Bioorganic & Medicinal Chemistry Letters 18 (2008) 5648–5652
Bioorganic & Medicinal Chemistry Letters
Design and synthesis of 2-amino-pyrazolopyridines as Polo-like kinase
1 inhibitors
Raymond V. Fucini a, , , Emily J. Hanan b, , Michael J. Romanowski c, Robert A. Elling c, Willard Lew b,
*
Kenneth J. Barr b, Jiang Zhu b, Joshua C. Yoburn b, Yang Liu b, Bruce T. Fahr b, Junfa Fan b, Yafan Lu b,
Phuongly Pham b, Ingrid C. Choong b, Erica C. VanderPorten a, Minna Bui b, Hans E. Purkey b,
Marc J. Evanchik a, Wenjin Yang b
a Department of Biology, Sunesis Pharmaceuticals, Inc., 395 Oyster Point Boulevard Suite 400, South San Francisco, CA 94080, USA
b Department of Chemistry, Sunesis Pharmaceuticals, Inc., 395 Oyster Point Boulevard Suite 400, South San Francisco, CA 94080, USA
c Department of Structural Biology, Sunesis Pharmaceuticals, Inc., 395 Oyster Point Boulevard Suite 400, South San Francisco, CA 94080, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 2-amino-pyrazolopyridines was designed and synthesized as Polo-like kinase (Plk) inhibitors
based on a low micromolar hit. The SAR was developed to provide compounds exhibiting low nanomolar
inhibitory activity of Plk1; the phenotype of treated cells is consistent with Plk1 inhibition. A co-crystal
structure of one of these compounds with zPlk1 confirms an ATP-competitive binding mode.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 12 June 2008
Revised 25 August 2008
Accepted 26 August 2008
Available online 29 August 2008
Keywords:
2-Amino-pyrazolopyridine
Polo-like kinase (Plk)
SAR
Kinase inhibitor
Polo-like kinases (Plks) belong to a highly conserved family of
serine/threonine kinases that play critical roles during multiple
stages of mitosis.1–3 Four Plks have been identified in mammals
(Plk1–4), each containing an N-terminal catalytic domain and a
C-terminal domain containing one or two highly conserved
sequences called polo box domains (PBDs).1 The best-characterized
human Plk is Plk1. Plk1 is expressed maximally during late G2 and
M phases of the cell cycle and contributes to the regulation of
centrosome maturation, bipolar spindle formation, and
cytokinesis.1–4 Inhibition of Plk1 activity with small molecules
leads to the formation of aberrant mitotic spindle poles, which in
turn triggers the spindle assembly checkpoint (SAC), resulting in
mitotic arrest and strong induction of apoptosis.1,5–7 Plk1 is
overexpressed in a broad range of human tumors and this
overexpression is positively correlated with aggressiveness and
poor prognosis in many cancers.1,4,5 Plk1 is thus considered a good
target for chemotherapeutic intervention.1,4,5
in complex with the kinase domain of zebrafish Plk1 (zPlk1), a
homolog that possesses a single conserved amino acid substitution
(zPlk1 Ile118 = hPlk1 Leu132) for a residue lining the active site
compared to human Plk1.16
The identification of 1 as a Plk1 inhibitor prompted further
investigation into this class of compounds. A SAR study was ini-
tiated, with a particular focus on the phenyl group linked to the
pyrazole nitrogen. Compounds 1–22 were synthesized as shown
in Scheme 1. Commercially available 2,4-dichloro-nicotinic acid
methyl ester was first hydrolyzed to the carboxylic acid and
then converted to acyl chloride 2 by reaction with oxalyl chlo-
ride. Treatment of 2 with copper iodide and methyl lithium at
low temperature yielded methyl ketone 3.8 Cyclization of 3 to
pyrazole
4 was effected through treatment with hydrazine.
Reaction of 4 with aryl halides in the presence of copper iodide,
potassium carbonate, and N,N-dimethylglycine resulted in N-
arylation of the pyrazole ring.9 Standard Buchwald–Hartwig
amination conditions provided the desired compounds 1 and
6–22.10
As part of our drug discovery efforts, we identified 2-amino-
pyrazolopyridine
1 as a moderate inhibitor of Plk1. In this
communication, we describe the synthesis and structure–activity
relationship (SAR) development of this novel chemical series, and
present an X-ray crystal structure of a small-molecule inhibitor
The SAR of these compounds is detailed in Table 1. Moving a
methyl or fluoro group around the phenyl ring indicated that
substitution at the 3-position is preferred. However, the
electron-donating methoxy group is not well tolerated except in
the 4-position. Further exploration of the 3-position confirmed
the preference for relatively small electron withdrawing groups,
with the 3-chloro (compound 15) being the most active as
* Corresponding author. Tel.: +1 650 266 3500; fax: +1 650 266 3505.
These authors contributed equally to this work.
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.08.095