Implants

IMPLANTS

Implants are small, sterile cylinders of drug, inserted beneath the skin or into muscle tissue to provide slow absorption and prolonged action therapy. This is principally based on the fact that such drugs, invariably hormones, are almost insoluble in water and yet the implant provides a rate of dissolution sufficient for a therapeutic effect. Implants are manufactured from the pure drug made into tablet form by compression or fusion. No other ingredient can be included because this may be insoluble or toxic, or, most importantly, may influence the rate of drug release. Copolymers such as polylactic acid/polyglycolic acid may be used as the implant matrix to provide a controlled rate of drug delivery.

Compression of sterile drugs must be conducted under aseptic conditions using sterile machine parts and materials. After manufacture, the outer surface of the implant is sterilized by immersion in 0.002% w/v phenylmercuric nitrate at 75 °C for 12 hours. After the surface has been dried, each implant is placed aseptically into a sterile glass vial with a cotton wool plug at both ends. This prevents damage and reduces the risk of glass specules, formed when the vial is opened, adhering to the implant. This compression process is not ideal and fusion processes may be used provided that the drug is heatstable. The pure drug is melted at 5–10 °C above its melting temperature and poured into moulds. The interior of the implant will be automatically sterilized by this process if the melting temperature is high enough. It is also possible to dry heat sterilize the implant after packaging provided that the melting temperature is above 160 °C. Clearly, it is easier to manufacture sterile implants by fusion as the process does not require presterilized ingredients or aseptic processing. The implant hardness is also very convenient.