Self-Nanoemulsifying Drug Delivery Systems (Snedds): Formulation Principles, Mechanistic Insights, and Biopharmaceutic Perspectives for Enhanced Oral Bioavailability

Poor aqueous solubility affects roughly 40% of newly discovered drug candidates, most of them BCS Class II, and remains a principal limitation on oral bioavailability. Self-nanoemulsifying drug delivery systems (SNEDDS) — isotropic, anhydrous pre-concentrates of oil, surfactant, and co-solvent that spontaneously form nanoemulsions (<200 nm) on contact with gastrointestinal fluids — address this limitation by keeping the drug solubilized and thermodynamically active throughout GI transit, thereby increasing dissolution rate and transmembrane flux. This review provides a mechanistically grounded synthesis of SNEDDS science: the biopharmaceutical rationale for their use, the thermodynamic basis of self-emulsification, systematic excipient selection, preparation and characterization methods, and Quality by Design (QbD)-driven optimization. Advanced platforms — solid SNEDDS, supersaturable systems exploiting the spring-and-parachute mechanism, and mucoadhesive/controlled-release SNEDDS — are evaluated for their mechanistic basis and translational limitations. The review concludes by identifying the unresolved in vitro–in vivo correlation (IVIVC) gap, the immaturity of SNEDDS-specific PBPK modeling, and the regulatory and long-term stability data required before SNEDDS can realize their full clinical potential.