Many groups are investigating the use of polymers for the controlled delivery of a variety of therapeutics. Polymers play an integral role in drug delivery and are desirable for controlled release therapeutics, tailored drug release kinetics, increasing the half-life of a drug in a biological system and systems that facilitate the placement and localization of a drug within close proximity of a desired tissue. Controlled drug release is desirable because it can eliminate the potential for both under- and over-dosing. Controlled drug delivery also allows one to more easily maintain consistent drug levels at the desired tissue site, permits less frequent and fewer overall administrations of the drug, and improves patient compliance. Polymer drug delivery systems generally have some disadvantages, including toxicity or non-biocompatibility of the materials used, undesirable degradation by-products, discomfort from implantable polymer drug delivery systems, initial burst release, synthesis and processing conditions that compromise the integrity of the drug or the biocompatibility of the materials, a disconnect between the degradation time of the polymer and the release period of the drug, and the high cost of controlled polymer drug release systems in comparison to standard pharmaceutical compositions.
Controlled delivery of a drug from a polymer occurs when the biologically active ingredient is released from the polymer in a predesigned manner. The release may be periodic over time, it may be constant, or it may be triggered by external events. The drug release kinetics are generally dependent upon the properties of the polymer as well as the drug. Drug may be liberated from the polymer carrier through a variety of methods, including diffusion of drug out the polymer matrix, erosion of the polymer matrix, chemical degradation of the polymer matrix, chemical degradation of a linker between the drug and the polymer matrix, or reduction of an attractive force between the drug and the polymer matrix. Controlled delivery can be preferred, or necessary, when frequent, repeated administration of traditional dosage forms is not feasible or desirable. In many cases, controlled release kinetics provide a therapeutic benefit. The delivery can be tailored so that water-soluble drugs are slowly released and low-solubility drugs are released quickly. The drug delivery can be specific for a target site, or the hydrogel prodrug systems can be designed to be quickly dissolved or degraded for fast elimination. Ideally, the polymer drug system should have an inert backbone, biocompatibility, biodegradability, and the capability of containing a high drug load without the dangers of accidental release. Furthermore, the polymer drug system should be simple to administer and easy to manufacture.
The goal for a polymer controlled drug delivery system is to achieve a delivery profile yielding a constant level of drug in a system or in a target tissue, or periodic drug release in a system or in a target tissue over a determined amount of time, or externally triggered drug release in a system or in a target tissue. In traditional drug dosing, such as oral administration of tablets or injected formulations, the levels of drug initially rise then subsequently fall until a second administration. The metabolism of a patient or the environment in which the drug is placed can lead to an undesired fast degradation, clearance, or waste of the drug before a second administration of drug can occur, and can lead to over- or under-dosing. As such, a polymer system, which achieves a constant level of drug over a given period of time, is greatly needed.