Concentration-Response Plots and IC50 Determination

The IC50 can thus be accurately determined by fitting the concentration-response data to Equation (5.1) through nonlinear curve-fitting methods. Some investigators prefer to plot data in terms of % inhibition rather than fractional activity. Using the mass-balance relationships discussed above, we can easily recast Equation (5.1) as follows  

Competitive Noncompetitive (a 1) Noncompetitive (a = 1) Noncompetitive (a 1) Uncompetitive Increases linearly with increasing [S]b Increases curvilinearly with increasing [S] No change with increasing [S] Decreases curvilinearly with increasing [S] Decreases curvilinearly with increasing [S] 

A question that often arises is how large a difference in IC50 between two compounds is considered significant. This can be addressed statistically using a standard Student /-test (Spence et al 1976). 

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Plot p24 against concentration to generate a dose response curve and determine the IC50 concentration (Fig. 3). 

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