Impact of apoptotic circulating tumor cells (aCTC) in metastatic breast cancer

PURPOSE: While intact circulating tumor cells (iCTC) have independent negative prognostic impact on patients with metastatic breast cancer (MBC), the prognostic relevance of apoptotic CTC (aCTC) has not been validated in larger patient cohorts. This study assessed aCTC and iCTC statuses at baseline (CTCBL) and CTC kinetics (CTCKIN) as changes from CTCBL to one completed treatment cycle for their utility in predicting response, progression-free survival (PFS), and overall survival (OS) in MBC.

METHODS: Status of iCTC and aCTC was prospectively assessed in 442 patients using the CellSearch™ system. Different cutoffs were analyzed both for iCTC and aCTC (≥5, ≥10, ≥25 and ≥50 CTC/7.5 ml). CTCKIN were characterized by ≥25 % changes in CTC counts.

RESULTS: Numbers of iCTC and aCTC at baseline correlated strongly (r = 0.7). For iCTCBL positive patients, additional detection of aCTCBL had a significant prognostic impact on OS (aCTCBL positive 10.3 vs. aCTCBL negative 16.4 months, p = 0.012). Worst prognosis for OS was observed in patients with ≥50 iCTC/7.5 ml and simultaneously detected aCTC. Determination of aCTCKIN showed stronger discriminating power than iCTCKIN, with higher PFS and OS for the group with decreasing CTCs (PFS 7.7 vs. 6.1; OS 22.2 vs. 16.4).

CONCLUSIONS: Intact and aCTC are predictive of outcome in MBC. Apoptotic CTC counts ≥ 5/7.5 ml in conjunction with iCTC at baseline have an independent unfavorable prognostic impact on OS. Decreasing aCTCKIN at ≥ 5/7.5 ml in serial enumeration is associated with favorable outcome. Therefore, separate enumeration of iCTC and aCTC is useful in tailoring systemic treatment.