Before I wrote any of the other posts on this blog, I should have written a post on what exactly a potency assay is.  To make up for this oversight, here is that post now.

In the context of biologics development, potency can be defined as the ability of a drug or treatment to elicit a particular response at a certain dose.  In other words, potency is a measure of a drug’s activity in a biological system.

If possible to perform, the purest readout of potency is a direct measure of clinical efficacy.  However, this method makes for a very inconvenient way to assay the material.  That’s why we need a potency assay. The goal of the potency assay is to measure the material in a more convenient system.  The potency assay should also be a predictor of the activity of the drug in the final recipient.  In order to do that, the potency assay has to be based on a biologically relevant measure.  You need to be able to draw on scientific knowledge and on clinical efficacy data to make that link. Potency is always a relative measure.  We calculate potency based on the activity of a reference standard (often with a link to a clinical result).

Ok, that’s a clear definition, but what does that mean in practice?  It means that at each dose of an assay the shift in the relative response between the sample and reference should be the same.  Let’s look at a couple of pictures to make this clearer:

As you can see in these two images, whether the dose response curve is linear or not, potency is nothing more than a shift in the activity between the reference and the standard.  If the activity at a certain dose is higher than the reference, the sample is said to be more potent and if it has less activity it is said to be less potent.

The measure of potency is calculated as a ratio of activity between the two curves, usually expressed as a percentage.  In the example above, the potency of the samples is 200% since the distance between the two curves is a two-fold increase in the dose.

There you have it, this is what we talk about when we discuss potency assays.

As you might have noticed from the images above, the calculation of potency is only meaningful if the two curves have exactly the same shape, otherwise the distance between them won’t be constant.  This property is called parallelism or similarity.   This restriction causes problems in the real world since potency assays can often be quite variable and we therefore only have an estimate of the true dose response curve.  We therefore have to make sure that we can say with a particular level of certainty that the true curves are actually parallel.  But that discussion deserves a post of it’s own at a later date.

Thanks for reading,

Dan

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