Review
Targeted Molecular Imaging and Therapy Based on Nuclear and Optical Technologies
2024 Volume 47 Issue 6 Pages 1066-1071
Details
2024 Volume 47 Issue 6 Pages 1066-1071
Both nuclear and optical imaging are used for in vivo molecular imaging. Nuclear imaging displays superior quantitativity, and it permits imaging in deep tissues. Thus, this method is widely used clinically. Conversely, because of the low permeability of visible to near-IR light in living animals, it is difficult to visualize deep tissues via optical imaging. However, the light at these wavelengths has no ionizing effect, and it can be used without any restrictions in terms of location. Furthermore, optical signals can be controlled in vivo to accomplish target-specific imaging. Nuclear medicine and phototherapy have also evolved to permit targeted-specific imaging. In targeted nuclear therapy, beta emitters are conventionally used, but alpha emitters have received significant attention recently. Concerning phototherapy, photoimmunotherapy with near-IR light was approved in Japan in 2020. In this article, target-specific imaging and molecular targeted therapy utilizing nuclear medicine and optical technologies are discussed.
The term “molecular imaging” has been frequently used since approximately 2000 to describe the imaging of biomolecules in vivo. Various imaging agents and methods have been developed, and molecular imaging technology has achieved sufficient maturity for use in both basic studies and clinical applications. Nuclear imaging and optical imaging are representative methods for molecular imaging. Sensitivity is a key factor for detecting biomolecules in vivo. These imaging modalities are superior to other in vivo imaging modalities in terms of sensitivity; that is, fewer chemical doses of imaging agents are required.
Nuclear imaging using electromagnetic waves with short wavelengths is characterized by the high permeability of electromagnetic waves through the body and excellent quantitativity. Optical imaging using electromagnetic waves in the visible to near-IR range has no ionizing effect, allowing its use without any restrictions concerning location. Furthermore, the optical signal can be controlled in vivo to achieve high specificity for the molecular target.
Nuclear medicine and phototherapy have also evolved concerning their molecular targeting nature. By replacing the signal molecule for imaging with therapeutic molecules, molecular targeted therapy is possible.
This review describes the properties and examples of target-specific imaging and molecular targeted therapies that can be applied in the clinic.
Nuclear medicine imaging modalities, such as positron emission tomography (PET) and single-photon emission tomography (SPECT), are non-invasive, highly quantitative imaging methods. Radiopharmaceuticals with positron emitters and single-photon emitters are used for PET and SPECT, respectively. Nuclear medicine imaging permits the quantitative evaluation of molecules in a living organism, and it can be used to detect diseases and evaluate the disease status. In addition, its non-invasive nature allows images to be obtained over time in the same individual. Thus, PET and SPECT can be used to evaluate treatment efficacy over time and as a companion diagnostic prior to treatment.
For example, PET is used to evaluate amyloid beta (Aβ) deposition before treatment with the U.S. Food and Drug Administration (FDA)-approved anti-Aβ drug lecanemab, as this drug is approved for patients with confirmed Aβ deposition. In addition, decreases in Aβ deposition during treatment can be monitored by PET. Several [18F]-labeled imaging probes that bind to Aβ plaque are used for this purpose.1) Cerebrospinal fluid testing is also used to evaluate Aβ deposition, but non-invasive evaluation is possible with PET.
We applied PET to monitor the therapeutic effects of drugs for atherosclerosis. Atherosclerotic plaques are classified into two types, namely stable and vulnerable. Stable plaques with thick fibrous cap do not easily rupture, whereas vulnerable plaques can rupture and induce thrombosis, potentially leading to stroke or heart attack. Vulnerable plaques are characterized by a lipid-rich core with a thin fibrous cap and macrophage infiltration. We focused on the elevation of glucose metabolism in macrophages, and [18F]FDG was used for vulnerable plaque detection.2) Watanabe heritable hyperlipidemic (WHHL) rabbits served as the atherosclerotic animal model for this study. Atherosclerotic plaques were successfully visualized by [18F]FDG-PET, and the level of [18F]FDG accumulation was correlated with the macrophage infiltration rate (Fig. 1). Thus, it was elucidated that [18F]FDG can detect plaques depending on their vulnerability. This method was applied to monitor the therapeutic efficacy of probucol, which is used for hyperlipidemia therapy. Plaque vulnerability was monitored by [18F]FDG-PET, and [18F]FDG accumulation was reduced time-dependently in probucol-treated WHHL rabbits3) Histopathological studies revealed the reduction of inflammatory plaques. Blood cholesterol levels were not reduced in these rabbits during this observation period. Thus, the results illustrated that [18F]FDG-PET can assess plaque vulnerability at early time points with greater sensitivity than blood tests in this model. This method is currently used to evaluate the efficacy of investigational drugs in clinical trials.4–6)
Plaque vulnerability was evaluated by [18F]FDG. PET-CT, positron emission tomography-computed tomography; WHHL, Watanabe heritable hyperlipidemic.
In addition to inflammatory responses in atherosclerotic plaques, calcification is also important factor for plaque rapture. Computed tomography is widely used to detect calcification in atherosclerosis; however, the detection of microcalcification by [18F]NaF at an earlier stage has been revealed to be important for early diagnosis and event prevention. Several studies have compared [18F]FDG and [18F]NaF, as well as other radiotracers, for atherosclerosis imaging.7–9) Because PET can non-invasively detect pathological and biomolecular changes in vivo, it should be helpful in formulating treatment plans, including drug selection.
Radiolabeled compounds with beta emitters, such as Sr-89, Y-90, I-131, and Lu-177, have been used for targeted nuclear therapy. Recently, targeted therapy using alpha emitters has received substantial attention. Because alpha rays have a shorter radiation range (40–100 µm) than beta rays (0.05–12 mm),10) their efficiency is considered insufficient for tumor therapy. However, it was reported in 2016 that targeted alpha therapy with Ac-225–labeled prostate-specific membrane antigen ligands exerted significant effects on metastatic prostate cancer compared to Lu-177–labeled compounds in clinical studies.11) One reason for the strong therapeutic effects of alpha emitters could be their greater ability to induce DNA double-strand breaks than beta emitters. In addition, the induction of cancer immunity is attracted attention in targeted alpha therapy.12,13)
Astatine-211 (At-211) is a promising alpha emitter for nuclear therapy. A nuclear reactor is not needed for At-211 production, and it can be produced by a cyclotron from the naturally abundant element Bi-209. Clinical studies of At-211–labeled compounds are ongoing in several countries including Japan. In addition, the development of new At-211–labeled compounds and labeling methods is progressing. We recently reported a new At-211 chemical method using iodonium ylide as a labeling precursor14) (Fig. 2). With this method, nucleophilic reaction with At-211 anions is used for radiolabeling instead of At-211 cations, which are sometimes unstable because of their various oxidized forms. Targeted alpha therapy is expected to be further developed in the future.
Reproduced from14) with permission from the Royal Society of Chemistry.
Targeted therapy using Auger electrons, which have a shorter radiation range (2–500 nm) than alpha rays,10) is also attracting attention. DNA damage can be induced by binding the Auger emitter Pt-191 directly to DNA.15) Because Auger electrons have an extremely short radiation range, organelle-specific damage can be induced.
Optical imaging is an easy-to-use technique because, unlike nuclear technology, it does not use ionizing radiation. Thus, it is widely used in basic sciences for both in vitro and in vivo experiments. For targeted in vivo fluorescence imaging, organic fluorescent molecules are typically used in combination with targeting molecules such as peptides, proteins, and nanoparticles. The advantages of fluorescence imaging include the ability to control the optical signal in vivo, and the signal can be activated specifically at the target tissues. With such activatable probes, target-specific imaging is possible with low background signals. Conversely, radiation from PET and SPECT imaging agents is not controllable in vivo, and thus, the background signal sometimes precludes target-specific imaging. Several approaches for constructing activatable probes have been reported, such as photo-induced electron transfer and fluorescence resonance energy transfer.16) We have been developing activatable imaging agents that exhibit fluorescence when accumulated by cancer cells using π-electron stacking as a switch for fluorescence control.17) That is, the formation of H-dimer, in which the dipole moment of the dye is coupled in the opposite direction due to π-electron interactions, is used to deactivate fluorescence. The fluorescence is activated after internalization of the imaging agents and conformational changes in the targeting molecule to release the π-electron interactions of the fluorescent dye. With this system, high contrast imaging was accomplished with low background signal. Thus, small tumor nodules were successfully visualized by fluorescence endoscopy (Fig. 3).
Fluorescence is quenched by H-type dimerization and is activated by conformational changes of the protein; b) Fluorescence endoscopy with a conventional always-ON probe. The background signal is observed as well as tumor nodules; c) Fluorescence endoscopy with an activatable probe. Tumor nodules are clearly visualized with a low background signal. Reprinted with permission from.17) Copyright 2009 American Chemical Society.
Optical imaging is widely used in small animal studies; however, it is difficult to visualize deep tissues because of the limited penetration of the optical signal. Near-IR light penetrates tissue better than visible light, but its penetrance is still limited to 1–2 cm. Photoacoustic imaging has attracted attention to overcome this issue. In photoacoustic imaging, the dye is excited by light, and produced acoustic signals from the excited dye are used for detection.18) Because acoustic signals have greater ability to permeate living organisms than light, it is possible to observe deeper tissue. Some molecular imaging agents have been developed including activatable type dyes.19–21)
In phototherapy, it is impossible to treat the tumor in areas in which light cannot be delivered, and therefore, in contrast to radiotherapy, the drugs are not toxic in areas not exposed to light. Thus, off-target effects can be reduced. In cancer treatment, photodynamic therapy kills cancer cells by producing singlet oxygen. We developed a new molecular targeted cancer therapy termed photoimmunotherapy, which uses the photosensitizing dye IR700 conjugated to an antibody.22) The IR700-conjugated antibody (Ab-IR700) binds to the target cancer cell membrane, and exposure to near-IR light induces target-specific cell disruption.
In photoimmunotherapy, singlet oxygen is not a key molecule in cell death, and the aggregation formation of Ab-IR700 on the cancer cell membrane induces cell toxicity23,24) (Fig. 4). IR700 is a Si-phthalocyanine derivative with hydrophilic axial ligands and a large absorption band (Q-band) in the near-IR region (690 nm). A frontal diffuser is used for the surface irradiation of near-IR light, and a cylindrical diffuser is used for in-tissue irradiation in combination with a needle catheter. After IR700 absorbs the light, it receives electron from the electron donor such as cysteine, and IR700 radical anion is produced25) (Fig. 5). The radical anion is then protonated, followed by Si-O bond elongation. Then, the hydrophilic axial ligands are cleaved, and π-electron–rich hydrophobic molecules are produced. The molecules bind to each other to form aggregates. This aggregation can be induced when IR700 is conjugated to an antibody. When antibodies on the cell membrane aggregate, cell membranes are pulled apart and damaged. Then, small molecules such as water and inorganic ions flow into the cell, causing it to burst. These processes can activate cancer immunity, and metastatic tumors can also be treated without direct irradiation.26) Since liposomes are lipid bilayers as cell membranes, it would be possible to disrupt the liposome for photo-responsive drug delivery system. Our study revealed that IR700-conjugated liposomes can successfully release the encapsulated drug upon light irradiation27) (Fig. 6).
An antibody–IR700 conjugate is used for photoimmunotherapy. IR700 aggregates in response to near-IR light irradiation, leading to cell membrane damage.
Reproduced from25) under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/).
In normoxic conditions, singlet oxygen is produced. Membrane permeability is elevated, but the membrane is not destroyed. By contrast, in hypoxic conditions, the membrane is disrupted by IR700 aggregation. Reprinted with permission from.27) Copyright 2020 The Pharmaceutical Society of Japan.
Nuclear imaging has long been utilized in the clinical field because of its high signal transparency and high sensitivity. However, it has not been frequently used in basic research, especially in mouse studies, because of its low resolution. With the development of dedicated equipment for small animals, nuclear imaging has been increasingly used in basic research in recent years. However, optical imaging has been widely used in basic research because of its easy-to-use features. Fluorescence imaging, luminescence imaging, in vivo imaging, and in vitro imaging are essential methods in basic research. As previously mentioned, various R&D efforts are underway to use optical imaging in humans. In addition, multimodal imaging combining the advantages of various techniques while overcoming their disadvantages, has been developed.28)
In terms of treatment, nuclear medicine uses ionizing radiation, and phototherapy uses non-harmful light. That is, each method has different mechanisms of action, and therefore, they have different treatment courses. However, these therapies, which use biological targeting molecules and physical energy, have the potential to facilitate more specific and effective treatment than conventional therapies.
Effective diagnosis and treatment in the living body can be achieved with the appropriate use of physical energy and chemical compounds according to their properties.
These studies were performed at Kyoto University, National Institute for Longevity Sciences, National Cardiovascular Center, Hamamatsu Medical University, Hokkaido University in Japan and US National Institute of Health. I express my sincere appreciation to Professors H. Saji, Y. Magata, and H. Kobayashi for fruitful discussions and encouragement. I additionally thank past and present staff and students.
This work was financially supported in part by the Japan Society for the Promotion of Science KAKENHI Grant Nos. 10920037, 11829573, 13058465, 16669376, 19159571, 19160876), and PRESTO (No. 15664344) and CREST (No. 19205052) of the Japan Science and Technology Agency.
The author declares no conflict of interest.
This review of the author’s work was written by the author upon receiving the 2023 Pharmaceutical Society of Japan Award for Divisional Scientific Promotion.
