INTRODUCTION AND DRIVERS FOR DEPOT DELIVERY Depot delivery systems, also known as sustained-release systems, are parenteral formulations containing multiple doses of drug that, when introduced into the body, are designed to release the drug over a specified, often prolonged, period of time. Depot formulations come in many forms, designed for several different administration routes, and have been in use for over half a century. In addition to the many depot pharmaceuticals approved for use today, the development of novel systems remains an active area of research because of the ability of depot systems to overcome several well-recognized challenges often associated with conventional delivery. These include variations in drug plasma levels between doses that can lead to adverse effects or compromised efficacy, poor patient compliance due to frequent dosing, and difficulty localizing exposures to the target organ or tissue.