Prospects for fluorescence molecular in vivo liquid biopsy of circulating tumor cells

We discuss our work in development of 'Diffuse in vivo Flow Cytometry' (DiFC) for non-invasive fluorescence enumeration of circulating tumor cells (CTCs) and describe recent progress towards human translation of DiFC. DiFC is an emerging technique wherein highly-scattered light is used to non-invasively sample blood flowing in large deep-seated blood vessels and detect fluorescently-labeled cells. The key advantages are that it allows continuous sampling of large circulating blood volumes and enumeration of rare cells over time. We discuss progress in development an application of near-infrared fluorescent molecular contrast agents for sensitive and specific labeling of CTCs directly in vivo. Candidate contrast agents include a folate receptor-targeted probe (OTL38, Cytalux), as well as new, purpose-designed pan-epithelial CTC-specific probes. We also discuss relevant tissue optics and instrumentation considerations for potential future human translation. Ultimately, DiFC could represent a new method for continuously enumerating CTCs without drawing blood samples that may enable early detection of cancer metastasis or monitoring of response to cancer therapies.