Monitoring and Characterization of CTC in Cancer Patients

Concomitant with the rapidly growing number of treatment options for patients with metastatic carcinomas is the need for biomarkers to guide and monitor their use. Tumor cells shed into the blood during metastasis may help meet this need by serving as generic biomarkers for a variety of carcinomas. Indeed, assessment of circulating tumor cells (CTC) has already been shown to provide prognostic information regarding progression free and overall survival in metastatic breast (MBC), colorectal (MCRC), and prostate cancer (MPC). Th is has been made possible by the introduction of the CellTracks® system, which has provided a standardized method for the enumeration and characterization of CTC. In addition to predicting outcomes and monitoring treatment, CTC have also been used to detect the presence of treatment targets. Th e use of CTC as a source of tumor tissue can provide real-time information helpful in selecting specifi c therapies or families of therapies making CTC an invaluable tool in the practice of evidence-based personalized medicine. Background and Materials Th e CellSearchTM system has been used to enumerate CTC in three separate prospective multi-center registration studies involving 177 MBC, 430 MCRC and 231 MPC patients respectively. 7.5 mL of blood was drawn from patients before starting a new line of therapy and at monthly intervals after initiation of therapy1-6. All patients in the MBC and MCRC trials had measurable disease, while all patients in the MPC study had castration resistant disease – defi ned as two consecutive increases in PSA despite standard hormonal management. All patients were enrolled on an Intent-to-Treat basis. Progression Free Survival and Overall Survival were measured from the time of the baseline blood draw to the diagnosis of progression, time of death or last recorded contact. Additional studies were performed to assess the presence of treatment targets on CTC. Clinical and Translational Leads Th e frequency in blood of CTC derived from the primary or metastatic tumors is extremely low. Th e technology developed to detect CTC is based on immunomagnetic enrichment of cells of epithelial origin from 7.5 mL blood in combination with immunofl uorescent labeling followed by semi-automated microscopic identifi cation7,8. Assay characteristics were determined by preclinical testing. Th e CellTracks assay demonstrated linear recovery for 1-1000 cells from carcinoma cell lines spiked in 7.5 mL of blood. Assay specifi city was demonstrated by the absence of tumor cells in the blood of healthy donors and patients with benign diseases9. Author Affi liations: Immunicon Corporation, Huntingdon Valley, Pennsylvania ©2008 American Association for Cancer Research AACR Education Book • http://educationbook.aacrjournals.org 617 Circulating Tumor Cells 618 AACR Annual Meeting 2008 • April 12–16, 2008 Th e relation between the presence of CTC before initiation of therapy and overall survival of the MBC, MCRC and MPC patients is illustrated in Figure 1. Th e white bars in Panels A, B and C show the median overall survival (mOS) for patients with 0 CTC in 7.5 mL of blood, while the gray bars indicate mOS for patients with increasing numbers of CTC. Th e percentage of patients in each group is indicated on top of each bar. Error bars indicate the 95% confi dence interval around the median survival. Th e median survival of patients with no CTC prior to the initiation of therapy is signifi cantly longer compared to those patients with 1 or more CTC. At larger CTC numbers the mOS is further decreasing. While the survival prospects for patients with and without CTC are remarkably similar between the three cancers, the proportion of patients with tumor cells as well as the average number of CTC is quite diff erent. For MBC and MPC a threshold of 5 CTC/7.5 mL and for MCRC a threshold of 3 CTC/ 7.5 mL were used to stratify patients into those with Favorable outcomes (CTC <3 or <5) and those with Unfavorable outcomes (CTC 33 or 35). Th e Kaplan Meier curves in Figure 2 show the probability of overall survival for patients with Favorable and Unfavorable CTC counts at monthly intervals after initiation of therapy. At all time points tested the diff erence in survival between the Favorable and Unfavorable groups is highly signifi cant (logrank p-values <=0.0070). Implications of these fi ndings are that, as CTC predict outcomes at any time during treatment, they can be used to monitor treatment – any treatment. Specifi cally, they suggest that patients with Unfavorable CTC counts after initiation of therapy are on a futile therapy. To answer the question as to whether a change in CTC “after” treatment has begun alters survival prospects, further Kaplan Meier analyses were performed. Patients were divided into four groups: 1) those with CTC that remained Favorable; 2) those that remained Unfavorable; 3) those that changed from favorable to Unfavorable; and 4) patients whose CTC changed from Unfavorable to Favorable during the course of therapy (Figure 3). In CTCs / 7.5mL of Blood M ed ia n S u rv iv al ( M o n th s) Colorectal n=413 Breast n=177 38% 42% 45% 46% 47% 50% 58%