Synthesis, cleavage profile, and antitumor efficacy of an albumin-binding prodrug of methotrexate that is cleaved by plasmin and cathepsin B

Cathepsin B and plasmin are intra- or extracellular proteases that are overexpressed by several solid tumors. In order to exploit both proteases as molecular targets for tumor-specific cleavage of prodrugs, an albumin-binding formulation of methotrexate was developed that incorporated the peptide sequence D-Ala-Phe-Lys as the protease substrate. Albumin is a suitable carrier for cytostatic agents due to passive accumulation in solid tumors. Synthesis was performed by coupling the peptide linker ENIC-D-Ala-Phe-Lys(Boc)-Lys-OH (EMC = epsilon-maleimidocaproic acid) to the gamma-COOH group of alpha-tert-butyl protected methotrexate. After cleavage of the protective groups and purification on reverse phase HPLC, a highly water-soluble methotrexate-peptide derivative was obtained that binds rapidly and selectively to human serum albumin. The albumin-bound form of the prodrug was shown to be efficiently cleaved by cathepsin B and plasmin as well as in an ovarian carcinoma homogenate (OVCAR-3) liberating a methotrexate-lysine derivative. In an OVCAR-3 xenograft model, the prodrug at a dose of 4 x 15 mg/kg methotrexate equivalents demonstrated distinctly superior antitumor efficacy compared to free methotrexate at a dose of 4 x 100 mg/kg [T/C(%) for MTX = 69; T/C(%) for MTX prodrug = 29]. The data provide a further proof of concept for the development of albumin-binding, enzymatically cleavable prodrugs of anticancer drugs.