Peptide- and protein-based therapeutics offer realized and potential benefits to health, due to their potent bioactivity, high specificity, and favorable safety characteristics. However, their widespread clinical application is constrained by inherent limitations, including rapid enzymatic degradation, poor membrane permeability, and a reliance on parenteral administration, which reduces patient adherence. To overcome these challenges, extensive research has explored non-invasive delivery strategies, including topical, transdermal, and oral formulations. Despite promising advances in these delivery strategies, they are yet to overcome substantial biological and physicochemical barriers in peptide and protein therapeutics, such as enzymatic degradation in the gastrointestinal tract, limited epithelial transport, and inherently low systemic bioavailability. This review provides a comprehensive and up-to-date analysis of the structural and physiological barriers influencing peptide and protein bioavailability and therapeutic efficacy. It critically examines key challenges associated with various administration routes, including topical, transdermal, oral (including delivery targeting the brain), and others. Furthermore, it explores innovative strategies to enhance peptide and protein stability and bioavailability, including chemical modifications, enzyme inhibitors, penetration enhancers, physical delivery technologies, and advanced nanoparticulate formulations. Additionally, emerging trends in formulation optimization, regulatory considerations, and translational pathways for clinical implementation are discussed. By addressing these critical challenges and highlighting recent advances, this review serves as a roadmap for the development of next-generation peptide and protein therapeutics with improved stability and efficacy, and enhanced patient adherence, which is needed to fully realize the true potential of this class of therapeutics.

The aim of this study was to evaluate (+)-catechin and (-)-epigallocatechin gallate (EGCG) cellular uptake and transport across human intestinal Caco-2 cell monolayer in both the absence and presence of niosomal carrier in variable conditions. The effect of free drugs and drug-loaded niosomes on the growth of Caco-2 cells was studied. The effects of time, temperature, and concentration on drug cellular uptake in the absence or presence of its niosomal delivery systems were investigated. The intestinal epithelial membrane transport of the drug-loaded niosomes was examined using the monolayer of the human Caco-2 cells. The kinetics of transport, and the effect of temperature, adenosine triphosphate inhibitor, permeability glycoprotein inhibitor, multidrug resistance-associated protein 2 inhibitor, and the absorption enhancer on transport mechanism were investigated. It was found that the uptake of catechin, EGCG, and their niosomes by Caco-2 cells was 1.22 ± 0.16, 0.90 ± 0.14, 3.25 ± 0.37, and 1.92 ± 0.22 μg/mg protein, respectively (n=3). The apparent permeability coefficient values of catechin, EGCG, and their niosomes were 1.68 ± 0.16, 0.88 ± 0.09, 2.39 ± 0.31, and 1.42 ± 0.24 cm/second (n=3) at 37°C, respectively. The transport was temperature- and energy-dependent. The inhibitors of permeability glycoprotein and multidrug resistance-associated protein 2 and the absorption enhancer significantly enhanced the uptake amount. Compared with the free drugs, niosomal formulation significantly enhanced drug absorption. Additionally, drug-loaded niosomes exhibited stronger stability and lower toxicity. These findings showed that the oral absorption of tea flavonoids could be improved by using the novel drug delivery systems.