Definition and examples
When a drug is administered, for example, a tablet is swallowed or an injection is injected into a vein, the active pharmaceutical ingredients subsequently spread throughout the body. This process is called distribution. Active ingredients distribute throughout the bloodstream, into tissues, and are eliminated through metabolism and excretion. Mathematically, the volume of distribution is defined as follows: Vd = D / C0 Where Vd represents the volume of distribution in liters (L), D represents the intravenous dose (mg), and C0 represents the plasma concentration at time 0 (mg/L). Plasma concentration is extrapolated retrograde. It is also often expressed in liters per kg body weight because the value is thus independent of weight and a better comparison can be made. The distribution volume is not a defined anatomical space in the organism, but rather an estimate of the extent to which the active substance is distributed in the tissues. Large distribution volumes are found, for example, with lipophilic active substances, as these distribute into fatty tissue. Conversely, polar substances and substances with a high protein binding and a high molecular weight generally have a small volume of distribution because they remain in the blood. Incidentally, a large volume of distribution does not necessarily mean that the active substance also distributes to the site of action. Examples:
| Insulin | 0.085 L/kg |
| Amoxicillin | 0.3 L/kg |
| Fentanyl | 3 L/kg |
| Imatinib | 4.9 L/kg |
| Digoxin | 6 L/kg |
| Mefloquine | 20 L/kg |
| Chloroquine | 400 L/kg |
See also
ADME, Distribution
