SAR studies of 2-arylthiazolidine-4-carboxylic acid amides: A novel class of cytotoxic agents for prostate cancer

Article abstract of DOI:10.1016/j.bmcl.2005.06.032

In our continuing efforts to develop novel chemotherapeutic agents for prostate cancer, recently we reported the discovery of 2-arylthiazolidine-4- carboxylic acid amides (ATCAAs) as a new class of cytotoxic agents. Several of them were very effective in

Full text of DOI:10.1016/j.bmcl.2005.06.032

Bioorganic & Medicinal Chemistry Letters 15 (2005) 4010–4013
SAR studies of 2-arylthiazolidine-4-carboxylic acid amides: A
novel class of cytotoxic agents for prostate cancer
Veeresa Gududuru,a Eunju Hurh,b Joshua Sullivan,a James T. Daltonb and
Duane D. Millera,*
aDepartment of Pharmaceutical Sciences, College of Pharmacy, University of Tennessee Health Science Center,
Memphis, TN 38163, USA
bDivision of Pharmaceutics, College of Pharmacy, The Ohio State University, Columbus, OH 43210, USA
Received 26 April 2005; revised 7 June 2005; accepted 7 June 2005
Available online 6 July 2005
Abstract—In our continuing efforts to develop novel chemotherapeutic agents for prostate cancer, recently we reported the discov-
ery of 2-arylthiazolidine-4-carboxylic acid amides (ATCAAs) as a new class of cytotoxic agents. Several of them were very effective
in killing specific human prostate cancer cell lines with low/sub-micromolar cytotoxicity and high selectivity against control cells in
our sulforhodamine B assay. Encouraged with these preliminary results, we decided to further optimize this new scaffold to enhance
the potency and selectivity. Current work describes the synthesis, SAR, and biological evaluation of new compounds for their ability
to inhibit the growth of five human prostate cancer cell lines. The cytotoxicity data demonstrated that ATCAAs are sensitive to
simple modifications or changes, which allowed us to understand the minimum structural requirements of this class of compounds
to exhibit potent and selective anticancer activity against prostate cancer cells.
Ó 2005 Elsevier Ltd. All rights reserved.
Prostate cancer is the most common cancer and is the
second leading cause of cancer-related deaths in North
America.1 According to American Cancer Society,
approximately 30,000 men will die from prostate cancer
in the United States in 2005.2 One out of nine men over
65 years of age is frequently diagnosed with prostate
cancer in the United States.3 Age and hormone are
two known factors influencing the incidence of prostate
cancer. Recently, dietary pattern has been identified as
a major factor for the difference in prostate cancer
incidence between Western and Asian countries.3–5
Hormonal ablation, the basis of systemic therapy, will
invariably fail to control the progression of metastatic
prostate cancer in the long run.6 Patients with advanced
or metastatic prostate cancer develop hormone-refracto-
ry status that becomes fatal because of the growth of
androgen-independent tumor cells and the emergence
of tumor clones. Agents that induce apoptosis in meta-
static prostate cancer are necessary for the cancer che-
motherapy and are urgent for the clinical treatment.
Recent signal transduction research has raised the idea
that intracellular signaling mechanisms triggered by
extracellular hormonal factors acting through heterotri-
meric guanine nucleotide-binding protein (G protein)-
coupled receptors (GPCRs) can mediate and sustain
prostate cancer pathologic process.7 Patients with ad-
vanced prostate cancer express elevated levels of GPCRs
and GPCR ligands, suggesting that the GPCR system is
activated in the cancerous gland and may contribute to
tumor growth.8 Importantly, inhibition of G protein sig-
naling attenuates prostate cancer cell growth in animal
models.7 However, the nature of intracellular signaling
pathways mediating mitogenic effects of GPCRs in pros-
tate cancer is poorly defined.
Apoptosis represents a general and delicately efficient
cellular suicide pathway. Most of the currently available
cytotoxic anticancer drugs mediate their effect via apop-
tosis induction in cancer cells.9 Apoptosis is suggested as
one of the major mechanisms for targeted therapy of
various cancers including prostate cancer.10–12 However,
cancer cells become resistant to apoptosis in case of ad-
vanced prostate cancer and do not respond to cytotoxic
chemotherapeutic agents.13 Thus, agents that induce
apoptotic death of hormone-refractory prostate cancer
Keywords: Prostate cancer; GPCRs; Synthesis; SAR.
*
Corresponding author. Tel.: +1 901 4486026; fax: +1 901 4483446;
e-mail: dmiller@utmem.edu
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.06.032

V. Gududuru et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4010–4013
4011
cells could be useful for the treatment of this
malignancy.
ture to give cyclized products (7–11), which were con-
verted to the corresponding Boc derivatives 12–16 as
shown in Scheme 1. Reaction of Boc-protected car-
boxylic acids 12–16 with octadecyl or di-n-octyl amine
using EDC/HOBt gave corresponding amides, which
were treated with TFA to form the target compounds
17–22. All new compounds18 were characterized by
spectroscopy and, in certain cases, by elemental anal-
ysis. The structure and antiproliferative effects of syn-
thesized compounds along with previously reported
ATCAAs (for comparison) are listed in Table 1.
Recently, we reported the discovery of 2-arylthiazoli-
dine-4-carboxylic acid amides (ATCAAs) as a new class
of cytotoxic agents for prostate cancer14 (Fig. 1). These
compounds were obtained as third-generation antican-
cer agents derived from lysophosphatidic acid (LPA),
a small bioactive phospholipid that stimulates cell pro-
liferation, migration, and survival by acting on its cog-
nate GPCRs.15 Accumulating evidence suggests that
LPAÕs actions are concordant with many of the hall-
marks of cancer,16 indicating an important role for
LPA in the initiation or progression of malignant dis-
ease. Indeed, LPA levels are significantly increased in
malignant effusions, and its receptors (LPA1/2/3) are
aberrantly expressed in prostate cancer cells.17 Further,
we showed that ATCAAs induce apoptosis in LNCaP
and PC-3 cells.14 Therefore, we hypothesize that AT-
CAAs represent a novel class of anti-prostate cancer
agents, which were very effective in the inhibition of
growth of human prostate cancer cell lines and capable
of inducing apoptosis. To further understand the struc-
tural features and their anticancer activity, we herein
propose synthetic optimization of ATCAAs toward
potency and selectivity. In this paper, we report the syn-
thesis, structure–activity relationship, and antiprolifera-
tive activity of new ATCAAs for prostate cancer.
The prepared compounds were tested for their potency
and selectivity against five human prostate cancer cell
lines (DU-145, PC-3, LNCaP, PPC-1, and TSU-Pr1)
and RH7777 cells (control cell line) using the sulforhoda-
mine B assay according to a previously reported proce-
dure.14 RH7777 cells are rat hepatoma cells that does
not express LPL receptors. These cells were used as neg-
ative controls to understand whether the antiprolifera-
tive activity of ATCAAs was mediated through
inhibition of LPL receptors. To validate their use as neg-
ative controls, we also examined LPL receptor expres-
sion in these cells and showed that none of the LPL
receptors were expressed in RH7777 cells by RT-
PCR.14 5-Fluorouracil (5-FU) was used as a reference
drug. Analog 17 containing 4-hydroxyphenyl head group
was equally active in all five prostate cancer cell lines, but
was not selective compared to 1 (with 3-hydroxyphenyl
group) against RH7777 cells. Comparison of the IC50
values of 2 and 18 suggests that an increase in the alkyl
chain length of the ether leads to decreased cytotoxicity.
Examination of the cytotoxicity data of ATCAAs sug-
gests that electron-donating substituents on the 2-phenyl
ring increases the biological activity, and compound 3
with 3,4,5-trimethoxyphenyl head group emerged as
one of the most potent and selective cytotoxic agents
from our previous study.14 It was also observed that
3,4,5-trimethoxyphenyl analog was more active than
3,4-dimethoxy and 4-methoxyphenyl derivatives. To fur-
ther optimize the substitution pattern of methoxy groups
on the phenyl ring, 19 was synthesized which showed a
decrease in the potency compared to 3 in all prostate can-
cer cell lines.
The general synthesis of target compounds is shown in
Scheme 1. Accordingly, L-cysteine (5a) or L-penicill-
amine (5b) was allowed to react with appropriate
benzaldehydes (6a–6e) in ethanol at ambient tempera-
Figure 1.
Scheme 1.

4012
V. Gududuru et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4010–4013
Table 1. Antiproliferative effects of ATCAAs
Compound
Structure
IC50 (lM)
RH7777a
31.0
DU-145b
5.7
PC-3b
LNCaPb
PPC-1b
1.2
TSU-Pr1b
4.0
1c
6.7
19.0
4.0
1.7
2c
>20
11.4
>20
5.4
8.7
3.9
2.1
0.82
1.6
1.6
6.1
2.4
1.5
19.6
2.4
3c
0.48
1.1
4c
5.3
6.0
3.0
17
18
19
3.1
5.3
0.82
4.4
3.0
>20
>20
>20
>20
>20
7.3
10.6
0.83
6.1
20
>20
>20
>20
>20
>20
8.7
14.1
18.8
>20
>20
21
>20
>20
>20
22
>20
>20
11.9
>20
>20
>20
5-FU
NDd
12.0
4.9
6.4
3.6
a Control cell line.
b Prostate cancer cell lines.
c ATCAAs for comparison.
d ND, not determined.
We showed that ATCAAs have demonstrated chain
length (lipophilic side chain)-dependent cytotoxicity
with shorter alkyl chain length containing compounds
being less active.14 However, the effect of branching in
the lipophilic tail region of ATCAAs on the biological
activity was not examined before. To investigate the
significance of amide group in ATCAAs, we decided
to replace the amide hydrogen with an alkyl group.
For these two reasons, compound 20 was synthesized
and tested against five human prostate cancer cell
lines. Analog 20 failed to demonstrate cytotoxicity at
concentration below 20 lM in three prostate cancer
cell lines except LNCaP and PPC-1 cells. Central
thiazolidine core in ATCAAs with two chiral centers
plays an important role in providing potency and
selectivity.14 We observed that replacement of the
thiazolidine ring with more stable thiazole ring result-
ed in loss of cytotoxicity.14 Compounds 21 and 22
were prepared to further optimize the central thiazoli-
dine core by dimethyl substitution at C-5 position.
However, this simple structural modification did not
improve the activity. Indeed, 21 and 22 were active
only above 20 lM against all tested five human pros-
tate cancer cell lines.

V. Gududuru et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4010–4013
7. Daaka, Y. Sci. STKE 2004, 2004, 2.
4013
In conclusion, 2-arylthiazolidine-4-carboxylic acid
amides represent a new class of cytotoxic agents for pros-
tate cancer. Furthermore, the anticancer activity of these
analogs is attributed to their ability to induce apoptosis in
prostate cancer cells. In our continued efforts to optimize
ATCAAs toward potency and selectivity, we have pre-
pared and evaluated a new set of compounds for their
ability to inhibit the growth of five human prostate cancer
cell lines. The SAR study revealed that (1) antiprolifera-
tive activity of ATCAAs is sensitive to the position of
the substituents on the phenyl ring, (2) introduction of
dialkyl (i.e., dioctyl) amide group into the tail region
decreases the potency, and (3) modifications to the central
thiazolidine core are not favorable. The present data com-
bined with our earlier SAR results provided an insight
into the important structural requirements of ATCAAs
for their anti-prostate cancer activity. On the basis of
these results, we conclude that our next focus will be to-
wards the synthesis of pure stereoisomers of 3 and their
pharmacological characterization in animal models, the
results of which will be reported in due course.
8. Kue, P.; Daaka, Y. J. Urol. 2000, 164, 2162.
9. Lowe, S. W. Carcinogenesis 2000, 21, 485.
10. Gurumurthy, S.; Vasudevan, K. M.; Rangnekar, V. M.
Cancer Metastasis Rev. 2001, 20, 225.
11. Guseva, N. V.; Taghiyev, A. F.; Rokhlin, O. W.; Cohen,
M. B. J. Cell. Biochem. 2004, 91, 70.
12. Jiang, C.; Wang, Z.; Ganther, H.; Lu, J. Cancer Res. 2001,
61, 3062.
13. Pilat, M.J.; Kamradt, J.M.; Pienta, K.J. Cancer Metastasis
1998–1999, 17, 373.
14. Gududuru, V.; Hurh, E.; Dalton, J. T.; Miller, D. D.
J. Med. Chem. 2005, 48, 2584.
15. Mills, G. B.; Moolenaar, W. Nat. Rev. Cancer 2003, 3,
582.
16. Hanahan, D.; Weinberg, R. A. Cell 2000, 100, 57.
17. Daaka, Y. Biochim. Biophys. Acta 2002, 1582, 265.
18. Characteristic data for some compounds are given
below. Compound 18: 1H NMR (300 MHz, CDCl3) d:
0.89 (t, J = 6.6 Hz, 3H), 1.27 (s, 32H), 1.4–1.43 (m, 3H),
3.29–3.34 (m, 2H), 3.38–3.41 (m, 1H), 3.71 (dd, J = 11.1,
3.9 Hz, 1H), 4.01–4.09 (m, 2H), 4.32–4.36 (m, 1H), 5.30
(d, J = 12 Hz, 0.7H), 5.59 (d, J = 10.2 Hz, 0.3H), 6.87–
6.92 (m, 2H), 7.39–7.46 (m, 2H); 13C NMR (75 MHz,
DMSO-d6) d: 13.7, 14.4, 22.0, 26.3, 28.6, 28.8, 28.9,
29.0, 31.2, 37.0, 62.8, 65.6, 66.1, 70.6, 71.6, 113.7, 113.9,
128.1, 158.0, 158.3, 169.6, 170.2; MS (ESI) m/z 505
[M+1].Compound 19: 1H NMR (300 MHz, CDCl3) d:
0.89 (t, J = 6.6 Hz, 3H), 1.26 (s, 32H), 3.12–3.45 (m,
3H), 3.72 (dd, J = 7.5, 4.5 Hz, 1H), 3.82 (d, J = 2.1 Hz,
3H), 3.84 (s, 6H), 4.14 (br s, 1H), 4.34 (d, J = 6 Hz,
1H), 5.86 (d, J = 7.5 Hz, 1H), 6.16 (d, J = 3.9 Hz, 2H),
7.38 (m, 1H); 13C NMR (75 MHz, DMSO-d6) d: 13.8,
22.0, 26.1, 26.2, 28.6, 28.9, 31.2, 35.8, 55.2, 55.9, 62.5,
63.8, 66.0, 66.5, 91.5, 105.7, 158.9, 160.7, 169.6, 170.3;
MS (ESI) m/z 551 [M+1].Compound 22: 1H NMR
(300 MHz, CDCl3) d: 0.89 (t, J = 9 Hz, 3H), 1.27 (s,
32H), 1.46 (s, 3H), 1.50 (s, 3H), 2.97 (s, 6H), 3.19–3.30
(m, 2H), 3.58 (s, 0.6H), 3.95 (s, Hz, 0.4H), 5.59 (s,
0.5H), 5.64 (s, 0.5H), 6.25 (t, J = 6 Hz, 1H), 6.72 (dd,
J = 8.7, 1.8 Hz, 2H), 7.38–7.43 (m, 2H); 13C NMR
(75 MHz, CDCl3) d: 13.5, 22.1, 25.5, 26.4, 27.2, 27.7,
28.7, 28.8, 28.9, 29.0, 29.1, 31.3, 38.4, 38.7, 39.9, 67.2,
67.9, 73.3, 74.2, 111.8, 111.9, 127.6, 127.9, 168.0, 169.6;
MS (ESI) m/z 533 [M+1].
Acknowledgment
This research was supported by a grant from the
Department of Defense (DAMD17-01-1-0830).
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