Subcutaneous

What the route is

Subcutaneous means under the skin. On a drug label it is a route of administration, listed next to intramuscular and intravenous, not a class of molecule. The Hospira bacteriostatic-water label names subcutaneous injection as one of the routes for which the diluent may be used, and only according to the instructions of the drug being dissolved.

Richter and Jacobsen's 2014 minireview is the clean pharmacokinetic description for biotherapeutics. Many protein drugs, marketed or in development, are given this way. Compared with intravenous administration, subcutaneous delivery can be more practical. The absorption that follows is relatively slow and mostly incomplete. [1] PMID: 25100673

Why absorption is slow

The same review locates the delay in the tissue, not in a slogan about "fat". Transport through the subcutis to the absorbing blood capillaries or lymph capillaries is a major reason subcutaneous absorption is slow. Larger proteins, above about 20 kDa, are mostly absorbed via the lymphatic system, with species differences that are not fully settled. Presystemic catabolism — breakdown before the molecule reaches the circulation — is part of why bioavailability is incomplete. [2] PMID: 25100673

That last point is why this entry links to bioavailability and half-life. A subcutaneous injection does not put the full labeled mass into plasma at once. The fraction that arrives, and the time course of that arrival, are pharmacokinetic facts. They are not implied by the word subcutaneous itself.

What it is not

It is not intramuscular. Intramuscular places the solution in muscle. The two routes can appear on the same label and still produce different absorption. It is not intranasal, oral, or intravenous. Intravenous administration skips the absorption step; subcutaneous administration is defined by that step.

It is not a dose. "Subcutaneous" does not say how much, how often, or whether a particular peptide has been studied by that route in humans. Bowers reported that one early growth-hormone-releasing hexapeptide was active by intravenous, subcutaneous, or intraperitoneal injection in rats. [3] PMID: 6714155 That is a finding about that peptide in that experiment. It is not a general rule for every research peptide in a vial.

Where to read it on this site

The administration-route guide and the dosing guide are the pages that use this word in context. This entry only fixes the meaning: a route into the tissue under the skin, with slow and often incomplete absorption for larger proteins, as the 2014 review describes it.