Synthesis of new boswellic acid derivatives as potential antiproliferative agents

Article abstract of DOI:10.1080/14786419.2018.1564295

In the current investigation, a series of heterocyclic derivatives of boswellic acids were prepared along with new monomers of 3-O-acetyl-11-keto-β-boswellic acid (AKBA, 1) 11-keto-β-boswellic acid (KBA, 2) and several new bis-AKBA and KBA homodimers and AKBA-KBA heterodimers. The effects of these compounds on the proliferation of different human cancer cell lines, viz., FaDu (pharynx carcinoma), A2780 (ovarian carcinoma), HT29 (colon adenocarcinoma), and A375 (malignant melanoma), have been evaluated. Thus, KBA homodimer 21 effectively inhibited the growth of FaDu, A2780, HT29, and A375 cells with EC₅₀ values below 9 μM. In addition, compounds 7, 8, 11, 12, 15, 16, and 17 also exhibited cytotoxic effects for A2780, HT29, and A375 cancer cells. In particular, the pyrazine analog 8 was highly cytotoxic for A375 cancer cells with an EC₅₀ value of 2.1 μM.

Full text of DOI:10.1080/14786419.2018.1564295

Natural Product Research
Formerly Natural Product Letters
ISSN: 1478-6419 (Print) 1478-6427 (Online) Journal homepage: https://www.tandfonline.com/loi/gnpl20
Synthesis of new boswellic acid derivatives as
potential antiproliferative agents
Umair Shamraiz, Hidayat Hussain, Najeeb Ur Rehman, Sulaiman Al-
Shidhani, Aasim Saeed, Husain Yar Khan, Ajmal Khan, Lucie Fischer, René
Csuk, Amin Badshah, Ahmed Al-Rawahi, Javid Hussain & Ahmed Al-Harrasi
To cite this article: Umair Shamraiz, Hidayat Hussain, Najeeb Ur Rehman, Sulaiman Al-Shidhani,
Aasim Saeed, Husain Yar Khan, Ajmal Khan, Lucie Fischer, René Csuk, Amin Badshah, Ahmed Al-
Rawahi, Javid Hussain & Ahmed Al-Harrasi (2019): Synthesis of new boswellic acid derivatives as
potential antiproliferative agents, Natural Product Research, DOI: 10.1080/14786419.2018.1564295
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NATURAL PRODUCT RESEARCH
https://doi.org/10.1080/14786419.2018.1564295
Synthesis of new boswellic acid derivatives as potential
antiproliferative agents
Umair Shamraiza,b, Hidayat Hussaina,c, Najeeb Ur Rehmana, Sulaiman
Al-Shidhania, Aasim Saeeda,b, Husain Yar Khana, Ajmal Khana, Lucie Fischerd,
d
b
a
a
ꢀ
Rene Csuk , Amin Badshah , Ahmed Al-Rawahi , Javid Hussain and Ahmed
Al-Harrasia
aUoN Chair of Oman’s Medicinal Plants and Marine Natural Products, University of Nizwa, Nizwa,
b
Sultanate of Oman; Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan;
cDepartment of Bioorganic Chemistry, Leibniz Institute of Plant Biochemistry, Halle, Germany;
dOrganic Chemistry, Martin-Luther-University Halle-Wittenberg, Halle, Germany
ABSTRACT
ARTICLE HISTORY
Received 8 October 2018
Accepted 19 December 2018
In the current investigation, a series of heterocyclic derivatives of
boswellic acids were prepared along with new monomers of 3-O-
acetyl-11-keto-b-boswellic acid (AKBA, 1) 11-keto-b-boswellic acid
(KBA, 2) and several new bis-AKBA and KBA homodimers and
AKBA-KBA heterodimers. The effects of these compounds on the
proliferation of different human cancer cell lines, viz., FaDu (phar-
ynx carcinoma), A2780 (ovarian carcinoma), HT29 (colon adeno-
carcinoma), and A375 (malignant melanoma), have been
evaluated. Thus, KBA homodimer 21 effectively inhibited the
growth of FaDu, A2780, HT29, and A375 cells with EC50 values
below 9 lM. In addition, compounds 7, 8, 11, 12, 15, 16, and 17
also exhibited cytotoxic effects for A2780, HT29, and A375 cancer
cells. In particular, the pyrazine analog 8 was highly cytotoxic for
A375 cancer cells with an EC50 value of 2.1 lM.
KEYWORDS
Boswellic acid; AKBA-KBA
heterodimers; cytotoxicity
CONTACT Hidayat Hussain
hussainchem3@gmail.com; Ahmed Al-Harrasi
aharrasi@unizwa.edu.om
Supplemental material for this article can be accessed at https://doi.org/10.1080/14786419.2018.1564295.
ß 2019 Informa UK Limited, trading as Taylor & Francis Group

2
U. SHAMRAIZ ET AL.
1. Introduction
Cancer is considered one of the most dreadful diseases in the world. This disease
caused nearly 8.2 million deaths in 2012 and around 14.1 million new cases (Roy et al.
2016) were repoted. Unfortunately, current chemotherapeutic agents are often linked
with various side effects, and especially the development of chemo-resistance towards
these clinical drugs impedes an efficient and sustainable treatment of cancer (Roy
et al. 2016). Frankincense has traditionally been used to treat cancer and several
inflammatory diseases such as chronic pain syndrome, asthma, cerebral edema, arth-
ritis, and chronic bowel disease (Takahashi et al. 2012; Roy et al. 2016). In addition, the
gum resin of Boswellia sacra (frankincense) has also traditionally been used, for
example, in Oman to treat severe muscle pain, colds, cough, dental infections, and
stomach aches but also fever (Al-Ghassany 2008).
Frankincense consists – among many other compounds—of four major triterpenic
acids known as boswellic acids, viz., 3-O-acetyl-11-keto-b-boswellic acid (AKBA, 1,
Figure 1), 11-keto-b-boswellic acid (KBA, 2), b-boswellic acid (BA, 3), and 3-O-acetyl-
b-boswellic acid (ABA, 4). These boswellic acids (BAs) showed interesting in vitro cyto-
toxic effects for various cancer cell lines, among them cancer of bladder (Frank et al.
2009), brain (Glaser et al. 1999), cervix (Qurishi et al. 2012), colon, liver (Liu et al.
2002), colorectum (Takahashi et al. 2012), leukemia (Shao et al. 1998), lung (Ravanan
et al. 2011), melanoma (Zhao et al. 2003), meningioma (Park et al. 2002), myeloma
(Kunnumakkara et al. 2009), neuroblastoma (Qurishi et al. 2012), pancreatic (Park et al.
2011), and prostate cancer (Syrovets et al. 2005). In addition, boswellic acids also dem-
onstrated in vivo anticancer effects towards various cancers, for example colon cancer
(Yuan et al. 2013), Ehrlich tumor (Agrawal et al. 2011), colorectal cancer (Takahashi
et al. 2012), glioma (Ravanan et al. 2011), leukemia (Huang et al. 2000), pancreatic can-
cer (Park et al. 2011), and prostate cancer (Syrovets et al. 2005). Some clinical studies
have also reported boswellic acids for treatment of brain tumor, lung cancer and
breast cancer (Huang et al. 2000). An extensive metabolism and poor absorption, how-
ever, narrow the bioavailability of AKBA (Cheng et al. 2015), and therefore there is an
urgent need of new derivatives of AKBA with effective biological actions and improved
€
bioavailability. Dimers (Bednarczyk-Cwynar and Gunther 2017) of oleanolic acid have
been previously reported by Cheng et al. (2015) and Medina-O’Donnell et al. (2018).
Figure 1. Structure of the most important b-boswellic acids.

NATURAL PRODUCT RESEARCH
3
Scheme 1. (Interconversion of the boswellic acids 1–4: a) PtO2, HOAc, H2, 85 bar, 25 ꢀC, 12 h, 87%;
b) AcCl, pyridine, DMAP, 25 ꢀC, 6 h, 91%; c) NaOH, EtOH, 25 ꢀC, 12 h, 98%; d) AcCl, pyridine, DMAP,
25 ꢀC, 6 h, 89%; e) NaOH (aq.), 25 ꢀC, 12 h, 93%; f) PtO2, HOAc, H2, 85 bar, 25 ꢀC, 12 h, 87% (Jauch
and Bergmann2003; Wolfram et al.2017).
These dimers showed significant antiproliferative effects against various cancer cell
lines (Cheng et al. 2015). Moreover, quite recently we have prepared several mono-
mers and dimers of the triterpene acid myrrhanone B, many of which have shown
potent anticancer effects against the different human cancer cell lines FaDu, A2780,
HT29 and A375 (Saeed et al. 2018). These interesting results from oleanolic acid
(dimers) and myrrhanone B (monomers and dimers) as well as the well-demonstrated
anticancer activity of boswellic acids reported against different types of cancer cells
has prompted us to synthesize various derivatives and dimers of some boswellic acids.
Unfortunately, the concentration of the most active boswellic acid AKBA in extracts
from Boswellia resins is in the range of 0.1–3%. Consequently, we used the reported
procedure described by Jauch and Bergmann (2003) to obtain AKBA and KBA in larger
amount (Scheme 1). This allowed the synthesis of several analogs of boswellic acids
that were subjected to an in vitro cytotoxicity screening utilizing different human can-
cer cell lines [FaDu (pharynx carcinoma), A2780 (ovarian carcinoma), HT29 (colon
adenocarcinoma) and A375 (malignant melanoma)] to identify new cytotoxic
lead molecules.
2. Results and discussion
2.1. Synthesis
The concentration of AKBA (1) in B. sacra is very low while the concentration of KBA
(2), BA, (3), or ABA, (4) is relatively higher. To prepare KBA in larger quantities we
followed the procedure reported by Jauch and Bergmann (2003) to obtain AKBA. All
boswellic acids 1–4 can easily be interconverted (Wolfram et al. 2017).
KBA (2) was esterified with BnBr/K2CO3 in DMF, and ester 5 was obtained in 91%
yield (Scheme 2). Jones oxidation (CrO3/aq. H2SO4) in acetone of the C-3 hydroxyl
group gave 3-keto 6 in 87% yield. Compound 6 served as a valuable starting material

4
U. SHAMRAIZ ET AL.
Scheme 2. Synthesis of compounds 5–10: a) BnBr, K2CO3, DMF, rt, 12 h, 91%; b) Jones reagent, rt,
t
2 h, 87%; c) ethane-1,2-diamine, morpholine, S, reflux, 18 h, 7 (45%), 8 (42%); d) aniline, BuOK,
DMSO, rt, 48%; e) thiosemicarbazide, EtOH, reflux, 81%.
for the syntheses to follow. Thus, synthesis of the pyrazine derivatives (Csuk et al.
2011; Zorina et al. 2018) of AKBA (compounds 7 and 8) was accomplished with 45%
and 42% yield by reacting ketone 6 with ethylene diamine in morpholine as the solv-
ent (Bhandari et al. 2014). Furthermore, ketone 6 gave upon reaction with potassium
tert-butoxide in the presence of aniline not the corresponding indole but this reaction
ended up with an opening of ring A and provided aldehyde 9 in 48% yield. The syn-
thesis of semithiocarbazone 10 was accomplished by reacting 6 with semicarbazide.
Monomers of boswellic acids 11–16 (Scheme 3) were prepared by esterifying boswellic
acids 1 and 2 with three equivalents of the linkers 1,2-dibromoethane, 1,3-dibromo-
propane, and 1,4-dibromobutane in the presence of K2CO3 in DMF, respectively (Saeed
et al. 2018). The bis and hetero dimers of boswellic acids 17–25 (ester-linked) were
obtained in good yields by reacting the monomers 11–16 in the presence of K2CO3 in
DMF with compounds 1 and 2.
2.2.Cytotoxic activity and SAR studies
The compounds were subsequently screened for their cytotoxic activity for human
cancer cell lines FaDu, A2780, HT29, and A375. The EC50 values of the boswellic acid
analogs and dimers for these cell lines are summarized in Table 1. Compounds having
EC50 values over 30 mM were considered as not active.
As evident from the data presented in Table 1, pyrazine analogs 7 and 8 demon-
strated cytotoxic effects for A2780 (7: EC50 ¼ 15.7 lM; 8: EC50 ¼ 13.7 lM), HT29 (7:

NATURAL PRODUCT RESEARCH
5
Scheme 3. Reagents and conditions: a) 1,2-dibromoethane (3 eq) or 1,3-dibromopropane (3 eq) or
1,4-dibromobutane (3 eq), K2CO3, DMF; b) compounds 1 or 2 (1 eq), 1,2-dibromoethane (1 eq) or
1,3-dibromopropane (1 eq) or 1,4-dibromobutane (1 eq), K2CO3, DMF.
Table 1. EC50 values (in lM, from sulforhodamine B assays performed at least thrice with three
technical replicates each) of novel Boswellic acid derivatives and dimers against various cancer
cell lines.
Compound
FaDu
A2780
HT29
A375
7
8
9
10
11
12, 13
14
15
16
>30
>30
>30
29.5 2.1
>30
>30
17.2 1.3
16.3 1.5
21.7 2.7
>30
8.6 1.2
>30
15.7 1.7
13.7 1.9
17.9 1.4
9.3 0.9
16.3 1.7
>30
13.3 1.3
12.1 1.5
13.2 1.6
>30
22.7 2.2
12.2 1.8
>30
11.3 0.8
27.7 3.1
>30
12.5 1.5
12.8 1.9
14.7 2.2
>30
6.2 1.1
>30
12.8 1.6
2.1 0.4
19.4 2.0
10.8 1.9
12.1 1.7
>30
13.9 1.6
12.6 1.4
16.7 2.0
>30
17–19
20
21–24
6.6 1.2
>30
8.2 1.7
>30
Each value represents the mean SD; FaDu ¼ pharynx carcinoma, A2780 ¼ ovarian carcinoma), HT29 ¼ colon
adenocarcinoma, and A375 ¼ melanoma human cancer cell lines; EC50 values >30 mM were considered inactive.
EC50 ¼ 22.7 lM; 8: EC50 ¼ 12.2 lM) and A375 (7: EC50 ¼ 12.8 lM; 8: EC50 ¼ 2.1 lM)
tumor cells. It is interesting to note that compound 8 exhibited a much better activity
than compound 7. In particular, AKBA analog 8 showed six times stronger cytotoxic
effect for A375 cancer cells (8: EC50 ¼ 2.1 lM vs 7: EC50 ¼ 12.8 lM). These results sug-
gest that an addition of a pyrazine group has a significant impact on the activity of
boswellic acids. On the other hand, ring A modified seco-structure 9 was only active
against cell lines A2780 (EC50 ¼ 17.9 lM) and A375 (EC50 ¼ 19.4 lM). This also indi-
cated that a cleavage of ring A did not enhance activity. This parallels previous

6
U. SHAMRAIZ ET AL.
findings for other triterpenoic acids (Csuk et al. 2011). Compound 7 (being decarboxy-
lated at position C-23 compared to its parent compound), and compound 9, which
holds a modified ring A, have shown diminished activities as compared to compound
8. This indicates that the presence of a carboxyl group and of an intact ring A is cru-
cial for retaining the cytotoxic effects of the boswellic acid analogs. Further, the semi-
thiocarbazone 10 demonstrated good cytotoxic effects for the three cancer cell lines
A2780 (EC50 ¼ 9.3 lM), HT29 (EC50 ¼ 11.3 lM) and A375 (EC50 ¼ 10.8 lM). This result
revealed that an incorporation of extra nitrogen into the AKBA skeleton enhances
activity; this finding is in excellent agreement with previous findings (Wolfram
et al. 2017).
Cytotoxic effects of the three monomers of AKBA 11–13 were not impressive at all.
Monomers 12 and 13, holding a bromopropyl or a bromobutyl group, were found to
be inactive for all of the tested cancer cell lines (EC50 > 30 lM). It was only monomer
11, carrying a bromoethyl group, that was found to be active against at least three
types of cancer cells: A2780 (EC50 ¼ 16.3 lM), HT29 (EC50 ¼ 27.7 lM), and A375 (EC50
¼ 12.1 lM). On the other hand, the growth inhibitory activity of the three monomers
of KBA viz., compounds 14–16 was very significant inasmuch as all these monomers
were active against the tested tumor cell lines (FaDu, A2780, HT29 and A375) with
EC50 values ranging from 12.1 to 21.7 lM.
Homodimers of AKBA 17–19 holding an ethylene, a trimethylene or a tetramethy-
lene spacer were found to be inactive (EC50 > 30 lM) against all the four cancer cell
lines tested. Similarly, the two homodimers of KBA 21 and 22 having a trimethylene
or a tetramethylene linker also turned out to be inactive (EC50 > 30 lM). However, the
KBA homodimer 20, holding an ethylene group as linker, proved to be the most
potent cytotoxic compound. This compound showed significant inhibitory effects on
the proliferation of all the four types of cancer cells viz., FaDu (8.6 lM), A2780 (EC50
¼
6.6 lM), HT29 (EC50 ¼ 6.2 lM), and A375 (EC50 ¼ 8.2 lM). Finally, it may be noted that
among all the tested compounds, only compounds 14–16 and 20 were effective in
inhibiting the proliferation of pharynx carcinoma (FaDu) cells.
3. Conclusion
We have prepared ring A modified AKBA analogs (compounds 7, 8, 9, and 10), AKBA
monomers (11–13) and KBA monomers (14–16) as well as homo- and heterodimers of
AKBA and KBA (17–24). The most remarkable anti-proliferative activity was displayed
by the pyrazine analog 8 against A375 (malignant melanoma) cells with an EC50 value
as low as 2.1 lM. SAR studies revealed that the presence of an amine enhances the
activity of boswellic acids. Moreover, it was also observed that decarboxylation or
cleavage of ring A led to reduced cytotoxic activity. KBA homodimer 20 and KBA-
AKBA heterodimer 23 exhibited cytotoxic effects while all the other dimers turned out
to be inactive. In fact, 20 was the most potent compound showing significant cyto-
toxic effects against all the four cancer cell lines with EC50 ꢁ 11.0 lM. It is interesting
to note that compound 20 (holding an ethylene group as linker in its dimeric struc-
ture) was cytotoxic. This suggests that the length of the spacer plays a significant role
in establishing the cytotoxic effects of KBA homodimers.

NATURAL PRODUCT RESEARCH
7
Acknowledgements
The authors acknowledge the Oman Research Council ORG/HSS/14/004 to A. A.-H.), H.H. thanks
the Alexander-von-Humboldt Foundation for a Humboldt Research fellowship. Financial support
by the “Science Campus Halle” (W13004216 to R.C.) is gratefully acknowledged.
Disclosure statement
The authors declared no conflict of interest.
Funding
This study was supported by “Science Campus Halle” (W13004216 to R.C.).
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