Activatable Off-on Near-Infrared QCy7-based Fluorogenic Probes for Bioimaging

Fluorogenic polymethine dyes by intramolecular cyclization

Fluorescence imaging plays a pivotal role in the study of biological processes, and cell-permeable fluorogenic dyes are crucial to visualize intracellular structures with high specificity. Polymethine dyes are vitally important fluorophores in single-molecule localization microscopy and in vivo imaging, but their use in live cells has been limited by high background fluorescence and low membrane permeability. In this review, we summarize recent advances in the development of fluorogenic polymethine dyes via intramolecular cyclization. Finally, we offer an outlook on the prospects of fluorogenic polymethine dyes for bioimaging.

An Efficient Strategy for Constructing Fluorescent Nanoprobes for Prolonged and Accurate Tumor Imaging

Activatable near-infrared (NIR) fluorescent probes possess advantages of high selectivity, sensitivity, and deep imaging depth, holding great potential in the early diagnosis and prognosis assessment of tumors. However, small-molecule fluorescent probes are largely limited due to the rapid diffusion and metabolic clearance of activated fluorophores in vivo. Herein, we propose an efficient and reproducible novel strategy to construct activatable fluorescent nanoprobes through bioorthogonal reactions and the strong gold-sulfur (Au-S) interactions to achieve an enhanced permeability and retention (EPR) effect, thereby achieving prolonged and high-contrast tumor imaging in vivo. To demonstrate the merits of this strategy, we prepared an activatable nanoprobe, hCy-ALP@AuNP, for imaging alkaline phosphatase (ALP) activity in vivo, whose nanoscale properties facilitate accumulation and long-term retention in tumor lesions. Tumor-overexpressed ALP significantly increased the fluorescence signal of hCy-ALP@AuNP in the NIR region. More importantly, compared with the small-molecule probe hCy-ALP-N3, the nanoprobe hCy-ALP@AuNP significantly improved the distribution and retention time in the tumor, thus improving the imaging window and accuracy. Therefore, this nanoprobe platform has great potential in the efficient construction of biomarker-responsive fluorescent nanoprobes to realize precise tumor diagnosis in vivo.

RNA-Selective Small-Molecule Ligands: Recent Advances in Live-Cell Imaging and Drug Discovery

RNA structures, including those formed from coding and noncoding RNAs, alternative to protein-based drug targets, could be a promising target of small molecules for drug discovery against various human diseases, particularly in anticancer, antibacterial and antivirus development. The normal cellular activity of cells is critically dependent on the function of various RNA molecules generated from DNA transcription. Moreover, many studies support that mRNA-targeting small molecules may regulate the synthesis of disease-related proteins via the non-covalent mRNA-ligand interactions that do not involve gene modification. RNA-ligand interaction is thus an attractive approach to address the challenge of "undruggable" proteins in drug discovery because the intracellular activity of these proteins is hard to be suppressed with small molecule ligands. We selectively surveyed a specific area of RNA structure-selective small molecule ligands in fluorescence live cell imaging and drug discovery because the area was currently underexplored. This state-of-the-art review thus mainly focuses on the research published within the past three years and aims to provide the most recent information on this research area; hopefully, it could be complementary to the previously reported reviews and give new insights into the future development on RNA-specific small molecule ligands for live cell imaging and drug discovery.

Liver-Targeted Near-Infrared Fluorescence/Photoacoustic Dual-Modal Probe for Real-Time Imaging of In Situ Hepatic Inflammation

Early diagnosis of hepatic inflammation is the key to timely treatment and avoid the worsening of liver inflammation. Near-infrared fluorescence (NIRF) probes have high sensitivity but low spatial resolution in lesion imaging, while photoacoustic (PA) imaging has good spatial location information. Therefore, the development of a NIRF/PA dual-modal probe integrated with high sensitivity and spatial location feedback can achieve an accurate early diagnosis of hepatic inflammation. Here, we report an activatable NIRF/PA dual-modal probe (hCy-Tf-CA) for the detection of the superoxide anion (O₂^·-) in early hepatic inflammation. hCy-Tf-CA showed high selectivity and sensitivity for detecting O₂^·- fluctuation in vitro. More importantly, by introducing hepatocyte-targeting cholic acid (CA), the probe successfully achieved accurate in situ imaging of acute inflammatory liver injury (AILI) and autoimmune hepatitis (AIH) in vivo. The introduced CA not only promotes the hepatic targeting accumulation of probes but also improves the performance of low background dual-modal imaging in vivo. Therefore, hCy-Tf-CA provides an effective strategy for significantly improving in situ imaging performance and holds great potential for early, sensitive, and accurate diagnosis of hepatic inflammation.

Molecular fluorescent probes for liver tumor imaging

Liver cancer is a malignant tumor with both high morbidity and mortality. Traditional treatment method is mainly based on hepatectomy for liver tumor. However, it is difficult to accurately distinguish the tumor tissue and its boundary with the naked eye and palpation, leading to an ambiguous resection result, finally causes high recurrence of liver cancer. Molecular fluorescent probes possess lots of advantages, such as non-invasive, high sensitivity, and real-time imaging have been extensively studied in liver cancer imaging and therapy. In this minireview, we briefly introduce the recent developments of always on and activatable fluorescent probes in the liver cancer image and therapy. Future potential challenges of the fluorescent probes for liver tumor are also discussed. We expect that this minireview would improve the fluorescent probes development for real clinical application of liver cancer disease.

Enzyme-activated near-infrared fluorogenic probe with high-efficiency intrahepatic targeting ability for visualization of drug-induced liver injury

Hepatotoxicity is a serious problem faced by thousands of clinical drugs, and drug-induced liver injury (DILI) caused by chronic administration or overdose has become a major biosafety issue. However, the near-infrared (NIR) fluorescent probes currently used for liver injury detection still suffer from poor liver targeting ability and low sensitivity. Enzyme-activated fluorogenic probes with powerful in situ targeting ability are the key to improving the imaging effect of liver injury. Herein, we rationally designed a leucine aminopeptidase (LAP) activated fluorogenic probe hCy-CA-LAP, which greatly improved the hepatocyte-targeting capability by introducing a cholic acid group. The probe hCy-CA-LAP is converted into a high-emission hCy-CA fluorophore in the presence of LAP, showing high selectivity, high sensitivity and low detection limit (0.0067 U mL-1) for LAP, and successfully realizes the sensitive detection of small fluctuations of LAP in living cells. Moreover, the probe can achieve effective in situ accumulation in the liver, thereby achieving precise imaging and evaluation of two different types of drug-induced hepatotoxicity in vivo. Therefore, the probe hCy-CA-LAP may be a potential tool for exploring the roles of LAP and evaluating the degree of DILI.