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

PARP-Targeted Radiotheranostics with Auger Electrons: An Updated Overview

Auger electrons (AEs) represent an intriguing topic in the field of radionuclide therapy. They are emitted by several radionuclides commonly used in nuclear medicine (indium-111, iodine-123, iodine-125), allowing for highly localized energy deposition and thus exerting a radiotoxic effect on specific cellular and sub-cellular targets. However, due to their short range in matter, AEs have had limited use in therapeutic applications so far. In recent years, the synthesis of various radiopharmaceuticals capable of binding to the enzyme poly(ADP-ribose) polymerase 1 has reignited interest in this type of therapy, laying the groundwork for a theranostic approach based on radionuclides emitting AEs. The enzyme PARP-1 operates enzymatically in close proximity to DNA that represents the prime target of radionuclide therapies. Following this trend, several PARP-targeted radiopharmaceuticals for AE-based theranostics have been developed. We provide an updated overview of preclinical studies focused on the applications of this new theranostic approach in glioblastoma, breast, prostate and ovarian carcinoma, and pancreatic adenocarcinoma.

"Antibacterial properties of nine indium(III) complexes of substituted pyrazoles/pyrazolate and the structural and solution characterization of the mer- and trans‑indium(III) complexes of 4-Me-pzH"

Two indium(III) complexes of formula mer-[InIIICl3(4-Me-pzH)3] and trans-[InIIICl2(4-Me-pzH)4]Cl·(4-Me-pzH)2·(H2O) were isolated from the same reaction mixture and crystallographically characterized. The two complexes exist in dynamic equilibrium and their dynamic behavior was probed by variable temperature 1H NMR spectroscopy in the 202 to 296 K range. Powder X-ray diffraction of the batch confirmed existence of both complexes in a 1:2 ratio. Antibacterial properties of both new complexes, in addition to seven other previously published indium(III) complexes, were investigated against three Gram-positive and four Gram-negative pathogenic bacterial strains. The results showed potential for the development of indium(III)-based antipseudomonal and antituberculosis drugs, with mer-[InCl3(4-Ph-pzH)3] being especially effective.

DNA Repair Inhibitors: Potential Targets and Partners for Targeted Radionuclide Therapy

The present review aims to explore the potential targets/partners for future targeted radionuclide therapy (TRT) strategies, wherein cancer cells often are not killed effectively, despite receiving a high average tumor radiation dose. Here, we shall discuss the key factors in the cancer genome, especially those related to DNA damage response/repair and maintenance systems for escaping cell death in cancer cells. To overcome the current limitations of TRT effectiveness due to radiation/drug-tolerant cells and tumor heterogeneity, and to make TRT more effective, we propose that a promising strategy would be to target the DNA maintenance factors that are crucial for cancer survival. Considering their cancer-specific DNA damage response/repair ability and dysregulated transcription/epigenetic system, key factors such as PARP, ATM/ATR, amplified/overexpressed transcription factors, and DNA methyltransferases have the potential to be molecular targets for Auger electron therapy; moreover, their inhibition by non-radioactive molecules could be a partnering component for enhancing the therapeutic response of TRT.

Prospects of Using Protein Engineering for Selective Drug Delivery into a Specific Compartment of Target Cells

A large number of proteins are successfully used to treat various diseases. These include natural polypeptide hormones, their synthetic analogues, antibodies, antibody mimetics, enzymes, and other drugs based on them. Many of them are demanded in clinical settings and commercially successful, mainly for cancer treatment. The targets for most of the aforementioned drugs are located at the cell surface. Meanwhile, the vast majority of therapeutic targets, which are usually regulatory macromolecules, are located inside the cell. Traditional low molecular weight drugs freely penetrate all cells, causing side effects in non-target cells. In addition, it is often difficult to elaborate a small molecule that can specifically affect protein interactions. Modern technologies make it possible to obtain proteins capable of interacting with almost any target. However, proteins, like other macromolecules, cannot, as a rule, freely penetrate into the desired cellular compartment. Recent studies allow us to design multifunctional proteins that solve these problems. This review considers the scope of application of such artificial constructs for the targeted delivery of both protein-based and traditional low molecular weight drugs, the obstacles met on the way of their transport to the specified intracellular compartment of the target cells after their systemic bloodstream administration, and the means to overcome those difficulties.

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.

Determination of distribution coefficients of mercury and gold on selected extraction chromatographic resins - towards an improved separation method of mercury-197 from proton-irradiated gold targets

Radioisotope mercury-197g (^197gHg, half-life: 64.14 h) along with its metastable isomer (^197mHg, half-life: 23.8 h) are potential candidates for targeted Meitner-Auger electron therapy due to their suitable decay properties. Their production can be achieved via proton irradiation of a natural gold target, but the number of studies surrounding their separation from an irradiated gold target is limited. This study focuses on the determination of distribution coefficients (K_d) of gold (III) and mercury (II) on seven extraction chromatographic resins. Mercury K_d were measured by means of radiotracers and Inductively Coupled Plasma Mass Spectrometry (ICP_MS); values obtained from the two methods were generally in good agreement. These results can provide insight on Hg and Au chemistry and aid in the design of improved separation system(s).

Mouse Syngeneic Melanoma Model with Human Epidermal Growth Factor Receptor Expression

The development of epidermal growth factor receptor (EGFR)-targeting agents for the treatment of malignant melanoma requires cheap and easy animal tumor models for high-throughput in vivo screening. Thus, the aim of this study was to develop mouse syngeneic melanoma model that expresses human EGFR. Cloudman S91 clone M3 mouse melanoma cells were transduced with lentiviral particles carrying the human EGFR gene followed by a multistep selection process. The resulting M3-EGFR has been tested for EGFR expression and functionality in vitro and in vivo. Radioligand assay confirmed the presence of 13,900 ± 1500 EGF binding sites per cell at a dissociation constant of 5.3 ± 1.4 nM. M3-EGFR demonstrated the ability to bind and internalize specifically and provide the anticipated intracellular nuclear import of three different EGFR-targeted modular nanotransporters designed for specific anti-cancer drug delivery. Introduction of the human EGFR gene did not alter the tumorigenicity of the offspring M3-EGFR cells in host immunocompetent DBA/2J mice. Preservation of the expression of EGFR in vivo was confirmed by immunohistochemistry. To sum up, we successfully developed the first mouse syngeneic melanoma model with preserved in vivo expression of human EGFR.

Mononuclear, hexanuclear and polymeric indium(III) pyrazolido complexes; structural characterization, dynamic solution studies and luminescent properties

A new family of six mononuclear indium(III) complexes of formula mer-[In^IIICl₃(pz*H)₃]-pz*H = pyrazole (pzH), or substituted pyrazoles: 4-Cl-pzH, 4-Br-pzH, 4-I-pzH, 4-Ph-pzH and 3,5-Me₂-pzH-were synthesized by addition reactions of InCl₃ and pz*H and crystallographically characterized. The fluxional behaviour of the complexes, probed by variable temperature ¹H NMR spectroscopy in the 328 K to 173 K range, was attributed to (at least) four simultaneous processes: pyrazole N-H proton dissociation/association, cis/trans-pyrazole exchange, and N¹/N² tautomerization of the cis- and of the trans-pyrazoles. Three novel trianionic hexanuclear complexes of general formula (pipH)₃[In₆Cl₆(μ₃-OH_0.5)₂(μ-OH)₆(μ-pz*)₆]-pz* = pz, 4-Cl-pz and 4-Ph-pz-showing μ-hydroxo and μ-oxo bridges were synthesized from the corresponding mer-[In^IIICl₃(pz*H)₃] and characterized by single crystal X-ray diffraction and ¹H NMR. Under different solvent conditions, multicolour emitting polymeric complexes of general formula [In(μ-pz*)₃]_n-pz* = pz, 4-Cl-pz, 4-I-pz and 4-Ph-pz-were obtained also from mer-[In^IIICl₃(pz*H)₃] after addition of a base. Luminescence and lifetime calculations were performed for all polymers formed.

Post-genomic platform for development of oligonucleotide vaccines against RNA viruses: diamond cuts diamond

The coronavirus pandemic has starkly demonstrated the need to create highly effective vaccines against various viral diseases. The emerging new platforms for vaccine creation (adenovirus vectors and mRNA vaccines) have shown their worth in the fight against the prevention of coronavirus infection. However, adenovirus vectors and mRNA vaccines have a serious disadvantage: as a rule, only the S protein of the coronavirus is presented as an antigen. This tactic for preventing infection allows the ever-mutating virus to escape quickly from the immunity protection provided by such vaccines. Today, viral genomic databases are well-developed, which makes it possible to create new vaccines on a fundamentally new post-genomic platform. In addition, the technology for the synthesis of nucleic acids is currently experiencing an upsurge in demand in various fields of molecular biology. The accumulated experience suggests that the unique genomic sequences of viruses can act as antigens that trigger powerful humoral and cellular immunity. To achieve this effect, the following conditions must be created: the structure of the nucleic acid must be single-stranded, have a permanent 3D nanostructure, and have a unique sequence absent in the vaccinated organism. Oligonucleotide vaccines are able to resist the rapidly changing genomic sequences of RNA viruses by using conserved regions of their genomes to generate a long-term immune response, acting according to the adage that a diamond cuts a diamond. In addition, oligonucleotide vaccines will not contribute to antibody-dependent enhanced infection, since the nucleic acid of the coronavirus is inside the viral particle. It is obvious that new epidemics and pandemics caused by RNA viruses will continue to arise periodically in the human population. The creation of new, safe, and effective platforms for the production of vaccines that can flexibly change and adapt to new subtypes of viruses is very urgent and at this moment should be considered as a strategically necessary task.

Individualization of Radionuclide Therapies: Challenges and Prospects

Simple Summary

Currently, patient-specific treatment plans and dosimetry calculations are not routinely performed for radionuclide therapies. In external beam radiotherapy, it is quite the opposite. As a result, a small fraction of patients receives optimal radioactivity. This conservative approach provides “radiation safety” to healthy tissues but delivers a lower than indicated absorbed dose to the tumors, resulting in a lower response rate and a higher disease relapse rate. Evidence shows that better and more predictable outcomes can be achieved with patient-individualized dose assessment. Therefore, the incorporation of individual planning into radionuclide therapies is a high priority for nuclear medicine physicians and medical physicists alike. Internal dosimetry is used in tumor therapy to optimize the absorbed dose to the target tissue. The main reasons for the difficulties in incorporating patients’ internal dosimetry into routine clinical practice are discussed. The article presents the prospects for the routine implementation of personalized radionuclide therapies.

Abstract

The article presents the problems of clinical implementation of personalized radioisotope therapy. The use of radioactive drugs in the treatment of malignant and benign diseases is rapidly expanding. Currently, in the majority of nuclear medicine departments worldwide, patients receive standard activities of therapeutic radiopharmaceuticals. Intensively conducted clinical trials constantly provide more evidence of a close relationship between the dose of radiopharmaceutical absorbed in pathological tissues and the therapeutic effect of radioisotope therapy. Due to the lack of individual internal dosimetry (based on the quantitative analysis of a series of diagnostic images) before or during the treatment, only a small fraction of patients receives optimal radioactivity. The vast majority of patients receive too-low doses of ionizing radiation to the target tissues. This conservative approach provides “radiation safety” to healthy tissues, but also delivers lower radiopharmaceutical activity to the neoplastic tissue, resulting in a low level of response and a higher relapse rate. The article presents information on the currently used radionuclides in individual radioisotope therapies and on radionuclides newly introduced to the therapeutic market. It discusses the causes of difficulties with the implementation of individualized radioisotope therapies as well as possible changes in the current clinical situation.

Searching for a Paradigm Shift in Auger-Electron Cancer Therapy with Tumor-Specific Radiopeptides Targeting the Mitochondria and/or the Cell Nucleus

Although 99mTc is not an ideal Auger electron (AE) emitter for Targeted Radionuclide Therapy (TRT) due to its relatively low Auger electron yield, it can be considered a readily available “model” radionuclide useful to validate the design of new classes of AE-emitting radioconjugates. With this in mind, we performed a detailed study of the radiobiological effects and mechanisms of cell death induced by the dual-targeted radioconjugates 99mTc-TPP-BBN and 99mTc-AO-BBN (TPP = triphenylphosphonium; AO = acridine orange; BBN = bombesin derivative) in human prostate cancer PC3 cells. 99mTc-TPP-BBN and 99mTc-AO-BBN caused a remarkably high reduction of the survival of PC3 cells when compared with the single-targeted congener 99mTc-BBN, leading to an augmented formation of γH2AX foci and micronuclei. 99mTc-TPP-BBN also caused a reduction of the mtDNA copy number, although it enhanced the ATP production by PC3 cells. These differences can be attributed to the augmented uptake of 99mTc-TPP-BBN in the mitochondria and enhanced uptake of 99mTc-AO-BBN in the nucleus, allowing the irradiation of these radiosensitive organelles with the short path-length AEs emitted by 99mTc. In particular, the results obtained for 99mTc-TPP-BBN reinforce the relevance of targeting the mitochondria to promote stronger radiobiological effects by AE-emitting radioconjugates.

The Chemical Scaffold of Theranostic Radiopharmaceuticals: Radionuclide, Bifunctional Chelator, and Pharmacokinetics Modifying Linker

Therapeutic radiopharmaceuticals have been researched extensively in the last decade as a result of the growing research interest in personalized medicine to improve diagnostic accuracy and intensify intensive therapy while limiting side effects. Radiometal-based drugs are of substantial interest because of their greater versatility for clinical translation compared to non-metal radionuclides. This paper comprehensively discusses various components commonly used as chemical scaffolds to build radiopharmaceutical agents, i.e., radionuclides, pharmacokinetic-modifying linkers, and chelators, whose characteristics are explained and can be used as a guide for the researcher.

Auger Emitter Conjugated PARP Inhibitor for Therapy in Triple Negative Breast Cancers: A Comparative In-Vitro Study

Simple Summary

Triple negative breast cancer (TNBC) is an aggressive subtype of breast cancer, with a high recurrence rate. Since treatment of BRCA^mut TNBC patients with PARP inhibitor (PARPi), targeting the nuclear protein PARP1, shows varied responses, its therapeutic efficacy is currently evaluated in combination with chemotherapy. Auger emitters (AEs) are radionuclides that can cause DNA damage when delivered close to the DNA. Due to the nuclear location of PARP1, radiolabelling of PARPi with AEs provide an efficient nuclear delivery mechanism. This study shows the radiosynthesis of an AE radiolabelled PARPi ([¹²⁵I]-PARPi-01) and its therapeutic effect as monotherapy or in combination with chemotherapeutics in a panel of TNBC cell lines. We found that [¹²⁵I]-PARPi-01 efficiently induces DNA damage with therapeutic effect irrespective of BRCA mutation. All responsive cell lines have homologous recombination deficiency. Short pretreatment with doxorubicin significantly reduces clonogenic survival of both responsive and resistant cell lines.

Abstract

PARP1 inhibitors (PARPi) are currently approved for BRCA^mut metastatic breast cancer, but they have shown limited response in triple negative breast cancer (TNBC) patients. Combination of an Auger emitter with PARPis enables PARP inhibition and DNA strand break induction simultaneously. This will enhance cytotoxicity and additionally allow a theranostic approach. This study presents the radiosynthesis of the Auger emitter [¹²⁵I] coupled olaparib derivative: [¹²⁵I]-PARPi-01, and its therapeutic evaluation in a panel of TNBC cell lines. Specificity was tested by a blocking assay. DNA strand break induction was analysed by γH2AX immunofluorescence staining. Cell cycle analysis and apoptosis assays were studied using flow cytometry in TNBC cell lines (BRCA^wt/mut). Anchorage independent growth potential was evaluated using soft agar assay. [¹²⁵I]-PARPi-01 showed PARP1-specificity and higher cytotoxicity than olaparib in TNBC cell lines irrespective of BRCA their status. Cell lines harbouring DNA repair deficiency showed response to [¹²⁵I]-PARPi-01 monotherapy. Combined treatment with Dox-NP further enhanced therapeutic efficiency in metastatic resistant BRCA^wt cell lines. The clonogenic survival was significantly reduced after treatment with [¹²⁵I]-PARPi-01 in all TNBC lines investigated. Therapeutic efficacy was further enhanced after combined treatment with chemotherapeutics. [¹²⁵I]-PARPi-01 is a promising radiotherapeutic agent for low radiation dosages, and mono/combined therapies of TNBC.

Targeted Radionuclide Therapy Using Auger Electron Emitters: The Quest for the Right Vector and the Right Radionuclide

Auger electron emitters (AEEs) are attractive tools in targeted radionuclide therapy to specifically irradiate tumour cells while sparing healthy tissues. However, because of their short range, AEEs need to be brought close to sensitive targets, particularly nuclear DNA, and to a lower extent, cell membrane. Therefore, radioimmunoconjugates (RIC) have been developed for specific tumour cell targeting and transportation to the nucleus. Herein, we assessed, in A-431_CEA-luc and SK-OV-3_1B9 cancer cells that express low and high levels of HER2 receptors, two ¹¹¹In-RIC consisting of the anti-HER2 antibody trastuzumab conjugated to NLS or TAT peptides for nuclear delivery. We found that NLS and TAT peptides improved the nuclear uptake of ¹¹¹In-trastuzumab conjugates, but this effect was limited and non-specific. Moreover, it did not result in a drastic decrease of clonogenic survival. Indium-111 also contributed to non-specific cytotoxicity in vitro due to conversion electrons (30% of the cell killing). Comparison with [¹²⁵I]I-UdR showed that the energy released in the cell nucleus by increasing the RIC’s nuclear uptake or by choosing an AEE that releases more energy per decay should be 5 to 10 times higher to observe a significant therapeutic effect. Therefore, new Auger-based radiopharmaceuticals need to be developed.

In Vitro Evaluation of No-Carrier-Added Radiolabeled Cisplatin ([189, 191Pt]cisplatin) Emitting Auger Electrons

Due to their short-range (2–500 nm), Auger electrons (Auger e⁻) have the potential to induce nano-scale physiochemical damage to biomolecules. Although DNA is the primary target of Auger e⁻, it remains challenging to maximize the interaction between Auger e⁻ and DNA. To assess the DNA-damaging effect of Auger e⁻ released as close as possible to DNA without chemical damage, we radio-synthesized no-carrier-added (n.c.a.) [^{189, 191}Pt]cisplatin and evaluated both its in vitro properties and DNA-damaging effect. Cellular uptake, intracellular distribution, and DNA binding were investigated, and DNA double-strand breaks (DSBs) were evaluated by immunofluorescence staining of γH2AX and gel electrophoresis of plasmid DNA. Approximately 20% of intracellular radio-Pt was in a nucleus, and about 2% of intra-nucleus radio-Pt bound to DNA, although uptake of n.c.a. radio-cisplatin was low (0.6% incubated dose after 25-h incubation), resulting in the frequency of cells with γH2AX foci was low (1%). Nevertheless, some cells treated with radio-cisplatin had γH2AX aggregates unlike non-radioactive cisplatin. These findings suggest n.c.a. radio-cisplatin binding to DNA causes severe DSBs by the release of Auger e⁻ very close to DNA without chemical damage by carriers. Efficient radio-drug delivery to DNA is necessary for successful clinical application of Auger e⁻.

Synthesis of no-carrier-added [188, 189, 191Pt]cisplatin from a cyclotron produced 188, 189, 191PtCl42- complex

We developed a novel method for production of no-carrier-added (n.c.a.) [^{188, 189, 191}Pt]Pt^IICl₄^2- from an Ir target material, and then synthesized n.c.a. [*Pt]cis-[Pt^IICl₂(NH₃)₂] ([*Pt]cisplatin) from [*Pt]Pt^IICl₄^2-. [*Pt]Pt^IICl₄^2- was prepared as a synthetic precursor of n.c.a. *Pt complex by a combination of resin extraction and anion-exchange chromatography after the selective reduction of Ir^IVCl₆^2- with ascorbic acid. The ligand-substitution reaction of Cl with NH₃ was promoted by treating n.c.a. [*Pt]Pt^IICl₄^2- with excess NH₃ and heating the reaction mixture, and n.c.a. [*Pt]cisplatin was successfully produced without employing precipitation routes. After this treatment, [*Pt]cisplatin was isolated through preparative HPLC with a radiochemical purity of 99 + % at the end of synthesis (EOS).

Epidermal Growth Factor Receptor: Key to Selective Intracellular Delivery

Epidermal growth factor receptor (EGFR) is an integral surface protein mediating cellular response to a number of growth factors. Its overexpression and increased activation due to mutations is one of the most common traits of many types of cancer. Development and clinical use of the agents, which block EGFR activation, became a prime example of the personalized targeted medicine. However, despite the obvious success in this area, cancer cure remains unattainable in most cases. Because of that, as well as the result of the search for possible ways to overcome the difficulties of treatment, a huge number of new treatment methods relying on the use of EGFR overexpression and its changes to destroy cancer cells. Modern data on the structure, functioning, and intracellular transport of EGFR, its natural ligands, as well as signaling cascades triggered by the EGFR activation, peculiarities of the EGFR expression and activation in oncological disorders, as well as applied therapeutic approaches aimed at blocking EGFR signaling pathway are summarized and analyzed in this review. Approaches to the targeted delivery of various chemotherapeutic agents, radionuclides, immunotoxins, photosensitizers, as well as the prospects for gene therapy aimed at cancer cells with EGFR overexpression are reviewed in detail. It should be noted that increasing attention is being paid nowadays to the development of multifunctional systems, either carrying several different active agents, or possessing several environment-dependent transport functions. Potentials of the systems based on receptor-mediated endocytosis of EGFR and their possible advantages and limitations are discussed.

Potent candidates for Targeted Auger Therapy: Production and radiochemical considerations

Targeted Auger Therapy represents great potential for the therapy of diseases which require a high degree of selectivity on the cellular level (e.g. for therapy of metastatic cancers). Due to their high Linear Energy Transfer (LET), Auger emitters, combined with selective biological systems which enable delivery of radionuclides close to the DNA of the targeting cell, can be extremely selective and powerful treatment tools. There are two main aspects associated with the development of efficient radiopharmaceuticals based on Auger Emitters: a) the availability of suitable Auger-emitting radionuclides for therapy and b) the design of targeting vectors which can deliver Auger emitters into/close to the nucleus. In the present review, we address the first aspect by defining important parameters for the selection of radionuclides for application to Targeted Auger Therapy and form a categorized list of the most promising radionuclides, their possible production routes, and their use in the synthesis of radiopharmaceuticals.

Use of radioiodine in nuclear medicine-A brief overview

Radioiodines have a long history in nuclear medicine. Herein, we discuss the production, properties and applications of these versatile iodine-based imaging and theragnostic agents. There are 38 isotopes of iodine (I) including one stable form (127 I). The most common radionuclides used in medical imaging and treatment, including Iodine-123 (123 I), Iodine-124 (124 I), Iodine-125 (125 I) and Iodine-131 (131 I), are discussed in this review.

Meitner-Auger Electron Emitters for Targeted Radionuclide Therapy: Mercury-197m/g and Antimony-119

Targeted Radionuclide Therapies (TRTs) based on Auger emitting radionuclides have the potential to deliver extremely selective therapeutic payloads on the cellular level. However, to fully exploit this potential, suitable radionuclides need to be applied in combination with appropriate delivery systems. In this review, we summarize the state-of-the-art production, purification, chelation and applications of two promising candidates for Targeted Auger Therapy, namely antimony- 119 (119Sb) and mercury-197 (197Hg). Both radionuclides have great potential to become efficient tools for TRT. We also highlight our current progress on the production of both radionuclides at TRIUMF and the University of Wisconsin.

Conventional Nanosized Drug Delivery Systems for Cancer Applications

Clinical responses and tolerability of conventional nanocarriers (NCs) are sometimes different from those expected in anticancer therapy. Thus, new smart drug delivery systems (DDSs) with stimuli-responsive properties and novel materials have been developed. Several clinical trials demonstrated that these DDSs have better clinical therapeutic efficacy in the treatment of many cancers than free drugs. Composition of DDSs and their surface properties increase the specific targeting of therapeutics versus cancer cells, without affecting healthy tissues, and thus limiting their toxicity versus unspecific tissues. Herein, an extensive revision of literature on NCs used as DDSs for cancer applications has been performed using the available bibliographic databases.

Low-resolution structures of modular nanotransporters shed light on their functional activity

Modular nanotransporters (MNTs) are multifunctional chimeric polypeptides for the multistep transport of locally acting cytotoxic agents into the nuclei of cancer target cells. MNTs consist of several polypeptide domains (functional modules) for the recognition of a cell-surface internalizable receptor, pH-dependent endosomal escape and subsequent transport into the nucleus through the nuclear pores. MNTs are a promising means for cancer treatment. As has been shown previously, all of the modules of MNTs retain their functionalities. Despite their importance, there is no structural information available about these chimeric polypeptides, which hampers the creation of new MNT variants. Here, a low-resolution 3D structure of an MNT is presented which was obtained by atomic force microscopy, transmission electron microscopy and small-angle X-ray scattering coupled to size-exclusion chromatography. The data suggest that the MNT can adopt two main conformations, but in both conformations the protein N- and C-termini are distanced and do not influence each other. The change in the MNT conformation during acidification of the medium was also studied. It was shown that the fraction of the elongated conformation increases upon acidification. The results of this work will be useful for the development of MNTs that are suitable for clinical trials and possible therapeutic applications.

Combination Strategies to Improve Targeted Radionuclide Therapy

In recent years, targeted radionuclide therapy (TRT) has emerged as a promising strategy for cancer treatment. In contrast to conventional radiotherapy, TRT delivers ionizing radiation to tumors in a targeted manner, reducing the dose that healthy tissues are exposed to. Existing TRT strategies include the use of 177Lu-DOTATATE, 131I-metaiodobenzylguanidine, Bexxar, and Zevalin, clinically approved agents for the treatment of neuroendocrine tumors, neuroblastoma, and non-Hodgkin lymphoma, respectively. Although promising results have been obtained with these agents, clinical evidence acquired to date suggests that only a small percentage of patients achieves complete response. Consequently, there have been attempts to improve TRT outcomes through combinations with other therapeutic agents; such strategies include administering concurrent TRT and chemotherapy, and the use of TRT with known or putative radiosensitizers such as poly(adenosine diphosphate ribose) polymerase and mammalian-target-of-rapamycin inhibitors. In addition to potentially achieving greater therapeutic effects than the respective monotherapies, these strategies may lead to lower dosages or numbers of cycles required and, in turn, reduce unwanted toxicities. As of now, several clinical trials have been conducted to assess the benefits of TRT-based combination therapies, sometimes despite limited preclinical evidence being available in the public domain to support their use. Although some clinical trials have yielded promising results, others have shown no clear survival benefit from particular combination treatments. Here, we present a comprehensive review of combination strategies with TRT reported in the literature to date and evaluate their therapeutic potential.

Advancements in the use of Auger electrons in science and medicine during the period 2015-2019

Auger electrons can be highly radiotoxic when they are used to irradiate specific molecular sites. This has spurred basic science investigations of their radiobiological effects and clinical investigations of their potential for therapy. Focused symposia on the biophysical aspects of Auger processes have been held quadrennially. This 9th International Symposium on Physical, Molecular, Cellular, and Medical Aspects of Auger Processes at Oxford University brought together scientists from many different fields to review past findings, discuss the latest studies, and plot the future work to be done. This review article examines the research in this field that was published during the years 2015-2019 which corresponds to the period since the last meeting in Japan. In addition, this article points to future work yet to be done. There have been a plethora of advancements in our understanding of Auger processes. These advancements range from basic atomic and molecular physics to new ways to implement Auger electron emitters in radiopharmaceutical therapy. The highly localized doses of radiation that are deposited within a 10 nm of the decay site make them precision tools for discovery across the physical, chemical, biological, and medical sciences.

The Delivery of Biologically Active Agents into the Nuclei of Target Cells for the Purposes of Translational Medicine

Development of vehicles for the subcellular targeted delivery of biologically active agents is very promising for the purposes of translational medicine. This review summarizes the results obtained by researchers from the Laboratory of Molecular Genetics of Intracellular Transport, Institute of Gene Biology RAS, which allowed them to design the core technology: modular nanotransporters. This approach ensures high efficacy and cell specificity for different anti-cancer agents, as they are delivered into the most vulnerable subcellular compartment within the cells of interest and makes it possible for antibody mimetics to penetrate into a compartment of interest within the target cells ("diving antibodies"). Furthermore, polyplexes, complexes of polycationic block copolymers of DNA, have been developed and characterized. These complexes are efficient both in vitro and in vivo and demonstrate predominant transfection of actively dividing cells.

Membrane Trafficking and Subcellular Drug Targeting Pathways

The movement of micro and macro molecules into and within a cell significantly governs several of their pharmacokinetic and pharmacodynamic parameters, thus regulating the cellular response to exogenous and endogenous stimuli. Trafficking of various pharmacological agents and other bioactive molecules throughout and within the cell is necessary for the fidelity of the cells but has been poorly investigated. Novel strategies against cancer and microbial infections need a deeper understanding of membrane as well as subcellular trafficking pathways and essentially regulate several aspects of the initiation and spread of anti-microbial and anti-cancer drug resistance. Furthermore, in order to avail the maximum possible bioavailability and therapeutic efficacy and to restrict the unwanted toxicity of pharmacological bioactives, these sometimes need to be functionalized with targeting ligands to regulate the subcellular trafficking and to enhance the localization. In the recent past the scenario drug targeting has primarily focused on targeting tissue components and cell vicinities, however, it is the membranous and subcellular trafficking system that directs the molecules to plausible locations. The effectiveness of the delivery platforms largely depends on their physicochemical nature, intracellular barriers, and biodistribution of the drugs, pharmacokinetics and pharmacodynamic paradigms. Most subcellular organelles possess some peculiar characteristics by which membranous and subcellular targeting can be manipulated, such as negative transmembrane potential in mitochondria, intraluminal delta pH in a lysosome, and many others. Many specialized methods, which positively promote the subcellular targeting and restrict the off-targeting of the bioactive molecules, exist. Recent advancements in designing the carrier molecules enable the handling of membrane trafficking to facilitate the delivery of active compounds to subcellular localizations. This review aims to cover membrane trafficking pathways which promote the delivery of the active molecule in to the subcellular locations, the associated pathways of the subcellular drug delivery system, and the role of the carrier system in drug delivery techniques.

New Recombinant Carriers Binding Specifically to the Epidermal Growth Factor Receptor

New recombinant carriers-modular nanotransporters (MNTs)-with N-terminal ligand module to the epidermal growth factor receptor (EGFR) were developed and characterized. Human epidermal growth factor (hEGF) and antibody-like protein Z₁₉₀₇ were used as a ligand module. We demonstrated that MNTs are able to internalize in a receptor-specific manner into the target cancer cells and to accumulate in the target cell nuclei. Conjugation of MNTs with the Auger electron emitter ¹¹¹In significantly enhanced the cytotoxic effect of ¹¹¹In on the target cells. It was found that the transfer of EGF from the C-terminus to the N-terminus of the MNT enhanced the proliferation of target cells, whereas the use of Z₁₉₀₇ did not have a similar effect.

Stabilization of Modular Nanotransporters by Embedding Hemin in Them in a New Strain with Heme Receptor Expression

It was found that the use of a new strain-producer Escherichia coli, expressing the heme receptor ChuA, enables obtaining a hemin-containing modular nanotransporter (MNT) for drug delivery into the nuclei of target cells. The hemin-containing MNT becomes stabilized, which leads to an increase in its thermal stability and prevents aggregation of this protein.

Targeted Delivery of 111In Into the Nuclei of EGFR Overexpressing Cells via Modular Nanotransporters With Anti-EGFR Affibody

Since cell nucleus is one of the most vulnerable compartments, the maximum therapeutic effect from a variety of locally acting agents, such as photosensitizers, alfa-emitters, Auger electron emitters, will be expected when they get there. Therefore, the targeted delivery of these agents into the nuclei of target tumor cells is necessary for their anticancer effects and minimization of side effects. Modular nanotransporters (MNT) are artificial polypeptides comprising several predefined modules that recognize target cell, launching their subsequent internalization, escape from endosomes, and transport the drug load to the nucleus. This technology significantly enhances the cytotoxicity of locally acting drugs in vitro and in vivo. Epidermal growth factor receptors (EGFR) are useful molecular targets as they are overexpressed in glioblastoma, head-and-neck cancer, bladder cancer, and other malignancies. Here, we examined the possibility of using internalizable anti-EGFR affibody as an EGFR-targeting MNT module for drug transport into the cancer cell nuclei. It binds to both murine and human EGFR facilitating preclinical studies. We showed that MNT with affibody on the N-terminus (MNT_N-affibody) effectively delivered the Auger electron emitter ¹¹¹In to target cell nuclei and had pronounced cytotoxic efficacy against EGFR-overexpressing human A431 epidermoid carcinoma cells. Using EGFR-expressing human adenocarcinoma MCF-7 cells, we demonstrated that in contrast to MNT with N-terminal epidermal growth factor (EGF), MNT_N-affibody and MNT with EGF on the C-terminus did not stimulate cancer cell proliferation.

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.