A novel antitumor prodrug platform designed to be cleaved by the endoprotease legumain

Chemotherapeutic treatment of neoplastic diseases is often restricted by adverse systemic toxicity, which limits the dose of drug that can be administered, or by the appearance of drug resistance. Therefore, novel targeted therapeutic approaches are being developed to improve current conventional therapy in order to increase specificity and biocompatibility, and decrease toxicity. Legumain represents a recently identified lysosomal protease that has been reported to be overexpressed in the majority of human solid tumors, to promote cell migration and is associated with enhanced tissue invasion and metastases. Therefore, it serves as a promising candidate for prodrug therapy. We synthesized a novel legumain-cleavable prodrug, carbobenzyloxy-alanine-alanine-asparagine-ethylenediamine-etoposide, which releases the chemotherapeutic agent, etoposide, as the active drug. The prodrug was characterized and analyzed by (1)H NMR and HPLC. 293 Human embryonic kidney (293 HEK) cells were stably transfected with human legumain, to achieve overexpression in vitro (293 HEK-Leg). 293 HEK-Leg cells expressed both active and inactive legumain and secreted it to the medium. Legumain expression was found to be elevated because of serum starvation in both 293 HEK cells and PC3 human prostate carcinoma cells. The commercial substrate of legumain, carbobenzyloxy-alanine-alanine-asparagine-amino-4-methyl coumarin (CBZ-Ala-Ala-Asn-AMC) and the synthesized prodrug were both cleaved by recombinant human legumain (rhlegumain) and legumain expressed in the 293 HEK-Leg cell lysate. Upon cleavage by rhlegumain, the prodrug showed an inhibitory effect on the proliferation of 293 HEK and 293 HEK-Leg cells. This study suggests a novel platform for prodrug therapy activated by legumain as a promising approach for cancer therapy.