In 2022, it was estimated that there were just under 300,000 new breast cancer cases1. Conventional therapies are effective in reducing or eliminating primary tumors, however, metastases can develop years or decades later in tissues far from the primary tumor. Such metastatic cancer is the cause of 90% of cancer deaths2–4. Prior to metastatic tumor growth, disseminated tumor cells (DTCs) must first successfully engraft in these distant metastatic sites and then, eventually, regain a rapidly proliferating phenotype. However, relatively little is known about how dormant DTC’s regain their proliferative phenotype. There is a staggering lack of models that can capture the dormant niche and the critical transition in DTC’s to a proliferative state, limiting progress towards effective treatment for disseminated cancer5.
Cancer-environment interaction is pivotal in understanding metastatic progression. The seed and soil hypothesis, first described by Paget in 1889, posits that successful metastasis depends on cancer cell “seeds” modifying the “soil” of the distant microenvironment. The bottleneck of microenvironment modification as described by the seed and soil hypothesis means, clinically, that circulating tumor cells (CTC’s) largely invade a select few tissues such as the bone, brain, liver, and lung in the case of breast cancer. The specificity with which CTC’s invade secondary sites points to a robust intercommunication between cancer cells and these distant niches. Before colonizing distant tissue, cancer cells modify these secondary metastatic sites extensively through secretion of growth factors and extracellular vesicles which enhance vascularization, inflammation, ECM remodeling, and inflammatory cell recruitment6,7. Through these modifications, tumors form pre-metastatic niches (PMN’s) in secondary sites that are amenable to tumor cell colonization in which CTC’s invade. Rather than immediately begin proliferating, the cancer cells become DTC’s and remain dormant for years.
Evidence suggests that the primary breast tumor begins shedding tumor cells early on in its development, which may limit future therapeutic strategies focused on preventing metastasis8–10. Furthermore, there is a significant body of evidence that conventional cancer treatments enhance dissemination11,12. Instead, developing therapeutic strategies centered around preventing DTC’s from regaining a proliferative phenotype or simply eliminating DTC’s before then may prove to be more feasible. Still, this strategy faces a multitude of challenges. DTC’s are often highly resistant to therapy13–16. A number of studies have shown it is possible to overcome this resistance and sensitize these dormant cancer cells to further treatment in certain circumstances17. Alternatively, rather than target DTC’s themself, it may be more effective to reeducate their microenvironment to eliminate pro-tumorigenic features such as inflammation and tumor derived ECM composition18. Regardless, developing more effective strategies depends on accurate modeling of the clinical scenario in which DTC’s inhabit a realistic secondary environment.
Navigating these therapeutic hurdles necessitates new models that accurately mimic the clinical scenario. A surgical intervention model demonstrated that mice with implanted polyvinyl acetate sponges to mimic wounds had a tumor incidence of 60% as compared to unwounded mice with 10% suggesting that surgery-driven inflammation is implicated in tumor growth 19. Administration of anti-inflammatory meloxicam resulted in tumor diameter dropping from 5mm down to 2mm, again suggesting inflammation as the critical mediator in tumor incidence. Mechanistically, proinflammatory molecules such as S100A9 have been linked to tumor cell reactivation via myeloid derived suppressor cell activity20. In-situ management of the tumor environment has similarly revealed that local anti-inflammatory administration and immunomodulatory drugs can decrease local recurrence following tumor resection21,22. Anti-recurrence treatment on the basis of niche regulation is a potentially valuable therapeutic strategy, however a model capable of translating such results to scenarios of distant metastasis is needed to develop such strategies.
Contrastingly, tissue engineered PMN’s leverage material-host interaction to simulate distant tissue colonization.23–25. Tissue engineered models are capable of recruiting immune cells, cancer cells, and stromal cells to develop realistic PMN’s with highly tunable properties including ECM composition and pore size26–31. For example, researchers have taken ECM from metastatic tissues and coated engineered scaffolds to provide a more realistic environment while maintaining the scaffold’s ability to support DTC’s26. In this work, the researchers decellularized lung tissue with sodium dodecyl sulfate, minced, lyophilized, dissolved in a solution of pepsin and 0.1 M of hydrochloric acid, and coated onto PCL scaffolds. Such artificial PMN’s enable deep investigation into the processes by which CTC’s infiltrate the niche, become dormant, and re-emerge later, replicating the critical junctures in metastatic cancer progression32.
While it has been demonstrated that implanted PMN’s are capable of rapid CTC’s accumulation33, long-term PMN evolution is comparatively less understood. Long-term models are required in order to fully investigate emerging evidence that conventional therapies may actually hasten the rate at which DTCs regain proliferative capabilities12,34–41. Negative niche changes due to treatment such as inflammation and ECM remodeling are implicated in driving metastasis42,43. In surgical wounding models, targeting inflammatory pathways or otherwise using anti-inflammatory drugs and immune landscape engineering abrogated the increase in tumor burden following surgery19,21–23,44. Often, long-term PMN study is limited by host animal morbidity before metastatic recurrence can be observed. A compelling solution to this critical limitation is serially transplanting the niche to a secondary host to isolate the post-dissemination niche from the primary tumor. This serial transplantation strategy allowed for the observation of the effect that subsequently injected peripheral blood mononuclear cells had on the post-dissemination niche45. Models specifically designed to enable investigation of such disruptions within the post dissemination cancer niche are needed to understand how treatments can potentially hasten metastatic recurrence.
In this paper, we aim to develop and demonstrate a model of disseminated cancer using serially transplanted hydrogel materials to sequester circulating tumor cells. It is demonstrated that these transplantable cancer niches are capable of recruiting cancer cells, which retain proliferative capacity after serial transplantation. In order to investigate the effects of inflammation on the post dissemination niche, we biopsy punched serially transplanted scaffolds. We conducted this biopsy punch procedure, additionally, on scaffolds that had been exposed to doxorubicin in order to more closely mimic post-treatment clinical scenarios. These studies were conducted using immunocompetent mice in order to maintain the immunological dimension of the DTC niche which many models must eliminate. Together, this data suggests that this model system obviates key limitations in studying the post dissemination niche, and enables deep investigation into the dynamics of DTC’s.