Serial biopsies of solid tumors can be challenging, not always without risk to the patient and do not capture intratumoral heterogeneity. ‘Liquid Biopsies’ in the form of Circulating Tumor DNA (ctDNA) and Circulating Tumor Cells (CTCs) offer minimally invasive means to stratify patients for therapy and to routinely monitor therapy responses and anticipate emergent therapy resistance. Whilst technically more challenging than ctDNA, CTCs offer a wider range of biomarker application allowing RNA and protein measurements within single and pools of CTCs following enrichment from the blood. In addition, we have focussed on the development of CTC derived explant models (CDX), particularly in small cell lung cancer (SCLC), where biopsies are particularly difficult to obtain (and often small and necrotic) to study the biology of this aggressive disease and to test new therapies. We have generated 17 SCLC CDX models to date where CTCs, enriched using negative depletion of blood cells, are grown subcutaneously in immune-compromised mice. The take rate is ∼45% and is broadly correlated to the number of CellSearch detected EpCam positive CTCs measured in a paired blood sample. CDX models are derived from SCLC patients at presentation before chemotherapy, where most patients are initially chemosensitive but ∼20% are chemorefractory. For some patients, a matched CDX has been derived following chemotherapy response and subsequent relapse with progressive disease and where a biopsy is rarely acquired. CDX histopathology mimics the diagnostic biopsy of the donor patient and recapitulates the donor patient's response to chemotherapy. CDX can be passaged with maintained growth characteristics and as few as 5-10 CDX cells can re-establish the tumor consistent with a high frequency of tumor initiating cells. We are exploiting CDX models to explore the biology of SCLC, mechanisms of tumor cell dissemination and chemotherapy resistance. The CDX models are also being used to facilitate drug development. Therapies that show promise in CDX models can be rapidly translated to the clinic where the high prevalence of CTCs in SCLC patients provide minimally invasive, proof of mechanism pharmacodynamic biomarkers and CTC number as a surrogate of tumor response. Once passaged in vivo, tumor cells from disaggregated CDX can be cultured ex vivo over 2-3 weeks for initial and more rapid drug screening. SCLC CDX are also being used to assess the importance of vascular mimicry (VM) - a manifestation of tumor cell plasticity that endows endothelial cell behaviour, and we have shown that VE-Cadherin plays a functional role in SCLC VM, altering tumor growth kinetics and intratumor delivery of cisplatin. The generation of s.c CDX from NSCLC patients is proving more challenging. We have generated a CDX from a NSCLC patient with no CellSearch EpCam positive CTCs in their 7.5ml blood sample but whose blood did contain >150 CTCs/ml blood detected by filtration using an immunofluorescence assay for Epithelial to Mesenchymal Transition. The resultant CDX and the majority of this patient's CTCs were mesenchymal. Alternative approaches including intrathoracic implantation of enriched NSCLC CTCs are being explored. Subcutaneous CDX have been successfully derived from advanced melanoma patients and can now complement other approaches and extend studies where metastatic melanoma biopsies are unforthcoming. In summary, the development of CDX models yields new opportunities to study advanced disease and examine treatment response and resistance. We are currently developing approaches that will allow the genetic manipulation of CDX cultures to underpin mechanistic studies and we are exploring the generation of CDX across other tumor types.

Citation Format: Caroline Dive, Kris Frese, Melanie Galvin, Alice Lallo, Cassandra Hodgkinson, Christopher Morrow, Kathryn Simpson, Francesca Trappani, Stuart Williamson, Lynsey Priest, Romina Girotti, Crispin Miller, Ged Brady, Richard Marais, Fiona Blackhall. Developing circulating tumor cell derived explant models (CDX). [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr PL04-04.