ABSTRACT
The ultimate goal of drug therapy is to deliver an active pharmaceutical agent to the site(s)
within the body where it can exert its desired action. Once a drug is absorbed or delivered
to the bloodstream, multiple steps are involved in the process of subsequently delivering
the drug to its active site, and the steps involved are commonly referred to as drug
distribution. These distribution steps rely on the same transfer processes that are involved
in drug absorption: dissolution within the body fluids or tissues; partitioning, active or
facilitated transport across cell membranes; and bulk flow within the vascular system. The
accessibility to each of these processes and the order in which they are encountered play a
significant role in determining the final distribution of the drug within the body. As an
example, when hydrocortisone, an anti-inflammatory corticosteroid with significant
immunosuppressive activity, is administered intravenously (as the water-soluble sodium
phosphate or sodium succinate ester), it rapidly reaches many of the tissues within the
body and exerts multiple pharmacologic effects. These ester prodrugs can also be injected
intramuscularly, and similar systemic effects are observed because of the rapid absorption
of the drug into the vascular system. Hydrocortisone can also be administered orally via
liquid or solid dosage forms, but since the orally absorbed drug first travels via the
bloodstream to the liver before it reaches the rest of the body, some of the drug is
metabolized, and not all of the absorbed drug is able (or “available”) to reach the other
body tissues. The amount of hydrocortisone that can be administered via these systemic
delivery routes is limited by the wide variety of pharmacologic effects it exerts throughout
the body, so other modes of administration have been developed. To maximize its anti-
inflammatory effects while limiting secondary effects at distant body sites, methods to
deliver hydrocortisone “locally” have also been developed. Hydrocortisone acetate is
injected directly into joint spaces (intra-articular or intrasynovial administration) or into
soft tissue spaces. Topical applications of various forms of hydrocortisone (typically ester
derivatives) directly to the skin surface in creams, ointments, and lotions; to the eye in
combination with antibiotics; or even rectally in suppositories or as a foam have been
marketed.
