A Review on Long Acting Parenteral Depot Systems

Parenteral drug delivery refers to administration by injection which takes the drug directly into the tissue fluid or blood without having to cross the intestinal mucosa. A depot injection is an injection, usually subcutaneous or intramuscular, of a pharmacological agent which releases its active compound in a consistent way over a long period of time. Long acting parenteral drug formulation are designed ideally to provide slow constant, sustained and prolonged action. Generally, parenterally administered drugs are advantageous because they can provide rapid and reliable drug systemic effects, long-term drug delivery and targeted drug delivery. It is achieved by two approaches. The first approach involves chemical modification of a parent compound to a derivative which is activated only at the targeted site. The second approach utilizes carriers such as liposomes, microspheres, injectable implants, injectable in-situ gels to direct the drug to its site of action. This article gives an overview of nanosuspensions, nanoemulsions, injectable implants, microspheres, liposomes, niosomes, injectable in-situ gels.

Number of drug delivery systems has been developed over the years, parenteral drug delivery system being one of them. Parenteral drug delivery refers to administration by injection which takes the drug directly into the tissue fluid or blood without having to cross the intestinal mucosa. The limitations of oral route are circumvented. Parenteral dosage forms with prolonged action are of medical and economic importance. Action is faster and surer (valuable in emergency). Gastric irritation and vomiting is not provoked. It can be employed even in unconscious, uncooperative or vomitose patient. There are no chances of interference by food or digestive juices. Liver is also bypassed by this route. But this route specifically requires that the drug delivery system should be sterile, besides being invasive and painful, assistance of other person often being required (though self injection is possible, e.g. insulin by diabetics), there are chances of local injury and being more risky. Once administered, the action is difficult to revert back in case of side effects or toxicity (Malik et al., 2010).

Parenteral preparations are sterile preparations intended for administration by injection, infusion or implantation into the human or animal body. Different muscle tissues have different blood flow. For example, blood flow to the deltoid muscle is higher than blood flow to the gluteus muscle. Intramuscular injections may be formulated to have a faster or slower drug release by changing the vehicle of the injection preparation. Aqueous solutions release drug more rapidly and the drug is more rapidly absorbed from the injection site, whereas a viscous, oily or suspension vehicle may result in a slow drug release, consequently slow and sustained drug absorption. Viscous vehicles generally slow down drug diffusion and distribution. A drug in an oily vehicle must partition into an aqueous phase before systemic absorption. A drug that is very soluble in oil and relatively insoluble in water may have a relatively long and sustained release from the absorption site because of slow partitioning. A change in a parenteral drug product from a solution to an emulsion, liposome etc will alter the drug distribution and pharmacokinetic profile. Clinical Example, Haloperidol (Haldol®) is a butyrophenone antipsychotic agent with pharmacologic effects similar to the piperazine phenothiazines. Haloperidol is available for oral and IM administration. Two IM preparations of haloperidol are available, including haloperidol lactate in an aqueous vehicle and haloperidol deconate in a nonaqueous sesame oil vehicle shows the tmax and elimination half-life of haloperidol after the oral, IM or IV Administration (Shargel et al., 2004).