Tyrosine kinase inhibitors (TKIs) block chemical messengers (enzymes) called tyrosine kinases. Tyrosine kinases help to send growth signals in cells, so blocking them stops the cell growing and dividing. Cancer growth blockers can block one type of tyrosine kinase or more than one type. Tyrosine kinase inhibitors (TKIs) inhibit corresponding kinases from phosphorylating tyrosine residues of their substrates and then block the activation of downstream signaling pathways. Tyrosine kinase enzymes (TKs) can be categorized into receptor tyrosine kinases (RTKs), non-receptor tyrosine kinases (NRTKs), and a small group of dual-specificity kinases (DSK) which can phosphorylate serine, threonine, and tyrosine residues. RTKs are transmembrane receptor that includes vascular endothelial growth factor receptors (VEGFR), platelet-derived growth factor receptors (PDGFR), insulin receptor (InsR) family, and the ErbB receptor family, which includes epidermal growth factor receptors (EGFR) and the human epidermal growth factor receptor-2 (HER2). NRTKs are cytoplasmic proteins that consist of nine families, including Abl, Ack, Csk, Fak, Fes/Fer, Jak, Src, Syk/Zap70, and Tec, with the addition of Brl/Sik, Rak/Frk, Rlk/Txk, and Srm, which fall outside the nine defined families. The most notable example of DSKs is the mitogen-activated protein kinase kinases (MEKs), which are principally involved in the MAP pathways. Kinase inhibitors are either irreversible or reversible. The irreversible kinase inhibitors tend to covalently bind and block the ATP site resulting in irreversible inhibition. The reversible kinase inhibitors can further subdivide into four major subtypes based on the confirmation of the binding pocket as well as the DFG motif. Different binding modes of TKIs include Type I inhibitors: competitively bind to the ATP-binding site of active TKs. The arrangement of the DFG motif in type I inhibitors has the aspartate residue facing the catalytic site of the kinase. Type II inhibitors: bind to inactive kinases, usually at the ATP-binding site. The DFG motif in type II inhibitors protrudes outward away from the ATP-binding site. Due to the outward rotation of the DFG motif, many type II inhibitors can also exploit regions adjacent to the ATP-binding site that would otherwise be inaccessible. Type III inhibitors: do not interact with the ATP-binding pocket. Type III inhibitors exclusively bind to allosteric pockets adjacent to the ATP-binding region. Type IV inhibitors: bind allosteric sites far removed from the ATP-binding pocket. Type V inhibitors: refer to a proposed subset of kinase inhibitors that exhibit multiple binding modes

Tyrosine-Kinase Inhibition of BCR-ABL Pathway References