We observed pretreatment thrombocytosis, defined as platelet count ≥ 400,000/mm³, in 18.9% of the patients with epithelial ovarian cancer in stage I-IV, being in line with previous reports of the same cutoff value as ours (7.4–42.5%)(10–12, 15). Our analyses for relationships between thrombocytosis and clinicopathologic parameters showed that thrombocytosis was significantly associated with MHA, primary treatment, FIGO stage, histologic subtype, operation achievement, non-malignant inflammatory condition, CA125, and TFI (Tables 2 and 3). Among these significant factors, FIGO stage, CA125, operation achievement, and primary treatment are considered to reflect the tumor extent, which has been reportedly associated with pretreatment thrombocytosis(6, 20). MHA and non-malignant inflammatory condition are clinically well known to induce thrombocytosis. We subsequently conducted the univariate and multivariate analyses for associations with thrombocytosis in the patients who relapsed after adjuvant chemotherapy, excluding the 2 factors, CA125 and primary treatment, which are considered to be closely related with FIGO stage. We found that MHA and TFI were significantly and independently associated with thrombocytosis (Table 3). Accordingly, thrombocytosis is suggested to possibly contribute to chemoresistance, as TFI is known to be an important surrogate marker for chemosensitivity of ovarian cancer(21–23). As regards MHA, iron deficiency anemia caused by intratumoral hemorrhage in ovarian cancer is likely to be involved.
Our survival analyses showed that patients with thrombocytosis had worse PFS and OS compared to those without thrombocytosis (Figs. 1A, B). Besides, when the analysis was confined to the stage-III/IV patients, there was still significant difference in PFS and OS (Figs. 2A, B), whereas the stage-I/II patients showed no difference in survival according to presence/absence of pretreatment thrombocytosis (Figs. 2C, D). These findings indicate that thrombocytosis affects survival mainly in advanced diseases, being consistent with our above finding that thrombocytosis was significantly and independently associated with TFI, an established predictor for chemosensitivity in recurrence therapies, as recurrence is prone to occur in advanced diseases. Furthermore, our multivariate analysis for prognostic factors demonstrated that thrombocytosis was significant for unfavorable PFS and OS independently of age, histology, and FIGO stage (Table 4). These findings indicate that pretreatment thrombocytosis may be an ideal predictive biomarker for treatment outcome and a reasonable therapeutic target in epithelial ovarian cancer.
Tumor cells firstly increase and activate platelets via various cytokines including interleukin-6 (IL-6)(16). Activated platelets in turn facilitate tumor growth and angiogenesis through growth factors and angiogenic factors including VEGF and PDGF(16, 24). Activated platelets also promote metastasis through epithelial mesenchymal transition (EMT) and defense by platelet-tumor interaction against blood flow and immune system including NK cells in circulation(16, 24). Besides, platelets contribute to chemoresistance through MAPK and PI3-kinase/Akt pathways and drug efflux proteins(24). Therefore, thrombocytosis can possibly affect patient prognosis via both tumor progression and chemoresistance. However, we found that thrombocytosis was significantly and independently associated with TFI, but not with FIGO stage (Table 3), and that thrombocytosis was significantly associated with PFS independently of FIGO stage (Table 4). These findings suggest that prognostic impact of thrombocytosis may be independent of tumor extent but rather attributed to chemoresistance. Indeed, platelets have been reported to be involved in chemoresistance in ovarian cancer by basic studies in vitro and in vivo. Radziwon-Balicka et al. reported that platelets decreased paclitaxel-induced apoptosis of human ovarian adenocarcinoma cells in vitro(25). Bottsford-Miller et al. reported that combined administration of platelet-depleting antibody with docetaxel caused 62% decrease in tumor weight compared to docetaxel treatment in orthotopic mouse models of human ovarian cancer(6). They further found that platelet transfusion blocked the effect of docetaxel on tumor growth, and aspirinization blocked the effect of transfusion. However, clinical evidence suggesting the link between thrombocytosis and chemoresistance in ovarian cancer is very few, as most studies only correlated thrombocytosis with survival after chemotherapy. Bottsford-Miller et al. reported on change of platelet count during first-line chemotherapy in the responsive and refractory groups matched for stage, histology, grade, and primary therapy(26). In patients with durable response, only 50% had pretreatment thrombocytosis and all of them achieved normal platelet count during therapy, whereas all had pretreatment thrombocytosis and only 50% of them achieved normal count during therapy in patients with refractory disease. However, the possibility that platelet count only reflects the real-time residual tumor amount cannot be excluded. Feng et al. reported that preoperative thrombocytosis was significantly associated with chemoresistance determined based on the interval between disease progression and adjuvant chemotherapy in high-grade serous ovarian cancer(20). However, thrombocytosis was not significant after stratification based on residual tumor after surgery. In our study, pretreatment thrombocytosis was not associated with operation achievement, and was significantly associated with TFI independently of FIGO stage (Table 3). Moreover, pretreatment thrombocytosis was a significant prognostic factor for poor PFS and OS independently of FIGO stage and operation achievement (Table 4). These observations strongly support the involvement of thrombocytosis in chemoresistance, implicating that molecular therapy targeting thrombocytosis may improve prognosis via attenuating chemoresistance. Based on the current findings, we assume that combination of chemotherapeutics and antiplatelet therapies may be efficacious for the ovarian cancer patients with thrombocytosis. Notably, patients with MHA or non-malignant inflammatory condition may have to be excluded from the treatment subjects, as the pathways for thrombocytosis in those patients must be different from paraneoplastic thrombocytosis.
Stone et al. proposed that increased hepatic thrombopoietin synthesis in response to tumor-derived IL-6 was a mechanism for paraneoplastic thrombocytosis(27). They further reported that treatment with siltuximab, anti-IL-6 antibody, significantly enhanced therapeutic efficacy of paclitaxel in mouse models of epithelial ovarian cancer. As regards clinical trials, a phase II study in patients with platinum-resistant ovarian cancer reported that siltuximab treatment showed partial response in one patient and disease stabilization in 7 among 18 evaluable patients(28). Regarding combination with chemotherapeutics, a phase I trial in patients with recurrent epithelial ovarian cancer reported that combined carboplatin/doxorubicin with tocilizumab, anti-IL-6 receptor antibody, and interferon-α2b showed complete response in 3, partial response in 8, and stable disease in 6 among 21 evaluable patients, and that toxicity was tolerable(29). More clinical trials and examination of clinical samples are warranted to evaluate usefulness and to investigate the underlying mechanism of anti-IL-6 therapies in ovarian cancer.
Our study has the following limitations. First, the sample size of the subset analyses is relatively small. Second, the strengthening of our hypothesis by basic study data is lacking. Third, the retrospective study design potentially causes selection biases. Prospective studies are required to verify our proposal.