Serial enumeration of circulating tumor cells predicts treatment response and prognosis in metastatic breast cancer: a prospective study in 393 patients

Background: To prospectively assess circulating tumor cell (CTC) status at baseline (CTCBL) and after one cycle of a new line of systemic therapy (CTC1C), and changes from CTCBL to CTC1C (CTC kinetics, CTCKIN) for their utility in predicting response, progression-free (PFS) and overall survival (OS) in metastatic breast cancer (MBC). Methods: CTCBL and CTC1C status was determined as negative (-) or positive (+) for < 5 or >= 5 CTCs/7.5 ml blood using CellSearch (TM) (Veridex). CTCKIN was categorized as favorable (CTC1C-) or unfavorable (CTC1C+). Tumor response was to be assessed every 2-3 months using the Response Evaluation Criteria in Solid Tumors (RECIST) criteria. Statistical analysis focused on the relation between CTC status and CTCKIN, and response, PFS, and OS. Results: 133/393 (34%) patients enrolled were CTCBL+. CTC1C status after one cycle and radiological tumor response were assessed after median (range) periods of 1.2 (0.5-3.2) and 2.9 (0.5-4.8) months, respectively. 57/201 (28%) were CTC1C+. Median [95% confidence interval] PFS and OS (months) were significantly reduced in CTCBL+ vs. CTCBL-patients (PFS 4.7 [3.7-6.1] vs. 7.8 [6.4-9.2]; OS 10.4 [7.9-15.0] vs. 27.2 [22.3-29.9]), and for CTC1C+ vs. CTC1C-patients (PFS 4.3 [3.6-6.0] vs. 8.5 [6.6-10.4]; OS 7.7 [6.4-13.9] vs. 30.6 [22.6-not available]). Unfavorable CTCKIN was significantly associated with progressive disease. Multivariate Cox regression analysis revealed prognostic factors for shorter PFS (CTCBL+, persistent CTCs after one cycle, = 3rd-line therapy, and triple-negative receptor status) and shorter OS (CTCBL+, persistent CTCs after one cycle, bone-and-visceral/local metastases, = 3rd-line therapy, and triple-negative receptor status). Conclusions: CTCBL, CTC1C, and CTCKIN are predictive of outcome in MBC. Serial CTC enumeration is useful in tailoring systemic treatment of MBC.