PMC

Cancer develops in a complex and dynamic TME, which exerts profound impacts to disease progression. Cancer cells are in close relationship with diverse non-cancer cell types within the TME, forming a functional nexus that facilitates tumor initiation, survival, and exacerbation. Cytotoxicity generated by treatments including chemotherapy, radiation, and targeted therapy eliminates many malignant cells within the cancer cell population; however, surviving cells are frequently retained in specific TME niches. Such protection minimizes the sensitivity to anti-cancer agents and generates resistant subclones through distinct mechanisms, prominently through acquired resistance conferred by a large body of soluble factors released from damaged or remodeled stroma. Alternatively, BMDCs, including MSCs and Tregs, mediate immunomodulation and prevent inflammation by restraining the activity of cytotoxic T cells, correlating with poor prognosis. Either acquired resistance or immunosurveillance evasion promotes cancer cell survival and subsequent expansion, allowing development of more advanced phenotypes, including tumor relapse, distant metastasis, and therapeutic failure, eventually causing high mortality in clinical settings. CAF, Carcinoma-associated fibroblast; MSC, Mesenchymal stem cell; BMDC, Bone marrow-derived cell; Treg cell, Regulatory T cell; EC, Endothelial cell; ECM, Extracellular matrix; TAM, Tumor-associated macrophage.

Illustrative models for the preclinical evaluation of novel anticancer regimes that incorporate TME-targeting agents. (A) Route 1 (singular), tumors develop in transgenic mice before the preclinical administration of chemotherapy or targeted therapy is applied as a singular agent. Dramatic cancer resistance is observed in such a therapeutic approach, with only limited efficacy available. (B) Route 2 (combinational), in contrast to route 1, an updated regime incorporating the novel agents (small molecule inhibitor or monoclonal antibodies) into the treatment program, which allows targeting both the tumor and TME. Significant disease regression follows after several cycles of the novel treatments, with much higher preclinical index achieved. (C) Route 3 (singular), tumors develop in the immunocompetent (wild type) mice xenografted with cancer cells and stromal cells from the same genetic and/or strain background as the host. Upon exposure to treatments as in Route A, a low outcome is observed. (D) Route 4 (combinational), tumors develop in the xenograft mice as in C, harboring implanted cancer and stromal components. Once receiving the same treatments as in Route B, animals present significantly improved therapeutic efficacy. (Note, in routes C and D, the preclinical paradigm in prospective trials exclude PDX, although it is a highly recommended model for many cancer studies).