Tumor imaging in mice revolutionized the ability to detect and quantify tumor burden in pre-clinical studies. However, this technology has been limited for the most part to the use of cancer cell lines manipulated to express specific fluorescence or bioluminescence in xenograft models. Recent advances in the generation of genetically engineered mouse (GEM) models have resulted in notable improvements as platforms for pre-clinical models. Variables such as initial tumor detection, size and location have influenced and limited objectivity in study design and interpretation. In this study we introduce a new modality for determining tumor burden using a commercially available recombinant EGF polypeptide labeled with a near infrared (NIR) fluoroprobe to identify and quantify tumor burden in a knockout mouse model. We evaluated the sensitivity and accuracy EGF-labeled IRDye using our prostate specific PTEN conditional gene-targeting mouse model using intact and castrated mice.

Mice were administered the EGF-labeled IRDye by tail vein injection 72 h prior to imaging in vivo imaging. The genitourinary tact (GUT), consisting of the prostate, urethra, bladder, seminal vesicles and coagulating gland, was collected, weighed, fixed in formalin and imaged and cryosectioned. EGFR expression was detected by immunohistochemistry and expression levels were measured by image analysis. To determine tumor burden, adjacent cryosections were analyzed by NIR fluorescence and by histology for size and distribution of tumor glands in H&E stained slides.

Closed abdomen in vivo imaging was not possible due to low signal and interference of the preputial gland. However, prostates surgically exposed revealed the presence of tumor nests in the prostate as small as 0.2 mm. Furthermore, formalin fixed GUTs retained fluorescence and were imageable up to 48 h post-fixation. EGFR overexpression was found to be concentrated in cancerous glands and expression was directly proportional to the NIR fluorescent signal. Tumor burden determined by GUT weight correlated poorly to NIR expression in castrated mice. Our data shows that EGF-labeled IRDye NIR imaging is a precise and sensitive method for the determination of tumor burden; however, at the present time, this technology is limited to invasive imaging or ex vivo applications. This study is the first to use NIR fluorescence to measure prostate tumor growth and response in GEM and thus provides a feasible alternative to conventional methods.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5246.

©2010 American Association for Cancer Research
2010