Throughout the many stages of drug discovery, various imaging techniques play an important role in identifying drug targets, testing candidate efficacy, and the drug’s biodistribution and pharmacokinetics.
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Drug discovery
There are various stages of drug development. Firstly, basic research is carried out to understand the pathogenesis of a disease. Inhibition or activation of proteins and pathways that result in a therapeutic effect identifies potential drug targets.
Target identification is usually carried out by high throughput screening. After selecting a range of potential targets, the targets are validated by a range of techniques, such as genetic manipulation and expression profile analysis, to demonstrate that the drug has an effect on the target and resulting in a desirable therapeutic outcome.
Secondary analysis is then carried out. This includes in vitro and ex vivo assays to characterize the drug’s selectivity and liability. Also, in vivo animal models are used to determine the toxicity and pharmacology of the drug. After completing the drug discovery pipeline above, the drug can proceed to clinical trials.
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Imaging techniques
Imaging techniques allow non-invasive visualization, characterization, and quantification of biological events within organisms in response to drugs. These techniques make use of the properties of probes or the tissue itself and are important for disease research and drug discoveries.
Imaging relies on developing agents that are selectively taken up or retained by the biological process of interest. And this agent generates a signal that can be visualized by radiography, ultrasound, magnetic resonance, and optical (bioluminescence or fluorescence) imaging.
Target identification
Identifying pathways previously unknown to contribute to disease pathogenesis catalyzes drug discovery. Receptors and protein expressions on cells can be up or downregulated in diseases.
Labeling receptors proposed to contribute to diseases and visualizing them via imaging techniques can provide insight into potential drug targets.
For example, in the context of atherosclerosis, imaging phosphatidylserine expression on macrophages using annexin-V probes are used to detect macrophage program cell death in atherosclerosis plaques.
This is valuable for discovering abnormalities in cell membrane protein expression and its association with the onset of the disease. This is an example of optical imaging where annexin-V emits fluorescence and helps visualize the biological process leading to the onset of diseases, and hence paves the way for drug discovery.
Also, positron emission tomography (PET) imaging of a chemical (18F-fluoromisonidazole) which is retained inside of cells in hypoxic environments sheds light on the hypoxic microenvironment in atherosclerosis. This provides crucial information on designing the effectiveness of a drug in such environments.
Testing candidate efficacy
To characterize on-target effects and proof mechanisms of actions of drugs, imaging techniques are used to demonstrate the effects of a drug on the biological process targeted by the drug.
An example of optical imaging in testing drug candidates is by transfecting a luminescent protein or enzyme into cells used for drug screening. In cancer cells, the activity of tyrosine kinase is suggested to play a role in the pathophysiology of cancer.
When tyrosine kinase is tagged with a luminescent protein, luciferase, higher luminescent shows tyrosine kinase activity and vice versa. When the cell is treated with a range of potential tyrosine kinase inhibitors, drugs that induce a low luciferase luminescent signal are selected for further testing.
Moreover, magnetic resonance imaging visualizing nanoparticles that are taken up by plaques in atherosclerosis can show the sizes and locations of plaques before and after treatment.
Hence, the efficacy of the drug can be deduced from treatment-related plaque reduction.
Biodistribution and pharmacokinetics
The distribution, absorption, metabolism, and excretion of a drug in a biological system has to be examined to understand drug safety, dosing, and the probability of therapeutic success.
Most commonly, radionucleotide imaging with PET allows visualization of the whole-body distribution of the drug candidate. One of the advantages would be that labeling the drug does not change its properties.
For example, in neurological diseases, PET imaging is used to examine the transport of drugs across the blood-brain barrier. This can also provide information on brain receptor occupancy to adjust the dosing of the drug.
Another strategy is to use agents that are activated in the presence of a drug or by a specific interaction of two proteins. This is usually done by a bioluminescence protein split into two subunits, and only in the presence of the drug would the reporter protein change in conformation and produce a signal. This allows imaging of the drug biodistribution.
References:
- Lindner Jonathan R. & Jeanne, L. (2018) Molecular Imaging in Drug Discovery and Development. Circulation: Cardiovascular Imaging. 11 (2), e005355.
- Kalimuthu, S., Jeong, J. H., Oh, J. M., & Ahn, B. C. (2017). Drug Discovery by Molecular Imaging and Monitoring Therapy Response in Lymphoma. International journal of molecular sciences, 18(8), 1639. https://doi.org/10.3390/ijms18081639
- Hughes, J. P., Rees, S., Kalindjian, S. B., & Philpott, K. L. (2011). Principles of early drug discovery. British journal of pharmacology, 162(6), 1239–1249. https://doi.org/10.1111/j.1476-5381.2010.01127.x
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