Propolis is a chemically complex natural product with recognized antioxidant potential, but its compositional heterogeneity and poor aqueous solubility complicate formulation and interpretation of in vitro release behavior. In this study, a nanostructured lipid carrier (NLC) based on Gelucire^® 44/14 was developed as a physicochemical platform to modulate the accessibility of a selected Chilean ethanolic propolis extract. Propolis extracts from different origins were first screened using complementary antioxidant assays (DPPH, ABTS, ORAC, FRAP), leading to the selection of the Peñaflor extract, which exhibited the highest phenolic content (~41 mg GAE/g) and antioxidant capacity. The selected extract was incorporated into NLCs with encapsulation efficiencies above 90%, a narrow size distribution (~200 nm), and high stability over 90 days. Under simple aqueous conditions, propolis release remained limited (<15% over 6 h), consistent with diffusion- and partition-controlled transport. In simulated gastrointestinal media containing bile components, pronounced pH- and composition-dependent effects were observed. While fed-state intestinal conditions induced extensive morphological remodeling without increasing the analytically accessible fraction (<3% at 4 h), fasted-state intestinal media promoted a higher accessible fraction (~14% within 1 h) without complete carrier disruption, as confirmed by transmission electron microscopy. Preliminary cytocompatibility studies in HepG2 cells showed acceptable viability at 10–40 µg/mL and concentration-dependent effects at higher doses. Overall, this work demonstrates that bile components modulate propolis accessibility through dynamic partitioning and colloidal reorganization rather than simple carrier breakdown, providing a physicochemical framework for future digestion and absorption studies.

Background: The increasing prevalence of multidrug-resistant bacterial infections highlights the need for drug-delivery strategies that improve antimicrobial exposure and sustain therapeutic activity. In this study, ciprofloxacin-loaded nanostructured lipid carriers (NLC-CIP) were developed and evaluated to better understand how formulation-dependent release behavior influences antibacterial performance against Escherichia coli. Methods: NLC-CIP were prepared and characterized in terms of size, polydispersity, encapsulation efficiency, and colloidal stability. In vitro release profiles were evaluated across different pH conditions, followed by kinetic modeling. Stability under refrigerated storage was assessed. Antibacterial performance was determined through IC₅₀ measurements and dynamic growth-kinetic analyses, while cytotoxicity was evaluated in HepG2 cells. Results: Ciprofloxacin incorporation increased hydrodynamic diameter (~116 to 194 nm) while preserving low polydispersity (PdI~0.04), high colloidal stability, and encapsulation efficiency (96%). Release studies showed medium-dependent behavior, with rapid release at pH 1.2, 4.5, and 7.4, and more sustained profile at pH 6.8, consistent with diffusion-controlled kinetics (Weibull model). Refrigerated storage preserved release profiles while slowing early-stage kinetics. NLC-CIP showed improved apparent antibacterial activity, reducing the IC₅₀ from 4.9 to 1.2 ng/mL, and sustained bacterial suppression by decreasing growth rates and prolonging doubling times. Unloaded NLCs showed no antibacterial activity, and cytotoxicity assays confirmed favorable biocompatibility. Conclusions: Overall, these results show that NLC-based encapsulation can modulate ciprofloxacin release and reshape drug exposure over time, thereby improving antibacterial performance under the tested conditions. This study supports integrated release and growth-kinetic analyses as a more informative framework for evaluating lipid-based antibiotic delivery systems.