Investigating the potential toxicity of any new drug is vital for its progress into the marketplace; however, the precise moment at which toxicity studies are performed during the drug development process can ultimately dictate its success or failure.
In vitro toxicity assays are some of the most useful preclinical methods of not only determining a drug’s safety profile but also providing an opportunity for researchers to improve its properties before reaching clinical trials.
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Importance of toxicity studies
The drug discovery process begins with the initial investigation of potential new drugs and ends with its entrance into the marketplace. Although this process may appear to be simple, any single new medication will go through several drug discovery phase requirements for a minimum of ten years before it can be approved for normal clinical use. The drug discovery and development process are long, tedious and costly.
Unfortunately, in the United States, it is estimated that only about 14% of drugs that reach the clinical trial stage will end up getting approved by the Food and Drug Administration (FDA). The main reasons for these drug failures include lack of efficacy and unfavorable safety profiles; therefore, more robust predictive safety assays must be used to reduce unnecessary costs in the drug discovery process.
Improving the role of toxicology
Although drug developers have traditionally used toxicity studies at the earliest stages of the drug development process, research has shown that frontloading these toxicity studies is not sufficient.
Instead, toxicologists have emphasized the importance of integrating toxicity studies throughout multiple stages of the drug discovery process. Rather than retrospectively trying to modify a drug’s structure, researchers could instead proactively alter the design of a novel drug improving their safety in a similar manner to which other drug properties are altered during early drug development.
Generic and customized in vitro assays
The most common and impactful safety concerns that result in the termination of any drug development programs include toxicities of the cardiovascular, liver and central nervous system (CNS). Even so, the specific target of a new drug can largely dictate its adverse effects. For example, drugs that are aimed to treat respiratory diseases can have a higher risk of causing respiratory toxicity.
This variability in a drug’s potential toxicity profile supports the use of both generic and customized in vitro safety screening assays. Generic assays provide an overview of the most significant risk factors of a given drug, such as potential genetic toxicity and carcinogenicity, as well as its secondary pharmacology profile.
Customized in vitro screening assays are instead used to identify project- or target-specific safety questions. For example, since most CNS drugs are specifically designed to enter the brain, bypassing the otherwise impermeable blood-brain barrier (BBB), there is an increased risk that these drugs will distribute to other areas of the CNS and cause undesirable effects.
Furthermore, customized assays can also take specific characteristics of the target patient population into consideration, such as disease pathophysiology, comorbidities, and comedications; all of which can all alter a new drug’s safety profile in the clinical setting.
Desirable in vitro assay properties
To ensure that the in vitro assays used in the small-molecular drug discovery process are valuable, it is crucial that these assays can be performed rapidly without risking any compromise on their throughput capabilities.
More importantly, the predictivity of any in vitro assay to determine its risk assessment for clinical safety must also be well established. Also, an in vitro assay that provides optimal predictivity should not comprise its sensitivity and specificity, as this can lead to unnecessary rejection or safety optimization of new drugs.
Novel in vitro toxicity assays
As previously mentioned, the detection of cardiovascular toxicity is one of the primary reasons for a new drug’s failure in both preclinical and clinical drug development. One of the most common cardiotoxicity events includes a prolongation of the QT interval or arrhythmias.
To improve the specificity of current cardiotoxicity assays, the Comprehensive In vitro Proarrhythmia Assay (CiPA) initiative has successfully allowed researchers to integrate their experimental results into its effects on specific cardiac ion channels and the action potentials in cardiomyocytes to establish more accurate proarrhythmic predictions.
Additionally, recent advancements in the understanding of stem cell-derived cardiomyocytes have allowed researchers to develop more accurate and reproducible in vitro assays that are based on human cells.
Future work in this area hopes to use stem cell-derived cardiomyocytes that have been integrated into three-dimensional structures for an additional layer of a drug safety screening.
Another major cause of drug attrition includes both acute and idiosyncratic drug-induced liver injury (DILI). Traditionally, the in vitro toxicity assays that have been used to identify molecules that carry drug-related DILI risk factors include two-dimensional (2D) cell systems and other standard preclinical models.
Additional assays that can be used for DILI detection include those that investigate a drug’s mitochondrial toxicity and cytotoxicity, in addition to its potential to alter bile acid homeostasis and oxidative stress levels.
Unfortunately, many of these conventional in vitro toxicity models are unable to assess the complex interplay of both drug- and patient-related risk factors for DILI. To this end, several novel hepatotoxicity models and micro physiological liver systems have emerged as potential solutions to such safety screening issues.
Johansson, J., Larsson, M. H., & Hornberg, J. J. (2019). Predictive in vitro toxicology screening to guide chemical design in drug discovery. Current Opinion in Toxicology 15; 99-108. DOI: 10.1016/j.cotox.2019.08.005.