Q&A Report: How to Better Predict Potential Drug Interactions Early in the Discovery Process
The answers to these questions have been provided by:
Jack Vanden Heuvel, PhD
- Professor, Veterinary and Biomedical Sciences, Penn State University
- Co-Founder & Chief Scientific Officer, INDIGO Biosciences
Is there a specific order in which studies should be performed? For example, should reporter assays be done before gene expression, or vice versa?
This largely depends on your level of expertise and what your overall interest is. There really is no “correct” order. So, you could first start with the reporter assays to determine if your drug is interacting with PXR, FXR, or CAR, and then you could follow that up with a gene expression assay to show that what you saw on the reporter assay is leading to a change as anticipated in the upcyte® cells or the hepatocyte system that you use, and that would confirm the molecular initiating event. You could also go the opposite direction where you have data that shows that you’re getting alteration in particular drug metabolism enzymes and you want to know which receptor is responsible, or you want to see if there are species differences. So let’s say you did a rat study of gene expression and you saw that CYP1A1 was induced. You can then look at the rat aryl hydrocarbon receptor (AhR) and show if that is a target, and then you can take that the next step and see if humans respond similarly to this drug where you could get that species extrapolation.
In your webinar, you used the term “triage assay.” What do you mean by that?
A triage assay is simply used to prioritize drug candidates to the extreme. So, say you’re trying to get an idea of which drug candidates you should put forward into animal studies. You may decide that there is a line you don’t want to cross, for example, you don’t want to see PXR activation or you don’t want to see CAR activation because you know it’s going to cause problems with some other drugs due to a drug-drug interaction. So, that would be prioritizing to such an extent that you are removing drug candidates from going forward at all.
Suppose my drug candidate activates PXR. How would that help me to determine what other drugs may be impacted?
There are a lot of great resources out there that will tell you which drugs are metabolized by a variety of specific Phase 1, Phase 2, and Phase 3 enzymes. So, for CYP3A4, we have a long list of the different pharmaceuticals that are metabolized by CYP3A4. We also know that that enzyme is regulated by PXR, so by inference, if you activate PXR, that whole list of chemicals are CYP3A4 substrates and are going to be affected. You can do this with each of the different nuclear receptors, you can look at all the lists of different enzymes. So you’re able to take advantage of all the resources and information that we have on some of the more typical prescription drugs that are on the market.
Is it a good idea to target nuclear receptors as a therapy?
There are many nuclear receptors that are targets for therapy, with around 15% of all prescription drugs targeting a nuclear receptor. The steroid receptors (e.g., AR, ER, MR, PGR, and GR), RARs, and TRs are very common targets. There are also emerging targets including ROR, the PPARs and LXR.
Of the three receptors you focused on in the webinar—PXR, FXR, and CAR—which is the most important?
PXR is the most important triage target, for two main reasons. First, it is the most important inducer of CYP3A4, an enzyme that metabolizes about 50% of all prescription drugs. Second, it has a large binding pocket and is very promiscuous with many xenobiotic ligands.