A comparative study of the antitumor efficacy of peptide-doxorubicin conjugates with different linkers

The peptide-drug conjugates caused much attention currently. The purpose of present study was to elucidate the possible synergistic effect between ligand peptide and stimuli-responsive linkage in amphiphilic peptide-drug conjugates (APDCs) with different linkers. Especially, the superiority of each strategy as well as the synergistic effect between them was carefully investigated via the parallel comparisons of the three systems throughout of the whole study. Here, we synthesized three APDCs, namely, cRGD-SS-DOX (RSSDOX), cRGD-S-DOX (RSDOX) and cRGD-VC-DOX (RVCDOX), using doxorubicin (DOX) as a model cytotoxic agent, cRGDfC as a homing peptide, and reduction cleavable disulfide (SS), noncleavable single thioether (S) or cathepsin B cleavable valine-citrulline dipeptide (VC) as linker. The APDCs showed high drug loading capacity and they were evaluated in vitro in the integrin αvβ3-overexpressing B16 cells and in vivo in tumor-bearing C57BL/6 mice. Endocytosis mechanism assay demonstrated that three types of APDCs internalized into cells through adynamin and actin depolymerizing-mediated pathway following receptor-mediated endocytosis. Notably, RSDOX or RVCDOX induced stronger antitumor efficacy, which depended on their cellular uptake levels, intracellular trafficking and the colocalization rates with lysosomes. The in vivo efficacy of RSDOX or RVCDOX was 1.4-1.7 fold of free DOX and 1.7-2.0 fold of RSSDOX, respectively. In addition, RSDOX or RVCDOX demonstrated acceptable system, tissue and blood compatibility. The compromised efficacy of RSSDOX might be due to the generation of DOX-SH during degradation of prodrug, but not DOX. Taken together, our studies suggest that certain type of APDCs can significantly decrease the toxicity of free DOX and improve therapy outcome, which provides insight for the design of peptide-drug conjugates integrating ligand peptide and stimuli-responsive linkage.