A model-based assay design to reproduce in vivo patterns of acute drug-induced toxicity
For more than a decade pharmaceutical R&D has been hampered by considerable attrition rates during clinical trials. The main reasons for drug failure is related to the lack of efficacy, limitations with respect to ADME (absorption, distribution, metabolism and excretion) properties, and—in approximately 30% of the cases—unforeseen toxicity (Kola and Landis 2004). The majority of adverse drug reactions observed in the clinical phase refer to organ injuries, e.g. of the cardiovascular system, the liver, the central nervous system and skeletal muscle (Cook et al. 2014). This clearly demonstrates the limited predictive accuracy of current preclinical models such as the rodent bioassay in evaluating repeated dose toxicity for predicting human toxic risks. It has been argued that overall, only 43% of toxic effects in humans may be correctly predicted by applying rodent-based safety evaluation protocols due to the fact that these assays tend to generate relatively large numbers of false negative as well as false positive read outs (Hartung 2009).
Toxicological dose descriptors
Obviously, to some extent inter-species differences in toxicant susceptibility may account for this lack of predictive accuracy of preclinical animal models. This consideration has initiated tremendous global efforts in developing alternative approaches for evaluating chemical safety such as Tox21 in the USA, TG-GATEs in Japan, and, within the EU, amongst others, the SEURAT programme, all of which use in vitro human cell models for developing accurate and non-animal based assays for predicting human organ toxicity risks.
A second argument for explaining the observed lack of predictive capacity of animal toxicity models is attributable to the application of fairly high doses of the compound in rodent studies (the OECD Test Guideline for the 28 days repeated dose toxicity study requires a highest dose level which should induce toxic effects but not yet death or severe suffering) which are unlikely to be reached in patients during clinical trials of new drug candidates, or, upon market introduction, in drug-treated patients. Moreover, it has been criticized that in general, in vitro models for assessing toxicity also tend to apply relatively high incubation concentrations of test compounds which do not reflect blood levels achieved in experimental animals, or in patients, for assessing toxicity (Wambaugh et al. 2015). In order to cope with this discrepancy, currently ongoing EU research programmes, e.g. HeCaToS and EUToxRisk, aim to apply physiologically relevant toxicant doses in vitro calculated from—preferably human—kinetic data of the compounds under investigation.
There are already numerous toxicological dose descriptors such as the maximum plasma concentration (Cmax), the average concentration across time (Caverage) or the area (integral) under the concentration–time curve (AUC) (Muller and Milton 2012) which all compare drug exposure to the intensity of specific adverse events. The establishment of such concentration–response correlations requires the systematic application of escalation studies to characterize the dose-dependent effect of a toxicant. However, toxicological dose descriptors inevitably reflect an underlying experimental setup, for example the drug concentration in the incubation media or the duration of drug exposure in an in vitro assay. In this regard, it should also be noted that toxicological dose descriptors usually quantify pharmacokinetic (PK) drug exposure in the venous plasma which is the routine sampling site in clinical practice. However, these plasma concentrations are only surrogate markers for the actual tissue level where the adverse event ultimately occurs. Depending on the physicochemistry or the biological properties of a particular drug such tissue levels may differ significantly in different organs. Alternatively, physiologically based pharmacokinetic (PBPK) models provide a possibility to describe the physiology of the body at a large level of detail. Different organs are explicitly represented in PBPK models to account for their specific physiological role in drug ADME (Kuepfer et al. 2016). The organs are further subdivided into the intracellular and the interstitial space as well as into blood plasma and red blood cells, respectively. Mass transfer inbetween the different sub-compartments is estimated from physicochemical properties of the drug such as the lipophilicity or the molecular weight. The simulation of drug concentration profiles in specific organ compartments allows predicting the concentration profile in the extracellular environment which corresponds to either the interstitial space of an organ or the incubation media of an assay (Hamon et al. 2015). Notably, in vitro–in vivo correlations are directly possible through this equivalence of drug exposure in the assay and the PBPK model, respectively. Likewise, in vitro dose descriptors may be directly translated to an in vivo situation to allow for the application of pharmacokinetics/pharmacodynamics (PK/PD) concepts (Derendorf and Meibohm 1999). The concept of model-based assay design developed in HeCaToS (Hepatic and Cardiac Toxicity Systems modelling) will be introduced in the following.
Model-based assay design
In summary, the HeCaToS project aims to establish better prediction models for human heart and liver toxicity, by challenging 3D human cardiac and hepatic cell models with physiologically relevant doses of cardio- and hepatotoxicants mimicking in vivo PK profiles. The comprehensive analysis of multi-scale deregulation of cell function through cross-omics approaches compared with molecular data from heart or liver biopsies from patients treated with the same toxicants for model validation can be expected to significantly enhance the relevance and predictivity of in vitro preclinical assays in the near future.
The authors acknowledge financial support by the European Union Seventh Framework Programme HeCaToS (FP7/2007-2013) under the Grant agreement no. 602156.
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