Remote-loading of liposomes with manganese-52 and in vivo evaluation of the stabilities of 52Mn-DOTA and 64Cu-DOTA using radiolabelled liposomes and PET imaging

Radiochemistry and Complex Formation of the Cyclen-Derived Chelator DOTI-Me with Mn2+, Cu2+, Zn2+, Ga3+, In3+, Tb3+, and Lu3

In this work, we describe the complex formation and radiochemistry of the cyclen-based chelator DOTI-Me bearing four methylimidazole arms. Radiolabeling properties were evaluated for 52gMn, 64Cu, 68Ga, 111In, 161Tb, and 177Lu, and DOTI-Me showed distinct differences to the structurally related H4DOTA. While radiochemical conversions (RCCs) for 52gMn and 111In were comparable to those of H4DOTA, DOTI-Me was not suited for 68Ga. Conversely, quantitative RCCs were achieved for 64Cu at ambient temperature, while elevated temperatures were required for complexation with H4DOTA. For 161Tb and 177Lu, good but not quantitative RCCs were obtained with DOTI-Me. With the exemption of 68Ga3+, radiolabeled complexes showed high stability in ligand challenge experiments and in human serum. X-ray analysis of the nonradioactive complexes revealed the formation of 8-coordinate Mn2+ and In3+ DOTI-Me complexes. Cu2+ adopted a unique distorted square-pyramidal 2 + 3 with the neutral DOTI-Me ligand and a Jahn-Teller distorted 4 + 2 coordination geometry for the diprotonated H2DOTI-Me2+ cation, respectively. For Zn2+, the complex with HDOTI-Me+ showed a distorted 4 + 3 pentagonal bipyramidal geometry. Summarizing, the ligand DOTI-Me may be an interesting alternative to H4DOTA for 52gMn, 64Cu, 111In, 161Tb, and 177Lu, covering diagnostic as well as therapeutic radionuclides. Further studies of targeted radiopharmaceuticals based on the DOTI-Me scaffold in combination with the set of radiometals presented herein are thus warranted.

A Dual-Recognition Fluorescence Enzyme-Linked Immunosorbent Assay for Specific Detection of Intact Lipid Nanoparticles via a Localized Scaffolding Autocatalytic DNA Circuit Amplifier

Lipid nanoparticles (LNPs) are emerging as one of the most promising drug delivery systems. The long-circulating effect of intact LNPs (i-LNPs) is the key to efficacy and toxicity in vivo. However, the significant challenge is specific and sensitive detection of i-LNPs. Herein, a dual-recognition fluorescence enzyme-linked immunosorbent assay (DR-FELISA) was developed to directly isolate and detect i-LNPs by combining dual-recognition separation with a one-step signal amplification strategy. The microplates captured and enriched i-LNPs through antibody-antigen reaction. Dual-chol probes were spontaneously introduced into the lipid bilayer of captured i-LNPs, converting the detection of i-LNPs into the detection of double-cholesterol probes. Finally, the end of the dual-chol probes initiated the localized scaffolding autocatalytic DNA circuits (SADC) system for further signal amplification. The SADC system provides a sensitive and efficient amplifier through localized network structures and self-assembled triggers. Simultaneous recognition of i-LNPs surface PEG-lipid and lipid bilayer structures significantly eliminates interference from biological samples. i-LNPs were detected with high selectivity, ranging from 0.2 to 1.25 mg/mL with a limit of detection of 0.1 mg/mL. Moreover, this method allows the isolation and quantitative analysis of different formulations of i-LNPs in serum samples with a satisfactory recovery rate ranging from 94.8 to 116.3%. Thus, the DR-FELISA method provides an advanced platform for the exclusive and sensitive detection of i-LNPs, providing new insights for the study of the quality and intracorporal process of complex formulations.

The Influence of Size on the Intracranial Distribution of Biomedical Nanoparticles Administered by Convection-enhanced Delivery in Minipigs

Because of the blood-brain barrier (BBB), successful drug delivery to the brain has long been a key objective for the medical community, calling for pioneering technologies to overcome this challenge. Convection-enhanced delivery (CED), a form of direct intraparenchymal microinfusion, shows promise but requires optimal infusate design and real-time distribution monitoring. The size of the infused substances appears to be especially critical, with current knowledge being limited. Herein, we examined the intracranial administration of polyethylene glycol (PEG)-coated nanoparticles (NPs) of various sizes using CED in groups of healthy minipigs (n = 3). We employed stealth liposomes (LIPs, 130 nm) and two gold nanoparticle designs (AuNPs) of different diameters (8 and 40 nm). All were labeled with copper-64 for quantitative and real-time monitoring of the infusion via positron emission tomography (PET). NPs were infused via two catheters inserted bilaterally in the putaminal regions of the animals. Our results suggest CED with NPs holds promise for precise brain drug delivery, with larger LIPs exhibiting superior distribution volumes and intracranial retention over smaller AuNPs. PET imaging alongside CED enabled dynamic visualization of the process, target coverage, timely detection of suboptimal infusion, and quantification of distribution volumes and concentration gradients. These findings may augment the therapeutic efficacy of the delivery procedure while mitigating unwarranted side effects associated with nonvisually monitored delivery approaches. This is of vital importance, especially for chronic intermittent infusions through implanted catheters, as this information enables informed decisions for modulating targeted infusion volumes on a catheter-by-catheter, patient-by-patient basis.

[52Mn]Mn-BPPA-Trastuzumab: A Promising HER2-Specific PET Radiotracer

This study aimed to develop a novel radiotracer using trastuzumab and the long-lived [52Mn]Mn isotope for HER2-targeted therapy selection and monitoring. A new Mn(II) chelator, BPPA, synthesized from a rigid bispyclen platform possessing a picolinate pendant arm, formed a stable and inert Mn(II) complex with favorable relaxation properties. BPPA was converted into a bifunctional chelator (BFC), conjugated to trastuzumab, and labeled with [52Mn]Mn isotope. In comparison to DOTA-GA-trastuzumab, the BPPA-trastuzumab conjugate exhibits a labeling efficiency with [52Mn]Mn approximately 2 orders of magnitude higher. In female CB17 SCID mice bearing 4T1 (HER2-) and MDA-MB-HER2+ (HER2+) xenografts, [52Mn]Mn-BPPA-trastuzumab demonstrated superior uptake in HER2+ cells on day 3, with a 3-4 fold difference observed on day 7. Overall, the hexadentate BPPA chelator proves to be exceptional in binding Mn(II). Upon coupling with trastuzumab as a BFC ligand, it becomes an excellent imaging probe for HER2-positive tumors. [52Mn]Mn-BPPA-trastuzumab enables an extended imaging time window and earlier detection of HER2-positive tumors with superior tumor-to-background contrast.

Nano-therapeutics: The upcoming nanomedicine to treat cancer

Nanotechnology is considered a successful approach for cancer diagnosis and treatment. Preferentially, cancer cell recognition and drug targeting via nano-delivery system include the penetration of anticancer agents into the cell membrane to damage the cancer cell by protein modification, DNA oxidation, or mitochondrial dysfunction. The past research on nano-delivery systems and their target has proven the beneficial achievement in a malignant tumor. Modern perceptions using inventive nanomaterials for cancer management have been offered by a multifunctional platform based on various nano-carriers with the probability of imaging and cancer therapy simultaneously. Emerging nano-delivery systems in cancer therapy still lack knowledge of the biological functions behind the interaction between nanoparticles and cancer cells. Since the potential of engineered nanoparticles addresses the various challenges, limiting the success of cancer therapy subsequently, it is a must to review the molecular targeting of a nano-delivery system to enhance the therapeutic efficacy of cancer. This review focuses on using a nano-delivery system, an imaging system, and encapsulated nanoparticles for cancer therapy.

A Targeted Multi-Crystalline Manganese Oxide as a Tumor-Selective Nano-Sized MRI Contrast Agent for Early and Accurate Diagnosis of Tumors

Introduction

Magnetic resonance imaging (MRI) is an important tool for the accurate diagnosis of malignant tumors in clinical settings. However, the lack of tumor-specific MRI contrast agents limits diagnostic accuracy.

Methods

Herein, we developed αv integrin receptor-targeting multi-crystalline manganese oxide (MCMO) as a novel MRI contrast agent for accurate diagnosis of tumors by coupling iRGD cyclopeptide PEGylation polymer onto the surface of MCMO (iRGD-pMCMO).

Results

The MCMO consisted of numerous small crystals and exhibited an oval structure of 200 nm in size. The iRGD-pMCMO actively recognizes tumor cells and effectively accumulates at the tumor site, consequently releasing abundant Mn2+ ions in a weakly acidic and high-GSH-expressing tumor microenvironment. Subsequently, Mn2+ ions interact with cellular GSH to form Mn-GSH chelates, enabling efficient T1-weighted MR contrast imaging. In vivo experiments indicated that iRGD-pMCMO significantly improved T1-weighted images, achieving an accurate diagnosis of subcutaneous and orthotopic tumors. The results verified that the T1 contrast effect of iRGD-pMCMO was closely associated with the expression of GSH in tumor cells.

Conclusion

Altogether, the novel tumor-targeting, highly sensitive MRI contrast agent developed in this study can improve the accuracy of MRI for tumor diagnosis.

Chelation chemistry of manganese-52 for PET imaging applications

INTRODUCTION

Due to its decay and chemical properties, interest in manganese-52 has increased for development of long-lived PET radiopharmaceuticals. Its long half-life of 5.6 days, low average positron energy (242 keV), and sufficient positron decay branching ratio make it suitable for radiolabeling macromolecules for investigating slow biological processes. This work aims to establish suitable chelators for manganese-52 that can be radiolabeled at mild conditions through the evaluation of commercially available chelators.

METHODS

Manganese-52 was produced through the nuclear reaction NatCr(p,n)52Mn by irradiation of natural chromium targets on a TR24 cyclotron followed by purification through ion exchange chromatography. The radiolabeling efficiencies of chelators: DOTA, DiAmsar, TETA, DO3A, NOTA, 4'-Formylbenzo-15-crown-5, Oxo-DO3A, and DFO, were assessed by investigating the impact of pH, buffer type, and temperature. In vitro stability of [52Mn]Mn(DO3A)-, [52Mn]Mn(Oxo-DO3A)-, and [52Mn]Mn(DOTA)2- were evaluated in mouse serum. The radiocomplexes were also evaluated in vivo in mice. Crystals of [Mn(Oxo-DO3A)]- were synthesized by reacting Oxo-DO3A with MnCl2 and characterized by single crystal X-ray diffraction.

RESULTS

Yields of 185 ± 19 MBq (5.0 ± 0.5 mCi) (n = 4) of manganese-52 were produced at the end of a 4 h, 15 μA, bombardment with 12.5 MeV protons. NOTA, DO3A, DOTA, and Oxo-DO3A chelators were readily radiolabeled with >96 % radiochemical purity at all conditions. Manganese radiocomplexes of Oxo-DO3A, DOTA, and DO3A remained stable in vitro up to 5 days and exhibited different biodistribution profiles compared to [52Mn]MnCl2. The solid-state structure of Mn-Oxo-DO3A complex was determined by single-crystal X-ray diffraction.

CONCLUSIONS

DO3A and Oxo-DO3A are suitable chelators for manganese-52 which are readily radiolabeled at mild conditions with high molar activity, and demonstrate both in vitro and in vivo stability.

Nanoparticles labeled with gamma-emitting radioisotopes: an attractive approach for in vivo tracking using SPECT imaging

Providing accurate molecular imaging of the body and biological process is critical for diagnosing disease and personalizing treatment with the minimum side effects. Recently, diagnostic radiopharmaceuticals have gained more attention in precise molecular imaging due to their high sensitivity and appropriate tissue penetration depth. The fate of these radiopharmaceuticals throughout the body can be traced using nuclear imaging systems, including single-photon emission computed tomography (SPECT) and positron emission tomography (PET) modalities. In this regard, nanoparticles are attractive platforms for delivering radionuclides into targets because they can directly interfere with the cell membranes and subcellular organelles. Moreover, applying radiolabeled nanomaterials can decrease their toxicity concerns because radiopharmaceuticals are usually administrated at low doses. Therefore, incorporating gamma-emitting radionuclides into nanomaterials can provide imaging probes with valuable additional properties compared to the other carriers. Herein, we aim to review (1) the gamma-emitting radionuclides used for labeling different nanomaterials, (2) the approaches and conditions adopted for their radiolabeling, and (3) their application. This study can help researchers to compare different radiolabeling methods in terms of stability and efficiency and choose the best way for each nanosystem.

Radiolabeled Liposomes for Nuclear Imaging Probes

Quantitative nuclear imaging techniques are in high demand for various disease diagnostics and cancer theranostics. The non-invasive imaging modality requires radiotracing through the radioactive decay emission of the radionuclide. Current preclinical and clinical radiotracers, so-called nuclear imaging probes, are radioisotope-labeled small molecules. Liposomal radiotracers have been rapidly developing as novel nuclear imaging probes. The physicochemical properties and structural characteristics of liposomes have been elucidated to address their long circulation and stability as radiopharmaceuticals. Various radiolabeling methods for synthesizing radionuclides onto liposomes and synthesis strategies have been summarized to render them biocompatible and enable specific targeting. Through a variety of radionuclide labeling methods, radiolabeled liposomes for use as nuclear imaging probes can be obtained for in vivo biodistribution and specific targeting studies. The advantages of radiolabeled liposomes including their use as potential clinical nuclear imaging probes have been highlighted. This review is a comprehensive overview of all recently published liposomal SPECT and PET imaging probes.

Nanoparticles and Radioisotopes: A Long Story in a Nutshell

The purpose of this narrative review was to assess the use of nanoparticles (NPs) to deliver radionuclides to targets, focusing on systems that have been tested in pre-clinical and, when available, clinical settings. A literature search was conducted in PubMed and Web of Science databases using the following terms: “radionuclides” AND “liposomes” or “PLGA nanoparticles” or “gold nanoparticles” or “iron oxide nanoparticles” or “silica nanoparticles” or “micelles” or “dendrimers”. No filters were applied, apart from a minimum limit of 10 patients enrolled for clinical studies. Data from some significant studies from pre-clinical and clinical settings were retrieved, and we briefly describe the information available. All the selected seven classes of nanoparticles were highly tested in clinical trials, but they all present many drawbacks. Liposomes are the only ones that have been tested for clinical applications, though they have never been commercialized. In conclusion, the application of NPs for imaging has been the object of much interest over the years, albeit mainly in pre-clinical settings. Thus, we think that, based on the current state, radiolabeled NPs must be investigated longer before finding their place in nuclear medicine.

Ligand conjugated lipid-based nanocarriers for cancer theranostics

Cancer is one of the major health-related issues affecting the population worldwide and subsequently accounts for the second-largest death. Genetic and epigenetic modifications in oncogenes or tumor suppressor genes affect the regulatory systems that lead to the initiation and progression of cancer. Conventional methods, including chemotherapy/radiotherapy/appropriate combinational therapy and surgery, are being widely used for theranostics of cancer patients. Surgery is useful in treating localized tumors, but it is ineffective in treating metastatic tumors, which spread to other organs and result in a high recurrence rate and death. Also, the therapeutic application of free drugs is related to substantial issues such as poor absorption, solubility, bioavailability, high degradation rate, short shelf-life, and low therapeutic index. Therefore, these issues can be sorted out using nano lipid-based carriers (NLBCs) as promising drug delivery carriers. Still, at most, they fail to achieve site targeted drug delivery and detection. This can be achieved by selecting a specific ligand/antibody for its cognate receptor molecule expressed on the surface of cancer cell. In this review, we have mainly discussed the various types of ligands used to decorate NLBCs. A list of the ligands used to design nanocarriers to target malignant cells has been extensively undertaken. The approved ligand decorated lipid-based nanomedicines with their clinical status has been explained in tabulated form to provide a wider scope to the readers regarding ligand coupled NLBCs. This article is protected by copyright. All rights reserved.

Metal complex-based liposomes: Applications and prospects in cancer diagnostics and therapeutics

Metal complexes are of increasing interest as pharmaceutical agents in cancer diagnostics and therapeutics, while some of them suffer from issues such as limited water solubility and severe systemic toxicity. These drawbacks severely hampered their efficacy and clinical applications. Liposomes hold promise as delivery vehicles for constructing metal complex-based liposomes to maximize the therapeutic efficacy and minimize the side effects of metal complexes. This review provides an overview on the latest advances of metal complex-based liposomal delivery systems. First, the development of metal complex-mediated liposomal encapsulation is briefly introduced. Next, applications of metal complex-based liposomes in a variety of fields are overviewed, where drug delivery, cancer imaging (single photon emission computed tomography (SPECT), positron emission tomography (PET), and magnetic resonance imaging (MRI)), and cancer therapy (chemotherapy, phototherapy, and radiotherapy) were involved. Moreover, the potential toxicity, action of toxic mechanisms, immunological effects of metal complexes as well as the advantages of metal complex-liposomes in this content are also discussed. In the end, the future expectations and challenges of metal complex-based liposomes in clinical cancer therapy are tentatively proposed.

Manganese in Diagnostics: A Preformulatory Study

This investigation aims to find lipid-based nanosystems to be used as tools to deliver manganese for diagnostic purposes in multimodal imaging techniques. In particular, the study describes the production and characterization of aqueous dispersions of anionic liposomes as delivery systems for two model manganese-based compounds, namely manganese chloride and manganese acetylacetonate. Negatively charged liposomes were obtained using four different anionic surfactants, namely sodium docusate (SD), N-lauroylsarcosine (NLS), Protelan AG8 (PAG) and sodium lauroyl lactylate (SLL). Liposomes were produced by the direct hydration method followed by extrusion and characterized in terms of size, polydispersity, surface charge and stability over time. After extrusion, liposomes are homogeneous and monodispersed with an average diameter not exceeding 200 nm and a negative surface charge as confirmed by ζ potential measurement. Moreover, as indicated by atomic absorption spectroscopy analyses, the loading of manganese-based compounds was almost quantitative. Liposomes containing NLS or SLL were the most stable over time and the presence of manganese-based compounds did not affect their size distribution. Liposomes containing PAG and SD were instable and therefore discarded. The in vitro cytotoxicity of the selected anionic liposomes was evaluated by MTT assay on human keratinocyte. The obtained results highlighted that the toxicity of the formulations is dose dependent.

Accelerated blood clearance and hypersensitivity by PEGylated liposomes containing TLR agonists

Systemic administration of toll-like receptor (TLR) agonists have demonstrated impressive preclinical results as an anti-cancer therapy due to their potent innate immune-stimulatory properties. The clinical advancement has, however, been hindered by severe adverse effects due to systemic activation of the immune system. Liposomal drug delivery systems may modify biodistribution, cellular uptake, and extend blood circulation, and thus, potentially enable systemic administration of TLR agonists at therapeutic doses. In this study, we investigated potential barriers for the administration of TLR agonists formulated in polyethylene glycosylated (PEGylated) liposomes with regards to liposome formulation, TLR agonist, administration route, administration schedule, biodistribution, blood clearance, and anti-PEG antibodies. We found that administration of TLR agonists formulated in PEGylated liposomes led to high anti-PEG antibody titers, which upon multiple intravenous administrations, resulted in accelerated blood clearance and acute hypersensitivity reactions. The latter was found to be associated with anti-PEG IgG antibody and not anti-PEG IgM antibody opsonization. This study highlights the need to carefully design and evaluate nanoparticle delivery systems for immunotherapy as anti-nanoparticle immune responses may challenge the therapeutic application.

Inorganic radiopharmaceutical chemistry of oxine

8-Hydroxyquinoline (8-HQ, oxine) is a small, monoprotic, bicyclic aromatic compound and its relative donor group orientation imparts impressive bidentate metal chelating abilities that have been exploited in a vast array of applications over decades. 8-HQ and its derivatives have been explored in medicinal applications including anti-neurodegeneration, anticancer properties, and antimicrobial activities. One long established use of 8-HQ in medicinal inorganic chemistry is the coordination of radioactive isotopes of metal ions in nuclear medicine. The metal-oxine complex with the single photon emission computed tomography (SPECT) imaging isotope [¹¹¹In]In³⁺ was developed in the 1970s and 1980s to radiolabel leukocytes for inflammation and infection imaging. The [¹¹¹In][In(oxine)₃] complex functions as an ionophore: a moderately stable lipophilic complex to enter cells; however, inside the cell environment [¹¹¹In]In³⁺ undergoes exchange and remains localized. As new developments have progressed towards radiopharmaceuticals capable of both imaging and therapy (theranostics), 8-HQ has been re-explored in recent years to investigate its potential to chelate larger radiometal ions with longer half-lives and different indications. Further, metal-oxine complexes have been used to study liposomes and other nanomaterials by tracking these nanomedicines in vivo. Expanding 8-HQ to multidentate ligands for highly thermodynamically stable and kinetically inert complexes has increased the possibilities of this small molecule in nuclear medicine. This article outlines the historic use of metal-oxine complexes in inorganic radiopharmaceutical chemistry, with a focus on recent advances highlighting the possibilities of developing higher denticity, targeted bifunctional chelators with 8-HQ.

Enhancing adoptive CD8 T cell therapy by systemic delivery of tumor associated antigens

Adoptive T-cell transfer (ACT) offers a curative therapeutic option for subsets of melanoma and hematological cancer patients. To increase response rates and broaden the applicability of ACT, it is necessary to improve the post-infusion performance of the transferred T cells. The design of improved treatment strategies includes transfer of cells with a less differentiated phenotype. Such T cell subsets have high proliferative potential but require stimulatory signals in vivo to differentiate into tumor-reactive effector T cells. Thus, combination strategies are needed to support the therapeutic implementation of less differentiated T cells. Here we show that systemic delivery of tumor-associated antigens (TAAs) facilitates in vivo priming and expansion of previously non-activated T cells and enhance the cytotoxicity of activated T cells. To achieve this in vivo priming, we use flexible delivery vehicles of TAAs and a TLR7/8 agonist. Contrasting subcutaneous delivery systems, these vehicles accumulate TAAs in the spleen, thereby achieving close proximity to both cross-presenting dendritic cells and transferred T cells, resulting in robust T-cell expansion and anti-tumor reactivity. This TAA delivery platform offers a strategy to safely potentiate the post-infusion performance of T cells using low doses of antigen and TLR7/8 agonist, and thereby enhance the effect of ACT.

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).

Radiolabelling of nanomaterials for medical imaging and therapy

Nanomaterials offer unique physical, chemical and biological properties of interest for medical imaging and therapy. Over the last two decades, there has been an increasing effort to translate nanomaterial-based medicinal products (so-called nanomedicines) into clinical practice and, although multiple nanoparticle-based formulations are clinically available, there is still a disparity between the number of pre-clinical products and those that reach clinical approval. To facilitate the efficient clinical translation of nanomedicinal-drugs, it is important to study their whole-body biodistribution and pharmacokinetics from the early stages of their development. Integrating this knowledge with that of their therapeutic profile and/or toxicity should provide a powerful combination to efficiently inform nanomedicine trials and allow early selection of the most promising candidates. In this context, radiolabelling nanomaterials allows whole-body and non-invasive in vivo tracking by the sensitive clinical imaging techniques positron emission tomography (PET), and single photon emission computed tomography (SPECT). Furthermore, certain radionuclides with specific nuclear emissions can elicit therapeutic effects by themselves, leading to radionuclide-based therapy. To ensure robust information during the development of nanomaterials for PET/SPECT imaging and/or radionuclide therapy, selection of the most appropriate radiolabelling method and knowledge of its limitations are critical. Different radiolabelling strategies are available depending on the type of material, the radionuclide and/or the final application. In this review we describe the different radiolabelling strategies currently available, with a critical vision over their advantages and disadvantages. The final aim is to review the most relevant and up-to-date knowledge available in this field, and support the efficient clinical translation of future nanomedicinal products for in vivo imaging and/or therapy.

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.

The dose threshold for nanoparticle tumour delivery

Nanoparticle delivery to solid tumours over the past ten years has stagnated at a median of 0.7% of the injected dose. Varying nanoparticle designs and strategies have yielded only minor improvements. Here we discovered a dose threshold for improving nanoparticle tumour delivery: 1 trillion nanoparticles in mice. Doses above this threshold overwhelmed Kupffer cell uptake rates, nonlinearly decreased liver clearance, prolonged circulation and increased nanoparticle tumour delivery. This enabled up to 12% tumour delivery efficiency and delivery to 93% of cells in tumours, and also improved the therapeutic efficacy of Caelyx/Doxil. This threshold was robust across different nanoparticle types, tumour models and studies across ten years of the literature. Our results have implications for human translation and highlight a simple, but powerful, principle for designing nanoparticle cancer treatments.

Copper-64 labeled nanoparticles for positron emission tomography imaging: a review of the recent literature

INTRODUCTION

Nuclear medicine plays a crucial role for personalized therapy, mainly in oncology. Chemotherapy and radiotherapy present some disadvantages and research is shifting toward nanotechnology with significant improvements in therapy and diagnosis of several cancers. Indeed, nanoparticles can be tagged with different radioisotopes for single photon emission computed tomography (SPECT) and positron emission tomography (PET) imaging and for therapy. This review describes the current state of the art of ⁶⁴Copper-labeled nanoparticles for PET imaging of cancer.

EVIDENCE ACQUISITION

We performed a systematic analysis of literature using the terms "64CuCl<inf>2</inf>," "64Cu," "Copper" AND "nanoparticle" AND "PET" in online databases: i.e. PubMed/MEDLINE and Scopus. The search was limited to English papers and original articles. We excluded articles not in English language, abstracts, case reports, review articles and meeting presentations.

EVIDENCE SYNTHESIS

Amongst the 116 articles retrieved, 88 were excluded because reviews, or not in English, or only in-vitro studies or meeting presentations. We considered only 28 original papers. The most used nanoparticles are liposomes and they are mainly used in breast cancer although other animal models of cancer have been also investigated.

CONCLUSIONS

The results showed that nanoparticles can be considered a promising radiopharmaceutical for PET imaging of different type of cancer.

Radiolabeled liposomes and lipoproteins as lipidic nanoparticles for imaging and therapy

Radiolabeled lipidic nanoparticles, particularly liposomes and lipoproteins, are of great interest as agents for imaging and therapy, due not only to their peculiar physicochemical and biological properties, but also to their great versatility and the ability to manipulate them to obtain the desired properties. This review provides an overview of radionuclide labeling strategies for preparing diagnostic and therapeutic nanoparticles based on liposomes and lipoproteins that have been developed to date, as well as the main quality control methods and in vivo applications.

Manganese in PET imaging: Opportunities and challenges

Several radionuclides of the transition metal manganese are known and accessible. Three of them, ⁵¹Mn, ^52mMn, and ^52gMn, are positron emitters that are potentially interesting for positron emission tomography (PET) applications and, thus, have caught the interest of the radiochemical/radiopharmaceutical and nuclear medicine communities. This mini‐review provides an overview of the production routes and physical properties of these radionuclides. For medical imaging, the focus is on the longer‐living ^52gMn and its application for the radiolabelling of molecules and other entities exhibiting long biological half‐lives, the imaging of manganese‐dependent biological processes, and the development of bimodal PET/magnetic resonance imaging (MRI) probes in combination with paramagnetic ^natMn as a contrast agent.

Preclinical evaluation of cationic DOTA-triarginine-lipid conjugates for theranostic liquid brachytherapy

Liquid brachytherapy is an emerging technology for internal radiation therapy where liquids containing radionuclides are administered directly into solid tumors. These technologies are less invasive than conventional brachytherapy, and can potentially improve the dose coverage and homogeneity of the radioactivity distribution within the tumor. For this purpose, we have developed a novel cationic micelle system for delivery of a range of radionuclides. The system is applicable for emitters of alpha, beta or photon radiation, and enables dose-mapping via theranostic nuclear imaging. Methods: The cationic micelles were developed as linear surfactants comprising the chelator DOTA, a triarginine sequence and a palmitoyl or stearoyl fatty acid chain. The critical micelle concentration of the surfactants was determined, and the micelles were radiolabelled with ⁶⁴Cu or ¹⁷⁷Lu in high radiochemical purity (>95%). The tumor retention and biodistribution of the ⁶⁴Cu-radiolabeled surfactants, administered as micelles or formulated in liposomes, were investigated in vivo by PET/CT in a tumor bearing mouse model. Results: The interaction of the micelles with anionic lipid membranes was demonstrated to be favourable, using a liposome partition assay. In vivo, the surfactants formulated both as cationic micelles and liposomes displayed the best intratumoral retention, with micelles providing more homogeneous activity distribution. Conclusion: A cationic, surfactant-based drug delivery system was developed and demonstrated promise as a vehicle for liquid brachytherapy when formulated as micelles or in liposomes. The system enables accurate dosimetry due to the flexible radiochemistry of DOTA.

Integrating Artificial Intelligence and Nanotechnology for Precision Cancer Medicine

Artificial intelligence (AI) and nanotechnology are two fields that are instrumental in realizing the goal of precision medicine-tailoring the best treatment for each cancer patient. Recent conversion between these two fields is enabling better patient data acquisition and improved design of nanomaterials for precision cancer medicine. Diagnostic nanomaterials are used to assemble a patient-specific disease profile, which is then leveraged, through a set of therapeutic nanotechnologies, to improve the treatment outcome. However, high intratumor and interpatient heterogeneities make the rational design of diagnostic and therapeutic platforms, and analysis of their output, extremely difficult. Integration of AI approaches can bridge this gap, using pattern analysis and classification algorithms for improved diagnostic and therapeutic accuracy. Nanomedicine design also benefits from the application of AI, by optimizing material properties according to predicted interactions with the target drug, biological fluids, immune system, vasculature, and cell membranes, all affecting therapeutic efficacy. Here, fundamental concepts in AI are described and the contributions and promise of nanotechnology coupled with AI to the future of precision cancer medicine are reviewed.

Indium-111 labelling of liposomal HEGF for radionuclide delivery via ultrasound-induced cavitation

The purpose of this exploratory study was to investigate the combination of a radiopharmaceutical, nanoparticles and ultrasound (US) enhanced delivery to develop a clinically viable therapeutic strategy for tumours overexpressing the epidermal growth factor receptor (EGFR). Molecularly targeted radionuclides have great potential for cancer therapy but are sometimes associated with insufficient delivery resulting in sub-cytotoxic amounts of radioactivity being delivered to the tumour. Liposome formulations are currently used in the clinic to reduce the side effects and improve the pharmacokinetic profile of chemotherapeutic drugs. However, in contrast to non-radioactive agents, loading and release of radiotherapeutics from liposomes can be challenging in the clinical setting. US-activated cavitation agents such as microbubbles (MBs) have been used to release therapeutics from liposomes to enhance the distribution/delivery in a target area. In an effort to harness the benefits of these techniques, the development of a liposome loaded radiopharmaceutical construct for enhanced delivery via acoustic cavitation was studied. The liposomal formulation was loaded with peptide, human epidermal growth factor (HEGF), coupled to a chelator for subsequent radiolabelling with ¹¹¹Indium ([¹¹¹In]In³⁺), in a manner designed to be compatible with preparation in a radiopharmacy. Liposomes were efficiently radiolabelled (57%) within 1 h, with release of ~12% of the radiopeptide following a 20 s exposure to US-mediated cavitation in vitro. In clonogenic studies this level of release resulted in cytotoxicity specifically in cells over-expressing the epidermal growth factor receptor (EGFR), with over 99% reduction in colony survival compared to controls. The formulation extended the circulation time and changed the biodistribution compared to the non-liposomal radiopeptide in vivo, although interestingly the biodistribution did not resemble that of liposome constructs currently used in the clinic. Cavitation of MBs co-injected with liposomes into tumours expressing high levels of EGFR resulted in a 2-fold enhancement in tumour uptake within 20 min. However, owing to the poor vascularisation of the tumour model used the same level of uptake was achieved without US after 24 h. By combining acoustic-cavitation-sensitive liposomes with radiopharmaceuticals this research represents a new concept in achieving targeted delivery of radiopharmaceuticals.

PET Imaging of Liposomal Glucocorticoids using 89Zr-oxine: Theranostic Applications in Inflammatory Arthritis

The encapsulation of Glucocorticoids (GCs) into long-circulating liposomes (LCLs) is a proven strategy to reduce the side effects of glucocorticoids and improve the treatment of inflammatory diseases, such as rheumatoid arthritis (RA). With the aim of supporting the development of GC-loaded LCLs, and potentially predict patient response to therapy clinically, we evaluated a direct PET imaging radiolabelling approach for preformed GC-LCLs in an animal model of human inflammatory arthritis. Methods: A preformed PEGylated liposomal methylprednisolone hemisuccinate (NSSL-MPS) nanomedicine was radiolabelled using [89Zr]Zr(oxinate)4 (89Zr-oxine), characterised and tracked in vivo using PET imaging in a K/BxN serum-transfer arthritis (STA) mouse model of inflammatory arthritis and non-inflamed controls. Histology and joint size measurements were used to confirm inflammation. The biodistribution of 89Zr-NSSL-MPS was compared to that of free 89Zr in the same model. A therapeutic study using NSSL-MPS using the same time points as the PET/CT imaging was carried out. Results: The radiolabelling efficiency of NSSL-MPS with [89Zr]Zr(oxinate)4 was 69 ± 8 %. PET/CT imaging of 89Zr-NSSL-MPS showed high uptake (3.6 ± 1.5 % ID; 17.4 ± 9.3 % ID/mL) at inflamed joints, with low activity present in non-inflamed joints (0.5 ± 0.1 % ID; 2.7 ± 1.1 % ID/mL). Importantly, a clear correlation between joint swelling and high 89Zr-NSSL-MPS uptake was observed, which was not observed with free 89Zr. STA mice receiving a therapeutic dose of NSSL-MPS showed a reduction in inflammation at the time points used for the PET/CT imaging compared with the control group. Conclusions: PET imaging was used for the first time to track a liposomal glucocorticoid, showing high uptake at visible and occult inflamed sites and a good correlation with the degree of inflammation. A subsequent therapeutic response matching imaging time points in the same model demonstrated the potential of this radiolabeling method as a theranostic tool for the prediction of therapeutic response - with NSSL-MPS and similar nanomedicines - in the treatment of inflammatory diseases.

Translational considerations in nanomedicine: The oncology perspective

Nanoparticles can provide effective control of the release rate and tissue distribution of their drug payload, leading to major pharmacokinetic and pharmacodynamic changes vis-à-vis the conventional administration of free drugs. In the last two decades, we have witnessed major progress in the synthesis and characterization of engineered nanoparticles for imaging and treatment of cancers, resulting in the approval for clinical use of several products and in new and promising approaches. Despite these advances, clinical applications of nanoparticle-based therapeutic and imaging agents remain limited due to biological, immunological, and translational barriers. There is a need to make high impact advances toward translation. In this review, we address biological, toxicological, immunological, and translational aspects of nanomedicine and discuss approaches to move the field forward productively. Overcoming these barriers may dramatically improve the development potential and role of nanomedicines in the oncology field and help meet the high expectations.

Radiolabeling nanomaterials for multimodality imaging: New insights into nuclear medicine and cancer diagnosis

Nuclear medicine imaging has been developed as a powerful diagnostic approach for cancers by detecting gamma rays directly or indirectly from radionuclides to construct images with beneficial characteristics of high sensitivity, infinite penetration depth and quantitative capability. Current nuclear medicine imaging modalities mainly include single-photon emission computed tomography (SPECT) and positron emission tomography (PET) that require administration of radioactive tracers. In recent years, a vast number of radioactive tracers have been designed and constructed to improve nuclear medicine imaging performance toward early and accurate diagnosis of cancers. This review will discuss recent progress of nuclear medicine imaging tracers and associated biomedical imaging applications. Radiolabeling nanomaterials for rational development of tracers will be comprehensively reviewed with highlights on radiolabeling approaches (surface coupling, inner incorporation and interface engineering), providing profound understanding on radiolabeling chemistry and the associated imaging functionalities. The applications of radiolabeled nanomaterials in nuclear medicine imaging-related multimodality imaging will also be summarized with typical paradigms described. Finally, key challenges and new directions for future research will be discussed to guide further advancement and practical use of radiolabeled nanomaterials for imaging of cancers.

Nano-carriers for targeted delivery and biomedical imaging enhancement

Theranostic approaches using nanotechnology have been a hot research area for the past decade. All nano drug delivery techniques and architectures have some limitations, as do diagnostic nano-approaches. Thus, combining nano drug delivery strategies with diagnostic techniques using nanoparticles for improving imaging modalities has been the key to fill up those gaps. In the past decade, lots of approaches have been made with different combinations of biomaterials fabricated/synthesized to nanostructures with modified surface functionalization to improve their overall theranostic properties. This article summarizes recent research works based on the biomaterials used for fabricating these nanostructures. Their combinations with other biomaterials have been demonstrated with their overall advantages and limitations.

Liposome-based probes for molecular imaging: from basic research to the bedside

Molecular imaging is very important in disease diagnosis and prognosis. Liposomes are excellent carriers for different types of molecular imaging probes. In this work, we summarize current developments in liposome-based probes used for molecular imaging and their applications in image-guided drug delivery and tumour surgery, including computed tomography (CT), ultrasound imaging (USI), magnetic resonance imaging (MRI), positron emission tomography (PET), fluorescence imaging (FLI) and photoacoustic imaging (PAI). We also summarized liposome-based multimodal imaging probes and new targeting strategies for liposomes. This work will offer guidance for the design of liposome-based imaging probes for future clinical applications.

Nuclear imaging of liposomal drug delivery systems: A critical review of radiolabelling methods and applications in nanomedicine

The integration of nuclear imaging with nanomedicine is a powerful tool for efficient development and clinical translation of liposomal drug delivery systems. Furthermore, it may allow highly efficient imaging-guided personalised treatments. In this article, we critically review methods available for radiolabelling liposomes. We discuss the influence that the radiolabelling methods can have on their biodistribution and highlight the often-overlooked possibility of misinterpretation of results due to decomposition in vivo. We stress the need for knowing the biodistribution/pharmacokinetics of both the radiolabelled liposomal components and free radionuclides in order to confidently evaluate the images, as they often share excretion pathways with intact liposomes (e.g. phospholipids, metallic radionuclides) and even show significant tumour uptake by themselves (e.g. some radionuclides). Finally, we describe preclinical and clinical studies using radiolabelled liposomes and discuss their impact in supporting liposomal drug development and clinical translation in several diseases, including personalised nanomedicine approaches.

Pharmacokinetic evaluation of liposomal nanoparticle-encapsulated nucleic acid drug: A combined study of dynamic PET imaging and LC/MS/MS analysis

In vivo biodistribution analyses, especially in tumors, of nucleic acids delivered with nanoparticles are important to develop drug delivery technologies for medical use. We previously developed wrapsome® (WS), an ~100 nm liposomal nanoparticle that can encapsulate siRNA, and reported that WS accumulates in tumors in vivo and inhibits their growth by an enhanced permeability and retention effect. In the present study, we evaluated the pharmacokinetics of nucleic acid-containing nanoparticles by combining dynamic positron emission tomography (PET) imaging and liquid chromatography-tandem mass spectrometry (LC/MS/MS) analysis. An 18-mer phosphorothioate oligodeoxynucleotide (ODN), trabedersen, was used as a model drug and was encapsulated in WS. Dynamic PET imaging and time-activity curve analysis of WS-encapsulated ⁶⁴Cu-labeled ODNs administered to mice with MIA PaCa-2 subcutaneous xenograft tumors showed tumor accumulation (~3% injected dose per gram (%ID/g)) and liver accumulation (~30 %ID/g) at 24 h. Under these conditions, LC/MS/MS analysis showed that the level of intact ODNs was 1.62 %ID/g in the tumor and 1.70 %ID/g in the liver. From these pharmacokinetic data, the intact/accumulated ODN ratios were calculated using the following equation: intact/accumulated ODN ratio (%) = %ID/g _{LC/MS/MS, tissue, mean}/%ID/g _{PET, tissue, mean} × 100. Interestingly, the ratios for the tumor and kidney were maintained at 20-50% over 48 h after administration of the WS-encapsulated form. In contrast, the ratio for the liver rapidly decreased at 24 h, showing the same pattern as that for naked ODN. These different patterns indicate that WS effectively protected the ODN in the tumor and kidney, but protected it less efficiently in the liver. A combined approach of dynamic PET imaging and LC/MS/MS analysis will assist the development of nanoparticle-encapsulated nucleic acid drugs, such as those using WSs, to determine their detailed pharmacokinetics.

Remote loading of liposomes with a 124I-radioiodinated compound and their in vivo evaluation by PET/CT in a murine tumor model

Long circulating liposomes entrapping iodinated and radioiodinated compounds offer a highly versatile theranostic platform. Here we report a new methodology for efficient and high-yield loading of such compounds into liposomes, enabling CT/SPECT/PET imaging and ¹³¹I-radiotherapy.

Methods: The CT contrast agent diatrizoate was synthetically functionalized with a primary amine, which enabled its remote loading into PEGylated liposomes by either an ammonium sulfate- or a citrate-based pH transmembrane gradient. Further, the amino-diatrizoate was radiolabeled with either ¹²⁴I (t_1/2 = 4.18 days) for PET or ¹²⁵I (t_1/2 = 59.5 days) for SPECT, through an aromatic Finkelstein reaction.

Results: Quantitative loading efficiencies (>99%) were achieved at optimized conditions. The ¹²⁴I-labeled compound was remote-loaded into liposomes, with an overall radiolabeling efficiency of 77 ± 1%, and imaged in vivo in a CT26 murine colon cancer tumor model by PET/CT. A prolonged blood circulation half-life of 19.5 h was observed for the radiolabeled liposomes, whereas injections of the free compound were rapidly cleared. Lower accumulation was observed in the spleen, liver, kidney and tumor than what is usually seen for long-circulating liposomes.

Conclusion: The lower accumulation was interpreted as release of the tracer from the liposomes within these organs after accumulation. These results may guide the design of systems for controlled release of remote loadable drugs from liposomes.

Radioactive Transition Metals for Imaging and Therapy

Nuclear medicine is composed of two complementary areas, imaging and therapy. Positron emission tomography (PET) and single-photon imaging, including single-photon emission computed tomography (SPECT), comprise the imaging component of nuclear medicine. These areas are distinct in that they exploit different nuclear decay processes and also different imaging technologies. In PET, images are created from the 511 keV photons produced when the positron emitted by a radionuclide encounters an electron and is annihilated. In contrast, in single-photon imaging, images are created from the γ rays (and occasionally X-rays) directly emitted by the nucleus. Therapeutic nuclear medicine uses particulate radiation such as Auger or conversion electrons or β^- or α particles. All three of these technologies are linked by the requirement that the radionuclide must be attached to a suitable vector that can deliver it to its target. It is imperative that the radionuclide remain attached to the vector before it is delivered to its target as well as after it reaches its target or else the resulting image (or therapeutic outcome) will not reflect the biological process of interest. Radiochemistry is at the core of this process, and radiometals offer radiopharmaceutical chemists a tremendous range of options with which to accomplish these goals. They also offer a wide range of options in terms of radionuclide half-lives and emission properties, providing the ability to carefully match the decay properties with the desired outcome. This Review provides an overview of some of the ways this can be accomplished as well as several historical examples of some of the limitations of earlier metalloradiopharmaceuticals and the ways that new technologies, primarily related to radionuclide production, have provided solutions to these problems.

Manganese-52: applications in cell radiolabelling and liposomal nanomedicine PET imaging using oxine (8-hydroxyquinoline) as an ionophore

The ionophore 8-hydroxyquinoline (oxine) has been used to radiolabel cells and liposomal medicines with 111In and, more recently, 89Zr, for medical nuclear imaging applications. Oxine has also shown promising ionophore activity for the positron-emitting radionuclide 52Mn that should allow imaging of labelled cells and nanomedicines for long periods of time (>14 days). However, to date, the radiometal complex formed and its full labelling capabilities have not been fully characterised. Here, we provide supporting evidence of the formation of [52Mn]Mn(oxinate)2 as the metastable complex responsible for its ionophore activity. The cell labelling properties of [52Mn]Mn(oxinate)2 were investigated with various cell lines. The liposomal nanomedicine, DOXIL® (Caelyx) was also labelled with [52Mn]Mn(oxinate)2 and imaged in vivo using PET imaging. [52Mn]Mn(oxinate)2 was able to label various cell lines with moderate efficiency (15-53%), however low cellular retention of 52Mn (21-25% after 24 h) was observed which was shown not to be due to cell death. PET imaging of [52Mn]Mn-DOXIL at 1 h and 24 h post-injection showed the expected pharmacokinetics and biodistribution of this stealth liposome, but at 72 h post-injection showed a profile matching that of free 52Mn, consistent with drug release. We conclude that oxine is an effective ionophore for 52Mn, but high cellular efflux of the isotope limits its use for prolonged cell tracking. [52Mn]Mn(oxinate)2 is effective for labelling and tracking DOXIL in vivo. The release of free radionuclide after liposome extravasation could provide a non-invasive method to monitor drug release in vivo.

Liposomes: Clinical Applications and Potential for Image-Guided Drug Delivery

Liposomes have been extensively studied and are used in the treatment of several diseases. Liposomes improve the therapeutic efficacy by enhancing drug absorption while avoiding or minimizing rapid degradation and side effects, prolonging the biological half-life and reducing toxicity. The unique feature of liposomes is that they are biocompatible and biodegradable lipids, and are inert and non-immunogenic. Liposomes can compartmentalize and solubilize both hydrophilic and hydrophobic materials. All these properties of liposomes and their flexibility for surface modification to add targeting moieties make liposomes more attractive candidates for use as drug delivery vehicles. There are many novel liposomal formulations that are in various stages of development, to enhance therapeutic effectiveness of new and established drugs that are in preclinical and clinical trials. Recent developments in multimodality imaging to better diagnose disease and monitor treatments embarked on using liposomes as diagnostic tool. Conjugating liposomes with different labeling probes enables precise localization of these liposomal formulations using various modalities such as PET, SPECT, and MRI. In this review, we will briefly review the clinical applications of liposomal formulation and their potential imaging properties.