Aryl tetrahydropyridine inhibitors of farnesyltransferase: Glycine, phenylalanine and histidine derivatives

Article abstract of DOI:10.1016/S0960-894X(03)00095-7

Inhibitors of farnesyltransferase are effective against a variety of tumors in mouse models of cancer. Clinical trials to evaluate these agents in humans are ongoing. In our effort to develop new farnesyltransferase inhibitors, we have discovered a series of aryl tetrahydropyridines that incorporate substituted glycine, phenylalanine and histidine residues. The design, synthesis, SAR and biological properties of these compounds will be discussed.

Full text of DOI:10.1016/S0960-894X(03)00095-7

Bioorganic & Medicinal Chemistry Letters 13 (2003) 1359–1362
Aryl Tetrahydropyridine Inhibitors of Farnesyltransferase:
Glycine, Phenylalanine and Histidine Derivatives
Stephen L. Gwaltney, II,* Stephen J. O’Connor, Lissa T. J. Nelson, Gerard M. Sullivan,
Hovis Imade, Weibo Wang, Lisa Hasvold, Qun Li, Jerome Cohen, Wen-Zhen Gu,
Stephen K. Tahir, Joy Bauch, Kennan Marsh, Shi-Chung Ng, David J. Frost,
Haiying Zhang, Steve Muchmore, Clarissa G. Jakob, Vincent Stoll, Charles Hutchins,
Saul H. Rosenberg and Hing L. Sham
Pharmaceutical Discovery, R47B, Building AP-10, Abbott Laboratories, Abbott Park, IL 60064-6101, USA
Received 12 August 2002; accepted 2 January 2003
Abstract—Inhibitors of farnesyltransferase are effective against a variety of tumors in mouse models of cancer. Clinical trials to
evaluate these agents in humans are ongoing. In our effort to develop new farnesyltransferase inhibitors, we have discovered a series
of aryl tetrahydropyridines that incorporate substituted glycine, phenylalanine and histidine residues. The design, synthesis, SAR
and biological properties of these compounds will be discussed.
# 2003 Elsevier Science Ltd. All rights reserved.
Ras oncogene mutations are the most frequently identified
in human tumors, especially those of the pancreas,
colon, lung and bladder. Mutations in ras genes lead to
uncontrolled cell growth. Post-translational farnesylation
of Ras (alkylation of the protein by a C-15 isoprenoid)
is required for the association of mature proteins with
the cell membrane. This association allows for the
transmission of extracellular signals to the nucleus and
leads to cell proliferation. Farnesylation is catalyzed by
protein farnesyltransferase (FT). A related enzyme,
geranylgeranyltransferase-I (GGT-I) carries out ger-
anylgeranylation (alkylation of the protein by a C-20
isoprenoid).1
Since GGT-I is responsible for the majority of protein
prenylations and it has been shown that concomitant
inhibition of FT and GGT-I in vivo leads to substantial
toxicity,3 we sought a selective inhibitor of FT. Initi-
ally, all our compounds were tested for their ability to
inhibit FT and GGT-I in vitro. Potent compounds
were then tested in a cellular assay that measured
inhibition of the farnesylation of Ras (Ras processing,
RP).4
Abbott’s first generation FTI, ABT-839, is shown in
Figure 1. This compound is a potent inhibitor of FT
(IC50=1.1 nM) and is selective for FT over GGT-I
(75,000 fold). The compound also demonstrates good
cellular activity (Ras processing EC50=16 nM) and in
vivo efficacy. However, the compound suffers from poor
pharmacokinetics.
Farnesyltransferase inhibitors (FTIs) were designed to
exploit the requirement of post-translational farnesyla-
tion of Ras in oncogenesis. It is now recognized that
protein targets other than Ras, such as Rho B, are
important for the antitumor effects of farnesyltransfer-
ase inhibitors (FTIs).1 While the mechanism of action of
these agents is still debated, the compounds have shown
impressive efficacy in preclinical models of human can-
cer. Clinical trials of these agents have also shown that
they possess activity alone and in combination with
cytotoxic agents.2
*Corresponding author. Tel.: +1-858-731-3562; fax: +1-858-550-
0526; e-mail: stephen.gwaltney@syrrx.com
Figure 1. ABT-839.
0960-894X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0960-894X(03)00095-7

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The crystal structure of ABT-839 bound to FT is shown
in Figure 2.5 Note that the compound does not interact
strongly with the active site zinc.
Intermediate 6 was also used in the preparation of FTIs
11 and 14. Acylation of 6 with the carbamoyl chloride
97 gave ester 10. Saponification then provided the target
compound. The synthesis of 14 began with known thia-
zolyl aldehyde 12.8 Treatment with cyclohexylethyl
Grignard gave alcohol 13, which was activated and
coupled to 6. Saponification then gave 14.
In an effort to discover new FTIs, a tetrahydropyridine
core was used to replace the biphenyl core of ABT-839.
This provided for easy substitution on nitrogen and
increased hydrophilicity. The synthesis of the tetra-
hydropyridine core is shown in Scheme 1. Esterification
of acid 1 followed by diazotization and iodination gave
the known iodoester 2.6 Suzuki coupling utilizing
o-tolylboronic acid led to tolylpyridine 3 in 98% yield.
Quaternization of the pyridine followed by reduction
gave the benzyltetrahydropyridine 4.
While these compounds were potent, they suffered from
poor pharmacokinetics. The methionine residue proved
to be a metabolic liability, suffering oxidation on sulfur.
After substantial work in another series of FTIs at
Abbott, it was discovered that a nitrile could serve as a
suitable replacement for the methionine. Armed with
this knowledge, we set out to prepare a cyano-sub-
stituted tetrahydropyridine core. Syntheses of the cyano
substituted tetrahydropyridine core are shown in
Schemes 3 and 4.
This core was then used to prepare the methionine con-
taining tetrahydropyridines as shown in Scheme 2.
Treatment of 4 with 1-chloroethylchloroformate (ACE-
Cl) in the presence of di-t-butyldicarbonate led to the
Boc-protected amine with concomitant cleavage of the
methyl ester. EDC coupling of 5 to methionine methyl
ester followed by removal of the Boc group gave 6.
Treatment of 6 with chloroacetylchloride followed by
addition of 77 gave the methyl ester which was saponi-
fied to FTI 8.
Figure 2. ABT-839 bound to farnesyltransferase.
Scheme 2. Synthesis of methionine containing tetrahydropyridine
FTIs.
Scheme 1. Synthesis of compound 4.

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1361
Scheme 3. Synthesis of compound 18.
Scheme 5. Glycine derivatives.
Scheme 4. Improved synthesis of compound 18.
Chlorination9 of 15 followed by a Suzuki coupling gave
aryl pyridine 16. Quaternization and reduction as
described in Scheme 1 gave the protected tetra-
hydropyridine, which was deprotected using ACE-Cl.
Scheme 6. Phenylalanine derivatives.
While the method shown in Scheme 3 provided the tar-
get, it was found to be inconvenient. Workup of the
chlorination proved tedious and this difficult isolation
led to lowered yields. In addition, the reduction of the
pyridiniumsalt proceeded in low yield and led to
regioisomeric by-products. The overall yield for this
sequence is 2–8%. In an effort to improve on this
method, the sequence shown in Scheme 4 was adopted.
Alkylation of the commercially available 19 with 20 was
followed by a Dieckmann cyclization to give enol 21.
Triflation followed by Suzuki coupling then provided
the protected tetrahydropyridine. Cleavage of the ben-
zyl group was then accomplished as shown earlier. This
method proceeds in approximately 35% overall yield,
can be carried out on large scale easily and allows for
variation of the aryl group.
similar potency. We noted little difference in potency
between the l- and d-phenylalanine derivatives.
We then examined analogues derived from histidine
(Scheme 7). Again, much the same chemistry was used
for this work. We found it convenient to install both the
N-benzyl and N-methyl groups using reductive ami-
nation. The first two compounds in this series showed
surprisingly different potency based on the stereo-
chemistry at the alpha carbon, with the d-histidine
derivative proving 100 times more potent than the
l-histidine derivative. This result is in sharp contrast
with that seen in the phenylalanine containing com-
pounds discussed above.
A crystal structure of compound 33 was obtained and
shows that this inhibitor overlays well with ABT-839 in
the FT active site (Fig. 3).10 In contrast to the binding
of ABT-839, compound 33 makes a good interaction
with the zinc atom.
With the core tetrahydropyridine in hand, various series
of derivatives were prepared. We first examined the
N,N-disubstituted glycine derivatives shown in Scheme
5. These were prepared fromthe tetrahydropyridine by
first carrying out an EDC coupling with a protected
glycine followed by deprotection. Reductive amination
then installed the benzyl and imidazolyl methyl groups.
Compound 33, while potent and selective for FT was
markedly less potent in the cellular assay. In an effort to
modulate the polarity11 of the compound and improve
cellular activity, we prepared the N-methyl derivative,
37. This compound was of similar in vitro potency as
the parent, but did not realize a significant gain in
potency in the cellular assay.
These compounds did not possess the potency required
and were not evaluated further.
We next examined inhibitors containing para-cyano-
phenylalanine derived substituents on the tetrahydro-
pyridine (Scheme 6). These compounds were prepared
in the same way as the glycine derivatives and were of
The farnesyltransferase inhibitors described here,12
while potent in vitro, did not possess the cellular activity

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required. However, the efficient synthesis of the tetra-
hydropyridine core provided a useful template that can
be easily modified to give potent FTIs. In the following
paper, we describe further efforts to improve the cellular
activity of these inhibitors and to discover compounds
with good pharmacokinetics.
Acknowledgements
Crystallographic data were collected at beamline 17-ID
in the facilities of the Industrial Macromolecular Crys-
tallography Association Collaborative Access Team
(IMCA-CAT) at the Advanced Photon Source. These
facilities are supported by the companies of the Indus-
trial Macromolecular Crystallography Association
through a contract with Illinois Institute of Technology
(IIT), executed through IIT’s Center for Synchrotron
Radiation Research and Instrumentation. Use of the
Advanced Photon Source was supported by the US
Department of Energy, Basic Energy Sciences, Office of
Science, under Contract No. W-31-109-Eng-38.
References and Notes
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Scheme 7. Histidine derivatives.
2. Bell, I. M. Exp. Opin. Ther. Patents 2000, 10, 1813.
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5. Coordinates deposited (PDB ID 1N94).
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10. Coordinates deposited (PDB ID 1N95).
11. C log P calculations were performed with Biobyte 1.5.
12. All compounds reported have physical data consistent
with the assigned structures.
Figure 3. Superimposed X-ray crystal structures of ABT-839 and
compound 33.