Pre-clinical Safety Assessment of Pharmaceutical Lead Candidates

In early drug development, identifying predictable safety issues of pharmaceutical lead candidates is crucial for reducing safety-related attrition later in the process. Approximately 75% of all adverse drug reactions (ADR) have been dose-dependent, which can be predicted on the basis of pharmacology profiling and safety evaluation studies. In this post, we will focus on the initial strategies and list risk assessment methods for some of the most frequent causes of drug termination or withdrawal.

Following physiochemical characterizations and pharmacological profiling of candidate drugs – such as target binding affinity, potency in cellular assays, solubility, stability, and initial exposure data – lead compounds are selected. To address the issue of what to screen for regarding safety profiling, researchers often start with information searches using scientific literature (e.g., PubMed), pathway analysis tools (e.g., Qiagen), and drug approval databases (e.g., Elsevier) to generate an overview of safety aspects that could be associated with the target of interest.

The following are some of the predictive methods used for evaluating safety hazards that most frequently impact drug development:

Liver toxicity

Although most drugs that were withdrawn for liver-related toxicity caused idiosyncratic hepatotoxicity, which largely depends on individual risk factors and is challenging to predict pre-clinically, the current strategy is to identify and eliminate drug-related risk factors including cellular toxicity, mitochondria toxicity, oxidative stress, glutathione depletion, bile transporter, etc.

For example, thiazolidinediones have been associated with mitochondria impairment and contribute to observed hepatotoxicity. To avoid mitochondrial liabilities, routine screens – such as assays measuring mitochondrial respiratory complex activities and permeability transition – need to be implemented during the drug development process.

A few cases of liver toxicity and possibly idiosyncratic toxicity were caused by reactive metabolites. To address this issue, glutathione-binding assays along with genetic toxicity tests can be incorporated into the testing scheme to assess the risk of reactive metabolite formation. When positive results are obtained, quantitative analysis (e.g., lowest toxic dose) can be further pursued.

Cardiology safety

Similar to liver toxicity, cardiology safety is a major cause of ADRs and accounts for approximately 65% of market withdrawals when taken into account with hepatotoxicity. Because of this reason, it is recommended that initial assays be conducted for evaluating the cardiac conduction function and systemic hemodynamic properties of test compounds. By using in vivo models, safety parameters – such as QT interval, arterial blood pressure, cardiac output, changes in morphology, and the presence of arrhythmias – may be included in early testing.

Blocking the hERG potassium channel has been associated with the occurrence of Torsades de Pointes, a rare but potentially life threatening drug-induced ventricular tachyarrhythmia. During the discovery phase, this issue can be explored by performing either patch-camp assays that assess hERG channel function in transfected cells or hERG binding assays that measure the ability of a test compound to compete with a radiolabelled hERG-binding control compound.

Off-target toxicity

Off-target interactions could also be causative for adverse reactions in animal models or clinical studies; identification of off-target interactions of pharmaceutical lead candidates early in the drug discovery process may be essential for reducing ADR incidences. One initial approach – conducting high-throughput affinity assays on a broad range of targets: receptors, ion channels, enzymes, and transporters – would allow researchers to search for off-target sites that are distinct from the intended therapeutic target(s).

To determine whether the positive results from the initial screening could potentially become high-impact events, the characterization can proceed with a more robust evaluation of the dose-response relationship (e.g., IC50) followed by a functional assay to demonstrate whether binding to the protein leads to a functional result (i.e., functional agonist or antagonist).

Because it is impossible to assess the risk of all potential ADRs during the early drug discovery stage, a clear strategy must be in place to appropriately manage pre-clinical toxicity. Early knowledge of the toxicities will enable researchers to build customized testing schemes to discover superior candidates in early development.

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Categories: Toxicology and Pharmacology

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