A novel in situ gel system for potentiating doxorubicin efficacy through reserpine co-therapy

Breast cancer remains a leading cause of female mortality, and doxorubicin widely employed as a primary chemotherapeutic agent. However, its effectiveness is significantly reduced by the development of multidrug resistance, along with concerns of systemic toxicity. To address these limitations, a thermoresponsive in situ gel was developed first time by incorporating doxorubicin together with reserpine, as a P-glycoprotein (P-gp) and CD44 inhibitor. In silico docking confirmed strong binding affinities of reserpine to both targets, supporting its role in modulating drug efflux and resistance pathways. Among the screened systems, formulation F2 (22% P407 + 15% P188) was optimized on the basis of its physicochemical properties, including a tumor-compatible pH, low viscosity at 4-8 °C, and suitable viscosity at 25 °C ensuring injectability. The system exhibited rapid solgel transition at tumor-relevant temperatures, adequate gel strength, high drug loading (96%), and entrapment efficiency (91.6%). FTIR confirmed drug peaks without chemical interaction ensuring encapsulation, while DSC and XRD revealed loss of crystalline endotherms, indicating amorphous molecular dispersion within the gel matrix. Particle size analysis (90.22 nm, PDI = 0.492) and SEM images demonstrated a uniform porous network favorable for sustained release. Drug release was pH triggered and thermoresponsive at pH 5.5, also mimicking the tumor microenvironment. Cytotoxicity (MTT assay, MCF-7 cells) showed markedly higher activity of F2 versus free drugs. In vivo acute toxicity revealed no mortality, organ toxicity, or histopathological abnormalities. In conclusion, F2 demonstrated structural stability, tumor selective release including P-gp blocker, enhanced cytotoxicity, and excellent safety, highlighting its potential for localized intratumoral breast cancer targeted therapy.