The unbound fraction of a drug is responsible for therapeutic actions, and it is this fraction which undergoes metabolism and clearance.
Half-life is dependent on several factors including volume of distribution, and clearance. So a highly protein bound drug will tend to have a low volume of distribution and clearance. On the contrary, low protein binding drugs tend to have high volume of distribution, and rapid clearance (which also depends on the rate of drug metabolism).
My question is, if a drug is highly protein bound (>80%), does it mean that it will have a long half-life? Drugs which have low protein binding (5%) seems to have long biological half-life, for instance, Dactinomycin (Protein Binding-5%, T1/2- 36 hrs). Also, low protein binding drugs like phenobarbitone have long T half-life of >53 hrs. Can anyone explain how this happens?
Reverse relationship, that's mean when plasma protein binding increases, the free concentration of drug reduced and half-life of drug reduced also and the reverse is true .
Warm Regards
That is true, reverse relationship between protein binding and the T1/2.
High protein binding drug has low volume of distribution and clearance and low free effective drug concentration in the blood, thus low T1/2
T1/2=( 0.7*Vd)/ clearance
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