Abstract 873: Correlation of plasma biomarker levels with early-stage tumor viability in an orthotopic ovarian cancer mouse model

Abstract

Abstract Introduction: The detection of proteins or other biomarkers secreted or released (shed) from tumor cells into blood may facilitate early cancer screening, particularly for lethal cancers such as ovarian carcinoma, which typically remains asymptomatic and undetected until advanced stages. But the utility of blood-based assays is confounded because it is unknown whether cancer biomarker levels correlate with viable tumor burden, and it is unclear how highly shed a biomarker must be to enable detection of early (< 1 cm diameter) tumors. No clinical or animal data is available to correlate blood biomarker levels with cancer state, before and after disease diagnosis, for multiple time points in the same subject, without intervention or treatment. Aim: To determine whether cancer blood biomarkers may be used as measures of tumor viability in an early-stage ovarian cancer mouse model. Methods: We genetically engineered human ovarian cancer cell line 2008 to stably coexpress a secretory reporter (secreted alkaline phosphatase, SEAP) not produced endogenously in any living animal, and a bifusion optical imaging reporter (firefly luciferase-eGFP, FL-eGFP) to enable in vivo bioluminescence imaging of tumor viability. We orthotopically implanted as few as 3×103 2008-SEAP-FL-eGFP cells into the ovary of 6-week-old female Nu/Nu mice, and monitored subsequent tumor growth for up to 28 days using serial 25-μl plasma SEAP sampling to assess biomarker shedding, in vivo bioluminescence imaging to quantify tumor viability, and in vivo ultrasound imaging to assess tumor volume. Plasma SEAP concentration was measured using a chemiluminescence assay (Clontech). Mice were sacrificed when tumor volume reached 1-cm diameter (or by Day 28) and tumors were excised for histological analysis. Results and Discussion: This is the first study to show that cancer plasma biomarker measurements may be used as surrogates of tumor viability during the early tumor growth stages in a living animal model. Orthotopic implantation of at least 3×104 transfected cells in the ovary resulted in tumor growth (confirmed by ultrasound imaging) that was detectable by plasma SEAP levels and FL imaging within 4 hours and up to 28 days. Clinical translation of this preclinical work was aided by the use of mathematical modeling, which yielded novel estimates of biomarker shedding rates never before measured in vivo (2×10-5 ng/day/cell) and were consistent with predicted rates from cell culture. Plasma SEAP levels correlated well with increasing FL average radiance during the first 15 days post-implantation (R2=0.86), indicating that a cancer-specific biomarker, when shed highly, may reflect early tumor viability. This integrative mouse model and mathematical modeling approach may be used to further elucidate the biology of cancer biomarker shedding in vivo, thereby helping to define guidelines for selecting better biomarkers for early cancer detection. Citation Format: Sharon S. Hori, Amelie M. Lutz, Ramasamy Paulmurugan, Sanjiv S. Gambhir. Correlation of plasma biomarker levels with early-stage tumor viability in an orthotopic ovarian cancer mouse model. [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 873. doi:10.1158/1538-7445.AM2014-873