In literature many types of sustained release formulations have been described like injectable suspension depots, in situ forming depots, polymeric implants (either biodegradable or non-biodegradable), injectable hydrogels, biodegradable microspheres etc. In principle these formulations can be classified into three major groups:
Suspension Depot Formulations
A suspension depot is usually administered subcutaneously or intramuscularly and allows slow release of drug and gradual absorption to the body. The release rate and duration of the depot is controlled by the dissolution rate in the interstitial fluid and diffusivity over the lipid membranes. The release rate and corresponding plasma levels can tuned by varying the injection volume, particle size and drug concentration.
In case the physicochemical properties of the drug are not suitable for development of a suspension depot formulation, the drug can also be incorporated in a carrier matrix. In the carrier matrix the drug is dispersed, either molecularly or as solid drug particles. Matrix delivery systems can be categorized in:
- Diffusion based designsDissolution based designs
- Erosion based designs
- Combination of diffusion/dissolution based designs
The release rate of the drug from the matrix delivery system is either controlled by the diffusion rate of drug in the matrix and/or the dissolution/erosion rate of the matrix.
In a reservoir device the drug is incorporated in a core that is surrounded by a rate limiting membrane. In the core the drug is dispersed, either molecularly or as solid drug particles. The presence of a release rate controlling membrane normally provides a better release profile. The release rate is controlled by the solubility and diffusivity in both core and membrane and by the thickness of the membrane.