Nanoemulsion of Zanthoxylum rhetsa (Roxb.) DC: Formulation, release characteristics and its potential use for anti-inflammation applications

Nanoemulsions of Zanthoxylum rhetsa (Roxb.) DC fruit extract combined with eucalyptus oil (ZRNEs) were formulated using mixed surfactants at various Tween 80/Span 80 mass ratios of 20/80 (ZRNE-6), 50/50 (ZRNE-10), and 80/20 (ZRNE-13). These formulations had corresponding hydrophilic-lipophilic balance (HLB) values of 6, 10, and 13, respectively. Effect of the HLB on properties of the nanoemulsions were evaluated. Among these formulations, ZRNE-10, was found to be optimal for loading ZR extract with eucalyptus. It exhibited the smallest particle size of 402 ± 12 nm with a PDI of 0.41 ± 0.01, as analyzed by dynamic light scattering technique. ZRNE-10 showed the highest encapsulation efficiency for terpinen-4-ol at 83.42 ± 0.51%. It demonstrated a gradual release profile, releasing 30% of terpinen-4-ol within 48 h in a pH 7.4 PBS buffer at 37 °C. Interestingly, higher HLB values accelerated the release rate of terpinen-4-ol. The release kinetics of ZRNE-10 were best described by the Higuchi model (R2 = 0.903). ZRNE-10 displayed robust antioxidant activity (59.5 ± 1.6%) and effective inhibition of nitric oxide (76.9 ± 0.8%). It also exhibited low cytotoxicity on normal human skin fibroblasts (IC50 ≈ 3 mg/mL). Stability assessments revealed that all ZRNE formulations maintained their physical attributes and bioactivities even after one month of storage at 25 °C, and even at an elevated temperature of 45 °C. Notably, the antioxidant activities of ZRNEs surpassed those of unencapsulated ZR extract. Given their favorable anti-inflammatory properties, low cytotoxicity, and maintained efficacy, ZRNEs hold promise as potential ingredients for topical use.