Imaging of pulmonary embolism and t-PA therapy effects using MDCT and liposomal iohexol blood pool agent: preliminary results in a rabbit model

Liposome-camouflaged iodinated mesoporous silica nanoparticles with high loading capacity, high hemodynamic stability, high biocompatibility and high radiopacity

Due to their low osmolality and high tolerability, the highly water-soluble nonionic iodinated contrast agents, such as Ioversol (IV), are widely used as clinical agents for CT imaging. However, their clinical applications still are severely limited by the rapid renal excretion, serious adverse effects especially contrast-induced nephropathy and inefficient targetability. Various nanocarriers have demonstrated tremendous potential for achieving high imaging efficiency and low side effects. However, few nanoparticulate contrast agents can simultaneously integrate the desirable functions for imaging, including high loading capacity of iodine, high structure stability for systemic circulation, high biocompatibility and high radiopacity. Herein, we designed and prepared a kind of new radiopaque liposome-camouflaged iodinated mesoporous silica nanoparticles (OIV-MSNs@Liposomes) as contrast agents in CT imaging. Their composition, structure, morphology, biocompatibility and physicochemical properties as well as in vitro radiopacity were investigated in detail. The results indicated that OIV-MSNs@Liposomes can integrate their individual advantages of liposomes and MSNs, thus exhibiting great potential for use in the CT imaging. Considering the simple preparation process and readily available starting materials as well as enhanced biosafety and high performance in X-ray attenuation, the strategy reported here offers a versatile route to efficiently deliver highly water-soluble nonionic iodinated contrast agents for enhanced CT imaging, which are unattainable by traditional means.

Facilitated encapsulation of a nonionic contrast agent for X-ray computed tomography into lipid vesicles by the multiple emulsification-solvent evaporation method

We studied the encapsulation of iohexol (Ihex), a nonionic contrast agent used for X-ray computational tomography, into lipid vesicles using the multiple emulsification-solvent evaporation method to formulate a nanosized contrast agent. This lipid vesicle preparation method consists of three steps: (1) primary emulsification for producing water-in-oil (W/O) emulsions containing fine water droplets that will be converted to the internal water phase of the lipid vesicles, (2) secondary emulsification for formulating multiple water-in-oil-in-water (W/O/W) emulsions encapsulating the fine water droplets containing Ihex, and (3) solvent evaporation to remove the oil phase solvent (n-hexane) and to form lipid bilayers surrounding the fine inner droplets, resulting in the formation of lipid vesicles encapsulating Ihex. As the diameter and Ihex concentration of the primary W/O emulsion droplets decreased, a higher Ihex encapsulation yield was obtained for the final lipid vesicles. The entrapment yield of Ihex in the final lipid vesicles varied significantly with the emulsifier (Pluronic® F-68) concentration in the external water phase of W/O/W emulsion, and the highest yield (65%) was obtained when the emulsifier concentration was 0.1 wt%. We also investigated the powderization of lipid vesicles encapsulating Ihex via lyophilization. The powderized vesicles were dispersed in water after rehydration and maintained their controlled diameters. The entrapment yield of Ihex in powderized lipid vesicles was maintained for over 1 month at 25 ˚C, while significant leakage of Ihex was observed in the lipid vesicles suspended in the aqueous phase.

Nanoparticle Contrast-enhanced MRI for Visualization of Retroplacental Clear Space Disruption in a Mouse Model of Placental Accreta Spectrum (PAS)

INTRODUCTION

Prior preclinical studies established the utility of liposomal nanoparticle blood-pool contrast agents in visualizing the retroplacental clear space (RPCS), a marker of normal placentation, while sparing fetuses from exposure because the agent does not cross the placental barrier. In this work, we characterized RPCS disruption in a mouse model of placenta accreta spectrum (PAS) using these agents.

MATERIALS AND METHODS

Contrast-enhanced MRI (CE-MRI) and computed tomography (CE-CT) using liposomal nanoparticles bearing gadolinium (liposomal-Gd) and iodine were performed in pregnant Gab3^-/- and wild type (WT) mice at day 16 of gestation. CE-MRI was performed on a 1T scanner using a 2D T1-weighted sequence (100×100×600 µm³ voxels) and CE-CT was performed at a higher resolution (70×70×70 µm³ voxels). Animals were euthanized post-imaging and feto-placental units (FPUs) were harvested for histological examination. RPCS conspicuity was scored through blinded assessment of images.

RESULTS

Pregnant Gab3^-/- mice showed elevated rates of complicated pregnancy. Contrast-enhanced imaging demonstrated frank infiltration of the RPCS of Gab3^-/- FPUs. RPCS in Gab3^-/- FPUs was smaller in volume, demonstrated a heterogeneous signal profile, and received lower conspicuity scores than WT FPUs. Histology confirmed in vivo findings and demonstrated staining consistent with a thinner RPCS in Gab3^-/- FPUs.

DISCUSSION

Imaging of the Gab3^-/- mouse model at late gestation with liposomal contrast agents enabled in vivo characterization of morphological differences in the RPCS that could cause the observed pregnancy complications. An MRI-based method for visualizing the RPCS would be valuable for early detection of invasive placentation.

Rodent models of pulmonary embolism and chronic thromboembolic pulmonary hypertension

Pulmonary embolism (PE) is the third most prevalent cardiovascular disease. It is associated with high in-hospital mortality and the development of acute and chronic complications. New approaches aimed at improving the prognosis of patients with PE are largely dependent on reliable animal models. Mice, rats, hamsters, and rabbits, are currently most commonly used for PE modeling because of their ethical acceptability and economic feasibility. This article provides an overview of the main approaches to PE modeling, and the advantages and disadvantages of each method. Special attention is paid to experimental endpoints, including morphological, functional, and molecular endpoints. All approaches to PE modeling can be broadly divided into three main groups: 1) induction of thromboembolism, either by thrombus formation in vivo or by injection of in vitro prepared blood clots; 2) introduction of particles of non-thrombotic origin; and 3) surgical procedures. The choice of a specific model and animal species is determined based on the objectives of the study. Rodent models of chronic thromboembolic pulmonary hypertension (CTEPH), which is the most devastating complication of PE, are also described. CTEPH models are especially challenging because of insufficient knowledge about the pathogenesis and high fibrinolytic activity of rodent plasma. The CTEPH model should demonstrate a persistent increase in pulmonary artery pressure and stable reduction of the vascular bed due to recurrent embolism. Based on the analysis of available evidence, one might conclude that currently, there is no single optimal method for modeling PE and CTEPH.

Polymers for Improved Delivery of Iodinated Contrast Agents

X-ray computed tomography (CT), as one of the most widely used noninvasive imaging modalities, can provide three-dimensional anatomic details with high resolution, which plays a key role in disease diagnosis and treatment assessment. However, although they are the most prevalent and FDA-approved contrast agents, iodinated water-soluble molecules still face some challenges in clinical applications, such as fast clearance, serious adverse effects, nonspecific distribution, and low sensitivity. Because of their high biocompatibility, tunable designability, controllable biodegradation, facile synthesis, and modification capability, the polymers have demonstrated great potential for efficient delivery of iodinated contrast agents (ICAs). Herein, we comprehensively summarized the applications of multifunctional polymeric materials for ICA delivery in terms of increasing circulation time, decreasing nephrotoxicity, and improving the specificity and sensitivity of ICAs for CT imaging. We mainly focused on various iodinated polymers from the aspects of preparation, functionalization, and application in medical diagnosis. Future perspectives for achieving better imaging and clinical translation are also discussed to motivate new technologies and solutions.

Organic Nanoplatforms for Iodinated Contrast Media in CT Imaging

X-ray computed tomography (CT) imaging can produce three-dimensional and high-resolution anatomical images without invasion, which is extremely useful for disease diagnosis in the clinic. However, its applications are still severely limited by the intrinsic drawbacks of contrast media (mainly iodinated water-soluble molecules), such as rapid clearance, serious toxicity, inefficient targetability and poor sensitivity. Due to their high biocompatibility, flexibility in preparation and modification and simplicity for drug loading, organic nanoparticles (NPs), including liposomes, nanoemulsions, micelles, polymersomes, dendrimers, polymer conjugates and polymeric particles, have demonstrated tremendous potential for use in the efficient delivery of iodinated contrast media (ICMs). Herein, we comprehensively summarized the strategies and applications of organic NPs, especially polymer-based NPs, for the delivery of ICMs in CT imaging. We mainly focused on the use of polymeric nanoplatforms to prolong circulation time, reduce toxicity and enhance the targetability of ICMs. The emergence of some new technologies, such as theragnostic NPs and multimodal imaging and their clinical translations, are also discussed.

Microfluidic-assisted synthesis of multifunctional iodinated contrast agent polymeric nanoplatforms

Contrast Induced Nephropathy is the most severe side-effect arising after non-ionic iodinated contrast agents (CAs) intravenous administration. The use of antioxidants (i.e., N-Acetylcysteine; NAC) is one of the attempted prevention approaches. Herein, we describe the microfluidic-assisted synthesis of iodinated polymeric nanoparticles (NPs) as new multifunctional blood pool CA. The aim of this research is to co-encapsulate Iohexol (IOX; iodinated CA) and NAC (preventive agent) into poly-D,L-lactide-co-glycolide (PLGA) and PEGylated-PLGA (PLGA-PEG) NPs to exploit CA diagnostic proprieties and NAC preventing antioxidant activity. A microfluidic-assisted nanoprecipitation protocol has been set-up for PLGA and PLGA-PEG NPs, evaluating the effect of formulation and microfluidic parameters by analysing the size, PDI and IOX and NAC encapsulation efficiency. The optimized NPs (PLGA-PEG, L:G 50:50, 5% PEG, Mw 90 kDa) formulated with a size of 67 ± 2.8 nm with PDI < 0.2, spherical shape, and an IOX and NAC encapsulation efficiency of 38% and 20%, respectively. The IOX and NAC encapsulation was confirmed by FTIR and DSC. In vitro release study showed an IOX retention into the polymeric matrix and NAC sustained release up to 24-48 h stating microfluidics as powerful tool for the formulation of multifunctional nanoplatforms. Finally, the protective effect of NPs and NAC were preliminary assessed on human kidney cells.

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.

Evaluation of X-ray tomography contrast agents: A review of production, protocols, and biological applications

X-ray computed tomography is a strong tool that finds many applications both in medical applications and in the investigation of biological and nonbiological samples. In the clinics, X-ray tomography is widely used for diagnostic purposes whose three-dimensional imaging in high resolution helps physicians to obtain detailed image of investigated regions. Researchers in biological sciences and engineering use X-ray tomography because it is a nondestructive method to assess the structure of their samples. In both medical and biological applications, visualization of soft tissues and structures requires special treatment, in which special contrast agents are used. In this detailed report, molecule-based and nanoparticle-based contrast agents used in biological applications to enhance the image quality were compiled and reported. Special contrast agent applications and protocols to enhance the contrast for the biological applications and works to develop nanoparticle contrast agents to enhance the contrast for targeted drug delivery and general imaging applications were also assessed and listed.

Small, Long Blood Half-Life Iodine Nanoparticle for Vascular and Tumor Imaging

Standard clinical X-ray contrast agents are small iodine-containing molecules that are rapidly cleared by the kidneys and provide robust imaging for only a few seconds, thereby limiting more extensive vascular and tissue biodistribution imaging as well as optimal tumor uptake. They are also not generally useful for preclinical microCT imaging where longer scan times are required for high resolution image acquisition. We here describe a new iodine nanoparticle contrast agent that has a unique combination of properties: 20 nm hydrodynamic diameter, covalent PEG coating, 40 hour blood half-life, 50% liver clearance after six months, accumulation in tumors, and well-tolerated to at least 4 g iodine/kg body weight after intravenous administration in mice. These characteristics are unique among the other iodine nanoparticles that have been previously reported and provide extended-time high contrast vascular imaging and tumor loading. As such, it is useful for preclinical MicroCT animal studies. Potential human applications might include X-ray radiation dose enhancement for cancer therapy and vascular imaging for life-threatening situations where high levels of contrast are needed for extended periods of time.

Albumin-mediated platinum nanocrystals for in vivo enhanced computed tomography imaging

Contrast agents play a vital role in the enhanced examination of computed tomography (CT) imaging. However, traditional clinical small-molecule agents face a variety of drawbacks, such as low blood circulating time, difficult modification and potential toxic and side effects. Herein, a simple albumin-directed fabrication of platinum (Pt) nanocrystals was achieved for exploring the utilization in CT imaging. Ultrasmall nanoagents with a mean core size of 2.1 nm were obtained through a facile one-pot synthesis by the reduction of chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O) using bovine serum albumin (BSA) as the biotemplate under room temperature. These synthesized well-dispersed nanocrystals exhibited good haemocompatibility and biocompatibility. Interestingly, it was demonstrated that the nanocrystals could serve as potential new and potent CT contrast agents, especially vital for in vivo imaging with prominent enhancement and metabolizable behaviours due to the combination of the higher X-ray attenuation property and prolonged imaging time, perhaps caused by the BSA modification. Furthermore, such ultrasmall platinum nanocrystals obtained from a feasible mild aqueous synthetic route for CT imaging has not been reported before. Thereby, this work also gives new insights for the protein-templated growth of biocompatible nanoparticulate contrast agents in future nanomedicines.

Neuroimaging in Alzheimer's disease: preclinical challenges toward clinical efficacy

The scope of this review focuses on recent applications in preclinical and clinical magnetic resonance imaging (MRI) toward accomplishing the goals of early detection and responses to therapy in animal models of Alzheimer's disease (AD). Driven by the outstanding efforts of the Alzheimer's Disease Neuroimaging Initiative (ADNI), a truly invaluable resource, the initial use of MRI in AD imaging has been to assess changes in brain anatomy, specifically assessing brain shrinkage and regional changes in white matter tractography using diffusion tensor imaging. However, advances in MRI have led to multiple efforts toward imaging amyloid beta plaques first without and then with the use of MRI contrast agents. These technological advancements have met with limited success and are not yet appropriate for the clinic. Recent developments in molecular imaging inclusive of high-power liposomal-based MRI contrast agents as well as fluorine 19 ((19)F) MRI and manganese enhanced MRI have begun to propel promising advances toward not only plaque imaging but also using MRI to detect perturbations in subcellular processes occurring within the neuron. This review concludes with a discussion about the necessity for the development of novel preclinical models of AD that better recapitulate human AD for the imaging to truly be meaningful and for substantive progress to be made toward understanding and effectively treating AD. Furthermore, the continued support of outstanding programs such as ADNI as well as the development of novel molecular imaging agents and MRI fast scanning sequences will also be requisite to effectively translate preclinical findings to the clinic.

Advances in nanoparticle imaging technology for vascular pathologies

Nanoparticle imaging agents for vascular pathologies are in development, and some agents are already in clinical trials. Untargeted agents, with long circulation, are excellent blood-pool agents, but molecularly targeted agents have significant advantages due to the signal enhancement possible with nanoparticle presentation of the contrast agent molecules. Molecular targets that are accessible directly from the vasculature are optimal for such agents. Targets that are removed from the vasculature, such as those on tumor cell surfaces, have limited accessibility owing to the enhanced permeation and retention effect. Yet, efforts at molecular targeting have tested small molecules, peptides, antibodies, and most recently aptamers as possible targeting ligands. The future is bright for nanoparticle-based imaging of vascular pathologies.

Contrast agents for preclinical targeted X-ray imaging

Micro-computed tomography (micro-CT) is an X-ray based instrument that it is specifically designed for biomedical research at a preclinical stage for live imaging of small animals. This imaging modality is cost-effective, fast, and produces remarkable high-resolution images of X-ray opaque skeleton. Administration of biocompatible X-ray opaque contrast agent allows delineation of the blood vessels, and internal organs and even detection of tumor metastases as small as 300 μm. However, the main limitation of micro-CT lies in the poor efficacy or toxicity of the contrast agents. Moreover, contrast agents for micro-CT have to be stealth nanoparticulate systems, i.e. preventing their rapid renal clearance. The chemical composition and physicochemical properties will condition their uptake and elimination pathways, and therefore all the biological fluids, organs, and tissues trough this elimination route of the nanoparticles will be contrasted. Furthermore, several technologies playing on the nanoparticle properties, aim to influence these biological pathways in order to induce their accumulation onto given targeted sites, organs of tumors. In function of the methodologies carried out, taking benefit or not of the action of immune system, of the natural response of the organism like hepatocyte uptake or enhanced permeation and retention effect, or even accumulation due to ligand/receptor interactions, the technologies are called passive or active targeted imaging. The present review presents the most recent advances in the development of specific contrast agents for targeted X-ray imaging micro-CT, discussing the recent advance of in vivo targeting of nanoparticulate contrast agents, and the influence of the formulations, nature of the nanocarrier, nature and concentration of the X-ray contrasting materials, effect of the surface properties, functionalization and bioconjugation. The pharmacokinetic and versatility of nanometric systems appear particularly advantageous for addressing the versatile biomedical research needs. State of the art investigations are on going to propose contrast agents with tumor accumulating properties and will contribute for development of safer cancer medicine having detection and therapeutic modalities.

Cost-effectiveness of a novel blood-pool contrast agent in the setting of chest pain evaluation in an emergency department

OBJECTIVE

We evaluated three diagnostic strategies with the objective of comparing the current standard of care for individuals presenting acute chest pain and no history of coronary artery disease (CAD) with a novel diagnostic strategy using an emerging technology (blood-pool contrast agent [BPCA]) to identify the potential benefits and cost reductions.

MATERIALS AND METHODS

A decision analytic model of diagnostic strategies and outcomes using a BPCA and a conventional agent for CT angiography (CTA) in patients with acute chest pain was built. The model was used to evaluate three diagnostic strategies: CTA using a BPCA followed by invasive coronary angiography (ICA), CTA using a conventional agent followed by ICA, and ICA alone.

RESULTS

The use of the two CTA-based triage tests before ICA in a population with a CAD prevalence of less than 47% was predicted to be more cost-effective than ICA alone. Using the base-case values and a cost premium for BPCA over the conventional CT agent (cost of BPCA ≈ 5× that of a conventional agent) showed that CTA with a BPCA before ICA resulted in the most cost-effective strategy; the other strategies were ruled out by simple dominance. The model strongly depends on the rates of complications from the diagnostic tests included in the model. In a population with an elevated risk of contrast-induced nephropathy (CIN), a significant premium cost per BPCA dose still resulted in the alternative whereby CTA using BPCA was more cost-effective than CTA using a conventional agent. A similar effect was observed for potential complications resulting from the BPCA injection. Conversely, in the presence of a similar complication rate from BPCA, the diagnostic strategy of CTA using a conventional agent would be the optimal alternative.

CONCLUSION

BPCAs could have a significant impact in the diagnosis of acute chest pain, in particular for populations with high incidences of CIN. In addition, a BPCA strategy could garner further savings if currently excluded phenomena including renal disease and incidental findings were included in the decision model.

Nano-sized CT contrast agents

Computed tomography (CT) is one of the most widely used clinical imaging modalities. In order to increase the sensitivity of CT, small iodinated compounds are used as injectable contrast agents. However, the iodinated contrast agents are excreted through the kidney and have short circulation times. This rapid renal clearance not only restricts in vivo applications that require long circulation times but also sometimes induces serious adverse effects related to the excretion pathway. In addition, the X-ray attenuation of iodine is not efficient for clinical CT that uses high-energy X-ray. Due to these limitations, nano-sized iodinated CT contrast agents have been developed that can increase the circulation time and decrease the adverse effects. In addition to iodine, nanoparticles based on heavy atoms such as gold, lanthanides, and tantalum are used as more efficient CT contrast agents. In this review, we summarize the recent progresses made in nano-sized CT contrast agents.

Dual-energy computed tomography imaging of atherosclerotic plaques in a mouse model using a liposomal-iodine nanoparticle contrast agent

BACKGROUND

The accumulation of macrophages in inflamed atherosclerotic plaques has long been recognized. In an attempt to develop an imaging agent for detection of vulnerable plaques, we evaluated the feasibility of a liposomal-iodine nanoparticle contrast agent for computed tomography imaging of macrophage-rich atherosclerotic plaques in a mouse model.

METHODS AND RESULTS

Liposomal-iodine formulations varying in particle size and polyethylene glycol coating were fabricated and shown to stably encapsulate the iodine compound. In vitro uptake studies using optical and computed tomography imaging in the RAW 264.7 macrophage cell line identified the formulation that promoted maximal uptake. Dual-energy computed tomography imaging using this formulation in apolipoprotein E-deficient (ApoE(-/-)) mice (n=8) and control C57BL/6 mice (n=6) followed by spectral decomposition of the dual-energy images enabled imaging of the liposomes localized in the plaque. Imaging cytometry confirmed the presence of liposomes in the plaque and their colocalization with a small fraction (≈2%) of the macrophages in the plaque.

CONCLUSIONS

The results demonstrate the feasibility of imaging macrophage-rich atherosclerotic plaques using a liposomal-iodine nanoparticle contrast agent and dual-energy computed tomography.

High-resolution CT vascular imaging using blood pool contrast agents

While the evolution of computed tomography imaging in the last 2 decades has been driven almost exclusively by improvements in the instrumentation and processing algorithms, there have been comparatively modest advances in contrast agent technology.A notable change in the last decade has been the development of blood pool contrast agents based on nanoparticle technology.While not yet ready for clinical use, the stable and uniform opacification provided by these agents in normal vasculature and controlled extravasation in compromised vasculature enables novel techniques for imaging and diagnosis of pathologies. This manuscript presents preclinical examples demonstrating cardiovascular pathologies and tumor characterization by high-resolution computed tomography imaging.

Iodinated nano-emulsions as contrast agents for preclinical X-ray imaging: Impact of the free surfactants on the pharmacokinetics

This study presents new important aspects in the design of contrast agents for X-ray preclinical imaging. The first one is a new simple formulation of long circulating contrast agents, formulated from a commercial iodinated oil, and resulting in CT contrast agents containing more than twice the iodine concentration commercial contrast agents. The second point is a methodological aspect, utilizing tangential filtration for reducing the residual surfactants in the bulk phase and serving as well for concentrating droplets (and iodine) in the suspension. The last point is a more general aspect regarding the influence of the free surfactant on the pharmacokinetics and biodistribution of the nano-emulsion droplets on mice. We showed that cross-flow filtration is efficient for concentrating the droplets and reducing the concentration of free surfactant from 10wt.% to 1wt.%, without any changes in the nano-emulsion droplet morphologies or surface properties. We also showed that the presence of free surfactant has a significant impact on the elimination way of the nano-emulsion droplets, shared between liver and kidneys. The purified nano-emulsions are preferentially eliminated by the kidneys in contrast to raw nano-emulsions, predominantly eliminated by the liver. In practice, for two similar suspensions, half-life decreases from 4.1±1.10h to 2.5±0.77h before and after purification. Since the design and development of long circulating systems are critical in numerous domains, and not for preclinical CT imaging, this study presents important results in that field, taken under a formulation and technical point of view.

CT imaging with iopromide liposomes in a rabbit model

The aim of this study was to evaluate the computed tomography (CT)-imaging potential of iopromide-carrying liposomes (SPC/CH/SPG, 6:3:1) of approximately 200 nm in diameter in healthy rabbits and in rabbits with implanted liver tumors in an intraindividual comparison with iopromide. Normal rabbits and animals with VX2 tumors implanted into the liver received iopromide (600 mg of iodine/kg, bolus injection) and, 1 or 2 days later, iopromide liposomes (300 mg of iodine/kg, bolus injection or 10-minute infusion). CT imaging up to 1 hour after administration was performed, focusing on the aorta, vena cava, kidney, spleen, and liver. Pharmacokinetic parameters for CT enhancement were calculated. Detectability and delineation of liver lesions were assessed on a 4-grade scale, and differences were evaluated statistically. Using half the iodine dose, iopromide liposomes achieved similar blood-pool enhancement as iopromide. Detectability and delineation of liver lesions were easy/good in the arterial phase after iopromide injection, but poor in the venous and equilibration phases. Iopromide liposomes resulted in a long-lasting, good detectability and delineation of liver lesions similar or superior to that observed after iopromide in the arterial phase.