Abstract by Cristiana Cunha

Oral administration is the preferred drug delivery route, but peptide therapeutics are challenged by low oral bioavailability due to poor gastrointestinal mucosa permeation. A common strategy to address this is to include functional excipients with permeation-enhancing properties, often referred to as permeation enhancers (PEs). While PEs have enabled recent clinical successes, their mechanisms of action are incompletely understood, particularly with respect to mucus-excipient interactions and the role of excipient colloidal behavior. This thesis investigates how functional excipients, with a focus on polymer- and lipid-based systems, modulate the intestinal mucosal barrier and influence peptide permeation. Complementary ex vivo and in vitro models were employed, including isolated porcine intestinal mucus, ex vivo mucus-covered epithelial cell models, epithelium-mimetic lipid membranes, and pH-regulated Ussing chambers on ex vivo rat jejunum.

In mucus permeation studies, molecular size emerged as the primary determinant of permeation, followed by hydrophilicity and, to a lesser extent, net charge of peptides. Functional excipients were shown to significantly modulate mucus architecture, and most of them reduced permeability at clinically relevant concentrations. Chitosan tightened the mucus network and reduced permeability in a concentration-dependent manner, while quaternized derivatives exhibited lesser effects, highlighting that polymer-mucin interactions extend beyond simple electrostatics.

Lipid-based PEs displayed pronounced structure- and peptide-dependent effects, with excipient lipophilicity correlating with increased mucus permeability independently of bulk mucus viscoelasticity. Additionally, the colloidal state of sodium decanoate (C10 or C10-Na) fundamentally modulates its mechanism of action. Micellar C10 enhanced permeation primarily via rapid epithelial membrane disruption, whereas vesicular C10 interacted strongly with mucins, increased mucus retention, and enhanced permeation by membrane fluidization without complete barrier loss. Importantly, pH-controlled EpiStat Ussing chamber experiments improved mechanistic understanding and demonstrated the tissues’ capacity to regulate pH. This research contributes to the understanding of functional excipients as active and critical components in oral peptide formulations and provides a framework for rational excipient selection and formulation design, thus enhancing the potential for successful development of oral peptide therapeutics.