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Biomedical Engineering pres.

PhD Defense: Joel Tan

Nanoparticle-enabled In Vivo Photoacoustic Molecular Imaging of Cancer

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NOTICE: This PhD defense will be taking place via Blue Jeans. Link below.

Blue Jeans:
Chair: Dr. Xueding Wang

Photoacoustic (PA) imaging is an emerging biomedical imaging modality that combines optical and ultrasound imaging technologies. PA imaging relies on the absorption of electromagnetic energy (usually in the form of visible or near-infrared light) leading to the generation of acoustic waves by thermoelastic expansion, which can be detected with an ultrasound detector. PA imaging can be used to detect endogenous chromophores such as deoxyhemoglobin and oxyhemoglobin, or can be used together with external nanosensors for added functionality. The former is used to measure things like blood oxygenation, while the latter opens up many possibilities for PA imaging, limited only to the availability of optical nanosensors. In this dissertation, I employ the use of PA nanosensors for contrast enhancement and molecular imaging in in vivo small animal cancer models.

In the first section, I introduce a novel PA background reduction technique called the transient triplet differential (TTD) method. The TTD method exploits the fact that phosphorescent dyes possess a triplet state with a unique red-shifted absorption wavelength, distinct from its ordinary singlet state absorption profile. By pumping these dyes into the triplet state and comparing the signal to the unpumped dyes, a differential signal can be obtained which solely originates from these dyes. Since intrinsic chromophores of biological tissue are not able to undergo intersystem crossing and enter the triplet state, the TTD method can facilitate “true” background free molecular imaging by excluding the signals from every other chromophore outside the phosphorescent dye. Here, I demonstrate up to an order of magnitude better sensitivity of the TTD method compared to other existing contrast enhancement techniques in both in vitro experiments and in vivo cancer models.

In the second section, I explore the use of a nanoparticle formulation of a repurposed FDA-approved drug called clofazimine for diagnosis of prostate cancer. Clofazimine nanoparticles have a high optical absorbance at 495 nm and has been known to specifically accumulate in macrophages as they form stable crystal-like inclusions once they are uptaken by macrophages. Due to the presence of tumor associated macrophages, it is expected that clofazimine would accumulate in much higher quantities in the cancerous prostate compared to normal prostates. Here, I show that there was indeed a significantly higher accumulation of clofazimine nanoparticles in cancerous prostates compared to normal prostates in a transgenic mouse model, which was detectable both using histology and ex vivo PA imaging.

In the third and final section, I explore the use of a potassium (K+) nanosensor together with PA imaging in measuring the in vivo K+ distribution in the tumor microenvironment (TME). K+ is the most abundant ion in the body and has recently been shown to be at a significantly higher concentration in the tumor. The reported 5-10 fold elevation (25-50 mM compared to 5 mM) in the tumor has been shown to inhibit immune cell efficacy, and thus immunotherapy. Despite the abundance and importance of K+ in the body, few ways exist to measure it in vivo. In this study, a solvatochromic dye K+ nanoparticle (SDKNP) was used together with PA imaging to quantitatively measure the in vivo distribution of K+ in the TME. Significantly elevated K+ levels were found in the TME, with an average concentration of approximately 29 mM, matching the values found by the previous study. The results were then verified using mass spectrometry.
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