The invention provides a method of sustained delivery of a tertiary amine-containing parent drug comprising administering to a patient an effective amount of a prodrug compound of the invention wherein upon administration to the patient, release of the parent drug from the prodrug is sustained release. Prodrug compounds suitable for use in the methods of the invention are labile quaternary ammonium salts of tertiary amine-containing parent drugs (or tertiary imine-containing parent drugs) that are derivatized through aldehyde-linked prodrug moieties that reduce the solubility of the prodrug compound at a reference pH as compared to the parent drug. The physical, chemical and solubility properties of these derivatives can be further modulated by the choice of counterion X−. In one embodiment, the present invention provides a prodrug compound of Formula I: where R1-R5 are defined in the written description of the invention. The prodrug compounds of the invention can be used to treat any condition for which the tertiary amine-containing parent drug or tertiary imine-containing parent drug is useful as a treatment.