In vitro drug-drug interaction (DDI) regulation updates in the FDA’s 2020 guidance: A summary

In January 2020 the Food and Drug Administration (FDA) reviewed its 2017 draft content and published finalized guidance for in vitro and clinical drug interaction studies. We’ve summarized the the key changes and implications for in vitro drug-drug interactions (DDI) testing in this post, but if you would like more detail on specific changes, review our in-depth online whitepaper, comparing FDA, EMA and PMDA DDI regulations.

DDI and associated adverse drug reactions (ADRs) are integral to drug development and can greatly affect how successful a drug is in the market. Severe or highly frequent ADRs suggest an unfavorable risk/benefit profile and can result in limitations such as dose restrictions, ultimately limiting the success of a drug.  

Compared to the draft 2017 in vitro FDA DDI guidance, only a few key changes were included in the finalized 2020 publication.

DDI studies are now part of the IND package

One of the biggest changes to the updated guidelines is that the FDA now clearly recommends a wider panel of in vitro DDI studies be performed before clinical development begins. As such, DDI studies now form part of the investigational new drug (IND)-enabling program.

One reason for this change is to allow for better characterization of multiple clearance pathways in special populations, helping to ensure the appropriate inclusion or exclusion of all relevant genotypes during clinical trials.

That said, even with the IND-package change, the 2020 FDA update has minimal impact on development timelines. To conduct DDI tests that are compliant with FDA, European Medical Agency (EMA) and Japan’s Pharmaceuticals and Medical Devices Agency (PMDA) guidelines, you should allow 4–6 months, including reporting time. But these studies can be run alongside preclinical safety studies to minimize delays.

Substrate victim studies: Validated methods & wider range of enzymes

While there are no major changes to substrate victim studies from the FDA draft DDI guidance in 2017 to the FDA 2020 final guidance, a roust anlaytical method must be used when measuring concentrations of the test item in these studies (normally by liquid chromatography–mass spectrometry [LC-MS] unless the test item is radiolabeled). This guidance is described as a ‘validated method’ in the new regulations but with no further definition. However, this validated method is interpreted to mean that robust, precise and reproducible analytical methods must be developed and used to measure concentrations of the test item in the in vitro system. 

In addition, the range of enzymes to be included in enzymology phenotyping studies has increased from the standard cytochromes P450 (CYP) and uridine 5′-diphospho-glucuronosyltransferase (UGT) enzymes.

Considering a larger number of enzymes allows for a greater assessment of potential DDIs and inter-individual variations in drug efficacy. A drug metabolized by only one enzyme has a higher DDI potential as a victim drug than a drug metabolized by multiple enzymes, particularly if it has a narrow therapeutic window.

Enzymes for assessing potential metabolism interactions
CYP Aldehyde oxidases (AO)
Carboxylesterases (CES)
Monoamine oxidase (MAO)
Flavin monooxygenase (FMO)
Xanthine oxidase (XO)
Alcohol dehydrogenase (ADH)
Aldehyde dehydrogenase (ALDH)
UGT Sulfotransferases (SULTs)

Therefore, the primary updates to note for substrate victim studies include:

  • Using validated methods for measuring test drug concentrations
  • Including appropriate positive controls to show positive enzyme activity when using recombinant enzymes.

Perpetrator studies: Simplified inhibition studies

Similar to victim studies, no major changes from the FDA draft DDI guidance in 2017 to the finalized 2020 guidance occurred. The one notable change that will simplify inhibition studies is the use of IC50 values for risk assessments instead of Ki values (under specific experimental conditions), which often involve much larger experiments.

Other minor changes to perpetrator studies include:

  • Enzyme activity can now be used as well as, or in place of, mRNA analysis as the index of induction in CYP induction studies
  • Cut-off values for being classified as positive for interaction have been aligned for MATE transporters, bringing them in line with other transporter cut-off values
Box 1: Practical considerations for conducting studies  

Radiolabeling in victim studies
Radiolabeled compounds are generally unavailable during early in vitro metabolism studies; however, their availability does offer certain advantages. This technique can be used to readily determine concentrations of unbound/free drug, solubility and non-specific binding in the in vitro test systems, negating the need to use LC-MS. Using radiolabeled items also allows quantification of metabolites for phenotyping/enzyme identification studies without the need for analytical standards.  

Good laboratory practice (GLP)
Typically, in vitro DDI studies do not require GLP status; however, “validated analytical methods” should be used. The use of liquid scintillation counting with a radiolabeled compound fulfils this requirement.

Finally, a preincubation phase has been added to OATP1B1/1B3 testing to test for potential time-dependent inhibition (a relatively recent phenomenon observed with these anionic uptake transporters).

FDA now aligns closer with international regulatory agencies for DDI guidance

The finalized FDA 2020 guidance brings more harmony and alignment between international agencies, including the the EMA and PMDA. For example, the recommendation of using unbound concentrations of test drug for risk assessment (and not total drug) and the inclusion of fold-change as a method to assess CYP induction aligns closer with these agencies.

Therefore, as it stands now, international agency requirements differ only slightly between each other. The improved alignment between international guidance should make it easier to develop a study program acceptable to multiple global regulatory bodies, simplifying filing and reducing development time and costs.

The EMA is currently reviewing their 2013 DDI guideline and, after issuing a concept document in 2017, the general expectation is that the advice on cut-off values and which enzymes and transporters to test will align even more closely with the 2020 FDA guidelines. In addition, the FDA’s recommendations on the use of IC50 values rather than Ki (under specific experimental conditions) for the risk assessment in (reversible) CYP inhibition and transporter studies may also see further alignment in the finalized EMA guidance.


Perhaps the most important change to remember from the finalized FDA guidance is that DDI studies must now be conducted as part of the IND-enabling program before clinical development begins. Other more minor changes will hopefully simplify the process of victim and perpetrator studies, for example, by allowing the use of IC50 values in place of Ki values.

If you want a more in-depth assessment of the finalized in vitro FDA 2020 DDI guidance, please read our comprehensive online whitepaper that details the regulatory differences between the FDA, EMA and PMDA for DDI guidance.

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