Clinical Advances and Perspectives in Targeted Radionuclide Therapy.

Targeted radionuclide therapy has become increasingly prominent as a nuclear medicine subspecialty. For many decades, treatment with radionuclides has been mainly restricted to the use of iodine-131 in thyroid disorders. Currently, radiopharmaceuticals, consisting of a radionuclide coupled to a vector that binds to a desired biological target with high specificity, are being developed. The objective is to be as selective as possible at the tumor level, while limiting the dose received at the healthy tissue level. In recent years, a better understanding of molecular mechanisms of cancer, as well as the appearance of innovative targeting agents (antibodies, peptides, and small molecules) and the availability of new radioisotopes, have enabled considerable advances in the field of vectorized internal radiotherapy with a better therapeutic efficacy, radiation safety and personalized treatments. For instance, targeting the tumor microenvironment, instead of the cancer cells, now appears particularly attractive. Several radiopharmaceuticals for therapeutic targeting have shown clinical value in several types of tumors and have been or will soon be approved and authorized for clinical use. Following their clinical and commercial success, research in that domain is particularly growing, with the clinical pipeline appearing as a promising target. This review aims to provide an overview of current research on targeting radionuclide therapy.

DTPA-Coated Liposomes as a New Delivery Vehicle for Plutonium Decorporation.

Administration of diethylenetriaminepentaacetic acid (DTPA) is the treatment approach used to promote the decorporation of internalized plutonium. Here we evaluated the efficacy of PEGylated liposomes coated with DTPA, primarily designed to prevent enhanced plutonium accumulation in bones, compared to marketed nonliposomal DTPA and liposomes encapsulating DTPA. The comparative effects were examined in terms of reduction of activity in tissues of plutonium-injected rats. The prompt treatment with DTPA-coated liposomes elicited an even greater efficacy than that with liposome-encapsulated DTPA in limiting skeletal plutonium. This advantage, undoubtedly due to the anchorage of DTPA to the outer layer of liposomes, is discussed, as well as the reason for the loss of this superiority at delayed times after contamination. Plutonium complexed with DTPA-coated liposomes in extracellular compartments was partly diverted into the liver and the spleen. These complexes and those directly formed inside hepatic and splenic cells appeared to be degraded, then released from cells at extremely slow rates. This transitory accumulation of activity, which could not be counteracted by combining both liposomal forms, entailed an underestimation of the efficacy of DTPA-coated liposomes on soft tissue plutonium until total elimination probably more than one month after treatment. DTPA-coated liposomes may provide the best delivery vehicle of DTPA for preventing plutonium deposition in tissues, especially in bone where nuclides become nearly impossible to remove once fixed. Additional development efforts are needed to limit the diversion or to accelerate cell release of plutonium bound to DTPA-coated liposomes, using a labile bond for DTPA attachment.

Cell Tracking in Cancer Immunotherapy.

The impressive development of cancer immunotherapy in the last few years originates from a more precise understanding of control mechanisms in the immune system leading to the discovery of new targets and new therapeutic tools. Since different stages of disease progression elicit different local and systemic inflammatory responses, the ability to longitudinally interrogate the migration and expansion of immune cells throughout the whole body will greatly facilitate disease characterization and guide selection of appropriate treatment regiments. While using radiolabeled white blood cells to detect inflammatory lesions has been a classical nuclear medicine technique for years, new non-invasive methods for monitoring the distribution and migration of biologically active cells in living organisms have emerged. They are designed to improve detection sensitivity and allow for a better preservation of cell activity and integrity. These methods include the monitoring of therapeutic cells but also of all cells related to a specific disease or therapeutic approach. Labeling of therapeutic cells for imaging may be performed in vitro, with some limitations on sensitivity and duration of observation. Alternatively, in vivo cell tracking may be performed by genetically engineering cells or mice so that may be revealed through imaging. In addition, SPECT or PET imaging based on monoclonal antibodies has been used to detect tumors in the human body for years. They may be used to detect and quantify the presence of specific cells within cancer lesions. These methods have been the object of several recent reviews that have concentrated on technical aspects, stressing the differences between direct and indirect labeling. They are briefly described here by distinguishing ex vivo (labeling cells with paramagnetic, radioactive, or fluorescent tracers) and in vivo (in vivo capture of injected radioactive, fluorescent or luminescent tracers, or by using labeled antibodies, ligands, or pre-targeted clickable substrates) imaging methods. This review focuses on cell tracking in specific therapeutic applications, namely cell therapy, and particularly CAR (Chimeric Antigen Receptor) T-cell therapy, which is a fast-growing research field with various therapeutic indications. The potential impact of imaging on the progress of these new therapeutic modalities is discussed.

Promising Scandium Radionuclides for Nuclear Medicine: A Review on the Production and Chemistry up to In Vivo Proofs of Concept.

Scandium radionuclides have been identified in the late 1990s as promising for nuclear medicine applications, but have been set aside for about 20 years. Among the different isotopes of scandium, 43Sc and 44Sc are interesting for positron emission tomography imaging, whereas 47Sc is interesting for therapy. The 44Sc/47Sc or 43Sc/47Sc pairs could be thus envisaged as true theranostic pairs. Another interesting aspect of scandium is that its chemistry is governed by the trivalent ion, Sc3+. When combined with its hardness and its size, it gives this element a lanthanide-like behavior. It is then also possible to use it in a theranostic approach in combination with 177Lu or other lanthanides. This article aims to review the progresses that have been made over the last decade on scandium isotope production and coordination chemistry. It also reviews the radiolabeling aspects and the first (pre) clinical studies performed.

Coupling a gamma-ray detector with asymmetrical flow field flow fractionation (AF4): Application to a drug-delivery system for alpha-therapy.

Alpha-particle-emitting radionuclides have been the subject of considerable investigation as cancer therapeutics, since they have the advantages of high potency and specificity. Among α-emitting radionuclides that are medically relevant and currently available, the lead-212/bismuth-212 radionuclide pair could constitute an in vivo generator. Considering its short half-life (T1/2 = 60.6 min), 212Bi can only be delivered using labelled carrier molecules that would rapidly accumulate in the target tumor. To expand the range of applications, an interesting method is to use its longer half-life parent 212Pb (T1/2 = 10.6 h) that decays to 212Bi. The challenge consists in keeping 212Bi bound to the vector after the 212Pb decay. Preclinical and clinical studies have shown that a variety of vectors may be used to target alpha-emitting radionuclides to cancer cells. Nanoparticles, notably liposomes, allow combined targeting options, achieving high specific activities, easier combination of imaging and therapy and development of multimodality therapeutic agents (e.g., radionuclide therapy plus chemotherapy). The aim of this work consists in assessing the in vitro stability of 212Pb/212Bi encapsulation in the liposomes. Indeed, the release of the radionuclide from the carrier molecules might causes toxicity to normal tissues. To reach this goal, Asymmetrical Flow Field-Flow Fractionation (AF4) coupled with a Multi-Angle Light Scattering detector (MALS) was used and coupling with a gamma (γ) ray detector was developed. AF4-MALS-γ was shown to be a powerful tool for monitoring the liposome size together with the incorporation of the high energy alpha emitter. This was successfully extended to assess the stability of 212Bi-radiolabelled liposomes in serum showing that more than 85% of 212Pb/212Bi is retained after 24 h of incubation at 37 °C.

Delivery of DTPA through Liposomes as a Good Strategy for Enhancing Plutonium Decorporation Regardless of Treatment Regimen.

In this study, we assessed the efficacy of unilamellar 110-nm liposomes encapsulating the chelating agent diethylenetriaminepentaacetic acid (DTPA) in plutonium-exposed rats. Rats were contaminated by intravenous administration of the soluble citrate form of plutonium. The comparative effects of liposomal and free DTPA at similar doses were examined in terms of limitation of alpha activity burden in rats receiving various treatment regimens. Liposomal DTPA given at 1 h after contamination more significantly prevented the accumulation of plutonium in tissues than did free DTPA. Also, when compared to free DTPA, liposome-entrapped DTPA was more efficient when given at late times for mobilization of deposited plutonium. In addition, repeated injections of liposomal DTPA further improved the removal of plutonium compared to single injection. Various possible mechanisms of action for DTPA delivered through liposomes are discussed. The advantage of liposomal DTPA over free DTPA was undoubtedly directly and indirectly due to the better cell penetration of DTPA when loaded within liposomes, mainly in the tissues of the mononuclear phagocytic system. The decorporation induced by liposomal DTPA may result first from intracellular chelation of plutonium deposited in soft tissues, predominantly in the liver. Afterwards, the slow release of free DTPA molecules from these same tissues may enable a sustained action of DTPA, probably mainly by extracellular chelation of plutonium available on bone surfaces. In conclusion, decorporation of plutonium can be significantly improved by liposomal encapsulation of DTPA regardless of the treatment regimen applied.

Improvement of the Targeting of Radiolabeled and Functionalized Liposomes with a Two-Step System Using a Bispecific Monoclonal Antibody (Anti-CEA × Anti-DTPA-In).

UNLABELLED: This study proposes liposomes as a new tool for pretargeted radioimmunotherapy (RIT) in solid tumors. Tumor pretargeting is obtained by using a bispecific monoclonal antibody [BsmAb, anti-CEA × anti-DTPA-indium complex (DTPA-In)] and pegylated radioactive liposomes containing a lipid-hapten conjugate (DSPE-PEG-DTPA-In). In this work, the immunospecificity of tumor targeting is demonstrated both in vitro by fluorescence microscopy and in vivo by biodistribution studies. METHODS: Carcinoembryonic antigen (CEA)-expressing cells (LS174T) were used either in cell culture or as xenografts in nude mice. Doubly fluorescent liposomes or doubly radiolabeled liposomes were, respectively, used for in vitro and in vivo studies. In each case, a tracer of the lipid bilayer [rhodamine or indium-111 ((111)In)] and a tracer of the aqueous phase [fluorescein or iodine-125 ((125)I)] were present. The targeting of liposomes was assessed with BsmAb for active targeting or without for passive targeting. RESULTS: Data obtained with the lipid bilayer tracer showed a fluorescent signal on cell membranes two to three times higher for active than for passive targeting. This immunospecificity was confirmed in vivo with tumor uptake of 7.5 ± 2.4% ID/g (percentage of injected dose per gram of tissue) for active targeting versus 4.5 ± 0.45% ID/g for passive targeting (p = 0.03). Regarding the aqueous phase tracer, results are slightly more contrasted. In vitro, the fluorescent tracer seems to be released in the extracellular matrix, which can be correlated with the in vivo data. Indeed, the tumor uptake of (125)I is lower than that of (111)In: 5.1 ± 2.5% ID/g for active targeting and 2.7 ± 0.6% ID/g for passive targeting, but resulted in more favorable tumor/organs ratios. CONCLUSION: This work demonstrated the tumor targeting immunospecificity of DSPE-PEG-DTPA-In liposomes by two different methods. This original and new approach suggests the potential of immunospecific targeting liposomes for the RIT of solid tumors.

Radioiodinated and astatinated NHC rhodium complexes: synthesis.

INTRODUCTION: The clinical development of radioimmunotherapy with astatine-211 is limited by the lack of a stable radiolabeling method for antibody fragments. An astatinated N-heterocyclic carbene (NHC) Rhodium complex was assessed for the improvement of radiolabeling methodologies with astatine. METHODS: Wet harvested astatine-211 in diisopropyl ether was used. Astatine was first reduced with cysteine then was reacted with a chlorinated Rh-NHC precursor to allow the formation of the astatinated analogue. Reaction conditions have been optimized. Astatine and iodine reactivity were also compared. Serum stability of the astatinated complex has been evaluated. RESULTS: Quantitative formation of astatide was observed when cysteine amounts higher than 46.2 nmol/µl of astatine solution were added. Nucleophilic substitution kinetics showed that high radiolabeling yields were obtained within 15 min at 60°C (88%) or within 5 min at 100°C (95%). Chromatographic characteristics of this new astatinated compound have been correlated with the cold iodinated analog ones. The radioiodinated complex was also synthesized from the same precursor (5 min. at 100°C, up to 85%) using [(125)I]NaI as a radiotracer. In vitro stability of the astatinated complex was controlled after 15 h incubation in human serum at 4°C and 37°C. No degradation was observed, indicating the good chemical and enzymatic stability. CONCLUSION: The astatinated complex was obtained in good yield and exhibited good chemical and enzymatic stability. These preliminary results demonstrate the interest of this new radiolabeling methodology, and further functionalizations should open new possibilities in astatine chemistry. ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE: Although there are many steps and pitfalls before clinical use for a new prosthetic group from the family of NHC complexes, this work may open a new path for astatine-211 targeting.

Antibody-Hapten Recognition at the Surface of Functionalized Liposomes Studied by SPR: Steric Hindrance of Pegylated Phospholipids in Stealth Liposomes Prepared for Targeted Radionuclide Delivery.

Targeted PEGylated liposomes could increase the amount of drugs or radionuclides delivered to tumor cells. They show favorable stability and pharmacokinetics, but steric hindrance of the PEG chains can block the binding of the targeting moiety. Here, specific interactions between an antihapten antibody (clone 734, specific for the DTPA-indium complex) and DTPA-indium-tagged liposomes were characterized by surface plasmon resonance (SPR). Non-PEGylated liposomes fused on CM5 chips whereas PEGylated liposomes did not. By contrast, both PEGylated and non-PEGylated liposomes attached to L1 chips without fusion. SPR binding kinetics showed that, in the absence of PEG, the antibody binds the hapten at the surface of lipid bilayers with the affinity of the soluble hapten. The incorporation of PEGylated lipids hinders antibody binding to extents depending on PEGylated lipid fraction and PEG molecular weight. SPR on immobilized liposomes thus appears as a useful technique to optimize formulations of liposomes for targeted therapy.

Contribution of [64Cu]-ATSM PET in molecular imaging of tumour hypoxia compared to classical [18F]-MISO--a selected review.

During the carcinogenesis process, tumour cells often have a more rapid proliferation potential than cells that participate in blood capillary formation by neoangiogenesis. As a consequence of the poorly organized vasculature of various solid tumours, a limited oxygen delivery is observed. This hypoxic mechanism frequently occurs in solid cancers and can lead to therapeutic resistance. The present selected literature review is focused on the comparison of two positron emitting radiopharmaceuticals agents, which are currently leaders in tumour hypoxia imaging by PET. {18F}-fluoromisonidazole (=FMISO) is most commonly used as an investigational PET agent with an investigational new drug exemption from the FDA, while {64Cu}-diacetyl-bis(N4-methylthiosemicarbazone) (64Cu-ATSM) has been presented as an alternative radiopharmaceutical not yet readily available. The comparison of these two radiopharmaceutical agents is particularly focused on isotope properties, radiopharmaceutical labelling process, pharmacological mechanisms, dosimetry data in patients, and clinical results in terms of image contrast. PET imaging has demonstrated a good efficacy in tumour hypoxia imaging with both FMISO and Cu-ATSM, but FMISO has presented too slow an in vivo accumulation and a weak image contrast of the hypoxia area. Despite a less favourable dosimetry, 64Cu-ATSM appears superior in terms of imaging performance, calling for industrial and clinical development of this innovative radiopharmaceutical.

Feasibility of the radioastatination of a monoclonal antibody with astatine-211 purified by wet extraction.

Astatine-211, a most promising α-particle emitter for targeted radiotherapy, is generally obtained by high-temperature distillation. However, a liquid-liquid extraction procedure (wet extraction) has also been described. The purpose of this study was to develop and optimize the labelling of the stannylated-activated ester N-hydroxysuccinimidyl-meta-trimethylstannylbenzoate ester (MeSTB) with astatine-211 extracted in di-isopropylether (DIPE) in the presence of the oxidant N-chlorosuccinimide (NCS). The effect of final volume, incubation duration and NCS amounts on radiolabelling yield was studied. The best yields (85-90%) of N-hydroxysuccinimidyl-meta-[(211)At]astatobenzoate ester (SAB) were obtained with 20 nmol of MeSTB, 100 nmol of NCS in 120 µL of DIPE after 15 min. The astatine-211-labelled-activated ester was then used to radiolabel a monoclonal antibody (mAb). The labelling yield was 20-25% and the radiochemical purity was 97-99%. These results show that mAbs may be efficiently labelled with astatine-211 obtained by wet extraction, a fully automatable technique that may prove to be a useful alternative to dry distillation for high activity labelling of radiopharmaceuticals. Copyright © 2008 John Wiley & Sons, Ltd.

High-activity radio-iodine labeling of conventional and stealth liposomes.

A new method to label preformed liposomes with high activities of radiohalogenated compounds has been developed. It uses activated esters of simple synthetic molecules that may be readily halogenated, such as Bolton-Hunter reagent (BH), and arginine-containing liposomes. BH, in the form of an activated ester, crosses the liposome membrane to react with arginine inside the liposomes, as demonstrated by thin-layer chromatography and by the fact that saline-containing liposomes, or hydrolyzed BH or the water soluble sulfo-BH afforded only marginal encapsulation yields. Under optimized conditions, between 37 and 55 degrees C, 62 +/- 4% (mean +/- SD) of radiolabeled BH were consistently encapsulated in the liposomes within 30 min. In molar amounts, this corresponds to a mean of 56 nmol of BH per micromol of lipids. Based on achievable specific activity, up to 2.8 GBq of iodine-131 could be entrapped per micromol of lipids. Leakage of radioactivity was very low, with less than 5% of the encapsulated activity released within 6 days at 4 degrees C in phosphate-buffered saline and less than 50% within 24 h in human serum at 37 degrees C. The labeling stability, and the fact that both conventional and PEGylated liposomes can be readily labeled with high doses of radioactivity, will make this technique useful for in vivo targeting applications, such as tumor detection (using iodine-123 or iodine-124) or therapy (with iodine-131 or astatine-211).