The stability of nine chemical deliverysystems (CDSs) for phenytoin (DPH) was studied in aqueous buffers and in biological materials. The systems were based on a dihydropyridine in equilibrium quaternary pyridinium salt redox pair attached to 3-(hydroxymethyl)phenytoin via an ester linkage. The pyridinium derivatives released DPH in aqueous buffers and their hydrolytic reactivity was consistent with