This study provides new evidence for a combination regimen in the second-line treatment of cancer of unknown primary (CUP), suggesting that baseline systemic eosinophil count may serve as a predictive biomarker for immunotherapy response, offering direct guidance for clinical strategies in tumor immunotherapy.
Literature Overview
The article titled 'Anti-PD-1 plus nab-paclitaxel and bevacizumab for second-line treatment of cancer of unknown primary (Fudan CUP-002): a phase II trial,' published in Nature Communications, systematically investigates the efficacy and safety of combining an anti-PD-1 antibody with nab-paclitaxel and bevacizumab as a second-line therapy in patients with cancer of unknown primary (CUP). The single-arm phase II trial enrolled 48 CUP patients who had progressed after first-line treatment. Results demonstrated that this triplet regimen achieved high objective response rates and durable disease control. Exploratory analyses further revealed a significant association between baseline peripheral blood eosinophil levels and treatment response, highlighting their potential value as a prognostic biomarker.Background Knowledge
Cancer of unknown primary (CUP) is a highly heterogeneous group of metastatic cancers for which the primary site remains unidentified despite standard diagnostic workups, accounting for 2–5% of all malignancies. These patients face extremely poor prognoses, with median overall survival ranging from only 3 to 16 months. Particularly after first-line therapy failure, there is no established standard second-line treatment. Although immune checkpoint inhibitors (ICIs) have shown some activity in subsets of CUP patients, the efficacy of monotherapy or chemotherapy combinations remains limited, with objective response rates (ORR) typically below 25%, and reliable predictive biomarkers are lacking. The current challenge in PD-1–targeted therapy lies in improving response rates, extending survival, and identifying the patient populations most likely to benefit. Anti-angiogenic agents such as bevacizumab inhibit the VEGF signaling pathway, which not only normalizes tumor vasculature and enhances drug delivery but also reverses the immunosuppressive tumor microenvironment and promotes T-cell infiltration, thereby synergizing with ICIs. This study builds on that mechanistic rationale, proposing a 'immunotherapy + chemotherapy + anti-angiogenesis' triplet strategy in CUP to overcome the current efficacy plateau in second-line treatment. It also explores the role of systemic inflammatory markers—including neutrophil-to-lymphocyte ratio (NLR), lymphocyte-to-monocyte ratio (LMR), and eosinophils—in predicting treatment outcomes, offering a new perspective for precision oncology.
Research Methods and Experiments
The study employed a single-center, single-arm phase II trial design (Fudan CUP-002, NCT04848597), enrolling 48 patients with CUP who had progressed after first-line systemic chemotherapy. Participants received treatment with an anti-PD-1 antibody (F520) combined with nab-paclitaxel (125 mg/m² on days 1 and 8) and bevacizumab (7.5 mg/kg) every three weeks, for up to 2 years or until disease progression. The primary endpoint was objective response rate (ORR) assessed by independent blinded review; secondary endpoints included progression-free survival (PFS), overall survival (OS), duration of response (DoR), and safety. Baseline peripheral white blood cell profiles were obtained via routine blood tests. Multiplex immunohistochemistry (mIHC) was used to evaluate CD8+ T cells, FOXP3+ Treg cells, and PD-1 expression within the tumor microenvironment (TME), alongside Sirius Red staining to assess tissue eosinophil infiltration. Additionally, a 90-gene expression profiling assay was performed in selected patients to predict potential primary tumor origins. Key findings showed an ORR of 54.2%, a disease control rate (DCR) of 95.8%, median PFS of 13.1 months, and median OS of 25.1 months—markedly superior to historical controls. More importantly, baseline eosinophil counts were significantly higher in responders than non-responders, and patients with elevated eosinophil counts exhibited significantly longer PFS (16.1 vs 8.6 months), indicating independent prognostic value. mIHC analyses revealed higher infiltration of CD8+ T cells and PD-1+ T cells, along with lower FOXP3+ Treg cell density, in responders. Spatial co-localization of eosinophils and CD8+ T cells further supported their协同 anti-tumor role.Key Conclusions and Perspectives
Research Significance and Prospects
This study sets a new benchmark for second-line therapy in CUP, a notoriously difficult-to-treat malignancy, with the triplet regimen significantly extending PFS and OS, positioning it as a potential future standard of care. From a research perspective, identifying eosinophils as a predictive biomarker offers a rapid, low-cost blood-based assay and provides new insights into the immune microenvironment heterogeneity of CUP. Future studies should further investigate eosinophil subsets and their molecular interactions with T cells, or use humanized mouse models for functional validation.
In drug development, these results support exploring bispecific antibodies targeting both VEGF and PD-1 (e.g., ivonescimab) in combination with chemotherapy to simplify regimens and enhance synergy. For clinical monitoring, baseline eosinophil assessment could be integrated into patient stratification strategies to enable personalized treatment decisions. Moreover, due to the lack of suitable CUP cell lines and animal models, this study underscores the urgent need to establish dedicated CUP disease modeling systems for deeper mechanistic investigations.
Conclusion
This study, through a prospective phase II trial, confirms the high efficacy and acceptable safety of combining anti-PD-1 with nab-paclitaxel and bevacizumab in second-line treatment of cancer of unknown primary (CUP), achieving an objective response rate exceeding 50% and a median overall survival beyond 25 months—substantially outperforming previous standard therapies. The research not only delivers a clinically translatable treatment strategy but, more importantly, for the first time identifies baseline peripheral blood eosinophil count as an independent predictive biomarker of response, offering a new tool for precision immunotherapy in CUP. Despite limitations related to the single-arm design and sample size, this finding lays the groundwork for future multicenter validation trials. Bridging laboratory and clinic, the study emphasizes the importance of integrating blood-based immune biomarkers with tumor microenvironment analysis, advancing CUP management from 'empirical treatment' toward 'biomarker-driven therapy.' For CUP care systems, this triplet regimen may become the new standard in second-line treatment, while the discovery of eosinophils offers a practical path for patient stratification and treatment optimization, marking a significant step forward in comprehensive CUP management.
