Demographic and disease characteristics of the 235 patients evaluated in this analysis are reported in Table . The median age of patients was 53 years (range 28 to 82 years). One hundred and fifty-one patients had hormone receptor-positive disease. Visceral metastases were detected in approximately 60% of patients. The baseline value of CTCs per 7.5 ml blood was < 5 in 141 (60%) and ≥ 5 in 94 (40%) patients. A higher percentage of patients with ≥ 5 CTCs at baseline had three or more metastatic sites of disease at baseline (45.7% vs. 29.8%, P = 0.011). No statistically significant difference in metastatic site (visceral vs. other) or in the distribution of immunohistochemistry-defined molecular subtypes was observed according to CTC value.
Patient characteristics stratified by baseline circulating tumor cell value
All patients received first-line systemic therapy for newly diagnosed MBC (Table ). Forty-seven patients (20%) received endocrine therapy, 109 (46.4%) were treated with chemotherapy alone, 39 (16.6%) received bevacizumab associated with taxane-based chemotherapy, and 40 patients (17%) with HER-2-overexpressed/amplified disease received chemotherapy combined with a HER-2 targeting drug, including trastuzumab and lapatinib.
Prognostic value of circulating tumor cells in the overall population
The median follow-up for all patients and for patients still alive was 17 months and 18 months, respectively. At the time of the analysis, 87 patients (37%) had died and 179 patients (76%) experienced disease progression. We found a remarkable correlation between the baseline value of CTCs and the outcome of all patients. The median PFS was 12.0 months (95% CI = 9.6 to 14.3) for patients with CTCs < 5 and 7.0 months (95% CI = 5.8 to 8.1) for those with CTCs ≥ 5 (log-rank P < 0.001). The median OS was 40.1 months (95% CI = 34.9 to 45.4) for women with a low CTC and 21.9 months (95% CI = 15.5 to 28.3) for those with CTCs ≥ 5 (log-rank P < 0.001; Figure ).
Figure 1 Prognostic value of circulating tumor cells in the overall population. Estimated (a) progression-free survival and (b) overall survival according to baseline circulating tumor cell (CTC) value (< 5 vs. ≥ 5) in the overall population. C.I., (more ...)
Furthermore, in multivariate analysis the baseline count of CTCs was confirmed to be an independent predictor of PFS and OS, regardless of hormone receptor status, HER-2 status, location or number of metastatic sites (Table ).
Multivariable Cox proportional hazards model
Effect of different treatments on the circulating tumor cell detection rate
We analyzed the effect of different systemic treatments on CTC count. For 144 patients (61%), a follow-up evaluation of CTCs was available. The median time between baseline and follow-up CTC evaluations was 10 weeks (15% of the patients had a follow-up count within week 6 from baseline, 53% from week 6 to week 12, 22% from week 13 to week 19, and 10% from week 20 to week 45). The effect of chemotherapy plus bevacizumab and chemotherapy plus HER-2-targeting drugs in patients with a high baseline CTC count was considerable, with a reduction of CTC number to below the threshold of 5 in 16 out of 17 (94%) and in nine out of nine (100%) subjects, respectively. Instead, chemotherapy or endocrine therapy were associated with a CTC reduction in 16 out of 40 cases (40%). Specifically, endocrine treatment was able to reduce CTCs under the threshold of 5 only in one out of 10 (10%) patients - whereas chemotherapy alone had a more pronounced effect, inducing a reduction of CTCs to < 5 in 15 out of 30 cases (50%).
Prognostic value of circulating tumor cells according to different first-line treatments
The differential ability of each modality of treatment to reduce the CTC number led us to evaluate whether the most effective therapies could impact the negative prognostic value associated with a high count of CTCs. We evaluated the CTC prognostic value in all treatment groups, including endocrine therapy, chemotherapy alone, chemotherapy plus bevacizumab, and chemotherapy plus HER-2-targeting drugs (Figure ).
Figure 2 Prognostic value of circulating tumor cells according with different first-line treatments. Estimated progression-free survival and overall survival according to baseline circulating tumor cell (CTC) value (< 5 vs. ≥ 5) in patients receiving (more ...)
CTCs remained a strong prognostic indicator in patients receiving endocrine treatment or chemotherapy alone. In both of these treatment groups, a high CTC count was associated with poor outcome, particularly in patients receiving endocrine therapy (median PFS 14.1 vs. 3.5 months for subjects with CTC < 5 and ≥ 5, respectively; log-rank P = 0.001; Figure ). Even when considering only the most effective chemotherapy regimens, including either a taxane as a single agent or in combination with other chemotherapeutic agents (that is, anthracyclines, capecitabine, gemcitabine, carboplatin), chemotherapy administered without any biologically targeted agent did not impact the negative prognostic value of CTCs (median PFS 12.6 months for patients with CTCs < 5 and 7.1 for those with CTCs ≥ 5, P = 0.03). Conversely, in patients with HER-2-overexpressed/amplified disease treated with an anti-HER-2-based treatment (trastuzumab n = 30, lapatinib n = 10), the prognostic value of CTCs was no longer sustainable as subjects with baseline CTCs ≥ 5 received a dramatic survival benefit from this therapy (median PFS 16.1 months, 95% CI = 4.1 to 28.1 months; Figure ). Furthermore, in women receiving taxane-based chemotherapy plus bevacizumab, CTCs < 5 were associated with neither a statistically significantly longer PFS nor OS in comparison with CTCs ≥ 5 (median PFS 9.6 months and 7.3 months for patients with CTCs < 5 and ≥ 5, respectively, P = 0.481; Figure ), suggesting a therapeutic benefit confined to the worse prognostic group.
Predictive value of circulating tumor cells
Of the 148 patients with HER-2 normal disease who were treated with chemotherapy, 64 (43.2%) received combination chemotherapy, 45 (30.4%) received single-agent chemotherapy, and 39 (26.4%) were treated with chemotherapy plus bevacizumab (Table ). Those treatments were selected according to patient characteristics (such as age, co-morbidity) and to the traditional predictive markers in use at the time of therapy administration.
We sought to explore a hypothetical predictive value for CTCs, comparing different treatments (combination chemotherapy versus monochemotherapy, and monochemotherapy plus bevacizumab vs. monochemotherapy alone) in patients with low (< 5) or high (≥ 5) baseline CTC counts. Combination chemotherapy was superior to single-agent chemotherapy, in terms of PFS, in both CTC groups, although the benefit provided by combination regimens was primarily confined to patients with CTCs ≥ 5 (test for heterogeneity P = 0.01; Figure ). With respect to OS, combination chemotherapy was superior to monochemotherapy only in patients with CTCs ≥ 5, but the heterogeneity between the two subgroups was not statistically significant (test for heterogeneity P = 0.16; Figure ). Furthermore, the association of chemotherapy with bevacizumab was superior to monochemotherapy, regarding PFS, but only in patients with a high baseline CTC count (hazard ratio = 0.88, 95% CI = 0.42 to 1.83 in patients with CTCs < 5; and hazard ratio = 0.28, 95% CI = 0.12 to 0.64, in those with CTCs ≥ 5; test for heterogeneity P = 0.04; Figure ).
Figure 3 Predictive value of circulating tumor cells. Comparison of different first-line treatments according to circulating tumor cell (CTC) baseline value. (a) Combination chemotherapy (poly-CTx) versus single-agent chemotherapy (mono-CTx). (b) Monochemotherapy (more ...)