Photoacoustic imaging of fibrosarcoma using RGD-Cy 3 as a targeted contrast agent

Photoacoustics (PA) combines the high contrast of optical imaging modalities with the high resolution of clinical ultrasound. Fluorescence mediated tomography (FMT), on the other hand, offers a very high molecular contrast specificity. Therefore, a combination of PA imaging and FMT might be fruitful: e.g. PA imaging could provide valuable a priori information for optical-tomography reconstruction algorithms thereby improving the resolution of FMT. The fusion of PA and FMT will be facilitated by contrast agents that are detectable by both modalities. This study investigates the usage of RGD-Cy 3 as a multimodal contrast agent specific for M21 and HT-1080 tumor cells in vivo (fluorescence reflectance imaging, FRI) and ex vivo (PA). 1.6 mu M of RGD-Cy 3 was Injected into tumor bearing mice (n=4 M21, n=6 HT-1080). The mice were subjected to in vivo FRI. The FRI showed a typical tracer washout time response, however a statistically significant difference between tumor tissue and muscle tissue could not be shown due to auto-fluorescence and hemoglobin absorption. Similar results were obtained 24 h after the injection of the tracer in biodistribution experiments conducted using harvested organs. Subsequently, the organs were PA imaged using a commercial ultrasound system (Sonix RP) and a Nd:YAG laser (532 nm). The kidney exhibited a mean contrast to the noise-floor of 23.97 dB ( +/- 2.65 standard error of the mean (SEM)), the hearts 23.45 dB (+/- 0.63 SEM), and the native tumor 26.09 dB. RGD-Cy 3 labeled tumors showed a gain of 0.26 dB (26.35 dB +/- 0.22 SEM) compared to unlabeled tumors, which was not statistically significant. These results were consistent with in vitro measurements of Cy 3-gelatine mixture phantoms that only showed a 9.4 dB gain compared to the noise-floor for a concentration 81 times higher than in vivo. These findings suggest that RGD-Cy 3 might not be a suitable contrast agent for in vivo PA imaging, although similar fluorochromes like Indocyanine Green were already successfully used for PA imaging.