Biopharmaceutics of Injectable Formulations

Most patients tend to prefer to take their medication orally. The most commonly used routes of injection for systemic exposure of pharmaceutical formulations are intravenous, subcutaneous (SC) and intramuscular (IM). SC injections are administered into the adipose layer of the skin. Doses administrable by SC or IM route are typically limited by the volume of injection and the ability to maintain injectability of the formulation. IM injections are administered into muscles around the body, usually the deltoid, quadricep or gluteal muscles. Drugs commonly injected by IM administration include lidocaine, cephalosporins, aminoglycosides, and diazepam, and phenytoin, insoluble salts of penicillin G, corticosteroids, narcotics, narcotic antagonists and contraceptive steroids. Subcutaneous injection targets the fibrous hyaluronic acid/collagen matrix within the subcutaneous tissue, which is bathed in interstitial fluid. In silico models for prediction of rate and extent of absorption for subcutaneously injected formulations are in their infancy relative to models for oral drug absorption.