The central role of iron in tumor progression and metastasis motivates the study of iron-binding compounds, such as thiosemicarbazone and aroylhydrazone chelators, as anticancer therapeutic candidates. Disulfide-based prochelators target intracellular iron because they are activated for iron coordination upon cellular uptake and reduction in the presence of elevated levels of glutathione. Herein, we report on the synthesis and characterization of bioconjugate prochelator systems designed to increase cancer selectivity. In a glycoconjugation strategy, disulfide linkages connect iron-binding units to carbohydrate moieties that exploit the avid glucose consumption of cancer cells (Warburg effect). We find that the glycoconjugate systems rely, at least in part, on glucose transporter GLUT1 for cellular uptake and that their antiproliferative activity is reduced in the presence of an inhibitor that blocks the transporter. In isogenic cell pairs, in which one cell line has been transduced to amplify GLUT1 expression, the IC<sub>50</sub> values of the glycoconjugate prochelators clearly correlate with GLUT1 expression (whereas those of aglycone controls remain unchanged). These findings indicate that glycoconjugation is a viable strategy to target malignant cells exhibiting high glucose consumption. Within this study of disulfide-based prochelators, we also found that compounds featuring alkylated thiosemicarbazone and imidazole-2-thione react with serum albumin, which is a major component of cell growth media and the most abundant protein in human blood. Native mass spectrometry experiments demonstrated a covalent modification of the protein consistent with thiol-disulfide exchange involving a reduced cysteine residue. In a panel of breast, ovarian and colorectal cancer cell lines, the constructs present antiproliferative activities at submicromolar concentrations - the best activity profiles within their class of disulfide-based prochelators. The new prochelators therefore combine a promising toxicity profile with albumin bioconjugation, which could confer enhanced lifetime in blood circulation and tumor accumulation.

Bioconjugate approaches to iron sequestration in cancer cells

ACS Spring 2022

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