Bioavailability
Bioavailability (F) refers to the fraction of an administered dose of drug that reaches the systemic circulation following absorption from the administration site. Usually used with reference to oral bioavailability. Generally, for drugs administered via a parenteral route, bioavailability is usually considered to be high (F ≈ 1), in the absence of evidence to the contrary. However, parenteral administration of a prodrug that is converted in the body to the active species may result in a bioavailability for the active metabolite that is far lower than 100% if a portion of the prodrug is cleared (hepatically or renally) before being converted to the active species. Sublingual or buccal administration may also result in bioavailability that is much lower than 100%, sometimes due to some of the administered dose being swallowed in saliva, and sometimes because of modest lipid solubility of the drug in question.
In considering bioavailability as a factor impacting dosing rate, particularly when converting from one administration route to another (when a change in drug formulation may be accompanied by a change in the salt used), it is important to consider the chemical formulation of the drug.
Example
The tetracycline antibiotic doxycycline is available as doxycycline hyclate (doxycycline hydrochloride hemiethanolate hemihydrate; empirical formula C22H24N2O8 · HCl · 0.5H2O · 0.5C2H6O; MW 512.94 g/mol) or as doxycycline monohydrate (empirical formula C22H24N2O8 · H2O; MW 462.45 g/mol). A dose of 100 mg of doxycycline monohydrate contains 96 mg of doxycycline, whereas a dose of 100 mg of doxycycline hyclate contains 87 mg of doxycycline. To administer the equivalent dose of doxycycline (96 mg), it would be necessary to use 110 mg of the hyclate salt.
Clinical Context
Understanding bioavailability of medications can be important for many reasons, including determining what route of administration may be required. For example, medications that are metabolised very quickly may require administration via a continuous infusion to be effective. You may choose to bypass first-pass metabolism using buccal or rectal routes (venous blood from the lower rectum drains into the inferior vena cava, bypassing the liver, although upper rectal venous drainage enters the portal circulation). Though you may not have intravenous access, an antibiotic such as levofloxacin that may be used to treat a hospitalised patient for pneumonia,could be prescribed orally, as its bioavailability approaches 100%. Knowing the bioavailability of oral antibiotics can also help in deciding whether patients can be stepped down from intravenous to oral medications, allowing earlier discharge from hospital. Commonly-prescribed medications such as ondansetron, dexamethasone, metronidazole and levofloxacin all have excellent bioavailability.
