111In- or 99mTc-labeled recombinant VEGF bioconjugates: in vitro evaluation of their cytotoxicity on porcine aortic endothelial cells overexpressing Flt-1 receptors

99mTc(I)-Labeled His-Tagged Proteins: Impact in the Development of Novel Imaging Probes and in Drug Discovery

Technetium-99 m (99mTc) remains the cornerstone of nuclear medicine for single photon emission computed tomography (SPECT) due to its widespread availability and chemical and physical features. Its multiple oxidation states allow for the design and production of radiopharmaceuticals with versatile properties, namely in terms of pharmacokinetic profile. 99mTc(V) is the most common oxidation state, but 99mTc(I) gained traction after the pioneering work of Alberto and colleagues, which resulted in the introduction of the organometallic core fac-[99mTc(CO)3(H2O)3]+. This core is readily available from [99mTcO4]- and displays three labile water molecules that can be easily swapped for ligands with different denticity and/or donor atoms in aqueous environment. This makes it possible to radiolabel small molecules as well as high molecular weight molecules, such as antibodies or other proteins, while assuring biological activity. Direct radiolabelling of those proteins with fac-[99mTc(CO)3]+ under mild conditions is accomplished through incorporation of a polyhistidine tag (His-tag), a commonly used tag for purification of recombinant proteins. This review aims to address the direct radiolabelling of His-tagged macromolecules with fac-[99mTc(CO)3]+ for development of molecular imaging agents and the impact of this technology in the discovery and development of imaging and/or therapeutic agents towards clinical application.

Strategies for the Nuclear Delivery of Metal Complexes to Cancer Cells

The nucleus is an essential organelle for the function of cells. It holds most of the genetic material and plays a crucial role in the regulation of cell growth and proliferation. Since many antitumoral therapies target nucleic acids to induce cell death, tumor-specific nuclear drug delivery could potentiate therapeutic effects and prevent potential off-target side effects on healthy tissue. Due to their great structural variety, good biocompatibility, and unique physico-chemical properties, organometallic complexes and other metal-based compounds have sparked great interest as promising anticancer agents. In this review, strategies for specific nuclear delivery of metal complexes are summarized and discussed to highlight crucial parameters to consider for the design of new metal complexes as anticancer drug candidates. Moreover, the existing opportunities and challenges of tumor-specific, nucleus-targeting metal complexes are emphasized to outline some new perspectives and help in the design of new cancer treatments.

Molecular and functional imaging in cancer-targeted therapy: current applications and future directions

Targeted anticancer drugs block cancer cell growth by interfering with specific signaling pathways vital to carcinogenesis and tumor growth rather than harming all rapidly dividing cells as in cytotoxic chemotherapy. The Response Evaluation Criteria in Solid Tumor (RECIST) system has been used to assess tumor response to therapy via changes in the size of target lesions as measured by calipers, conventional anatomically based imaging modalities such as computed tomography (CT), and magnetic resonance imaging (MRI), and other imaging methods. However, RECIST is sometimes inaccurate in assessing the efficacy of targeted therapy drugs because of the poor correlation between tumor size and treatment-induced tumor necrosis or shrinkage. This approach might also result in delayed identification of response when the therapy does confer a reduction in tumor size. Innovative molecular imaging techniques have rapidly gained importance in the dawning era of targeted therapy as they can visualize, characterize, and quantify biological processes at the cellular, subcellular, or even molecular level rather than at the anatomical level. This review summarizes different targeted cell signaling pathways, various molecular imaging techniques, and developed probes. Moreover, the application of molecular imaging for evaluating treatment response and related clinical outcome is also systematically outlined. In the future, more attention should be paid to promoting the clinical translation of molecular imaging in evaluating the sensitivity to targeted therapy with biocompatible probes. In particular, multimodal imaging technologies incorporating advanced artificial intelligence should be developed to comprehensively and accurately assess cancer-targeted therapy, in addition to RECIST-based methods.

Exploiting active nuclear import for efficient delivery of Auger electron emitters into the cell nucleus

BACKGROUND

The most attractive features of Auger electrons (AEs) in cancer therapy are their extremely short range and sufficiently high linear energy transfer (LET) for a majority of them. The cytotoxic effects of AE emitters can be realized only in close vicinity to sensitive cellular targets and they are negligible if the emitters are located outside the cell. The nucleus is considered the compartment most sensitive to high LET particles. Therefore, the use of AE emitters could be most useful in specific recognition of a cancer cell and delivery of AE emitters into its nucleus.

PURPOSE

This review describes the studies aimed at developing effective anticancer agents for the delivery of AE emitters to the nuclei of target cancer cells. The use of peptide-based conjugates, nanoparticles, recombinant proteins, and other constructs for AE emitter targeted intranuclear delivery as well as their advantages and limitations are discussed.

CONCLUSION

Transport from the cytoplasm to the nucleus along with binding to the cancer cell is one of the key stages in the delivery of AE emitters; therefore, several constructs for exploitation of this transport have been developed. The transport is carried out through a nuclear pore complex (NPC) with the use of specific amino acid nuclear localization sequences (NLS) and carrier proteins named importins, which are located in the cytosol. Therefore, the effectiveness of NLS-containing delivery constructs designed to provide energy-dependent transport of AE emitter into the nuclei of cancer cells also depends on their efficient entry into the cytosol of the target cell.

The Role of VEGF Receptors as Molecular Target in Nuclear Medicine for Cancer Diagnosis and Combination Therapy

Simple Summary

The rapid development of diagnostic and therapeutic methods of the cancer treatment causes that these diseases are becoming better known and the fight against them is more and more effective. Substantial contribution in this development has nuclear medicine that enables very early cancer diagnosis and early start of the so-called targeted therapy. This therapeutic concept compared to the currently used chemotherapy, causes much fewer undesirable side effects, due to targeting a specific lesion in the body. This review article discusses the possible applications of radionuclide-labelled tracers (peptides, antibodies or synthetic organic molecules) that can visualise cancer cells through pathological blood vessel system in close tumour microenvironment. Hence, at a very early step of oncological disease, targeted therapy can involve in tumour formation and growth.

Abstract

One approach to anticancer treatment is targeted anti-angiogenic therapy (AAT) based on prevention of blood vessel formation around the developing cancer cells. It is known that vascular endothelial growth factor (VEGF) and vascular endothelial growth factor receptors (VEGFRs) play a pivotal role in angiogenesis process; hence, application of angiogenesis inhibitors can be an effective approach in anticancer combination therapeutic strategies. Currently, several types of molecules have been utilised in targeted VEGF/VEGFR anticancer therapy, including human VEGF ligands themselves and their derivatives, anti-VEGF or anti-VEGFR monoclonal antibodies, VEGF binding peptides and small molecular inhibitors of VEGFR tyrosine kinases. These molecules labelled with diagnostic or therapeutic radionuclides can become, respectively, diagnostic or therapeutic receptor radiopharmaceuticals. In targeted anti-angiogenic therapy, diagnostic radioagents play a unique role, allowing the determination of the emerging tumour, to monitor the course of treatment, to predict the treatment outcomes and, first of all, to refer patients for AAT. This review provides an overview of design, synthesis and study of radiolabelled VEGF/VEGFR targeting and imaging agents to date. Additionally, we will briefly discuss their physicochemical properties and possible application in combination targeted radionuclide tumour therapy.

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

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

A stochastic cascade model for Auger-electron emitting radionuclides

To benchmark a Monte Carlo model of the Auger cascade that has been developed at the Australian National University (ANU) against the literature data. The model is applicable to any Auger-electron emitting radionuclide with nuclear structure data in the format of the Evaluated Nuclear Structure Data File (ENSDF). Schönfeld's algorithms and the BrIcc code were incorporated to obtain initial vacancy distributions due to electron capture (EC) and internal conversion (IC), respectively. Atomic transition probabilities were adopted from the Evaluated Atomic Data Library (EADL) for elements with atomic number, Z = 1-100. Atomic transition energies were evaluated using a relativistic Dirac-Fock method. An energy-restriction protocol was implemented to eliminate energetically forbidden transitions from the simulations. Calculated initial vacancy distributions and average energy spectra of ¹²³I, ¹²⁴I, and ¹²⁵I were compared with the literature data. In addition, simulated kinetic energy spectra and frequency distributions of the number of emitted electrons and photons of the three iodine radionuclides are presented. Some examples of radiation spectra of individual decays are also given. Good agreement with the published data was achieved except for the outer-shell Auger and Coster-Kronig transitions. Nevertheless, the model needs to be compared with experimental data in a future study.

New radiotracers for imaging of vascular targets in angiogenesis-related diseases

Tremendous advances over the last several decades in positron emission tomography (PET) and single photon emission computed tomography (SPECT) allow for targeted imaging of molecular and cellular events in the living systems. Angiogenesis, a multistep process regulated by the network of different angiogenic factors, has attracted world-wide interests, due to its pivotal role in the formation and progression of different diseases including cancer, cardiovascular diseases (CVD), and inflammation. In this review article, we will summarize the recent progress in PET or SPECT imaging of a wide variety of vascular targets in three major angiogenesis-related diseases: cancer, cardiovascular diseases, and inflammation. Faster drug development and patient stratification for a specific therapy will become possible with the facilitation of PET or SPECT imaging and it will be critical for the maximum benefit of patients.

Modular nanotransporters: a versatile approach for enhancing nuclear delivery and cytotoxicity of Auger electron-emitting 125I

Background

This study evaluates the potential utility of a modular nanotransporter (MNT) for enhancing the nuclear delivery and cytotoxicity of the Auger electron emitter ¹²⁵I in cancer cells that overexpress the epidermal growth factor receptor (EGFR).

Methods

MNTs are recombinant multifunctional polypeptides that we have developed for achieving selective delivery of short-range therapeutics into cancer cells. MNTs contain functional modules for receptor binding, internalization, endosomal escape and nuclear translocation, thereby facilitating the transport of drugs from the cell surface to the nucleus. The MNT described herein utilized EGF as the targeting ligand and was labeled with ¹²⁵I using N-succinimidyl-4-guanidinomethyl-3-[¹²⁵I]iodobenzoate (SGMIB). Membrane binding, intracellular and nuclear accumulation kinetics, and clonogenic survival assays were performed using the EGFR-expressing A431 epidermoid carcinoma and D247 MG glioma cell lines.

Results

[¹²⁵I]SGMIB-MNT bound to A431 and D247 MG cells with an affinity comparable to that of native EGF. More than 60% of internalized [¹²⁵I]SGMIB-MNT radioactivity accumulated in the cell nuclei after a 1-h incubation. The cytotoxic effectiveness of [¹²⁵I]SGMIB-MNT compared with ¹²⁵I-labeled bovine serum albumin control was enhanced by a factor of 60 for D247 MG cells and more than 1,000-fold for A431 cells, which express higher levels of EGFR.

Conclusions

MNT can be utilized to deliver ¹²⁵I into the nuclei of cancer cells overexpressing EGFR, significantly enhancing cytotoxicity. Further evaluation of [¹²⁵I]SGMIB-MNT as a targeted radiotherapeutic for EGFR-expressing cancer cells appears warranted.

Production and dosimetric aspects of the potent Auger emitter 58mCo for targeted radionuclide therapy of small tumors

PURPOSE

Based on theoretical calculations, the Auger emitter 58mCo has been identified as a potent nuclide for targeted radionuclide therapy of small tumors. During the production of this isotope, the coproduction of the long-lived ground state 58gCo is unfortunately unavoidable, as is ingrowth of the ground state following the isomeric decay of 58mCo. The impact of 58Co as a beta(+)- and gamma-emitting impurity should be included in the dosimetric analysis. The purpose of this study was to investigate this critical part of dosimetry based on experimentally determined production yields of 58mCo and 58gCo using a low-energy cyclotron. Also, the cellular S-values for 58mCo have been calculated and are presented here for the first time.

METHODS

58mCo was produced via the 58Fe(p,n)58mCo nuclear reaction on highly enriched 58Fe metal. In addition, radiochemical separations of produced radio-cobalt from natFe target material were performed. The theoretical subcellular dosimetry calculations for 58mCo and 58gCo were performed using the MIRD formalism, and the impact of the increasing ground state impurity on the tumor-to-normal-tissue dose ratios (TND) per disintegration as a function of time after end of bombardment (EOB) was calculated.

RESULTS

192 +/- 8 MBq of 58mCo was produced in the irradiation corresponding to a production yield of 10.7 MBq/microAh. The activity of 58gCo was measured to be 0.85% +/- 0.04% of the produced 58mCo activity at EOB. The radio-cobalt yields in the rapid separations were measured to be > 97% with no detectable iron contaminations in the cobalt fractions. Due to the unavoidable coproduction and ingrowth of the long-lived ground state 58gCo, the TND and the potency of the 58mCo decrease with time after EOB. If a future treatment with a 58mCo labeled compound is not initiated before, e.g., 21 h after EOB, the resulting TND will be approximately 50% of the TND of 'pure' 58mCo as a result of the increased normal tissue dose from the ground state.

CONCLUSIONS

The Auger emitter 58mCo is a potent radioisotope for targeted radionuclide therapy, and the production of therapeutic quantities should be achievable using a small biomedical cyclotron. However, the unavoidable coproduction and ingrowth of the long-lived ground state 58gCo requires fast radiochemical processing and use of future 58mCo-labeled radiopharmaceuticals in order to exploit the high achievable TND of 58mCo.

Nuclear targeting with cell-specific multifunctional tricarbonyl M(I) (M is Re, (99m)Tc) complexes: synthesis, characterization, and cell studies

Auger-emitting radionuclides such as (99m)Tc have been the focus of recent studies aiming at finding more selective therapeutic approaches. To explore the potential usefulness of (99m)Tc as an Auger emitter, we have synthesized and biologically evaluated novel multifunctional structures comprising (1) a pyrazolyl-diamine framework bearing a set of donor atoms to stabilize the [M(CO)(3)](+) (M is Re, (99m)Tc) core; (2) a DNA intercalating moiety of the acridine orange type to ensure close proximity of the radionuclide to DNA and to follow the internalization and subcellular trafficking of the compounds by confocal fluorescence microscopy; and (3) a bombesin (BBN) analogue of the type X-BBN[7-14] (where X is SGS, GGG) to provide specificity towards cells expressing the gastrin releasing peptide receptor (GRPr). Of the evaluated (99m)Tc complexes, Tc ( 3 ) containing the GGG-BBN[7-14] peptide showed the highest cellular internalization in GRPr-positive PC3 human prostate tumor cells, presenting a remarkably high nuclear uptake in the same cell line. Live-cell confocal imaging microscopy studies with the congener Re complex, Re ( 3 ), showed a considerable accumulation of fluorescence in the nucleus, with kinetics of uptake similar to that exhibited by Tc ( 3 ). Together, these data show that the acridine orange intercalator and the metal fragment are colocalized in the nucleus, which indicates that they remain connected despite the lysosomal degradation of Tc ( 3 )/Re ( 3 ). These compounds are the first examples of (99m)Tc bioconjugates that combine specific cell targeting with nuclear internalization, a crucial issue to explore use of (99m)Tc in Auger therapy.

Trifunctional 99mTc based radiopharmaceuticals: metal-mediated conjugation of a peptide with a nucleus targeting intercalator

The development of molecular imaging agents with multiple functions has become a major trend in radiopharmaceutical chemistry. We present herein the syntheses of trifunctional compounds, combining an acridine orange (AO) based intercalator with a GRP receptor specific bombesin like peptide (BBN). Metal-mediated conjugation of these two functions via the [2 + 1] approach to the third function, the [M(CO)(3)](+) (M = (99m)Tc, Re) moiety, yielded the final trifunctional molecules. The strongly fluorescent acridine orange, a nuclear targeting agent, has been derivatised with 4-imidazolecarboxylate as a bidentate ligand and bombesin with an isonitrile group as a monodentate ligand. For cell and nuclear uptake studies, [Re(L(1)-BBN)(L(2)-Ical)(CO)(3)] type complexes were synthesized and characterized. For radiopharmaceutical purposes, the (99m)Tc analogues have been prepared in a stepwise synthesis. Fluorescence microscopy studies on PC-3 cells, bearing the BBN receptor, showed high and rapid uptake into the cytoplasm. For the bifunctional molecule, lacking the BBN peptide, no internalization was observed.