The development of orally administered peptide therapeutics has been hindered by poor oral bioavailability, necessitating administration via injection. Understanding the sites and mechanisms of absorption of peptides from the gastrointestinal tract is essential for designing more effective delivery systems. This study examines the absorption sites and lymphatic uptake of two functionalised peptides with varying molecular weight and functionalisation: PTG-01 (2794 Da functionalised with fatty acid) and PTG-02 (1845 Da functionalised with small-molecule albumin-binding moiety).

Peptides were administered to anaesthetised rats into the small intestine or stomach with/out pylorus ligation and serum samples collected over time to identify the primary absorption site: stomach or intestine. The absorption route was examined by comparing lymph and serum pharmacokinetics after intestinal delivery of the peptides in mesenteric- or thoracic lymph-duct cannulated rats. 

After oral administration, PTG-01 serum concentrations were lower in rats with stomach ligation compared to stomach-intact, suggesting small intestine absorption. Conversely, PTG-02 serum concentrations were higher with stomach ligation compared to stomach-intact, suggesting stomach absorption. PTG-02 serum concentrations also increased after dosing in the small intestine versus stomach with/out ligation, indicating absorption in both stomach and intestine. Lymphatic uptake of PTG-02 and PTG-01 was relatively low, but higher peptide concentrations were observed in lymph compared to serum for some rats and time points, particularly for PTG-02, suggesting potential direct lymph uptake from the intestine. Co-administration with oleic acid enhanced PTG-01 absorption into blood and lymph, suggesting a strategy for improved oral bioavailability.

PTG-01 was primarily absorbed in the intestine, while PTG-02 was absorbed in both the stomach and small intestine. Additionally, there is evidence of lymphatic uptake, potentially due to association with albumin. Future studies will investigate absorption mechanisms, optimal conjugations and formulations for enhanced absorption and lymphatic uptake, and increased delivery to target sites in lymphatics, such as immune cells.