In the current study, we explored the expression of cell-free miRNAs in malignant effusion samples, and found that malignant effusions have higher expression levels of miR-24, miR-26a and miR-30d. This suggests that malignant effusions may have a different cell-free miRNA expression profile. miR-24, miR-26a and miR-30d are differentially expressed in a panel of cancers [14–17]. Chen et al  demonstrated that these three miRNAs were highly expressed in the serum of lung cancer patients compared to healthy control. Our result is in accordance with the result in the serum of lung cancer patients.
This study also revealed that effusion samples with tumor cells resistant to docetaxel contain lower levels of cell-free miR-152 than those from chemo-sensitive tumor cells. We demonstrated for the first time that cell-free miRNAs may be potential diagnostic biomarkers for diagnosis and drug sensitivity.
Currently used biomarkers mainly rely on tumor specific peptides derived from proteomic profiling . However, as tumor-associated proteins constitute only a minor fraction compared with normal proteins, the sensitivity for these "tumor proteins" may be limited. Cell-free nucleic acids have recently attracted interest as the diagnostic biomarkers for cancer. Several studies have focused on cell-free nucleic acids from fluid samples and explored their potential application as diagnostic biomarkers , . Up to now, cell-free miRNAs have been studied well in plasma, serum, urine , saliva  and sputum . Recently published reports proved that cell-free miRNAs in plasma or serum are stable and can be used to distinguish cancer patients from healthy subjects [8, 10, 23]. In view of the success of cell-free miRNAs of other body fluids in cancer diagnosis and the results of this study, cell-free miRNAs may serve as novel diagnostic biomarkers in the diagnosis of body cavity effusions with minimal invasiveness and sample requirement.
Docetaxel is a tubulin-binding agent that induces cell death through stabilizing microtubules after binding to β-tubulin. Interogation of miRGen, a web tool for miRNA target prediction and function , revealed a group of microtubule related genes that may be potential targets of miR-152, including β- tubulin2b, β-tubulin 4q-chain, β-tubulin 6 and β-tubulin 8. A group of ATP-binding cassettes (ABC) transporters was also identified as potential targets of miR-152, including ABCA1, ABCB7 and ABCD3. Chemotherapeutic resistance of tumor cells may be due to the drug pumps, which are mainly made up of ABC proteins . These two types of possible targets may explain partly the reason for lower expression of cell-free miR-152 in docetaxel resistant group. However, data is still scarce and further research is needed.