Quantification of the concentration gradient of biomarkers between ovarian carcinoma interstitial fluid and blood

BACKGROUND: Tumor interstitial fluid (TIF) rather than plasma should be used in cancer biomarker discovery because of the anticipated higher concentration of locally produced proteins in the tumor microenvironment. Nevertheless, the actual TIF-to-plasma gradient of tumor specific proteins has not been quantified. We present the proof-of-concept for the quantification of the postulated gradient between TIF and plasma. METHODS: TIF was collected by centrifugation from serous (n = 19), endometrioid (n = 9) and clear cell (n = 3) ovarian carcinomas with early (n = 15) and late stage (n = 16) disease in grades 1 (n = 2), 2 (n = 8) and 3 (n = 17), and ELISA was used for the determination of CA-125, osteopontin and VEGF-A. RESULTS: All three markers were significantly up-regulated in TIF compared with plasma (p < 0.0001). The TIF-to-plasma ratio of the ovarian cancer biomarker CA-125 ranged from 1.4 to 24,300 (median = 194) and was inversely correlated to stage (p = 0.0006). The cancer related osteopontin and VEGF-A had TIF-to-plasma ratios ranging from 1 to 62 (median = 15) and 2 to 1040 (median = 59), respectively. The ratios were not affected by tumor stage, indicative of more widespread protein expression. CONCLUSION: We present absolute quantitative data on the TIF-to-plasma gradient of selected proteins in the tumor microenvironment, and demonstrate a substantial and stage dependent gradient for CA-125 between TIF and plasma, suggesting a relation between total tumor burden and tissue-to-plasma gradient. GENERAL SIGNIFICANCE: We present novel quantitative data on biomarker concentration in the tumor microenvironment, and a new strategy for biomarker selection, applicable in future biomarker studies.