Imaging Acidosis In Tumors Using PH-activated Near-infrared Fluorescence Probes

Cancer is one of the major diseases threatening human health. Recent studies reveal the close relationship between the tumor microenvironment and the occurrence, development and metastasis of tumors. The tumor cells and its microenvironment are mutual promotion, mutual assistance and mutual constraint. Therefore, dynamically visualizing intratumoral microenvironment not only holds the promise to clarify the role of the tumor microenvironment in tumor migration, invasion and metastasis, but also helps to develop the personalized cancer therapeutic strategies.The tumor microenvironment is the cellular environment in which the tumor exists, including the surrounding blood vessels, fibroblasts and signaling molecules. Dysregulated pH is emerging as a hallmark of the solid tumors. In normal differentiated adult cells, intracellular pH (pHi) is generally-7.2, lower than the extracellular pHe (pHe) of～7.4. However, cancer cells have a higher pHi of>7.4and a lower pHe of-6.7-7.1. An increased pHi is permissive for cell proliferation and the evasion of apoptosis, facilitates metabolic adaptation and is obligatory for efficient directed cell migration. A decreased pHe promotes extracellular matrix (ECM) remodelling and stimulates acid-activated proteases to facilitate tumor cell invasion and dissemination. This’reversed’pH gradient creates a perfect storm for tumor occurence, development and metastatic progression.Optical imaging, especially the near-infrared (NIR) fluorescence imaging, demonstrates superior sensitivity (pM-nM), improved tissue penetration depth, fast-acquisition time (ms-s) and no radioactive irradiation. Compared with the pHi, pHe shows the advantages including large pH fluctuation range, convenience for detection and close relationship with tumor metastasis. Therefore, we developed a series of pH activatable NIR fluorescence probes to sense the the intratumoral acidic pHs. These pH activatable NIR probes not only hold the promise to visualize the intratumoral acidosis with high spatial resolution and sensitivity, but aslo provide a novel tool to understand the relationship between the tumor microenvironment and the tumor migration and metastasis.In the paper, we first established a novel small molecule pH-activatable fluorescence probe that was made up of two heptamethine cyanine fluorophores conjugated via an acid liable hydrazone bond. In normal tissues, the bound NIR fluorophores with hydrophobic core intend to form a face to face aggregate and their fluorescence is fully suppressed via intermolecular nonradioactive decay. Under intratumoral acidic pHs, the cleavage of hydrazone bond leads to the remarkable enhancement of fluorescence intensity. This probe not only visualized subcutaneous human breast MDA-MB-231tumor xenograft with high target to background signal ratio in vivo, but also successfully delineated the acidic tumor volume in3D reconstructed NIR fluorescence images. In this work, we found the tumor acidosis is not distributed homogeneously, but as "hot lumps" in a certain area. To further improve the sensitive of the probe to physiological acidosis, we optimized the fluorophore by extend the π-conjugated system, which remarkably increases the qutuam yield and photochemical stability. Overall, the pH acitvatable probe holds the promise to visualize the tumors in their early stage, predict the tumor invasion/metastasis potential and evaluate the therapeutic response by dynamically visualizing intratumoral acidosis.