New horizons for imaging lymphatic function

Optical imaging

Non-invasive optical imaging techniques, such as fluorescence imaging (FI) or bioluminescence imaging (BLI) have emerged as important tools in biomedical research. As demonstrated in different animal disease models, they enable visualization of physiological and pathophysiological processes at the cellular and molecular level in vivo with high specificity. Optical techniques are easy to use, fast, and affordable. Furthermore, they are characterized by their high sensitivity. In FI, very low amounts of the imaging agent (nano- to femtomol or even less) can be detected. Due to the absorption and scattering of light in tissue, optical techniques exhibit a comparably low spatial resolution in the millimeter range and a depth limit of a few centimeters. However, non-invasive imaging of biological processes in small animals and in outer or inner surfaces as well as during surgery even in humans is feasible. Currently two agents for fluorescence imaging are clinically approved, namely indocyanine green (ICG) and 5-aminolevulinic acid (5-ALA). In the past years, a number of new optical imaging agents for FI and reporter systems for BLI have been developed and successfully tested in animal models. Some of the FI agents might promise the application in clinical oncology. In this chapter, we describe the basic principles of non-invasive optical imaging techniques, give examples for the visualization of biological processes in animal models of cancer, and discuss potential clinical applications in oncology.

MR lymphography of lymphatic vessels in lower extremity with gynecologic oncology-related lymphedema

Objective

To characterize lymphatic vessel morphology in lower extremity lymphedema using MR lymphography at 3T.

Study Design

Forty females with lower extremity lymphedema secondary to gynecologic carcinoma treatment underwent MR lymphography (MRL) at 3T. Lymphatic vessel morphology in normal and affected limbs was compared.

Results

The median diameter of the lymphatic vessels in swollen calf and thigh were significantly larger than that in the contralateral calf and thigh, respectively (p<0.05). The median number of lymphatic vessels visualized in normal calf was less than that in the lymphedematous calf (p<0.01), while no significant difference was found between the normal thigh and swollen thigh. Lymphatic vessel number in the affected calf was significantly greater than that in affected thigh and the mean diameter of affected calf was also significantly wider than that of affected thigh (p<0.01). Mean diameter of lymphatic vessels in the affected calf was significantly different between stage I and stage III (p<0.05), but not significantly different between stages I and II, and between stages II and III (p>0.05). The median number of lymphatic vessels for affected calf showed significant difference between stage I and stage III, and between stage II and stage III (p<0.05), but no significant difference between stage I and stage II (p>0.05). There was no significant difference in mean diameter or median number of lymphatic vessels in the affected thigh found between different stages (p>0.05).

Conclusion

There are significant differences in the number or diameter of lymphatic vessels between normal and affected limbs and there are significant differences for affected calf between early and late stages of lymphedema; therefore, MR lymphography can be helpful in diagnosis or clinical staging for lower extremity with gynecologic oncology-related lymphedema.

Lymphatic abnormalities in the normal contralateral arms of subjects with breast cancer-related lymphedema as assessed by near-infrared fluorescent imaging

Current treatment of unilateral breast cancer-related lymphedema (BCRL) is only directed to the afflicted arm. Near-infrared fluorescent imaging (NIRF) of arm lymphatic vessel architecture and function in BCRL and control subjects revealed a trend of increased lymphatic abnormalities in both the afflicted and unafflicted arms with increasing time after lymphedema onset. These pilot results show that BCRL may progress to affect the clinically "normal" arm, and suggest that cancer-related lymphedema may become a systemic, rather than local, malady. These findings support further study to understand the etiology of cancer-related lymphedema and lead to better diagnostics and therapeutics directed to the systemic lymphatic system.

Albumin-binding domain conjugate for near-infrared fluorescence lymphatic imaging

PURPOSE

The aim of this study was to develop and characterize a novel peptide imaging agent for noninvasive near-infrared fluorescence imaging of protein transport by the lymphatics. An imaging agent consisting of a cyclic albumin-binding domain (cABD) peptide, with sequence, Arg-Leu-Ile-Glu-Asp-Ile-Cys-Leu-Pro-Arg-Trp-Gly-Cys-Leu-Trp-Glu-Asp-Asp-Lys, was conjugated to a near-infrared fluorophore, IRDye800CW, allowing for enhanced vascular uptake, retention, and fluorescence imaging.

PROCEDURE

Characterization of the cABD-IRDye800 peptide conjugate was performed using fluorescence spectroscopy to assess optical properties and SDS-PAGE and Biacore binding assays to determine binding affinity and specificity. Fluorescence imaging of normal C57BL/6 mice was conducted to monitor lymphatic uptake and retention.

RESULTS

cABD-IRDye800 exhibited approximately six times greater fluorescent yield and greater stability than indocyanine green, an agent previously used in humans to image lymphatic vasculature. The agent exhibited affinity for albumin with IC(50) and Kd in the nanomolar range and demonstrated superior retention characteristics within mouse lymphatics when compared with IRDye800CW.

CONCLUSIONS

cABD-IRDye800 has utility for assessing lymphatic function in mouse models of human lymphatic disease and the potential for use in clinical diagnostic imaging of the lymphatic vasculature.

Imaging tumor-induced sentinel lymph node lymphangiogenesis with LyP-1 peptide

Lymphangiogenesis in tumor-draining lymph nodes (LNs) starts before the onset of metastasis and is associated with metastasis to distant LNs and organs. In this study, we aimed to visualize tumor-induced lymphangiogenesis with a tumor lymphatics-specific peptide LyP-1. The LyP-1 peptide was labeled with a near-infrared fluorophore (Cy5.5) for optical imaging. At days 3, 7, 14 and 21 after subcutaneous 4T1 tumor inoculation, Cy5.5-LyP-1 was administered through the middle phalanges of the upper extremities of the tumor-bearing mice. At 45 min and 24 h postinjection, brachial LN fluorescence imaging was performed. Ex vivo fluorescence images were acquired for quantitative analysis of the fluorescence intensity. Tumor-induced lymphangiogenesis was confirmed by LYVE-1 immunostaining and increased size of tumor side brachial LNs. Cy5.5-LyP-1 staining in LNs co-localized with LYVE-1, suggesting lymphatics-specific binding of LyP-1 peptide. The brachial LNs were clearly visualized by optical imaging at both time points. The tumor side LNs showed significantly higher fluorescence intensities than the contralateral brachial LNs at days 7, 14, and 21, but not day 3 after tumor inoculation. At day 21 after tumor inoculation, the average signal of tumor-draining LNs was 78.0±2.44, 24.3±5.43, 25.6±0.25 (×10(3) photon/cm2/s) using Cy5.5-LyP-1, Cy5.5-LyP-1 with blocking, and Cy5.5 only, respectively. Tumor-draining brachial LNs showed extensive growth of lymphatic sinuses throughout the cortex and medulla. Use of LyP-1 based imaging probes with optical imaging offers a useful tool for the study of tumor-induced lymphangiogenesis. LyP-1 may serve as a marker of lymphangiogenesis useful in detecting "high risk" LNs before tumor metastasis and after micro-metastasis, as well as for screening potential anti-lymphatic therapies.

Trafficking of a dual-modality magnetic resonance and fluorescence imaging superparamagnetic iron oxide-based nanoprobe to lymph nodes

PURPOSE

This study aims to develop and characterize the trafficking of a dual-modal agent that identifies primary draining or sentinel lymph node (LN).

PROCEDURE

Herein, a dual-reporting silica-coated iron oxide nanoparticle (SCION) is developed. Nude mice were imaged by magnetic resonance (MR) and optical imaging and axillary LNs were harvested for histological analysis. Trafficking through lymphatics was observed with intravital and ex vivo confocal microscopy of popliteal LNs in B6-albino, CD11c-EYFP, and lys-EGFP transgenic mice.

RESULTS

In vivo, SCION allows visualization of LNs. The particle's size and surface functionality play a role in its passive migration from the intradermal injection site and its minimal uptake by CD11c+ dendritic cells and CD169+ and lys+ macrophages.

CONCLUSIONS

After injection, SCION passively migrates to LNs without macrophage uptake and then can be used to image LN(s) by MRI and fluorescence. Thus, SCION can potentially be developed for use in sentinel node resections or for intralymphatic drug delivery.

Concentration of indocyanine green does not significantly influence lymphatic function as assessed by near-infrared imaging

BACKGROUND

Absorbance of near-infrared (600-800 nm) light by the tissue components water, melanin, and hemoglobin is minimal. This property allows the use of near-infrared-emitting fluorophores for noninvasive, in vivo, real-time imaging of tissue, without the interference of autofluorescence experienced with imaging in other wavelength ranges. Near-infrared (NIR) fluorescence imaging has been used to noninvasively image lymphatic architecture and pumping function in animals, as well as in humans. The effects of different doses of a NIR dye, indocyanine green (ICG), on lymphatic function have been questioned. This study aims to address these concerns in the context of a mouse inguinal-to-axillary lymphatic imaging model.

METHODS AND RESULTS

We measured lymph propulsive velocity and frequency using an imaging system composed of a laser diode for excitation of the dye, an image intensifier, and an intensified charge-coupled device (ICCD) camera to capture real-time images. At 0.32, 0.645, and 1.3 mM ICG, no significant differences in lymphatic propulsive velocity or frequency were observed. Additionally, the use of other NIR imaging agents did not result in significant differences.

CONCLUSIONS

The use of different concentrations of ICG and the use of other near-infrared fluorophores for optical imaging of lymphatics does not significantly affect lymphatic propulsive velocity or frequency.

Video-rate fluorescence diffuse optical tomography for in vivo sentinel lymph node imaging

We have developed a fiber-based, video-rate fluorescence diffuse optical tomography (DOT) system for noninvasive in vivo sentinel lymph node (SLN) mapping. Concurrent acquisition of fluorescence and reference signals allowed the efficient generation of ratio-metric data for 3D image reconstruction. Accurate depth localization and high sensitivity to fluorescent targets were established in to depths of >10 mm. In vivo accumulation of indocyanine green (ICG) dye was imaged in the region of the SLN following intradermal injection into the forepaw of rats. These results suggest that video-rate fluorescence DOT has significant potential as a clinical tool for noninvasive mapping of SLN.

Minimally invasive quantification of lymph flow in mice and rats by imaging depot clearance of near-infrared albumin

There is a lack of available methods to noninvasively quantify lymphatic function in small experimental animals, a necessity for studies on lymphatic system pathophysiology. We present a new method to quantify lymph flow in mice and rats, based on optically monitoring the depot clearance of near-infrared fluorescently labeled albumin and subsequent calculation of removal rate constants (k). BSA was conjugated with Alexa680 NHS ester and remained stable in protein-rich solutions without free dye dissociation. To assess lymph flow, mice or rats were imaged every 30 or 60 min during a 3- to 6-h period following an intradermal injection of 0.5 or 1 μl Alexa680-albumin. Mice were awake between measurements, whereas rats were anesthetized throughout the experiment. The k, a parameter defined as equivalent to lymph flow, was calculated from the slopes of the resultant log-linear washout curves and averaged -0.40 ± 0.03 and -0.30 ± 0.02%/min for control C57BL/6 and C3H mice, respectively. Local administration of the vasoconstrictor endothelin-1 in mice led to a significant reduction in k, whereas overhydration in rats increased k, reflecting the coupling between capillary filtration and lymph flow. Furthermore, k was 50% of wild type in lymphedema Chy mice where dermal lymphatics are absent. We conclude that lymph flow can be determined as its rate constant k by optical imaging of depot clearance of submicroliter amounts of Alexa680-albumin. The method offers a minimally invasive, reproducible, and simple alternative to assess lymphatic function in mice and rats.

Preclinical lymphatic imaging

Noninvasive in vivo imaging of lymphatic vessels and lymphatic nodes is expected to fulfill the purpose of analyzing lymphatic vessels and their function, understanding molecular mechanisms of lymphangiogenesis and lymphatic spread of tumors, and utilizing lymphatic molecular markers as a prognostic or diagnostic indicator. In this review, we provide a comprehensive summary of in vivo imaging modalities for detecting lymphatic vessels, lymphatic drainage, and lymphatic nodes, which include conventional lymphatic imaging techniques such as dyes and radionuclide scintigraphy as well as novel techniques for lymphatic imaging such as optical imaging, computed tomography, magnetic resonance imaging, ultrasound, positron emission tomography using lymphatic biomarkers, photoacoustic imaging, and combinations of multiple modalities. The field of lymphatic imaging is ever evolving, and technological advances, combined with the development of new contrast agents, continue to improve the research of lymphatic vascular system in health and disease states as well as to improve the accuracy of diagnosis in the relevant diseases.

Optical tecnology developments in biomedicine: history, current and future

Biomedical optics is a rapidly emerging field for medical imaging and diagnostics. This paper reviews several biomedical optical technologies that have been developed and translated for either clinical or pre-clinical applications. Specifically, we focus on the following technologies: 1) near-infrared spectroscopy and tomography, 2) optical coherence tomography, 3) fluorescence spectroscopy and imaging, and 4) optical molecular imaging. There representative biomedical applications are also discussed here.

Role of the lymphatics in cancer metastasis and chemotherapy applications

The lymphatic system was first described centuries ago. The recent discovery of various molecular markers has allowed for more in-depth research of the lymphatic system and its role in health and disease. The lymphatic system has recently been elucidated as playing an active role in cancer metastasis. The knowledge of the active processes involved in lymphatic metastasis provides novel treatment targets for various malignancies.

Development of membrane type-1 matrix metalloproteinase-specific activatable fluorescent probe for malignant tumor detection

Membrane type-1 matrix metalloproteinase (MT1-MMP) is a protease that activates pro-MMP-2 and pro-MMP13, which are related to tumor malignancy. Therefore, probes that specifically image MT1-MMP would be useful for malignant tumor diagnosis. In the present study, we prepared rhodamine X-conjugated anti-MT1- MMP antibody (anti-MT1-MMP mAb-ROX) as an activatable fluorescent probe and evaluated its usefulness for MT1-MMP-specific imaging. Anti-MT1-MMP mAb-ROX was obtained in a quenched form with approximately three ROX molecules per mAb. Its fluorescence intensity increased approximately 14-fold in the presence of detergent, which is suitable for activatable systems. C6 glioma cells and MCF-7 human breast adenocarcinoma cells were used as MT1-MMP-positive and MT1-MMP-negative models, respectively. The fluorescence intensity of C6 cells treated with anti-MT1-MMP mAb-ROX, but not ROX-conjugated isotype control antibody (NC Ab-ROX), increased with time and was significantly higher than that of MCF-7 cells at 6 h (P < 0.001). The fluorescence intensity of cells treated with anti-MT1-MMP mAb-ROX was also suppressed by pre-treatment with a MT1-MMP endocytosis inhibitor (P < 0.05). Furthermore, the probes were intravenously administered to C6 and MCF-7 xenografted mice. The tumor-to-muscle (T/M) ratio of the anti-MT1-MMP mAb-ROX group was 15.1 ± 3.2 at 48 h and was significantly higher than that of the NC Ab-ROX group (T/M ratio = 4.6 ± 3.0, P < 0.05) in C6 xenografted mice, while the T/M ratio of the anti-MT1-MMP mAb-ROX and NC Ab-ROX groups was not different in MCF-7 xenografted mice. These findings suggest that anti-MT1-MMP mAb-ROX is a promising probe for specifically detecting MT1-MMP-expressing tumors.

Assessment of lymphatic contractile function after manual lymphatic drainage using near-infrared fluorescence imaging

OBJECTIVE

To investigate the feasibility of assessing the efficacy of manual lymphatic drainage (MLD), a method for lymphedema (LE) management, by using near-infrared (NIR) fluorescence imaging.

DESIGN

Exploratory pilot study.

SETTING

Primary care unit.

PARTICIPANTS

Subjects (N=10; age, 18-68y) with a diagnosis of grade I or II LE and 12 healthy control subjects (age, 22-59y).

INTERVENTION

Indocyanine green (25 μg in 0.1 mL each) was injected intradermally in bilateral arms or legs of subjects. Diffused excitation light illuminated the limbs, and NIR fluorescence images were collected by using custom-built imaging systems. Subjects received MLD therapy, and imaging was performed pre- and posttherapy.

MAIN OUTCOME MEASURES

Apparent lymph velocities and periods between lymphatic propulsion events were computed from fluorescence images. Data collected pre- and post-MLD were compared and evaluated for differences.

RESULTS

By comparing pre-MLD lymphatic contractile function against post-MLD lymphatic function, results showed that average apparent lymph velocity increased in both the symptomatic (+23%) and asymptomatic (+25%) limbs of subjects with LE and control limbs (+28%) of healthy subjects. The average lymphatic propulsion period decreased in symptomatic (-9%) and asymptomatic (-20%) limbs of subjects with LE, as well as in control limbs (-23%).

CONCLUSIONS

We showed that NIR fluorescence imaging could be used to quantify immediate improvement of lymphatic contractile function after MLD.

Current trends and emerging future of indocyanine green usage in surgery and oncology: a literature review

Ever since Kitai first performed fluorescent navigation of sentinel lymph nodes (SLNs) using indocyanine green (ICG) dye with a charge-couple device and light emitting diodes, the intraoperative use of near infrared fluorescence has served a critical role in increasing our understanding in various fields of surgical oncology. Here the authors review the emerging role of the ICG fluorophore in the development of our comprehension of the lymphatic system and its use in SLN mapping and biopsy in various cancers. In addition, they introduce the novel role of ICG-guided video angiography as a new intraoperative method of assessing microvascular circulation. The authors attempt to discuss the promising potential in addition to assessing several challenges and limitations in the context of specific surgical procedures and ICG as a whole. PubMed and Medline literature databases were searched for ICG use in clinical surgical settings. Despite ICG's significant impact in various fields of surgical oncology, ICG is still in its nascent stages, and more in-depth studies need to be carried out to fully evaluate its potential and limitations.

Magnetic resonance lymphangiography with a nano-sized gadolinium-labeled dendrimer in small and large animal models

AIM

Imaging of the lymphatic system is critical in preoperative planning of resections of complex lymphatic malformations. However, safe, effective imaging methods with sufficient resolution to identify the lymphatics have been lacking. In this study, we demonstrate the use of gadolinium-labeled dendrimers to image the lymphatics in small and large animal models during magnetic resonance lymphangiography.

METHODS

Polyamidoamine G6-Gd_1B4M_N-hydroxysuccinimide was synthesized and administered intradermally in the extremities of normal mice and pigs at several doses.

RESULTS

The lymphatics were well demonstrated in both animal models and there was rapid uptake in the deep lymphatic system, including the thoracic duct. A significant dose reduction was achieved (1 µmol Gd/kg) in the 35-kg pig compared with mice, while still producing excellent results. No toxicity was observed and only minor inflammatory changes were observed at the injection site 30 days later.

CONCLUSION

We demonstrate that a low dose of a macromolecular magnetic resonance contrast agent can provide rapid imaging of the deep lymphatic system in both small and large animals. This data provides a basis to consider a similar agent in clinical trials.

Near-infrared fluorescent dye-doped semiconducting polymer dots

Near-infrared (NIR) fluorescence sensing is desirable for in vivo biological measurements, but the method is currently limited by the availability of NIR fluorescent markers as well as by their poor performance, such as self-aggregation and dim fluorescence, in a physiological environment. To address this issue, this paper describes a NIR fluorescent polymer dot (Pdot) that emits at 777 nm. This Pdot was comparable in size to a water-soluble NIR quantum dot that emits at 800 nm (ITK Qdot800) but was about four times brighter and with a narrower emission peak. We formed the NIR Pdot by doping the NIR dye, silicon 2,3-naphthalocyanine bis(trihexylsilyloxide) (NIR775), into the matrix of poly (9,9-dioctylfluorene-co-benzothiadiazole) (PFBT) as the Pdot formed using a nanoscale precipitation technique. Free molecules of NIR775 aggregate in aqueous solution, but encapsulating them into the hydrophobic Pdot matrix effectively introduced them into aqueous solution for use in biological studies. Most importantly, the brightness of NIR775 was dramatically enhanced because of the excellent light-harvesting ability of PFBT and the very efficient energy transfer from PFBT to NIR775. We anticipate this bright NIR Pdot will be useful in biological measurements and cellular imaging where strong NIR emission is beneficial.

Assessment of free dye in solutions of dual-labeled antibody conjugates for in vivo molecular imaging

PURPOSE

Recent preclinical and clinical studies show that dyes that excite and fluoresce in the near-infrared range may be used for tracking and detecting disease targets in vivo. A method for quantifying free dye molecules in antibody conjugate preparations is required for agent batch release and for translation into the clinic.

PROCEDURES

Herein, we developed and validated a SDS-PAGE method to determine the percentage of free IRDye 800 CW in (DTPA)(n)-trastuzumab-(IRDye 800)(m) conjugate sample preparations in which high-performance liquid chromatography (HPLC) assessment of free dye was not possible.

RESULTS

The SDS-PAGE assay was accurate and valid for free IRDye 800 CW amounts between 38 and 4 mol% of total dye. Gel sample preparation reagent affected the specificity of the assay, and lower and upper limits of quantitation and detection were determined.

CONCLUSION

This method may be applicable to other near-infrared dye-conjugated antibody-based imaging agents in which HPLC assessment of purity is not feasible. This validated method for quality assurance will facilitate the translation of dual-labeled antibody conjugates for nuclear and optical imaging.

Methods for lymphatic vessel culture and gene transfection

OBJECTIVE

To develop the techniques needed for the specific gene/protein targeting transfection experiments in isolated lymphatic vessels, we completed two major tasks: 1) optimize the experimental conditions to maintain the viability of isolated rat lymphatic vessels in culture for sufficiently long periods of time to permit knockdown or overexpression of selected proteins/genes and 2) develop effective transfection protocols for lymphatic muscle and endothelial cells in intact lymphatic vessels without nonspecific impairment of lymphatic contractile function due to the transfection protocol itself.

METHODS

Experimental protocols were developed for the maintenance of isolated lymphatic vessels under nonpressurized and pressurized conditions for 3-12 days in culture and for adenoviral gene transfection of the lymphatic muscle and endothelial cells.

RESULTS

The data demonstrate the effectiveness of the newly developed experimental protocols for the maintenance of isolated rat mesenteric lymphatic vessels and thoracic duct in culture up to 3-12 days without significant impairment of the parameters of their pumping and effective adenoviral/GFP transfection of lymphatic endothelial and muscle cells in isolated rat mesenteric lymphatic vessels.

CONCLUSIONS

These experimental techniques will extend the set of the modern experimental tools available to researchers investigating the physiology of lymphatic function.

Indocyanine green and lymphatic imaging: current problems

BACKGROUND

Near-infrared (NIR) fluorescence imaging using indocyanine green (ICG) has recently been presented as a comparatively easy and informative technique to image lymphatic channels in vivo. However, no data or references have been provided concerning the impact of ICG application on normal lymphatic contractility and lymph transport. Thus, the imaging agent and/or the method of administration may introduce a significant artifact.

METHODS AND RESULTS

Standard pharmacological tests were performed to investigate the influence of ICG on the spontaneous contractility of isolated, cannulated, and pressurized rat mesenteric lymphatic vessels. The data demonstrate that non-irradiated ICG dramatically and dynamically influences the contractility of rat lymphatic vessels in both a dose- and diluent-dependent manner with low ICG concentrations principally altering contractile frequency and higher ICG concentrations completely blocking lymphatic contractility.

CONCLUSIONS

Currently, both researchers and doctors should exercise caution in extrapolating the data obtained with ICG imaging to normal lymphatic function regardless of whether it was obtained in mice, pigs, or humans. Careful and extended pharmacological tests must be performed to evaluate the mechanism of action of ICG on the contractility and physiology of lymphatic vessels with consideration of dose, diluent, and duration of irradiation.

Optical coherence tomography: the intraoperative assessment of lymph nodes in breast cancer

During breast-conserving surgeries, axillary lymph nodes draining from the primary tumor site are removed for disease staging. Although a high number of lymph nodes are often resected during sentinel and lymph-node dissections, only a relatively small percentage of nodes are found to be metastatic, a fact that must be weighed against potential complications such as lymphedema. Without a real-time in vivo or in situ intraoperative imaging tool to provide a microscopic assessment of the nodes, postoperative paraffin section histopathological analysis currently remains the gold standard in assessing the status of lymph nodes. This paper investigates the use of optical coherence tomography (OCT), a high-resolution real-time microscopic optical-imaging technique, for the intraoperative ex vivo imaging and assessment of axillary lymph nodes. Normal (13), reactive (1), and metastatic (3) lymph nodes from 17 human patients with breast cancer were imaged intraoperatively with OCT. These preliminary clinical studies have identified scattering changes in the cortex, relative to the capsule, which can be used to differentiate normal from reactive and metastatic nodes. These optical scattering changes are correlated with inflammatory and immunological changes observed in the follicles and germinal centers. These results suggest that intraoperative OCT has the potential to assess the real-time node status in situ, without having to physically resect and histologically process specimens to visualize microscopic features.

Human Lymphatic Architecture and Dynamic Transport Imaged Using Near-infrared Fluorescence

BACKGROUND

Although the importance of lymphatic function is well recognized, the lack of real-time imaging modalities limits our understanding of its role in many diseases. In a phase 0 exploratory study, we used dynamic, near-infrared (NIR) fluorescence imaging to assess the extremes of lymphatic architecture and transport in healthy human subjects and in subjects clinically diagnosed with unilateral lymphedema (LE), a disease that can be prevalent in cancer survivors.

METHODS AND RESULTS

Active lymphatic propulsion was imaged after intradermal injections of 25 µg of indocyanine green (total maximum dose ≤400 µg) bilaterally in the arms or legs of control and subjects. Images show well-defined lymphatic structures with propulsive dye transport in limbs of healthy subjects. In LE subjects, we observed extravascular dye accumulation, networks of fluorescent lymphatic capillaries, and/or tortuous lymphatic vessels in all symptomatic and some asymptomatic limbs. Statistical models indicate that disease status and/or limb significantly affect parameters of apparent lymph propagation velocity and contractile frequency.

CONCLUSIONS

These clinical research studies demonstrate the potential of NIR fluorescence imaging as a diagnostic measure of functional lymphatics and as a new tool in translational research studies to decipher the role of the lymphatic system in cancer and other diseases.

In vivo imaging of inflammation- and tumor-induced lymph node lymphangiogenesis by immuno-positron emission tomography

Metastasis to regional lymph nodes (LN) is a prognostic indicator for cancer progression. There is a great demand for sensitive and noninvasive methods to detect metastasis to LNs. Whereas conventional in vivo imaging approaches have focused on the detection of cancer cells, lymphangiogenesis within tumor-draining LNs might be the earliest sign of metastasis. In mouse models of LN lymphangiogenesis, we found that systemically injected antibodies to lymphatic epitopes accumulated in the lymphatic vasculature in tissues and LNs. Using a (124)I-labeled antibody against the lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1), we imaged, for the first time, inflammation- and tumor-draining LNs with expanded lymphatic networks in vivo by positron emission tomography (PET). Anti-LYVE-1 immuno-PET enabled visualization of lymphatic vessel expansion in LNs bearing metastases that were not detected by [(18)F]fluorodeoxyglucose-PET, which is clinically applied to detect cancer metastases. Immuno-PET with lymphatic-specific antibodies may open up new avenues for the early detection of metastasis, and the images obtained might be used as biomarkers for the progression of diseases associated with lymphangiogenesis.

Uniform silica coated fluorescent nanoparticles: synthetic method, improved light stability and application to visualize lymph network tracer

Background

The sentinel lymph node biopsy (SLNB) was developed as a new modality in the surgical diagnosis of lymph node metastases. Dye and radioisotope are major tracers for the detection of sentinel lymph nodes (SLN). Dye tends to excessively infiltrate into the interstitium due to their small size (less than several nanometers), resulting in difficulties in maintaining clear surgical fields. Radioisotopes are available in limited number of hospitals. Fluorescent nanoparticles are good candidates for SLN tracer to solve these problems, as we can choose suitable particle size and fluorescence wavelength of near-infrared. However, the use of nanoparticles faces safety issues, and many attempts have been performed by giving insulating coats on nanoparticles. In addition, the preparation of the uniform insulating layer is important to decrease variations in the quality as an SLN tracer.

Methodology/Principal Findings

We herein succeeded in coating fluorescent polystyrene nanoparticles of 40 nm with uniform silica layer of 13 nm by the modified Stöber method. The light stability of silica coated nanoparticles was 1.3-fold greater than noncoated nanoparticles. The popliteal lymph node could be visualized by the silica coated nanoparticles with injection in the rat feet.

Conclusions/Significance

The silica coated nanoparticles in lymph nodes could be observed by transmission electron microscope, suggesting that our silica coating method is useful as a SLN tracer with highly precise distribution of nanoparticles in histological evaluation. We also demonstrated for the first time that a prolonged enhancement of SLN is caused by the phagocytosis of fluorescent nanoparticles by both macrophages and dendritic cells.

Quantitative imaging of lymphatic function with liposomal indocyanine green

Lymphatic vessels play a major role in cancer progression and in postsurgical lymphedema, and several new therapeutic approaches targeting lymphatics are currently being developed. Thus, there is a critical need for quantitative imaging methods to measure lymphatic flow. Indocyanine green (ICG) has been used for optical imaging of the lymphatic system, but it is unstable in solution and may rapidly enter venous capillaries after local injection. We developed a novel liposomal formulation of ICG (LP-ICG), resulting in vastly improved stability in solution and an increased fluorescence signal with a shift toward longer wavelength absorption and emission. When injected intradermally to mice, LP-ICG was specifically taken up by lymphatic vessels and allowed improved visualization of deep lymph nodes. In a genetic mouse model of lymphatic dysfunction, injection of LP-ICG showed no enhancement of draining lymph nodes and slower clearance from the injection site. In mice bearing B16 luciferase-expressing melanomas expressing vascular endothelial growth factor-C (VEGF-C), sequential near-IR imaging of intradermally injected LP-ICG enabled quantification of lymphatic flow. Increased flow through draining lymph nodes was observed in mice bearing VEGF-C-expressing tumors without metastases, whereas a decreased flow pattern was seen in mice with a higher lymph node tumor burden. This new method will likely facilitate quantitative studies of lymphatic function in preclinical investigations and may also have potential for imaging of lymphedema or improved sentinel lymph detection in cancer.

Functional lymphatic imaging in tumor-bearing mice

The lymphatic system provides a route for dissemination of metastatic cancer cells. Yet to date transient changes in lymphatic drainage pathways and function as a result of tumor growth and metastasis have not been completely elucidated. Herein, we non-invasively imaged functional and architectural lymphatic changes in mice with regional, palpable lymph node (LN) involvement using dynamic near-infrared (NIR) fluorescence imaging with intradermal injection of indocyanine green (ICG) to both tumor-free mice and mice bearing C6/LacZ rat glioma tumors in the tail or hindlimb. We found that lymphatic drainage pathways were transiently altered and the contractile function of regional conducting lymphatic vessels was reduced or lost with progressive disease. Therefore, transient changes in the regional lymphatic architecture and function that occur with progressive disease, can be imaged using NIR fluorescence, and may provide a new method to stage disease.

Near-Infrared Fluorescence Imaging in Humans with Indocyanine Green: A Review and Update

Near-infrared (NIR) fluorescence imaging clinical studies have been reported in the literature with six different devices that employ various doses of indocyanine green (ICG) as a non-specific contrast agent. To date, clinical applications range from (i) angiography, intraoperative assessment of vessel patency, and tumor/metastasis delineation following intravenous administration of ICG, and (ii) imaging lymphatic architecture and function following subcutaneous and intradermal ICG administration. In the latter case, NIR fluorescence imaging may enable new discoveries associated with lymphatic function due to (i) a unique niche that is not met by any other conventional imaging technology and (ii) its exquisite sensitivity enabling high spatial and temporal resolution. Herein, we (i) review the basics of clinical NIR fluorescence imaging, (ii) survey the literature on clinical application of investigational devices using ICG fluorescent contrast, (iii) provide an update of non-invasive dynamic lymphatic imaging conducted with our FDPM device, and finally, (iv) comment on the future NIR fluorescence imaging for non-invasive and intraoperative use given recent demonstrations showing capabilities for imaging following microdose administration of contrast agent.

Dual-modality molecular imaging using antibodies labeled with activatable fluorescence and a radionuclide for specific and quantitative targeted cancer detection

Multimodality molecular imaging should have potential for compensating the disadvantages and enhancing the advantages of each modality. Nuclear imaging is superior to optical imaging in whole body imaging and in quantification due to good tissue penetration of gamma rays. However, target specificity can be compromised by high background signal due to the always signal ON feature of nuclear probes. In contrast, optical imaging can be superior in target-specific imaging by employing target-specific signal activation systems, although it is not quantitative because of signal attenuation. In this study, to take advantage of the mutual cooperation of each modality, multimodality imaging was performed by a combination of quantitative radiolabeled probe and an activatable optical probe. The monoclonal antibodies, panitumumab (anti-HER1) and trastuzumab (anti-HER2), were labeled with 111In and ICG and tested in both HER1 and HER2 tumor bearing mice by the cocktail injection of radiolabeled and optical probes and by the single injection of a dual-labeled probe. The optical and nuclear images were obtained over 6 days after the conjugates injection. The fluorescence activation properties of ICG labeled antibodies were also investigated by in vitro microscopy. In vitro microscopy demonstrated that there was no fluorescence signal with either panitumumab-ICG or trastuzumab-ICG, when the probes were bound to cell surface antigens but were not yet internalized. After the conjugates were internalized into the cells, both conjugates showed bright fluorescence signal only in the target cells. These results show that both conjugates work as activatable probes. In in vivo multimodality imaging by injection of a cocktail of radio-optical probes, only the target specific tumor was visualized by optical imaging. Meanwhile, the biodistribution profile of the injected antibody was provided by nuclear imaging. Similar results were obtained with radio and optical dual-labeled probes, and it is confirmed that pharmacokinetic properties did not affect the results above. Here, we could characterize the molecular targets by activatable optical probes and visualize the delivery of targeting molecules quantitatively by radioactive probes. Multimodality molecular imaging combining activatable optical and radioactive probes has great potential for simultaneous visualization, characterization, and measurement of biological processes.

Chylothorax: diagnosis and management in children

Chylothorax is the accumulation of chyle in the pleural space, as a result of damage to the thoracic duct. Chyle is milky fluid enriched with fat secreted from the intestinal cells and lymphatic fluid. Chylothorax in children, is most commonly seen as a complication of cardiothoracic surgery but may occur in newborns or conditions associated with abnormal lymphatics. The diagnosis is based on biochemical analysis of the pleural fluid, which contains chylomicrons, high levels of triglycerides and lymphocytes. Investigations to outline the lymphatic channels can prove helpful in some cases. Initial treatment consists of drainage, dietary modifications, total parenteral nutrition and time for the thoracic duct to heal. Somatostatin and its analogue octreotide may be useful in some cases. Surgery should be considered for patients who fail these initial steps, or in whom complications such as electrolyte and fluid imbalance, malnutrition or immunodeficiency persist. Surgical intervention may be attempted thoracoscopically with repair or ligation of the thoracic duct.

In vivo real-time, multicolor, quantum dot lymphatic imaging

The lymphatic network is complex and difficult to visualize in real-time in vivo. Moreover, the direction of flow within lymphatic networks is often unpredictable especially in areas with well-developed "watershed" or overlapping lymphatics. Herein, we report a method of in vivo real-time multicolor lymphatic imaging using cadmium-selenium quantum dots (Qdots) with a fluorescence imaging system that enables the simultaneous visualization of up to five distinct lymphatic basins in real-time. Five visually well-distinguishable carboxyl-Qdots (Qdot 545, 565, 585, 605, and 655) were selected and injected subdermally into mice at five different sites, and serially imaged in vivo or in situ under surgery with real-time multicolor lymphatic imaging. In all seven mice, in vivo lymphatic images successfully distinguished all five lymphatic basins with different colors in real-time. These visualizations of lymph node lasted up to at least 7 days. This method could have a considerable potential in lymphatic research for studying the anatomy and flow within the lymphatic system as well as in some limited clinical settings where real-time visible fluorescence could facilitate procedures under surgery or endoscopy.

Lymphangiogenesis and cancer metastasis

Metastasis is a characteristic trait of most tumour types and the cause for the majority of cancer deaths. Many tumour types, including melanoma and breast and prostate cancers, first metastasize via lymphatic vessels to their regional lymph nodes. Although the connection between lymph node metastases and shorter survival times of patients was made decades ago, the active involvement of the lymphatic system in cancer, metastasis has been unravelled only recently, after molecular markers of lymphatic vessels were identified. A growing body of evidence indicates that tumour-induced lymphangiogenesis is a predictive indicator of metastasis to lymph nodes and might also be a target for prevention of metastasis. This article reviews the current understanding of lymphangiogenesis in cancer anti-lymphangiogenic strategies for prevention and therapy of metastatic disease, quantification of lymphangiogenesis for the prognosis and diagnosis of metastasis and in vivo imaging technologies for the assessment of lymphatic vessels, drainage and lymph nodes.

Regional nodal staging for early stage colon cancer in the era of endoscopic resection and N.O.T.E.S

Advanced endoscopic technologies and techniques capable of providing localized resection of colonic primaries are entering clinical practice. As much as Natural Orifice Transluminal Endoscopic Surgery (N.O.T.E.S.) may ultimately provide for transmural resection with narrow margins, intraluminal techniques such as endoscopic submucosal resection can now effect excision of early stage tumors from within the colon. However, the limit on the application of these approaches is oncological providence as current staging requires en bloc mesenteric resection in every case to ensure that adequate nodal assessment is assured. Furthermore, this requirement is also a limiting factor on the advance of innovative procedures such as Single-Incision Laparoscopic Surgery and N.O.T.E.S.-hybrid techniques as these approaches, while likely adept at the definitive management of the primary, have limitations regarding their ability to provide full base mesenteric resection (due mostly to constraints on retraction capacity as well as operating field space and exposure). Therefore a means to accurately and efficiently identify those patients who are truly node negative (and so in whom radical mesenteric lymphadenectomy could be avoided) would allow all of these techniques to advance with a clear focus on address of the primary. This review analyses the current state of the art of regional staging in the colonic mesentery in place of formal lymphadenectomy. It includes deliberation of both preoperative non-invasive testing as well as novel means of employing N.O.T.E.S. approaches to allow direct determination of lymph node status (in particular that of sentinel nodes) by either rapid histopathological examination or by emerging technologies such as Optical Coherence Tomography that may provide optical or 'virtual' biopsy.

Lymphatic imaging in humans with near-infrared fluorescence

While the lymphatic system is increasingly associated with diseases of prevalence, study of these diseases is difficult owing to the paucity of imaging techniques with the sensitivity and temporal resolution to discriminate lymphatic function. Herein, we review the known, pertinent features of the human lymphatic system in health and disease and set the context for a number of emerging studies that use near-infrared fluorescence imaging to non-invasively assess tumor draining lymphatic basins in cancer patients, intraoperatively guide resection of first draining lymph nodes, and to interrogate the difference between normal and aberrant lymphatic structure and function.

In vivo molecular imaging of cancer with a quenching near-infrared fluorescent probe using conjugates of monoclonal antibodies and indocyanine green

Near-infrared (NIR) fluorophores have several advantages over visible fluorophores, including improved tissue penetration and lower autofluorescence; however, only indocyanine green (ICG) is clinically approved. Its use in molecular imaging probes is limited because it loses its fluorescence after protein binding. This property can be harnessed to create an activatable NIR probe. After cell binding and internalization, ICG dissociates from the targeting antibody, thus activating fluorescence. ICG was conjugated to the antibodies daclizumab (Dac), trastuzumab (Tra), or panitumumab (Pan). The conjugates had almost no fluorescence in PBS but became fluorescent after SDS and 2-mercaptoethanol, with a quenching capacity of 10-fold for 1:1 conjugates and 40- to 50-fold for 1:5 conjugates. In vitro microscopy showed activation within the endolysosomes in target cells. In vivo imaging in mice showed that CD25-expressing tumors were specifically visualized with Dac-ICG. Furthermore, tumors overexpressing HER1 and HER2 were successfully characterized in vivo by using Pan-ICG(1:5) and Tra-ICG(1:5), respectively. Thus, we have developed an activatable NIR optical probe that "switches on" only in target cells. Because both the antibody and the fluorophore are Food and Drug Administration approved, the likelihood of clinical translation is improved.