Mikako Ogawa

New Strategies for Fluorescent Probe Design in Medical Diagnostic Imaging

In vivo medical imaging has made great progress due to advances in the engineering of imaging devices and developments in the chemistry of imaging probes. Several modalities have been utilized for medical imaging, including X-ray radiography and computed tomography (x-ray CT), radionuclide imaging using single photons and positrons, magnetic resonance imaging (MRI), ultrasonography (US), and optical imaging. In order to extract more information from imaging, “contrast agents” have been employed. For example, organic iodine compounds have been used in X-ray radiography and computed tomography, superparamagnetic or paramagnetic metals have been used in MRI, and microbubbles have been used in ultrasonography. Most of these, however, are non-targeted reagents. Molecular imaging is widely considered the future for medical imaging. Molecular imaging has been defined as the in vivo characterization and measurement of biologic process at the cellular and molecular level1, or more broadly as a technique to directly or indirectly monitor and record the spatio-temporal distribution of molecular or cellular processes for biochemical, biologic, diagnostic, or therapeutic application2. Molecular imaging is the logical next step in the evolution of medical imaging after anatomic imaging (e.g. x-rays) and functional imaging (e.g. MRI). In order to attain truly targeted imaging of specific molecules which exist in relatively low concentrations in living tissues, the imaging techniques must be highly sensitive. Although MRI, US, and x-ray CT are often listed as molecular imaging modalities, in truth, radionuclide and optical imaging are the most practical modalities, for molecular imaging, because of their sensitivity and the specificity for target detection. Radionuclide imaging, including gamma scintigraphy and positron emission tomography (PET), are highly sensitive, quantitative, and offer the potential for whole body scanning. However, radionuclide imaging methods have the disadvantages of poor spatial and temporal resolution3. Additionally, they require radioactive compounds which have an intrinsically limited half life, and which expose the patient and practitioner to ionizing radiation and are therefore subject to a variety of stringent safety regulations which limit their repeated use4. Optical imaging, on the other hand, has comparable sensitivity to radionuclide imaging, and can be “targeted” if the emitting fluorophore is conjugated to a targeting ligand3. Optical imaging, by virtue of being “switchable”, can result in very high target to background ratios. “Switchable” or activatable optical probes are unique in the field of molecular imaging since these agents can be turned on in specific environments but otherwise remain undetectable. This improves the achievable target to background ratios, enabling the detection of small tumors against a dark background5,6. This advantage must be balanced against the lack of quantitation with optical imaging due to unpredictable light scattering and absorption, especially when the object of interest is deep within the tissue. Visualization through the skin is limited to superficial tissues such as the breast7-9 or lymph nodes10,11 The fluorescence signal from the bright GFP-expressing tumors can be seen in the deep organ only in the nude mice 12,13. However, optical molecular imaging can also be employed during endoscopy14 or surgery 15,16.

Cancer cell-selective in vivo near infrared photoimmunotherapy targeting specific membrane molecules.

Three major modes of cancer therapy (surgery, radiation and chemotherapy) are the mainstay of modern oncologic therapy. To minimize the side effects of these therapies, molecular-targeted cancer therapies, including armed antibody therapy, have been developed with limited success. In this study, we have developed a new type of molecular-targeted cancer therapy, photoimmunotherapy (PIT), that uses a target-specific photosensitizer based on a near-infrared (NIR) phthalocyanine dye, IR700, conjugated to monoclonal antibodies (mAbs) targeting epidermal growth factor receptors. Cell death was induced immediately after irradiating mAb-IR700–bound target cells with NIR light. We observed in vivo tumor shrinkage after irradiation with NIR light in target cells expressing the epidermal growth factor receptor. The mAb-IR700 conjugates were most effective when bound to the cell membrane and produced no phototoxicity when not bound, suggesting a different mechanism for PIT as compared to conventional photodynamic therapies. Target-selective PIT enables treatment of cancer based on mAb binding to the cell membrane.

Rapid Cancer Detection by Topically Spraying a γ-Glutamyltranspeptidase–Activated Fluorescent Probe

A spirocyclic-caged, small-molecule imaging probe fluoresces upon cleavage by a cancer-specific enzyme and can be used during surgical or endoscopic tumor removal procedures. No Tumor Left Behind Although quick action with spray paint usually conjures images of a schoolboy prank, researchers now show that spray painting of tiny tumors might save lives by illuminating these troublemakers that are often overlooked by the naked eye. Ovarian cancer is a deadly gynecological disease, considering its propensity for invading the peritoneal cavity and depositing tumors throughout. Surgeons can miss these disseminated tumors during surgical removal of cancerous lesions, owing to their small size (~1 mm) and unclear borders. To help surgeons visualize and eliminate these clandestine killers, Urano et al. have developed a small-molecule aminopeptidase probe that fluoresces upon contact with cancer cells. The probe—γ-glutamyl hydroxymethyl rhodamine green (gGlu-HMRG)—is intramolecularly caged, so that it is quenched (nonfluorescent) in its “off” state. When the probe encounters cancer cells, which overexpress the enzyme γ-glutamyltranspeptidase (GGT), the gGlu is cleaved, simultaneously turning “on” the fluorescent HMRG. Urano and colleagues first tested the probe in 11 human ovarian cancer cell lines in vitro and observed rapid fluorescence within 10 min after addition of the imaging agent to the cell cultures. They next moved into several mouse models of disseminated human peritoneal ovarian cancer, using a spray formulation of the probe that allowed the researchers to topically apply the probe during surgery or endoscopy. Within 1 min of spraying the tumors, gGlu-HMRG was enzymatically cleaved, revealing a bright fluorescent region of the peritoneal cavity in which the cancerous lesions were located. These small nodules were quickly and completely removed from living animals with forceps, demonstrating the power of rapid fluorescence-guided tumor resection. This gGlu-based fluorescent probe as well as several other aminopeptidase–based reagents identified by the authors could help surgeons to track down tiny tumors dispersed throughout body cavities, ensuring that no residual tumor is left behind. Complete obliteration of disseminated tumors should improve cancer outcomes after surgery. The ability of the unaided human eye to detect small cancer foci or accurate borders between cancer and normal tissue during surgery or endoscopy is limited. Fluorescent probes are useful for enhancing visualization of small tumors but are typically limited by either high background signal or the requirement for administration hours to days before use. We synthesized a rapidly activatable, cancer-selective fluorescence imaging probe, γ-glutamyl hydroxymethyl rhodamine green (gGlu-HMRG), with intramolecular spirocyclic caging for complete quenching. Activation occurs by rapid one-step cleavage of glutamate with γ-glutamyltranspeptidase (GGT), which is not expressed in normal tissue, but is overexpressed on the cell membrane of various cancer cells, thus leading to complete uncaging and dequenching of the fluorescence probe. In vitro activation of gGlu-HMRG was evident in 11 human ovarian cancer cell lines tested. In vivo in mouse models of disseminated human peritoneal ovarian cancer, activation of gGlu-HMRG occurred within 1 min of topically spraying the tumor, creating high signal contrast between the tumor and the background. The gGlu-HMRG probe is practical for clinical application during surgical or endoscopic procedures because of its rapid and strong activation upon contact with GGT on the surface of cancer cells.

Toxicity of organic fluorophores used in molecular imaging: literature review.

Fluorophores are potentially useful for in vivo cancer diagnosis. Using relatively inexpensive and portable equipment, optical imaging with fluorophores permits real-time detection of cancer. However, fluorophores can be toxic and must be investigated before they can be administered safely to patients. A review of published literature on the toxicity of 19 widely used fluorophores was conducted by searching 26 comprehensive biomedical and chemical literature databases and analyzing the retrieved material. These fluorophores included Alexa Fluor 488 and 514, BODIPY FL, BODIPY R6G, Cy 5.5, Cy 7, cypate, fluorescein, indocyanine green, Oregon green, 8-phenyl BODIPY, rhodamine 110, rhodamine 6G, rhodamine X, rhodol, TAMRA, Texas red, and Tokyo green. Information regarding cytotoxicity, tissue toxicity, in vivo toxicity, and mutagenicity was included. Considerable toxicity-related information was available for the Food and Drug Administration (FDA)-approved compounds indocyanine green and fluorescein, but published information on many of the non-FDA-approved fluorophores was limited. The information located was encouraging because the amounts of fluorophore used in molecular imaging probes are typically much lower than the toxic doses described in the literature. Ultimately, the most effective and appropriate probes for use in patients will be determined by their fluorescent characteristics and the safety of the conjugates.

Rational chemical design of the next generation of molecular imaging probes based on physics and biology: mixing modalities, colors and signals

In recent years, numerous in vivo molecular imaging probes have been developed. As a consequence, much has been published on the design and synthesis of molecular imaging probes focusing on each modality, each type of material, or each target disease. More recently, second generation molecular imaging probes with unique, multi-functional, or multiplexed characteristics have been designed. This critical review focuses on (i) molecular imaging using combinations of modalities and signals that employ the full range of the electromagnetic spectra, (ii) optimized chemical design of molecular imaging probes for in vivo kinetics based on biology and physiology across a range of physical sizes, (iii) practical examples of second generation molecular imaging probes designed to extract complementary data from targets using multiple modalities, color, and comprehensive signals (277 references).

Biologically optimized nanosized molecules and particles: more than just size.

The expanded biological and medical applications of nanomaterials place a premium on better understanding of the chemical and physical determinants of in vivo particles. Nanotechnology allows us to design a vast array of molecules with distinct chemical and biological characteristics, each with a specific size, charge, hydrophilicity, shape, and flexibility. To date, much research has focused on the role of particle size as a determinant of biodistribution and clearance. Additionally, much of what we know about the relationship between nanoparticle traits and pharmacokinetics has involved research limited to the gross average hydrodynamic size. Yet, other features such as particle shape and flexibility affect in vivo behavior and become increasingly important for designing and synthesizing nanosized molecules. Herein, we discuss determinants of in vivo behavior of nanosized molecules used as imaging agents with a focus on dendrimer-based contrast agents. We aim to discuss often overlooked or, yet to be considered, factors that affect in vivo behavior of synthetic nanosized molecules, as well as aim to highlight important gaps in current understanding.

X-ray computed tomography contrast agents prepared by seeded growth of gold nanoparticles in PEGylated dendrimer

Gold nanoparticles (Au NPs) are a potential x-ray computed tomography (CT) contrast agent. A biocompatible and bioinactive surface is necessary for application of gold nanoparticle to CT imaging. Polyethylene glycol (PEG)-attached dendrimers have been used as a drug carrier with long blood circulation. In this study, the Au NPs were grown in the PEGylated dendrimer to produce a CT contrast agent. The Au NPs were grown by adding gold ions and ascorbic acid at various equivalents to the Au NP-encapsulated dendrimer solution. Both size and surface plasmon absorption of the grown Au NPs increased with adding a large number of gold ions. The x-ray attenuation of the Au NPs also increased after the seeded growth. The Au NPs grown in the PEG-attached dendrimer at the maximum under our conditions exhibited a similar CT value to a commercial iodine agent, iopamidol, in vitro. The Au NP-loaded PEGylated dendrimer and iopamidol were injected into mice and CT images were obtained at different times. The Au NP-loaded PEGylated dendrimer achieved a blood pool imaging, which was greater than a commercial iodine agent. Even though iopamidol was excreted rapidly, the PEGylated dendrimer loading the grown Au NP was accumulated in the liver.

In vivo multiple color lymphatic imaging using upconverting nanocrystals

Upconverting nanocrystals are unique nano-sized particles that emit light at shorter wavelengths (visible and near infrared) after excitation in the near infrared that dramatically reduces background autofluorescence in in vivo two color lymphatic imaging for depicting the lymphatic channels and nodes.

Targeting of Lectinlike Oxidized Low-Density Lipoprotein Receptor 1 (LOX-1) with 99mTc-Labeled Anti–LOX-1 Antibody: Potential Agent for Imaging of Vulnerable Plaque

Lectinlike oxidized low-density lipoprotein (LDL) receptor 1 (LOX-1), a cell surface receptor for oxidized LDL, has been implicated in vascular cell dysfunction related to plaque instability, which could be a potential target for an atherosclerosis imaging tracer. In this study, we designed and prepared 99mTc-labeled anti–LOX-1 monoclonal IgG and investigated its usefulness as an atherosclerosis imaging agent. Methods: Anti–LOX-1 monoclonal IgG and control mouse IgG2a were labeled with 99mTc after derivatization with 6-hydrazinonicotinic acid to yield 99mTc-LOX-1-mAb and 99mTc-IgG2a, respectively. Myocardial infarction–prone Watanabe heritable hyperlipidemic (WHHLMI) rabbits (atherosclerosis model) and control rabbits were injected intravenously with these probes, and in vivo planar imaging was performed. At 24 h after the injection, the aortas were removed, and radioactivity was measured. Autoradiography and histologic studies were performed with serial aortic sections. Results: The level of 99mTc-LOX-1-mAb accumulation was 2.0-fold higher than the level of 99mTc-IgG2a accumulation in WHHLMI rabbit aortas, and the level of 99mTc-LOX-1-mAb accumulation in WHHLMI rabbit aortas was 10.0-fold higher than the level of 99mTc-LOX-1-mAb accumulation in control rabbit aortas. In vivo imaging clearly visualized the atherosclerotic aortas of WHHLMI rabbits. Autoradiography and histologic studies revealed that regional 99mTc-IgG2a accumulation was independent of the histologic grade of the lesions; however, regional 99mTc-LOX-1-mAb accumulation was significantly correlated with LOX-1 expression density and the vulnerability index. The highest level of 99mTc-LOX-1-mAb accumulation, expressed as {radioactivity in region of interest (Bq/mm2)/[injected radioactivity (Bq)/animal body weight (g)]} × 102, was found in atheromatous lesions (3.8 ± 1.1 [mean ± SD]), followed in decreasing order by fibroatheromatous lesions (2.0 ± 1.0), collagen-rich lesions (1.6 ± 0.8), and neointimal lesions (1.4 ± 0.7). Conclusion: The level of 99mTc-LOX-1-mAb accumulation in grade IV atheroma was higher than that in neointimal lesions or other, more stable lesions. Nuclear imaging of LOX-1 expression with 99mTc-LOX-1-mAb may be a useful means for predicting atheroma at high risk for rupture.

Sensitive β-galactosidase-targeting fluorescence probe for visualizing small peritoneal metastatic tumours in vivo.

Fluorescence-guided diagnostics is one of the most promising approaches for facile detection of cancer in situ. Here we focus on β-galactosidase, which is overexpressed in primary ovarian cancers, as a molecular target for visualizing peritoneal metastases from ovarian cancers. As existing fluorescence probes are unsuitable, we have designed membrane-permeable HMRef-βGal, in which the optimized intramolecular spirocyclic function affords >1,400-fold fluorescence enhancement on activation. We confirm that HMRef-βGal sensitively detects intracellular β-galactosidase activity in several ovarian cancer lines. In vivo, this probe visualizes metastases as small as <1 mm in diameter in seven mouse models of disseminated human peritoneal ovarian cancer (SHIN3, SKOV3, OVK18, OVCAR3, OVCAR4, OVCAR5 and OVCAR8). Because of its high brightness, real-time detection of metastases with the naked eye is possible. Endoscopic fluorescence detection of metastases is also demonstrated. The results clearly indicate preclinical potential value of the probe for fluorescence-guided diagnosis of peritoneal metastases from ovarian cancers.