What is EC/IC Anything?

The potency of a drug is commonly quantified as the EC₅₀, the concentration that leads to 50% maximal response.  But in some systems you might be more interested in the EC₈₀ or the EC₉₀ or some other value. You can compute these values from the EC₅₀ to provide EC_anything.

If your response goes down with increasing dose (so the Hillslope is negative), watch out for the terminology. The EC₁₀ (for example) is defined as the concentration that gives you a response 10% of the way between the Bottom and Top plateaus. In situations where the response goes down as you increase dose, you may think of this as the "IC₉₀" since it gives 90% of the maximum inhibition.

Calculating EC/IC Anything from the EC₅₀

The calculation for the EC/IC anything is deriving the value from the EC₅₀ using the following formula, where F is the value you wish to set:

Let's say you set F=80. Then the EC_F for inhibitory data would be the concentration (X value) required to bring the curve down to 80%. If you actually want the concentration that brings the curve down by 80%, to 20%, then you'd need to set F equal to 20 in the formula above.

We have summarized these ideas in the following table for your future reference:

Please see the source materials and a further discussion of EC/IC anything calculations on the GraphPad Prism website

How to Calculate EC/IC Anything in FCS Express

With FCS Express you can easily calculate your own value for EC or IC. View our example EC/IC anything calculation in our example layout. You may download the example layout here.

EC/IC Anything is calculated within FCS Express using regression plots. We can take certain variables as calculated from the best-fit curve. We then plug the variables into a formula within our internal spreadsheet.

Step by Step Considerations

• The first step is to ensure that your data is represented within a spreadsheet in FCS Express. You will require two columns, one for concentration and one for a read out of response.
• From here, you can create a scatter with regression plot. This will need to be formatted to a FiveParmLogistic (Five Parameter Logistic) regression Model.

• Two statistics are required from this plot (the IC₅₀ and the Hillslope).
  Note: EC₅₀ and IC₅₀ are equal, within FCS Express, the statistic is named "IC50", which can be used for both values.
  • One way to report these statistics is by right-clicking on the Scatter with Regression plot > Statistics > Regression Parameter Statistics
  • These statistics can be directly reported in an integrated spreadsheet by:
    • Right-clicking inside a Spreadsheet Cell >Insert Token > Statistic
    • Selecting the correct regression plot to source the statistics from.

• • • Then, in the Statistic menu, selecting the Regression Parameter Statistics from the drop down menu.

• In addition to these statistics, we recommend entering the F value into the spreadsheet.
• These three values, the IC50, the Hillslope and the F value, can be plugged into a formula to retrieve your desired EC/IC anything value.

If you would like multiple EC/ICs it is simply a matter of creating additional cells utilising the same formula.