Intrinsic Proteolytic Activities from Cancer Cells Are Sufficient to Activate Alkoxyamine Prodrugs and Induce Cell Death

In search of better specificity and lower chances of resistance, protease-activatable alkoxyamine prodrugs to fight cancer have been proposed. These molecules are made of a peptide linked to an alkoxyamine. Proteolysis of the peptide converts the stable prodrug at 37 °C to a metastable alkoxyamine that spontaneously homolyzes into two free radicals: a stable nitroxide and a very reactive alkyl radical. The alkyl radical induces apoptosis in the surrounding cells by inducing random chemical alterations. Here, we show that varying the peptide moiety from succinyl-Ala-Ala-Pro-Val- to PyroGlu-Gly-Arg- or PyroGlu-Gly-Lys- is effective in switching the activating enzyme from elastase to urokinase. Furthermore, these prodrugs induce the death of HT-1080 cells, a cell line that secretes several active proteases in culture. This cytotoxic activity can be suppressed by protease inhibitors and does not affect cell lines devoid of active urokinase. We thus provide examples of alkoxyamine prodrugs that are efficiently activated by the limited intrinsic protease activity and that succeed in the destruction of cancer cell lines and cancer cells from tumor explants.