The active DFG-in conformation of a kinase is characterized by the conserved
DFG Asp residue facing the ATP-binding site. This Asp coordinates to Mg
2+ ions,
forming a bridge contact with the ATP’s triphosphate group, while the side chain of
the Phe residue is contained in a hydrophobic adjacent pocket (Fig. 1) [5, 7].
In turn, in the so-called DFG-out conformation, the side chain of Asp is placed
away from the ATP pocket through a rearrangement of the DFG loop, preventing
ATP binding and prompting the phenyl ring to leave the aforementioned adjacent
hydrophobic pocket (Fig. 1) [5, 9].
Several cancer types are associated with genetic modifications or abnormalities in
RTKs, resulting in increased activity, abundance, and/or cellular distribution. Therefore, many efforts have been dedicated to the search of novel drugs able to block or
attenuate RTK signaling pathways, with a number of RTK inhibitors already
approved by the US Food and Drug Administration (FDA) [2, 7, 10].
The modulation strategies include small-molecule tyrosine kinase inhibitors
(TKIs), targeting the ATP-binding site of the intracellular TK domain [7, 10], and
monoclonal antibodies directed to the extracellular domain, interfering with RTK
activation and/or marking RTK-expressing cells for destruction by the immune
system [1].
TKIs can be, in turn, subdivided according to their preferential interactions with
active DFG-in or inactive DFG-out target conformations. Type I TKIs interact in and
around the ATP-binding region of the target in a DFG-in conformation, performing
hydrogen bonds with the hinge residue and additional hydrophobic and electrostatic
interactions with the adjacent amino acid residues, which can differ between the TK
family members [11]. In contrast, type II TKIs bind and stabilize the inactive
DFG-out conformation of the target RTK, not only interacting with the
ATP-binding site but also occupying the adjacent hydrophobic pocket available in
this conformation as an allosteric binding site (Fig. 1) [6, 7]. This chapter focuses
particularly on the discussion of several case studies on the development of smallmolecule tyrosine kinase inhibitors (TKIs) directed to clinically relevant RTKs and
the subset of advances in this field.
2 Epidermal Growth Factor Receptor (EGFR)
Epidermal growth factor receptor (EGFR, ErbB-1, HER-1) is a RTK member of the
ErbB receptor family, which is related to cellular growth, differentiation, and
survival. The other members of the ErbB RTK family are ErbB-2 (HER-2), ErbB3 (HER-3), and ErbB-4 (HER-4) [12, 13]. The EGFR activation process is initiated
by extracellular ligand recognition, inducing receptor dimerization with formation of
homodimers of EGFR or heterodimers between EGFR and other ErbB family
members [14, 15], among which ErbB-2 (HER-2) is the most common EGFR
heterodimerization partner [16].
Ligands of the EGFR extracellular binding domain comprise epidermal growth
factor (EGF), transforming growth factor alpha (TGF-α), amphiregulin (AR), epigen
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DFG Asp residue facing the ATP-binding site. This Asp coordinates to Mg
2+ ions,
forming a bridge contact with the ATP’s triphosphate group, while the side chain of
the Phe residue is contained in a hydrophobic adjacent pocket (Fig. 1) [5, 7].
In turn, in the so-called DFG-out conformation, the side chain of Asp is placed
away from the ATP pocket through a rearrangement of the DFG loop, preventing
ATP binding and prompting the phenyl ring to leave the aforementioned adjacent
hydrophobic pocket (Fig. 1) [5, 9].
Several cancer types are associated with genetic modifications or abnormalities in
RTKs, resulting in increased activity, abundance, and/or cellular distribution. Therefore, many efforts have been dedicated to the search of novel drugs able to block or
attenuate RTK signaling pathways, with a number of RTK inhibitors already
approved by the US Food and Drug Administration (FDA) [2, 7, 10].
The modulation strategies include small-molecule tyrosine kinase inhibitors
(TKIs), targeting the ATP-binding site of the intracellular TK domain [7, 10], and
monoclonal antibodies directed to the extracellular domain, interfering with RTK
activation and/or marking RTK-expressing cells for destruction by the immune
system [1].
TKIs can be, in turn, subdivided according to their preferential interactions with
active DFG-in or inactive DFG-out target conformations. Type I TKIs interact in and
around the ATP-binding region of the target in a DFG-in conformation, performing
hydrogen bonds with the hinge residue and additional hydrophobic and electrostatic
interactions with the adjacent amino acid residues, which can differ between the TK
family members [11]. In contrast, type II TKIs bind and stabilize the inactive
DFG-out conformation of the target RTK, not only interacting with the
ATP-binding site but also occupying the adjacent hydrophobic pocket available in
this conformation as an allosteric binding site (Fig. 1) [6, 7]. This chapter focuses
particularly on the discussion of several case studies on the development of smallmolecule tyrosine kinase inhibitors (TKIs) directed to clinically relevant RTKs and
the subset of advances in this field.
2 Epidermal Growth Factor Receptor (EGFR)
Epidermal growth factor receptor (EGFR, ErbB-1, HER-1) is a RTK member of the
ErbB receptor family, which is related to cellular growth, differentiation, and
survival. The other members of the ErbB RTK family are ErbB-2 (HER-2), ErbB3 (HER-3), and ErbB-4 (HER-4) [12, 13]. The EGFR activation process is initiated
by extracellular ligand recognition, inducing receptor dimerization with formation of
homodimers of EGFR or heterodimers between EGFR and other ErbB family
members [14, 15], among which ErbB-2 (HER-2) is the most common EGFR
heterodimerization partner [16].
Ligands of the EGFR extracellular binding domain comprise epidermal growth
factor (EGF), transforming growth factor alpha (TGF-α), amphiregulin (AR), epigen
158
L. M. Lima et al.