''-DTPA-Bevacizumab as a radioimmunotherapy agent targeting VEGF expressing cancers

This study aimed at the preparation and evaluation of (177)Lu-CHX-A''-DTPA-Bevacizumab for targeting VEGF over-expressing cancers. Bevacizumab conjugated to p-NCS-Bn-CHX-A''-DTPA was radiolabeled with (177)Lu. The radioimmunoconjugate characterized by SE-HPLC exhibited radiochemical purity of 98.0±0.6%. In vitro stability was retained upto 4 days at 37°C. In vitro cell binding studies showed good uptake by VEGF expressing U937 tumor cells. Biodistribution studies in melanoma model showed significant uptake and retention of (177)Lu-CHX-A''-DTPA-Bevacizumab in tumor with reduction in uptake in presence of cold Bevacizumab confirming its specificity to VEGF.

Underscoring the influence of inorganic chemistry on nuclear imaging with radiometals

Over the past several decades, radionuclides have matured from largely esoteric and experimental technologies to indispensible components of medical diagnostics. Driving this transition, in part, have been mutually necessary advances in biomedical engineering, nuclear medicine, and cancer biology. Somewhat unsung has been the seminal role of inorganic chemistry in fostering the development of new radiotracers. In this regard, the purpose of this Forum Article is to more visibly highlight the significant contributions of inorganic chemistry to nuclear imaging by detailing the development of five metal-based imaging agents: (64)Cu-ATSM, (68)Ga-DOTATOC, (89)Zr-transferrin, (99m)Tc-sestamibi, and (99m)Tc-colloids. In a concluding section, several unmet needs both in and out of the laboratory will be discussed to stimulate conversation between inorganic chemists and the imaging community.

Modular syntheses of H4octapa and H2dedpa, and yttrium coordination chemistry relevant to86Y/90Y radiopharmaceuticals

The ligands H₂dedpa and H₄octapa have been synthesized using labiletert-butyl ester protection, and H₄octapa has been studied with yttrium.

Matching chelators to radiometals for radiopharmaceuticals

Radiometals comprise many useful radioactive isotopes of various metallic elements. When properly harnessed, these have valuable emission properties that can be used for diagnostic imaging techniques, such as single photon emission computed tomography (SPECT, e.g.(67)Ga, (99m)Tc, (111)In, (177)Lu) and positron emission tomography (PET, e.g.(68)Ga, (64)Cu, (44)Sc, (86)Y, (89)Zr), as well as therapeutic applications (e.g.(47)Sc, (114m)In, (177)Lu, (90)Y, (212/213)Bi, (212)Pb, (225)Ac, (186/188)Re). A fundamental critical component of a radiometal-based radiopharmaceutical is the chelator, the ligand system that binds the radiometal ion in a tight stable coordination complex so that it can be properly directed to a desirable molecular target in vivo. This article is a guide for selecting the optimal match between chelator and radiometal for use in these systems. The article briefly introduces a selection of relevant and high impact radiometals, and their potential utility to the fields of radiochemistry, nuclear medicine, and molecular imaging. A description of radiometal-based radiopharmaceuticals is provided, and several key design considerations are discussed. The experimental methods by which chelators are assessed for their suitability with a variety of radiometal ions is explained, and a large selection of the most common and most promising chelators are evaluated and discussed for their potential use with a variety of radiometals. Comprehensive tables have been assembled to provide a convenient and accessible overview of the field of radiometal chelating agents.

H(2)azapa: a versatile acyclic multifunctional chelator for (67)Ga, (64)Cu, (111)In, and (177)Lu

Preliminary experiments with the novel acyclic triazole-containing bifunctional chelator H2azapa and the radiometals (64)Cu, (67)Ga, (111)In, and (177)Lu have established its significant versatile potential as an alternative to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) for metal-based radiopharmaceuticals. Unlike DOTA, H2azapa radiolabels quantitatively with (64)Cu, (67)Ga, (111)In, and (177)Lu in 10 min at room temperature. In vitro competition experiments with human blood serum show that (64)Cu remained predominantly chelate-bound, with only 2% transchelated to serum proteins after 20 h. Biodistribution experiments with [(64)Cu(azapa)] in mice reveal uptake in various organs, particularly in the liver, lungs, heart, intestines, and kidneys. When compared to [(64)Cu(DOTA)](2-), the lipophilic neutral [(64)Cu(azapa)] was cleared through the gastrointestinal tract and accumulated in the liver, which is common for lipophilic compounds or free (64)Cu. The chelator H2azapa is a model complex for a click-based bifunctional chelating agent, and the lipophilic benzyl "place-holders" will be replaced by hydrophilic peptides to modulate the pharmacokinetics and direct activity away from the liver and gut. The solid-state molecular structure of [In(azapa)(H2O)][ClO4] reveals a very rare eight-coordinate distorted square antiprismatic geometry with one triazole arm bound, and the structure of [(64)Cu(azapa)] shows a distorted octahedral geometry. The present study demonstrates significant potential for bioconjugates of H2azapa as alternatives to DOTA in copper-based radiopharmaceuticals, with the highly modular and "clickable" molecular scaffold of H2azapa easily modified into a variety of bioconjugates. H2azapa is a versatile addition to the "pa" family, joining the previously published H2dedpa ((67/68)Ga and (64)Cu), H4octapa ((111)In, (177)Lu, and (90)Y), and H5decapa ((225)Ac) to cover a wide range of important nuclides.

Antibody delivery of drugs and radionuclides: factors influencing clinical pharmacology

The therapeutic rationale of antibody conjugates is the selective delivery of a cytotoxin to tumor cells via binding and internalization of the monoclonal antibodies to a specific cell-surface antigen, thereby enhancing the therapeutic index of the cytotoxin. The key structural and functional components of an antibody conjugate are the antibody, the linker and the cytotoxin (chemical or radionuclide) with each component being critical for the successful development of the conjugate. Considerable efforts have been made in understanding the pharmacokinetics, pharmacodynamics, tissue distribution, metabolism and pharmacologic effects of these complex macromolecular entities. The purpose of this article is to discuss the properties and various structural components of antibody conjugates that influence their clinical pharmacology.

Radiopharmaceutical development of radiolabelled peptides

Receptor targeting with radiolabelled peptides has become very important in nuclear medicine and oncology in the past few years. The overexpression of many peptide receptors in numerous cancers, compared to their relatively low density in physiological organs, represents the molecular basis for in vivo imaging and targeted radionuclide therapy with radiolabelled peptide-based probes. The prototypes are analogs of somatostatin which are routinely used in the clinic. More recent developments include somatostatin analogs with a broader receptor subtype profile or with antagonistic properties. Many other peptide families such as bombesin, cholecystokinin/gastrin, glucagon-like peptide-1 (GLP-1)/exendin, arginine-glycine-aspartic acid (RGD) etc. have been explored during the last few years and quite a number of potential radiolabelled probes have been derived from them. On the other hand, a variety of strategies and optimized protocols for efficient labelling of peptides with clinically relevant radionuclides such as (99m)Tc, M(3+) radiometals ((111)In, (86/90)Y, (177)Lu, (67/68)Ga), (64/67)Cu, (18)F or radioisotopes of iodine have been developed. The labelling approaches include direct labelling, the use of bifunctional chelators or prosthetic groups. The choice of the labelling approach is driven by the nature and the chemical properties of the radionuclide. Additionally, chemical strategies, including modification of the amino acid sequence and introduction of linkers/spacers with different characteristics, have been explored for the improvement of the overall performance of the radiopeptides, e.g. metabolic stability and pharmacokinetics. Herein, we discuss the development of peptides as radiopharmaceuticals starting from the choice of the labelling method and the conditions to the design and optimization of the peptide probe, as well as some recent developments, focusing on a selected list of peptide families, including somatostatin, bombesin, cholecystokinin/gastrin, GLP-1/exendin and RGD.

86Y based PET radiopharmaceuticals: radiochemistry and biological applications

Development of targeted radionuclide therapy with (90)Y labeled antibodies and peptides has gained momentum in the past decade due to the successes of (90)Y-ibritumomab tiuxetan and (90)Y-DOTA-Phe(1)-Tyr(3)-octreotide in treatment of cancer. (90)Y is a pure β(-)-emitter and cannot be imaged for patient-specific dosimetry which is essential for pre-therapeutic treatment planning and accurate absorbed dose estimation in individual patients to mitigate radiation related risks. This review article describes the utility of (86)Y, a positron emitter (33%) with a 14.7-h half-life that can be imaged by positron emission tomography and used as an isotopically matched surrogate radionuclide for (90)Y radiation doses estimations. This review discusses various aspects involved in the development of (86)Y labeled radiopharmaceuticals with the specific emphasis on the radiochemistry and biological applications with antibodies and peptides.

A practical guide to the construction of radiometallated bioconjugates for positron emission tomography

Positron emission tomography (PET) has become a vital imaging modality in the diagnosis and treatment of disease, most notably cancer. A wide array of small molecule PET radiotracers have been developed that employ the short half-life radionuclides (11)C, (13)N, (15)O, and (18)F. However, PET radiopharmaceuticals based on biomolecular targeting vectors have been the subject of dramatically increased research in both the laboratory and the clinic. Typically based on antibodies, oligopeptides, or oligonucleotides, these tracers have longer biological half-lives than their small molecule counterparts and thus require labeling with radionuclides with longer, complementary radioactive half-lives, such as the metallic isotopes (64)Cu, (68)Ga, (86)Y, and (89)Zr. Each bioconjugate radiopharmaceutical has four component parts: biomolecular vector, radiometal, chelator, and covalent link between chelator and biomolecule. With the exception of the radiometal, a tremendous variety of choices exists for each of these pieces, and a plethora of different chelation, conjugation, and radiometallation strategies have been utilized to create agents ranging from (68)Ga-labeled pentapeptides to (89)Zr-labeled monoclonal antibodies. Herein, the authors present a practical guide to the construction of radiometal-based PET bioconjugates, in which the design choices and synthetic details of a wide range of biomolecular tracers from the literature are collected in a single reference. In assembling this information, the authors hope both to illuminate the diverse methods employed in the synthesis of these agents and also to create a useful reference for molecular imaging researchers both experienced and new to the field.

The rise of metal radionuclides in medical imaging: copper-64, zirconium-89 and yttrium-86

Positron emission tomography, with its high sensitivity and resolution, is growing rapidly as an imaging technology for the diagnosis of many disease states. The success of this modality is reliant on instrumentation and the development of effective and novel targeted probes. Initially, research in this area was focused on what we will define in this article as ‘standard’ PET isotopes (carbon-11, nitrogen-13, oxygen-15 and fluorine-18), but the short half-lives of these isotopes limit radiopharmaceutical development to those that probe rapid biological processes. To overcome these limitations, there has been a rise in nonstandard isotope probe development in recent years. This review focuses on the biological probes and processes that have been examined, in additiom to the preclinical and clinical findings with nonstandard radiometals: copper-64, zirconium-89, and yttrium-86.

Development of a new radioligand for cholecystokinin receptor subtype 2 scintigraphy: from molecular modeling to in vivo evaluation

To improve the targeting to tumors expressing the cholecystokinin receptor subtype 2 (CCK2R) with limited kidney uptake, we synthesized a novel cholecystokinin C-terminal tetrapeptide (CCK4)-based derivative conjugated to an original bipyridine-chelator (BPCA), 111In-BPCA-(Ahx)2-CCK4. To our knowledge this is the first CCK4-based radioligand that presents a high affinity for the CCK2R, a high and specific tumor uptake, a low renal accumulation and a very good visualization of tumors in vivo compared with an internal control, 111Indium-trans-cyclohexyldiethylenetriaminepenta-acetic acid-cholecystokinin octapeptide (111In-CHX-A''-DTPA-CCK8). These properties make 111In-BPCA-(Ahx)2-CCK4, a promising candidate for imaging and peptide receptor radionuclide therapy of CCK2R positive tumors.

Melanoma imaging using (111)In-, (86)Y- and (68)Ga-labeled CHX-A''-Re(Arg11)CCMSH

INTRODUCTION

A novel alpha-melanocyte-stimulating hormone peptide analog CHX-A''-Re(Arg(11))CCMSH, which targeted the melanocortin-1 receptor (MC1-R) overexpressed on melanoma cells, was investigated for its biodistribution and tumor imaging properties.

METHODS

The metal bifunctional chelator CHX-A'' was conjugated to the melanoma targeting peptide (Arg(11))CCMSH and cyclized by Re incorporation to yield CHX-A''-Re(Arg(11))CCMSH. CHX-A''-Re(Arg(11))CCMSH was labeled with (111)In, (86)Y and (68)Ga, and the radiolabeled peptides were examined in B16/F1 melanoma-bearing mice for their pharmacokinetic as well as their tumor targeting properties using small animal SPECT and PET.

RESULTS

The radiolabeling efficiencies of the (111)In-, (86)Y- and (68)Ga-labeled CHX-A''-Re(Arg(11))CCMSH peptides were >95%, resulting in specific activities of 4.44, 3.7 and 1.85 MBq/microg, respectively. Tumor uptake of the (111)In-, (86)Y- and (68)Ga-labeled peptides was rapid with 4.17+/-0.94, 4.68+/-1.02 and 2.68+/-0.69 %ID/g present in the tumors 2 h postinjection, respectively. Disappearance of radioactivity from the normal organs and tissues was rapid with the exception of the kidneys. Melanoma tumors were imaged with all three radiolabeled peptides 2 h postinjection. MC1-R-specific uptake was confirmed by competitive receptor blocking studies.

CONCLUSIONS

Melanoma tumor uptake and imaging was exhibited by the (111)In-, (86)Y- and (68)Ga-labeled Re(Arg(11))CCMSH peptides, although the tumor uptake was moderated by low specific activity. The facile radiolabeling properties of CHX-A''-Re(Arg(11))CCMSH allow it to be employed as a melanoma imaging agent with little or no purification after (111)In, (86)Y and (68)Ga labeling.

Evaluation of a maleimido derivative of CHX-A'' DTPA for site-specific labeling of affibody molecules

Affibody molecules are a new class of small targeting proteins based on a common three-helix bundle structure. Affibody molecules binding a desired target may be selected using phage-display technology. An Affibody molecule Z HER2:342 binding with subnanomolar affinity to the tumor antigen HER2 has recently been developed for radionuclide imaging in vivo. Introduction of a single cysteine into the cysteine-free Affibody scaffold provides a unique thiol group for site-specific labeling of recombinant Affibody molecules. The recently developed maleimido-CHX-A'' DTPA was site-specifically conjugated at the C-terminal cysteine of Z HER2:2395-C, a variant of Z HER2:342, providing a homogeneous conjugate with a dissociation constant of 56 pM. The yield of labeling with (111)In was >99% after 10 min at room temperature. In vitro cell tests demonstrated specific binding of (111)In-CHX-A'' DTPA-Z 2395-C to HER2-expressing cell-line SKOV-3 and good cellular retention of radioactivity. In normal mice, the conjugate demonstrated rapid clearance from all nonspecific organs except kidney. In mice bearing SKOV-3 xenografts, the tumor uptake of (111)In-CHX-A'' DTPA-Z 2395-C was 17.3 +/- 4.8% IA/g and the tumor-to-blood ratio 86 +/- 46 (4 h postinjection). HER2-expressing xenografts were clearly visualized 1 h postinjection. In conclusion, coupling of maleimido-CHX-A'' DTPA to cysteine-containing Affibody molecules provides a well-defined uniform conjugate, which can be rapidly labeled at room temperature and provides high-contrast imaging of molecular targets in vivo.

Synthesis of a cross-bridged cyclam derivative for peptide conjugation and 64Cu radiolabeling

The increased use of copper radioisotopes in radiopharmaceutical applications has created a need for bifunctional chelators (BFCs) that form stable radiocopper complexes and allow covalent attachment to biological molecules. Previous studies have established that 4,11-bis-(carbo- tert-butoxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (H 2CB-TE2A), a member of the ethylene "cross-bridged" cyclam (CB-cyclam) class of bicyclic tetraaza macrocycles, forms highly kinetically stable complexes with Cu(II) and is less susceptible to in vivo transchelation than its nonbridged analogue, 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (TETA). Herein, we report a convenient synthesis of a novel cross-bridged BFC that is structurally analogous to CB-TE2A in that it possesses two coordinating acetate arms, but in addition possesses a third orthogonally protected arm for conjugation to peptides and other targeting agents. Application of this strategy to cross-bridged chelators may also enable the development of even further improved agents for (64)Cu-mediated diagnostic positron emission tomography (PET) imaging as well as for targeted radiotherapeutic applications.

Synthesis, characterization, and biological evaluation of integrin alphavbeta3-targeted PAMAM dendrimers

Ligand size and valency strongly influence the receptor uptake and clearance of tumor angiogenesis imaging agents. The structures of successful imaging agents exhibit a high degree of variability, encompassing small monovalent arginine-glycine-aspartic acid (RGD)-containing peptides, multivalent RGD-oligomers, and a monoclonal antibody against integrin alpha-v-beta-3 (alpha-v-beta-3). We have pursued a nanoscale approach to imaging of angiogenesis using rationally designed polyamidoamine (PAMAM) dendrimers covalently adorned with RGD-cyclopeptides. An orthogonal oxime-ligation strategy was applied to chemoselectively effect conjugation of the PAMAM dendrimers with RGD-cyclopeptides for targeting alpha vbeta 3. Fluorescent dyes for optical imaging and chelates for gadolinium-based magnetic resonance (MR) imaging were subsequently appended to create robust multimodal macromolecular imaging agents. Fluorescence microscopy revealed selective binding of the resulting RGD peptide-bearing dendrimer with empty chelates to alpha-v-beta-3-expressing cells, but somewhat reduced selectivity was observed following Gd(III) complexation. The expected incomplete saturation of chelates with Gd(III) ions permitted radiometal complexation, and an in vivo tissue distribution of the resulting agent in M21 melanoma tumor-bearing mice showed mostly renal and reticuloendothelial accumulation, with the tumor:blood ratio peaking (3.30+/-0.03) at 2 h postinjection.

A novel bifunctional maleimido CHX-A'' chelator for conjugation to thiol-containing biomolecules

A novel bifunctional maleimido CHX-A'' DTPA chelator 5 was developed and conjugated to the monoclonal antibody trastuzumab (Herceptin) and subsequently radiolabeled with (111)In. The resulting (111)In labeled immunoconjugate 2 was demonstrated to bind to SKOV-3 ovarian cancer cells comparably to an isothiocyanato CHX-A'' DTPA modified native trastuzumab, 1. Through efficient thiol-maleimide chemistry, antibodies, peptides or other targeting vectors can now be modified with an established radioactive metal chelating agent CHX-A'' DTPA for imaging and/or therapies of cancer.

Development of radioimmunotherapeutic and diagnostic antibodies: an inside-out view

Only a handful of radiolabeled antibodies (Abs) have gained US Food and Drug Administration (FDA) approval for use in clinical oncology, including four immunodiagnostic agents and two targeted radioimmunotherapeutic agents. Despite the advent of nonimmunogenic Abs and the availability of a diverse library of radionuclides, progress beyond early Phase II radioimmunotherapy (RIT) studies in solid tumors has been marginal. Furthermore, [18F]fluorodeoxyglucose continues to dominate the molecular imaging domain, underscored by a decade-long absence of any newly approved Ab-based imaging agent (none since 1996). Why has the development of clinically successful Abs for RIT been limited to lymphoma? What obstacles must be overcome to allow the FDA approval of immuno-positron emission tomography (immuno-PET) imaging agents? How can we address the unique challenges that have thus far prevented the introduction of Ab-based imaging agents and therapeutics for solid tumors? Many poor decisions have been made regarding radiolabeled Abs, but useful insight can be gained from these mistakes. The following review addresses the physical, chemical, biological, clinical, regulatory and financial limitations that impede the progress of this increasingly important class of drugs.

Design, synthesis, and characterization of a dual modality positron emission tomography and fluorescence imaging agent for monoclonal antibody tumor-targeted imaging

A novel lysine-based trifunctional chelate 3 was designed, synthesized, and characterized and bears both a chelating moiety (CHX-A' ') for sequestering radiometals (86Y or 111In) and the near-infrared dye Cy5.5 for dual modality PET (or SPECT) and fluorescence imaging, respectively. Successful conjugation of 3 to the monoclonal antibody trastuzumab (Herceptin) was achieved by efficient thiol-maleimide chemistry, thereby yielding immunoconjugate 2. Analysis of 2 by flow cytometry and competitive binding assay demonstrates that immunoconjugate 2 binds to SKOV3 tumor cells comparably to native trastuzumab and, thus, may be used as a tumor-targeted monoclonal antibody probe for multimodality imaging.

The emerging role of molecular imaging and targeted therapeutics in peritoneal carcinomatosis

Peritoneal carcinomatosis is a common and often fatal late-stage complication of many gastrointestinal and gynecologic malignancies. This review discusses the ongoing evolution of diagnostic and treatment strategies for peritoneal carcinomatosis and the role that molecular imaging and radioimmunotherapy may play in improving patient survival. An overview of recent developments in targeted imaging and therapeutics for peritoneal carcinomatosis, as well as the authors' opinions as to future developments in this field is also provided.

Infrared-based module for the synthesis of 68Ga-labeled radiotracers

INTRODUCTION

Generator-produced positron emission tomography tracers have gained much attention recently due to favorable imaging characteristics, accessibility and affordability. The focus of this study was to design and validate a semiautomated module for 68Ga-labeled chemistry utilizing infrared-based heating for rapid control of thermal cycle.

METHODS

A prototype module was built and installed in our laboratory. DOTA (1,4,7,10-tetra-azacyclododecane-1,4,7,10-tetra-acetic acid) was manually labeled (10-1000 nmol) with 68Ga to optimize synthesis conditions. For automation, 250 nmol of DOTA was labeled with 68Ga with reaction times of 5 min (n=5), 10 min (n=5) and 20 min (n=6). A dose calibrator and radio-thin-layer chromatography were used to access the product yield and quality of both manual and automated syntheses.

RESULTS

A semiautomated 68Ga synthesis module was developed. The system showed that software control could be used to drive a multistep radiochemical synthesis and to produce 68Ga-DOTA with >95% radiochemical purity, similar to that in manual synthesis. The device also showed that for a short reaction time of 5 min, decay-corrected radioactive yields of >70% could be achieved. The total synthesis was as short as 22 min, including 6-8 min for HCl evaporation. The temperature and pressure profiles of the process were consistent.

CONCLUSION

We demonstrated the use of a commercially available 68Ga/68Ge generator with a semiautomated module to successfully label the bifunctional chelator DOTA with 68Ga. Further investigation with different 68Ga-labeled bioconjugates is warranted to demonstrate the usefulness of the module as a tool for tracer development and imaging research.