Gold nanoparticles to aid in determining drug effectiveness

One of the critical steps in drug development is tracking how it gets into target cells and observing their action inside the cells. Therefore, analysis techniques form an essential part of successful drug development.

In collaboration with RIKEN, researchers from Osaka University have come up with a Raman microscopy-based approach for the drug analysis. Their findings, published in ACS Nano, use gold nanoparticles for visualizing small-molecule drugs.

Typically small drug molecules are tracked by attaching them to fluorescent probes, which are visible when exposed to light. A microscope can then be used for real-time visualization. However, there are few drawbacks to this method. The fluorescent molecules can be bulky and can affect the behavior of the drug molecule. Also, some fluorescent molecules can lose fluorescence when over-exposed to light, making long-duration studies difficult.

Alkyne labels are one of the alternatives to a fluorescent tag. Unlike fluorescent molecules, alkynes, composed of carbon-carbon triple bonds, are small and have minimal effect on the small-molecule behavior. Since the arrangement of atoms in alkynes is not found naturally in cells, they provide a highly specific marker.

Alkynes works differently—instead of emitting fluorescence, they produce a Raman signal that can be identified clearly. Raman spectroscopy is commonly used in chemistry to provide a structural fingerprint to identify a molecule. It relies upon a principle called inelastic scattering of photos called Raman scattering.

Even though the alkynes method provides a significant improvement over the fluorescence technique, it is tricky due to the low efficiency of Raman scattering. To solve this problem, researchers have combined alkyne-tagging with the use of gold nanoparticles. Surface-enhanced Raman scattering (SERS) microscopy can stimulate the gold nanoparticles to generate enhanced electric fields. These electric fields boost the alkyne’s Raman signal making them easier to detect.

Time-lapse 3D SERS imaging of small-molecule uptake by live cells. We successfully observed that the SERS signals of alkynes were initially detected around 10?15 min after drug administration, and the number of the signals gradually increased over time. The drug administration concentration was 20 µM. | Image: Osaka University

Gold nanoparticles get absorbed easily by different cells, making this the best technique for broader application. The nanoparticles enter the cell’s lysosome compartments and enhance the signal of the alkyne-tagged molecules as well as provide a surface for the alkynes to interact. The combined effect is a boost in the signal.

Quantitative detection of the number of SERS signals of alkynes at the single-cell level (left figure). The number of SERS signals detected per cell at each administration concentration over time (right). Using the quantitative SERS detection method we successfully observed that the uptake speed greatly depended on the drug concentration. Image: Osaka University

According to the study’s corresponding author Katsumasa Fujita, “Our SERS technique has the potential to be used with a variety of different cell types as well as a virtually limitless number of drug candidates.”

A better understanding of the drug dynamics in real-time is extremely valuable in drug development.

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