The use of amorphous solid dispersions (ASD) to overcome poor drug solubility has gained interest in the pharmaceutical industry over the past decade. ASDs are challenging to formulate because they are thermodynamically unstable, and the dispersed drugs tend to recrystallize. Until now, most research on ASDs has focused on immediate-release formulations, supersaturation, and stability; only a few studies have recently reported on the manufacturing of sustained-release ASDs. Sustained-release ASDs can minimize the frequency of administration and prevent high concentrations that can lead to toxicity. Sustained-release ASDs can also decrease the reprecipitation rate in the medium, which can lead to increased bioavailability. However, sustained-release ASDs also pose some significant challenges, such as intramatrix recrystallization, inhibition of drug release as a result of drug–polymer gelling, and low supersaturation due to a slow dissolution rate. This review details the challenges and the formulation approaches that have been investigated to manufacture sustained-release ASDs. In particular, the advantages and drawbacks of hydrophilic polymers, hydrophobic polymers, and lipid-based systems are discussed.
Recently, several approaches have been proposed for formulating the sustained release of drugs from ASDs. These studies highlight specific challenges, such as the recrystallization of drugs in their dosage form, which can be prevented using hydrophobic excipients or by eroding systems with reduced hydration of the dosage form. Other challenges have been reported, such as the low drug release caused by low levels of supersaturation, high hydrophobicity, or polymer and drug gelling. The physicochemical properties of a drug (e.g., solubility, propensity for recrystallization) have a dramatic impact on formulation performance. Therefore, they must be carefully considered when formulating a sustained-release form of adrug using an ASD. In addition, if properly formulated, the sustained release of a drug from an ASD can decrease the reprecipitation of the drug, thus improving its bioavailability in humans.