Tyrosine kinase inhibitors
July, 2014

Tyrosine Kinase
Protein tyrosine kinases (PTKs) carry out a process called phosphorylation. Tyrosine phosphorylation is one
of the key covalent modifications that occurs in multicellular organisms as a result of intercellular
communication during embryogenesis and maintenance of adult tissues. This activity catalyzes the transfer
of the phosphate of ATP to tyrosine residues on protein substrates. Phosphorylation of tyrosine residues
modulates enzymatic activity and creates binding sites for the recruitment of downstream signaling proteins.
Two classes of PTKs are present in cells: the transmembrane receptor PTKs and the nonreceptor PTKs. [5]

Tyrosine kinase receptors are a family of receptors with a similar structure. They each have a tyrosine kinse
domain (which phosphorylates proteins on tyrosine residues), a hormone binding domain, and a carboxyl
terminal segment with multiple tyrosines for autophosphorylation. When hormone binds to the extracellular
domain the receptors aggregate.

When the receptors aggregate, the tyrosine kinase domains phosphorylate the C terminal tyrosine residues.
This phosphorylation produces binding sites for proteins with SH2 domains. GRB2 is one of these proteins.
GRB2, with SOS bound to it, then binds to the receptor complex. This causes the activation of SOS. SOS is a
guanyl nucleotide-release protein (GNRP). When this is activated, it causes certain G proteins to release
GDP and exchange it for GTP. Ras is one of these proteins. When ras has GTP bound to it, it becomes
active. Activated ras then causes the activation of a cellular kinase called raf-1.  Raf-1 kinase then
phosphorylates another cellular kinase called MEK. This cause the activation of MEK. Activated MEK then
phosphorylates another protein kinase called MAPK causing its activation. This series of phosphylating
activations is called a kinase cascade. It results in amplification of the signal. Among the final targets of the
kinase cascade are transcriptions factors. Phosphorylation of these proteins causes them to become active
and bind to the DNA, causing changes in gene transcription. Dr. Donald F. Slish at State University of New
York at Plattsburgh has a nice animation on this process. If you are interested to view the animation, please,
click:
http://faculty.plattsburgh.edu/donald.slish/tyrosinekinase/TK1.html

Definition - Tyrosine Kinase Inhibitor
Tyrosine kinase inhibitor is a substance that blocks the action of enzymes called tyrosine kinases. Tyrosine
kinases are a part of many cell functions, including cell signaling, growth, and division. These enzymes may
be too active or found at high levels in some types of cancer cells, and blocking them may help keep cancer
cells from growing. Some tyrosine kinase inhibitors are used to treat cancer. [2]

How tyrosine kinase inhibitor works?
Targeted cancer therapies are drugs or other substances that interfere with specific molecules involved in
cancer cell growth and survival. Tyrosine kinase inhibitors are a type of targeted therapy.

How are they different from each other?
Tyrosine kinase inhibitors (TKI) are effective in the targeted treatment of different kinds of cancers. Imatinib
was the first used to treat cancers, and then it was followed by gefitinib, erlotinib, sorafenib, sunitinib, and
dasatinib. Although these tyrosine kinase inhibitors share the same mechanism of action - competitive ATP
inhibition at the catalytic binding site of tyrosine kinase, they differ from each other in the spectrum of
targeted kinases, their pharmacokinetics as well as substance-specific adverse effects. In general, tyrosine
kinase inhibitors cause skin toxicity, including folliculitis, in more than 50% of patients. The agents that target
EGFR, such as erlotinib and gefitinib, display the broadest spectrum of adverse effects on skin and hair,
including folliculitis, paronychia, facial hair growth, facial erythema, and varying forms of frontal alopecia. In
contrast, folliculitis is not common during administration of sorafenib and sunitinib, which target
VEGFR,
PDGFR, FLT3. Periorbital edema is a common adverse effect of imatinib. [1]
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Epidermal Growth Factor Receptor (EGFR) - The epidermal growth factor receptor (EGFR; ErbB-1;
HER1 in humans) is the cell-surface receptor for members of the epidermal growth factor family (EGF-family)
of extracellular protein ligands.

Vascular endothelial growth factor receptors (VEGFR) - Vascular endothelial growth factor receptors
(VEGFR) are considered essential for angiogenesis. The VEGFR-family proteins consist of VEGFR-1/Flt-1,
VEGFR-2/KDR/Flk-1, and VEGFR-3/Flt-4. Among these, VEGFR-2 is thought to be principally responsible
for angiogenesis.

Platelet-derived growth factor receptor (PDGFR) - Platelet-derived growth factor (PDGF) is a critical
regulator of mesenchymal cell migration and proliferation. PDGF has important functions for angiogenesis,
as well as development of kidney, brain, cardiovascular system and pulmonary alveoli during
embryogenesis. Clinical studies reveal that aberrant expression of PDGF and its receptor is often associated
with -atherosclerosis, fibroproliferative diseases of lungs, kidneys and joints, and neoplasia. PDGF
contributes to cancer development and progression by both autocrine and paracrine signaling mechanisms.
[3]

fms-like tyrosine kinase 3 (FLT3) The fms-like tyrosine kinase 3 (FLT3) plays an important role in both
normal and malignant hematopoiesis. Activating mutations in the FLT3 receptor can be detected in
approximately 30% of acute myeloid leukemias (AMLs) and are associated with a distinctly poor clinical
outcome for patients. [4]

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Reference
[1] Hartmann JT et al, Tyrosine kinase inhibitors - a review on pharmacology, metabolism and side effects.
Curr Drug Metab. 2009 Jun;10(5):470-81.
[2] NCI website, July 2014
[3] Yu J et al, Platelet-derived growth factor signaling and human cancer. J Biochem Mol Biol. 2003 Jan
31;36(1):49-59.
[4] Khaled el-Shami et al, FLT3 Inhibitors in Acute Myeloid Leukemia Expert Rev Hematol. 2008;1(2):153-160.
[5] Hubbard SR1, Till JH.Protein tyrosine kinase structure and function. Annu Rev Biochem. 2000;69:373-98.