[arhh]

Hi there,
Im studying for a test in Medicinal Chemistry and i cant answer this question, i think the answer is (E) because online it says that EC50 and IC50 values are comparable only in relation to an agonist drug....any help would be greatly appreciated, especially with an explanation as to why it is the one it is, or why the others are correct.

7. What is a false statement regarding enzyme inhibitors and their Ki and IC50 value?
(a) The lower the Ki value the better the inhibition
(b) The lower the IC50 value the better the inhibition
(c) If the Ki value is low it follows that the IC50 is also low
(d) If the IC50 value is low it follows that the Ki value is also low
(e) For enzyme inhibitors the EC50 and IC50 values are identical

[Babcock_Hall]

Cheng and Prusoff (1973) and Brandt et al (1987) both wrote papers on the relationships between K_i and IC50, but I don't have copies of either one yet.  I would say that c and d are a little questionable.  In the case of competitive inhibition, the equation given by Cheng and Prusoff indicates that the IC50 will be equal to or greater than K_i, in a way that depends on the ratio of [Substrate] to K_m.  However, I don't presently have their paper, and I am not sure about uncompetitive or noncompetitive inhibition.

[pgk]

What is the definition of EC50?
What is the definition of IC50?
And what is the Ki?
If you know or can find the above definitions then, the answer of this question, will come by itself.

[Babcock_Hall]

A more thorough perusal of Brandt and coworkers' paper indicates that the relationships between IC50 and the K_i do change from one type of inhibition to another (competitive versus uncompetitive, for example).  However, it looks to me that IC50 is ≥ K_i in a way that depends upon the ratio of [ S] to K_m.