ADME – how to get started?

During preclinical drug development, the number of compounds decreases from hundreds or thousands to the one entering the clinical trials. Understanding the ADME properties is essential to guide the decision makers and hence, ADME studies constitute an important part of preclinical drug development. When initiating this line of work for the first time, it may appear a bit puzzling. So, let’s have a simplified overview how get started with small molecules.

To get going with the actual absorption, distribution, metabolism and excretion (ADME) research, some general physicochemical characteristics of the compound, such as solubility, lipophilicity and ionisation properties should be known to enable planning the right conditions for the experiments. These give also an idea about the ADME properties of the compound. As an example, poor solubility is one of the factors limiting intestinal absorption and bioavailability.

Information about drug metabolism is needed at several stages of drug discovery and development and it is often also a good starting point for ADME studies. Results from in vitro drug metabolism studies can be used to predict hepatic clearance and extent of first pass metabolism. The first studies may be set up to screen metabolic stability and to identify metabolic soft spots to aid lead optimisation. Later on, it becomes important to have a deeper look on the metabolite profiles and to understand, which are the metabolite producing enzymes and in what extent they are responsible for the metabolism. In addition, safety of the formed metabolites has to be investigated. It is highly necessary to clarify, also from the regulatory point of view, whether the formed human metabolites have been adequately expressed in the preclinical species used in the safety assessment. Metabolites in safety testing (MIST) process starts with in vitro cross-species comparison of metabolism, to choose the right preclinical species for toxicological evaluation, and continues later by comparing plasma metabolite profiles, i.e. performing the MIST analysis when the first human samples are available from clinical studies.

Limited permeability generates challenges for good bioavailability and target exposure, and hence it is good to be evaluated early enough. The in vitro permeability models offer tools to screen compounds for their overall permeability or to evaluate their intestinal absorption, taking also the role of drug transporters into account. It must be noted that in addition to affecting the intestinal absorption after oral administration, drug transporters play a role in drug distribution and elimination. It is also good to keep in mind that only the unbound compound is able to distribute in the body and interact with the target. Therefore, plasma protein binding should be investigated. Information about the unbound fraction in circulation is also required in various ADME studies, when in vitro data is translated to predict in vivo behavior, or to aid choosing the relevant test concentrations for in vitro drug-drug interaction studies.

Despite comprehensive in vitro research, eventually it becomes necessary to perform animal experiments to provide deeper insight to drug metabolism and pharmacokinetics (DMPK). In in vivo, all the physiological factors contribute to the obtained data, enabling better overall understanding, instead of looking closely one particular aspect in an in vitro assay. Every now and then animal experiments may reveal ADME issues, which were not expected based on the earlier in vitro data, and then again, more laboratory work is required to guide the next steps of the project. Mice and rats are commonly used animal models at the early stages of preclinical drug development for accessing essential ADME properties, such as bioavailability, peak plasma concentration and half-life.

Simultaneous use of several medications increases the risk for drug-drug interactions, which may affect the pharmacokinetics and have clinically relevant outcomes. This is why the potential for drug-drug interactions has to be evaluated. The compound should be studied for its potential to act as an inhibitor or an inducer of the most important drug metabolising enzymes. To evaluate risk of a compound being a victim for drug-drug interactions, it is necessary to clarify which enzymes are responsible for its metabolism. In addition, drug-drug interactions may arise through drug transporters and it is required to evaluate whether the compound is a substrate or an inhibitor of relevant drug transporters. The earliest studies are usually set up for screening of compound series for their potential to inhibit or induce major drug metabolising enzymes, whereas in later stages more comprehensive studies are performed according the regulatory guidelines and the recommendations regarding e.g. the experimental approach, set up and analysis are followed.

In addition to ADME, the importance of toxicological studies cannot be highlighted too much. There are options for toxicity screening, which are useful to reveal possible tox liabilities in early drug discovery and development to support internal decision making. For example, formation of reactive metabolites, geno/cardiotoxicity and mutagenicity are often screened early due to their considerable role in drug safety. Furthermore, as many of the ADME studies utilise living cells, it is important to check that the obtained results are not affected by cytotoxic properties of the compound or its metabolites.

Traditional laboratory work can be complemented by various computer aided predictions and simulations to support drug development are used throughout all the stages. In drug discovery and preclinical development, the earliest ADME and toxicity predictions may be performed while screening compound libraries based on chemical structures. At later stages, as an example, the existing information from in vitro and animal PK studies may be used in physiologically based pharmacokinetic modelling to design first time in human studies.

Taken together, the earlier is the stage of the project, the higher number of compounds are screened. Later on, more comprehensive and regulatory compliant preclinical ADME studies are carried out usually for lower number of compounds, and definite human ADME properties elucidated when the compound has reached the clinical stage. Often some of the studies are performed in parallel, and there is not a definite path to follow, as everything depends on the compound in question and its specific challenges. It may also happen that prior proceeding to another set of studies, some relevant data is required and one needs to go back to e.g. metabolism or drug-drug interaction studies.

In general, the preclinical ADME studies do not have to be performed under GLP conditions but the data must be of high-quality. Using non-GLP quality systems is not only sufficient but also more cost-effective at early stages of drug discovery and development. A good CRO is able to advice how to proceed with the studies or how to dig deeper the research questions, find answers, understand the phenomena behind the results and give advice about the regulatory point of view. The better is the understanding and optimisation of the ADME/DMPK of the compound, the better are the chances for a successful drug development program!

Written by Miia Kovalainen

P.S. If you are interested to learn more about specific ADME-Tox topics, you may want to have a look on our e-books!