Metabolite Bioavailability

The systemic bioavailability of the metabolite is the fraction of the formed metabolite that reaches the systemic circulation.

When a drug is metabolized by any organ like the liver:


Drug

Liver

Metabolite

Secondary
metabolism

Metabolite
excretion in bile

Bioavailable
Metabolite

The metabolite can be metabolized further to a secondary metabolite before it reaches the systemic circulation.

The metabolite can be directly excreted in bile before it reaches the systemic circulation.

The fraction of the metabolite that escapes further metabolism or biliary excretion is the bioavailable metabolite.

The metabolite bioavailability is an additional factor that can make the plasma metabolite concentration lower than the expected concentration given the specific fm for the metabolic pathway. The plasma metabolite conc is expressed by:

Cp(m) =

fm FH (m) k D

Vd(m) (k(m)-k) (e-kt - e-k(m) t )
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