Chlorinated hemicyanine as a cancer cell selective phototheranostic agent

Cancer is the second leading cause of death worldwide. Even though the significant advancement achieved in medicine over the last decades, many affairs must be considered to improve the efficacy of cancer therapy. Among therapeutics, cancer theranostics is a convincing method that evaluates pertinent clinical guidance on the integration of encyclopedic in vitro diagnostics, in vivo molecular visualizations, and case-based treatments of various cancer types. This method provides a valuable methodology to eliminate multi-step therapies and diminishes delays in cancer treatment. Recently, phototheranostics has been received great attention among theranostics. Phototheranostics provides imaging and therapy by photoactivation of light-triggered theranostics. However, phototheranostics is still experiencing intricate impediments, as it might engender complicated and prolonged therapy, and rises unnecessary side effects and systemic toxicity to patients. Photothermal therapy (PTT) is among the most effective light-activated, near-infrared (NIR) photosensitizer-based therapeutic methods that can establish safe modalities of tumour ablation by penetrating tissues deeply while causing little damage to surrounding areas. In PTT, a photothermal agent converts photon energy to thermal energy and provides targeting hyperthermia for numerous cancers by employing metabolic signals, denaturing proteins/enzymes, and inducing cancer ablation. Within the family of organic NIR sensitizers, water-soluble tricarbocyanine dye, called hemicyanine (Hem), exhibited excellent and tunable absorption and emission wavelengths. Hem is an indocyanine green-based dye, which is the only U.S. Food and Drug Administration (FDA) approved fluorescent dye, and many inspections have been done to improve its photophysical properties. Primary amino acids including cysteine (Cys), glutathione (GSH) and homocysteine (Hcy), which take a significant role in the antioxidant defence of cancer cells, are found to be at high levels due to their involvement in overcoming the increased production of reactive oxygen species (ROS) in cancer cells. While Cys can be biosynthesized in the body in a low concentration under normal physiological conditions, cancer cells, which are characterized as highly proliferative, with increased metabolic rate and intensified oxidative pressure, produces a high concentration of Cys. Taking physiological activities of cancer cells, such as elevated levels of bio-thiols, into account, recently there is growing attention on designing activable-targeting agents that able to discriminate and selectively ablate cancer cells. Within this thesis, a Hem-based (Cl-Cys) activable photothranostic agent that can turn on its diagnostic signal and therapeutic action, simultaneously, only in response to Cys, is investigated. Cl-Cys employed an acrylate group (a responsive site for Cys) on the phenol group of the structure. At the same time, the protection diminished the electron-donating ability of the oxygen atom, which prevented its fluorescence activity. After incubation with Cys, uncaged Cl-Cys revealed in the form of chlorinated hemicyanine (Cl-Hem) with pKa of 6.4, which indicates its ability to be present as a phenolate in physiological pH of 7.2-7.4. Upon activation, Cl-Hem exhibited a significant red-shift (from 570 nm to 695 nm) in absorption signal and a great enhancement in emission intensity, due to improved intramolecular charge transfer (ICT), which assists better visualization. Cl-Cys triggered up to 20 °C temperature increase in solution upon activation by laser irradiation. To examine the sensitivity of the Cl-Cys to Cys, fluorescence imaging and cytotoxic experiments were conducted on the Cys-overexpressing HeLa and A549 cancer cells and the control healthy cells (L929). Results indicated strong NIR fluorescence signals, as well as hyperthermia in cancer cells. Cl-Cys not only marks the first example of a Cys activatable PTT agent but also introduces the first study that shows a chlorinated hemicyanine can be utilized as a PTT agent core.