We observed pretreatment thrombocytosis, defined as a platelet count > 400,000/mm³, in 18.9% of the patients with stage I-IV epithelial ovarian cancer, which is in line with previous reports using the same cutoff value as ours (7.4-42.5%)(10-12, 15). Our analyses of the relationships between thrombocytosis and clinicopathologic parameters showed that thrombocytosis was significantly associated with MHA, primary treatment, FIGO stage, histologic subtype, operation achievement, nonmalignant inflammatory condition, CA125 level, and TFI (Tables 2 and 3). Among these significant factors, FIGO stage, CA125 level, operation achievement, and primary treatment are considered to reflect the tumor extent, which has been reportedly associated with pretreatment thrombocytosis(6, 20). MHA and nonmalignant inflammatory condition are clinically well known to induce thrombocytosis. We subsequently conducted univariate and multivariate analyses for associations with thrombocytosis in patients who relapsed after adjuvant chemotherapy, excluding the 2 factors of CA125 level and primary treatment, which are considered to be closely related to 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 the chemosensitivity of ovarian cancer(21-23). Regarding 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 than those without thrombocytosis (Figures 1A, B). In addition, when the analysis was confined to stage III/IV patients, there was still a significant difference in PFS and OS (Figures 2A, B), whereas stage I/II patients showed no difference in survival according to the presence/absence of pretreatment thrombocytosis (Figures 2C, D). These findings indicate that thrombocytosis affects survival mainly in advanced diseases, consistent with our above finding that thrombocytosis was significantly and independently associated with TFI, an established predictor of chemosensitivity in the treatment of recurrence, 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 independent 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 first 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 interactions against blood flow and the immune system, including NK cells, in circulation(16, 24). In addition, platelets contribute to chemoresistance through MAPK and PI3-kinase/Akt pathways and drug efflux proteins(24). Moreover, chemoresistance in ovarian cancer cells is suggested to involve the interaction between the surrounding immune system and cancer stem cells in the tumor microenvironment, where platelets play key roles(25, 26). 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 independent of FIGO stage (Table 4). These findings suggest that the 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 in vitro and in vivo basic studies. Radziwon-Balicka et al. reported that platelets decreased paclitaxel-induced apoptosis in human ovarian adenocarcinoma cells in vitro(27). Bottsford-Miller et al. reported that the combined administration of platelet-depleting antibodies and docetaxel caused a 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 limited, as most studies only correlate thrombocytosis with survival after chemotherapy. Bottsford-Miller et al. reported changes in platelet counts during first-line chemotherapy in responsive and refractory groups matched for stage, histology, grade, and primary therapy(28). In patients with a durable response, only 50% had pretreatment thrombocytosis, and all of them achieved a normal platelet count during therapy, whereas all had pretreatment thrombocytosis, and only 50% achieved a 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 tumors after surgery. In our study, pretreatment thrombocytosis was not associated with operation achievement and was significantly associated with TFI independent of FIGO stage (Table 3). Moreover, pretreatment thrombocytosis was a significant prognostic factor for poor PFS and OS independent 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 by attenuating chemoresistance. Based on the current findings, we assume that the combination of chemotherapeutics and antiplatelet therapies may be efficacious for ovarian cancer patients with thrombocytosis. Notably, patients with MHA or nonmalignant inflammatory conditions may have to be excluded from the treatment subjects, as the pathways for thrombocytosis in these patients must be different from those for paraneoplastic thrombocytosis.
Stone et al. proposed that increased hepatic thrombopoietin synthesis in response to tumor-derived IL-6 was a mechanism for paraneoplastic thrombocytosis(29). They further reported that treatment with siltuximab, an anti-IL-6 antibody, significantly enhanced the therapeutic efficacy of paclitaxel in mouse models of epithelial ovarian cancer. Regarding clinical trials, a phase II study in patients with platinum-resistant ovarian cancer reported that siltuximab treatment showed a partial response in one patient and disease stabilization in 7 of 18 of the evaluated patients(30). Regarding the combination with chemotherapeutics, a phase I trial in patients with recurrent epithelial ovarian cancer reported that the combination of carboplatin/doxorubicin and tocilizumab, an anti-IL-6 receptor antibody, and interferon-α2b showed complete response in 3, partial response in 8, and stable disease in 6 of the 21 evaluated patients, and they showed that the toxicity was tolerable(31). Additional clinical trials and the examination of clinical samples are warranted to evaluate the 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 was relatively small. Second, the strengthening of our hypothesis by basic study data was lacking. Third, the retrospective study design potentially caused selection biases. Prospective studies are required to verify our findings.