Systemic anti-tumor immunity mediated by photodynamic therapy using injectable chitosan hydrogels for intra-tumoral and sustained drug delivery

Photodynamic therapy (PDT) is an anti-cancer therapy with proven efficacy; however, its application is often limited by prolonged skin photosensitivity and solubility issues associated with the phototherapeutic agents. Injectable hydrogels that can effectively provide intra-tumoral delivery of photosensitizers with sustained release is attracting increased interest for photodynamic cancer therapies. However, most of the hydrogels for PDT applications are based on systems with high complexity and often, pre-clinical validation is not provided. Herein, we provide a simple and reliable pH-sensitive hydrogel formulation that presents proper rheological properties for intra-tumoral injection. For this, Temoporfin (m-THPC), which is one of the most potent clinical photosensitizers, was chemically modified to introduce functional groups that act as cross-linkers in the formation of chitosan-based hydrogels. The introduction of -COOH groups gave rise to a water-soluble derivative, named as PS 2, that was the most promising candidate. Although PS 2 was not internalized by the target cells, its extracellular activation cause effective damage of the cancer cells which was likely mediated by lipid peroxidation. The injection of the hydrogel containing PS 2 in the tumors was monitored by high frequency ultrasounds and in vivo fluorescence imaging which confirmed the sustained release of PS 2 for at least 72 h. Following local administration, light exposure was conducted one (single irradiation protocol) or three (multiple irradiation protocol) times. The latter deliver the best therapeutic outcomes which included complete tumor regression and systemic anti-cancer immune responses. Immunological memory was developed as ~ 75% of the mice cured with our strategy rejected a second rechallenge with live cancer cells. Additionally, failure of PDT to treat immunocompromised mice bearing tumors reinforces the relevance of the host immune system. Finally, our strategy promotes anti-cancer immune responses that lead to the abscopal protection against distant metastases.