Bioequivalence and Bioavailability Forum

Hi T.,

❝ What is the difference between plasma half life and biological half life?

I think what is meant is effect half-life? Only a guess.

❝ If for a product the plasma half life is 3h while biological half life is 30h, do I need to consider biological half life for the wasout period?

If this is a compound that qualifies for PK-evaluation then I would say you can disregard effect considerations.

❝ Considering differences in half life, will there be differences in the other PK parameters as well--biological Tmax! biological Cmax! biological AUC!

❝ What is the relation between Tmax and halflife? What are the properties of a product with lesser Tmax and higher half life and viceversa?

❝

That stuff is complicated and it ultimately depends on the mechanism of action and ADME. For example, a Michaelis-Menten type of relationship between concentration and effect is often proven in vitro with some kind of easy assay but is very difficult to investigate in vivo. Throw in several receptors, various ED50-values and some biological variation and you quickly get data that is all over the place in view of the model.
In the world of PK Tmax and Cmax are often taken as indicators of rate of absorption (which isn't always the same as rate of effect production) whereas AUC is an indocator of extent. "Biological Cmax" must be a misnomer?!?
Tmax and half-life will typically not correlate meaningfully.