Rationale behind targeting fibroblast activation protein-expressing carcinoma-associated fibroblasts as a novel chemotherapeutic strategy

Abstract

The tumor microenvironment has emerged as a novel chemotherapeutic strategy in the treatment of cancer. This is most clearly exemplified by the antiangiogenesis class of compounds. Therapeutic strategies that target fibroblasts within the tumor stroma offer another treatment option. However, despite promising data obtained in preclinical models, such strategies have not been widely used in the clinical setting, largely due to a lack of effective treatments that specifically target this population of cells. The identification of fibroblast activation protein α (FAP) as a target selectively expressed on fibroblasts within the tumor stroma or on carcinoma-associated fibroblasts led to intensive efforts to exploit this novel cellular target for clinical benefit. FAP is a membrane-bound serine protease of the prolyl oligopeptidase family with unique post-prolyl endopeptidase activity. Until recently, the majority of FAP-based therapeutic approaches focused on the development of small-molecule inhibitors of enzymatic activity. Evidence suggests, however, that FAP's pathophysiological role in carcinogenesis may be highly contextual, depending on both the exact nature of the tumor microenvironment present and the cancer type in question to determine its tumor-promoting or tumor-suppressing phenotype. As an alternative strategy, we are taking advantage of FAP's restricted expression and unique substrate preferences to develop a FAP-activated prodrug to target the activation of a cytotoxic compound within the tumor stroma. Of note, this strategy would be effective independently of FAP's role in tumor progression because its therapeutic benefit would rely on FAP's localization and activity within the tumor microenvironment rather than strictly on inhibition of its function.

Figure 1

CAFs can promote tumorigenesis directly through multiple mechanisms, including increased angiogenesis, proliferation, invasion, and inhibition of tumor cell death. These effects are mediated through the expression and secretion of numerous growth factors, cytokines, proteases, and extracellular matrix proteins, such as SDF-1, FGF2, VEGF, TGF-β, HGF, tenascin-c, LOX, and the MMPs. CAFs can additionally influence tumorigenesis indirectly through effects on a multitude of other cell types, including adipocytes and inflammatory and immune cells. Furthermore, paracrine signals (examples listed around the perimeter of the web) derived from these accessory cells feed back to promote tumor growth. Ac, acetyl; AFC, 7-amino-4-(trifluoromethyl) coumarin; bFGF, basic fibroblast growth factor; CCL2, chemokine (C-C motif) ligand 2; Col, collagen; DPP-II (IV, 6, 7, 8, 9, 10), dipeptidyl peptidase-II (IV, 6, 7, 8, 9, 10); FN, fibronectin; GM-CSF, granulocyte macrophage colony-stimulating factor; HGF, hepatocyte growth factor; IGF2, insulin-like growth factor 2; LOX, lysyl oxidase; SDF-1, stromal cell-derived factor 1; SFRP-1, secreted frizzled-related protein 1; SPARC, secreted protein, acidic and rich in cysteine; TNC, tenascin-c.

Figure 2

Schematic of the FAP-activated prodrug strategy. A FAP-activated prodrug is administered systemically and circulates throughout the body in an inactive form. The inactive prodrug diffuses into the tumor microenvironment where it encounters CAFs expressing FAP, which selectively activate the cytotoxin by cleaving off the inactivating peptide containing the FAP recognition sequence. Once activated, the thapsigargin analog used in this example directly kills the FAP-expressing cells, and it also targets neighboring cancer and endothelial cells through a bystander effect. In theory, any cytotoxic agent can be used as the warhead in this strategy, provided that its structure allows it to be conjugated to a protease-accessible peptide substrate. TG, thapsigargin.