Targeting Acidity in Pancreatic Adenocarcinoma: Multispectral Optoacoustic Tomography Detects pH-Low Insertion Peptide Probes In Vivo

Abstract

Background pH-low Insertion Peptides (pHLIPs) can serve as a targeting moiety that enables pH-sensitive probes to detect solid tumors. Using these probes in conjunction with multispectral optoacoustic tomography (MSOT) is a promising approach to improve imaging for pancreatic cancer. Methods A pH-sensitive pHLIP (V7) was conjugated to 750 NIR fluorescent dye and evaluated as a targeted probe for pancreatic adenocarcinoma. The pH-insensitive K7 pHLIP served as an untargeted control. Probe binding was assessed in vitro at pH 7.4, 6.8, and 6.6 using human pancreatic cell lines S2VP10 and S2013. Using MSOT, semi-quantitative probe accumulation was then assessed in vivo with a murine orthotopic pancreatic adenocarcinoma model. Results In vitro, the V7–750 probe demonstrated significantly higher fluorescence at pH 6.6 compared to pH 7.4 (S2VP10, p=0.0119; S2013, p=0.0160), while no difference was observed with the K7–750 control (S2VP10, p=0.8783; S2013, p=0.921). In the in vivo S2VP10 model, V7–750 probe resulted in 782.5 MSOT a.u. signal compared to 5.3 MSOT a.u. in K7–750 control in tumor (p= 0.0001). Similarly, V7–750 probe signal was 578.3 MSOT a.u. in the S2013 model compared to K7–750 signal at 5.1 MSOT a.u. (p=0.0005). There was minimal off-target accumulation of the V7–750 probe within the liver or kidney, and probe distribution was confirmed with ex vivo imaging. Conclusion Compared to pH-insensitive controls, V7–750 pH-sensitive probe specifically targets pancreatic adenocarcinoma, and has minimal off-target accumulation. The non-invasive detection of pH-targeted probes by means of MSOT represents a promising modality to improve the detection and monitoring of pancreatic cancer.

Commentary on Photoacoustic Tomography

“…Bohndiek et al evaluated orthotopic ovarian tumors, emphasizing the depth resolution of the technique. Indeed, another popular site for PAT investigations has been orthotopic pancreatic tumors (11).…”

Current and Emerging Clinical Applications of Multispectral Optoacoustic Tomography (MSOT) in Oncology

“…Alternatively, optical imaging provides functional information and the ability to simultaneously detect multiple biomarkers as contrast agents; however, these approaches are restricted to superficial detection as light scattering degrades the spatial resolution at increased penetration depths. Multispectral optoacoustic tomography (MSOT) is emerging as an alternative modality that is not restricted by many of the limitations of the imaging used in the diagnosis and treatment of diseases (1, 2). MSOT was initially developed for research (1, 3, 4), but has been adapted for clinical uses (2).…”

“…Multispectral optoacoustic tomography (MSOT) is emerging as an alternative modality that is not restricted by many of the limitations of the imaging used in the diagnosis and treatment of diseases (1, 2). MSOT was initially developed for research (1, 3, 4), but has been adapted for clinical uses (2). …”

An in vitro study on the detection of ovarian cancer using a novel pH-specific targeting polypyrrole nanocontrasting agent.

“…However, with the need for further improvement of image resolution and quality, optoacoustic imaging has emerged as a promising novel biomedical imaging modality [10,11]. Unlike nuclear imaging, optoacoustic imaging does not solely utilize radio-labels and radioactive material [12] as a means to trace a contrasting agent, rather it emits light energy which then reflects off of the contrast agent and produces a sound wave which is then recorded by an ultrasonic transducer allowing for an ultrasound image with higher resolution [13][14][15][16].…”

“…Also, it is able to distinguish various contrasting agents already present within the tissue, e.g. hemoglobin [16,18]. Due to optoacoustic imaging's ability to detect absorbing molecules at depth, it provides a means of detection for absorbing nanoparticles which would allow for determination of the overall accumulation and biodistribution in real time.…”