4 Toxicology Approaches in Drug Discovery

The field of toxicology has changed significantly over the last decade, toxicology experts are stepping away from traditional techniques and are shifting towards novel methods for assessing toxicity. More specifically, they are embracing the advancing areas of cell culture, molecular biology and use of computer modelling for toxicological outcome prediction.

“…the most exciting aspects of toxicology today lie in the 21^st century paradigm... The addition of molecular endpoints in toxicological studies brings new insights into mechanistic effects and an improved predictive capability. 21^st century toxicology offers us the potential for more effective, efficient and reliable testing strategies, which has far reaching implications for industries such as pharmaceuticals and biotechnology, as well as for regulatory bodies...” Dr Ulrike Kogel, Senior Scientist, Systems Toxicology, Philip Morris International

There have been remarkable changes in recent years. Here we highlight four approaches used to assess toxicology within the field of drug discovery.

1. Toxicology in vivo  

Preclinical toxicology testing is conducted to determine the organ- and dose- specific effects of an investigational compound.¹ The use of animal models for the purposes of toxicity studies began in the 1920’s¹ but it wasn’t until the ‘thalidomide disaster’ in the 1960’s that the importance and profile of toxicology testing soared. Since then, regulatory agencies have emphasized the requirement for rigorous toxicity testing thorough the drug development process.

Over the past few years there has certainly been a shift towards in vitro toxicity testing as an alternative to in vivo methods. But why? As well as the ethical concerns of using animal models for the purposes of research, there are several additional factors that affect confidence in the validity of in vivo testing.

Firstly, stating the obvious, these models are not the same species as us. Different animals have different defence mechanisms that can lead to the generation of false- or misleading results. Unnatural conditions, human contact, and stress can all alter an animals’ response. Varied physiological responses to these factors may camouflage, accentuate, or trigger an indicator of toxicity.²

Stress is a particularly disruptive variable, able to interfere with critical pathways, ultimately jeopardizing the entire experimental model.¹ The distinct lack of this variable in in vitro models makes them a more reliable tool for predicting potential toxic effects.

2. Toxicology in vitro

“The most significant developments are in the area of advanced in vitro testing models. They are providing us with new, physiologically relevant testing capabilities and have the potential to reduce the necessity for in vivo models, offering more timely results and a deeper understanding of the biological processes underlying toxicity.” Dr Ulrike Kogel, Senior Scientist, Systems Toxicology, Philip Morris International

The development and validation of non-animal methods, namely in vitro and in silico models, have increased in recent years. Basic in vitro testing using two-dimensional cellular assays is already routinely used throughout the drug discovery pipeline, however these do come with limitations.

Three-dimensional systems have a greater potential to accurately reflect physiological systems. The ability to reconstruct human tissue responses using organ-on-a-chip models has revolutionized the way in vitro toxicology testing is conducted.

More traditional static well systems are being dominated by perfused fluidic systems which can mimic physiological conditions in a more sophisticated manner, providing superior representation of the toxicological effects of a compound.

3.Toxicology in silico 

In silico toxicology employs computational techniques to analyze, simulate, or predict the toxicity and ADME of a compound, acting as an alternative to ‘real-life’ (in vitro and in vivo) methods.^3,4 in silico methods are primarily based on quantitative structure–activity relationships (QSARs); the association between a compound’s chemical structure, and its biological activity.

“In association with computational modelling, the increasing sophistication and predictive power of toxicological testing has the potential to provide an improved understanding of adverse effects. QSAR modelling provides tools to predict drug targets as well as their off-targets.” Dr Ulrike Kogel, Senior Scientist, Systems Toxicology, Philip Morris International

In silico models take advantage of datasets generated from high-throughput screening during the drug discovery pipeline. Data is analyzed using bioinformatics and chemoinformatics and ultimately aims to identify and prioritize ‘lead’ compounds.⁴

4. Toxicogenomics

Toxicogenomics combines both toxicology and ‘omics’ technologies, spanning genomics, transcriptomics, proteomics, and metabolomics. These strategies can be used to determine the molecular mechanisms that control responses to toxic compounds.⁵

Genomics: DNA sequencing technologies have advanced significantly, resulting in the construction of many organisms’ genomes, including humans. These ‘genetic catalogues’ have limited usefulness when it comes to toxicology as they do not capture individual genetic variation — the factor ultimately responsible for an individual’s personalized response to a specific compound.

However, in some cases, organisms chosen as preclinical models of human toxicity do not accurately reflect the molecular and cellular responses seen in humans. This divergence could be due to genetic variation. Genome sequencing can establish evolutionary species-specific divergence to determine the biological relevance of a model organism.⁶

Transcriptomics: The sum of an organism’s RNA transcripts is defined as it’s transcriptome.⁷ Two key techniques for the analysis of the transcriptome include microarrays and RNA-Seq. Microarray analysis uses immobilized nucleotide probes that can hybridize labelled RNAs. RNA-Seq uses next generation sequencing to determine the presence and amount of RNA in a sample. RNA-seq enables characterization of a cell’s transcriptomic response upon exposure to compounds, shedding light on potential deleterious effects.

Proteomics and Metabolomics: Key advancements in the areas of mass spectrometry (MS) and nuclear magnetic resonance (NMR) have driven the expansion of proteomics and metabolomics as a means for determining toxicology.⁵ Studying an organism’s proteome allows you to gather cellular insights that are unattainable through genomics studies. Gel-based proteomics and ‘shotgun’ proteomics are the two main methods used.

Metabolomics involves the analysis of small molecules — metabolites. Metabolites can be affected by both environmental and genetic factors.⁸ Due to the dynamic nature of the metabolome, it can be difficult to analyse, multiple analytical technologies are routinely required to obtain adequate insights from metabolomics data.