Abstract 2047: Molecular photoacoustic imaging and serum diagnostics rapidly detect response to angiopoietin 1 and 2 blockade in ovarian cancer
Authors: Bohndiek Sarah E. , Sasportas Laura , Machtaler Steven , Jokerst Jesse V. , Hori Sharon , Gambhir Sanjiv S. 
Published: May 2015

Introduction: There is an urgent clinical need to develop noninvasive biomarkers that early detect tumor responses to anti-angiogenic therapy. Treatment response in ovarian cancer patients is assessed with transvaginal ultrasound imaging of tumor size and CA125 screening. We hypothesized that combining photoacoustic (PA) imaging to non-invasively visualize tumor vascular architecture, with serum diagnostics using endothelial biomarkers, would yield a sensitive and readily translatable approach for monitoring response to anti-angiogenic therapy.

Methods: Ovarian tumors were established in nude mice by orthotopic injection of OV2008 cells at day 0. At day 10 mice were randomized into groups of at least n = 10. Baseline PA imaging and submandibular blood draw were performed, then repeated 24h after dosing with a peptibody that prevents the interaction of secreted angiopoietin 1 (Ang1; agonist) and 2 (Ang2; antagonist) with the receptor tyrosine kinase Tie2, or vehicle, on days 13, 16 and 19. Mice were then sacrificed and tumors excised for histology.

Results: PA imaging uses endogenous hemoglobin light absorption to generate contrast. At 797nm, we found a 1.5-fold increase in hemoglobin-weighted signal in vehicle mice by day 19, while treated mice remained close to the baseline (n=9; p=0.008). The oxyhemoglobin-weighted PA signal (837nm/797nm) was elevated by 20% in treated mice but decreased 15% in vehicle mice (n=9; p&lt;0.01). This indicated treated mice had significantly lower tumor vessel density but improved function of residual vessels. To gain insight into the mechanism of drug action, we measured host serum angiopoietins over the treatment time course. Ang1 in vehicle mice increased by 1.5-fold (n=10; p&lt;0.0001) and was significantly higher than treated mice by day 19 (n=10; p=0.03). Ang2 was identical between treatment groups and healthy control mice (n=10; p=0.65). Elevated Ang1 in the circulation of vehicle mice is consistent with excess Ang1 promoting vascular remodeling and plasticity in tumors, while normalization in treated mice allows vessel stabilization. Immunofluorescence (IF) confirmed this by indicating that CD31 positive vessel density was 30% lower in treated mice but their tumor vessel coverage of desmin-positive pericytes was 2.5-fold higher.

Conclusions: We have shown for the first time that PA imaging and serum angiopoietin levels measure complementary parameters that indicate response to angiopoietin blockade in ovarian cancer. PA imaging noninvasively detected drug-induced vessel regression and normalization, giving a functional readout of response. This modality can be readily combined with transvaginal ultrasound. The drug action was mediated by normalization of mAng1 levels, indicated by serum measures and IF. Importantly, this work supports the combined use of in vitro and in vivo diagnostics to monitor response to anti-angiogenic therapy in clinical trials.

Citation Format: Sarah E. Bohndiek, Laura Sasportas, Steven Machtaler, Jesse V. Jokerst, Sharon Hori, Sanjiv S. Gambhir. Molecular photoacoustic imaging and serum diagnostics rapidly detect response to angiopoietin 1 and 2 blockade in ovarian cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2047. doi:10.1158/1538-7445.AM2014-2047