H. R. Heyman et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1246–1249
1249
Table 5. KDR fold-potency versus a series of tyrosine kinases
Compound FLT1 FLT3 KIT PDGFRb TIE2 FGFR SRC
In summary, a series of potent KDR thienopyridine
urea inhibitors has been identified. Optimal urea and
7-substitution afforded analogs (e.g., 2) with significant
enzymatic and cellular potency as well as selectivity
against the non-VEGF tyrosine kinases FGFR and
SRC. Homology modeling suggested that the predomi-
nant interactions include hydrogen-bonds between the
thienopyridine nucleus and the protein backbone as well
as the urea functionality with a glutamate residue and a
hydrophobic pocket.
7b
2
0.6
1
1
0.5
2
2
10
13
40
5
>1000 >1000
>440 >440
The selectivity of analogs 7b and 2 for KDR versus
other tyrosine kinases as a ratio of enzymatic IC50 val-
ues is shown in Table 5. These inhibitors were equipo-
tent against the kinases most homologous to KDR
(FLT1, FLT3, and KIT), modestly potent against
PDGFRb and TIE2, and much less active against
FGFR and the cytosolic kinase SRC.
References and notes
Further characterization of these inhibitors indicated
that, while 7-unsubstituted analogs such as 7b were only
modestly active in vivo (40% inhibition, 30 mg/kg dose,
po), as measured in an estradiol-induced mouse uterine
edema model,7 inhibitors such as 2 possessed excellent
potency (ED50 2.1 mg/kg). However, it was determined
that 2 had a disappointing pharmacokinetic profile in
mouse (t1/2 0.4 h, 3 mg/kg dose, iv) with low exposure
after oral dosing (1.1 lmol h/L, 10 mg/kg dose, po) which
precluded its assessment in a mouse tumor model. Pyridyl
2 was also a potent inhibitor of Cyp3A4 (IC50 ꢀ250 nM).
Homology modeling17 of inhibitor 2 bound to the ATP-
binding site of the ‘inactive’ conformation of KDR is
reminiscent of our earlier analyses of urea KDR inhibi-
tors (Fig. 1).7,12a In this analysis, two hydrogen bonds
were created with the kinase hinge: the exocyclic amino
group of 2 with the backbone carbonyl of Glu917; and
the proximal ring nitrogen of the thienopyridine with
the backbone N–H of Cys919. In this bound conforma-
tion the urea unit accessed the back hydrophobic pocket
adjacent to the ATP-binding site with the urea N–H
bonds interacting with Glu885 of KDR. The terminal
tolyl group rested in a hydrophobic region comprised
of the side chains of Ile888, Leu889, Leu1019, and
Val898. As mentioned, the 7-pyridyl of inhibitor 2 pro-
jects into a solvent-accessible region of the active site
without any specific contact with the protein and is con-
sistent with the fact that most of the 7-substituted ana-
logs (2 and 12a–f) are equipotent with 7b.
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ATP concentration of 1 mM as described in Ref. 7.
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16. The cellular IC50 values represent inhibition of KDR
phosphorylation in NIH3T3 cells stably transfected with
full length human KDR. The procedure was followed as
described in Ref. 12.
17. The computational analysis used to create this homology
model with a recently published crystal structure of KDR-
homolog KIT kinase (PDB entry 1T46) is described in
Ref. 7.
Figure 1. Model of thienopyridine 2 (green) bound to active site of
KDR (model, inactive conformation, in purple) with hinge hydrogen
bonds to Glu917 C@O and Cys919 N–H in black and urea hydrogen
bonds to Glu885 in brown.