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The urokinase plasminogen activator and its receptor (uPA/uPAR) are biomarkers for metastasis, especially in triple-negative breast cancer. We prepared anti-mitotic N-alkylisatin (N-AI)-loaded liposomes functionalized with the uPA/uPAR targeting ligand, plasminogen activator inhibitor type 2 (PAI-2/SerpinB2), and assessed liposome uptake in vitro and in vivo. Receptor-dependent uptake of PAI-2-functionalized liposomes was significantly higher in the uPA/uPAR overexpressing MDA-MB-231 breast cancer cell line relative to the low uPAR/uPAR expressing MCF-7 breast cancer cell line. Furthermore, N-AI cytotoxicity was enhanced in a receptor-dependent manner. In vivo, PAI-2 N-AI liposomes had a plasma half-life of 5.82 h and showed an increased accumulation at the primary tumor site in an orthotopic MDA-MB-231 BALB/c-Fox1nu/Ausb xenograft mouse model, relative to the non-functionalized liposomes, up to 6 h post-injection. These findings support the further development of N-AI-loaded PAI-2-functionalized liposomes for uPA/uPAR-positive breast cancer, especially against triple-negative breast cancer, for which the prognosis is poor and treatment is limited.

Abstract: We have previously reported a series of pH-sensitive imine-linked N-alkylisatin prodrugs that are stable at pH 7.4, but readily cleaved at pH 4.5. Herein, one of the most potent prodrugs, 5,7-dibromo-N-(p-methoxybenzyl)isatin (NAI), was functionalized with a para-phenylpropionic acid linker, and the resulting NAI–imine prodrug conjugated to transferrin (Tf) to form a NAI–imine–Tf conjugate. Cytotoxicity assays revealed the conjugate was equipotent to the free drug against MCF-7 breast cancer cells, with clear selectivity patterns based on TfR levels. These results suggest that this novel isatin-based cytotoxin conjugated to a tumor targeting protein via an acid-labile linker warrants further preclinical testing. [Copyright &y& Elsevier]