A biomimetic, one-step transformation of simple indolic compounds to malassezia-related alkaloids with high ahr potency and efficacy

Article abstract of DOI:10.1021/acs.chemrestox.9b00270

Malassezia furfur isolates from diseased skin preferentially biosynthesize compounds which are among the most active known aryl-hydrocarbon receptor (AhR) inducers, such as indirubin, tryptanthrin, indolo[3,2-b]carbazole, and 6-formylindolo[3,2-b]carbazole. In our effort to study their production from Malassezia spp., we investigated the role of indole-3-carbaldehyde (I3A), the most abundant metabolite of Malassezia when grown on tryptophan agar, as a possible starting material for the biosynthesis of the alkaloids. Treatment of I3A with H₂O₂ and use of catalysts like diphenyldiselenide resulted in the simultaneous one-step transformation of I3A to indirubin and tryptanthrin in good yields. The same reaction was first applied on simple indole and then on substituted indoles and indole-3-carbaldehydes, leading to a series of mono- A nd bisubstituted indirubins and tryptanthrins bearing halogens, alkyl, or carbomethoxy groups. Afterward, they were evaluated for their AhR agonist activity in recombinant human and mouse hepatoma cell lines containing a stably transfected AhR-response luciferase reporter gene. Among them, 3,9-dibromotryptanthrin was found to be equipotent to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as an AhR agonist, and 3-bromotryptanthrin was 10-times more potent than TCDD in the human HG2L7.5c1 cell line. In contrast, 3,9-dibromotryptanthrin and 3-bromotryptanthrin were 4000 and >10,000 times less potent than TCDD in the mouse H1L7.5c3 cell line, respectively, demonstrating that they are species-specific AhR agonists. Involvement of the AhR in the action of 3-bromotryptanthrin was confirmed by the ability of the AhR antagonists CH223191 and SR1 to inhibit 3-bromotryptanthrin-dependent reporter gene induction in human HG2L7.5c1 cells. In conclusion, I3A can be the starting material used by Malassezia for the production of both indirubin and tryptanthrin through an oxidation mechanism, and modification of these compounds can produce some highly potent, efficacious and species-selective AhR agonists.

Full text of DOI:10.1021/acs.chemrestox.9b00270

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Article
A biomimetic, one-step transformation of simple indolic compounds
to Malassezia-related alkaloids with high AhR potency and efficacy
Nikitia Mexia, Stamatis Koutrakis, Guochun He, Alexios-Leandros
Skaltsounis, Michael S. Denison, and PROKOPIOS MAGIATIS
Chem. Res. Toxicol., Just Accepted Manuscript • DOI: 10.1021/acs.chemrestox.9b00270 • Publication Date (Web): 24 Oct 2019
Downloaded from pubs.acs.org on October 25, 2019
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A biomimetic, one-step transformation of simple indolic compounds
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to Malassezia-related alkaloids with high AhR potency and efficacy
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Nikitia Mexia†, Stamatis Koutrakis†, Guochun He‡, Alexios-Leandros Skaltsounis†,
Michael S. Denison‡, Prokopios Magiatis†,‡,*
†Department of Pharmacognosy and Natural Products Chemistry, Faculty of Pharmacy,
University of Athens, Athens 15771, Greece;
‡Department of Environmental Toxicology, University of California, Davis 95616,
USA
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TABLE OF CONTENTS GRAPHIC
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NH
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O
N
NH
O
N
NH
NH
O
O
CHO
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NH
+ H2O2
+(PhSe)2
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ABSTRACT
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Malassezia furfur isolates from diseased skin preferentially biosynthesize compounds
which are among the most active known Aryl-hydrocarbon Receptor (AhR) inducers,
such as indirubin, tryptanthrin, indolo[3,2-b]carbazole and 6-formylindolo[3,2-
b]carbazole. In our effort to study their production from Malassezia spp., we
investigated the role of indole-3-carbaldehyde (I3A), the most abundant metabolite of
Malassezia when grown on tryptophan agar, as a possible starting material for the
biosynthesis of the alkaloids. Treatment of I3A with H2O2 and use of catalysts like
diphenyldiselenide resulted in the simultaneous one-step transformation of I3A to
indirubin and tryptanthrin in good yields. The same reaction was first applied on simple
indole and then on substituted indoles and indole-3-carbaldehydes, leading to a series
of mono- and bi-substituted indirubins and tryptanthrins bearing halogens, alkyl or
carbomethoxy groups. Afterwards, they were evaluated for their AhR agonist activity
in recombinant human and mouse hepatoma cell lines containing a stably transfected
AhR-response luciferase reporter gene. Among them, 3,9-dibromotryptanthrin was
found to be equipotent to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as an AhR
agonist and 3-bromotryptanthrin was 10-times more potent than TCDD in the human
HG2L7.5c1 cell line. In contrast, 3,9-dibromotryptanthrin and 3-bromotryptanthrin
were ~4,000- and >10,000-times less potent than TCDD in the mouse H1L7.5c3 cell
line, respectively, demonstrating that they are species-specific AhR agonists.
Involvement of the AhR in the action of 3-bromotryptanthrin was confirmed by the
ability of the AhR antagonists CH223191 and SR1 to inhibit 3-bromotryptanthrin-
dependent reporter gene induction in human HG2L7.5c1 cells. In conclusion, I3A can
be the starting material used by Malassezia for the production of both indirubin and
tryptanthrin through an oxidation mechanism and modification of these compounds can
produce some highly potent, efficacious and species-selective AhR agonists.
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INTRODUCTION
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Malassezia yeasts are part of the human skin microbiota that can become
pathogenic and cause serious infections such as seborrhoeic dermatitis, pytiriasis
versicolor, dandruff and psoriasis.1 Extensive studies have revealed the preferential
biosynthesis of indole derivatives by Malassezia furfur isolates from lessional skin
which are among the most potent Aryl-hydrocarbon Receptor (AhR) activators known,
e.g. indirubin, tryptanthrin, indolo[3,2-b]carbazole (ICZ) and 6-formylindolo[3,2-
b]carbazole (6-FICZ).2-4 Although the production of the above AhR activators seems
to be a general characteristic of the genus, specific Malasezia species and strains are
able to overproduce them and consequently their constant presence in the skin at
elevated concentrations has been proposed to be implicated in the development of skin
diseases.2,3 However up today there is no available data about the biosynthetic origin of
those compounds except that they are related with tryptophan.
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Having as ultimate target of future works the inhibition of their overproduction
and possibly the inhibition of their toxic effects for skin, we considered as a very
important first step to investigate the role of potential biosynthetic precursors and the
chemical mechanism that could lead to their production, mimicking what is potentially
happening with Malassezia.The first compound that was investigated as a possible
starting material for the biosynthesis of the alkaloids was indolo-3-carbaldehyde, I3A
(1), the most abundant metabolite of all studied Malassezia strains when cultured with
L-tryptophan as single nitrogen source.5, 6
During early trials, the treatment of I3A (1) with H2O2 and acetic acid resulted
in intensively colored solutions, suggesting thus the formation of indigoids. The major
product was indigo but other indole derivatives such as indirubin (2) could be detected
in traces by NMR spectroscopy. Further trials under Baeyer-Villiger oxidation
conditions led to increased yield of indirubin (2) . This type of reaction is commonly
used for the transformation of aromatic ketones and aldehydes to the corresponding
esters7 but in this case it led to more complicated structures. For realizing this reaction
we chose, among the available oxidants, hydrogen peroxide due to its eco-friendly
nature, its natural presence and that exposure to H2O2 occurs in most human cells.8 We
also investigated the use of a potential catalyst in order to promote the reaction and
enhance the yield of the desirable compounds. A thorough investigation of several
parameters led to the discovery of a biomimetic reaction that can possibly explain the
common presence of indirubin (2) and tryptanthrin (3) in Malassezia extracts.
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Complementary to this assumption is a recent publication that indicates the synthesis,
from indole-3-acetaldehyde upon treatment with H2O2, of 6-FICZ, another Malassezia
metabolite.9 Although we were unable to detect indole-3-acetaldehyde in the extracts
of Malassezia species, it is biosynthetically a probable product of tryptophan
metabolism.10
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At a next step, the above described reaction was applied to several substituted
indolic monomers for the preparation of a series of new or known indirubin and
tryptanthrin derivatives, having as target the investigation of the structure-activity
relationships concerning the AhR activation. Finally, all the synthesized analogues
were evaluated for their stimulate AhR-dependent gene expression, leading to the
identification of some compounds with significantly increased potency and efficacy.
EXPERIMENTAL PROCEDURES
Materials and Equipment
All the solvents used during the experimental procedure were of analytical grade
or distilled. Aluminum and glass plates, covered with silicon dioxide (Silica gel 60 F254)
were used for TLC and preparative TLC respectively and the obtained chromatograms
were observed in a CAMAG TLC Visualizer at 254 and 366 nm. All the one- and two-
dimensional NMR spectra of the purified compounds, as well as the calculations of the
reaction yields, were acquired in a Bruker UltrashieldTM Plus 600MHz NMR.
Initial methodology of the reaction on I3A (1)
0.34 mmol of I3A (1) were dissolved in 1.0 mL distilled DCM while 0.1 mL
H2O2 10% and 0.05 mmol of (PhSe)2 were then added in the reaction mixture. The flask
was covered with aluminum foil to prevent occurrence of any light-induced side-
reactions and the solution was stirred for 1.5 hours at room temperature (rt). Then, the
same quantities of catalyst and oxidative reagent were added again and the reaction
remained under mechanical stirring overnight at rt. After the formation of a dark-
colored solution, MeOH was added in the mixture and it was evaporated to dryness.
Then the crude mixture was submitted to column chromatography for the isolation of
the produced compounds.
Modification of the reaction and application on I3A (1) and indole (8)
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The modified protocol for the investigation of the above reaction was based on
a method described by Santoro et al.11 0.4 mmol of I3A (1) or indole (8) were added to
a mixture of 2.1 mL H2O2 6% and 0.7 mL acetonitrile containing a catalytic amount of
(PhSe)2 (0.04 mmol). The reaction remained under mechanical stirring for 72h at 23°C.
The precipitate was filtered, washed with water, then solubilized with THF, collected
and evaporated to dryness. The isolation of the synthesized compounds 2 and 3 was
performed with preparative TLC and their structure elucidation with NMR
spectroscopy.
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Optimization of the reaction conditions
For the optimization of the reaction, numerous trials were performed by
modifying and investigating parameters such as the oxidative means, the catalyst or the
quantity of the reagents. These efforts were carried out in parallel, under the same
environmental conditions and by modifying only one parameter at the time. In all cases
the starting material was I3A (1) and the mixture was stirred for 72h at 23°C. All the
attempts are summarized in Table 1, leading to some interesting remarks and
conclusions for this synthesis.
For further examination of the optimal conditions, the environmental conditions
were then modified, namely the temperature and the reaction time. The starting material
was once again I3A (1) and the reagents the same as the ones referred to entry 1 of
Table 1. These experiments are listed in Table 2.
Investigation of the reaction mechanism
To explore the mechanism of the reaction and its’ potentials, we then applied
the reaction on compounds that bear longer chains in the position 3 of the indolic
scaffold (e.g. tryptophan and tryptamine). Moreover, aiming to examine the necessity
and the nature of the nitrogen atom in position 1 of the aromatic core, we used
benzo[b]thiophene and N-Boc-indolo-3-carbaldehyde as staring materials. In all cases
the aforementioned developed protocol was applied.
General protocol for the synthesis of compounds 2-2h and 3-3h
A quantity (0.4 mmol) of the appropriately substituted indolic compound was
added to a mixture of 2.1 mL H2O2 6% and 0.7 mL acetonitrile containing a catalytic
amount of (PhSe)2 (0.04 mmol). The reaction remained under mechanical stirring for
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72h at 23°C. The produced precipitate was filtered, washed with water, then solubilized
with THF, collected and evaporated to dryness.
The isolation of the synthesized compounds was performed with preparative TLC
and their structure elucidation with NMR Spectroscopy. The synthesized compounds
and the reaction yields are presented in Tables 3 and 4.
Synthesis of indirubin (2) and tryptanthrin (3):
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The general method was followed using as starting material either I3A (1) or indole
(8). In both cases the produced alkaloids and their yields were equivalent.
Indirubin (2)
Purple solid; Yield: 5%; Rf=0.50 (cyclohexane:EtOAc – 1:1 + 1.5% acetic acid).
Spectral data are in accordance to Adachi et al.12 and are given in Supporting
Information.
Tryptanthrin (3)
Yellow solid; Yield: 12%; Rf=0.63 (cyclohexane:EtOAc – 1:1 + 1.5% acetic acid).
Spectral data are in accordance to Jao et al.13 and are given in Supporting Information.
Synthesis of 5,5’-difluoroindirubin (2a) and 2,8-difluorotryptanthrin (3a):
The general method was followed using as starting material the 5-fluoroindole.
5,5’-difluoroindirubin (2a)
Purple solid; Yield: 5%; Rf=0.18 (cyclohexane:EtOAc – 7:3 + 1.5% acetic acid.
Spectral data are in accordance to Wang et al.14 and are given in Supporting
Information.
2,8-difluorotryptanthrin (3a)
Yellow solid; Yield: 16%; Rf=0.53 (cyclohexane:EtOAc – 7:3 + 1.5% acetic acid).
Spectral data are in accordance to Wang et al.15 and are given in Supporting
Information.
Synthesis of 5,5’-dichloroindirubin (2b) and 2,8-dichlorotryptanthrin (3b):
The general method was followed using as starting material the 5-chloroindole.
5,5’-dichloroindirubin (2b)
Purple solid; Yield: 5%; Rf=0.21 (cyclohexane:EtOAc – 7:3 + 1.5% acetic acid).
Spectral data are in accordance to Riepl et Urmann16 and are given in Supporting
Information.
2,8-dichlorotryptanthrin (3b)
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Yellow solid; Yield: 10%; Rf=0.63 (cyclohexane:EtOAc – 7:3 + 1.5% acetic acid).
Spectral data are in accordance to Wang et al.15 and are given in Supporting
Information.
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Synthesis of 5,5’-dibromoindirubin (2c) and 2,8-dibromotryptanthrin (3c):
The general method was followed using as starting material the 5-bromoindole.
5,5’-dibromoindirubin (2c)
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Purple solid; Yield: 6%; Rf=0.44 (cyclohexane:EtOAc – 7:3 + 1.5% acetic acid).
Spectral data are in accordance to Beauchard et al.17 and are given in Supporting
Information.
2,8-dibromotryptanthrin (3c)
Yellow solid; Yield: 12%; Rf=0.64 (cyclohexane:EtOAc – 7:3 + 1.5% acetic
acid). Spectral data are in accordance to Wang et al.15 and are given in Supporting
Information.
Synthesis of 5,5’-dimethylindirubin (2d) and 2,8-dimethylotryptanthrin (3d):
The general method was followed using as starting material the 5-methylindole.
5,5’-dimethylindirubin (2d)
Purple solid; Yield: 6%; Rf=0.23 (cyclohexane:EtOAc – 7:3 + 1.5% acetic acid
Spectral data are in accordance to Wang et al.14 and are given in Supporting
Information.
2,8-dimethylotryptanthrin (3d)
Yellow solid; Yield: 15%; Rf=0.46 (cyclohexane:EtOAc – 7:3 + 1.5% acetic acid).
Spectral data are in accordance to Wang et al.15 and are given in Supporting
Information.
Synthesis
of
5,5’-dicarbomethylindirubin
(2e)
and
2,8-
dicarboxymethylotryptanthrin (3e):
The general method was followed using as starting material the 5-carboxymethylindole.
5,5’-dicarboxymethylindirubin (2e)
Red solid; Yield: 17%; Rf=0.27 (cyclohexane:EtOAc – 1:1 + 1.5% acetic acid).
1H-NMR ((CD3)2SO, 600MHz): δ 11.44 (brs, -NH); 11.34 (brs, -NH); 9.45 (d, H-4,
J=1.7Hz); 8.18 (brs, H-4’); 8.16 (dd, H-6’, J=8.5Hz, 1.6Hz); 7.93 (dd, H-6, J=8.2Hz,
1.7Hz); 7.54 (d, H-7’, J=8.5Hz); 7.02 (d, H-7, J=8.2Hz); 3.88 (s, 3H, -CH3); 3.86 (s,
3H, -CH3).
2,8-dicarboxymethylotryptanthrin (3e)
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Yellow solid; Yield: 27%; Rf=0.52 (cyclohexane:EtOAc – 1:1 + 1.5% acetic acid).
1H-NMR (CDCl3, 600MHz): δ 9.11 (d, H-1, J=1.8Hz); 8.73 (d, H-10, J=8.4Hz); 8.59
(d, H-7, J=1.4Hz); 8.51 (dd, H-3, J=8.4Hz, 1.8Hz); 8.49 (dd, H-9, J=8.4Hz, 1.4Hz);
8.10 (d, H-4, J=8.4Hz); 4.02 (s, 3H, -CH3); 3.99 (s, 3H, -CH3)
Synthesis of 6,6’-dibromoindirubin (2f) and 3,9-dibromotryptanthrin (3f):
The general method was followed using as starting material the 6-bromoindole-3-
carbaldehyde.
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6,6’-dibromoindirubin (2f)
Red solid; Yield: 5%; Rf=0.28 (cyclohexane:EtOAc – 7:3 + 1.5% acetic acid).
Spectral data are in accordance to Clark et Cooksey18 and are given in Supporting
Information.
3,9-dibromotryptanthrin (3f)
Yellow solid; Yield: 13%; Rf=0.65 (cyclohexane:EtOAc – 7:3 + 1.5% acetic
acid). 1H-NMR (CDCl3, 600MHz): δ 8.84 (d, H-10, J=1.6Hz); 8.28 (d, H-1, J=8.5Hz);
8.19 (d, H-4, J=1.8Hz); 7.80 (dd, H-2, J=8.5Hz, 1.8Hz); 7.70 (d, H-7, J=8.1Hz); 7.61
(dd, H-8, J=8.1Hz, 1.6Hz)
Synthesis
of
6,6’-dicarbomethylindirubin
(2g)
and
3,9-
dicarboxymethylotryptanthrin (3g):
The general method was followed using as starting material the 6-carboxymethylindole.
6,6’-dicarboxymethylindirubin (2g)
Purple solid; Yield: 14%; Rf=0.40 (cyclohexane:EtOAc – 1:1 + 1.5% acetic
1
acid). H-NMR ((CD3)2SO, 600MHz): δ 11.45 (brs, -NH); 11.08 (brs, -NH); 8.82 (d,
H-4, J=8.2Hz); 8.06 (brs, H-7’); 7.79 (d, H-4’, J=8.0Hz); 7.67 (d, H-5’, J=8.0Hz); 7.60
(d, H-5, J=8.2Hz); 7.43 (brs, H-7); 3.90 (s, 3H, -CH3); 3.86 (s, 3H, -CH3)
3,9-dicarboxymethylotryptanthrin (3g)
Yellow solid; Yield: 17%; Rf=0.59 (cyclohexane:EtOAc – 1:1 + 1.5% acetic
acid). Spectral data are in accordance to Wang et al.15 and are given in Supporting
Information.
Synthesis of 6’-bromoindirubin (2h) and 3-bromotryptanthrin (3h):
The general method was followed using as starting material equivalent quantities of
I3A (1) and 6-bromoindole-3-carbaldehyde (8). The quantities of all the reagents were
doubled.
6’-bromoindirubin (2h)
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Purple solid; Yield: 5%; Rf=0.38 (cyclohexane:EtOAc – 7:3). Spectral data are
in accordance to Clark et Cooksey18 and are given in Supporting Information.
3-bromotryptanthrin (3h)
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Yellow solid; Yield: 12%; Rf=0.53 (cyclohexane:EtOAc – 7:3). Spectral data
are in accordance to Li et al.19 and are given in Supporting Information.
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Calculation of the reaction yields with NMR Spectroscopy
1
The calculation of the reaction yields was performed using quantitative H-
NMR spectroscopy in a similar way as previously described for metabolite quantitation
in complex natural mixtures.20 The spectra were acquired in solution of the reaction
mixtures after the work up described above at specific concentration in (CD3)2SO,
where a known quantity of syringaldehyde was added as Internal Standard. Using the
peak of the aldehyde group of syringaldehyde at 9.79 ppm for comparison, the ratio of
the reactants/products was measured in the reaction mixture.
Protocol for the evaluation of AhR activity
The synthesized alkaloids were evaluated for their ability to activate AhR-
dependent gene expression in two different recombinant cell lines (human hepatoma
(HG2L7.5c1) and mouse hepatoma (H1L7.5c3),) containing the identical stably
transfected AhR-responsive luciferase reporter gene plasmid, pGudLuc7.5.21, 22 The
results (EC50 (M)) calculated from concentration-response experiments were compared
to those of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the prototypical and highly
potent AhR agonist. The AhR activity protocol was derived from He et al.23 and was as
follows:
Human and mouse cell lines were grown in 100-mm tissue culture plates
(Corning Glass Works; Corning, NY) using sterile techniques, maintained in culture
medium (alpha-minimum essential media (α-MEM) supplemented with 10% (v/v) fetal
bovine serum (FBS) and G418 (1 mg/mL)) and incubated at >80% humidity and 37°C.
For AhR-dependent gene expression analysis, plates of human or mouse cells
(approximately 80–100% confluent) were trypsinized and resuspended in 20 mL
culture media. An aliquot (100 μL) of the indicated cell suspension was added into
sterile 96-well tissue culture plates (Corning) and plates were incubated for 24 h prior
to chemical exposure, allowing cells to attach and reach confluence. Prior to ligand
addition, wells were washed with 1X phosphate-buffered saline (PBS), and then cells
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were incubated with culture media containing DMSO (10 μL/mL) or the indicated
concentration of TCDD or test compounds in DMSO for 6h or 24h at 37°C. For AhR
antagonist studies, cells were incubated with TCDD or 3-bromotryptanthrin in the
absence or presence of the indicated concentration of the AhR antagonist CH223191
(Chembridge Corporation; San Diego, CA) or StemRegenin1 (SR1) (Cayman
Chemicals; Ann Arbor, MI)) for 6h at 37°C.
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After incubation cells were washed twice with PBS, 100 μL of 1X Lysis buffer
(Promega; Madison, WI) was added to each well and the plate shaken at room
temperature until cells were lysed (approximately 20 min). Luciferase activity was
measured using an automated microplate luminometer (Anthos Lucy2, Austria) in
enhanced flash mode with the automatic injection of 50 μL of Promega stabilized
luciferase reagent.
RESULTS
Initial methodology of the reaction on I3A (1)
The performance of the initial reaction led to the formation and isolation of
indirubin (2) after treatment of I3A (1) with H2O2 under Baeyer-Villiger oxidation
conditions (Figure 1). However, several practical difficulties were observed in the
above reaction such as the need for renewal of the reagents in the course of the reaction
or the way of evaluating its success. Therefore, we investigated the possible
ameliorations that could increase the reaction yield.
Modification of the reaction and application on I3A (1) and indole (8)
A reaction described by Santoro et al.11 was selected as alternative synthetic
approach. It seemed preferable mainly due to its reagents, which were coinciding with
the ones used for the transformation of I3A (1) to indirubin (2). According to this
protocol, H2O2 has to be at a concentration of 6%, the solvent H2O:ACN in a ratio 3:1
and the optimal conditions are achieved when the selenium derivative is added in a
catalytic amount (10%) and the reaction mixture is stirred for 72h at 23°C. This
synthetic path is being characterized as an “eco-friendly olefin dihydroxylation”,
probably due to the eco-friendly reagents used, and is proposed for the formation of vic
diols (cis- or trans-) from the corresponding alkenes.
When the above conditions were applied on I3A (1), they resulted once again
in the production of indirubin (2) along with a mixture of several co-products where a
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fluorescent yellow compound was predominant. The yellow compound was isolated
and identified as indolo[2,1-b]quinazolin-6,12-dione or tryptanthrin (3) after
comparison of its spectroscopic data with literature13. Tryptanthrin (3) is an already
known AhR inducer which possesses also significant anti-inflammatory, antimicrobial
and cytostatic activity.24-30 It has been isolated from many plants as well as the yeast
Candida (Yarrowia) lipolytica31-33 and has also been detected and isolated in
Malassezia furfur yeasts.3, 4 Additionally, the spectroscopic analysis of the mixture
showed the formation of indolo[3,2-b]carbazole (4), another known Malassezia
metabolite, in traces (Figure 2).
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Those results prompted us to investigate further the optimal conditions for the
reaction as well as its mechanism. During this effort, the reaction was also applied on
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simple indole (8), another abundant metabolite of Malassezia spp. and interestingly
the formation of the metabolites 2 and 3 was also achieved, while indolo[3,2-
b]carbazole (4) was not detected.
Optimization of the reaction conditions
The successful application of the oxidation on indole (8) and I3A (1) paved the
road for a further investigation of the optimal reaction conditions as well as for the role
of each reagent. For this reason, several trials were performed and the attempts are
summarized in Table 1. Further details on the optimization of the reaction conditions
are presented as supporting information.
Synthesis of tryptanthrin and indirubin derivatives
After the described synthesis was successfully applied on indole (8) and I3A (1)
and the optimal reaction conditions were clarified, the next step was the application on
substituted indoles and indole-3-carbaldehydes. A wide variety of substituted indole
compounds bearing halogens, alkyl-, hydroxyl-, alkoxy-, carboxymethyl- and amino-
groups in positions 4, 5, 6 or 7 of the indole ring was chosen to be studied.
The most active among them seemed to be the compounds with substituents in
positions 5 or 6 of the core and, especially, the ones that bore halogens, alkyl- and
carboxymethyl- groups. This observation let us proceed in the synthesis of all the
corresponding bisubstituted indirubins and tryptanthrins in yields that varied from 5 to
17% for indirubins (2-2g) and from 10 to 27% for tryptanthrins (3-3g), depending on
the substitution. The highest yields for both molecules were achieved when 5- and 6-
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carboxymethylindoles were used as reactants. On the contrary, the application of this
protocol on indoles bearing easily oxidizable groups (e.g. amino-, cyano-, hydroxyl- or
methoxy- groups), led to the formation of dark colored, non-soluble precipitates.
Furthermore, the reaction didn’t evolve as predicted when applied on either azaindoles
or compounds with substituents in positions 4 or 7 of the indolic core.
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Yet, another reaction performed on the mixture of I3A (1) and 6-bromo-I3A
resulted in the simultaneous synthesis of all 8 bis-, mono- and unsubstituted analogues
of the two alkaloids. Amongst the monosubstituted compounds, the formation of 6’-
bromoindirubin and 3-bromotryptanthrin was favored by the reaction conditions,
probably due to differences in the kinetics of the reactions.
Evaluation of AhR Agonist Activity
The AhR agonist activity of 15 out of the 18 synthesized compounds was
evaluated in recombinant HG2L7.5c1 human hepatoma cells containing a stably
transfected AhR-responsive luciferase reporter gene.22 Indirubins were tested only in
the human cell line due to previous evidence for the high selectivity of indirubins for
the human AhR as compared to the mouse AhR.3, 34, 35 In contrast, tryptanthrin
analogues were tested in both stably transfected human (HG2L7.5c1) and mouse
(H1L7.5c3)hepatoma cell lines, not only to determine their relative potencies and
efficacies, but also to evaluate whether these analogues also exhibited a greater
selectivity for human AhR compared to mouse AhR. The 6 hour concentration-response
results of tryptanthrins obtained using the human HG2L7.5c1 cells are presented in
Figure 3. The EC50 (M) results from concentration response curve for the indirubins
and tryptanthrins were calculated and compared to the EC50 (M) obtained from TCDD
concentration response analysis (Tables 5 and 6).
While most of the synthesized indirubins showed very strong activity (two of
them (2b, 2f) had potencies comparable to or greater than TCDD), insertion of
substituents reduced activity in comparison to the highly potent unsubstituted indirubin
(Table 5). Concerning the tryptanthrin analogues, it is clear that this skeleton can afford
extremely active derivatives with very simple modifications.
Among the tryptanthrin derivatives (Table 6), 3,9-dibromotryptanthrin (3f)
interestingly showed a 350-fold increased potency compared to tryptanthrin (3) in the
human HG2L7.5c1 cells and was comparable to the potency of TCDD when evaluated
at 6h (EC50 5x10-10M. In contrast, the monosubstituted analogue 3h (3-
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bromotryptanthrin) was 3,000-times more potent than tryptanthrin and 10 times more
potent than TCDD in the human cells. Given the previously documented high degree
of selectivity of indirubin for human AhR, as compared to mouse AhR,3,34,35 and
similarities in several structural aspects of indirubin and tryptanthrin, the relative
potencies of tryptanthrin analogues in human and mouse cells were compared. These
analyses revealed that while most tryptanthrin analogues were about 10-fold less potent
in the mouse cell line, as compared to that of the human cell line, all were significantly
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less potent than TCDD in the mouse cells (Table 6, Figure 4).
Interesting, 3,9-
dibromotryptanthrin and 3-bromotryptanthrin were 4,000- and 10,000-fold less potent
in the mouse cells, respectively, than in the human cells, where these compounds were
equipotent to or more potent than TCDD (Table 6, Figure 4). These results clearly
demonstrate that 3,9-dibromotryptanthrin and 3-bromotryptanthrin are highly potent
and efficacious human selective AhR agonists.
To confirm that the induction of reporter gene expression in human cells was
mediated by the AhR, the effect of AhR antagonists (CH223191 and SR136,37 on the
induction of reporter gene expression by 3-bromotryptanthrin was examined in the
human HG2L7.5c1 cells (Figure 4). These results clearly demonstrate that AhR
antagonists inhibit both TCDD- and 3-bromotryptanthrin-dependent reporter gene
induction in the human cell line, consistent with these responses being mediated by the
AhR
The gene induction experiments were carried out at 6h of incubation due to the
metabolic lability of the tryptanthrins.
At 24h of incubation, the EC50 for gene induction in human cells by TCDD
remained as low as 3x10-10 M but the potency of tryptanthrins was significantly reduced
(EC50s of 10-6 to 10-7) and this results from differences in the metabolism of these
compounds. While TCDD cannot be metabolized to any significant degree and
therefore its relative potency is retained at 24h, tryptanthrins are metabolized by the
CYP enzymes induced in these cells, resulting in induction of AhR-dependent gene
expression that is transient, with a maximum peak between 6h and 12h.
Discussion
Here we describe the one-step transformation of the aldehyde 1 and/or indole
(8) to the alkaloids indirubin (2) and tryptanthrin (3), under Baeyer-Villiger oxidation
conditions.
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As already analyzed, the established reaction protocol is the result of an amelioration
process, with one of the most important and consistent characteristics to be the catalyst
used, diphenyl diselenide, the reaction of which with H2O2 results in the in situ
formation of the corresponding seleninic acid (aka Syper method of activation).38-40 The
necessity for further investigation and optimization of the procedure led us to perform
an extensive bibliographic search on oxidation of indoles and on reactions using either
H2O2 as an oxidative means or organic derivatives of selenium as a catalyst, identified
a number of different relevant publications.
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We chose to follow the procedure as it is described by Santoro et al.11 as it
combined the use of H2O2 and diphenyl diselenide with the organoselenic derivatives
to be a category of catalysts preferred in several organic reactions due to their easy
synthesis and their stability under various conditions. Throughout this protocol, we
achieved the one-step transformation of the aldehyde 1 to the alkaloids 2 and 3 in good
yields and the compound 4 in traces. This can be considered as a biomimetic reaction,
showing for the first time the possible common biosynthetic origin of the three
metabolites (Figure 2), assumption that is verified by the synthesis of the same two out
of three alkaloids when indole (8), also a metabolite found in abundance in Malassezia
cultures, was used as starting material. The one-pot conversion of both I3A (1) and
indole (8) to indirubin (2) and tryptanthrin (3) simultaneously via the same oxidative
reaction has never been reported.
The up-to-date data on the oxidation of indoles refer to studies conducted in
many different ways, using a variety of reagents (usually expensive and difficult to
handle) and leading mainly to the formation of the corresponding 2-indolinones or
48,49
oxindoles.41,42,45,47 In some cases also isatin50 or indigo were formed.
Additional
research on the attempted syntheses of the two alkaloids revealed that both are formed
by the coupling of an isatin (5) molecule with either 3-acetoxyindole for indirubin (2)
53or isatoic anhydride for tryptanthrin (3).31,54,55 Many other alternative paths have been
used for the synthesis of 3, including even electrochemical methods and condensation
of isatin (5).31, 56-60 A review on the synthetic and medicinal advances for tryptanthrin
has been recently published, summarizing most of the available methods for
tryptanthrin synthesis.61 Among the most recent publications, some newer interesting
methods for the synthesis of thryptanthrin (3) were described that are very similar to
the aforementioned biomimetic reaction. In two of them, I3A (1) was submitted to
62
oxidative dimerization using either Oxone in a mixture of water and ACN or urea
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peroxide in toluene.63 Especially the last one is very efficient for tryptanthrin but not
for indirubin and probably this is related with the use of toluene and heat that make the
reaction conditions much more different than those used in our case which are closer to
those of a living organism (aqueous medium and ambient temperature). Three more
publications have used indole (8) as starting material which is dimerized either with
copper iodide as a catalyst under a continuous flux of oxygen15 or via irradiation by
LED light combined with the use of selected catalysts and simultaneous exposure to
oxygen.19,64
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However, the developed synthetic protocol followed in our case seems to have
certain advantages. This synthetic path is being described as an “eco-friendly olefin
dihydroxylation” leading to the formation of vic diols from the corresponding alkenes.11
The eco-friendly character of the reaction is of great importance and reassured by the
solvents (water and ACN) and the reagents (H2O2 and (PhSe)2) used, that are not
harmful for the environment, especially after the treatment of the residue. Other
advantages are the simple overall procedure, the low-cost, easily accessible reagents
and the possibility of application on a significant number of substrates, comprising both
indoles and indole-3-carbaldehydes. All these features, along with the simultaneous
formation of two different categories of alkaloids, establish the reaction as an attractive
alternative for their synthesis. To the very best of our knowledge, the successful
application of the same dimerization reaction on both indole (8) and I3A (1) hasn’t been
reported before whereas the parallel synthesis of both indirubin (2) and tryptanthrin (3)
has been mentioned only once again,62 where indirubin (2) is referred to as one of the
byproducts of the reaction.
The mechanism that mediates the oxidative transformation of I3A (1) and indole
(8) is difficult to be confirmed with certainty. A primary concept can be based on the
mechanism described by Santoro, involving the formation of an intermediary epoxide
which then opens to the cis- or trans- diol11 and the catalyst is reborn through this cycle.
In accordance to the above assumption, we came up with a hypothesis involving
the synthesis of 3-indolyl formate (5), which would be originally expected under Bayer-
Villiger conditions, and the subsequent formation of an intermediary epoxide on the
double bond of the positions 2 and 3. Hydrolysis of the intermediate results in isatin
(6), creating thus an equilibrium between the indole derivatives 5 and 6, the coupling
of which gives indirubin (3). This mechanism is depicted in Figure 5 and is close to a
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well-known methodology of indirubin (3) synthesis53 which involves a coupling
reaction between 3-acetoxyindole and isatin (6).
A quite similar mechanism is proposed for the synthesis of tryptanthrin (3)
which is thought to be produced by the coupling reaction between anthranilic acid (7)
and isatin (6). As mentioned, isatin (6) is formed in the reaction medium, whereas
anthranilic acid (7) can result by the hyperoxidation of either I3A (1) or isatin (6) and
subsequent in situ formation of the isatoic anhydride which will be hydrolyzed to yield
the acid 7, a hypothesis supported by previous bibliographic data.60 This synthetic path
is shown in Figure 6 and the concept behind this proposed mechanism is similar to the
ones suggested by the most relevant publications.15, 62, 63,
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It’s worth mentioning that, like in the case of indirubin (2), the reaction between
anthranilic acid (7) and isatin (6) is a common synthetic method for the production of
tryptanthrin (3),65,66 with distinct reference to the biosynthesis of the alkaloid.31
However, when an ACN solution of the proposed intermediates isatin (6) and
anthranilic acid (7) was stirred for 72h at 23°C none of the desirable alkaloids were
formed, suggesting the necessity of an oxidative means in the reaction medium, an
assumption that is also in accordance with bibliographic data.60, 65, 66
The aforementioned synthetic schemes represent largely the assumed
mechanisms for the transformation of indole (8) to the two alkaloids when the exact
same reaction conditions. The hypothesis is slightly altered, involving the condensation
of two molecules of the produced isatin (6). This is supported also by recent data that
describe the synthesis of indirubins by isatin coupling in reductive conditions.14 The
proposed synthetic scheme for this reaction is in Figure 7.
As far as the formation of indolo[3,2-b]carbazole (4) is concerned, a different
mechanism is proposed, involving the direct coupling of two I3A (1) units (Figure 8)
and is based on relevant bibliographic data.9, 67-70 As it has already been mentioned,
compound 4 is formed only in the reaction of I3A (1). This observation can be easily
explained as indolo[3,2-b]carbazole (4) is a molecule with 18 carbon atoms whilst the
dimerization of indole (8), can offer only up to 16 carbon atoms in the reaction. On the
other hand, in the reaction of I3A (1) the carbon atoms participating are sufficient
enough for the formation indolo[3,2-b]carbazole (4).
Summing up the details of the reaction mechanism, we need to state the
importance of the order of addition of the reagents as, according to the general reaction
mechanism proposed 11, the catalyst has first to be activated meaning that the reaction
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of the oxidative means with the catalyst has to take place prior to its reaction with the
starting material. For this reason, the addition of H2O2 in the solution of the catalyst
before the reactant is mandatory.
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Finally, examining the products resulting from the reaction on the mixture of
I3A (1) and 6-bromo-I3A (8) we observed the favored synthesis of certain compounds;
combined with the mechanisms proposed, they can lead to some remarks for the
kinetics of the transformations. Given the bibliographic data31, 71 the 6’-substitution on
indirubin scaffold derives from the substitution of acetoxyindole and, respectively, the
3-subsitution on tryptanthrin from the anthranilic acid (7). Our observation can suggest
that the reaction for the formation of 6-bromoisatin (10) progresses more slowly than
the corresponding acetoxyindole (9) and anthranilic acid (11), favoring thus the one for
the non-acylated compound (Figure 9). According to this hypothesis, the synthesis of
6’-bromoindirubin (2h) and 3-bromotryptanthrin (3h) seems to be favored compared to
the 6- and 9- substituted compounds.
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The final step of this work comprised the evaluation of the AhR agonist activity
of the purified indirubin and tryptanthrin analogues and revealed some interesting
results for these classes of alkaloids. Although most of the synthesized indirubin
analogues were relatively potent AhR agonists, all of the resulting compounds were
less potent than that of unsubstituted indirubin. Additionally, it was clear that even
simple modification of the tryptanthrin skeleton could produce analogues that are
extremely potent and efficacious AhR agonists, particularly in the human cell line.
The tryptanthrin structure activity relationship bioassay results in human cells
revealed that insertion of bromine at positions 3 and 9 (3f) not only increased the
relative potency (EC50) of unsubstituted tryptanthrin (3) from 170 nM to 490 pM, but
induced maximal AhR-dependent reporter gene activity (Figure 3). These observations
are significant in that the relative potency and efficacy of the 3,9-dibromotryptanthrin
as an AhR agonist in human cells is comparable to that of TCDD, the prototypical and
one of the most potent known AhR agonists. Even more significant is that a tryptanthrin
analogue with only a single bromine at position 3 (3-bromotryptanthrin (3h)) had a
relative potency in human cells that was even 10-fold higher (57 pM), not only
suggesting that the additional bromine at position 9 in compound 3f actually reduced
the overall relative AhR affinity/potency compared to that of compound 3h, but
producing a compound that was 10-fold more potent than TCDD. 3-Bromotryptanthrin
(3h) was also found to be a highly efficacious AhR agonist, stimulating AhR-dependent
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reporter gene expression to a level comparable to that produced by a maximally
inducing concentration of TCDD (Figure 3). Previous mutagenesis and molecular
docking analysis studies have clearly demonstrated that some specific interactions of a
ligand with residues within the AhR ligand binding site are critical and can drive ligand-
selective interactions,72-75 such that a simple modification like the insertion of a
carbomethoxy group in the same scaffold positions (3g) instead of bromine can lead to
a completely inactive derivative. While induction of reporter gene expression in the
human cells by a given compound strongly suggests that it is an AhR agonist and
activates reporter gene expression via the AhR, the role of the AhR in the ability of the
most potent tryptanthrin analogue (3-bromotryptanthrin (3h)) was confirmed by
demonstrating the ability of two AhR antagonists (CH223191 and SR1) to inhibit the
induction response (Figure 4). Together, these analyses have identified two new and
novel tryptanthrin analogues (3-bromotryptanthrin and 3,9-dibromotryptanthrin) as
highly potent and efficacious agonists of the human AhR and AhR signaling pathway.
Studies from several laboratories have demonstrated that indirubin is a highly
potent and efficacious human-selective AhR agonist that induces AhR-dependent gene
expression in human cells with an EC50 that is ~10-fold greater than that of TCDD.3, 34,
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35, 76 In contrast, in mice or mouse cells, indirubin is ~100-less less potent than TCDD,
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but still highly efficacious.
Site-directed mutagenesis and functional analysis
studies have identified specific amino acid residues within the mouse and human AhR
ligand binding pockets that contributes to this differential specificity and
responsiveness.35 The similarities in structual aspects of indirubins and tryptanthrins
suggest that similar species differences in tryptanthrin ligand specificity may also exist.
Comparison of the relative potencies of the indirubins (3-3h) to stimulate AhR-
dependent reporter gene analysis in both human and mouse hepatoma cell lines
containing the identical reporter gene plasmid (Table 6) revealed that while the potency
of tryptanthrin (3) was comparable between human and mouse cell lines, all tryptanthrin
analogues were less potent in the mouse cell lines. The divergent species-specificity
was particularly evident for the tryptanthin analogues that were most potent human
AhR agonists (i.e., 3-bromotryptanthrin (3h) and 3,9-dibromotryptanthrin (3f)). While
3-bromotryptanthrin (3h) was 10-times more potent than TCDD in human cells, it was
10,000-times less potent than TCDD in the mouse cells; the difference in relative AhR
agonist potency between the human and mouse cells was >17,000-fold (Table 6).
Similarly, although 3,9-dibromotryptanthrin (3h) was equipotent to that of TCDD in
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human cells, it was 4,000-times less potent than TCDD in the mouse cells; the
difference in relative AhR agonist potency between the human and mouse cells was
~800-fold (Table 6). These results confirm that these two tryptanthrin analogues, like
indirubin, are human-selective AhR agonists. However, the AhR antagonist results with
3-bromotryptanthrin suggest that it interacts with residues within the AhR ligand
binding pocket in a manner that is distinctly different than that of indirubin. Previous
studies have demonstrated that while the AhR antagonist CH223191 could effectively
inhibit the ability of TCDD to bind to and activate the AhR, this antagonist had little
effect on AhR activation by indirubin,37 suggesting that TCDD and indirubin
differentially interact with residues within the AhR ligand binding domain. The ability
of CH223191 to inhibit AhR-dependent reporter gene activity by both TCDD and 3-
bromotryptanthrin (Figure 5) suggests that binding of 3-bromotryptanthrin to the AhR
is more similar to that of TCDD than indirubin, which is not inhibited by CH223191.
However, since 3-bromotryptanthrin is a more potent AhR activator of the human AhR
than TCDD and a much less potent activator of the mouse AhR than TCDD, 3-
bromotryptanthrin must interact with the AhR ligand bidning pocket in a manner
similar to, but distinctly different from that of TCDD; the differences in potency of
TCDD between the human and mouse cell lines was only ~6-fold (Table 6). These
induction and inhibition responses suggest that 3-bromotryptanthrin (and perhaps other
tryptanthrins) may represent a novel group of AhR agonists that interact with residues
within the AhR ligand binding pocket in a manner distinctly different from TCDD,
indirubin and other ligands characterized to date.34, 35, 73-78. However, further detailed
QSAR and docking analysis of these analogues into both human and mouse AhR ligand
binding sites generated by molecular modeling are needed to understand the molecular
mechanisms responsible for the high affinity/potency of selected tryptanthrins and the
dramatic species differences in ligand specificity.
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Conclusion
The total of the collected data is of significant chemical and toxicological value.
The synthesis of all three alkaloids -indirubin (2), tryptanthrin (3) and indolo[3,2-
b]carbazole (4)- through the same oxidative mechanism from I3A (1), the main product
of L-tryptophan metabolism in Malassezia yeasts, seems to be used also by the fungus
for the production of these secondary metabolites which are implicated in the
Malassezia-related skin diseases. Taking into consideration that indirubin (2) and
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tryptanthrin (3) are often together isolated from several plant species, like Isatis spp.
and Polygonum tinctorium,78,79 the existence of a common biosynthetic path for the two
metabolites is a reasonable hypothesis. The inhibition of the oxidative transformation
of I3A to indirubin and tryptanthrin by Malassezia could be a potential future target for
the inhibition of Malassezia toxicity to skin.
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Supporting Information
Analytical data for the synthesized compounds and results of the optimization trials and
reaction mechanism investigation
Author Information
* Corresponding author. Tel.: +30 210 7274052. E-mail: magiatis@pharm.uoa.gr
Abbreviations
AhR: Aryl-hydrocarbon Receptor
I3A: Indole-3-carbaldehyde
H2O2: Hydrogen Peroxide
ICZ: Indolo[3,2-b]carbazole
6-FICZ: 6-Formylindolo[3,2-b]carbazole
NMR: Nuclear Magnetic Resonance
TLC: Thin Layer Chromatography
DCM: Dichrolomethane
(PhSe)2: Diphenyl diselenide
THF: Tetrahydrofuran
Rf: Retardation factor
EtOAc: Ethyl Acetate
(CD3)2SO: Deuterated Dimethyl Sulfoxide
CDCl3: Deuterated Chloroform
EC50: Half maximal effective concentration
TCDD: 2,3,7,8-Tetrachlorodibenzodioxin
α-MEM: alpha-minimum essential media
FBS: Fetal bovine serum
PBS: phosphate-buffered saline
DMSO: Dimethyl Sulfoxide
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ACN: Acetonitrile
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6
CYP: Cytochrome P450
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Table 1: Trials of the biomimetic reaction with modification of the reagents’ quantities
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9
Entry
Oxidative
Means
Catalyst
Quantity
of catalyst
10%
Solvent
Results
1
2
H2O2 6%
(PhSe)2
(PhSe)2
-
ACN
-
Indirubin&Tryptanthrin
Hyperoxidation
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H2O2 6%
H2O2 6%
H2O2 6%
H2O2 6%
H2O212%
H2O212%
H2O212%
H2O212%
H2O212%
10%
-
3
ACN
ACN
ACN
ACN
-
n.d.
4
(PhSe)2
(PhSe)2
(PhSe)2
(PhSe)2
-
20%
50%
10%
10%
-
Indirubin&Tryptanthrin
Indirubin&Tryptanthrin
Indirubin&Tryptanthrin
Hyperoxidation
5
6
7
8
ACN
ACN
ACN
n.d.
9
(PhSe)2
(PhSe)2
20%
50%
Indirubin&Tryptanthrin
Indirubin&Tryptanthrin
10
n.d.: not detected
Table 2: Trials of the biomimetic reaction with modification of the conditions
Entry Temperature
Reaction
Time
72h
Result
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12
13
14
0-23°C
~10°C
40°C
Indirubin&Tryptanthrin
Lower yield
72h
72h
Lower yield
23°C
120h
Indirubin&Tryptanthrin
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Table 3: Synthetic Analogues of Indirubin (2)
R5
R6
6
7
8
9
O
R5
R6
NH
NH
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O
Substitution
Yield
2
R5=R6=R5’=R6’=H
5%
5%
2a R5=R5’=F, R6=R6’=H
2b R5=R5’=Cl, R6=R6’=H
2c R5=R5’=Br, R6=R6’=H
5%
6%
2d R5=R5’=CH3, R6=R6’=H
6%
2e R5=R5’=COOCH3, R6=R6’=H
2f R5=R5’=H, R6=R6’=Br
17%
5%
2g R5=R5’=H, R6=R6’=COOCH3
2h R5=R5’= R6=H, R6’=Br
14%
5%
Table 4: Synthetic Analogues of Tryptanthrin (3)
R9
O
R8
R2
N
R3
N
O
Substitution
Yield
3
R2=R3=R8=R9=H
12%
16%
10%
12%
15%
27%
13%
18%
12%
3a R2=R8=F, R3=R9=H
3b R2=R8=Cl, R3=R9=H
3c R2=R8=Br, R3=R9=H
3d R2=R8=CH3, R3=R9=H
3e R2=R8=COOCH3, R3=R9=H
3f R2=R8=H, R3=R9=Br
3g R2=R8=H, R3=R9=COOCH3
3h R2=R8= R9=H, R3=Br
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Table 5: EC50values for the induction of AhR-dependent gene expression in human hepatoma
(HG2L7.5c1) cells by Indirubins
6
7
R5
R6
8
O
9
R5
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NH
NH
R6
O
EC50, 6h Human
2
2a
2.8x10-11
1.0x10-9
3.0x10-10
8.2x10-8
8.6x10-10
1.0x10-6
5.7x10-10
2b
2e
2f
2g
TCDD
Table 6: EC50values for the induction of AhR-dependent gene expression in human
(HG2L7.5c1) and mouse (H1L7.5c3) hepatoma cell lines by tryptanthrins.
R9
O
R8
R2
R3
N
N
O
EC50, 6h
EC50, 6h
Human Cells Mouse Cells
3
1.7x10-7
5.4x10-8
8.4x10-8
5.6x10-7
6.8x10-8
4.5x10-7
4.9x10-10
Inactive
5.7x10-11
8.1x10-7
2.0x10-6
9.1x10-7
3.6x10-6
7.2x10-7
3.7x10-6
3.9x10-7
Inactive
>1.0x10-6
3a
3b
3c
3d
3e
3f
3g
3h
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TCDD
5.7x10-10
1.0x10-10
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O
CHO
Se
Se
6
7
8
NH
CH2Cl2 dist.
H2O2 10%
NH
NH
9
O
2
1
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Figure 1: Initial attempt for the formation of indirubin (2)
O
2
CHO
NH
NH
Se
O
Se
O
3
4
N
ACN:H2O2 6% -
1:3
N
O
NH
NH
1
NH
Figure 2: General scheme of the biomimetic reaction
TCDD
3
3f
3c
3a
3e
3h
Figure 3: Concentration-response analysis of the induction of AhR-dependent luciferase
reporter gene expression in human hepatoma (HG2L7.5c1) cells by TCDD, tryptanthrin and
tryptanthrin analogues. See Table 4 for specific chemical substitutions.
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0
Figure 4: The AhR antagonists CH223191 (CH) and StemRegenin1 (SR1) inhibit TCDD-
and 3BT-dependent induction of luciferase reporter gene expression in recombinant human
hepatoma (HG2L7.5c1) cells.
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+
ACN:H2O2 6% - 1:3
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Figure 5: Proposed mechanism for the formation of Indirubin (2)
+
ACN:H2O2 6% - 1:3
1
7
6
3
Figure 6: Proposed mechanism for the formation of Tryptanthrin (3)
6
2
ACN:H2O2 6% - 1:3
8
+
6
7
3
Figure 7: Proposed mechanism for the transformation of indole (8)
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