Abstract 3736: Application of circulating tumor cells and circulating cell-free DNA to assess the pharmacodynamic response to chemotherapy in xenograft models

Circulating tumor cells (CTCs) are found in various cancer patients and are a prognostic factor for survival in some tumor indications. Circulating cell-free DNA is elevated in cancer patients and has been shown to decrease in response to treatments that result in clinical benefit in cancer patients. CTCs and circulating DNA have the potential to serve as minimally invasive methods that allow monitoring response of tumor to anticancer treatment. Our objectives were to 1) establish analytical methods for enumeration and characterization of CTCs from human xenografts in mice, and 2) evaluate CTCs and circulating DNA as pharmacodynamic (PD) markers/proof of mechanism biomarkers for anticancer therapy in xenograft models. Methods: the Veridex CellSearch™ method was modified for quantifying CTCs (defined as nucleated cells positive for cytokeratin but negative for mouse CD45) in mouse whole blood. The assay was characterized and validated by spiking humor tumor cells into mouse whole blood. Additional markers of Ki-67 and cleaved caspase 3 were added to the assay for assessment of proliferation and apoptosis of CTCs. Circulating nucleosomal DNA was measured in plasma by anti-histone H3 ELISA assay. Results: CTCs were detectable in blood from mice with tumor xenografts of PC3-M, MDA-MB-468, and H358. The number of CTCs was not correlated with the volume of the primary tumor. To evaluate changes of CTCs and circulating DNA in response to anticancer therapy, mice with tumor xenografts were treated with taxotere (0, 2.5, 7.5 or 15 mg/kg) to assess dose response or following a single dose (15 mg/kg) to assess response kinetics at 1, 3, 5 and 8 days. Treatment of tumor-bearing mice with taxotere increased the total number of CTCs but decreased the percentage of proliferating Ki-67 positive CTCs, which was consistent with decreased levels of Ki-67 positive cells observed in tumor sections. The increase in the number of CTCs was transient, peaking on day 3 and then returning to a low level by day 5 post-treatment. Circulating DNA levels were significantly elevated in tumor-bearing mice as compared with naive mice and were positively correlated with tumor growth. Taxotere treatment further increased the level of circulating DNA in a dose-dependent manner. The elevation of the level of circulating DNA peaked on day 5 and returned to pre-treatment level on day 8. These results suggest that the release of tumor cells into circulation with decreased proliferative potential combined with increased circulating cell-free DNA may represent markers for the early response to effective cancer treatments. Studies in mice support the use of CTCs, particularly if characterized for proliferative potential, in addition to circulating DNA, as potential early response/proof of mechanism biomarkers for anticancer drug candidates and warrant further evaluation in human clinical studies. 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 3736.