Bioorganic & Medicinal Chemistry Letters 22 (2012) 431–435

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Bioorganic & Medicinal Chemistry Letters

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Acyl derivatives of boswellic acids as inhibitors of NF-jB and STATs

Ajay Kumar, Bhahwal A. Shah ⇑, Samar Singh, Abid Hamid, Shashank K. Singh, Vijay K. Sethi,Ajit K. Saxena, Jaswant Singh, Subhash C. Taneja ⇑Natural Product Microbes Division, Indian Institute of Integrative Medicine (CSIR), Canal Road, Jammu Tawi 180 001, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 14 June 2011Revised 14 October 2011Accepted 31 October 2011Available online 9 November 2011

Keywords:Boswellic acidsAcyl derivativesAnti-cancerNF-jBSTAT

0960-894X/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.bmcl.2011.10.112

⇑ Corresponding authors.E-mail addresses: bashah@iiim.ac.in (B.A. Shah), sct

Boswellic acid acylates including their epimers were synthesized and screened against a panel of humancancer cell lines. They exhibited a range of cytotoxicity against various human cancer cell lines therebyleading to the development of a possible SAR. One of the identified lead compounds was found to be aninhibitor of the NF-jB and STAT proteins, warranting further investigations to be developed into a poten-tial anticancer lead.

� 2011 Elsevier Ltd. All rights reserved.

The gum exudate of Boswellia serrata comprises of four majortriterpenic acids known as boswellic acids (BAs), that is, b-boswellicacid (BA), 3-O-a-acetyl-b-boswellic acid (ABA), 11-keto-b-boswellicacid (KBA) and 3-O-a-acetyl-11-keto-b-boswellic acid (AKBA). Thegum resin of B. serrata is traditionally used in Ayurvedic system ofmedicine for the treatment of various ailments.1 In the past manyreports have appeared related to their activities against inflamma-tion,2,3 arthritis,4 ulcerative colitis,5 chronic colitis,6 asthma7 andhepatitis.8 However, it is the anticancer activity that has attractedthe most attention in the recent past due to their ability to induceapoptosis.9,10 They have been reported to inhibit the growth and in-duce apoptosis in brain tumors,11 malignant glioma cells,12 coloncancer cells13 and leukemic cells.14 They have also been found tobe more potent inhibitors of topoisomerase I and II in comparisonto camptothecin, amsacrine or etoposide, using pure topoisomeraseassay.15 Thus, prompted by the potential of BAs and our interest inthe identification and development of potent anticancer leads basedon the natural products16 including BAs,17 we herein report ourwork on alkyl–acyl derivatives of BAs and their potential as inducersof apoptosis. We further envisaged exploring the effect of the mostactive molecule/s on NF-jB (nuclear factor-kappa B) and somemembers of STAT (signal transducer and activator of transcriptionprotein) family. Many previous studies have shown that NF-jBand some STATs (Stat3, Stat5 and Stat6) are persistently active inmany types of leukemia and other tumor types. Stat5 and Stat6are found to be persistently activated in various haematopoietic

ll rights reserved.

aneja@iiim.ac.in (S.C. Taneja).

malignancies, facilitating cell proliferation and inhibition of apopto-sis.18 Stat3 and Stat6 have also been implicated in promoting immu-nosuppressive myeloid derived suppressor cells (MDSCs).19,20

About 50% of acute myeloid leukemia (AML) cases also show consti-tutively active Stat3. Inhibition of constitutively active Stat3 hasbeen shown to induce AML cell apoptosis.21 NF-jB is known to reg-ulate the expression of several anti-apoptotic genes such as IAPs,Bcl-2, Bcl-xL and survivin.22 In addition, NF-jB regulates the expres-sion of several genes involved in cancer cell proliferation andinflammation; these include COX-2, TNF-a, IL-1b, IL-6, IL-23, etc.Interestingly, some of these genes are also regulated by Stat3. Par-ticularly, the association of NF-jB and Stat3 plays an important rolein all aspects of cancer development. In fact Stat3 and NF-jB are clo-sely linked and both of these two transcription factors require eachother for their persistent activation in the cancer cells.18 Stat3 isfound to be crucial for growth of several cancer types but its expres-sion is dispensable for normal cells in postembryonic stage.23 Thismakes NF-jB and Stat3 an ideal target for the development of anti-cancer therapeutics.

The idea behind the preparation of the alkyl–acyl analogs was tostudy the effect of alkyl–acyl groups of varying carbon length on 3-hydroxyl functionality as well as on its stereochemistry for the bio-activity of BAs, which consequently may help in the establishment ofa possible SAR and suggest directions for future studies. Thus, vari-ous acyl analogs of boswellic acids and their epimers were preparedusing acylating agents. In order to prepare the specific 3-O-a-for-myl-b-boswellic acid (5), BA was added slowly to the formylating re-agent (prepared by adding formic acid to acetic anhydride) withconstant stirring at room temperature. The other acyl analogs of

432 A. Kumar et al. / Bioorg. Med. Chem. Lett. 22 (2012) 431–435

BA were prepared by standard method of using anhydrides, that is,acetic, propanoic and n-butyric anhydrides in presence of dimethylamino pyridine (DMAP) as a catalyst in dry DCM to produce 3-O-a-acetyl-b-boswellic acid (3), 3-O-a-propionyl-b-boswellic acid (6)and 3-O-a-n-butyryl-b-boswellic acid (7), respectively, in almostquantitative yields. Likewise, 3-O-a-formyl-11-keto-b-boswellicacid (9), 3-O-a-acetyl-11-keto-b-boswellic acid (4), 3-O-a-propio-nyl-11-keto-b-boswellic acid (10) and 3-O-a-n-butyryl-11-keto-b-boswellic acid (11) were also prepared in high yields. Besides, itwas also envisaged to improve hydrophilicity by suitably changingthe functionality. Therefore, additional carboxylic acid functionalityat C-3 was introduced through the formation of hemisuccinates. Thereaction was accomplished by reacting succinic anhydride with 1/2in presence of pyridine to afford 3-O-a-hemisuccinyl-b-boswellicacid (8) and 3-O-a-hemisuccinyl-11-keto-b-boswellic acid (12),respectively, in 85% yield (Scheme 1).

The configuration of hydroxyl function in the naturally occurringBAs at C-3 is a. Therefore, to study the effect of change of configura-tion at C-3 on cytotoxicity, we envisaged the preparation of 3-epianalogs. To achieve it, we first protected the carboxyl function in1/2 by esterification via reaction with diazomethane (CH2N2) indiethyl ether to afford methyl-3-a-hydroxyurs-12-en-24-oate (13)and methyl-3-a-hydroxy-11-oxours-12-en-24-oate (14), respec-tively, in 95% yield. The formation of esters was followed by theiroxidation with pyridinium chlorochromate (PCC) in dichlorometh-ane to convert them to corresponding 3-keto derivatives, that is,methyl-3-oxours-12-en-24-oate (15) and methyl-3, 11-dioxours-12-en-24-oate (16), respectively, in 85% yield. The 3-keto deriva-tives were then quantitatively reduced by NaBH4 in methanol tostereoselectively produce methyl-3-b-hydroxyurs-12-en-24-oate(17) and methyl-3-b-hydroxy-11-oxours-12-en-24-oate (18),respectively, having (S) or b-configuration at C-3, as the steric hin-drance caused by the b-substituent groups mainly in the A/B ring(e.g., 25-Me, 24-COOMe) facilitated the approach of hydride reagentfrom a side. The methyl esters thus obtained were hydrolyzed withKOH/MeOH in a high pressure reaction vessel at 98–100 �C to afford3-epi-b-boswellic acid (19) and 3-epi-11-keto-b-boswellic acid (20),respectively in 90% yield (Scheme 2).

The 3-O-acyl analogs of 3-b-hydroxy-b-boswellic acid were sim-ilarly prepared using alkyl anhydrides in dry DCM using DMAP as acatalyst to obtain 3-O-b-acetyl-boswellic acid (21), 3-O-b-propio-nyl-b-boswellic acid (22) and 3-O-b-n-butyryl-b-boswellic acid(23), respectively. Similarly, 3-O-b-acetyl-11-keto-b-boswellic acid(25), 3-O-b-propionyl-11-keto-b-boswellic acid (26) and 3-O-b-n-butyryl-11-keto-b-boswellic acid (27) were also prepared. The3-epi BAs were also subjected to hemi succinate formation usingsuccinic anhydride and pyridine to produce 3-O-b-hemisuccinyl-b-boswellic acid (24) and 3-O-b-hemisuccinyl-11-keto-b-boswellicacid (28) in almost quantitative yields (Scheme 2).

RO

(i)

5. R = -COH, R1=R2= H3. R = -COCH3, R1=R2= H6. R = -COC2H5, R1=R2= H7. R = -COC3H7, R1=R2= H8. R = -COCH2CH2COOH, R

HOCOOH

R1R2

1 / 2

Scheme 1. Reagents: (i) R2

The preliminary screening results of BAs analogs at 50 lM con-centration showed that the propionate 6, 11-ketobutyrate 11 and3-epi propionate 22 derivatives potently inhibited the growth(84–98%) of all the cell lines used [HT-29 (colon), SW-620 (colon),Colo-205 (colon), Hep-2 (larynx), DU-145 (prostate), PC-3 (pros-tate)], whereas the natural compound 1 effectively inhibited thecell growth (92–100%) in prostate cancer cell lines DU-145 andPC-3 (Table 1). The n-butyrate 7 also showed high cytotoxic activ-ity (>88%) in all the cancer cell lines except in PC-3 where itshowed 77% growth inhibition (Table 1). The 11-keto propionate10 as well was found to exhibit significantly enhanced cytotoxicitywhen compared to 1, displaying >85% growth inhibition in HT-29,SW-620 and Colo-205 cell lines. Its growth inhibitory activity wasimproved against larynx Hep-2 cancer cell line; however, it wasless effective in DU-145 and PC-3 as compared to 1.

Deregulation of apoptosis is a hallmark of all cancer cells and thecompounds that are capable of inducing apoptosis in cancer cells areconsidered to be important in anticancer therapeutics. On the basisof preliminary screening of toxicity in different cancer cell lines, thederivatives 6, 7, 10, 11 and 22 were selected to study their apoptoticfunctioning. We used a p53 ‘null’ human myeloid leukemia HL-60cell line for apoptotic screening because of the reason that p53 isa tumor suppressor gene that provides a vital genetic defenceagainst development of cancer24 and is reported to be mutated inabout half of the cancers.25 Thus, a compound with an ability to in-duce apoptosis in HL-60 cells may have wider applicability in othercancers as well. Apoptosis is a complex phenomenon, which is reg-ulated by genetic mechanisms and is principally characterized bymorphological and biochemical changes in the nucleus, includinginternucleosomal DNA fragmentation. The most active compoundswere thus analyzed for their potential to induce DNA fragmentationin HL-60 cells. All the selected compounds caused DNA fragmenta-tion at 50 lM concentration in HL-60 cells, which was visible in theform of a ladder of 180–200 bp fragments or multiples of it (Fig. 1A).In order to identify the most active molecule we used MTT assay tocalculate the IC50 values and relative cytotoxicity of selected analogsin HL-60 cells. The results showed IC50 value of 11 is lowest at 11 lM(Fig. 1B) whereas, the IC50 values of other analogs were found to behigher than 11 (Fig. 1B). Normal cell line hGF was also treated with11 for comparative studies on toxicity. The IC50 of 11 in hGF cellswas calculated as 80 lM (Fig. 1C). Therefore, comparatively low tox-icity in normal cell line (hGF) further strengthened the candidatureof 11 for detailed studies.

In order to analyze whether the toxicity caused by these deriv-atives is purely due to apoptosis or other types of cell death alsoplays some role (necrosis and autophagy), we employed flowcytometer based analysis after staining the HL-60 cells withannexin-v-FITC/propidium iodide (to analyze apoptosis and necro-sis) and acridine orange (to analyze autophagy). It was observed

COOH

1=R2= H

R1R2

9. R = -COH, R1+R2 = O4. R = -COCH3, R1+R2 = O10. R = -COC2H5,R1+R2 = O11. R = -COC3H7, R1+R2 = O12. R = -COCH2CH2COOH, R1+R2 = O

O, DMAP, DCM (95%).

Table 1In vitro cytotoxicity of natural BAs and their analogs against different human cancer cell lines. All the cell lines were treated with 50 lM of test compounds for 48 h, 5-flurouracil(10 lM), mitomycin c (10 lM) and adriamycin (2 lM) were used as positive control. Data are shown as mean values of three similar experiments ± SEM (standard error of themean).

Compound Conc (lM) HT-29 colon SW-620 Colo-205 Hep-2 larynx DU-145 prostate PC-3

% Growth inhibition ± SEM

1 50 66 ± 0.88 64 ± 0.83 85 ± 1.01 43 ± 0.69 92 ± 0.88 1002 50 39 ± 0.38 28 ± 0.83 52 ± 0.66 35 ± 0.50 95 ± 1.2 1003 50 93 ± 0.88 93 ± 1.26 91 ± 1.15 85 ± 0.76 3 ± 0.19 44 ± 0.694 50 88 ± 0.76 88 ± 1.01 85 ± 1.20 59 ± 1 85 ± 0.83 76 ± 1.025 50 76 ± 0.83 87 ± 1.38 90 ± 1.17 72 ± 0.88 90 ± 1.26 95 ± 1.336 50 93 ± 1.17 94 ± 1.57 84 ± 1.17 86 ± 1.07 91 ± 1.45 93 ± 1.027 50 95 ± 1 93 ± 1.83 88 ± 1.01 94 ± 1.57 89 ± 0.83 77 ± 1.268 50 72 ± 0.38 67 ± 1.26 88 ± 1.34 55 ± 0.88 93 ± 1.01 90 ± 0.849 50 63 ± 0.57 69 ± 0.38 73 ± 0.88 54 ± 0.66 83 ± 1.2 95 ± 1.3510 50 89 ± 1 85 ± 1.34 87 ± 1.34 69 ± 0.88 25 ± 0.33 64 ± 0.8811 50 96 ± 0.66 97 ± 0.69 94 ± 0.83 98 ± 0.50 93 ± 0.66 96 ± 0.6912 50 5 ± 0.50 4 ± 0.38 2 ± 0.19 6 ± 0.38 51 ± 0.69 38 ± 0.5113 50 35 ± 0.57 46 ± 0.96 43 ± 0.88 26 ± 0.66 85 ± 1.01 54 ± 0.8814 50 67 ± 0.96 67 ± 0.83 70 ± 1 50 ± 0.69 65 ± 1 87 ± 0.6915 50 33 ± 0.38 34 ± 0.83 34 ± 0.66 7 ± 0.33 50 ± 0.88 44 ± 0.8416 50 17 ± 0.57 11 ± 0.19 30 ± 0.69 10 ± 0.50 23 ± 0.33 3 ± 0.3317 50 5 ± 0.38 9 ± 0.19 22 ± 0.69 0 3 ± 0.19 7 ± 0.1918 50 8 ± 0.19 10 ± 0.38 21 ± 0.33 0 4 ± 0.19 5 ± 0.1919 50 70 ± 0.76 63 ± 0.69 81 ± 1.07 69 ± 0.88 89 ± 1.01 72 ± 0.8820 50 44 ± 0.57 45 ± 1.26 49 ± 0.76 51 ± 0.69 84 ± 0.83 70 ± 0.6721 50 31 ± 0.76 35 ± 0.76 32 ± 0.57 42 ± 0.50 87 ± 0.69 56 ± 0.5122 50 92 ± 0.66 89 ± 1.45 86 ± 0.83 87 ± 0.33 88 ± 0.66 95 ± 1.1723 50 57 ± 0.88 79 ± 1.34 67 ± 1.07 63 ± 0.69 89 ± 1.26 83 ± 0.8424 50 13 ± 0.57 5 ± 0.57 19 ± 0.38 11 ± 0.33 48 ± 0.69 35 ± 0.8825 50 44 ± 0.38 44 ± 1.07 54 ± 0.69 47 ± 0.83 83 ± 0.83 62 ± 0.5126 50 64 ± 0.96 72 ± 0.96 83 ± 1 63 ± 0.69 89 ± 1.33 96 ± 1.1727 50 62 ± 0.69 79 ± 1.17 91 ± 0.88 66 ± 0.66 92 ± 0.88 94 ± 0.6928 50 17 ± 0.38 4 ± 0.19 28 ± 0.33 19 ± 0.50 83±1.07 62 ± 0.845FU 10 41 ± 1.55 44 ± 1.02 29 ± 0.37 29 ± 0.22 35 ± 0.50 16±0.33Mitomycin C 10 60 ± 0.92 55 ± 1.22 43 ± 0.83 58 ± 0.69 48 ± 0.56 55 ± 0.88Adriamycin 02 88 ± 1.01 72 ± 1.07 81 ± 1.45 64 ± 1.02 90±1.25 86 ± 1.25

HOCOOH

HOCOOMe

(i) (ii)

(iii)

(iv)

R1R2

R1R2

COOMe

R1R2

HOCOOMe

R1R2

HOCOOH

R1R2

O

1. R1 = R2= H2. R1 + R2 = O

13. R1 = R2 = H14. R1 + R2 = O

15. R1 = R2 = H16. R1 + R2 = O

17. R1 = R2 = H18. R1 + R2 = O

19. R1 = R2 = H20. R1 + R2 = O

H HH

HH

H HH

HH

ROCOOH

21. R = -COCH3, R1=R2= H22. R = -COC2H5, R1=R2= H23. R = -COC3H7, R1=R2= H24. R = -COCH2CH2COOH, R1=R2= H25. R = -COCH3, R1+R2 = O26. R = -COC2H5, R1+R2 = O27. R = -COC3H7, R1+R2 = O28. R = -COCH2CH2COOH, R1+R2 = O

R1R2

(v)

Scheme 2. Reagents and conditions: (i) CH2N2, ether (95%) (ii) PCC, DCM, rt (80%) (iii) NaBH4, MeOH, rt (90%) (iv) KOH, MeOH (88%) (v) R2O, DMAP, DCM (95%)

A. Kumar et al. / Bioorg. Med. Chem. Lett. 22 (2012) 431–435 433

Fig. 1. (A) DNA fragmentation induced by selected dervatives of boswellic acids. Apoptotic DNA ladder was observed when 2 � 106 HL-60 cells were treated with variousderivatives of BAs at 50 lM concentration for 6 h. The cells treated for 6 h with 4 lM camptothecin were used as positive control. (B) Inhibition of cell proliferation byselected dervatives of BAs in HL-60 cells and (C) in hGF cells. The cell proliferation analysis was performed in HL-60 and hGF cells by using MTT. The cells were incubated witheach analog for 48 h at indicated concentrations and thereafter mitochondrial competence of MTT reduction as an index of cell viability determined. OD of untreated controlwas taken as 100% viability. IC50 values of all the compounds are depicted in the B and C.

434 A. Kumar et al. / Bioorg. Med. Chem. Lett. 22 (2012) 431–435

that all the derivatives at concentration 50 lM induced cell deathpredominantly by apoptosis, whereas the number of cells undergo-ing necrosis was relatively insignificant after 6 h exposure. None ofthe compounds produced autophagic death in HL-60 cells (data notshown). However, as expected, 11 was found to be the most effec-tive of all the analogs producing approximately 80% of apoptoticcell population at the end of 6 h treatment, suggesting that itmay be probably a potential candidate for further developmentto an anti-cancer therapeutic lead. The butyrate analog 11 exhib-ited relatively lower IC50 values and showed greater potential forinducing apoptosis, therefore, warranted more studies to under-stand its molecular mechanism of action (Fig. 2).

For the validation of the effectiveness of 11 against few amongmany putative molecular targets in cancer cells, we analyzed theexpression of NF-jB and some anti-apoptotic members of STATfamily in HL-60 cells. The transcription factors NF-jB and Stat3are persistently active in most of the cancers and are involved inthe regulation of genes required for cancer cell proliferation, sur-vival, angiogenesis and invasion. Both these proteins work in con-junction with each other for the growth of cancer cells and requireeach other for their constitutive activity. In view of the importanceof NF-jB and Stat3 and other procancerous members of STAT fam-ily, we analyzed the effect of 11 on the expression of these proteinsin HL-60 cells. The control cells showed a high level of NF-jB (p65)and NF-jB (p50) in the whole cell fraction, whereas 6 h treatment

Fig. 2. Effect of selected boswellic acid analogs on the relative efficiency of induction of50 lM concentration for 6 h and stained with annexin v-FITC/PI. The dot plots are therepresent the apoptotic population, in the upper right as post apoptotic and in the uppershown are representative of one of the three similar experiments.

with 11 reduced the expression of NF-jB (p65) by about 90% irre-spective of the concentration of 11 (20 and 50 lM), while theexpression of NF-jB (p50) was more potently inhibited at 50 lMconcentration of 11 (Fig. 3A).

The anti-apoptotic members of STAT family contribute to thecancer cell growth by employing different mechanisms. Both theunphosphorylated and phosphorylated forms of STAT proteinswork together for cancer cell survival. The analysis of anti-apopto-tic members of Stat family (Stat3, Stat5 and Stat6) revealed differ-ential expression of these proteins in HL-60 cells. Theunphosphorylated Stat3a, Stat3b, Stat5 and Stat6 were highly ex-pressed in control HL-60 cells (Figs. 3B and C). However, treatmentof HL-60 cells with 11 for 6 h strongly reduced the expression of allthe unphosphorylated Stat proteins. The expression analysis ofphosphorylated STAT proteins revealed that HL-60 cells show veryhigh level of pStat3 (Ser 727) (Fig. 3B), which signifies the impor-tance of pStat3 in the proliferation of HL-60 cells. Other STAT pro-teins showed varied expression in the control cells, where pStat3(Ser 727) and pStat3 (Thr 705) had significantly high expression.pStat5 (Tyr 694/699) and pStat6 (Tyr 641) showed significantlylow level of protein (Fig. 3C). However, the treatment with 11strongly reduced the expression of phosphorylated forms of allthe members of STAT family studied. The data clearly suggests thatcytotoxic effect of 11 is mediated by the inhibition of NF-jB-Stat3tango along with other anti-apoptotic members of STAT family.

apoptosis and necrosis in HL-60 cells. The cells were incubated with each analog atrepresentative of one of the three experiments. Values in the lower right quadrantleft as necrotic. Other conditions are the same as described in the methods. The data

Fig. 3. Effect of 11 on the expression of NF-jB and procancerous STAT proteins in HL-60 cells. (A) The cells were treated with or without 11 (20 and 50 lM) for 6 h. Total celllysates were prepared for immunoblot analysis of NF-jB (p50 and p65). Protein samples (70 lg) were loaded on 10 % SDS–PAGE gel for western blot analysis. b-Actin wasused as internal control to represent the same amount of protein applied for SDS–PAGE. Data are representative of one of three similar experiments. (B and C) Western blotanalysis for STAT proteins as indicated in the figure. 8% SDS–PAGE was used to resolve the STAT proteins; other conditions are same as in A.

A. Kumar et al. / Bioorg. Med. Chem. Lett. 22 (2012) 431–435 435

Thus, based on this data, compound 11 can be further studied as aneffective anti-cancer lead.

In conclusion, the alkyl–acyl derivatives of BAs were synthe-sized and screened for cytotoxicity against a panel of human can-cer cell lines. The semi-synthetic compounds exhibited a range ofactivities, thereby establishing a partial SAR which is summarizedas follows. (i) The alkyl–acyl analogs in general displayed improvedactivity than the parent BAs, that is, higher acyl homologues (3–7and 9–11) displayed better cytotoxicity indicating free hydroxylis not essential for activity, (ii) the attempts to increase hydrophi-licity by way of C-3 hemi-succinate formation (8, 12, 24 and 28)did not improve cytotoxicity, (iii) the esterification of acid group(13–18) led to the reduction/ loss of activity, implying that C-24acid moiety is important for activity and (iv) the 3-epi BAs(19–28) did not display any significant improvement in activity,indicating that a-configuration at C-3 is more important for cyto-toxicity. Thus, screening of BA derivatives lead to the identificationof compound 11 (3-O-n-butyryl-11-keto-b-boswellic acid) as apromising lead that exhibited anti-cancer activity by inhibitingthe NF-jB and STAT proteins which may be further developed intoa potential anti-cancer therapeutic.

Supplementary data

Supplementary data (experimental procedures and character-isation data) associated with this article can be found, in the onlineversion, at doi:10.1016/j.bmcl.2011.10.112.

References and notes

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Bioorg. Med. Chem. Lett. 2007, 17, 3706; (b) Singh, S.; Khajuria, A.; Taneja, S. C.;Johri, R. K.; Singh, J.; Qazi, G. N. Phytomedicine 2008, 15, 400.

3. Ammon, H. P. Planta Med. 2006, 72, 1100.4. Khanna, D.; Sethi, G.; Ahn, K. S.; Pandey, M. K.; Kunnumakkara, A. B.; Sung, B.;

Aggarwal, A.; Aggarwal, B. B. Curr. Opin. Pharmacol. 2007, 7, 344.5. (a) Gupta, I.; Parihar, A.; Malhotra, P.; Singh, G. B.; Ludtke, R.; Safayhi, H.;

Ammon, H. P. Eur. J. Med. Res. 1997, 2, 37; (b) Singh, S.; Khajuria, A.; Taneja, S.C.; Khajuria, R. K.; Singh, J.; Johri, R. K.; Qazi, G. N. Phytomedicine 2008, 15, 408.

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