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The new FDA draft guidance on DDI studies – how does it affect in vitro studies?

1 December 2017

Recently FDA released new draft guidance on drug-drug interaction (DDI) studies. Compared to the previous one issued in 2012, also the structure is revised as there is now two complementary documents. The “In Vitro Metabolism- and Transporter –Mediated Drug-Drug Interaction Studies” focuses on in vitro approaches to evaluate potential interaction risk,

while “Clinical Drug Interaction Studies – Study Design, Data Analysis and Clinical Implications” provides information on when and how the clinical DDI study should be conducted.

The new in vitro guidance brings only subtle changes to metabolism mediated DDI studies, while the impact on transporter studies is more significant. Common for all in vitro DDI studies the revised guideline now encourages to conduct the studies earlier, generally before the clinical phase.

For the enzyme phenotyping studies the guideline continues to emphasize the importance of using two methods; enzyme specific inhibitors with human liver microsomes or hepatocytes and recombinant enzymes, bearing in mind their proportional amounts and activities compared to the human liver. The measurement of the parent drug depletion is preferred over the measurement of metabolite formation when evaluating the contribution of an individual enzyme to the overall metabolism, although monitoring individual metabolites gives valuable additional mechanistic information. As majority of the chemical inhibitors used in the assays are not fully specific for individual enzymes the new guidance highlights the importance of validating the selectivity and potency of the selected inhibitors in the experimental conditions.

For the enzyme inhibition studies the new guidance doesn’t provide any major changes on how the experimental work should be conducted. However, when evaluating the clinical relevance of in vitro observed inhibition the guideline emphasizes utilizing the free fraction of the compound in the calculations. Additionally, the guideline suggests considering the usage of primary human hepatocytes enriched with human plasma as an in vitro model representing better the physiological conditions compared to more conservative assay with human liver microsomes. The importance of well validated analytical assay is underlined when measuring the formation of a probe substrate’s metabolite for enzyme inhibition assay, like it is emphasized throughout the guideline.

In 2012 the revised guideline brought a major change for CYP induction assay, as mRNA levels became the preferred endpoint instead of enzyme activity levels. Now the new guidance is taking a step backwards, as it lists both endpoints as accepted ones, although it is acknowledged that measuring mRNA levels reveal enzyme induction also in the cases where it cannot be detected at enzyme activity level due to the concomitant inhibition. However, if inhibition profile is known the usage of enzyme activity level as endpoint measurement is accepted. According to the new guideline the concentration of the parent drug should be measured at several time points during the last day of the incubation as the actual drug concentration is important for the in vitro – in vivo extrapolation of the results.

For predicting the in vivo effect of a drug as an enzyme modulator the guideline refers to basic, static mechanistic and dynamic mechanistic models, similar to the previous guidance. However, using the maximal unbound plasma concentration of the interacting drug for the basic model calculations is now advised instead of the total concentration, making the FDA guidance more uniform with EMA’s “Guideline on the Investigation of Drug Interactions”.

Generally the guideline provides information in more detailed and reader-friendly manner than the previous one issued in 2012. FDA is welcoming industry comments for both draft guidances before January 28, 2018 and the implementation of the changes should get started to meet the revised expectations of regulatory authorities.

Written by Minna

PS. For your convenience I thought to share with you a link to two posters; “Comparison of N-in-one CYP inhibition assays for 7 major CYP enzymes between human hepatocytes and liver microsomes” and “High resolution mass spectrometry (HR-MS) with N-in-one CYP IC50 shift TDI assay”