Abstract CN1-1: Target mutation: The dark side of targeted therapies
Authors: Pierotti Marco A. 
Published: January 2011

Abstract
Receptor tyrosine kinases (RTK) are cell–surface receptors responsible for the transmission of extracellular signals across the plasma membrane and for the regulation of intracellular signal transduction pathways, playing an important role in many processes including cell metabolism, cell cycle, cell survival, cell migration and development.
Gastrointestinal Stromal Tumors (GISTs) consist of a proliferation of spindle, or, less frequently, epithelioid cells, commonly expressing the KIT protein (CD117). The key molecular event determining the development of GIST is the activation of the KIT protein and its pathway. Alternatively, a relatively small percentage of GIST patients have an activated RTK, the platelet-derived growth factor receptor alfa (PDGFRA).
In physiological conditions, these receptors become activated-phosphorylated when they are bound to their specific ligands. The binding confers a conformational changing in their structure, resulting in protein dimerization, intramolecular phosphorylation and TK activation.
In pathological conditions, the activating event for KIT or PDGFRA receptors appears to be a mutation in peculiar exons, more frequently affecting their transmembrane regions or their kinase domains.
Imatinib mesylate (Gleevec®, Novartis Pharmaceuticals) has been demonstrated to be the most effective chemotherapy for metastatic GISTs. Imatinib is a competitive inhibitor of the KIT tyrosine kinase as well as other tyrosine kinases, BCR-ABL and PDGFR. It competes with ATP for binding to the KIT/PDGFRA tyrosine kinases, preventing the transfer of the gamma-phosphate group, and is able to inhibit their downstream pathways.
Evidence derived from the clinical experience points out that different c-Kit mutation types correlate with a different response rate to Imatinib and that GISTs carrying mutations in exon 11 respond well to the drug, even if with a gradient of response. However, the development of acquired resistance in GIST indicates that second line therapies are needed, and the knowledge of the peculiar inhibition mechanism of these molecules may contribute to understanding why these receptors are no longer sensitive to the drug. To date, the presence of additional point mutations affecting the ATP pocket of the kinases suggests that this compound is not able to inhibit the receptor efficiently. For this reason, the use of new molecules designed to inhibit the non responding KIT/PDGFRA receptors or, alternatively, to ensure the pharmacological interruption of their pathological signaling at a downstream level, may represent a promising approach to overcome the acquired resistance in GISTs treatment.
Molecular modeling is a sophisticated technique based on the crystallographic coordinates of the active KIT structure; it enables to calculate the binding affinity of a molecule to the receptor and can predict if a new drug will be effective against all the mutated RTKs.
A multidimensional approach is therefore proposed: it combines molecular modeling analysis (in silico), genetical egineering cell line (in vitro) and, in terms of endpoint assessment, in patients approaches (in vivo) in order to predict the response of personalized therapy. Finally, the in silico molecular modeling could be used as a screening methodology to identify effective anticancer molecules.
Citation Information: Clin Cancer Res 2010;16(7 Suppl):CN1-1