Fish oil-based microemulsion can efficiently deliver oral peptide blocking PD-1/PD-L1 and simultaneously induce ferroptosis for cancer immunotherapy

Peptide immune checkpoint inhibitors in cancer immunotherapy have attracted great attention recently, but oral delivery of these peptides remains a huge challenge due to the harsh gastrointestinal environment, large molecular size, high hydrophilic, and poor transmembrane permeability. Here, for the first time, a fish oil-based microemulsion was developed for oral delivery of programmed death-1/programmed cell death-ligand 1 (PD-1/PD-L1) blocking model peptide, OPBP-1. The delivery system was characterized, in vitro and in vivo studies were conducted to evaluate its overall implication. As a result, this nutraceutical microemulsion was easily formed without the need of co-surfactants, and it appeared light yellow, transparent, good flowability with a particle size of 152 ± 0.73 nm, with a sustained drug release manner of 56.45 ± 0.36% over 24 h and a great stability within the harsh intestinal environment. It enhanced intestinal drug uptake and transportation over human intestinal epithelial Caco-2 cells, and drastically elevated the oral peptide bioavailability of 4.1-fold higher than that of OPBP-1 solution. Meanwhile, the mechanism of these dietary droplets permeated over the intestinal enterocytic membrane was found via clathrin and caveolae-mediated endocytic pathways. From the in vivo studies, the microemulsion facilitated the infiltration of CD8+ T lymphocytes in tumors, with increased interferon-γ (IFN-γ) secretion. Thus, it manifested a promising immune anti-tumor effect and significantly inhibited the growth of murine colonic carcinoma (CT26). Furthermore, it was found that the fish oil could induce ferroptosis in tumor cells and exhibited synergistic effect with OPBP-1 for cancer immunotherapy. In conclusion, this fish oil-based formulation demonstrated great potential for oral delivery of peptides with its natural property in reactive oxygen species (ROS)-related ferroptosis of tumor cells, which provides a great platform for functional green oral delivery system in cancer immunotherapy.