Abstract 3920: In vivo tyrosinase reporter gene imaging with multispectral photoacoustic technology.

Abstract

Abstract Photoacoustic imaging is a powerful tool for assessing tumor vasculature and oxygen saturation as well as detecting contrast agents for molecular imaging. Another exciting possibility is to visualize gene expression by using a reporter which produces a photoacoustic contrast agent. Tyrosinase is such a reporter in that its expression results in the production of melanin, which gives a strong photoacoustic signal. Here we describe the use of a commercially available photoacoustic (PA) imaging system (Vevo LAZR, VisualSonics, Toronto) to image inducible gene expression in subcutaneous xenograft tumors using tyrosinase as a reporter gene. The photoacoustic imaging system generated light from a tunable laser (680 - 970 nm) which was delivered through fiber optic bundles integrated into a linear array transducer (LZ-250, fc = 21 MHz), mounted to a linear stepper motor for 3D imaging. Animals (n=3) having MCF-7 xenograft tumors on either flank transfected with (+TYR) or without (-TYR) doxycycline-regulated tyrosinase were imaged before and one week after the induction of tyrosinase expression. 3D images of the tumors were acquired using multiple wavelengths (680, 750, 800, 850, 900 and 950nm) and 2D images were acquired across the entire wavelength range of the laser to generate absorption curves for blood and melanin. To confirm the presence of melanin and distinguish it from the endogenous blood signal, one animal was exsanguinated and the tumor was imaged again. Approximately 1mm-thick slices of the tumors were taken and photographed for visual confirmation of the presence of melanin. Comparison of pre- and post-doxycycline images of +TYR tumors clearly showed enhanced contrast in the tumor which closely matched the absorption spectrum of melanin. This signal persisted upon exsanguination. -TYR tumors showed signal which corresponded with the spectra of oxy and deoxy hemoglobin and changed little over the course of the experiment. Visual inspection of the excised sliced tumors revealed pigmentation in the +TYR tumors which was absent in the -TYR tumors. Here we have shown the ability of the Vevo LAZR photoacoustic imaging system to visualize inducible reporter gene expression in vivo. This has implications for assessing genetically controlled cellular processes and their response to anti-cancer drugs but also for monitoring gene therapy for cancer treatment non-invasively. Citation Format: Andrew Heinmiller, Minalini Lakshman, Dave Bates, Andrew Needles, Catherine Theodoropoulos, Robert J. Paproski, Roger J. Zemp. In vivo tyrosinase reporter gene imaging with multispectral photoacoustic technology. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3920. doi:10.1158/1538-7445.AM2013-3920