International Journal of Molecular Sciences IF 4.556
Targeted Cancer Therapy and Mechanisms of Resistance
Institute for Experimental Endocrinology and Oncology, IEOS
National Research Council, CNR
31 March 2021 Article/Review
Message from the Guest Editor
Tumor cells commonly exhibit dependence on a single (often the initiating) activated oncogenic pathway or protein to maintain their malignant proliferation and survival, a phenomenon that is called “oncogene addiction”. According to this concept, protein kinases have been elected as promising molecular targets for cancer therapy. There are several possibilities to target these proteins in cancer, including monoclonal antibodies that can bind to the extracellular domain of the RTK, compounds able to favor the proteolytic degradation of the kinase and, finally, small molecule protein kinase inhibitors (PKIs). In addition to targeting oncogenes, new anticancer treatments have been increasingly developed towards tumor suppressor genes and RNA interference.
Despite promising results in cancer treatment with targeted cancer drugs, clinical experience has shown that only a fraction of patients respond to targeted therapies, even if their tumor expresses the altered target. This kind of resistance is known as primary resistance. Moreover, secondary or acquired resistance to the treatment arises almost invariably when tumors are treated with cancer drugs. Acquired resistance mechanisms can be divided into two main categories: 1) target-dependent and 2) target-independent mechanisms.
Target-dependent resistance typically occurs through genetic modifications of the target. Such genetic modifications may include point mutations and copy number amplifications. The acquisition of mutations conferring drug resistance has been documented for several PKIs, such as drugs against BCR/ABL, EGFR, FLT3, KIT and PDGFR. Evidence suggests mutation may pre-exist in a minority of cancer cells, and it is then selected upon treatment. This suggests that secondary PKI that can also bind the mutated kinase can be used to overcome resistance. Gene amplification is another major mechanism of target-dependent resistance. The selective pressure of the drug can drive amplification of the target gene, thus leading to additional overexpression of the encoded protein.
Instead, target-independent mechanisms occur through activation of alternative pathways that allow the bypass of the drug-mediated block. In other words, cancer cells escape treatment by switching to an alternative signaling pathway that is not inhibited by the drug.
Other mechanisms of resistance can exploit the enormous genome plasticity of cancer cells by modulating miRNA expression or remodeling chromatin. Finally, though not as commonly as with classical cytotoxic drugs, other resistance mechanisms can cause a decrease of the effective intracellular concentration of the targeted cancer drug.
Dr. Valentina De Falco
IJMS Topic Editor