Abstract 3098: The first circulating tumor cell-derived explant (CDX) model of a Merkel cell carcinoma

NeuroEndocrine Carcinomas (NECs) are rare yet lethal diseases; they are frequently metastatic and have limited treatment options. Whilst progress has been made in understanding the biology of Small Cell Lung Cancer (SCLC), the most studied NEC, research on NECs from other anatomical sites lags behind. In SCLC, circulating tumor cells (CTCs) can be used to generate patient-relevant mouse models; termed CTC-Derived eXplants (CDX) which are faithful in biology and chemotherapy response to donor patients1,2. A pilot prospective study is on-going to assess the potential utility of cell-free DNA (cfDNA) and/or CellSearch (CS) CTCs (EpCAM/cytokeratin positive) as liquid biopsies and feasibility of CDX generation from patients with advanced stage extrapulmonary NECs undergoing palliative chemotherapy. CS CTCs are enumerated before and during treatment and marker-independent RosetteSep-enriched CTCs from parallel blood samples are implanted in immunodeficient mice to attempt CDX generation. Twenty-seven patients were recruited from Jun-19 to Nov-21. CS-CTCs were present in 19/27; 70% pre-treatment samples (mean 57, median 2, range 0-685 CTCs per 7.5mL of blood). CDX generation was successful from a pre-treatment CTC sample from a patient originally diagnosed with a NEC of unknown origin who responded to Platinum/Etoposide. Whole-exome sequencing of this CDX and matched pre-treatment cfDNA from the donor revealed largely overlapping copy number changes and common mutations including an activating PIK3CA mutation. In addition, there was no evidence of TP53 or RB1 loss (hallmarks of SCLC) in neither the CDX nor the donor’s cfDNA. Immunohistochemical (IHC) analysis of this CDX showed a small cell NEC morphology, 90% Ki-67 positive cells, expression of EpCAM, pan-cytokeratins and neuroendocrine (NE) diagnostic markers, concordant with the donor’s biopsy. Further IHC and RNA sequencing revealed expression of the NE transcription factor atonal homolog 1 (ATOH1)2, CK20 and abundant Merkel cell polyomavirus (MCPyV) transcripts. These characteristics prompted reclassification of the original patient diagnosis as Merkel cell carcinoma (MCC). There was only 1 CS-CTC in the parallel pre-treatment blood sample implying that EpCAM-negative CTCs may have given rise to the CDX. The CDX showed in vivo sensitivity to Platinum/Etoposide, mirroring treatment response of the donor. Ex vivo cultures from CDX-derived cells grow as suspension clusters similar to ‘classic’ SCLC and MCPyV-po