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An introduction to optoacoustic imaging in drug development

Milou Noltes, MD, PhD

Just like nuclear and fluorescent imaging, you can use optoacoustic imaging in drug development. Especially, for monitoring your drug's delivery and therapeutic effects in your target population. So, how does it exactly work?

The optoacoustic effect.

Optoacoustic imaging is a non-invasive imaging technique based on the optoacoustic effect. The optoacoustic effect is the formation of sound waves as a result of light absorption in a material sample.

This effect was first discovered by Alexander Graham Bell in 1880 when he experimented with long-distance sound transmission. When experimenting with the “photophone”, Bell discovered that sound waves were produced directly from a solid sample when exposed to a beam of sunlight that was rapidly interrupted.

Alexander Graham Bell's photo phone lays the basis of optoacoustic imaging

Alexander Graham Bell’s photo phone | Universal History Archive

How does optoacoustic imaging work?

In optoacoustic imaging, you use non-ionizing laser pulses to illuminate your tissue of interest. For example, a tumor. The tissue absorbs the light pulse and converts it into heat. In turn this results in a temperature rise and subsequently thermal expansion of the tissue.

Because you use a pulsating laser the tissue repeatedly expands and shrinks. This leads to the formation of pressure (sound) waves. You can then use an ultrasound transducer to detect these sound waves and converts them into an image. In other words, it converts absorbed optical energy in acoustic energy.

Different tissue types absorb light at different wavelengths. This makes it possible for you to visualize different tissue components (e.g., hemoglobin, deoxyhemoglobin, lipid, melanin, and water) or exogenous contrast agents. You can also add ultrasound. Ultrasound provides you with the anatomical structures and optoacoustic imaging the molecular information about tissue composition and functionality.

Optoacoustic imaging versus nuclear and fluorescent imaging.

The main advantage of optoacoustic imaging is that it is non-invasive. At the same time, you avoid using ionizing radiation while still getting high resolution and strong molecular contrast in real-time with deep penetration depth.

As opposed to fluorescence imaging, the optoacoustic technique achieves contrast without the need for exogenous dye administration. Additionally, with you can achieve a deeper penetration depth and less scattering compared to fluorescence imaging.

However, in comparison with nuclear and fluorescence imaging modalities optoacoustic imaging results are more difficult to analyze and quantify.

Optoacoustic imaging in drug development.

Besides diagnosing, you can also use optoacoustic imaging for monitoring drug delivery and therapeutic effects. A current challenge in drug development is to monitor the biodistribution and pharmacokinetics in a non-invasive manner in real-time. Currently, PET and CT are often used for this goal. However, this way you expose your patients to ionizing radiation.

Recent research shows that optoacoustic imaging has the potential to monitor drug delivery without ionizing radiation. For example, in 2019 Jeevarathinam et al. used the imaging modality to image the anticoagulant drug heparin. The authors suggest that the optoacoustic technique monitors both heparin concentration and activity. In the future, if this technology would be incorporated in an implantable catheter, you could measure real-time heparin concentrations in vivo.

Additionally, optoacoustic imaging is also able to monitor therapeutic effects. For example, in oncology you can monitor the effect of trebanabib, a drug that disrupts angiogenesis. Angiogenesis is the formation of new blood vessels and is often related to the progression of a tumor. In 2015, Bohndiek et al. showed that optoacoustic imaging could monitor the anti-angiogenesis efficacy of trebanabib. To do this they monitored the tumor hemoglobin concentration and oxygenation in a mouse model with ovarian cancer.

Optoacoustic imaging provides you with an early readout of your drug’s response. It is expected that with development of novel optoacoustic tracers, sensitivity will be enhanced for the detection of optoacoustic-labelled drugs.

The capabilities of optoacoustic imaging keep growing rapidly. It holds great promise for drug development. All thanks to the clinical appeal and easy application of the modality. Want to explore if it suits your drug development? Contact us here.

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