Download The Why and How of Absorption, Distribution, Metabolism, Excretion

The Why and How of Absorption, Distribution, Metabolism, Excretion,
and Toxicity Research☆
H van de Waterbeemd, Consultant in Small Molecule Drug Discovery, Saint André, France
B Testa, Pharmacy Dept, Lausanne University Hospital, Lausanne, Switzerland
ã 2013 Elsevier Inc. All rights reserved.
Evolving Paradigm in Drug R&D
The Increasing Role of ADME-Tox in Pharmaceutical R&D
Issues in ADMET
Transporters
Metabolite identification
Drug-drug interactions
Toxicology and safety prediction
Technological Issues
In vitro screening
In silico ADME
Simulations: PBPK and PK/PD
Conclusion
References
Nomenclature
ADMET
CYP
DDI
DMPK
EMEA
FDA
IT
Absorption, distribution, metabolism, excretion
and toxicity
Cytochrome P450
Drug-drug interaction
Drug metabolism and pharmacokinetics
European Agency for the Evaluation of Medicinal
Products
Food and Drug Administration (USA)
Information technology
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2
3
3
3
3
4
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4
4
4
5
5
NCE
PAMPA
PBPK
PD
P-gp
PK
QSAR
R&D
New chemical entity
Parallel artificial membrane permeability assay
Physiologically-based pharmacokinetics
Pharmacodynamics
P-glycoprotein
Pharmacokinetics
Quantitative structure-activity relationship
Research and development
Evolving Paradigm in Drug R&D
Not very long ago, pharmacokinetics, drug metabolism and toxicology of selected clinical candidates were studied mainly during
preclinical and clinical development. In those days the mission of medicinal chemistry was to discover and supply very potent
compounds, with less interest being given to their behavior in the body. However, the R&D paradigm in the pharmaceutical
industry has undergone dramatic changes since the 1970’s and particularly since the mid-1990’s. High-throughput biological
assays were developed which have enabled large series of compounds to be screened. This was driven by the increasing sizes of
proprietary depositories and the availability of new reagents and detection technologies.
Simultaneously, medicinal chemists have developed new synthetic strategies such as combinatorial chemistry and parallel
synthesis. The number of compounds synthesized increased dramatically. In addition, specialized biotech companies as well as
universities began offering compound collections and focused libraries. As a result, much attention is currently being paid to the
design and/or purchase criteria of lead- and drug-like compounds.1–3 Increasingly this includes considerations on ADME-related
physicochemical properties as well as to ADME properties themselves. The concept of property-based design,4 in addition to
structure-based design where target structures are available, is now commonly used to address ADME issues as early as possible.
Thus the former traditional in vivo animal ADME evaluation could no longer cope with the demand and in vitro ADME screens
became widely used. Despite rapid advances in the use of automation and robotics to increase the throughput of the in vitro ADME
assays,5 screening all available compounds was not necessarily the most efficient and cost-effective strategy. It thus became
reasonable and even essential to develop in silico tools to predict and simulate various physicochemical and ADME properties
and to balance these in decision making processes together with combined in vivo and in vitro approaches (in combo).
☆
Change History: June 2013. H van de Waterbeemd updated the references, deleted figure 3b, deleted the ‘Concept of the Volume’ section and replaced it with a
conclusion.
Reference Module in Chemistry, Molecular Sciences and Chemical Engineering
http://dx.doi.org/10.1016/B978-0-12-409547-2.02586-5
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The Why and How of Absorption, Distribution, Metabolism, Excretion, and Toxicity Research
Figure 1 The two basic modes of interaction between bioactive agents and biological systems, namely pharmacodynamic events (activity and toxicity)
and pharmacokinetic events (ADME) (Reproduced with modifications from Testa, B.; Vistoli, G.; Pedretti, A., Chem. Biodiv. 2005, 2, 1411–1427
with the kind permission of the copyright owner, Verlag Helvetica Chimica Acta in Zurich).
Rigorous analyses of the root causes of attrition during development revealed that lack of efficacy, toxicity, as well as
inappropriate absorption, distribution, metabolism, and excretion (ADME) are among the major determinants of the failure of
candidates.6,7 Lack of efficacy, in addition to insufficient response of the target, may of course be caused by poor absorption,
inadequate distribution and/or rapid metabolism, leading to too low drug concentrations at the target site.8 And since toxicity is
also a major factor of attrition, the development of compounds is often halted even before a detailed human pharmacokinetics or
efficacy study can be performed. Hence, the real impact of ADMET processes remains somewhat hidden in (incomplete) attrition
data. During the 1990’s, it became good practice to collect ADME and toxicity data during the drug discovery stage in order to use
them in decision making to select the best clinical candidates.9,10Today, the drug discovery process has also become strongly
dependent on departments providing data, guidance and insight on issues of drug metabolism and pharmacokinetics (DMPK),
toxicology and safety.
Yet the success of this move to early involvement of DMPK has been questioned. According to some practitioners the main
contribution of a discovery (research) department of drug metabolism and pharmacokinetics has been to enable the design of
pharmacokinetically adequate rather than optimal compounds and thus to make it possible to work on difficult targets.11 Yet
despite all preclinical efforts there will always remain an essential need for extensive clinical pharmacokinetics to lay the ground for
safe prescription once the drug is on the market.11
A bird’s-eye view allows the above historical summary to be summarized in two statements. First and in a more industrial
perspective, it is now entirely clear that ADMET profiling must be initiated as early as possible in the discovery process, using highthroughput and in silico methods characterized by the best possible balance between good relevance to clinical properties on the
one hand, and high speed, efficiency and capacity on the other hand.
Second and in a more fundamental perspective, it took decades for pharmacologists and biologists to realize that there is an
unseverable relation between pharmacodynamic effects (what the drug does to the organism) and pharmacokinetic effects (what
the organism does to the drug) (Figure 1).12–14 For much more than a century, these two components of the interaction between
drug and organism were investigated separately and in complete ignorance of any influence the other component might have. The
importance now given to early ADME screening is a belated recognition the interdependence of pharmacodynamic and pharmacokinetic effects. Indeed, the influence of pharmacokinetic effects on a drug’s actions is common knowledge, be it in the duration
and intensity of these actions, or even in their nature when active metabolites are produced. As for the changes in its disposition
that result directly from a drug’s pharmacodynamic effects, these may be due to modifications in blood flow, gastrointestinal transit
time or enzyme responses, to name a few.
The Increasing Role of ADME-Tox in Pharmaceutical R&D
ADME studies aim at obtaining an early estimate of human pharmacokinetic and metabolic profiles.15 But drug behavior in the
body is a highly complex process involving numerous components, as presented in very simplified form in Figure 2.16 This
diversity and complexity is reflected in the ADMET studies themselves, which include absorption, bioavailability, clearance and its
mechanism, volume of distribution, plasma half-life, involvement of major metabolizing enzymes, nature and level of metabolites,
dose estimates, dose intervals, potential for drug-drug interactions, etc.17,18
Early toxicology and safety studies should weed out compounds before they enter lengthy and costly clinical trials. As a result of
the recent withdrawal of a number of marketed drugs, more pressure is now put on pharmaceutical companies by regulatory
agencies such as the FDA and the EMEA, on safety evaluations including pharmacological and toxicological safety. Investigation of
the potential to cause QT prolongation is now routine. However, the interpretation of data in not straightforward, since many
marketed drugs can prolong the QT interval.19
The increased role of early and preclinical ADME and safety/tox studies has led to an important growth of the supporting
departments and a considerable development of various technologies to address the key issues. A short overview of hot issues is
The Why and How of Absorption, Distribution, Metabolism, Excretion, and Toxicity Research
3
Figure 2 A schematic description of the major processes of drug disposition, showing absorption (passive and active), distribution (passive including
binding, and active including efflux), metabolism (¼ biotransformation), and excretion (passive and active including efflux). Elimination is not
indicated explicitly, since metabolism is chemical elimination and excretion is physical elimination (Reproduced from. Van de Waterbeemd, H.; Testa, B.,
The Why and How of Absorption, Distribution, Metabolism, Excretion, and Toxicity Research. In Comprehensive Medicinal Chemistry II; Taylor, J.B.
and Triggle, D.J. Ed.; Elsevier: Oxford, 2007, Vol.5, Chapter 5.01; pp. 1–9)
given below,20 where we highlight first important challenges in investigating the disposition of new chemical entities (NCEs) and
candidates, followed by some technological issues.
Issues in ADMET
Transporters
Transporter proteins constitute a significant fraction of membrane-bound proteins. They are typically expressed in all organs involved
in the uptake, distribution and elimination of drugs, including the gastrointestinal tract, the blood–brain barrier, the liver and the
kidneys.21 There is hope that in the near future we might get experimental 3D structures of the key transporters. Interaction of drugs
with transporters can alter their behavior in membrane transport, which may result in e.g. active uptake, efflux and rapid elimination.
In other words, the pharmacokinetics of a drug may be influenced by transporters. Apart from a metabolic component (see subsection
Drug-drug interactions), drug-drug and drug-nutrient interactions may involve transporters.22,23 But while the basic knowledge on
transporters is rapidly growing, their real clinical significance remains open to debate.24 A number of P-glycoprotein (P-gp) assays
have been developed, as well as some double- and triple-transfected assays. More transporter assays will soon be available.
The challenge will be to translate the flood of experimental data into relevant information for drug design projects.
Metabolite identification
With increasing resolution of mass spectrometry and NMR, it is now feasible to detect minute amounts of metabolites.25,26 Debate
is ongoing to define major vs. minor metabolites.27 Some metabolites might be pharmacologically active and contribute to the
overall PK. Reactive metabolites28,29 might bind to proteins and cause idiosyncratic reactions.30 The challenge remains to detect
these as early as possible. It has been suggested that time-dependent inhibition should be in standard in vitro screening protocols.31
Good progress has been made in metabolite prediction.32,33
Drug-drug interactions
Regulatory authorities require information to be submitted on the potential for interactions to cause adverse effects. With the
availability of in vitro systems this aspect is therefore often considered at early stages of discovery, including hit evaluation.
Oxidative metabolism by cytochromes P450 (CYPs) is the major route of elimination of most drugs. Since CYPs are also able to
metabolize multiple substrates, their inhibition is the major focus of drug-drug interaction (DDI) studies. Thus, CYP3A4 is not only
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The Why and How of Absorption, Distribution, Metabolism, Excretion, and Toxicity Research
the most abundant hepatic CYP, but is also present in the gut wall and is responsible for the metabolism of 50-60% of all drugs.
This enzyme is therefore highly susceptible to both reversible and irreversible (mechanism-based) inhibition.34Most CYP3A4
substrates or inhibitors are also P-glycoprotein substrates or inhibitors. It is believed that CYP3A4 and P-gp in the gastrointestinal
tract work in concert to limit uptake of xenobiotics including drugs.35 Current inhibition studies are based on Ki and IC50? values,36
but more quantitative approaches would be a benefit.37 Great progress has been made in the reliable simulation of DDIs, even
taking into account variability in the population.38 A further question is how metabolites contribute to DDIs; a better understanding of allosteric kinetics of CYPs is also needed.39
Although clinically somewhat less important than enzyme inhibition, enzyme induction is also an unescapable issue and
adequate protocols are being developed for its characterization. It remains to be agreed when to carry out such assays or screens
during the discovery process.31
Toxicology and safety prediction
Drug safety is a great concern to patients, medical professionals and regulatory bodies. As a result, early toxicity predictions and
safety estimates are receiving ever increasing attention in all drug discovery programs.40 Simple in vitro screening assays for, e.g.
hERG and other cardiac ion channels, genetic toxicology, and cytotoxicity now are routinely added to the growing battery of
biology, ADME and tox/safety screens. Despite encouraging progress,41 in silico predictive toxicology is still in its infancy,42 but gets
funding from e.g. EU Framework projects to underline its importance. A promising tool is the integration of ADME, tox and
pharmacology data to predict side-effects, as in the BioPrint approach.43
Technological Issues
In vitro screening
Based on the experience gained with pharmacodynamic high-throughput screening, many in vitro ADME screens can now run in
medium or high-throughput modes using automation, robotics and miniaturisation.44–46 Physicochemical properties are now
recognized to play a key role in modulating DMPK properties,4,47–49 and their assessment and understanding are therefore
receiving greater attention.50 Due to the nature of many high-throughput physicochemistry and ADME assays, the typical analytical
endpoint is often LC-MS. Cell-based assays such as the Caco-2 screen for permeability/absorption are quite expensive due to
considerable reagent costs, particularly when run in screening mode with many compounds. The trend is to investigate in either
cheaper in vitro alternatives such as PAMPA (parallel artificial membrane permeability assay) method or to move to in silico
approaches. A proper synergistic hybrid combination of in vitro and in silico methods,51 which has been called the in combo
approach,52 may be the most cost-effective approach to ADME screening in drug discovery.
In silico ADME
Prediction and simulation of various ADME properties is considerable cheaper than in vitro screening. Therefore great efforts have
been made to turn all available data into predictive computational models using quantitative structure-activity relationship (QSAR)
methods and molecular modeling.52–58 In vitro data are now generated for many ADME and physicochemical endpoints and can be
used to build more robust models. Model updating will need to be automated and fitted in the data generation cycle.59 There is a
need for both local (project-specific) as well as global (general, encompassing a wide range of chemotypes) models. Unfortunately,
there is still a paucity of human in vivo data, and thus models based upon these will needed to be handled with care. Of course
interindividual variability within the population is another key factor to take into account for human predictions. Predictions will
be ranges rather than hard numbers. Many drug companies have web-based cheminformatics and ADME predictions deployed via
their intranet giving the medicinal chemist easy access to “web screening”.60 For the development of potential drugs, predictions
may contribute to high-throughput pharmaceutics and rational drug delivery.61
Simulations: PBPK and PK/PD
Physiologically-based pharmacokinetic (PBPK) models rely on principles well known in chemical engineering and describe the
human or animal body as a series of pipes and tanks.62,63 Convenient softwares are now available and make PBPK modeling more
accessible to drug discovery and development to predict various PK parameters and concentration-time profiles in the body.
Population variability64 and specific groups such as children and elderly can also be taken into account. The more experimental
data are available, the better the simulations, making these approaches of interest in drug discovery and clinical development.
Some commercial programs have put considerable effort in simulating the absorption process, which is of interest in optimizing
pharmaceutical formulations. Pharmacokinetic/pharmacokinetic (PK/PD) modelling links dose-concentration relationships (PK)
to concentration-effect relationships (PD). This approach helps to simulate the time course of drug effects depending on the dose
regimen.65 Specialized softwares and a better understanding of the applications may help to speed up clinical development. As a
whole the pharmaceutical industry is still far behind in using prediction, modeling and simulation as compared to engineeringbased industries such as car and airplane manufacturers. Often resources still remain allocated to producing experimental data
rather than developing the information technology (IT) tools to move toward in silico pharma, in other words toward web
screening rather than wet screening and to a “will do” rather than a “can do” attitude.66
The Why and How of Absorption, Distribution, Metabolism, Excretion, and Toxicity Research
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Figure 3 The major steps in drug R&D, showing target validation, discovery and development, followed by the clinical phases and the post marketing
phase. Note the overlaps, since many knowledge-expanding investigations are carried out after the next phases have begun, e.g. in vivo research in
animals continues during the clinical phases, and clinical studies continue post-marketing for confirmatory and indication-enlarging purposes.
Preclinical R&D has two objective, activity and ADMET. These translate as efficacy and tolerance in the early clinical phases, to merge subsequently
into the global objective of utility.
Conclusion
ADME-Tox issues were far from the forefront in the early days of drug discovery and development. Innumerable and fast
developments since three decades or more have completely changed the picture, with activity/efficacy assessment running in
parallel with ADMET/tolerance assessment, both lines merging in the global paradigm of ‘utility’ (Figure 3).
In this view, pharmacodynamics (i.e., activity) is obviously the prime object of study, but it is so markedly influenced by
pharmacokinetic events (Figure 1) that drawing a separation line becomes a challenge. This is why both lines of inquiry must run
together, a further condition of success being for research directors and supervisors to share full information with high intensity and
great constancy. This is particularly true for medicinal chemists, who find themselves in progressing in close collaboration with
pharmacologists, biologists, biochemists, bioanalysts, physicochemists, computer scientists and other experts.
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