Novel synovial targeting peptide-sinomenine conjugates as a potential strategy for the treatment of rheumatoid arthritis.

Sinomenine (SIN) is an effective anti-inflammatory agent, but its therapeutic efficacy is limited by its short half-life and the high dosage required. Tissue-specific strategies have the potential to overcome these limitations. The synovial homing peptide (CKSTHDRLC) was identified to have high synovial endothelium targeting affinity. In this work, two peptide-drug conjugates (PDCs), conjugate (L) and conjugate (C), were synthesized, in which SIN was covalently connected to the linear and cyclic synovial homing peptide, respectively, via a 6-aminocaproic acid linker. An evaluation of biostability showed that conjugate (C) was more stable in mouse serum and inflammatory joint homogenate than conjugate (L). The two conjugates gradually released free SIN. Interestingly, conjugate (L) self-cyclized via a disulfide bridge in a biological environment, which significantly impacted its biostability. It had an almost equipotent half-life in serum but faster degradation in the inflammatory joint than conjugate (C). Therefore, conjugate (C) exhibited better therapeutic efficacy and tissue targeting. All the results indicated that PDCs particularly in its cyclic form might be more efficient for targeted deliver and represent a potential strategy for the treatment of rheumatoid arthritis.