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ADME and Drug-Drug Interactions for the Toxicologist

Highlights from the recent webinar presented by our newest expert consultant, Dr. Pallavi Limaye

In her recent webinar (now available for free access to registrants), Dr. Pallavi Limaye discusses in vitro absorption, distribution, metabolism, and excretion (ADME) and drug-drug interaction (DDI) studies and their utility in preclinical safety evaluation as well as early decision making for Investigational New Drug (IND)-enabling preclinical studies and overall drug development. Dr. Limaye is a new addition to our network of expert consultants. She has more than 15 years of experience in toxicology and in vitro drug metabolism studies, bringing a unique perspective to the team and clients navigating their drugs’ development.

In her webinar,“In Vitro ADME and Drug-Drug Interaction Considerations for Toxicologists,” Dr. Limaye outlines major in vitro DDI / ADME study types and important considerations for drug developers conducting these experiments, including why, when, and how. She also highlights several important historical case studies to illustrate the importance of early, thorough investigation to avoid attrition or late-stage failure.

Key points discussed:

  • Why do in vitro DDI studies
  • Types of IND-enabling in vitro DDI studies
  • Proper study design
  • Strategic timing

Why do in vitro DDI studies?

“Are in vitro DDI studies in preclinical development standard box-checking studies?”

Dr. Limaye says simply, no. Investigating a drug’s ADME properties and in vitro DDI potential serves several purposes in pre-IND risk assessment, including building an understanding of a molecule’s metabolism, choosing appropriate in vivo animal species for nonclinical toxicology studies, predicting first in-human (FIH) study dose, identifying drug-drug interaction (DDI) concerns, and preparing for clinical DDI studies. Data from these studies can help drug developers assess risk early and plan follow-up investigations when necessary.

Typical in vitro ADME / DDI studies to elucidate the risk of clinical ddis

ADME componentType of in vitro study
Drug Metabolism (M, E)1. Inter-species comparative metabolism
2. Metabolite ID – Qualitative analysis of metabolite profile
3. Reaction phenotyping – Determine which CYPs are metabolizing
Drug Metabolizing Enzymes (M, E)1. CYP Inhibition – Profile specific CYP inhibitions
2. CYP induction – Induction potential for specific CYPs
Drug Transporters (A, D, E)1. Transporter substrate – Determine Transporter substrate profile
2. Transporter inhibition – Profile specific inhibition of major Transporters

“The ADME of a drug is driven by two alterable cellular elements—drug transporters, which are involved in absorption, distribution, and excretion; and drug metabolizing enzymes which metabolize or clear drugs from systemic circulation.” In the case of a drug-drug interaction, one drug (perpetrator) prescribed concomitantly with another (victim) may affect drug transporters or drug-metabolizing enzymes involved in the victim drug’s ADME, causing an increased or decreased exposure of the victim drug, potentially leading to adverse effects (i.e., increased exposure of parent drug, production of toxic metabolites or loss of efficacy).

Because polypharmacy (when a patient takes multiple medications) is a common practice, predicting a drug’s potential for DDIs is crucial for safety considerations early in development. Profiling a drug candidate’s ADME properties can provide information to elucidate DDI risk by informing the drug developer of its metabolic pathway and metabolite formationinhibitory or substrate potential with drug transporters, and ability to inhibit or induce drug-metabolizing enzymes.

An overview of each study type listed in the table above is discussed in the webinar, click here to tune in!

How to avoid misleading data

Study design is extremely important in these experiments. For example, in a CYP inhibition study, false negative results can mislead drug developers into overlooking time-dependent inhibition. This can be avoided with proper design and adjustments in protein concentration and incubation time.

“Choosing the right test system based on [compound] structure is very important.” When choosing enzymes for a reaction phenotyping panel, a ‘box-checking’ mindset might lead you to limit the results. However, take ezetimibe for example. Only using human liver microsomes (HLM) and NADPH can be misleading. The molecular structure of the drug indicates that that it would be glucuronidated (or sulfated) because of the phenolic moiety, so the drug developer would need to look at test systems inclusive of other drug-metabolizing enzymes, such as human hepatocytesrecombinant enzymes, or simply HLMs supplemented with UDPGA. 

The quality of the test system can also skew results and you must be able to detect issues and interpret data accordingly. In CYP induction studies, you can get false results from hepatocyte cultures with low confluency and therefore low basal enzyme activity.  In such a case the fold induction would be skewed higher.

Each lot of our plateable hepatocytes are tested by our experts specifically for confluency and suitability for induction assays — see our lot selection here.

Timing your in vitro ADME / DDI studies

It is generally advised to get as much data early on in development as possible because without adequate DDI results before first in-human (FIH) studies, patients taking other medications may be excluded from clinical trials and approval could be delayed. However, planning of preclinical drug-drug interaction studies can be highly subjective and variable. Development strategy should consider de-risking requirements of the molecule as well as the drug developer’s risk tolerance; a large pharma company with many molecules in the pipeline may be less risk-averse than a small pharma company with a few or just one molecule in development.

When it comes to conducting each in vitro DDI study, risk can be reduced by ensuring studies are carried out by experienced experts in the field. Conduct of studies and interpretation of experimental results can be “deceptively simple.” Providing relevant information to a seasoned drug-drug interaction study specialist can ensure proper study design and guidance and minimize the risk of misinterpretation of data leading to missteps in decision-making. 

Q&A with Drs. Limaye and Ogilvie

After the presentation, follow-up questions from webinar participants were addressed by Dr. Limaye and Dr. Brian Ogilvie, VP of Scientific Consulting (answers can be heard on the recording), including:

  • “For pre-IND CYP inhibition and induction, what if we typically don’t have information for drug concentrations?”
  • “If we have a compound that is slowly metabolized, are there recommendations for the best test system?”
  • “Are there different recommendations for repurposing drugs (505-B2 application)?”
  • “If we see borderline induction results, is it necessary to plan for follow-up clinical investigation?”
  • “Is a time-dependent inhibition study required for any drug metabolizing enzyme inhibition studies (UGTs, SULTs)?”
  • “Is it always assumed that DDI risk of one drug can reflect risk of its entire class (e.g. statins)?”

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About the Authors

Madison Knapp received her BS from the University of Missouri – Columbia and became SEKISUI XenoTech’s Scientific Communications Coordinator in 2019 after serving in similar positions at CropLife America, Bond Life Sciences Center and the CAFNR Office of Communications.

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