Antigen-responsive molecular sensor enables real-time tumor-specific imaging

Activatable near-infrared fluorescence probe for real-time imaging of PD-L1 expression in tumors

In clinical practice, determining programmed death-ligand 1 (PD-L1) expression is crucial for selecting patients and monitoring immune checkpoint blockade therapies. Currently, PD-L1 expression is quantified using immunohistochemistry (IHC). However, IHC-based methods do not capture the heterogeneous and dynamic nature of PD-L1 expression. Thus, there is a pressing need for a rapid and efficient method for monitoring PD-L1 expression both in vitro and in vivo, which would considerably aid in prognosis and treatment selection. In this study, we present for the first time an activatable near-infrared (NIR) fluorescence imaging probe (Q-Atezol) for the real-time monitoring of PD-L1 expression in vitro and in vivo. The ability of Q-Atezol to detect PD-L1 expression quickly and in real-time was evaluated in both tumor spheroid and lung cancer xenograft models. An always-on optical probe (ON-Atezol) was synthesized and tested for comparison. In vivo NIR fluorescence imaging studies were conducted on A549 and H1975 tumor-bearing mice, and their tumor-to-background ratios (TBRs) were analyzed. The quenched NIR fluorescence of Q-Atezol is activated upon binding to PD-L1 proteins on the surface of cancer cells, thereby enabling PD-L1 detection in the three-dimensional (3D) tumor spheroids without a washing step. Notably, PD-L1-positive H1975 tumors were clearly visualized with a high TBR 6 hours after Q-Atezol injection, whereas ON-Atezol treatment could not detect H1975 tumors even 24 hours post-injection. The activatable fluorescence probe Q-Atezol demonstrated great potential as an exceptional sensor for assessing PD-L1 expression in 3D cell structures and for in vivo applications.

Quenched Zwitterionic Cyclic Arg-Gly-Asp-Containing Pentapeptide Probe for Real-Time Brain Tumor Imaging

The efficacy of glioblastoma treatment is closely associated with complete tumor resection. However, conventional surgical techniques often result in incomplete removal, leading to poor prognosis. A major challenge is the accurate delineation of tumor margins from healthy tissues. Imaging-guided surgery, particularly using fluorescent probes, is a promising solution for intraoperative guidance. The recently developed 'always-on' types of targeted fluorescence probes generate signals irrespective of their presence in tumor cells or in blood circulation, hampering their effectiveness. Here, we propose a novel activatable fluorescence imaging probe, Q-cRGD, that targets glioma cells via the specific binding of the cyclic Arg-Gly Asp-containing pentapeptide (cRGD) to integrins. The Q-cRGD probe was synthesized by conjugating a near-infrared (NIR) dye to a tryptophan quencher via a disulfide linkage, including a cRGD-targeting ligand. This activatable probe remained inactive until the redox-responsive cleavage of the disulfide linkage occurred within the target cell. The zwitterionic nature of NIR dyes minimizes nonspecific interactions with serum proteins, thereby enhancing the tumor-to-background signal ratio (TBR). An in vivo fluorescence imaging study demonstrated a TBR value of 2.65 within 3 h of the intravenous injection of Q-cRGD, confirming its potential utility in imaging-guided brain cancer surgery.

Novel Near-Infrared Fluorescent Probe for Hepatocyte Growth Factor in Vivo Imaging in Surgical Navigation of Colorectal Cancer

Despite recent advancements in colorectal cancer (CRC) treatment, the prognosis remains unfavorable primarily due to high recurrence and liver metastasis rates. Fluorescence molecular imaging technologies, combined with specific probes, have gained prominence in facilitating real-time tumor resection guided by fluorescence. Hepatocyte growth factor (HGF) is overexpressed in CRC, but the advancement of HGF fluorescent probes has been impeded by the absence of effective HGF-targeting small-molecular ligands. Herein, we present the targeted capabilities of the novel V-1-GGGK-MPA probe labeled with a near-infrared fluorescent dye, which targets HGF in CRC. The V-1-GGGK peptide exhibits high specificity and selectivity for HGF-positive in vitro tumor cells and in vivo tumors. Biodistribution analysis of V-1-GGGK-MPA revealed tumor-specific accumulation with low background uptake, yielding signal-to-noise ratio (SNR) values of tumor-to-colorectal >6 in multiple subcutaneous CRC models 12 h postinjection. Quantitative analysis confirmed the probe's high uptake in SW480 and HT29 orthotopic and liver metastatic models, with SNR values of tumor-to-colorectal and -liver being 5.6 ± 0.4, 4.6 ± 0.5, and 2.1 ± 0.3, 2.0 ± 0.5, respectively, enabling precise tumor visualization for surgical navigation. Pathological analysis demonstrated the excellent tumor boundaries discrimination capacity of the V-1-GGGK-MPA probe at the molecular level. With its rapid tumor targeting, sustained tumor retention, and precise tumor boundary delineation, V-1-GGGK-MPA merges as a promising HGF imaging agent, enriching the toolbox of intraoperative navigational fluorescent probes for CRC.

Preclinical evaluation of AGTR1-Targeting molecular probe for colorectal cancer imaging in orthotopic and liver metastasis mouse models

Despite advancements in colorectal cancer (CRC) treatment, the prognosis remains unfavorable for patients with distant liver metastasis. Fluorescence molecular imaging with specific probes is increasingly used to guide CRC surgical resection in real-time and treatment planning. Here, we demonstrate the targeted imaging capacity of an MPA-PEG4-N3-Ang II probe labeled with near-infrared (NIR) fluorescent dye targeting the angiotensin II (Ang II) type 1 receptor (AGTR1) that is significantly upregulated in CRC. MPA-PEG4-N3-Ang II was highly selective and specific to in vitro tumor cells and in vivo tumors in a mouse CRC xenograft model. The favorable ex vivo imaging and in vivo biodistribution of MPA-PEG4-N3-Ang II afforded tumor-specific accumulation with low background and >10 contrast tumor-to-colorectal values in multiple subcutaneous CRC models at 8 h following injection. Biodistribution analysis confirmed the probe's high uptake in HT29 and HCT116 orthotopic and liver metastatic models of CRC with signal-to-noise ratio (SNR) values of tumor-to-colorectal and -liver fluorescence of 5.8 ± 0.6, 5.3 ± 0.7, and 2.7 ± 0.5, 2.6 ± 0.5, respectively, enabling high-contrast intraoperative tumor visualization for surgical navigation. Given its rapid tumor targeting, precise tumor boundary delineation, durable tumor retention and docking study, MPA-PEG4-N3-Ang II is a promising high-contrast imaging agent for the clinical detection of CRC.

Effective synthesis, development and application of a highly fluorescent cyanine dye for antibody conjugation and microscopy imaging

An asymmetric cyanine-type fluorescent dye was designed and synthesized via a versatile, multi-step process, aiming to conjugate with an Her2+ receptor specific antibody by an azide-alkyne click reaction. The aromaticity and the excitation and relaxation energetics of the fluorophore were characterized by computational methods. The synthesized dye exhibited excellent fluorescence properties for confocal microscopy, offering efficient applicability in in vitro imaging due to its merits such as a high molar absorption coefficient (36 816 M-1 cm-1), excellent brightness, optimal wavelength (627 nm), larger Stokes shift (26 nm) and appropriate photostability compared to cyanines. The conjugated cyanine-trastuzumab was constructed via an effective, metal-free, strain-promoted azide-alkyne click reaction leading to a regulated number of dyes being conjugated. This novel cyanine-labelled antibody was successfully applied for in vitro confocal imaging and flow cytometry of Her2+ tumor cells.

Fast and Durable Intraoperative Near-infrared Imaging of Ovarian Cancer Using Ultrabright Squaraine Fluorophores

The residual tumor after surgery is the most significant prognostic factor of patients with epithelial ovarian cancer. Near-infrared (NIR) fluorescence-guided surgery is actively utilized for tumor localization and complete resection during surgery. However, currently available contrast-enhancing agents display low on-target binding, unfavorable pharmacokinetics, and toxicity, thus not ideal for clinical use. Here we report ultrabright and stable squaraine fluorophores with optimal pharmacokinetics by introducing an asymmetric molecular conformation and surface charges for rapid transporter-mediated cellular uptake. Among the tested, OCTL14 shows low serum binding and rapid distribution into cancer tissue via organic cation transporters (OCTs). Additionally, the charged squaraine fluorophores are retained in lysosomes, providing durable intraoperative imaging in a preclinical murine model of ovarian cancer up to 24 h post-injection. OCTL14 represents a significant departure from the current bioconjugation approach of using a non-targeted fluorophore and would provide surgeons with an indispensable tool to achieve optimal resection.

Quenched cetuximab conjugate for fast fluorescence imaging of EGFR-positive lung cancers

Cetuximab-dye conjugates have shown great potential for image-guided surgery of epidermal growth factor receptor (EGFR)-positive cancers in clinical trials. However, their long circulation half-life and prolonged generation of high background signals require the injection of antibody conjugates several days prior to imaging, which limits the clinical applications. Herein, we developed a cetuximab-ATTO655 conjugate (i.e., Q-Cetuximab) for fast and real-time fluorescence imaging of EGFR-positive lung cancers. The fluorescence intensity of Q-Cetuximab was quenched to just 6.9% of that of the unconjugated dye when only 2.14 ATTO655 dyes were conjugated to cetuximab. In vitro real-time cell imaging showed that EGFR-positive A549 cells emitted strong fluorescence at 10 min after Q-Cetuximab treatment in the absence of the washing step, implying target-specific activation of quenched Q-Cetuximab fluorescence upon binding with EGFR-positive cancer cells. When mice with orthotropic A549 tumors received intravenous injection of Q-Cetuximab, scattered microsized tumors in the lungs could be clearly identified from near-infrared fluorescence imaging with a tumor-to-background ratio of 4.28 at 8 h post-injection. For comparison, the cetuximab-Alexa647 conjugate (i.e., ON-Cetuximab), which does not show fluorescence quenching, was synthesized as an always-on type of probe. The ON-Cetuximab-treated mice expressed strong fluorescence throughout their body at 8 h post-injection; therefore, lung tumor sites could not be discriminated using fluorescence imaging. These results confirm the benefits of Q-Cetuximab for image-guided precision surgery of EGFR-positive lung cancers.

A Quenched Annexin V-Fluorophore for the Real-Time Fluorescence Imaging of Apoptotic Processes In Vitro and In Vivo

Annexin-based probes have long been used to study apoptotic cell death, which is of key importance to many areas of biological research, drug discovery, and clinical applications. Although apoptosis is a dynamic biological event with cell-to-cell variations, current annexin-based probes are impractical for monitoring apoptosis in real-time. Herein, a quenched annexin V-near-infrared fluorophore conjugate (Q-annexin V) is reported as the first OFF-ON annexin protein-based molecular sensor for real-time near-infrared fluorescence imaging of apoptosis. Q-annexin V is non-fluorescent in the extracellular region, due to photoinduced electron transfer interactions between the conjugated dye and amino acid quenchers (tryptophan and tyrosine). The probe becomes highly fluorescent when bound to phosphatidylserines on the outer layer of cell membranes during apoptosis, thereby enabling apoptosis to be monitored in real-time in 2D and 3D cell structures. In particular, Q-annexin V shows superior utility for in vivo apoptosis fluorescence imaging in animal models of cisplatin-induced acute kidney injury and cancer immune therapy, compared to the conventional polarity-sensitive pSIVA-IANBD or annexin V-Alexa647 conjugates.

Near-infrared fluorescence imaging in immunotherapy

Near-infrared (NIR) light possesses many suitable optophysical properties for medical imaging including low autofluorescence, deep tissue penetration, and minimal light scattering, which together allow for high-resolution imaging of biological tissue. NIR imaging has proven to be a noninvasive and effective real-time imaging methodology that provides a high signal-to-background ratio compared to other potential optical imaging modalities. In response to this, the use of NIR imaging has been extensively explored in the field of immunotherapy. To date, NIR fluorescence imaging has successfully offered reliable monitoring of the localization, dynamics, and function of immune responses, which are vital in assessing not only the efficacy but also the safety of treatments to design immunotherapies optimally. This review aims to provide an overview of the current research on NIR imaging of the immune response. We expect that the use of NIR imaging will expand further in response to the recent success in cancer immunotherapy. We will also offer our insights on how this technology will meet rapidly growing expectations in the future.

Recent progress in the molecular imaging of therapeutic monoclonal antibodies

Therapeutic monoclonal antibodies have become one of the central components of the healthcare system and continuous efforts are made to bring innovative antibody therapeutics to patients in need. It is equally critical to acquire sufficient knowledge of their molecular structure and biological functions to ensure the efficacy and safety by incorporating new detection approaches since new challenges like individual differences and resistance are presented. Conventional techniques for determining antibody disposition including plasma drug concentration measurements using LC-MS or ELISA, and tissue distribution using immunohistochemistry and immunofluorescence are now complemented with molecular imaging modalities like positron emission tomography and near-infrared fluorescence imaging to obtain more dynamic information, while methods for characterization of antibody's interaction with the target antigen as well as visualization of its cellular and intercellular behavior are still under development. Recent progress in detecting therapeutic antibodies, in particular, the development of methods suitable for illustrating the molecular dynamics, is described here.

Mini-Platform for Off-On Near-Infrared Fluorescence Imaging Using Peptide-Targeting Ligands

Here, we propose a zwitterionic near-infrared (NIR) fluorophore-tryptophan (Trp) conjugate with a cleavable linker as a minimal-sized versatile platform (MP) for the preparation of peptide ligand-based off-on type molecular probes. The zwitterionic NIR fluorophore in MP undergoes fluorescence quenching via a photoinduced electron transfer mechanism when in close proximity to tryptophan, and nonspecific binding with serum proteins is minimized by the zwitterionicity of the fluorophore. The linker can be cleaved inside cancer cells in response to tumor-associated stimuli. As a proof-of-concept experiment, ATTO655 was covalently linked with Trp via a diarginine linker to form an MP. A cyclic peptide consisting of Arg-Gly-Asp-d-Phe-Lys (cRGD) was used as a cancer-targeting ligand and was conjugated to the MP to form cRGD-MP. The NIR fluorescence of cRGD-MP could be selectively turned on inside the target cancer cells, thereby enabling specific fluorescence imaging of integrin α_vβ₃-overexpressing cancer cells in vitro and in vivo.

Comparison between Fractionated Dose and Single Dose of Cu-64 Trastuzumab Therapy in the NCI-N87 Gastric Cancer Mouse Model

For optimum radioimmunotherapy (RIT), deep penetration and uniform distribution into the tumor core is important. The solid tumor microenvironment, consisting of a highly fibrotic or desmoplastic tumor, abnormal tumor vasculature, high fluid pressure, and the absence of fluid lymphatics, limits the distribution of monoclonal antibodies mAbs to the tumor core. To investigate the optimal rationale for therapeutic mAbs administration and the microdistribution of mAbs, single and serial fractional dosage regimens of Cu-64-trastuzumab (TRZ) with paclitaxel were evaluated. Groups of nude mice were inoculated with gastric cancer cell line NCI-N87 tumor cells. When the tumor size reached 200 ± 20 mm³, the mice were divided into two groups for injection of Alexa-647-TRZ. One group (n = 5) was injected with 15 mg/kg in a single dose (SD), and the other group (n = 5) with two doses of 7.5 mg/kg (fractionated dose (FD)). In both cases, the injections were done intravenously in combination with intraperitoneal paclitaxel either as a SD of 70 mg/kg or fractionated into two doses of 40 and 30 mg/kg. Tumors were harvested, flash frozen, and sectioned (8 µm) five days after Alexa-647-TRZ injection. Rhodamine lectin (rhodamine-labeled Ricinus communis agglutinin I, 1 mg in 0.2 mL of phosphate-buffered saline (PBS)) was intravenously injected to delineate the functional vessel for a wait time of 5 min before animal euthanization. Microscopic images were acquired with an IN Cell Analyzer. The amount of TRZ that penetrated the tumor surface and the tumor vessel was calculated by area under the curve (AUC) analysis. For RIT efficacy (n = 21), Cu-64-TRZ was injected following the same dose schedule to observe tumor volume and survival ratio for 30 days. The SD and FD regimens of Alexa-647-TRZ were observed to have no significant difference in penetration of mAbs from the tumor edge and vessel, nor was the total accumulation across the whole tumor tissue significantly different. Additionally, the SD and FD regimens of Cu-64-TRZ were not proven to be significantly efficacious. Our study reveals that SD and FD in a treatment design with Cu-64-TRZ and paclitaxel shows no significant difference in therapeutic efficacy on tumor growth inhibition in vivo in mice bearing human gastric cancer xenografts overexpressing HER2 antigen.

Emerging Fluorescent Molecular Tracers to Guide Intra-Operative Surgical Decision-Making

Fluorescence imaging is an emerging technology that can provide real-time information about the operating field during cancer surgery. Non-specific fluorescent agents, used for the assessment of blood flow and sentinel lymph node detection, have so far dominated this field. However, over the last decade, several clinical studies have demonstrated the great potential of targeted fluorescent tracers to visualize tumor lesions in a more specific way. This has led to an exponential growth in the development of novel molecular fluorescent contrast agents. In this review, the design of fluorescent molecular tracers will be discussed, with particular attention for agents and approaches that are of interest for clinical translation.

Bifunctional aptamer-mediated catalytic hairpin assembly for the sensitive and homogenous detection of rare cancer cells

The presence of cancer cells in body fluids confirms the occurrence of metastasis and guides treatment. A simple, fast, and homogeneous fluorescent method was developed to detect cancer cells based on catalytic hairpin assembly (CHA) and bifunctional aptamers. The bifunctional aptamer had a recognition domain for binding to target cancer cells and an initiator domain for triggering the CHA reaction. In the presence of target cells, the bifunctional aptamer was released from the inhibitor and initiated a cascade reaction of assembly and disassembly of the hairpins. Separation of the fluorophores from the quenchers produced fluorescence signals. The proposed strategy showed high specificity for discriminating normal cells and leukocytes, and the detection limit was 10 cells/mL, which was lower than that of previous aptasensors. This assay was further tested using four kinds of clinical samples spiked with target cells to confirm its applicability. We developed a simple, rapid, and cost-effective method for the detection of cancer cells that did not require purification, and the approach holds great potential for bioanalysis and early diagnosis.