Utilization of Nanoparticles for Photoacoustic Chemical Imaging

Abstract

Tumors are known to have unique chemical properties, such as low pH (acidosis), high K+ (hyperkalemia), and low O2 (hypoxia). Tumor acidosis has been known to influence therapeutic activities of chemotherapeutic drugs. Another conventional cancer treatment, radiation therapy, is highly dependent on local oxygen concentrations. Hyperkalemia has been recently reported to suppress the immune response of activated T-cells. It is also believed that the spatial distribution of these analytes and its heterogeneity, are of relevance. Despite the importance of such chemical information on tumors, there are no clinically available tools for “quantitative” pH, K+, or tissue O2 imaging. Here, photoacoustic (PA) imaging is employed to provide chemical imaging of all these target analytes for cancer (pH, O2 and K+). As for pH, we report on an in vivo pH mapping nanotechnology. This subsurface chemical imaging is based on tumor-targeted, pH sensing nanoprobes and multi-wavelength photoacoustic imaging (PAI). The nanotechnology consists of an optical pH indicator, SNARF-5F, 5-(and-6)-Carboxylic Acid, encapsulated into polyacrylamide nanoparticles with surface modification for tumor targeting. Facilitated by multi-wavelength PAI plus a spectral unmixing technique, the accuracy of pH measurement inside the biological environment is not susceptible to the background optical absorption of biomolecules, i.e., hemoglobins. As a result, both the pH levels and the hemodynamic properties across the entire tumor can be quantitatively evaluated with high sensitivity and high spatial resolution in in vivo cancer models. For K+, we extend this technique to ion-selective photoacoustic optodes (ISPAOs) that serve at the same time as fluorescence-based ISOs, and apply it specifically to potassium (K+). However, unfortunately, sensors capable of providing potassium images in vivo are still a future proposition. Here, we prepared an ion-selective potassium nanosensor (NS) aimed at in vivo photoacoustic (PA) chemical imaging of the extracellular environment, while being also capable of fluorescence based intracellular ion-selective imaging. This potassium nanosensor (K+ NS) modulates its optical properties (absorbance and fluorescence) according to the potassium concentration. The K+ NS is capable of measuring potassium, in the range of 1 mM to 100 mM, with high sensitivity and selectivity, by ISPAO based measurements. Also, a near infrared dye surface modified K+ NS allows fluorescence-based potassium sensing in the range of 20 mM to 1 M. The K+ NS serves thus as both PA and fluorescence based nanosensor, with response across the biologically relevant K+ concentrations, from the extracellular 5 mM typical values (through PA imaging) to the intracellular 150 mM typical values (through fluorescence imaging). Lastly, nano-enabled tissue O2 monitoring by PA, called lifetime-based PA (PALT) imaging, was introduced and demonstrated. A known PALT oxygen indicator, Oxyphor G2, is conjugated into polyacrylamide nanoparticles, called G2-PAA NP. The oxygen sensing capability of the G2-PAA NP has been confirmed in vitro and in vivo studies. In an Appendix, we show how to monitor photodynamic therapy (PDT) using the PALT approach to measure the local oxygen depletion as a function of PDT time. Oxygen depletion during PDT is monitored using both oximeter and PALT spectroscopy in vitro. The latter is enabled by theranostic NPs of methylene blue (MB) conjugated PAA, used for both PALT and PDT. This synergistic approach has good potential for personalized medicine.