Non-invasive in vivo imaging of near infrared-labeled transferrin in breast cancer cells and tumors using fluorescence lifetime FRET

Fluorescence Lifetime Imaging for Quantification of Targeted Drug Delivery in Varying Tumor Microenvironments

Rationale

Trastuzumab (TZM) is a monoclonal antibody that targets the human epidermal growth factor receptor (HER2) and is clinically used for the treatment of HER2-positive breast tumors. However, the tumor microenvironment can limit the access of TZM to the HER2 targets across the whole tumor and thereby compromise TZM's therapeutic efficacy. An imaging methodology that can non-invasively quantify the binding of TZM-HER2, which is required for therapeutic action, and distribution within tumors with varying tumor microenvironments is much needed.

Methods

We performed near-infrared (NIR) fluorescence lifetime (FLI) Forster Resonance Energy Transfer (FRET) to measure TZM-HER2 binding, using in vitro microscopy and in vivo widefield macroscopy, in HER2 overexpressing breast and ovarian cancer cells and tumor xenografts, respectively. Immunohistochemistry was used to validate in vivo imaging results.

Results

NIR FLI FRET in vitro microscopy data show variations in intracellular distribution of bound TZM in HER2-positive breast AU565 and AU565 tumor-passaged XTM cell lines in comparison to SKOV-3 ovarian cancer cells. Macroscopy FLI (MFLI) FRET in vivo imaging data show that SKOV-3 tumors display reduced TZM binding compared to AU565 and XTM tumors, as validated by ex vivo immunohistochemistry. Moreover, AU565/XTM and SKOV-3 tumor xenografts display different amounts and distributions of TME components, such as collagen and vascularity. Therefore, these results suggest that SKOV-3 tumors are refractory to TZM delivery due to their disrupted vasculature and increased collagen content.

Conclusion

Our study demonstrates that FLI is a powerful analytical tool to monitor the delivery of antibody drug tumor both in cell cultures and in vivo live systems. Especially, MFLI FRET is a unique imaging modality that can directly quantify target engagement with potential to elucidate the role of the TME in drug delivery efficacy in intact live tumor xenografts.

Transferrin receptor in primary and metastatic breast cancer: Evaluation of expression and experimental modulation to improve molecular targeting

BACKGROUND

Conjugation of transferrin (Tf) to imaging or nanotherapeutic agents is a promising strategy to target breast cancer. Since the efficacy of these biomaterials often depends on the overexpression of the targeted receptor, we set out to survey expression of transferrin receptor (TfR) in primary and metastatic breast cancer samples, including metastases and relapse, and investigate its modulation in experimental models.

METHODS

Gene expression was investigated by datamining in twelve publicly-available datasets. Dedicated Tissue microarrays (TMAs) were generated to evaluate matched primary and bone metastases as well as and pre and post chemotherapy tumors from the same patient. TMA were stained with the FDA-approved MRQ-48 antibody against TfR and graded by staining intensity (H-score). Patient-derived xenografts (PDX) and isogenic metastatic mouse models were used to study in vivo TfR expression and uptake of transferrin.

RESULTS

TFRC gene and protein expression were high in breast cancer of all subtypes and stages, and in 60-85% of bone metastases. TfR was detectable after neoadjuvant chemotherapy, albeit with some variability. Fluorophore-conjugated transferrin iron chelator deferoxamine (DFO) enhanced TfR uptake in human breast cancer cells in vitro and proved transferrin localization at metastatic sites and correlation of tumor burden relative to untreated tumor mice.

CONCLUSIONS

TfR is expressed in breast cancer, primary, metastatic, and after neoadjuvant chemotherapy. Variability in expression of TfR suggests that evaluation of the expression of TfR in individual patients could identify the best candidates for targeting. Further, systemic iron chelation with DFO may upregulate receptor expression and improve uptake of therapeutics or tracers that use transferrin as a homing ligand.

Near-infrared spectroscopy and machine learning algorithms for rapid and non-invasive detection of Trichuris

BACKGROUND

Trichuris trichiura (whipworm) is one of the most prevalent soil transmitted helminths (STH) affecting 604-795 million people worldwide. Diagnostic tools that are affordable and rapid are required for detecting STH. Here, we assessed the performance of the near-infrared spectroscopy (NIRS) technique coupled with machine learning algorithms to detect Trichuris muris in faecal, blood, serum samples and non-invasively through the skin of mice.

METHODOLOGY

We orally infected 10 mice with 30 T. muris eggs (low dose group), 10 mice with 200 eggs (high dose group) and 10 mice were used as the control group. Using the NIRS technique, we scanned faecal, serum, whole blood samples and mice non-invasively through their skin over a period of 6 weeks post infection. Using artificial neural networks (ANN) and spectra of faecal, serum, blood and non-invasive scans from one experiment, we developed 4 algorithms to differentiate infected from uninfected mice. These models were validated on mice from a second independent experiment.

PRINCIPAL FINDINGS

NIRS and ANN differentiated mice into the three groups as early as 2 weeks post infection regardless of the sample used. These results correlated with those from concomitant serological and parasitological investigations.

SIGNIFICANCE

To our knowledge, this is the first study to demonstrate the potential of NIRS as a diagnostic tool for human STH infections. The technique could be further developed for large scale surveillance of soil transmitted helminths in human populations.

Probing organoid metabolism using fluorescence lifetime imaging microscopy (FLIM): The next frontier of drug discovery and disease understanding

Organoid models have been used to address important questions in developmental and cancer biology, tissue repair, advanced modelling of disease and therapies, among other bioengineering applications. Such 3D microenvironmental models can investigate the regulation of cell metabolism, and provide key insights into the mechanisms at the basis of cell growth, differentiation, communication, interactions with the environment and cell death. Their accessibility and complexity, based on 3D spatial and temporal heterogeneity, make organoids suitable for the application of novel, dynamic imaging microscopy methods, such as fluorescence lifetime imaging microscopy (FLIM) and related decay time-assessing readouts. Several biomarkers and assays have been proposed to study cell metabolism by FLIM in various organoid models. Herein, we present an expert-opinion discussion on the principles of FLIM and PLIM, instrumentation and data collection and analysis protocols, and general and emerging biosensor-based approaches, to highlight the pioneering work being performed in this field.

In vivo quantitative FRET small animal imaging: Intensity versus lifetime-based FRET

Förster resonance energy transfer (FRET) microscopy is used in numerous biophysical and biomedical applications to monitor inter- and intramolecular interactions and conformational changes in the 2-10 nm range. FRET is currently being extended to in vivo optical imaging, its main application being in quantifying drug-target engagement or drug release in animal models of cancer using organic dye or nanoparticle-labeled probes. Herein, we compared FRET quantification using intensity-based FRET (sensitized emission FRET analysis with the three-cube approach using an IVIS imager) and macroscopic fluorescence lifetime (MFLI) FRET using a custom system using a time-gated-intensified charge-coupled device, for small animal optical in vivo imaging. The analytical expressions and experimental protocols required to quantify the product f D E of the FRET efficiency E and the fraction of donor molecules involved in FRET, f D , are described in detail for both methodologies. Dynamic in vivo FRET quantification of transferrin receptor-transferrin binding was acquired in live intact nude mice upon intravenous injection of a near-infrared-labeled transferrin FRET pair and benchmarked against in vitro FRET using hybridized oligonucleotides. Even though both in vivo imaging techniques provided similar dynamic trends for receptor-ligand engagement, we demonstrate that MFLI-FRET has significant advantages. Whereas the sensitized emission FRET approach using the IVIS imager required nine measurements (six of which are used for calibration) acquired from three mice, MFLI-FRET needed only one measurement collected from a single mouse, although a control mouse might be needed in a more general situation. Based on our study, MFLI therefore represents the method of choice for longitudinal preclinical FRET studies such as that of targeted drug delivery in intact, live mice.

in vivo quantitative FRET small animal imaging: intensity versus lifetime-based FRET

Förster Resonance Energy Transfer (FRET) microscopy is used in numerous biophysical and biomedical applications to monitor inter- and intramolecular interactions and conformational changes in the 2-10 nm range. FRET is currently being extended to in vivo optical imaging, its main application being in quantifying drug-target engagement or drug release in animal models of cancer using organic dye or nanoparticle-labeled probes. Herein, we compared FRET quantification using intensity-based FRET (sensitized emission FRET analysis with the 3-cube approach using an IVIS imager) and macroscopic fluorescence lifetime (MFLI) FRET using a custom system using a time-gated ICCD, for small animal optical in vivo imaging. The analytical expressions and experimental protocols required to quantify the product f D E of the FRET efficiency E and the fraction of donor molecules involved in FRET, f D , are described in detail for both methodologies. Dynamic in vivo FRET quantification of transferrin receptor-transferrin binding was acquired in live intact nude mice upon intravenous injection of near infrared-labeled transferrin FRET pair and benchmarked against in vitro FRET using hybridized oligonucleotides. Even though both in vivo imaging techniques provided similar dynamic trends for receptor-ligand engagement, we demonstrate that MFLI FRET has significant advantages. Whereas the sensitized emission FRET approach using the IVIS imager required 9 measurements (6 of which are used for calibration) acquired from three mice, MFLI FRET needed only one measurement collected from a single mouse, although a control mouse might be needed in a more general situation. Based on our study, MFLI therefore represents the method of choice for longitudinal preclinical FRET studies such as that of targeted drug delivery in intact, live mice.

Single-Molecule Imaging of Membrane Proteins on Vascular Endothelial Cells

Transporting substances such as gases, nutrients, waste, and cells is the primary function of blood vessels. Vascular cells use membrane proteins to perform crucial endothelial functions, including molecular transport, immune cell infiltration, and angiogenesis. A thorough understanding of these membrane receptors from a clinical perspective is warranted to gain insights into the pathogenesis of vascular diseases and to develop effective methods for drug delivery through the vascular endothelium. This review summarizes state-of-the-art single-molecule imaging techniques, such as super-resolution microscopy, single-molecule tracking, and protein-protein interaction analysis, for observing and studying membrane proteins. Furthermore, recent single-molecule studies of membrane proteins such as cadherins, integrins, caveolins, transferrin receptors, vesicle-associated protein-1, and vascular endothelial growth factor receptor are discussed.

Malaria absorption peaks acquired through the skin of patients with infrared light can detect patients with varying parasitemia

To eliminate malaria, scalable tools that are rapid, affordable, and can detect patients with low parasitemia are required. Non-invasive diagnostic tools that are rapid, reagent-free, and affordable would also provide a justifiable platform for testing malaria in asymptomatic patients. However, non-invasive surveillance techniques for malaria remain a diagnostic gap. Here, we show near-infrared Plasmodium absorption peaks acquired non-invasively through the skin using a miniaturized hand-held near-infrared spectrometer. Using spectra from the ear, these absorption peaks and machine learning techniques enabled non-invasive detection of malaria-infected human subjects with varying parasitemia levels in less than 10 s.

Computational macroscopic lifetime imaging and concentration unmixing of autofluorescence

Single-pixel computational imaging can leverage highly sensitive detectors that concurrently acquire data across spectral and temporal domains. For molecular imaging, such methodology enables to collect rich intensity and lifetime multiplexed fluorescence datasets. Herein we report on the application of a single-pixel structured light-based platform for macroscopic imaging of tissue autofluorescence. The super-continuum visible excitation and hyperspectral single-pixel detection allow for parallel characterization of autofluorescence intensity and lifetime. Furthermore, we exploit a deep learning based data processing pipeline, to perform autofluorescence unmixing while yielding the autofluorophores' concentrations. The full scheme (setup and processing) is validated in silico and in vitro with clinically relevant autofluorophores flavin adenine dinucleotide, riboflavin, and protoporphyrin. The presented results demonstrate the potential of the methodology for macroscopically quantifying the intensity and lifetime of autofluorophores, with higher specificity for cases of mixed emissions, which are ubiquitous in autofluorescence and multiplexed in vivo imaging.

Design and characterization of a time-domain optical tomography platform for mesoscopic lifetime imaging

We report on the system design and instrumental characteristics of a novel time-domain mesoscopic fluorescence molecular tomography (TD-MFMT) system for multiplexed molecular imaging in turbid media. The system is equipped with a supercontinuum pulsed laser for broad spectral excitation, based on a high-density descanned raster scanning intensity-based acquisition for 2D and 3D imaging and augmented with a high-dynamical range linear time-resolved single-photon avalanche diode (SPAD) array for lifetime quantification. We report on the system's spatio-temporal and spectral characteristics and its sensitivity and specificity in controlled experimental settings. Also, a phantom study is undertaken to test the performance of the system to image deeply-seated fluorescence inclusions in tissue-like media. In addition, ex vivo tumor xenograft imaging is performed to validate the system's applicability to the biological sample. The characterization results manifest the capability to sense small fluorescence concentrations (on the order of nanomolar) while quantifying fluorescence lifetimes and lifetime-based parameters at high resolution. The phantom results demonstrate the system's potential to perform 3D multiplexed imaging thanks to spectral and lifetime contrast in the mesoscopic range (at millimeters depth). The ex vivo imaging exhibits the prospect of TD-MFMT to resolve intra-tumoral heterogeneity in a depth-dependent manner.

Macroscopic Fluorescence Lifetime Imaging for Monitoring of Drug-Target Engagement

Precision medicine promises to improve therapeutic efficacy while reducing adverse effects, especially in oncology. However, despite great progresses in recent years, precision medicine for cancer treatment is not always part of routine care. Indeed, the ability to specifically tailor therapies to distinct patient profiles requires still significant improvements in targeted therapy development as well as decreases in drug treatment failures. In this regard, preclinical animal research is fundamental to advance our understanding of tumor biology, and diagnostic and therapeutic response. Most importantly, the ability to measure drug-target engagement accurately in live and intact animals is critical in guiding the development and optimization of targeted therapy. However, a major limitation of preclinical molecular imaging modalities is their lack of capability to directly and quantitatively discriminate between drug accumulation and drug-target engagement at the pathological site. Recently, we have developed Macroscopic Fluorescence Lifetime Imaging (MFLI) as a unique feature of optical imaging to quantitate in vivo drug-target engagement. MFLI quantitatively reports on nanoscale interactions via lifetime-sensing of Förster Resonance Energy Transfer (FRET) in live, intact animals. Hence, MFLI FRET acts as a direct reporter of receptor dimerization and target engagement via the measurement of the fraction of labeled-donor entity undergoing binding to its respective receptor. MFLI is expected to greatly impact preclinical imaging and also adjacent fields such as image-guided surgery and drug development.

Visualizing and quantifying antimicrobial drug distribution in tissue

The biodistribution and pharmacokinetics of drugs are vital to the mechanistic understanding of their efficacy. Measuring antimicrobial drug efficacy has been challenging as plasma drug concentration is used as a surrogate for tissue drug concentration, yet typically does not reflect that at the intended site(s) of action. Utilizing an image-guided approach, it is feasible to accurately quantify the biodistribution and pharmacokinetics within the desired site(s) of action. We outline imaging modalities used in visualizing drug distribution with examples ranging from in vitro cellular drug uptake to clinical treatment of microbial infections. The imaging modalities of interest are: radio-labeling, magnetic resonance, mass spectrometry imaging, computed tomography, fluorescence, and Raman spectroscopy. We outline the progress, limitations, and future outlook for each methodology. Further advances in these optical approaches would benefit patients and researchers alike, as non-invasive imaging could yield more profound insights with a lower clinical burden than invasive measurement approaches used today.

Emerging role of ferroptosis in breast cancer: New dawn for overcoming tumor progression

Breast cancer has become a serious threat to women's health. Cancer progression is mainly derived from resistance to apoptosis induced by procedures or therapies. Therefore, new drugs or models that can overcome apoptosis resistance should be identified. Ferroptosis is a recently identified mode of cell death characterized by excess reactive oxygen species-induced lipid peroxidation. Since ferroptosis is distinct from apoptosis, necrosis and autophagy, its induction successfully eliminates cancer cells that are resistant to other modes of cell death. Therefore, ferroptosis may become a new direction around which to design breast cancer treatment. Unfortunately, the complete appearance of ferroptosis in breast cancer has not yet been fully elucidated. Furthermore, whether ferroptosis inducers can be used in combination with traditional anti- breast cancer drugs is still unknown. Moreover, a summary of ferroptosis in breast cancer progression and therapy is currently not available. In this review, we discuss the roles of ferroptosis-associated modulators glutathione, glutathione peroxidase 4, iron, nuclear factor erythroid-2 related factor-2, superoxide dismutases, lipoxygenase and coenzyme Q in breast cancer. Furthermore, we provide evidence that traditional drugs against breast cancer induce ferroptosis, and that ferroptosis inducers eliminate breast cancer cells. Finally, we put forward prospect of using ferroptosis inducers in breast cancer therapy, and predict possible obstacles and corresponding solutions. This review will deepen our understanding of the relationship between ferroptosis and breast cancer, and provide new insights into breast cancer-related therapeutic strategies.

Piperine-Loaded Glycyrrhizic Acid- and PLGA-Based Nanoparticles Modified with Transferrin for Antitumor : Piperine-Loaded Glycyrrhizic Acid- and PLGA-Based Nanoparticles

The purpose of this study was to enhance the antitumor effect of piperine by constructing the nanoparticles modified with transferrin (Tf-PIP-NPs) and evaluating their efficacy in vitro and in vivo. The Tf-PIP-NPs were prepared by the solvent evaporation method, and their properties were characterized. The effects of Tf-PIP-NPs on cytotoxicity, cell uptake, apoptosis, and mitochondrial membrane potential were evaluated in HepG2 cells, MDA-MB-231 cells, and 4T1 cells. In a 4T1 tumor-bearing mouse model, the antitumor efficacy of Tf-PIP-NPs was assessed in terms of tumor volumes, changes in body weight, HE staining, and immunohistochemical analysis. With a mean particle size of 112.2 ± 1.27 nm, the zeta potential of (- 28.0 ± 1.6 mV) Tf-PIP-NPs were rapidly internalized by tumor cells after 1 h through the transferrin receptor (TfR)-mediated endocytosis pathway, significantly inducing cellular apoptosis and mitochondrial membrane potential loss. Although Tf-PIP-NPs had no significant difference with PIP-NPs in tumor volume inhibition due to the presence of tumor microenvironment, it could significantly upregulate the expression of related pro-apoptotic proteins and induce tumor necrosis. We used the self-assembly properties of glycyrrhizic acid (GL) and polymer-PLGA to encapsulate piperine and modified with the transferrin, which provided a promising approach to improve the antitumor efficacy for anticarcinogen.

Iron-binding cellular profile of transferrin using label-free Raman hyperspectral imaging and singular value decomposition (SVD)

Serum transferrin (Tf) is the essential iron transport protein in the body. Transferrin is responsible for the sequestration of free iron in serum and the delivery of iron throughout the body and into cells, where iron is released inside a mildly acidified endosome. Altered iron distributions are associated with diseases such as iron-overload, cancer, and cardiovascular disease. The presence of free iron is linked to deleterious redox reactions, inside and outside cells and organelles. As Tf iron release is pH dependent, any changes in intraorganelle and extracellular pH, often associated with disease progression, could inhibit normal iron delivery or accelerate iron release in the wrong compartment. However, imaging approaches to monitor changes in the iron-bound state of Tf are lacking. Recently, Raman spectroscopy has been shown to measure iron-bound forms of Tf in solution, intact cells and tissue samples. Here, a biochemical Raman assay has been developed to identify iron-release from Tf following modification of chemical environment. Quantitative singular value decomposition (SVD) method has been applied to discriminate between iron-bound Tf samples during endocytic trafficking in intact cancer cells subjected to Raman hyperspectral confocal imaging. We demonstrate the strength of the SVD method to monitor pH-induced Tf iron-release using Raman hyperspectral imaging, providing the redox biology field with a novel tool that facilitates subcellular investigation of the iron-binding profile of transferrin in various disease models.

Luminescence lifetime imaging of three-dimensional biological objects

A major focus of current biological studies is to fill the knowledge gaps between cell, tissue and organism scales. To this end, a wide array of contemporary optical analytical tools enable multiparameter quantitative imaging of live and fixed cells, three-dimensional (3D) systems, tissues, organs and organisms in the context of their complex spatiotemporal biological and molecular features. In particular, the modalities of luminescence lifetime imaging, comprising fluorescence lifetime imaging (FLI) and phosphorescence lifetime imaging microscopy (PLIM), in synergy with Förster resonance energy transfer (FRET) assays, provide a wealth of information. On the application side, the luminescence lifetime of endogenous molecules inside cells and tissues, overexpressed fluorescent protein fusion biosensor constructs or probes delivered externally provide molecular insights at multiple scales into protein-protein interaction networks, cellular metabolism, dynamics of molecular oxygen and hypoxia, physiologically important ions, and other physical and physiological parameters. Luminescence lifetime imaging offers a unique window into the physiological and structural environment of cells and tissues, enabling a new level of functional and molecular analysis in addition to providing 3D spatially resolved and longitudinal measurements that can range from microscopic to macroscopic scale. We provide an overview of luminescence lifetime imaging and summarize key biological applications from cells and tissues to organisms.

Quantification of Trastuzumab-HER2 Engagement In Vitro and In Vivo

Human EGF Receptor 2 (HER2) is an important oncogene driving aggressive metastatic growth in up to 20% of breast cancer tumors. At the same time, it presents a target for passive immunotherapy such as trastuzumab (TZM). Although TZM has been widely used clinically since 1998, not all eligible patients benefit from this therapy due to primary and acquired drug resistance as well as potentially lack of drug exposure. Hence, it is critical to directly quantify TZM–HER2 binding dynamics, also known as cellular target engagement, in undisturbed tumor environments in live, intact tumor xenograft models. Herein, we report the direct measurement of TZM–HER2 binding in HER2-positive human breast cancer cells and tumor xenografts using fluorescence lifetime Forster Resonance Energy Transfer (FLI-FRET) via near-infrared (NIR) microscopy (FLIM-FRET) as well as macroscopy (MFLI-FRET) approaches. By sensing the reduction of fluorescence lifetime of donor-labeled TZM in the presence of acceptor-labeled TZM, we successfully quantified the fraction of HER2-bound and internalized TZM immunoconjugate both in cell culture and tumor xenografts in live animals. Ex vivo immunohistological analysis of tumors confirmed the binding and internalization of TZM–HER2 complex in breast cancer cells. Thus, FLI-FRET imaging presents a powerful analytical tool to monitor and quantify cellular target engagement and subsequent intracellular drug delivery in live HER2-positive tumor xenografts.

High compression deep learning based single-pixel hyperspectral macroscopic fluorescence lifetime imaging in vivo

Single pixel imaging frameworks facilitate the acquisition of high-dimensional optical data in biological applications with photon starved conditions. However, they are still limited to slow acquisition times and low pixel resolution. Herein, we propose a convolutional neural network for fluorescence lifetime imaging with compressed sensing at high compression (NetFLICS-CR), which enables in vivo applications at enhanced resolution, acquisition and processing speeds, without the need for experimental training datasets. NetFLICS-CR produces intensity and lifetime reconstructions at 128 × 128 pixel resolution over 16 spectral channels while using only up to 1% of the required measurements, therefore reducing acquisition times from ∼2.5 hours at 50% compression to ∼3 minutes at 99% compression. Its potential is demonstrated in silico, in vitro and for mice in vivo through the monitoring of receptor-ligand interactions in liver and bladder and further imaging of intracellular delivery of the clinical drug Trastuzumab to HER2-positive breast tumor xenografts. The data acquisition time and resolution improvement through NetFLICS-CR, facilitate the translation of single pixel macroscopic flurorescence lifetime imaging (SP-MFLI) for in vivo monitoring of lifetime properties and drug uptake.

Multiplexed non-invasive tumor imaging of glucose metabolism and receptor-ligand engagement using dark quencher FRET acceptor

Rationale: Following an ever-increased focus on personalized medicine, there is a continuing need to develop preclinical molecular imaging modalities to guide the development and optimization of targeted therapies. Near-Infrared (NIR) Macroscopic Fluorescence Lifetime Förster Resonance Energy Transfer (MFLI-FRET) imaging offers a unique method to robustly quantify receptor-ligand engagement in live intact animals, which is critical to assess the delivery efficacy of therapeutics. However, to date, non-invasive imaging approaches that can simultaneously measure cellular drug delivery efficacy and metabolic response are lacking. A major challenge for the implementation of concurrent optical and MFLI-FRET in vivo whole-body preclinical imaging is the spectral crowding and cross-contamination between fluorescent probes. Methods: We report on a strategy that relies on a dark quencher enabling simultaneous assessment of receptor-ligand engagement and tumor metabolism in intact live mice. Several optical imaging approaches, such as in vitro NIR FLI microscopy (FLIM) and in vivo wide-field MFLI, were used to validate a novel donor-dark quencher FRET pair. IRDye 800CW 2-deoxyglucose (2-DG) imaging was multiplexed with MFLI-FRET of NIR-labeled transferrin FRET pair (Tf-AF700/Tf-QC-1) to monitor tumor metabolism and probe uptake in breast tumor xenografts in intact live nude mice. Immunohistochemistry was used to validate in vivo imaging results. Results: First, we establish that IRDye QC-1 (QC-1) is an effective NIR dark acceptor for the FRET-induced quenching of donor Alexa Fluor 700 (AF700). Second, we report on simultaneous in vivo imaging of the metabolic probe 2-DG and MFLI-FRET imaging of Tf-AF700/Tf-QC-1 uptake in tumors. Such multiplexed imaging revealed an inverse relationship between 2-DG uptake and Tf intracellular delivery, suggesting that 2-DG signal may predict the efficacy of intracellular targeted delivery. Conclusions: Overall, our methodology enables for the first time simultaneous non-invasive monitoring of intracellular drug delivery and metabolic response in preclinical studies.

NIR Photodynamic Destruction of PDAC and HNSCC Nodules Using Triple-Receptor-Targeted Photoimmuno-Nanoconjugates: Targeting Heterogeneity in Cancer

Receptor heterogeneity in cancer is a major limitation of molecular targeting for cancer therapeutics. Single-receptor-targeted treatment exerts selection pressures that result in treatment escape for low-receptor-expressing tumor subpopulations. To overcome this potential for heterogeneity-driven resistance to molecular targeted photodynamic therapy (PDT), we present for the first time a triple-receptor-targeted photoimmuno-nanoconjugate (TR-PIN) platform. TR-PIN functionalization with cetuximab, holo-transferrin, and trastuzumab conferred specificity for epidermal growth factor receptor (EGFR), transferrin receptor (TfR), and human epidermal growth factor receptor 2 (HER-2), respectively. The TR-PINs exhibited up to a 24-fold improvement in cancer cell binding compared with EGFR-specific cetuximab-targeted PINs (Cet-PINs) in low-EGFR-expressing cell lines. Photodestruction using TR-PINs was significantly higher than the monotargeted Cet-PINs in heterocellular 3D in vitro models of heterogeneous pancreatic ductal adenocarcinoma (PDAC; MIA PaCa-2 cells) and heterogeneous head and neck squamous cell carcinoma (HNSCC, SCC9 cells) containing low-EGFR-expressing T47D (high TfR) or SKOV-3 (high HER-2) cells. Through their capacity for multiple tumor target recognition, TR-PINs can serve as a unique and amenable platform for the effective photodynamic eradication of diverse tumor subpopulations in heterogeneous cancers to mitigate escape for more complete and durable treatment responses.

UNMIX-ME: spectral and lifetime fluorescence unmixing via deep learning

Hyperspectral fluorescence lifetime imaging allows for the simultaneous acquisition of spectrally resolved temporal fluorescence emission decays. In turn, the acquired rich multidimensional data set enables simultaneous imaging of multiple fluorescent species for a comprehensive molecular assessment of biotissues. However, to enable quantitative imaging, inherent spectral overlap between the considered fluorescent probes and potential bleed-through must be considered. Such a task is performed via either spectral or lifetime unmixing, typically independently. Herein, we present "UNMIX-ME" (unmix multiple emissions), a deep learning-based fluorescence unmixing routine, capable of quantitative fluorophore unmixing by simultaneously using both spectral and temporal signatures. UNMIX-ME was trained and validated using an in silico framework replicating the data acquisition process of a compressive hyperspectral fluorescent lifetime imaging platform (HMFLI). It was benchmarked against a conventional LSQ method for tri and quadri-exponential simulated samples. Last, UNMIX-ME's potential was assessed for NIR FRET in vitro and in vivo preclinical applications.

Complex Rab4-Mediated Regulation of Endosomal Size and EGFR Activation

Early sorting endosomes are responsible for the trafficking and function of transferrin receptor (TfR) and EGFR. These receptors play important roles in iron uptake and signaling and are critical for breast cancer development. However, the role of morphology, receptor composition, and signaling of early endosomes in breast cancer remains poorly understood. A novel population of enlarged early endosomes was identified in breast cancer cells and tumor xenografts but not in noncancerous MCF10A cells. Quantitative analysis of endosomal morphology, cargo sorting, EGFR activation, and Rab GTPase regulation was performed using super-resolution and confocal microscopy followed by 3D rendering. MDA-MB-231 breast cancer cells have fewer, but larger EEA1-positive early endosomes compared with MCF10A cells. Live-cell imaging indicated dysregulated cargo sorting, because EGF and Tf traffic together via enlarged endosomes in MDA-MB-231, but not in MCF10A. Large EEA1-positive MDA-MB-231 endosomes exhibited prolonged and increased EGF-induced activation of EGFR upon phosphorylation at tyrosine-1068 (EGFR-p1068). Rab4A overexpression in MCF10A cells produced EEA1-positive enlarged endosomes that displayed prolonged and amplified EGF-induced EGFR-p1068 activation. Knockdown of Rab4A lead to increased endosomal size in MCF10A, but not in MDA-MB-231 cells. Nevertheless, Rab4A knockdown resulted in enhanced EGF-induced activation of EGFR-p1068 in MDA-MB-231 as well as downstream signaling in MCF10A cells. Altogether, this extensive characterization of early endosomes in breast cancer cells has identified a Rab4-modulated enlarged early endosomal compartment as the site of prolonged and increased EGFR activation. IMPLICATIONS: Enlarged early endosomes play a Rab4-modulated role in regulation of EGFR activation in breast cancer cells.

Fast fit-free analysis of fluorescence lifetime imaging via deep learning

Fluorescence lifetime imaging (FLI) provides unique quantitative information in biomedical and molecular biology studies but relies on complex data-fitting techniques to derive the quantities of interest. Herein, we propose a fit-free approach in FLI image formation that is based on deep learning (DL) to quantify fluorescence decays simultaneously over a whole image and at fast speeds. We report on a deep neural network (DNN) architecture, named fluorescence lifetime imaging network (FLI-Net) that is designed and trained for different classes of experiments, including visible FLI and near-infrared (NIR) FLI microscopy (FLIM) and NIR gated macroscopy FLI (MFLI). FLI-Net outputs quantitatively the spatially resolved lifetime-based parameters that are typically employed in the field. We validate the utility of the FLI-Net framework by performing quantitative microscopic and preclinical lifetime-based studies across the visible and NIR spectra, as well as across the 2 main data acquisition technologies. These results demonstrate that FLI-Net is well suited to accurately quantify complex fluorescence lifetimes in cells and, in real time, in intact animals without any parameter settings. Hence, FLI-Net paves the way to reproducible and quantitative lifetime studies at unprecedented speeds, for improved dissemination and impact of FLI in many important biomedical applications ranging from fundamental discoveries in molecular and cellular biology to clinical translation.

Novel time-resolved camera based on compressed sensing

Time-resolved cameras with high temporal resolution (down to ps) enable a huge set of novel applications ranging from biomedicine and environmental science to material and device characterization. In this work, we propose, and experimentally validate, a novel detection scheme for time-resolved imaging based on a compressed sampling approach. The proposed scheme unifies into a single element all the required operations, i.e. space modulation, space integration and time-resolved detection, paving the way to dramatic cost reduction, performance improvement and ease of use.

Fluorescence Lifetime-Based Tumor Contrast Enhancement Using an EGFR Antibody-Labeled Near-Infrared Fluorophore

PURPOSE

Imaging techniques for highly specific detection of cancer cells in vivo can have applications ranging from preclinical drug discovery studies to clinical cancer diagnosis and surgical therapy. Although fluorescence imaging using cancer-targeted antibodies has shown promise, nonspecific probe accumulation in tissue results in significant background fluorescence, reducing detection sensitivity using traditional intensity-based continuous-wave (CW) fluorescence imaging. Here we demonstrate that fluorescence lifetime (FLT) imaging can provide significant tumor contrast enhancement over CW intensity in preclinical models of human breast cancer.

EXPERIMENTAL DESIGN

Mice bearing MDA-MB-231 tumors were injected with anti-EGFR antibody conjugated to the fluorescent dye IRDye 800CW (anti-EGFR-800). Time domain fluorescence imaging was performed in vivo and in situ up to 48 hours after dye injection.

RESULTS

Mice injected with anti-EGFR-800 showed a significantly longer FLT (0.7 ± 0.03 ns) compared with the FLT of nonspecific probe uptake in liver (0.63 ± 0.05 ns), providing a dramatic improvement in sensitivity and specificity compared with CW intensity. IgG antibody-conjugated IRDye 800CW did not show an increased FLT compared with normal tissue, suggesting that the FLT increase of anti-EGFR-800 in tumors was associated with receptor expression. Using serial surgery, we show that FLT allows the detection of smaller residual tumors in the surgical bed than possible using CW intensity.

CONCLUSIONS

Our data suggest that FLT can significantly enhance tumor contrast using fluorescently labeled antibodies, thereby accelerating the efficient clinical application of these probes for margin assessment in image-guided surgery and for highly specific detection of tumor receptors in vivo.

Laser-based 3D bioprinting for spatial and size control of tumor spheroids and embryoid bodies

3D multicellular aggregates, and more advanced organotypic systems, have become central tools in recent years to study a wide variety of complex biological processes. Most notably, these model systems have become mainstream within oncology (multicellular tumor spheroids) and regenerative medicine (embryoid bodies) research. However, the biological behavior of these in vitro tissue surrogates is extremely sensitive to their aggregate size and geometry. Indeed, both of these geometrical parameters are key in producing pathophysiological gradients responsible for cellular and structural heterogeneity, replicating in vivo observations. Moreover, the fabrication techniques most widely used for producing these models lack the ability to accurately control cellular spatial location, an essential component for regulating homotypic and heterotypic cell signaling. Herein, we report on a 3D bioprinting technique, laser direct-write (LDW), that enables precise control of both spatial patterning and size of cell-encapsulating microbeads. The generated cell-laden beads are further processed into core-shelled structures, allowing for the growth and formation of self-contained, self-aggregating cells (e.g., breast cancer cells, embryonic stem cells). Within these structures we demonstrate our ability to produce multicellular tumor spheroids (MCTSs) and embryoid bodies (EBs) with well-controlled overall size and shape, that can be designed on demand. Furthermore, we investigated the impact of aggregate size on the uptake of a commonly employed ligand for receptor-mediated drug delivery, Transferrin, indicating that larger tumor spheroids exhibit greater spatial heterogeneity in ligand uptake. Taken together, these findings establish LDW as a versatile biomanufacturing platform for bioprinting and patterning core-shelled structures to generate size-controlled 3D multicellular aggregates. STATEMENT OF SIGNIFICANCE: Multicellular 3D aggregates are powerful in vitro models used to study a wide variety of complex biological processes, particularly within oncology and regenerative medicine. These tissue surrogates are fabricated using environments that encourage cellular self-assembly. However, specific applications require control of aggregate size and position to recapitulate key in vivo parameters (e.g., pathophysiological gradients and homotypic/heterotypic cell signaling). Herein, we demonstrate the ability to create and spatially pattern size-controlled embryoid bodies and tumor spheroids, using laser-based 3D bioprinting. Furthermore, we investigated the effect of tumor spheroid size on internalization of Transferrin, a common ligand for targeted therapy, finding greater spatial heterogeneity in our large aggregates. Overall, this technique offers incredible promise and flexibility for fabricating idealized 3D in vitro models.

Multiparametric Time-Correlated Single Photon Counting Luminescence Microscopy

Classic time-correlated single photon counting (TCSPC) technique involves detection of single photons of a periodic optical signal, registration of the photon arrival time in respect to the reference pulse, and construction of photon distribution with regard to the detection times. This technique achieves extremely high time resolution and near-ideal detection efficiency. Modern TCSPC is multi-dimensional, i.e., in addition to the photon arrival time relative to the excitation pulse, spatial coordinates within the image area, wavelength, time from the start of the experiment, and many other parameters are determined for each photon. Hence, the multi-dimensional TCSPC allows generation of photon distributions over these parameters. This review describes both classic and multi-dimensional types of TCSPC microscopy and their application for fluorescence lifetime imaging in different areas of biological studies.

In Vivo Biosensing Using Resonance Energy Transfer

Solution-phase and intracellular biosensing has substantially enhanced our understanding of molecular processes foundational to biology and pathology. Optical methods are favored because of the low cost of probes and instrumentation. While chromatographic methods are helpful, fluorescent biosensing further increases sensitivity and can be more effective in complex media. Resonance energy transfer (RET)-based sensors have been developed to use fluorescence, bioluminescence, or chemiluminescence (FRET, BRET, or CRET, respectively) as an energy donor, yielding changes in emission spectra, lifetime, or intensity in response to a molecular or environmental change. These methods hold great promise for expanding our understanding of molecular processes not just in solution and in vitro studies, but also in vivo, generating information about complex activities in a natural, organismal setting. In this review, we focus on dyes, fluorescent proteins, and nanoparticles used as energy transfer-based optical transducers in vivo in mice; there are examples of optical sensing using FRET, BRET, and in this mammalian model system. After a description of the energy transfer mechanisms and their contribution to in vivo imaging, we give a short perspective of RET-based in vivo sensors and the importance of imaging in the infrared for reduced tissue autofluorescence and improved sensitivity.

In vitro and in vivo phasor analysis of stoichiometry and pharmacokinetics using short-lifetime near-infrared dyes and time-gated imaging

We introduce a simple new approach for time-resolved multiplexed analysis of complex systems using near-infrared (NIR) dyes, applicable to in vitro and in vivo studies. We show that fast and precise in vitro quantification of NIR fluorophores' short (subnanosecond) lifetime and stoichiometry can be done using phasor analysis, a computationally efficient and user-friendly representation of complex fluorescence intensity decays obtained with pulsed laser excitation and time-gated camera imaging. We apply this approach to the study of binding equilibria by Förster resonant energy transfer using two different model systems: primary/secondary antibody binding in vitro and ligand/receptor binding in cell cultures. We then extend it to dynamic imaging of the pharmacokinetics of transferrin engagement with the transferrin receptor in live mice, elucidating the kinetics of differential transferrin accumulation in specific organs, straightforwardly differentiating specific from nonspecific binding. Our method, implemented in a freely-available software, has the advantage of time-resolved NIR imaging, including better tissue penetration and background-free imaging, but simplifies and considerably speeds up data processing and interpretation, while remaining quantitative. These advances make this method attractive and of broad applicability for in vitro and in vivo molecular imaging and could be extended to applications as diverse as image-guided surgery or optical tomography.

Surface-enhanced Raman spectroscopy (SERS) nanoprobes for ratiometric detection of cancer cells

A surface-enhanced Raman spectroscopic strategy is developed for ratiometric detection of cancer cells by quantifying the expression ratio of extracellular biomarkers.

Comparison of illumination geometry for lifetime-based measurements in whole-body preclinical imaging

Macroscopic fluorescence lifetime imaging (MFLI) has been proved to be an accurate tool to quantify Förster resonance energy transfer (FRET) lifetime-based assessment of receptor-ligand engagement in vitro and in vivo. Herein, we report on the quantitative comparison of MFLI for whole-body preclinical studies in transmittance and reflectance geometries. The comparative study was conducted for both in vitro and in vivo conditions. FRET quantification performance in both geometries was similar in high fluorescence concentration samples. However, the reflectance geometry performed better at low fluorescence concentration. In addition, reflectance geometry could acquire subsurface imaging of the main whole-body organs of small animals without being compromised by tissue attenuation.

Quantitative imaging of receptor-ligand engagement in intact live animals

Maintaining an intact tumor environment is critical for quantitation of receptor-ligand engagement in a targeted drug development pipeline. However, measuring receptor-ligand engagement in vivo and non-invasively in preclinical settings is extremely challenging. We found that quantitation of intracellular receptor-ligand binding can be achieved using whole-body macroscopic lifetime-based Förster Resonance Energy Transfer (FRET) imaging in intact, live animals bearing tumor xenografts. We determined that FRET levels report on ligand binding to transferrin receptors conversely to raw fluorescence intensity. FRET levels in heterogeneous tumors correlate with intracellular ligand binding but strikingly, not with ubiquitously used ex vivo receptor expression assessment. Hence, MFLI-FRET provides a direct measurement of systemic delivery, target availability and intracellular drug delivery in preclinical studies. Here, we have used MFLI to measure FRET longitudinally in intact and live animals. MFLI-FRET is well-suited for guiding the development of targeted drug therapy in heterogeneous tumors in intact, live small animals.

Assessing patterns for compressive fluorescence lifetime imaging

A novel hyperspectral single pixel system was used to compare different compressive basis patterns for intensity imaging, lifetime imaging, and FRET quantification. Six popular basis patterns were compared experimentally in a phantom containing two fluorescent dyes. The basis patterns that performed best for lifetime quantification were used to measure FRET occurrence in well-plate samples with varying acceptor-donor ratios. The ABS-WP approach using Haar patterns and the compressive sensing approach with Hadamard Ranked patterns displayed the best overall performances at a 50% compression ratio.

Review of structured light in diffuse optical imaging

Diffuse optical imaging probes deep living tissue enabling structural, functional, metabolic, and molecular imaging. Recently, due to the availability of spatial light modulators, wide-field quantitative diffuse optical techniques have been implemented, which benefit greatly from structured light methodologies. Such implementations facilitate the quantification and characterization of depth-resolved optical and physiological properties of thick and deep tissue at fast acquisition speeds. We summarize the current state of work and applications in the three main techniques leveraging structured light: spatial frequency-domain imaging, optical tomography, and single-pixel imaging. The theory, measurement, and analysis of spatial frequency-domain imaging are described. Then, advanced theories, processing, and imaging systems are summarized. Preclinical and clinical applications on physiological measurements for guidance and diagnosis are summarized. General theory and method development of tomographic approaches as well as applications including fluorescence molecular tomography are introduced. Lastly, recent developments of single-pixel imaging methodologies and applications are reviewed.

Improving mesoscopic fluorescence molecular tomography via preconditioning and regularization

Mesoscopic fluorescence molecular tomography (MFMT) is a novel imaging technique capable of obtaining 3-D distribution of molecular probes inside biological tissues at depths of a few millimeters with a resolution up to ~100 μm. However, the ill-conditioned nature of the MFMT inverse problem severely deteriorates its reconstruction performances. Furthermore, dense spatial sampling and fine discretization of the imaging volume required for high resolution reconstructions make the sensitivity matrix (Jacobian) highly correlated, which prevents even advanced algorithms from achieving optimal solutions. In this work, we propose two computational methods to respectively increase the incoherence of the sensitivity matrix and improve the convergence rate of the inverse solver. We first apply a compressed sensing (CS) based preconditioner on either the whole sensitivity matrix or sub sensitivity matrices to reduce the coherence between columns of the sensitivity matrix. Then we employed a regularization method based on the weight iterative improvement method (WIIM) to mitigate the ill-condition of the sensitivity matrix and to drive the iterative optimization process towards convergence at a faster rate. We performed numerical simulations and phantom experiments to validate the effectiveness of the proposed strategies. In both in silico and in vitro cases, we were able to improve the quality of MFMT reconstructions significantly.

Synergistic antitumor effects of combination PI3K/mTOR and MEK inhibition (SAR245409 and pimasertib) in mucinous ovarian carcinoma cells by fluorescence resonance energy transfer imaging

The aim of this study was to clarify the synergistic effects of dual inhibition of the PI3K/mTOR and MAPK pathways in ovarian mucinous carcinoma (OMC) cells, using fluorescence resonance energy transfer (FRET) imaging. We exposed 6 OMC cell lines to a PI3K/mTOR inhibitor (voxtalisib, SAR245409) and/or a MEK inhibitor (pimasertib), and evaluated synergistic effects using the Chou-Talalay method. Then, S6K (PI3K pathway) and ERK (MAPK pathway) kinase activities, and their individual proliferative or cytotoxic effects were calculated by time-lapse FRET imaging. In combination with SAR245409, pimasertib (30 nM) synergistically inhibited cell growth (combination indexes: 0.03-0.5) and induced apoptosis in all 6 OMC cell lines. FRET-imaging results demonstrated that ERK inhibition induced both anti-proliferation and apoptosis in a dose-dependent manner in both MCAS and OAW42 cells. However, S6K inhibition suppressed proliferation in a threshold manner in both cell lines, although apoptosis was only induced in OAW42 cells. These results demonstrated that combined PI3K/mTOR and MEK inhibition exhibited synergistic antitumor effects in OMC cells and that FRET imaging is useful for analyzing kinase activities in live cells and elucidating their cytostatic and cytotoxic effects.

Highly Fluorescent Metal-Organic-Framework Nanocomposites for Photonic Applications

Metal-organic frameworks (MOFs) are porous hybrid materials built up from organic ligands coordinated to metal ions or clusters by means of self-assembly strategies. The peculiarity of these materials is the possibility, according to specific synthetic routes, to manipulate both the composition and ligands arrangement in order to control their optical and energy-transport properties. Therefore, optimized MOFs nanocrystals (nano-MOFs) can potentially represent the next generation of luminescent materials with features similar to those of their inorganic predecessors, that is, the colloidal semiconductor quantum dots. The luminescence of fluorescent nano-MOFs is generated through the radiative recombination of ligand molecular excitons. The uniqueness of these nanocrystals is the possibility to pack the ligand chromophores close enough to allow a fast exciton diffusion but sufficiently far from each other preventing the aggregation-induced effects of the organic crystals. In particular, the formation of strongly coupled dimers or excimers is avoided, thus preserving the optical features of the isolated molecule. However, nano-MOFs have a very small fluorescence quantum yield (QY). In order to overcome this limitation and achieve highly emitting systems, we analyzed the fluorescence process in blue emitting nano-MOFs and modeled the diffusion and quenching mechanism of photogenerated singlet excitons. Our results demonstrate that the excitons quenching in nano-MOFs is mainly due to the presence of surface-located, nonradiative recombination centers. In analogy with their inorganic counterparts, we found that the passivation of the nano-MOF surfaces is a straightforward method to enhance the emission efficiency. By embedding the nanocrystals in an inert polymeric host, we observed a +200% increment of the fluorescence QY, thus recovering the emission properties of the isolated ligand in solution.

AlliGator: A Phasor Computational Platform for Fast in vivo Lifetime Analysis

A fast time-gated phasor analysis tool was developed and used to monitor kinetics of NIR FLIM-FRET in vitro and in vivo. The results were validated by comparison with standard two-component fluorescence decay fitting analysis.

Next generation NIR fluorophores for tumor imaging and fluorescence-guided surgery: A review

Cancer is a group of diseases responsible for the major causes of mortality and morbidity among people of all ages. Even though medical sciences have made enormous growth, complete treatment of this deadly disease is still a challenging task. Last few decades witnessed an impressive growth in the design and development of near infrared (NIR) fluorophores with and without recognition moieties for molecular recognitions, imaging and image guided surgeries. The present article reviews recently reported NIR emitting organic/inorganic fluorophores that targets and accumulates in organelle/organs specifically for molecular imaging of cancerous cells. Near infrared (NIR probe) with or without a tumor-targeting warhead have been considered and discussed for their applications in the field of cancer imaging. In addition, challenges persist in this area are also delineated in this review.

Compressive hyperspectral time-resolved wide-field fluorescence lifetime imaging

Spectrally resolved fluorescence lifetime imaging^1-3 and spatial multiplexing^1,4,5 have offered information content and collection-efficiency boosts in microscopy, but efficient implementations for macroscopic applications are still lacking. An imaging platform based on time-resolved structured light and hyperspectral single-pixel detection has been developed to perform quantitative macroscopic fluorescence lifetime imaging (MFLI) over a large field of view (FOV) and multiple spectral bands simultaneously. The system makes use of three digital micromirror device (DMD)-based spatial light modulators (SLMs) to generate spatial optical bases and reconstruct N by N images over 16 spectral channels with a time-resolved capability (~40 ps temporal resolution) using fewer than N² optical measurements. We demonstrate the potential of this new imaging platform by quantitatively imaging near-infrared (NIR) Förster resonance energy transfer (FRET) both in vitro and in vivo. The technique is well suited for quantitative hyperspectral lifetime imaging with a high sensitivity and paves the way for many important biomedical applications.

AAV Vectors for FRET-Based Analysis of Protein-Protein Interactions in Photoreceptor Outer Segments

Fluorescence resonance energy transfer (FRET) is a powerful method for the detection and quantification of stationary and dynamic protein-protein interactions. Technical limitations have hampered systematic in vivo FRET experiments to study protein-protein interactions in their native environment. Here, we describe a rapid and robust protocol that combines adeno-associated virus (AAV) vector-mediated in vivo delivery of genetically encoded FRET partners with ex vivo FRET measurements. The method was established on acutely isolated outer segments of murine rod and cone photoreceptors and relies on the high co-transduction efficiency of retinal photoreceptors by co-delivered AAV vectors. The procedure can be used for the systematic analysis of protein-protein interactions of wild type or mutant outer segment proteins in their native environment. Conclusively, our protocol can help to characterize the physiological and pathophysiological relevance of photoreceptor specific proteins and, in principle, should also be transferable to other cell types.

99mTc-labeled and gadolinium-chelated transferrin enhances the sensitivity and specificity of dual-modality SPECT/MR imaging of breast cancer

A dual-modal probe ^99mTc–Tf–DTPA–Gd could provide high spatial resolution and high sensitivity images of breast tumor.

Temporal Data Set Reduction Based on D-Optimality for Quantitative FLIM-FRET Imaging

Fluorescence lifetime imaging (FLIM) when paired with Förster resonance energy transfer (FLIM-FRET) enables the monitoring of nanoscale interactions in living biological samples. FLIM-FRET model-based estimation methods allow the quantitative retrieval of parameters such as the quenched (interacting) and unquenched (non-interacting) fractional populations of the donor fluorophore and/or the distance of the interactions. The quantitative accuracy of such model-based approaches is dependent on multiple factors such as signal-to-noise ratio and number of temporal points acquired when sampling the fluorescence decays. For high-throughput or in vivo applications of FLIM-FRET, it is desirable to acquire a limited number of temporal points for fast acquisition times. Yet, it is critical to acquire temporal data sets with sufficient information content to allow for accurate FLIM-FRET parameter estimation. Herein, an optimal experimental design approach based upon sensitivity analysis is presented in order to identify the time points that provide the best quantitative estimates of the parameters for a determined number of temporal sampling points. More specifically, the D-optimality criterion is employed to identify, within a sparse temporal data set, the set of time points leading to optimal estimations of the quenched fractional population of the donor fluorophore. Overall, a reduced set of 10 time points (compared to a typical complete set of 90 time points) was identified to have minimal impact on parameter estimation accuracy (≈5%), with in silico and in vivo experiment validations. This reduction of the number of needed time points by almost an order of magnitude allows the use of FLIM-FRET for certain high-throughput applications which would be infeasible if the entire number of time sampling points were used.

Real-time label-free quantitative fluorescence microscopy-based detection of ATP using a tunable fluorescent nano-aptasensor platform

Although real-time label-free fluorescent aptasensors based on nanomaterials are increasingly recognized as a useful strategy for the detection of target biomolecules with high fidelity, the lack of an imaging-based quantitative measurement platform limits their implementation with biological samples. Here we introduce an ensemble strategy for a real-time label-free fluorescent graphene (Gr) aptasensor platform. This platform employs aptamer length-dependent tunability, thus enabling the reagentless quantitative detection of biomolecules through computational processing coupled with real-time fluorescence imaging data. We demonstrate that this strategy effectively delivers dose-dependent quantitative readouts of adenosine triphosphate (ATP) concentration on chemical vapor deposited (CVD) Gr and reduced graphene oxide (rGO) surfaces, thereby providing cytotoxicity assessment. Compared with conventional fluorescence spectrometry methods, our highly efficient, universally applicable, and rational approach will facilitate broader implementation of imaging-based biosensing platforms for the quantitative evaluation of a range of target molecules.

Wide-field fluorescence molecular tomography with compressive sensing based preconditioning

Wide-field optical tomography based on structured light illumination and detection strategies enables efficient tomographic imaging of large tissues at very fast acquisition speeds. However, the optical inverse problem based on such instrumental approach is still ill-conditioned. Herein, we investigate the benefit of employing compressive sensing-based preconditioning to wide-field structured illumination and detection approaches. We assess the performances of Fluorescence Molecular Tomography (FMT) when using such preconditioning methods both in silico and with experimental data. Additionally, we demonstrate that such methodology could be used to select the subset of patterns that provides optimal reconstruction performances. Lastly, we compare preconditioning data collected using a normal base that offers good experimental SNR against that directly acquired with optimal designed base. An experimental phantom study is provided to validate the proposed technique.

Assessment of Gate Width Size on Lifetime-Based Förster Resonance Energy Transfer Parameter Estimation

Förster Resonance Energy Transfer (FRET) enables the observation of interactions at the nanoscale level through the use of fluorescence optical imaging techniques. In FRET, fluorescence lifetime imaging can be used to quantify the fluorescence lifetime changes of the donor molecule, which are associated with proximity between acceptor and donor molecules. Among the FRET parameters derived from fluorescence lifetime imaging, the percentage of donor that interacts with the acceptor (in proximity) can be estimated via model-based fitting. However, estimation of the lifetime parameters can be affected by the acquisition parameters such as the temporal characteristics of the imaging system. Herein, we investigate the effect of various gate widths on the accuracy of estimation of FRET parameters with focus on the near-infrared spectral window. Experiments were performed in silico, in vitro, and in vivo with gate width sizes ranging from 300 ps to 1000 ps in intervals of 100 ps. For all cases, the FRET parameters were retrieved accurately and the imaging acquisition time was decreased three-fold. These results indicate that increasing the gate width up to 1000 ps still allows for accurate quantification of FRET interactions even in the case of short lifetimes such as those encountered with near-infrared FRET pairs.

Fluorescent nanoparticles for the accurate detection of drug delivery

INTRODUCTION

The application of intravenously administered nanoparticle (NP) therapies is increasingly being explored for a variety of diseases. The key to their success lies in a thorough understanding of nanoparticle pharmacological behavior, specificity and elimination kinetics. Fluorescent imaging techniques provide exciting opportunities to gain insight into such NP behavior in complex biological systems, at macroscopic as well as microscopic levels.

AREAS COVERED

In this review, we will summarize NP labeling methods in relation to their established and emerging fluorescent imaging modalities for in vitro, in vivo, and ex vivo studies of NP behavior. We will highlight novel applications and discuss recent developments in techniques such as fluorescence molecular tomography (FMT), Förster resonance energy transfer (FRET), and Raman imaging. Finally, we will provide a perspective on the challenges and future directions of NP-based fluorescent labeling and imaging in nanotherapeutics.

EXPERT OPINION

Commonly used in preclinical applications, fluorescent imaging of NPs can be achieved with minimal invasiveness and low toxicity in a multiplex fashion. Increasingly applied in the study of NP biodistribution, dissociation, and elimination behavior, fluorescent imaging allows fluid longitudinal tracking and visualization of NP interactions at the level of the whole animal, target organs/tissues, and individual cells.