Subject: IC50 vs. Ki summary

From: [email-masked] (Ellen R. Laird)
Date: Fri, 20 Jan 95 8:08:24 EST

Greetings:

	Recently I posted a query regarding comparisons between IC50 
	and Ki data for receptor/ligand binding.  The original post
	and a summary of responses follows.  Many thanks to the 
	respondents for several in-depth replies.

	And now for related survey:  is there a consensus in the drug discovery
	community as to what constitutes an acceptable degree of variance in
	data for binding assays (i.e., "simple" ligand::protein binding
	experiments, not cell- or tissue- based).  I am especially interested
	in the experiences of others in the pharmaceutical industry who are
	trying to establish SAR.

	Thanks in advance, and I'll post a summary.

	- Ellen

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  ~~~~~~~~  					 Ellen R. Laird
 ~~~~~~~~~~					 [email-masked]
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   ~~~~~~ARIAD Pharmaceuticals, Inc.             26 Landsdowne St.
                                                 Cambridge MA  02139
                                                 (617) 494-0400, ext 234
                                                 fax:  494-8144
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Original Post:
==============

	It is well known that for two compounds competing for binding
	to the same protein with equilibrium dissociation constants Kd_1 and 
	Kd_2, the difference in the Gibbs free energy of binding is given by

			ddG(2-1) = -RT ln(Kd_2/Kd_1)
	
	It is common in the pharmaceutical industry to have assays that
	generate IC50 values rather than equilibrium constants.  Clearly, if
	the ratio IC50_2/IC50_1 = Kd_2/Kd_1 then the IC50 ratio may be used in
	the above equation.

	I am requesting opinions and references on the following:

	If this ratio equivalence has not been demonstrated (i.e., K_n have not
	been measured in a comparable assay), is there a general relationship
	that can be drawn between IC50 and Kd?

	I have read a straightforward treatment of this *for enzymes* by Cheng
	and Prusoff (Biochemical Pharmacology, 1973, 22, 3099 - thanks to Dr.
	James Blake of Pfizer Central Research for supplying this reference);
	according to their analysis, Kd for an inhibitor is equal to the IC50
	only when noncompetitive or uncompetitive kinetics apply.  

	Their analysis assumes simple Michaelis-Menten behavior, 
	and begins from that equation:  

		        V_max [S]
                V_0 =    -------
                        K_m + [S]    i.e., rates of catalysis are involved
	                                   in the derivation

	Unfortunately, I am dealing with binding to non-catalytic 
	protein domains; it is not clear to me how one could extend
	the Cheng and Prusoff arguments to compounds that simply inhibit
	receptor-ligand interactions.

	On a related issue - it is evident from the literature that the
	majority of researchers in medicinal chemistry use a direct comparison
	of IC50 values as sufficient for determining the relative efficacy
	among a set of inhibitors; other than a desire to validate ddG
	computed by via sophisticated simulations, do computational chemists
	involved in pharmaceutical research even care to consider IC50 
	data in terms of free energy relationships?

	Thanks in advance; I'll post a summary of responses.    - Ellen

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RESPONSES:
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