Cheng-Prusoff

This is a linear relation often referred to as the Cheng-Prusoff relationship [5], It is characteristic of competitive ligand-receptor interactions. An example is shown in Figure 4.6b. 

It can be seen that the IC50 increases with increasing values of [A ]/Kd in accordance with simple competitive antagonism. This can be tested by comparison of the data to the Cheng-Prusoff equation (Equation 4.12). The data in Table 12.2b... 

The ICS0 is used in a version of the Cheng-Prusoff equation for functional assays. Thus, the apparent Kb (apparent equilibrium dissociation constant for the antagonist-receptor complex) is given by... 

Cheng-Prusoff correction, published by Cheng and Prusoff (Biochem. Pharmacol. 22, 3099-3108, 1973), this method is used to derive the equilibrium dissociation constant of a ligand-receptor (or enzyme) pair from the experimentally observed ICso (concentration that produces 50% reduction in effect) for that molecule see Eqn 4.11. 

Several brevetoxins have been examined for their respective abilities to competitively displace tritiated brevetoxin PbTx-3 from its specific site of action in brain synaptosomes. Analysis of IC q values revealed no marked differences in the displacing abilities between any of the type-1 toxins, and similarly there was no apparent difference between displacing abilities of PbTx-1 or -7, both type-2 toxins. Although some specific details require correlation, a gross comparison indicates that sodium channels in brain are similar in the systems examined. In the system studied most extensively, the rat brain synaptosome, t-test analysis revealed no significant differences between PbTx-2 and PbTx-3 IC q, or between PbTx-1 and PbTx-7 IC q, but statistically significant differences were found between the two classes (P<0.01) (5). If the Cheng-Prusoff equation (15) is applied ... 

As we stated above, there is a risk involved in the use of the Cheng-Prusoff relationships for SAR studies, as it is possible that structural alterations of the lead analogues could change the inhibition modality. This can be check from time to time for compounds that represent the greatest structural excursions from the lead molecule. Additionally compounds that are destined for progression into cellular and animal models should have their inhibition modality and affinity confirmed by running the more comprehensive studies discussed in Section 5.3. 

The term 1 + ( I. KL) is often referred to as the Cheng-Prusoff correction. It is clear from this analysis that the IC50 does not give a direct estimate of Kt unless [L] is very low, when IC50 tends to Kv Just as with saturation experiments, the situation will be complicated by the presence of different classes of binding sites (e.g., receptor subtypes) and by the involvement of G-proteins in agonist binding. 

The affinity constant of the test compound K can in turn be calculated from the IC50 value, the Kj of the marker and the concentration of M according to Cheng-Prusoff [see Eq. (3)] [22]. Since the concentration of free marker (M) is usually not determined in radioligand binding assays, it should be noted that the calculation of Ki according to Eq. (3) based on Mtot yields only reliable results if the depletion of the marker is negligible (i.e. <5% to 10% [7, 17, 21]). In competition experiments performed under typical conditions (Ttot Mtot this criterion is fulfilled. 

Leff, P., and Dougall, I. G. (1993). Further concerns over Cheng-Prusoff analysis. Trends Pharmacol. Sci. 14 110-112. 

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