Fitting the pEC50

The pEC50 is defined as the negative logarithm of the EC50. If the EC50 equals 1 micromolar (10^-6 molar), the log(EC50) is -6 and the pEC50 is 6. The advantage to expressing potency this way is that drugs with larger pEC50 values are more potent. No negative numbers. In some fields, it has become customary to report the pEC50.

If you fit a dose response curve using one of the classic equations, Prism will report the logEC50. Multiply by -1 to obtain the pEC50.

If you want to fit the pEC50 directly, perhaps to embed into a table on a Prism graph, use the following equation.

Y=Bottom + (Top-Bottom)/(1+10^((-pEC50-X)*HillSlope))

Download this Prism file to view sample data to that equation. To add the equation to your user-defined equation list, go to the Parameters dialog, then go to the Fit tab, then click the button to view the equation. Click OK twice, and the equation will be on your list.