Background
Breast cancer (BC) is the most common malignancy in women. Once metastasis occurs, most patients become incurable. However, metastatic BC is a heterogeneous disease in which some women survive only a few weeks whereas others survive many years. Hormonal receptor status including estrogen receptor (ER) and progesterone receptor (PR) as well as human epidermal growth factor receptor 2 (HER2) have been shown to predict treatment response and survival [
1]. Patients with metastasis only to bone survive longer than those with visceral metastasis [
2]. Nevertheless, patients with bone metastasis still carry heterogeneous outcomes, and the survival time might become shorter when skeletal-related events (SRE) such as pathological fractures, radiation to bones, spinal cord compression, hypercalcemia, and bone surgery occur to patients [
3]. Biomarkers of bone metabolism are eagerly sought as a way to predict outcome in BC patients with bone metastasis [
3‐
5]. In a recent post hoc analysis from a randomized phase III trial comparing zoledronic acid and pamidronate in patients with bone metastasis, Lipton et al. demonstrated that early normalization of elevated baseline urinary N-telopeptide of type 1 collagen (NTX) levels was associated with longer event-free and overall survival (OS) times in BC patients receiving zoledronic acid[
6,
7]. Our previous studies demonstrated that NTX and tartrate-resistant acid phosphatase 5b (TRACP 5b) have similar clinical performance for the diagnosis of bone metastases and treatment monitoring [
8]. Furthermore, preanalytical variables such as diurnal rhythm and feeding influence NTX and other collagen biomarkers more than TRACP 5b activity [
8]. In most clinical practices it is difficult to standardize the time of blood draw. Therefore TRACP 5b has certain advantages over collagen biomarkers for assessment of bone metabolism.
TRACP 5b is an enzyme secreted by osteoclasts and its activity can be measured specifically in serum by immunoassay[
9]. Serum TRACP 5b activity is a marker of osteoclast number and becomes elevated in the serum of individuals with increased bone turnover rate such as pre-pubertal children and post-menopausal women [
9‐
11]. We and other investigators have found that serum TRACP 5b activity could become elevated in BC patients with bone metastasis because 80% of the bone metastasis lesions caused by BC are osteolytic in nature [
12,
13]. Furthermore elevated serum TRACP 5b activity in BC patients is positively correlated with the extent of bone metastasis [
8,
14]. We have also demonstrated that serum TRACP 5b activity can be potentially used as a marker to monitor treatment response in BC patients with bone metastasis and as an adjunct to bone scintigraphy for the diagnosis and follow-up of bone metastasis [
8,
14]. In the current study we hypothesize that an elevated serum TRACP 5b activity may reflect a higher tumor burden and further signify a worse outcome in BC patients with bone metastasis. Therefore we examine if baseline serum TRACP 5b activity and its interval change after treatment can be used as prognostic markers for overall survival time in BC patients with bone metastasis. Confounding factors including age, tumor markers including carcinoembryonic antigen (CEA) and CA 15.3, ER, HER2 over-expression, and the presence of visceral metastasis were adjusted.
Discussion
The survival of patients with metastatic BC varies from months to several years, and the most common metastatic site is bone. Metastasis to sites other than bone often dramatically shortens survival [
2,
4]. Bone metastasis is often related to ER-positivity and low grade tumor [
16]. Our findings were consistent with previous studies in that ER positivity and bone-only were associated with a longer survival [
17]. Tumor markers have been proven previously to have limited or no prognostic value [
2,
18], and they were not prognostic markers for survival in this study. Specifically HER2 status did not contribute as a prognostic marker perhaps due to the use of trastuzumab in half of our patients.
When marrow is invaded by BC cells, osteoclasts are recruited and differentiated at an increased rate by the production of high amounts of tumor-derived cytokines such as transforming growth factor β (TGF-β), interleukin-6 (IL-6), parathyroid hormone-related protein (PTHrP) [
19]. It is reasonable to hypothesize that a higher tumor burden in the marrow could be associated with higher cytokine levels, which could further result in increased numbers of mature osteoclasts and elevated serum TRACP 5b activity.
We have already shown that serum TRACP 5b activity correlates with extent of bone metastasis in BC patients [
13]. Since a higher tumor burden is related to a poorer survival, we tested the hypothesis that elevated serum TRACP 5b activity, as a measure of extent of bone disease, could be a marker of poorer prognosis in BC patients with bone metastasis. In our study, TRACP 5b activity was a significant prognostic marker for survival along with ER status and bone-only metastasis.
Nevertheless, serum TRACP 5b activity may not be elevated in all BC patients with bone metastasis, only in those with extensive metastasis [
14]. Previous receiver operative characteristic curve analysis showed that, although serum TRACP 5b activity is highly specific for bone metastasis, its sensitivity is only 80% [
12]. Other factors to weaken this association could be whether bone metastasis is osteoblastic, osteolytic or mixed. If the lesions were primarily osteoblastic, serum TRACP 5b activity might be lower than those with osteolytic or mixed lesions (Personal observations by TYC). Baseline bone metabolic rate of individual patients may also confound analysis because age and the presence of osteoporosis have been shown to have impact on serum TRACP 5b activity [
12]. For example, serum TRACP 5b activity may be lower in pre-menopausal patients with bone metastasis. Although we corrected for age, BC and bone metastasis afflicts younger woman in Asia than in the West. Also in this regard, many post-menopausal women take anti-resorptive drugs to preserve bone health, which may cause apoptosis of osteoclasts and decrease the serum TRACP 5b activity [
10]. All these confounding factors may have weakened the power of serum TRACP 5b activity in reflecting the tumor burden. In fact, some patients with a lower serum TRACP 5b activity may actually carry a high tumor burden.
Accordingly, we proposed that only in those patients with higher serum activity can TRACP 5b genuinely reflected higher tumor burden. Whereas in those patients with lower activities, serum TRACP 5b may not be useful as a prognostic factor. However, the real cutoff value to differentiate high from low serum TRACP 5b activity is unknown. Therefore we divided our patients into subgroups arbitrarily by the scale of baseline TRACP 5b activities, i.e. top 1/3 versus remaining 2/3 with the cutoff value of 5.736 U/L. We indeed found that those patients with baseline serum TRACP 5b activity higher than 5.736 U/L had a significantly shorter survival than the remainders.
Interval changes of bone markers after treatment in BC patients with bone metastases have been proposed as potential prognostic factors. Lipton
et al. reported that a persistently elevated urinary NTX level was related to poorer prognosis [
6,
7]. Early normalization of baseline elevated NTX was associated with longer event-free and overall survival times. Additionally, an extremely elevated baseline urinary NTX was associated with increased tumor burden in bone and an extremely aggressive disease. Our results showed that serum TRACP 5b activity interval change might also be a potential prognostic marker, but it needs to be validated independently. In those patients with baseline serum TRACP 5b activities higher than 5.736 U/L, a Δ
maxTRACP 5b greater than LSC after treatment had significantly longer OS compared to the rest of the group after adjustment of other prognostic factors. Serum TRACP 5b activity of more than 5.736 U/L is probably related to a significant volume of cancer cells. Furthermore, when interval change of a biological marker is considered to be of prognostic value for a specific clinical setting, the LSC of a marker should be taken into account while interpreting the results. In our study, the LSC for serum TRACP 5b activity was 38.59% determined from 15 early BC patients. When we dichotomized those patients whose serum TRACP 5b activities were higher than 5.736 U/L into 2 groups based on response, we were able to demonstrate that patients with a Δ
maxTRACP 5b > 38.59% had a significantly longer survival than those without. However, this correlation was not existent when all patients were considered together. Other confounding factors such as ER status and bone-only metastasis correlated more strongly with the OS. Δ
maxTRACP 5b lost its statistical significance as a prognostic marker for survival in the lower tertile groups. Our explanation is that serum TRACP 5b may not reflect the real tumor burden in those patients with lower activities. This may have biased the result when all patients were included. Another possible explanation might be that Δ
maxTRACP 5b only correlated with treatment response, which was not a strong prognostic marker for OS.
There are several limitations of this study. Firstly this is a retrospective study and patients did not receive uniform treatment. Nevertheless, it does reflect the real context of clinical practice, and effects due to specific treatments were not our primary objective. Secondly, the patient numbers are relatively small and the results could not be interpreted by stratification. Thirdly, tissues were not adequate enough or not available for determining ER status in 9 patients and HER2 in 25 patients. Fourthly, the cut-off value of 5.736 U/L was arbitrarily determined by dividing patients into top 1/3 and bottom 2/3 according to the scale of serum TRACP 5b activity. This cut-off may not be the same in different laboratories and with different assays. Fifthly, 19 of the 100 patients in this study did not have serial TRACP 5b data to calculate the association between its interval change and the overall survival time. This may have biased the final results.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
YYW analyzed the clinical information along with laboratory data and drafted the manuscript. CHK performed the statistical analysis. CPY reviewed the pathological slides. TYC designed and oversaw the progress of the study as well as finalized the manuscript. JCY and LTY gave criticisms to this manuscript. AJ set up the TRACP 5b immunoassay, critically reviewed and assisted in writing this manuscript. SHL and HYL carried out the biomarker immunoassays and were responsible for the quality assurance. All authors read and approved the final manuscript.