Development of a Doxorubicin−Sitosterol Conjugate as a Radiosensitizer for Cancer Therapy

Doxorubicin (Dox) is a widely used anthracycline anticancer drug. However, the clinical application is limited by severe dose-limiting cardiotoxicity and systemic toxicity. To overcome these challenges, recent studies have focused on chemoradiotherapy, reporting that combination nanoparticles with irradiation maximize the therapeutic efficiency. Herein, we developed a radiosensitizing chemotherapeutic agent by conjugating Dox and β-sitosterol (ST-Dox A) to enhance the radiotherapeutic efficacy while minimizing drug dose and toxicity. ST-Dox A self-assembled into 170 nm nanoparticles, which are expected to facilitate passive tumor accumulation via the enhanced permeability and retention (EPR) effect. Moreover, the positively charged nature of ST-Dox A (+22.7 mV) led to improved cytotoxicity and facilitated endosomal escape via the proton sponge effect, compared to the non-charged ST-Dox B. In addition, ST-Dox A showed prolonged intracellular retention, thereby enhancing its availability even at low dosage. In an animal study, a low dose of ST-Dox A (equivalent to 2 mg/kg Dox) combined with a single local irradiation (5 Gy) significantly suppressed tumor growth compared to free Dox or radio combination by increased apoptotic cell death. Consequently, the conjugation of β-sitosterol to Dox would enable passive targeting and sustained exposure of ST-Dox A, offering a promising strategy for a potent radiosensitizer. This approach achieves a significantly enhanced antitumor effect with a low dose of drug and irradiation, minimizing systemic toxicity.