Circulating tumor cells: The tumor trail left in the blood

Background: Metastasis in HNC patients is reflected by measurable levels of circulating tumor cells (CTCs) in the peripheral blood of cancer patients. CTCs represent cancer cells from the primary and metastatic sites, thereby providing a comprehensive representation of the tumor burden of an individual patient. For patients without CTCs at presentation, the detection of CTCs in the blood and analysis of biomarkers within them provide an opportunity to identify patients "at risk" of developing overt metastasis, accelerating targeted treatment in addition to routing care with the clear aim of improving cure. Methods: Our study aimed to assess whether CTCs from the blood of HNC patients attending the Princess Alexandra Hospital and Royal Brisbane and Women's Hospital provided early cues of distant metastases (n=250). Results: With significant advances in CTC isolation technologies, we could demonstrate a higher CTC capture efficiency using epitope-independent platforms. By assessment of single and clustered CTCs, our data showed that HNC patients can be identified 4-6 months prior to developing clinical/radiographically evident metastasis. In these patients, a window for treatment escalation could become a possibility. In a proof-of-principle study, using novel culture formulations and hypoxic conditions (1-2% O2), we were able to demonstrate, for the first time, short-term patient-derived CTC cultures ex vivo from 7/18 HNC samples (4/7 HPV-positive, oropharyngeal) in a clinically relevant time period. Recent advancements have shown that PD-1 immune checkpoint therapies have durable responses in metastatic HNC patients that fail 1st- and 2nd-line therapy. Our preliminary data suggest PD-L1 is frequently expressed on HNC CTCs, and an immunoscore may be able to stratify patients likely to respond to immunotherapy. Conclusion: Expanding CTCs outside the patient's body allows for the recapitulation of the molecular diversity present within the tumor, understanding the disease progression and testing of therapies. Patients with a high percentage of PD-L1+ CTCs could be potential candidates for anti-PD-L1 therapy, a promising new immunotherapy