Photodynamic chemotherapy using reactive oxygen species-degradable polymeric micelles

Abstract

Due to compromised biodistribution of tumor-targeted nanocarriers and release of the therapeutics, stimuli-sensitive nanocarriers become a popular solution that provide more controlled therapeutic release when stimulated by signals. Light is a non-invasive stimulus with high spatial and temporal resolution that is increasingly attractive in biological applications. Inappropriately, many phototherapies utilize harmful ultraviolet light or high-intensity near infrared (NIR) light above the skin maximum permissible exposure (MPE) or a high concentration of chemotherapeutics to eradicate cancer cells. Here, I introduce new light-triggered ROS-degradable nanocarriers for on-demand burst delivery of chemotherapeutics under its maximum tolerated dose (MTD) and the skin MPE. ROS-degradable micelles were fabricated from synthesized amphiphilic poly(ethylene glycol)-poly(1,4-phenyleneacetone dimethylene thioketal) block copolymer. Unfortunately, the degradation of the hydrophobic polythioketal micelles core by superoxide anion, an endogenous ROS, was slow because of the limited half-life and working distance of superoxide anion. A faster degradation of ROS-degradable micelles was achieved by encapsulating meso-tetraphenylporphine (TPP), a photosensitizer, into the micelles core, which generate ROS close to the ROS-degradable thioketal bonds. With visible light illumination at low intensity (55 mW $cm^{-2}$) at 650 nm, the ROS generated degraded the thioketal group at faster rates and created pores at the core of the micelles for the burst release of co-encapsulated paclitaxel. The folate-decorated visible light-degradable therapeutic micelles presented excellent cytotoxicity effect towards HeLa cervical cancer cells with half maximal inhibitory concentration (IC_{50}) about 4.28 % of the reported IC50 of paclitaxel in Taxol® formulation. The tumor-targeting micelles were also able to suppress tumor growth with only 1 mg $kg^{-1}$ paclitaxel, or 5 % of the MTD of paclitaxel, without causing any acute and chronic toxicity. The more superior therapeutic efficacy of the visible light-degradable polythioketal-based micelle, when compared to those of polycaprolactone-based micelles, signifies the use of ROS-degradable polymer for light-induced burst release of chemotherapeutics. To effectively deliver chemotherapeutics in the deeper tissues, silicon 2,3-naphthalocyanine bis(trihexylsilyloxide), a NIR light-sensitive photosensitizer, was encapsulated into the ROS-degradable micelles. The NIR light-degradable micelles were also broken down and released paclitaxel at a light intensity below the skin MPE (300 mW $cm^{-2}$). NIR light was directed through tissue-like phantoms before illuminating the micelles to simulate micelle degradation and paclitaxel release in the lungs orthotopically. Although the phantoms greatly reduce the light intensity reaching the micelles, degradation in the NIR light-degradable micelles was observed. The in vitro cytotoxicity of the biotin-decorated NIR light-degradable micelles was 2.5 % of the reported $IC_{50}$ of paclitaxel in Taxol® formulation and only 12 % higher than the reported $IC_{50}$ value under simulated deep tissue condition. Furthermore, the micelles were able to eliminate a tumor, both under subcutaneous and simulated deep tissue conditions, with only 1 mg $kg^{-1}$ paclitaxel, 5 % of the MTD of paclitaxel. Unfortunately, the encapsulation efficiency in both visible and NIR light-degradable therapeutic micelles was very low. To increase the incorporation of chemotherapeutics into the ROS-degradable micelles, we conjugated doxorubicin to PPADT and co-encapsulated the drug conjugate with TPP into biotin-decorated micelles. The encapsulation efficiency of Dox-PPADT was 19.1-fold higher than that of free doxorubicin. Overall, this thesis presents the therapeutic effect of burst released of chemotherapeutics from ROS-degradable micelles through the photodynamic reaction of co-encapsulated photosensitizers under low-intensity light illumination. This new approach allows effective elimination of cancer cells and tumor suppression using a low dose of chemotherapeutics an order below the reported $IC_{50}$ and MTD. The versatility of the ROS-degradable micelles is demonstrated by changing the amount and type of photosensitizer, chemotherapeutics, and targeting moiety.