Liquid CO₂ Formulated Mesoporous Silica Nanoparticles for pH-Responsive Oral Delivery of Meropenem

Meropenem (MER) is an effective broad-spectrum antibiotic currently only available in the parenteral form requiring frequent drug preparation and administration due to its extremely poor stability. The unavailability of oral Meropenem is primarily due to its ultrapoor handling and processing stability, hydrophilic nature that inhibits the passive diffusion across the gastrointestinal (GI) epithelium, degradation in the harsh gastric environment, and GI expulsion through enterocyte efflux glycoproteins. In this regard, we have developed an oral drug delivery system that confines MER into mesoporous silica nanoparticles (MSNs i.e, MCM-41 ∼141 nm) using a novel liquid carbon dioxide (CO₂) method. MER was efficiently encapsulated within pristine, phosphonate (negatively charged MSN), and amine (positively charged MSN) modified MSNs with loading capacity ranging between 25 wt % and 31 wt %. Next, the MER-MCM-NH₂ particles were electrostatically coated with Eudragit S100 enteric polymer that protected MER against gastric pH (pH 1.9) and enabled site-specific delivery in the small intestine (pH 6.8). Cellular uptake results in RAW 264.7 macrophage, Caco-2, and LS174T cells confirming the efficient cellular uptake of nanoparticles in all three cell lines. More importantly, the bidirectional transport (absorptive and secretory) of MER across Caco-2 monolayer was significantly improved for both MSN-based formulations, particularly MSNs coated with a polymer (Eud-MER-MCM-NH₂) where permeability was significantly enhanced (∼2.4-fold) for absorptive transport and significantly reduced (∼1.8-fold) for secretory transport. Finally, in vitro antibacterial activity [minimum inhibitory concentration (MIC)] and time-kill assay against S. aureus and P. aeruginosa showed that drug-loaded nanoparticles were able to retain antibacterial activity comparable to that of free MER in a solution at equivalent dose. Thus, Eudragit-coated silica nanoparticles could offer a promising and novel solution for oral delivery of Meropenem and other such drugs.