Implant

In 1861 Lafarge first introduced the concept of implantable systems for sustained drug administration. The concept was later used to produce solid implants containing  steroid hormonoes.initiating the use of implantable system for long- term delivery.these traditional pharmaceutical pellets consisted of pure drug with no added excipient and were defined as small. Rod shaped or ovoid shaped. sterile tablets consisting of the highly purified drug. usually compressed without excipient.

Intended for subcutaneous implantation in body tissue. The devices thus prepared had a high degree of hardness and virtually zero porosity. since water did not penetrate the matrix, drug release occurred principally by surface dissolution. Due to the inherently poor solubility of steroid drugs. this method provided a good form of depot medication.

There are still a few of the traditional implants in commercial use. including desoxycorticosterone acetate (percorten pellet. Ciba) . and estradiol and testosterone (Progynon and Oerton pellets. respectively . Schering).

For incorporation of a variety of therapeutic agents with different physicochemical properties and for better control of drug release a limited number of excipient is now used. Thus more recent implants usually contain the drug in a rate controlling systems.

These systems are available in a variety of sizes and shapes. Some of the recently approved implantation products include leuprolide acetate and nafarelin acetate in biodegradable dl-lactic and glycolic acidc copolymer for one-month release (Lupron depot micro spheres.
TAP and Zoladex pellot ICI, respectively).

Also new on the market is a silicone polymer capsule system containing levonorgestrone for five year contraception (Norplant. Wyeth-Ayerst). In addition, several implantable pumps for prolonged  drug delivery are in commercial use. With rapid advances in  implantation therapy and excipients to control the release pattern, the USP XXII identifies a much broader definition of implants, recognizing the presence of excipients and implantation in the body at sites other than subcutaneous.