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European Journal of Medicinal Chemistry 43 (2008) 2011e2015http://www.elsevier.com/locate/ejmech

Short communication

1,2,3,4-Tetrahydro-2-thioxopyrimidine analogs of combretastatin-A4

Lauren Lee a, Ryan Davis a, Jenna Vanderham a, Patrice Hills b,Hilary Mackay a, Toni Brown a, Susan L. Mooberry b, Moses Lee a,*

a Department of Chemistry and the Division of Natural and Applied Sciences, Hope College, 35E, 12th street, Holland, MI 49423, United Statesb Department of Physiology and Medicine, Southwest Foundation for Biomedical Research, San Antonio, TX 78227, United States

Received 23 September 2007; received in revised form 28 November 2007; accepted 29 November 2007

Available online 14 December 2007

Abstract

Eleven 1,2,3,4-tetrahydro-2-thioxopyrimidine analogs of combretastatin-A4 (CA-4) were synthesized and their cytotoxicity againstthe growth of two murine cancer cell lines (B16 melanoma and L1210 leukemia) in culture was determined using an MTT assay. Two 2-thioxopyrimidine analogs 8f and 9a exhibited significant activity (IC50< 1 mM for L1210 and <10 mM for B16 cells). Exposure of A-10 cellsto 8f and 9a produced a significant reduction in cellular microtubules in interphase cells, with an EC50 value of 4.4 and 2.9 mM, respectively, formicrotubule loss. Molecular modeling studies using MacSpartan indicated that the two active 2-thioxopyrimidine analogs preferably adopta twisted conformation, similar to CA-4, affirming that conformation and structure are connected to activity.� 2007 Elsevier Masson SAS. All rights reserved.

Keywords: Tubulin; Combretastatin; Chalcone; Thiourea; Cytotoxicity

Vascular targeting agents (VTAs), such as combretastatin-A4 (CA-4, 1, Fig. 1), are effective antitumor agents. VTAsrapidly and specifically disrupt the abnormal tumor vascula-ture, resulting in vascular collapse and tumor necrosis [1].There are data to suggest that the antivascular actions mightbe mediated through the vascular endothelial-cadherin signal-ing pathway [1b]. CA-4 (1) is a natural product extracted fromthe African Willow Tree, Combretum caffrum and it inhibitstubulin polymerization by interacting with the colchicine bind-ing site on tubulin [2]. This alters the morphology of endothe-lial cells and causes vascular shutdown and regression oftumor vasculature. However, the use of CA-4 (1) as a clinicalantitumor agent is limited by its low bioavailability and pooraqueous solubility [3]. These drawbacks have led to the devel-opment of water-soluble derivatives and analogs as depicted inFig. 1, which include a phosphate-containing pro-drug (CA-4P, 2), an amino analog 3 [5a] and an amino acid derivative(AVE-8062, 4) [4]. These structural analogs have proven to

* Corresponding author. Tel.: þ1 616 392 7190; fax: þ1 616 392 7923.

E-mail address: lee@hope.edu (M. Lee).

0223-5234/$ - see front matter � 2007 Elsevier Masson SAS. All rights reserved.

doi:10.1016/j.ejmech.2007.11.030

be effective VTAs and they have antitumor actions aloneand in combination with current cancer treatments such ascytotoxic chemotherapy, radiation, radio-immunotherapy, andanti-angiogenic agents [1].

Further examples of cytotoxic analogs of CA-4, 1, that havebeen reported include furanones [5b,6a], isoxazoles [6b], im-idazoles [6c], triazoles [6d], azetidinones [6e], pyrazoles(e.g., 5) [6f], pyrazolines (e.g., 6) [7], and cyclohexenones(e.g., 7) [8]. The latter three heterocyclic derivatives of CA-4 (1) were synthesized in the author’s laboratory and theirstructures are given in Fig. 1. The compounds showed varyingdegrees of cytotoxic potency, but the pyrazole-compound 5showed a significant loss in potency against murine cancercells growing in culture. The X-ray crystallography structureof a close analog of pyrazole 5 (in which the eOH group isreplaced with an eOCH3 group) [6f] revealed that the com-pound adopted a planar conformation, lacking the twisted ge-ometry of CA-4 needed to bind optimally to tubulin [9]. Theplanar conformation of 3,5-diarylpyrazoles was also predictedfrom molecular modeling studies [7,8]. Interestingly, the pyr-azoline 6 and cyclohexenone 7 compounds have significant cy-totoxicity and they cause a major loss of cellular microtubules

NHHN

H3CO

H3CO

OCH3

R1

R2

R3

R4

R5

S

8a, R1=R5=H; R2=R3=R4=OCH38b, R2=R4=H; R1=R3=R5=OCH38c, R1=R4=R5=H; R2=R3=OCH38d, R2=R3=R5=H; R1=R4=OCH38e, R1=R3=R4=R5=H; R2=OCH38f, R1=R4=R5=H; R3=OCH3; R2=OH8g, R1=R4=R5=H; R2=NO2; R3=OCH38h, R1=R2=R4=R5=H; R3=NO28i, R1=R2=R4=R5=H; R3=Cl

H3CO

H3CO OCH3

OR

OCH3

1, R=H (CA-4)2, R=PO3

-2 (CA-4P)

H3CO

H3CO OCH3

NHR

OCH3

3, R=H (AC-7739)4, R=amino acid (AVE-8062)

H3CO

H3CO

OCH3

NHNOH

OCH3

5

H3CO

H3CO

OCH3

NHNOH

OCH3

6

H3CO

H3CO

OCH3

OH

OCH3

O

7

NHHN

OCH3

R1

R2

R3

R4

R5

S

OCH3

9a, R2=R4=H; R1=R3=R5=OCH39b, R1=R4=R5=H; R2=NO2; R3=OCH3

A B

Type I

Type II

Fig. 1. Structures of combretastatin-A4 (CA-4, 1) and its water-soluble derivatives 2e4 and analogs 5e7, including pyrazole, pyrazoline and cyclohexenone de-

rivatives of chalcones, 5e7, respectively. Two general types of 1,2,3,4-tetrahydro-2-thioxopyrimidine, I for compounds 8aei and II for 9a,b.

2012 L. Lee et al. / European Journal of Medicinal Chemistry 43 (2008) 2011e2015

in A-10 cells [7,8]. Molecular modeling studies confirmed thatboth compounds 6 and 7 preferred twisted geometries [2].

Despite past successes in designing analogs of CA-4, 1, thatare more soluble in biological media, the task of creating mol-ecules that are equally potent as an anticancer agent as CA-4has been far more challenging. As part of a program to developnovel heterocyclic analogs of CA-4, a series of novel 1,2,3,4-tetrahydro-2-thioxopyrimidine analogs was designed with the

Fig. 2. Energy optimized structures of two 1,2,3,4-tetrahydro-2-thioxopy

purpose of examining their cytotoxic properties and to corre-late the results with the conformational ‘‘twist’’ of the mole-cules. An additional objective for synthesizing this class ofcompounds is to search for analogs of CA-4 that have good wa-ter solubility and are biologically active. 1,2,3,4-Tetrahydro-2-thioxopyrimidine analogs are attractive for our studies becauseonly a small handful of 1,2,3,4-tetrahydro-2-thioxopyrimidinehave been made and reported [10a], and none has been

rmidine analogs: 9a (left) and 8f (right), type II and I, respectively.

Table 1

Cytotoxicity of compounds 8aei (type I) and 9a,b (type II) against the growth

of murine cancer cells grown in culturea

Compound IC50 (mM) (type I)

B16 L1210

Ar¼

8a >100 >100

8b 6.3 5.7

8c >100 56.3

8d 36 32

8e 59.5 >100

8f 6.0 0.5

8g >100 >100

8h >100 >100

8i 55.5 48.3

Compound IC50 (mM) (type II)

B16 L1210

Ar¼

9a 6.1 0.4

9b 42 2.2

a IC50 of CA-4 (1) against L1210 and B16 are 0.002 and 0.003 mM, respec-

tively [13,14].

2013L. Lee et al. / European Journal of Medicinal Chemistry 43 (2008) 2011e2015

investigated as potential analogs of CA-4 [10b]. Eleven specificanalogs were synthesized and they are divided into two types: I,which contains a 3,4,5-trimethoxyphenyl moiety in the A-ring(8aei), and II, containing a 2,5-dimethoxyphenyl group in theA-ring (9a,b). The conformation of analogs, 8f and 9a, were as-sessed by molecular modeling studies using MacSpartan,a strategy that was reported earlier [7,8]. The molecular struc-tures were optimized by molecular mechanics (MMFF) usinga molecular equilibrium conformer procedure. The equilibriumgeometry was subsequently optimized using Hatree-Fock (3-21 G) calculations, followed by a density-functional calcula-tion (B3LYP and 6-31 G). The conformations of compounds8f and 9a are shown in Fig. 2, and it is evident that the mole-cules adopt a twisted geometry similar to that of CA-4 [9].

The 1,2,3,4-tetrahydro-2-thioxopyrimidine analogs 8aeiand 9a,b were synthesized by reaction of the appropriate chal-cone [11] with 2.5 mol equivalents of thiourea and potassiumcarbonate in refluxing ethanol (overnight), Scheme 1. Uponcompletion, the cooled reaction mixture was poured into ice-water and the precipitate was collected. When needed, theprecipitate was recrystallized from methanol. The chemicalyields were between 40e90% and the structures were ascer-tained by 400 MHz 1H NMR, infrared, and mass spectrometrymeasurements.

The cytotoxicity of the newly synthesized compounds wastested in an in vitro 72-h MTT assay [12]. The analogs weretested against L1210 and B16 cell lines (murine leukemiaand melanoma, respectively) and the compound concentration(mM) required to reduce the cell population by 50% relative toan untreated control (IC50 value) was determined. The resultsare shown in Table 1. It is worthy to note that the thioxopyr-imidine derivatives have good solubility in cell culture media,ca. >17.5 mM.

The cytotoxicity results showed three trends. First, three an-alogs (8b,f and 9a) displayed significant activity against bothcell lines, comparable to the pyrazoline [7] and cyclohexenone[8] analogs reported earlier; however, L1210 cells were gener-ally more sensitive to the agents. It is reasonable to expect com-pound 8f of the type I design to be active, since analogs 5[5b,6], 6 [7], and 7 [8], and CA-4, (1), which have a similar sub-stitution pattern are highly active. Second, the activity of theanalogs that contain a 2,4,6-trimethoxy substituted B-ring is in-teresting; particularly as compound 9a, a type II molecule,which has a completely different substitution pattern fromCA-4 (1), is as potent as 8f. Analysis of the molecular models

ThioureaK2CO3

NHHN

S

H3CO

H3CO

OCH3

OH

OCH3

O

H3CO

H3CO

OCH3

OH

OCH3EtOH, reflux,

overnight

8f

Scheme 1. A representative synthesis of 1,2,3,4-tetrahydro-2-thioxopyrimidine 8f.

2014 L. Lee et al. / European Journal of Medicinal Chemistry 43 (2008) 2011e2015

given in Fig. 2 provided a plausible explanation, in which theconformation of 9a is twisted due to steric hindrance betweenortho-methoxy groups on both phenyl groups. This explanationis consistent with the lack of activity for compound 8a, a type Imolecule that contains a 3,4,5-trimethoxy substituted pattern inthe B-ring. Finally, as with many of the analogs of CA-4 (1) thathave been synthesized and tested, the nitro-group containingcompounds were generally inactive even at 100 mM, exceptcompound 9b which gave IC50 values of 2.2 and 42 mM forL1210 and B16, respectively.

To gain insight into the mechanism of action for the 2-thioxopyrimidine analogs, the effects of compounds 8f and9a on interphase cellular microtubules were evaluated in A-10 aortic smooth muscle cells [8,]. The results given in Fig. 3provided clear evidence that compound 8f was very active,with an EC50 value of 4.4 mM (effective concentration to cause50% loss of cellular microtubules). Compound 9a was alsovery potent (data not shown) and it gave an EC50 value of2.9 mM. With the EC50 values being comparable to the cytotox-icity IC50 values, the data suggest that microtubule disruption isa likely mechanism for both compounds to exert their biologi-cal activity. For comparison, CA-4 is still significantly morepotent in disrupting microtubules under similar conditions,with an EC50 value of 0.007 mM [9a]. These results provide ad-ditional evidence to support our earlier suggestions [7,8] thatthe conformational ‘‘twist’’ of CA-4 analogs as well as thecomposition and positioning of substituents are important pa-rameters for biological activity.

Fig. 3. Effects on interphase microtubules in A-10 Cells: (A) control and (B)

10 mM 8f.

In summary, the novel 1,2,3,4-tetrahydro-2-thioxopyrimi-dine compounds 8aei and 9a,b, analogs of CA-4 (1), havebeen found to have significant cytotoxic activity against cancercells grown in culture. Even though the IC50 values of the ‘‘ac-tive’’ compounds are larger (or less potent) than CA-4 itself, 1,the new compounds have significant advantages: enhancedsolubility in aqueous biological media and ease of synthesis.

Acknowledgements

The authors are grateful to Taiho Pharmaceutical Co. ofJapan, Hope College and the William Randolph Hearst Foun-dation (SLM) for support.

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