Synthesis of Monensin Derivatives and Their Effect on the Activity of Ricin A-Chain Immunotoxins

Cell-specific cytotoxic heteroconjugates are made by linking bacterial toxins (e.g., diphtheria toxin, Pseudomonas exotoxin A) or plant toxins (e.g., ricin, abrin) to monoclonal antibody (MAb)/ligands that bind target antigens or receptors at the cell surface (1 -3 ). Toxins used for the synthesis of cytotoxic heteroconjugates are formed by two subunits: the A-chain, an enzyme that inhibits protein synthesis in the target cell; and the B-chain, able to bind ubiquitous cell surface structures and to help A-chain translocation to the cytosol (4 -5 ). Naturally occurring A-chain-like toxins (ribosome-inactivating proteins, or RIPs) have also been used as toxic components of cell-selective conjugates (6 -7 ). Unlike intact toxin conjugates, ricin toxin-A-chain (RTA) or RIP conjugates show high target-cell specificity but variable cytotoxicity, often too low to be of appreciable therapeutic value, particularly in solid tumor treatment (1 ). The cytotoxic activity of RTA conjugates can be potentiated in vitro by lysosomotropic amines (e.g., NH₄ Cl, chloroquine, methylamine, amantadine), ionophores (e.g., monensin, nigericin, lasalocid), or lysosomal enzyme inhibitors (e.g., leupeptin, pepstatin) (8 -13 ).