We have synthesized a large variety of test-tube models which simulate the enzyme-substrate complex by having the substrate frozen into a single, very favorable conformation and by having the interacting groups brought into the closest possible juxtaposition (stereopopulation control). These compounds undergo intra-molecular reactions at rates approaching those catalyzed by enzymes (but independently of any functional assistance). As part of our studies of practical applications of stereopopulation control, we have been exploring the use of various SPC-derivatives of biogenic amines and antibiotics as prodrugs. The intent is to facilitate transport from the gut to the circulatory system to the desired site of action by temporary masking of charge within the molecule, by improvement in lipo-philicity and by regeneration based simply on local pH variation in ligand sites or on local concentrations of potent reducing agents. Recent studies have concentrated on o- nitrophenylpropionic acids as carriers. To date, these carriers have been coupled to GABA, to protected DOPA derivatives and to indoleamines. The nitro group is reduced enzymatically, and the resulting amine attacks an amide bond intramolecularly to release the drug. Rapid attack by the amine is ensured by placing a gem-dimethyl group on the adjacent carbon. Kinetic studies have shown that chemical reduction of the nitro group occurs at the same rate with or without the gem-dimethyl group, but that drug release is greatly enhanced by the action of the gem-dimethyl. Surprisingly, xanthine oxidase reduced the hindered nitro compound more readily than the unhindered. This result reveals that the enzyme must be able to reach a face of the nitro group for reduction, and not just an edge. Response to other reducing agents is now being explored with a view to using such prodrugs to deliver mustards and other anticancer agents to hypoxic cells. Similar model systems have been designed with conformationally frozen indole and phenolic rings, in order to stimulate and study the tight charge transfer complexes achieved by tryptophan in proteins or by bioindoles in receptor binding.
