Background  Cell-penetrating peptides (CPPs) are a class of peptides with cell membrane penetrating ability, which can be used as drug carriers to transport macromolecules into cells. They have been widely used in medical imaging and therapeutic research because of their low cytotoxicity and stable penetrating ability in different tissue.

  Objective  To design an arginine-rich peptide cobalt protoporphyrin complex and evaluate its biocompatibility and effect on lens epithelial cells under oxidative stress damage.

  Methods  The cell-penetrating peptide was synthesized by the Fmoc solid-phase synthesis method and then linked with cobalt protoporphyrin (CoPP), the fluorescein isothiocyanate (FITC) was added as the label. The purity and relative molecular mass of the complex (R6-CoPP) was evaluated by high-performance liquid chromatography and mass spectrometry. The characterization, elemental distribution, particle diameter, and surface potential of R6-CoPP were evaluated by scanning transmission electron microscopy (STEM) surface/line scan and laser nanoparticle size potentiometry. The penetrating ability of R6-CoPP was evaluated by confocal microscopy and flow cytometry. R6-CoPP toxicity was investigated by the CCK-8 method using human lens epithelial cells. The oxidative damage model of lens epithelial cells was created by H₂O₂ to compare the antioxidant effects of R6-CoPP and CoPP.

  Results  The purity of R6-CoPP was 94.56% by high-performance liquid chromatography (HLPC), and the relative molecular weight was 2109.75 by mass spectrometry (MS). The particle size was 227.8 nm and the Zeta potential was + 13.9mV. STEM energy spectrum surface scan showed the distribution of cobalt elements within the complex. R6-CoPP was nontoxic in vitro with good biocompatibility. Cellular flow cytometry showed that the percentage of fluorescent positive cells could reach more than 80% after 2 h incubation. Confocal fluorescence microscopy showed that R6-CoPP could be detected inside the lens cell within 15 minutes of culturing, and the fluorescence intensity decreased with time. Compared to the oxidative damage model group, the cell survival rate was significantly higher after pretreatment with CoPP and R6-CoPP (P<0.05).

  Conclusion  The cell-penetrating peptide drug complex (R6-CoPP) is biocompatible and has strong cell membrane penetration ability, which also can attenuate the apoptosis of lens epithelial cells caused by oxidative injury in vitro.