Depot Formulation

A depot formulation trades immediate availability for duration by putting drug into a physical form it must escape before absorption. The usual constructions are biodegradable polymer microspheres, most often poly lactic-co-glycolic acid, which release as the polymer hydrolyses; in-situ forming gels that solidify on contact with tissue water; poorly soluble salts and crystalline suspensions; oil solutions of lipophilic esters; and solid implants. The pharmacokinetic signature is definitive: absorption is slower than elimination, so the apparent half-life reflects the release rate rather than the molecule, a pattern called flip-flop kinetics.

Peptides supply the best-documented examples. Leuprolide acetate has been marketed for decades as polymer microspheres in presentations designed to cover one, three, four and six months, and as a small implant. Octreotide and lanreotide are given as long-acting monthly preparations for acromegaly and neuroendocrine tumours. Exenatide extended-release applied the same microsphere principle to a twice-daily peptide.

Depots trade control for adherence. A single administration covering months removes the largest source of failure in chronic therapy and flattens the peak-to-trough swing that drives some side effects. What it removes in exchange is reversibility: once the depot is in tissue it cannot be retrieved, so an adverse reaction or a change of plan has to be managed for the life of the depot.

The error to watch for is reading a long interval as a property of the molecule. Leuprolide itself is cleared in hours; only the formulation is long-acting, and a different presentation of the identical peptide behaves completely differently. More seriously, grey-market material is often called long-acting on a supplier's word when nothing in it provides a controlled-release mechanism.