Injectable Lipid-Based Depot Formulations: Where Do We Stand?

The remarkable number of new molecular entities approved per year as parenteral drugs, such as biologics and complex active pharmaceutical ingredients, calls for innovative and tunable drug delivery systems. Besides making these classes of drugs available in the body, injectable depot formulations oer the unique advantage in the parenteral world of reducing the number of required injections, thus increasing eectiveness as well as patient compliance. To date, a plethora of excipients has been proposed to formulate depot systems, and among those, lipids stand out due to their unique biocompatibility properties and safety profile. Looking at the several long-acting drug delivery systems based on lipids designed so far, a legitimate question may arise: How far away are we from an ideal depot formulation? Here, we review sustained release lipid-based platforms developed in the last 5 years, namely oil-based solutions, liposomal systems, in situ forming systems, solid particles, and implants, and we critically discuss the requirements for an ideal depot formulation with respect to the used excipients, biocompatibility, and the challenges presented by the manufacturing process. Finally, we delve into lights and shadows originating from the current setups of in vitro release.

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Rahnfeld, L.; Luciani, P. Injectable Lipid-Based Depot Formulations: Where Do We Stand? Pharmaceutics 2020, 12, 567.

Excipients for Formulating Depots
Two major classes of chemical compounds are used to create drug delivery systems: polymers and lipids. Despite the fact that both serve the same purpose—encapsulating drugs for depot formulations—they differ in their fundamental properties like chemical structure, solubility, or biodegradability and biocompatibility. The focus of this review is exclusively on depot formulations based on lipids, although hybrid systems combining the benefits of lipid and polymeric excipients will also be discussed. For more details about polymeric drug delivery systems we refer the readers to dedicated comprehensive recent reviews.
Lipids are defined as hydrophobic or amphiphilic small molecules originating entirely or in part by carbanion-based condensations of thioesters and/or by carbocation-based condensations, and are classified by LIPID MAPS® into eight categories shown in the Table.