Compensation of optical heterogeneity-induced artifacts in fluorescence molecular tomography: theory and in vivo validation

Near-Infrared Fluorescent Probes as Imaging and Theranostic Modalities for Amyloid-Beta and Tau Aggregates in Alzheimer's Disease

A person suspected of having Alzheimer's disease (AD) is clinically diagnosed for the presence of principal biomarkers, especially misfolded amyloid-beta (Aβ) and tau proteins in the brain regions. Existing radiotracer diagnostic tools, such as PET imaging, are expensive and have limited availability for primary patient screening and pre-clinical animal studies. To change the status quo, small-molecular near-infrared (NIR) probes have been rapidly developed, which may serve as an inexpensive, handy imaging tool to comprehend the dynamics of pathogenic progression in AD and assess therapeutic efficacy in vivo. This Perspective summarizes the biochemistry of Aβ and tau proteins and then focuses on structurally diverse NIR probes with coverages of their spectroscopic properties, binding affinity toward Aβ and tau species, and theranostic effectiveness. With the summarized information and perspective discussions, we hope that this paper may serve as a guiding tool for designing novel in vivo imaging fluoroprobes with theranostic capabilities in the future.

Tomographic Imaging and Correlation to Quantify Vascular and Inflammatory Changes in an Experimental Spinal Cord Injury

Spinal cord injury (SCI) is a devastating condition causing the loss of sensory and motor functions. SCI pathology is multifaceted, encompassing inflammation, scarring, neuronal damage, and vascular and tissue remodeling. The dynamics of SCI rapidly transform from acute, sub-acute, and chronic phases. The rapidly changing environment necessitates the real-time monitoring of disease severity. Therefore, in this study, we used the IVIS spectrum, a noninvasive fluorescence imaging modality, to monitor the disease pathology in live animals. We used near-infrared fluorescence imaging agents including Angiosense 750 EX, a probe that detects vascular changes, and Cat B 680 FAST, a probe that detects inflammation at various day points post injury (DPI), that is, DPI-1, DPI-14, and DPI-28. We quantified the pathophysiological changes after SCI using IVIS in live animals. As a result, we observed distinct differences in the disease progression between injured and sham mice. Moreover, live imaging showed a good correlation with behavioral studies, protein expression, and immunohistological analysis. Hence, the goal of this study was to introduce a new optical imaging modality that offers a determination of disease severity and the advantage of accelerated imaging of the correlated biomarkers in a real-time and dynamic manner. This study concluded that Cat B 680 Fast and Angiosense 750 EX could be used to assess the disease severity after SCI. Furthermore, our study suggests that the noninvasive fluorescence optical imaging modality offers a unique approach in monitoring neuroinflammatory diseases in live animals.

Paradigms in Fluorescence Molecular Imaging: Maximizing Measurement of Biological Changes in Disease, Therapeutic Efficacy, and Toxicology/Safety

Fluorescence molecular imaging (MI) is an important concept in preclinical research that focuses on the visualization of cellular and biological function in a non-invasive fashion to better understand in vivo disease processes and treatment effects. MI differs fundamentally from traditional preclinical imaging strategies in that it generally relies on reporter probes specific for particular targets or pathways that can be used to reveal biological changes in situ, at the site(s) of disease. In contrast, the more established imaging modalities, like magnetic resonance imaging, X-ray, micro X-ray computed tomography, and ultrasound, historically have relied primarily on late-stage anatomical or physiologic changes. The practical application of fluorescence MI, however, has drifted somewhat from the emphasis on quantifying biology, and based on the publication record, it now appears to include any imaging in which a probe or contrast agent is used to non-invasively acquire in vivo endpoint information. Unfortunately, the mere use of a defined biologically specific probe, in the absence of careful study design, does not guarantee that any useful biological information is actually gained, although often useful endpoint results still can be achieved. This review proposes to add subcategories of MI, termed MI biological assessment (or MIBA), that emphasize a focus on obtaining early and clear biological changes associated with disease development, therapeutic efficacy, and drug-induced tissue changes. Proper selection of probes and careful study design are critical for maximizing the non-invasive assessment of in vivo biological changes, and applications of these critical elements are described.

Mechanistic profiling of the release kinetics of siRNA from lipidoid-polymer hybrid nanoparticles in vitro and in vivo after pulmonary administration

Understanding the release kinetics of siRNA from nanocarriers, their cellular uptake, their in vivo biodistribution and pharmacokinetics is a fundamental prerequisite for efficient optimisation of the design of nanocarriers for siRNA-based therapeutics. Thus, we investigated the influence of composition on the siRNA release from lipid-polymer hybrid nanoparticles (LPNs) consisting of cationic lipidoid 5 (L₅) and poly(DL-lactic-co-glycolic acid) (PLGA) intended for pulmonary administration. An array of siRNA-loaded LPNs was prepared by systematic variation of: (i) the L₅ content (10-20%, w/w), and (ii) the L₅:siRNA ratio (10,1-30:1, w/w). For comparative purposes, L₅-based lipoplexes, L₅-based stable nucleic acid lipid nanoparticles (SNALPs). and dioleoyltrimethylammoniumpropane (DOTAP)-modified LPNs loaded with siRNA were also prepared. Release studies in buffer and lung surfactant-containing medium showed that siRNA release is dependent on the presence of both surfactant and heparin (a displacing agent) in the release medium, since these interact with the lipid shell structure thereby facilitating decomplexation of L₅ and siRNA, as evident from the retarded siRNA release when the L₅ content and the L₅:siRNA ratio were increased. This confirms the hypothesis that siRNA loaded in LPNs is predominantly present as complexes with the cationic lipid and primarily is located near the particle surface. Cellular uptake and tolerability studies in the human macrophage cell line THP-1 and the type I-like human alveolar epithelial cell line hAELVi, which together represents a monolayer-based barrier model of lung epithelium, indicated that uptake of LPNs was much higher in THP-1 cells in agreement with their primary clearance role. In vivo biodistributions of formulations loaded with Alexa Fluor® 750-labelled siRNA after pulmonary administration in mice were compared by using quantitative fluorescence imaging tomography. The L₅-modified LPNs, SNALPs and DOTAP-modified LPNs displayed significantly increased lung retention of siRNA as compared to L₅-based lipoplexes, which had a biodistribution profile comparable to that of non-loaded siRNA, for which >50% of the siRNA dose permeated the air-blood barrier within 6 h and subsequently was excreted via the kidneys. Hence, the enhanced lung retention upon pulmonary administration of siRNA-loaded LPNs represents a promising characteristic that can be used to control the delivery of the siRNA cargo to lung tissue for local management of disease.

Optical tomographic imaging of near infrared imaging agents quantifies disease severity and immunomodulation of experimental autoimmune encephalomyelitis in vivo

Background

Experimental autoimmune encephalomyelitis (EAE) is an animal model that captures many of the hallmarks of human multiple sclerosis (MS), including blood–brain barrier (BBB) breakdown, inflammation, demyelination and axonal destruction. The standard clinical score measurement of disease severity and progression assesses functional changes in animal mobility; however, it does not offer information regarding the underlying pathophysiology of the disease in real time. The purpose of this study was to apply a novel optical imaging technique that offers the advantage of rapid imaging of relevant biomarkers in live animals.

Methods

Advances in non-invasive fluorescence molecular tomographic (FMT) imaging, in combination with a variety of biological imaging agents, offer a unique, sensitive and quantifiable approach to assessing disease biology in living animals. Using vascular (AngioSense 750EX) and protease-activatable cathepsin B (Cat B 680 FAST) near infrared (NIR) fluorescence imaging agents to detect BBB breakdown and inflammation, respectively, we quantified brain and spinal cord changes in mice with relapsing-remitting PLP_139-151-induced EAE and in response to tolerogenic therapy.

Results

FMT imaging and analysis techniques were carefully characterized and non-invasive imaging results corroborated by both ex vivo tissue imaging and comparison to clinical score results and histopathological analysis of CNS tissue. FMT imaging showed clear differences between control and diseased mice, and immune tolerance induction by antigen-coupled PLGA nanoparticles effectively blocked both disease induction and accumulation of imaging agents in the brain and spinal cord.

Conclusions

Cat B 680 FAST and AngioSense 750EX offered the combination best able to detect disease in both the brain and spinal cord, as well as the downregulation of disease by antigen-specific tolerance. Non-invasive optical tomographic imaging thus offers a unique approach to monitoring neuroinflammatory disease and therapeutic intervention in living mice with EAE.

Distribution of aerosols in mouse lobes by fluorescent imaging

Better methods are needed to quantify the distribution of drug among the airways of the lungs of small animals to facilitate the development of agents that can target specific airways. Mice were exposed to aerosols of aluminum phthalocyanine tetrasulfonic acid (AlPCS) that ranged in concentration and size (0.2-2.8 μm). The trachea and lobes were removed and placed between glass slides, and fluorescent images were obtained at two different compression thicknesses. The intensity, normalized by the area, exposure time, and thickness, was then plotted as a function of compression thickness, from which the concentration and attenuation coefficient were estimated for each lobe and then for each pixel of the image. The latter was then used to generate an image reflective of the concentration. The lobe volume, concentration, and tissue attenuation of AlPCS was consistent among the lobes. The deposition fraction increased with decreasing particle size. The network of lines in the concentration image indicated that connective tissue has a lower concentration. The central airways were clearly evident in the images of mice exposed to the very small and large aerosols. This approach provides a rapid, economical means to obtain high resolution images of mouse lungs from which detailed analysis of the distribution of deposited aerosol particles can be obtained.

Noninvasive in vivo quantification of neutrophil elastase activity in acute experimental mouse lung injury

We developed a neutrophil elastase-specific near-infrared fluorescence imaging agent, which, combined with fluorescence molecular tomographic imaging, allowed us to detect and quantify neutrophil elastase activity in vivo, in real time, and noninvasively in an acute model of lung injury (ALI). Significantly higher fluorescent signal was quantified in mice with LPS/fMLP-induced ALI as compared to healthy controls, correlating with increases in the number of bronchoalveolar lavage cells, neutrophils, and elastase activity. The agent was significantly activated ex vivo in lung sections from ALI but not from control mice, and this activation was ablated by the specific inhibitor sivelestat. Treatment with the specific inhibitor sivelestat significantly reduced lung signal in mice with ALI. These results underscore the unique ability of fluorescence molecular imaging to quantify specific molecular processes in vivo, crucial for understanding the mechanisms underlying disease progression and for assessing and monitoring novel pharmacological interventions.

Quantitative whole body biodistribution of fluorescent-labeled agents by non-invasive tomographic imaging

When small molecules or proteins are injected into live animals, their physical and chemical properties will significantly affect pharmacokinetics, tissue penetration, and the ultimate routes of metabolism and clearance. Fluorescence molecular tomography (FMT) offers the ability to non-invasively image and quantify temporal changes in fluorescence throughout the major organ systems of living animals, in a manner analogous to traditional approaches with radiolabeled agents. This approach is best used with biotherapeutics (therapeutic antibodies, or other large proteins) or large-scaffold drug-delivery vectors, that are minimally affected by low-level fluorophore conjugation. Application to small molecule drugs should take into account the significant impact of fluorophore labeling on size and physicochemical properties, however, the presents studies show that this technique is readily applied to small molecule agents developed for far-red (FR) or near infrared (NIR) imaging. Quantification by non-invasive FMT correlated well with both fluorescence from tissue homogenates as well as with planar (2D) fluorescence reflectance imaging of excised intact organs (r²  =  0.996 and 0.969, respectively). Dynamic FMT imaging (multiple times from 0 to 24 h) performed in live mice after the injection of four different FR/NIR-labeled agents, including immunoglobulin, 20-50 nm nanoparticles, a large vascular imaging agent, and a small molecule integrin antagonist, showed clear differences in the percentage of injected dose per gram of tissue (%ID/g) in liver, kidney, and bladder signal. Nanoparticles and IgG1 favored liver over kidney signal, the small molecule integrin-binding agent favored rapid kidney and bladder clearance, and the vascular agent, showed both liver and kidney clearance. Further assessment of the volume of distribution of these agents by fluorescent volume added information regarding their biodistribution and highlighted the relatively poor extravasation into tissue by IgG1. These studies demonstrate the ability of quantitative FMT imaging of FR/NIR agents to non-invasively visualize and quantify the biodistribution of different agents over time.

Measurement of the distribution of aerosols among mouse lobes by fluorescent imaging

Lung samples were prepared to investigate the perturbing effects of light absorption for quantifying the fluorescence signal of aluminum phthalocyanine tetrasulfonic acid (AlPCS). Standard solutions of known concentration and depth were imaged with different exposure times and analyzed. The intensity was found to be a linear function of concentration, depth, exposure time, and area. Mice were exposed to an aerosol of AlPCS with a mass median aerodynamic diameter of 390 nm and geometric standard deviation of 1.8. Images of intact lung lobes and lung homogenates were obtained and then analyzed to allow quantifying the concentration of AlPCS among the lung lobes and trachea. For the distribution of aerosols, the results indicate that the concentration was uniform among the different lobes. Combining the quantitative analysis of the concentration with image analysis of the area/thickness, the mass deposited in each lobe was readily determined. This approach provides a quantitative means to determine the selectivity of drug delivery to mouse lower respiratory tract.