Objective To enhance the cyclic peptide compound's membrane permeability,structural stability,and neuroprotective activity,based on the amino acid sequence of peptides of Tat-GluA2-3Y,by designing and synthesizing a serial of cyclic peptides through strategies including polypeptide cyclization,replacement of the cell-penetrating peptide,substitution with D-amino acids,and incorporation of mini polyethylene glycol fragments.Methods The target peptides were synthesized based on standard Fmoc solid-phase method,followed by analysis and purification via reverse phase high-performance liguid chromatography(RP-HPLC).The cytoprotective activity of the peptides was evaluated by using the HT-22 cell model.The transmembrane transport efficiency of the peptides was determined based on the Caco-2 monolayer intestinal epithelial cell model.Plasmatic plasma and metabolic stability of the peptides were measured by in vitro co-incubation experiments with rat plasma and human liver microsomes.Finally,the in vivo neuroprotective activity of the peptides was validated by using a mouse middle cerebral artery occlusion model.Results Seven cyclic peptides were successfully designed and synthesized by using the standard Fmoc solid-phase method,with purities exceeding 90%as confirmed by RP-HPLC.Cytoprotective activity assay demonstrated that both Tat-GluA2-3Y and CMT-C3Y exhibited activity at concentrations above 125 nmol/L,with CMT-C3Y showing superior activity as compared to Tat-GluA2-3Y.The results of the transmembrane assay demonstrated that,compared to Tat-GluA2-3Y,CMT-C3Y exhibited significant transmembrane capabilities at all tested concentrations(P<0.001).CMT-C3Y was classified as a highly permeable compound,whereas Tat-GluA2-3Y was categorized as a moderately permeable compound.Plasma stability studies indicated that over 50%of Tat-GluA2-3Y was metabolized after 4 h of co-incubation with rat plasma.After 8 h of coincubation with CMT-C3Y,the remaining amount was 88.1%,and no obvious degradation phenomenon occurred.In human liver microsomal stability tests,the half-life of Tat-GluA2-3Y was 26.1 min,as compared to 103.8 min for CMT-C3Y,highlighting the enhanced stability of CMT-C3Y.Tat-GluA2-3Y and CMT-C3Y were classified as a fast-metabolizing drug and a moderate-metabolizing drug,respectively.Animal experiments further demonstrated that at a dose of 8 mg/kg the neuroprotective activity of CMT-C3Y was significantly superior to that of Tat-GluA2-3Y(P<0.001).Conclusion The designed bicyclic peptide CMT-C3Y demonstrates significantly higher cell-penetrating efficiency and superior plasma stability as compared to Tat-GluA2-3Y,along with enhanced neuroprotective activity at both cellular and animal levels.