- Solid Implants: From the solid implants, the drug release is controlled by varying polymer composition and increase in polymer molecular weight. These implants exhibit biphasic drug release kinetics, with initial burst of drug due to the release of drug deposited on the surface of the implant and further constant zero-order kinetics is achieved. An example is polylactic acid co-glycolic acid sclera implants containing ganciclovir for the treatment of cytomegalovirus infection.
- In Situ Forming Implants: An attractive alternative to solid implants is represented by the biodegradable injectable in situ forming implant drug delivery systems. Based on the mechanism of depot formation, injectable in situ forming implants are classified into five categories:
- Thermoplastic Pastes: These are semisolid polymers that injected as a melt form a depot upon cooling to body temperature. They have been used extensively in the manufacture of surgical sutures and ocular implants. The limitation associated with this system is that when injected at temperatures above 60°C, it leads to pain and necrosis at the site of drug administration.
- In Situ Cross-linked Polymer Systems: These systems are formed by free radical reactions initiated by heat or ionic interactions between small cation and polymer anions. Examples are ion-mediated gelation using polymers such as alginates/calcium ions or chitosan/phosphate ions.
- In Situ Polymer Precipitation: In these systems, a water-insoluble and biodegradable polymer is dissolved in a biocompatible organic solvent to which a drug is added, forming a solution or suspension. When injected into the body, the organic solvent dissipates, leading to the penetration of water into the organic phase. This results in phase separation and precipitation of the polymer depot at the site of injection.
- Thermally Induced Gelling System: Certain polymers undergo physical changes as a function of environmental temperature. The thermoresponsive polymer poly(N-isopropylacrylamide)[poly(NIPAAM)] exhibits sharp lower critical solution temperature at about 32°C, which can be shifted to body temperature by formulating poly NIPAAM-based gels with salt and surfactant. Other polymers such as pluronics or poloxamers with high polymer strength solution when injected have shown gelation at body temperature.
- In Situ Solidifying Organogel: Organogels are composed of amphiphilic lipids, which are insoluble and swell in water to form various types of lyotropic liquid crystals. Examples of lipids used for the drug delivery are glycerol monopalmitostearate, glycerol monolinoleate, and sorbitan monostearate and different gelation modifiers such as Tween 20 and 80 in various organic solvents and oils.
Leave a Reply