Characterization of circulating tumor DNA in advanced prostate cancer

Background: Cell-free DNA (cfDNA) from tumors can be found in the blood of cancer patients. The clinical significance of these circulating nucleic acids is uncertain. The effects of chemotherapy on cfDNA is also not well understood, although tumor lysis may cause significant alterations in the cfDNA profile that could eventually translate into genomics-based blood tests for predicting treatment responses in cancer. Based on this premise, a pilot study was conducted testing the hypothesis that an increase in plasma cfDNA levels accompanies chemotherapy exposure. To explore the use of cfDNA profiling as a potential diagnostic complement to in-vivo molecular imaging, this pilot study was incorporated into a clinical trial of 18F-fluorocholine positron emission tomography (FCH PET) for assessing chemotherapeutic response in hormone refractory prostate cancer (HRPC). Results: Mean cfDNA levels increased significantly from a pre-chemotherapy level of 13.3 ng/ml to 46.8 ng/ml after the first chemotherapy cycle and 50.9 ng/ml after the 3rd chemotherapy cycle (ANOVA p=0.001). Plasma cfDNA fragments at baseline ranged in size from 160-180bp, consistent with DNA from apoptosis. Samples collected after chemotherapy showed additional cfDNA fragments ranging in size from 200bp-7kbs, suggesting DNA contributed by necrotic cells. All baseline samples demonstrated hypermethylation at GSTP1 or RARB2 suggesting that at least some DNA was of tumor origin. Treatment was associated with loss of GSTP1 or RARB2 methylation in 4/8 patients. Normalized plasma cfDNA level and mean tumor SUVmax were significantly correlated (r=-0.50, p=0.01) and lower cfDNA levels at baseline were associated with significant tumor metabolic responses on FCH PET after the 3rd chemotherapy cycle (8.0 ng/ml of cfDNA in PET responders vs. 16.4 ng/ml in PET non-responders, p=0.03). Conclusions: Exposure to docetaxel appears associated with quantitative and epigenetic changes in the plasma cfDNA of patients with HRPC. Preliminary findings support a relationship between cfDNA levels and tumor activity on FCH PET encouraging further exploration of these molecular markers as potential complementary measures of chemotherapeutic response.