The pharm + tox lecture series: #3 - Absorption, putting the A in ADME

When it comes to describing the journey that a drug takes through the body, pharmacologists like to employ a cute little acronym: ADME. It stands for Absorption, Distribution, Metabolism, and Elimination. I'll be covering the D, M, and E parts in future posts, but for now let's focus on the A.

Absorption, in the pharmacological sense, refers to the passage of a drug from the external environment into your body, usually into systemic circulation (i.e. your bloodstream). In essence, how a drug gets inside you. There are all sorts of interesting ways by which this can be accomplished, so I'll give you the rundown.

Topical administration consists of applying a drug directly to the part of the body where you want it do stuff. The drug does not get into your systemic circulation, either because it is given in such a small amount or it specially formatted so that it won't pass into your blood in appreciable amounts. Examples include skin ointments and creams, eye drops, nose sprays, ear drops, vaginal suppositories, as well as drugs injected into joints (e.g. corticosteroids to treat arthritis), the epidural space (e.g. an epidural), or the cerebrospinal fluid (intrathecal administration, e.g. to treat a brain infection with antibiotics).

Percutaneous or transdermal administration consists of applying a lipid soluble drug to your skin with the intent that it will dissolve through your dermis into your systemic circulation. Nicotine (hooray for quitting tobacco!), scopolamine (to prevent you from puking in moving vehicles) and birth control patches (which apparently make you fat) are primo examples. A similar route is via inhalation, with the drug passing through the lung epithelium instead of the skin. This tends to occur very rapidly. Things like inhalational general anaesthetics (e.g. desflurane or sweet, sweet xenon), medicinal marijuana (wink!), and asthma drugs (e.g. puffers) can be administered this way.

Enteral administration refers to giving a drug in such a manner that it enters your body at some point along your gastrointestinal (alimentary) tract. Sites include your oral mucosa (e.g. buccal or sublingual nitroglycerine drops), your stomach/intestines (referred to as the oral or p.o. route, any drug that you have to swallow), or your rectal mucosa (mmm, mmm, suppositories!).

Parenteral administration means giving a drug 'beside the gut'. In other words, injecting it somewhere with a needle so that it bypasses your gut to get into your bloodstream. The types of injections are fairly self-explanatory. Subcutaneous (s.c.) means under your skin, intramuscular (i.m.) means into a muscle, intravenous (i.v.) means into a vein (the medical world's usual target of choice), and intra-arterial (i.a.) means into an artery (used to direct anticancer drugs at high concentrations into tumours). Another variety of injection that is generally limited to lab animals due to the relatively high risk of infection or injury is intraperitoneal (i.p.) injection. This involves the delivery of a drug into the peritoneum, which has a large surface area and so allows rapid entry into the bloodstream. It's worth noting that some drugs, such as vancomycin or heparin, can only be given parenterally because their chemical structures are such that they are either broken down in the gut or else are not appreciably absorbed through gut walls into the bloodstream.