Enhanced Polarity of Sulfonamide Metformin Derivatives Increases Cellular Uptake and Apoptosis-Inducing Effects in Human Breast Cancer Cells

Multitarget agent metformin, a compound originally developed as an antidiabetic agent, has been extensively studied as a repurposed medicine for various diseases. Over the past few decades, the mechanisms by which metformin is transported across cell membranes have also been identified. These include various solute carriers (SLCs), such as plasma membrane monoamine transporter (PMAT), organic cation transporters 1-3 (OCT1-3), and multidrug and toxin extrusion 1-2 (MATE1-2), which can facilitate the bidirectional transport of metformin depending on the cell type. Since metformin is a highly polar and easily excreted compound, more lipophilic derivatives and prodrugs of metformin have been, in turn,developed to improve the targeted delivery, e.g., into cancer cells. However, the required interactions of novel metformin derivatives with cationic transporters are not yet well understood. In the present study, the cellular uptake of nine metformin sulfonamides with various polar substituents was explored in human breast adenocarcinoma cell lines, MCF-7 and MDA-MB-231. The interactions of the novel derivatives with OCT1 and OCT3 were investigated by docking and molecular dynamics simulations. Curiously, the highest cellular uptake was achieved with a compound that effectively released metformin and thus behaved as a prodrug (compound 6). This highlights that despite the molecular interactions with the protein, the greatest driving force into the cancer cells was the intracellular bioconversion. Subsequently, the cell viability and apoptosis-inducing effects of the most effectively uptaken compounds were evaluated, which revealed, in turn, that the prodrug approach may not be the most efficient strategy to attain anticancer effects with metformin. Some more stable metformin derivatives with polar substituents were many times more effective in inducing apoptosis with smaller intracellular concentrations compared with metformin released from the prodrug 6. Therefore, the rational design of novel metformin derivatives should focus on analogical structures of metformin with functionalities that can increase apoptosis-inducing effects while maintaining appropriate interactions with transmembrane proteins, and thus, have a balanced cellular uptake. The polar ring substituents in the sulfonamide moiety of metformin sulfonamides may offer a potential solution.