Drug metabolism and pharmacokinetics (DMPK) is a core discipline in drug development that considers the biotransformation of a drug compound and other pharmacokinetic properties to assess drug safety. DMPK studies allow drug developers to experimentally evaluate intrinsic properties of a drug candidate to validate that it can and will be cleared from the body, when administered to a patient, without producing harmful byproducts (metabolites), reaching dangerous exposure levels (toxicity), or causing adverse side effects. This class of in vitro and in vivo studies can also help drug developers to predict potential risks, early in drug development, as well as provide nonclinical evidence to justify decisions or explain clinical observations.
An important risk DMPK studies can help evaluate is the potential for drug-drug interactions (DDIs), occurring when adverse effects result from a patient taking multiple medications. Properly investigating a drug’s interactions with metabolizing enzymes and drug transporters in vitro can identify risks before first in-human trials and help the drug developer anticipate and plan clinical DDI studies or labeling restrictions if necessary.
Pharmacology is the study of the uses, effects and modes of action of a drug. Comprised of two complementary elements: pharmacodynamics (PD), describing what a drug does to the body once it reaches the site of action, and pharmacokinetics (PK), which put simply is what the body does to a drug along the way and afterward. Broadly speaking, pharmacokinetics tell the story of how a xenobiotic moves into, through, and out of the body.
Commonly, PK is illustrated by a plasma concentration-time curve, but the pharmacokinetic profile of a compound includes all the factors that shape it, including the basic processes of absorption, distribution, metabolism, and excretion (ADME). A drug’s ADME properties (along with other components of the pharmacokinetic profile) can make it more or less suitable as an effective therapeutic and can impact decision-making early in the drug development pipeline. Optimal pharmacokinetic properties demonstrate that a compound “administered at a given dose actually achieves the required concentration, for sufficient duration in the target tissue to achieve the desired biological effect while minimizing any undesired off target effects.”1 The major way to predict a drug’s pharmacokinetics before it gets administered to human volunteers is through appropriate in vitro and in vivo preclinical testing.
Additionally, pharmacokinetic properties can illuminate “soft spots” of drugs in major metabolic pathways and predict PD issues. For example, if results from in vitro and in vivo ADME studies of a drug candidate indicate high clearance, short half-life (t1/2), or low bioavailability following oral dosing, these ‘red flags’ can indicate sub-optimal pharmacodynamic effects.2
Drug metabolism describes the enzymatic conversion of a drug into other related compounds (metabolites), usually caused by drug-metabolizing enzymes. The main goal of this conversion process is to make a lipophilic molecule more hydrophilic (water-soluble) so it can be eliminated.
Metabolites of a drug can cause adverse effects, and humans and animals may metabolize a drug differently, so metabolism studies are an important part of preclinical drug development.
In vitro studies
In vitro test systems such as liver microsomes or cryopreserved hepatocytes contain relatively high concentrations of drug-metabolizing enzymes (DMEs) and allow drug developers to explore their drug candidate’s metabolism by collecting experimental data to answer specific, relevant questions about how a drug will interact with human or animal cells at primary sites of metabolism.
The three major in vitro drug metabolism studies are:
Metabolic stability screening in hepatocytes or subcellular fractions (e.g., microsomes or S9) first determines a drug’s ability to be converted to metabolites by measuring the rate of intrinsic clearance by drug-metabolizing enzymes.
Metabolite characterization and identification studies then allow a drug developer to find out which metabolites may be formed from the parent drug and if any are unique to humans or disproportionately higher in human than preclinical animal models. Qualitative metabolic profiles, and proposed biotransformation schemes are established in each species as well to determine which will be most similar to a drug’s metabolism in a human. Comparing metabolite formation in human and other species can help drug developers early in development choose an appropriate animal model (species) for definitive nonclinical studies.
Reaction phenotyping provides drug developers with insight into which cytochrome P450 (CYP) enzymes are responsible for the metabolism of a drug candidate and identifies its victim potential for metabolism-mediated DDIs. Definitive reaction phenotyping experiments often begin with a panel of recombinant human CYP enzymes to identify potential activity with specific CYP enzymes. Loss of parent drug or metabolite formation is measured and rate of metabolism is plotted. These data are then corroborated by incubation with human liver microsomes (HLM) and selective CYP inhibitors. Metabolism by non-CYP pathways can be detected as well, especially if hepatocytes are used, and can inform the need for follow-up studies.
Within each of the drug metabolism studies described above, sponsors often elect to use test systems from multiple animal species in addition to human. Comparing potential toxicological species to each other and to human results allows them to choose a species for in vivo and in vitro definitive studies that has a similar metabolic profile to humans (including metabolites formed and metabolic pathway) and allows them to identify any human-specific metabolites.
Common animal species a sponsor may consider including in in vitro metabolism studies include:
Metabolism-mediated drug interactions
Additional studies investigating drug-metabolizing enzymes as part of the determination of a compound’s potential for drug-drug interactions are:
These studies are specified in FDA’s guidance for industry documents as tools to predict a drug’s perpetrator potential in metabolism-mediated drug interactions. They are designed to use test systems with average enzyme expression (e.g., a pool of liver microsomes from 200 human donors, or several lots of human hepatocytes) to determine if a compound is likely to induce (upregulate) or inhibit drug-metabolizing enzymes which are responsible for the metabolism of other (victim) drugs which may also be prescribed to a patient.
In vivo studies
Animal ADME studies using radiolabeled drug are typically conducted pre-IND through Phase I of clinical trials.3 In vivo animal models reflect combined effects of permeability, distribution, metabolism, and elimination before administration to a human.
Animal studies are required to measure parent drug and metabolite exposure levels and determine potential toxicities, and can identify areas of concern such as low absorption or high clearance.4 Metabolism in particular represents a critical piece of the puzzle; metabolic profiling and metabolite identification in excreta and plasma help to validate choice of preclinical species, define the route of elimination of a drug from the body, and quantify the contribution of metabolism.5 Radiolabeled drug, as typically used in in vivo studies but not commonly for in vitro, allows drug developers to accurately identify metabolites and measure how much of each is found in plasma or excreta.
Leadership in DMPK science
For your DMPK studies, work with a provider you can trust. SEKISUI XenoTech was one of the very first drug metabolism specialty CROs on the market more than 25 years ago, and since then we have spent every day accumulating experience and expertise with every study and test system preparation to best help drug developers understand and evaluate risk for their compounds.
Our experts are committed to quality and superiority, providing a consultative approach to our products and services as well as strong dedication to thought leadership by consistently raising the bar for DMPK science.
Contact one of our product or service specialists to find out how we can support you in your drug’s journey through the development pipeline.
Learn more about how ADME fits in with DMPK and DDI in our ADME 101 overview webinar presented by VP of Scientific Operations, Dr. Joanna Barbara
Learn more about:
 Thomas D.Y. Chung, David B. Terry, and Layton H. Smith. “In Vitro and In Vivo Assessment of ADME and PK Properties During Lead Selection and Lead Optimization – Guidelines, Benchmarks and Rules of Thumb” September 2015. Assay Guidance Manual. https://www.ncbi.nlm.nih.gov/books/NBK326710/
 Zhang, Tang. “Drug metabolism in drug discovery and development” February 2018. Acta Oharmaceutica Sinica B. https://doi.org/10/1016/j.aspb.2018.04.003
 Page 550, Zhang et al. “Preclinical experimental models of drug metabolism and disposition in drug discovery and development.” September 2012. Institute of Materia Medica, Chinese Academy of Medical Sciences. http://dx.doi.org/10.1016/j.aspb.2012.10.004
 Page 555, Zhang et al. “Preclinical experimental models of drug metabolism and disposition in drug discovery and development.” September 2012. Institute of Materia Medica, Chinese Academy of Medical Sciences. http://dx.doi.org/10.1016/j.aspb.2012.10.004[
5] Page 518, Penner, Xu, and Prakash. “Radiolabeled Absorption, Distribution, Metabolism, and Excretion Studies in Drug Development: Why, When, and How?” 2012. Chemical Research in Toxicology, American Chemical Society. http://dx.doi.org/
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