Chlorin Conjugates in Photodynamic Chemotherapy for Triple-Negative Breast Cancer

Abstract Breast cancer (BC) is the most common type of cancer in women and the number of new cases in the US is still increasing each year. Triple-negative breast cancer (TNBC), which comprises 15-20% of all breast cancer, is a heterogeneous disease and is considered the most aggressive type of breast cancer due to lack of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) expressions for treatments. Traditional chemotherapy is the standard protocol for treatment of TNBC. Toxicity and multidrug resistance are major drawbacks for chemotherapy. The lack of molecular targets and poor prognosis for TNBC prompted an urgent need to discover novel therapeutic strategies to improve clinical outcomes and quality of life for patients. Photodynamic therapy (PDT) or light treatment is a binary anticancer procedure that uses a photosensitizer (PS) which upon light activation produces cytotoxic oxygen species destroying tumor cells. PDT is minimally-invasive and can be repeated few times without accumulating significant toxicity in the surrounding tissues. The primary goal of this study is to investigate in vitro photodynamic chemotherapy as a ternary combination therapy using our synthesized photosensitizers (chlorin-vitamin conjugates and their corresponding indium complexes) co-treated with known chemotherapeutic agents (taxol, doxorubicin, cisplatin, fluorouracil, or methotrexate) in the presence of light and determine the optimum conditions as pre-clinical study for enhanced tumoricidal effect against TNBC. Our results indicated that the best combination for effective chemophotodynamic effect involves a ternary treatment of the indium complex of chlorin lipoic acid conjugate (InCLA) co-treated with taxol which exhibited strong synergism at the nanomolar concentration when combined together in the presence of visible light irradiation. Other ternary combinations containing taxol with synergistic anti-tumor effect against TNBC include chlorin pantothenic acid (CPA) and chlorin biotin (CBTN) conjugates. Several other ternary combinations containing InCLA, CBTN, and CPA with either cisplatin, fluorouracil, or methotrexate were identified to generate a synergistic or additive effect. Light dosage remained constant but dosage of photosensitizers and chemotherapy drugs were varied to obtain the lowest possible concentration for the desired effect. Synergistic, additive or antagonistic effects of drug combinations were based on the Chou-Talalay method with InCLA-taxol having the lowest Combination Index (CI) of 0.25. Fluorescence and transmission electron microscopy (TEM) images provided evidence of apoptosis as the preferred mode of cell death pathway. Our study demonstrated the combination of PDT and chemotherapy as a potential treatment option for TNBC patients.