Development of a dual-locked fluorescent probe and an organelle-targeted activity-based photodynamic therapy agent

Cancer is a complex and devastating disease and continues to be a major global health issue. The traditional methods frequently employed for cancer treatment suffer from major drawbacks such as drug resistance, side effects, non-specific targeting, and high operational cost, in addition to limited treatment options for certain phenotypes. Photodynamic therapy (PDT) is an emerging treatment method that utilizes light-sensitive molecules, also known as photosensitizers (PSs), to generate reactive oxygen species (ROS). The production of superoxide anion and hydroxyl radicals through an electron transfer process is known as type I PDT, while the generation of singlet oxygen (1O2) by energy transfer to ground state tissue oxygen is classified as type II PDT. This action irreversibly disrupts cellular function due to the highly toxic nature of ROS, yet it is favored to perform type I PDT due to the hypoxic tumor microenvironment. PSs are tuned to function only within the cancer cells to minimize off-targeting. Cancer biomarkers are at the core of this approach as they quench the photophysical properties of PSs. However, the targeted analyte liberates the active PS, which results in photocytotoxicity and cancer-selective therapy. Hydrogen sulfide (H2S) is one of the biothiols and has recently been associated with cancer due to the high expression of the cystathionine β-synthase (CBS) enzyme responsible for its production. The overexpression of H2S in cancer cells paved the way for cancer-selective PDT. One approach to enhance PDT efficacy is harming the vital organelles to interrupt physiological processes. Among them, the endoplasmic reticulum (ER), which is linked to protein synthesis, calcium storage, and lipid metabolism, is one of the most crucial organelles in cells. The first chapter in this thesis features the first H2S-responsive and ER-targeted type I PDT agent (HEH), specified to treat neuroblastomas. The probe is designed to display superior selectivity over other biothiols and is further employed for ER-localized PDT in SH-SY5Y neuroblastoma cells. In contrast, HEH remained inert toward VERO normal cells, thereby facilitating cancer-selective PDT. Fluorescence imaging is one of the most common methods employed to monitor cancer and cancer-associated processes. This progression is commonly investigated by using mono-activatable fluorescent molecules. However, it is likely to observe non-specific activation, which subsequently leads to a potential false positive signal and results in misdiagnosis. To address this challenge, a new strategy is proposed in the second chapter to monitor melanomas precisely. This precision is achieved through a dual-locked strategy, which facilitates the harvesting of fluorescence emission only in the co-presence of two cancer biomarkers: H2S and tyrosinase. In light of this approach, the first-ever H2S and tyrosinase-responsive fluorescent probe (RHT) is devised to track melanomas. This technology enables selective melanoma detection, which is impractical using mono-locked probes as those probes also give false readouts in normal cells.