Which of the following is true regarding bioavailability?
Bioavailability
Bioavailability refers to the percentage of drug that is detected in the systemic circulation after its administration. Losses can be attributed to an inherent lack of absorption/passage into the systemic circulation and/or to metabolic clearance.
Imagine you are testing a drug in a trial. Call it drug X. X is administered orally and plasma levels reveal that only 60% reaches the systemic circulation. The IV administration of the drug leads to plasma levels of 100%. The bioavailability of drug X is 0.6 or 60%.
The rectal route bypasses around two thirds of the first-pass metabolism as the rectum's venous drainage is two thirds systemic (middle and inferior rectal vein) and one third portal (superior rectal vein). This means the drug will reach the circulatory system with significantly less alteration and in greater concentrations.
