Background

In the precision oncology era, monitoring treatment response using circulating blood-based biomarkers such as circulating tumor DNA (ctDNA) and circulating tumor cells (CTCs) is rapidly being evaluated and established. The leading cause of mortality in ovarian cancer patients is delayed diagnosis and the inability to effectively select patients for targeted therapies, including immune checkpoint blockade (ICB) agents. Treatment for ovarian cancer usually involves a combination of surgery and chemotherapy. However, postoperative resection and therapy with curative intent often fail to account for minimal cellular disease (MCD). The dissemination of circulating tumor cells (CTCs) represents minimal residual disease (MRD), which may diffuse and cause micro-metastasis through epithelial-to-mesenchymal transition (EMT) and bio-mechanistic activation in blood circulation. Simultaneous detection of overexpression of programmed death-ligand 1 (PD-L1) on CTCs as a dynamic biomarker may be useful for assessing patients for immune checkpoint inhibitor (ICI) therapy.

Methods

In a retrospective analysis of real-world data, peripheral blood samples from 75 ovarian cancer patients were analyzed for the presence of CTCs, with and without PD-L1 expression, and for the presence of CTC clusters. CTCs were detected using the CDSCO-approved OncoDiscover platform from 1.5 ml of peripheral blood. The platform is a multifunctional magneto-nanosystem mediated by anti-epithelial cellular adhesion molecule (EpCAM) antibodies. CTCs were identified as positive when EpCAM+, CK+, PD-L1+, DAPI+, and CD45- markers were present. PD-L1 expression on CTCs was analyzed based on the linear intensity gradients of fluorescence signals using image acquisition on an automated Zeiss microscope.

Results

Baseline sample analysis showed that 86% (n = 65) of patients had at least one CTC per 1.5 ml of blood. The CTC distribution ranged from 1 to 9 CTCs. Among the patients with CTCs, 46.15% (n = 30) showed PD-L1 expression. Notably, the highest number of CTCs (~26.7%, n = 23) was observed in the 41–50 age group. Additionally, 8% (n = 6) of the total patients showed the presence of CTC clusters. The presence of CTCs with PD-L1 expression and CTC clusters did not show a correlation with factors such as staging, follow-ups, metastasis, or disease-free survival (DFS) status.

Conclusions

We observed the presence of minimal cellular disease (MCD) and minimal residual disease (MRD) in ovarian cancer patients despite treatment with curative intent. Detection of CTCs, CTC clusters, and PD-L1 overexpression as real-time dynamic biomarkers may help assess early metastasis, disease progression, and regression. These biomarkers may also support the selection of patients suitable for immune checkpoint inhibitor (ICI) therapy when tissue samples are inadequate or unavailable, potentially improving clinical outcomes.