site of the intracellular TK domain, and monoclonal antibodies directed to the
extracellular domain, interfering with RTK activation and/or marking
RTK-expressing cells for destruction by the immune system. Even though these
drugs clearly represented an impressive breakthrough in the therapy of
RTK-addicted tumors, resistance development and detection of refractory tumors
have given rise to novel therapeutic challenges, pushing the drug discovery process
forward. This chapter focuses particularly on the discussion of several case studies
on the development of small-molecule tyrosine kinase inhibitors (TKIs) directed to
EGFR, VEGFR, and PDGFR, clinically relevant RTKs, and the subset of advances
in this field.
Keywords Cancer, Epidermal growth factor receptor (EGFR), Platelet-derived
growth factor receptor (PDGFR), Receptor tyrosine kinases (RTKs), Tyrosine
kinase inhibitors (TKIs), Vascular endothelial growth factor receptor (VEGFR)
1 Receptor Tyrosine Kinases and Inhibitors
Receptor tyrosine kinases (RTKs) are cell-surface proteins that trigger key cellular
responses, such as survival, proliferation, differentiation, migration, and cell-cycle
control [1, 2]. All RTKs belonging to 20 subfamilies encoded by human genome
share a common structural architecture, comprising an extracellular (EC) agonistbinding domain, a single transmembrane helix, and a cytoplasmic subunit with a
juxtamembrane (JM) region and a tyrosine kinase (TK) domain, which is in turn
subdivided into the ATP-binding region (TK1) and the phosphotransferase region
(TK2) [1, 3, 4].
Particularly the TK domain is responsible for transferring the terminal phosphate
of adenosine triphosphate (ATP) to the corresponding substrates. This catalytic
domain contains a typical bilobal architecture connected by the so-called hinge
region, located in the cleft between these two lobes (Fig. 1). The adenine core of
ATP forms two hydrogen bonds with the hinge amino acid backbone, while the
phosphate and ribose groups of ATP are directed to the solvent-accessible surface
area through a hydrophilic channel, while the side chain of the so-called gatekeeper
residue defines the total volume of the ATP-binding pocket among different kinases
[5–7]. The conformational topology of the glycine-rich region (GXGXXG) in the Nlobe, also known as flexible phosphate-binding loop (P-loop), is considered to be a
structurally determinant factor for the pocket configuration. Moreover, kinases
present an activation loop (A-loop), which starts with a flexible conserved amino
acid sequence Asp-Phe-Gly (DFG). Since the A-loop has flexible variations in size
and sequence, its conformation is critical for modulating kinase catalytic activity
[6, 8].
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L. M. Lima et al.
extracellular domain, interfering with RTK activation and/or marking
RTK-expressing cells for destruction by the immune system. Even though these
drugs clearly represented an impressive breakthrough in the therapy of
RTK-addicted tumors, resistance development and detection of refractory tumors
have given rise to novel therapeutic challenges, pushing the drug discovery process
forward. This chapter focuses particularly on the discussion of several case studies
on the development of small-molecule tyrosine kinase inhibitors (TKIs) directed to
EGFR, VEGFR, and PDGFR, clinically relevant RTKs, and the subset of advances
in this field.
Keywords Cancer, Epidermal growth factor receptor (EGFR), Platelet-derived
growth factor receptor (PDGFR), Receptor tyrosine kinases (RTKs), Tyrosine
kinase inhibitors (TKIs), Vascular endothelial growth factor receptor (VEGFR)
1 Receptor Tyrosine Kinases and Inhibitors
Receptor tyrosine kinases (RTKs) are cell-surface proteins that trigger key cellular
responses, such as survival, proliferation, differentiation, migration, and cell-cycle
control [1, 2]. All RTKs belonging to 20 subfamilies encoded by human genome
share a common structural architecture, comprising an extracellular (EC) agonistbinding domain, a single transmembrane helix, and a cytoplasmic subunit with a
juxtamembrane (JM) region and a tyrosine kinase (TK) domain, which is in turn
subdivided into the ATP-binding region (TK1) and the phosphotransferase region
(TK2) [1, 3, 4].
Particularly the TK domain is responsible for transferring the terminal phosphate
of adenosine triphosphate (ATP) to the corresponding substrates. This catalytic
domain contains a typical bilobal architecture connected by the so-called hinge
region, located in the cleft between these two lobes (Fig. 1). The adenine core of
ATP forms two hydrogen bonds with the hinge amino acid backbone, while the
phosphate and ribose groups of ATP are directed to the solvent-accessible surface
area through a hydrophilic channel, while the side chain of the so-called gatekeeper
residue defines the total volume of the ATP-binding pocket among different kinases
[5–7]. The conformational topology of the glycine-rich region (GXGXXG) in the Nlobe, also known as flexible phosphate-binding loop (P-loop), is considered to be a
structurally determinant factor for the pocket configuration. Moreover, kinases
present an activation loop (A-loop), which starts with a flexible conserved amino
acid sequence Asp-Phe-Gly (DFG). Since the A-loop has flexible variations in size
and sequence, its conformation is critical for modulating kinase catalytic activity
[6, 8].
156
L. M. Lima et al.