Thapsigargin and FAP
But, scientists say, while FAP cell-associated activities may be targets for diagnosis and treatment of various cancers, coming up with drugs to inhibit their activity has proven challenging.
Antibodies that can target or interrupt FAP activity are not yet available, and development of small molecule inhibitors of FAP is stymied because the endogenous substrates of FAP have not been identified. And researchers say, similarly as for other enzymatic targets such as kinases, the highly conserved structure of the catalytic domains of serine proteases, in particular the structural relatedness of FAP to other members of the DPP family proteases, can “defy the rationale of the highly specific small molecule inhibitors required to avoid off-target effects.”
But rather than targeting the protease directly with for example, small molecule enzyme inhibitors, investigators at the department of pharmacology and molecular sciences, Johns Hopkins University, say 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, W. Nathaniel Brennen, Ph.D., and colleagues are “taking advantage” of FAP’s restricted expression and unique substrate preferences by developing a FAP-activating prodrug that targets the activation of a cytotoxic compound within the tumor stroma.
In their test case, the investigators tested Thapsigargin (TG), a highly toxic natural plant product that triggers a rise in intracellular calcium levels with subsequent apoptosis. FAP is therefore, they said, a provocative target for the activation of prodrugs consisting of a FAP-specific peptide coupled to a potent cytotoxic analog of TG.
The investigators tested the efficacy of FAP-activated peptidyl-TG prodrug in vitro in cell proliferation assays and assessed its effects on intracellular calcium in human cancer cell lines. The effects of FAP-activated prodrugs on tumor growth and host toxicity were also tested in Balb-C nude MCF-7 and LNCaP xenograft mice. FAP-activated prodrugs killed human cancer cells at low nanomolar concentrations; amino acid-12ADT analogs from FAP-cleaved prodrugs, but not uncleaved prodrugs, produced a rapid rise in intracellular calcium within minutes of exposure.
Immunohistochemical analysis of xenografts exposed to FAP-prodrugs documented stromal-selective cell death of fibroblasts, pericytes, and endothelial cells of sufficient magnitude to inhibit growth of MCF-7 and LNCaP xenografts with minimal systemic toxicity, whereas non-FAP cleavable prodrugs proved inactive.
The authors comment that strategies that attempt to exploit cellular targets within the tumor stroma offer several potential advantages over traditional approaches. These include providing a more genetically stable target that is not only less heterogeneous than its malignant epithelial counterparts but also less likely to acquire resistance to a cytotoxic agent.
Targeting CAFs or stromal cells remains a “relatively underappreciated” treatment strategy, the scientists commented, largely due to a lack of evidence suggesting that it is a practical option in the clinical setting.
But, they say, the surge in both pharmaceutical and academic efforts to develop FAP as a therapeutic target offers a promising outlook for changing that statement in the not-so-distant future.
To read part II of this article, click here.