Dibutyltin(IV) complexes containing arylazobenzoate ligands: Chemistry, in vitro cytotoxic effects on human tumor cell lines and mode of interaction with some enzymes

Article abstract of DOI:10.1007/s10637-009-9360-3

Dibutyltin(IV) complexes of composition Bu₂Sn (LH)₂, where LH is a carboxylate residue derived from 2-[(E)- (5-tert-butyl-2- hydroxyphenyl)diazenyl]benzoate (L¹H) with water molecule (1), 4-[(E)-(5-tert-butyl-2-hydroxyphen

Full text of DOI:10.1007/s10637-009-9360-3

Invest New Drugs (2011) 29:285–299
DOI 10.1007/s10637-009-9360-3
PRECLINICAL STUDIES
Dibutyltin(IV) complexes containing arylazobenzoate
ligands: chemistry, in vitro cytotoxic effects on human tumor
cell lines and mode of interaction with some enzymes
Tushar S. Basu Baul & Anup Paul & Lorenzo Pellerito & Michelangelo Scopelliti &
Palwinder Singh & Pooja Verma & Andrew Duthie & Dick de Vos & Edward R. T. Tiekink
Received: 5 November 2009 /Accepted: 20 November 2009 /Published online: 11 December 2009
#
Springer Science+Business Media, LLC 2009
Summary Dibutyltin(IV) complexes of composition Bu2Sn
(LH)2, where LH is a carboxylate residue derived from 2-[(E)-
(5-tert-butyl-2-hydroxyphenyl)diazenyl]benzoate (L1H) with
water molecule (1), 4-[(E)-(5-tert-butyl-2-hydroxyphenyl)
diazenyl]benzoate (L2H) (2) and 4-[(E)-(4-hydroxy-5-
methylphenyl)diazenyl]benzoate (L3H) (3), were synthesized
and characterized by spectroscopic (1H, 13C and 119Sn NMR,
IR, 119Sn Mössbauer) techniques. A full characterization was
accomplished from the crystal structure of complex 1. The
molecular structures and geometries of the complexes (1a i.e.
1 without water molecule and 3) were fully optimized using
the quantum mechanical method (PM6). Complexes 1 and 3
were found to exhibit stronger cytotoxic activity in vitro
across a panel of human tumor cell lines viz., A498, EVSA-
T, H226, IGROV, M19 MEL, MCF-7 and WIDR. Com-
pound 3 is found to be four times superior for the A498,
EVSA-T and MCF-7 cell lines than CCDP (cisplatin), and
four, eight and sixteen times superior for the A498, H226
and MCF-7 cell lines, respectively, compared to ETO
(etoposide). The mechanistic role of cytotoxic activity of
test compounds is discussed in relation to the theoretical
results of docking studies with some key enzymes such as
ribonucleotide reductase, thymidylate synthase, thymidylate
phosphorylase and topoisomerase II associated with the
propagation of cancer.
Electronic supplementary material The online version of this article
(doi:10.1007/s10637-009-9360-3) contains supplementary material,
which is available to authorized users.
:
T. S. Basu Baul (*) A. Paul
Department of Chemistry, North-Eastern Hill University,
NEHU Permanent Campus, Umshing,
Shillong 793 022, India
e-mail: basubaul@nehu.ac.in
e-mail: basubaul@hotmail.com
:
. .
Keywords Anti-cancer drugs Dibutyltin(IV) compounds
L. Pellerito M. Scopelliti
Dipartimento di Chimica Inorganica e Analitica“Stanislao
Cannizzaro”, Università degli Studi di Palermo,
Viale delle Scienze, Parco D’Orleans II, Edificio 17,
90128 Palermo, Italy
.
.
.
Arylazobenzoate Spectroscopy Cell lines Docking studies
Introduction
:
P. Singh P. Verma
Department of Chemistry, Guru Nanak Dev University,
Amritsar 143005, India
Cisplatin [1–4] was the first inorganic cancer chemothera-
peutic agent, and remains a front-line treatment for
testicular, ovarian and other cancers. Despite heavy side
effects and drug resistance of tumor cells, cisplatin is
among the most widely used anti-cancer drugs, and other
platinum (II) complexes, such as carboplatin, oxaliplatin
and nedoplatin, with improved activity and/or reduced
toxicity, are being introduced in therapy. As a result,
platinum drugs are playing a major role within established
medical treatments of cancers [2, 5, 6]. The wide clinical
success of platinum(II) compounds has prompted a great
A. Duthie
School of Life & Environmental Science, Deakin University,
Geelong, Victoria 3217, Australia
D. de Vos
Pharmachemie BV,
PO Box 552, 2003 RN, Haarlem, The Netherlands
E. R. T. Tiekink
Department of Chemistry, University of Malaya,
50603 Kuala Lumpur, Malaysia

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Invest New Drugs (2011) 29:285–299
deal of interest in other platinum and non-platinum metal-
lodrugs (e.g., Sn, Ti, Au, Cu, Ru, Pd) that might exhibit
comparable cytotoxic properties accompanied by a different
pattern of antitumor specificities and by a more favorable
pharmacological and toxicological profile. The current
investigative efforts and the state-of-the-art in the field of
anticancer metal complexes have been documented in the
literature [7–15].
with non-azo ligands [27]. On the other hand, examples of
diorganotin(IV) arylazobenzoates of formulations R2Sn(L)2
and {[R2Sn(L)]2O}2 are scanty except for the recent ones
where L=5-[(E)-2-(aryl)-1-diazenyl]-2-hydroxybenzoate, for
which in vitro cytotoxic results were found to be quite
promising at least with respect to some of the standard drugs
[28, 29].
This investigation focuses on the synthesis and spectro-
scopic characterization of three new dibutyltin(IV) complexes
of compositions, viz., Bu2Sn(L1H)2.H2O (1), Bu2Sn(L2H)2
(2) and Bu2Sn(L3H)2 (3), in an attempt to see whether this is
a significant variable for docking with various enzymes and
to obtain better cytotoxic performances. The ligands used
herein show selected variations in the structures, for
example, the positions of the caboxylate group (ortho- and
para-) in 1 and 2 in which other substituents are held
constant, while in 3 nuclear substituent modification of the
coupling moiety would hopefully allow some useful con-
clusions to be made. The molecular structures and geome-
tries of the dibutyltin(IV) compounds (1a and 3) were fully
optimized using the semiempirical quantum chemistry
method. The molecular docking of 1a and 3 have been
investigated with some selected enzymes and preliminary in
vitro cytotoxicity data of complexes (1–3) are reported
across a panel of human tumor cell lines viz., A498 (renal
cancer), EVSA-T (mammary cancer), H226 (non-small-cell
lung cancer), IGROV (ovarian cancer), M19 MEL (melano-
ma), MCF-7 (mammary cancer) and WIDR (colon cancer).
Structural diversity of the organotin(IV) carboxylates is
well recognized and a variety of coordination geometries
leading to monomeric-, dimeric-, trimeric-, tetrameric-,
oligomeric- and polymeric structures have been reported
[16–18]. It is generally believed that such structures arise
because of combination of steric and electronic factors
[19–22]. Furthermore, it has been reported that the size of
the carboxylic acids used and stoichiometry of the reactant
also play important role in the formation of solid state
frameworks. Among the various types of organotin(IV)
carboxylates, generally triorganotin(IV) (R3SnL) and dio-
rganotin(IV) (R2SnL2 and {[R2Sn(L)]2O}2 ) carboxylates
have been investigated in greater extent for their cytotoxic
potential [23]. So far, triphenyltin(IV) carboxylates of
formulation Ph3SnL, where L=carboxylate residue, viz.,
-terebate, -steroidcarboxylate, -benzocrowncarboxylate [23]
and those derived from Schiff bases containing amino
acetates, e.g., 2-{[(2Z)-(3-hydroxy-1-methyl-2-butenylidene)]
amino}-4-methyl-pentanoate; 2-{[(E)-1-(2-hydroxyphenyl)
methylidene]amino}-4-methyl-pentanoate; 2-{[(E)-1-(2-
hydroxyphenyl)ethylidene]amino}-4-methyl-pentanoate
[24], 2-{[(2Z)-(3-hydroxy-1-methyl-2-butenylidene)]amino}
phenylpropionate; 2-{[(E)-1-(2-hydroxyphenyl)methylidene]-
amino}phenylpropionate; 2-{[(E)-1-(2-hydroxyphenyl)ethyli-
dene]amino}phenylpropionate [25] and arylazobenzoates,
e.g., 2-[(E)-2-(3-formyl-4-hydroxyphenyl)-1-diazenyl]benzo-
ate; 2-[(E)-2-(4-hydroxy-5-methylphenyl)-1-diazenyl]benzoate
[26], found to be very active when tested in vitro against
human tumor cell lines. In general, the ID50 values for the
triphenyltin(IV) arylazobenzoates [26] were found to be si-
milar to that of the triphenyltin(IV) complexes of Schiff bases
derived from amino acids (e.g., l-leucine and phenylalanine)
[24, 25] but their advantage lies in prolonged stability in both
the solution- and solid-state. Further, the docking studies of
triphenyltin(IV) 2-[(E)-2-(aryl)-1-diazenyl]benzoates indicated
that the azo group nitrogen atoms and formyl, carbonyl and
ester oxygen atoms in the ligand moiety also play an im-
portant role. They exhibit hydrogen bonding interactions with
the active site of amino acids of various enzymes such as
ribonucleotide reductase, thymidylate synthase and thymidy-
late phosphorylase. The higher activity was attributed to the
presence of the azo group in the molecules of triphenyltin(IV)
complexes [26]. Similar observations were also noted recently
for the ruthenium complexes containing azo-ligands which
have shown higher activity in comparison to the complexes
Experimental
Materials and methods
Dibutyltin oxide (Fluka) was used as received. The solvents
used in the reactions were of AR grade and dried using
standard literature procedures. Benzene was distilled from
sodium benzophenone ketyl.
Synthesis and characterization of ligands
Synthesis of 2-[(E)-(5-tert-butyl-2-hydroxyphenyl)diazenyl]
benzoic acid (L1HH’)
Ligand L1HH’ was prepared by reacting o-carboxybenze-
nediazonium chloride with 4-tert-butyl-phenol in alkaline
solution under cold conditions (0–5°C) by the method
described in our earlier report [30]. Several crystallizations
from methanol yielded red plates of L1HH’. M.p.: 180–
182°C. Anal. calc. for C17H18N2O3: C, 68.44; H, 6.08; N,
9.39 %. Found C 68.30; H, 6.28; N, 9.59 %. IR absorption
1
(cm−1): 1699 ν(OCO)asym. H and 13C NMR (in CDCl3)
gave satisfactory results [30].

Invest New Drugs (2011) 29:285–299
287
The other ligands, viz., L2HH’ and L3HH’ were prepared
analogously by reacting p-carboxybenzenediazonium chloride
with 4-tert-butyl-phenol and 2-methylphenol, respectively, in
alkaline solution under cold conditions. The characterization
and spectroscopic data are presented below.
isolated from the mother liquor and dried in vacuo. Yield:
46 %. M.p.: 128–130°C. Anal. Found: C, 60.90; H, 6.13; N,
6.90 %. Calc. for C42H54N4O7Sn: C, 60.66; H, 6.44; N,
1
6.63 %. IR (cm−1): 1595 ν(OCO)asym. H-NMR (CDCl3):
δH: Ligand skeleton: 12.5 [brs, 2H, OH], 8.24 [d, 8 Hz, 2H,
H3], 7.91 [d, 8 Hz, 2H, H6], 7.84 [d, 2.5 Hz, 2H, H6’], 7.58
[t, 8 Hz, 2H, H5], 7.46 [t, 8 Hz, 2H, H4], 7.31 [dd, 2.5, 8 Hz,
2H, H4’], 6.88 [d, 8 Hz, 1H, H3’], 1.30 [s, 18H, CH3];
Sn-Bu skeleton: 0.79 [t, 6H, H-4*], 1.35 [m, 4H, H-3*], 1.70
[m, 4H, H-2*], 1.83 [t, 4H, H-1*], ppm. 13C-NMR (CDCl3);
δ C: 175.3 [CO2], 150.8 [C2’], 149.9 [C1], 142.2 [C1’],
137.8 [C4’], 133.1 [C6’], 132.7 [C5], 131.4 [C3], 130.0
[C4], 129.9 [C5’], 127.2 [C2], 118.4 [C3’], 115.8 [C6], 34.1
[C-7’], 31.4 [CH3]; Sn-Bu skeleton: 13.6 [C-4*], 25.9
[C-3*], 26.3 [C-2*], 26.7 [C-1*] ppm. 119Sn-NMR (CDCl3):
−139.5 ppm. 119Sn Mössbauer, mm s−1: δ=1.54, Δ=4.39,
Γav=0.83, ρ =2.85.
Synthesis of 4-[(E)-(5-tert-butyl-2-hydroxyphenyl)diazenyl]
benzoic acid (L2HH’)
Recrystallized from methanol to give orange crystalline
material in 42 % yield. M.p.: 258–260°C. Anal. calc. for
C17H18N2O3: C, 68.44; H, 6.08; N, 9.39 %. Found C 68.18;
H, 6.18; N, 9.50 %. IR (cm−1): 1686 ν(OCO)asym. 1H-NMR
(DMSO-d6): δ H: 12.1 [brs, 1H, CO2H], 8.16 [dd, 2.5 Hz,
8.0 Hz, 2H, H2/6], 7.97 [dd, 2.5 Hz, 8.0 Hz, 2H, H3/5],
7.90 [d, 2.5 Hz, 1H, H6’], 7.45 [dd, 2.5, 8 Hz, 1H, H4’],
6.97 [d, 8 Hz, 1H, H3’], 1.19 [s, 9H, CH3] ppm. Signal for
phenol was exchanged due to presence of water in the solvent.
13C-NMR (DMSO-d6); δ C: 166.8 [CO2], 33.6 [C-7’], 30.8
[CH3], other carbons: 152.8, 150.4, 142.4, 136.7, 132.3,
131.4, 130.4, 126.9, 121.6, 125.9, 117.4 ppm.
Synthesis of Bu2Sn(L2H)2 (2)
Compound 2 was prepared analogously by following the
method and conditions described for 1 and using L2HH’
and Bu2SnO. The orange product was recrystallized from
benzene-hexane mixture. Yield: 33%. M.p.: 194–196°C.
Anal. Found: C, 61.16; H, 6.45; N, 6.85 %. Calc. for
C42H52N4O6Sn: C, 60.96; H, 6.33; N, 6.77 %. IR (cm−1):
Synthesis of 4-[(E)-(4-hydroxy-5-methylphenyl)diazenyl]
benzoic acid (L3HH’)
Recrystallized from methanol to give red-orange crystalline
material in 60 % yield. M.p.: 234–236°C. Anal. calc. for
C14H12N2O3: C, 65.62; H, 4.72; N, 10.93 %. Found. C,
1
1606 ν(OCO)asym . H-NMR (CDCl3); δH: Ligand skele-
ton: 12.6 [brs, 2H, OH], 8.30 [dd, 2.5 Hz, 8.0 Hz, 4H, H2/
6], 7.95 [m, 6H, H3/5 and H6’], 7.43 [dd, 2.5, 8 Hz, 2H,
H4’], 6.98 [d, 8 Hz, 2H, H3’], 1.39 [s, 9H, CH3]; Sn-Bu
skeleton: 0.92 [t, 6H, H-4*], 1.46 [m, 4H, H-3*], 1.79 [m,
4H, H-2*], 1.86 [t, 4H, H-1*] ppm. 13C-NMR (CDCl3); δ
C: Ligand skeleton: 175.1 [CO2], 34.1 [C-7’], 31.3 [CH3],
other carbons: 153.5, 150.7, 142.9, 137.1, 131.8, 131.7,
130.1, 128.9, 121.9, 117.8; Sn-Bu skeleton: 13.5 [C-4*],
25.7 [C-3*], 26.5 [C-2*], 26.7 [C-1*] ppm. 119Sn-NMR
(CDCl3): –146.3 ppm. 119Sn Mössbauer, mm s−1: δ=1.44,
Δ=3.64, Γav=0.99, ρ =2.53, C-Sn-C=147°.
65.70; H, 4.58; N, 10.88 %. IR (cm−1): 1684 ν(OCO)asym
.
1H-NMR (DMSO-d6): δ H: 13.3 [brs, 1H, CO2H], 10.4
[brs, 1H, OH], 8.08 [dd, 2.5 Hz, 8.0 Hz, 2H, H2/6], 7.83
[dd, 2.5 Hz, 8.0 Hz, 2H, H3/5], 7.70 [d, 2.5 Hz, 1H, H6’],
7.66 [dd, 2.5, 8 Hz, 1H, H2’], 6.96 [d, 8 Hz, 1H, H5’], 2.48
[s, 3H, CH3] ppm. 13C-NMR (DMSO-d6); δ C: 167.7
[CO2], 16.8 [CH3], other carbons: 160.7, 155.5, 146.0,
132.6, 131.4, 126.1, 125.8, 124.7, 122.8, 115.8 ppm.
Synthesis and characterization of dibutyltin(IV) complexes
Synthesis of Bu2Sn(L1H)2.H2O (1)
Synthesis of Bu2Sn(L3H)2 (3)
A suspension of Bu2SnO (0.21 g, 0.84 mmol) and L1HH’
(0.50 g, 1.67 mmol) in 50 ml anhydrous benzene was
refluxed for 6 h in a flask equipped with a Dean-Stark water
separator and a water cooled condenser. After the reaction,
a wine-red colored solution was obtained and filtered while
hot. The solvent was concentrated on a hot plate and
precipitated with petroleum ether. The red colored precipitate
was filtered and washed thoroughly with petroleum ether
(60–80°C) and dried in vacuo. The dried product was
dissolved in anhydrous benzene-hexane (v/v 2:1), filtered
off to remove any undissolved particles and the filtrate left to
crystallize at room temperature. The red–brown crystals were
Compound 3 was prepared analogously by following the
method and conditions described for 1 and using L3HH’
and Bu2SnO. The orange colored product was recrystallized
from ethanol. Yield: 53%. M.p.: 132–134°C. Anal. Found:
C, 58.28; H, 5.23; N, 7.50 %. Calc. for C36H40N4O6Sn: C,
58.16; H, 5.42; N, 7.54%. IR (cm−1): 1596 ν(OCO)asym.
1H-NMR (CDCl3); δ H: Ligand skeleton: not detected
[OH], 8.27 [dd, 2.5 Hz, 8.0 Hz, 4H, H2/6], 7.91 [dd,
2.5 Hz, 8.0 Hz, 4H, H3/5], 7.81 [d, 2.5 Hz, 2H, H6’], 7.74
[dd, 2.5, 8 Hz, 1H, H2’], 6.89 [d, 8 Hz, 2H, H5’], 2.34
[s, 6H, CH3]; Sn-Bu skeleton: 0.89 [t, 6H, H-4*], 1.46
[m, 4H, H-3*], 1.79 [m, 4H, H-2*], 1.87 [t, 4H, H-1*],

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Invest New Drugs (2011) 29:285–299
ppm. 13C-NMR (CDCl3); δ C: Ligand skeleton: 176.1
[CO2], 16.3 [CH3], other carbons: 158.2, 156.1, 147.4, 132.0,
131.2, 126.1, 125.5, 124.2, 122.8, 115.7; Sn-Bu skeleton:
13.9 [C-4*], 26.1 [C-3*], 26.8 [C-2*], 27.1 [C-1*] ppm.
119Sn-NMR (CDCl3): −142.8 ppm. 119Sn Mössbauer,
mm s−1: δ=1.41, Δ=3.34, Γav=0.82, ρ =2.37, C-Sn-C=138°.
supplemented with 10% FCS, penicillin 100 μg/ml and
streptomycin 100 μg/ml. The cells were mildly trypsinized
for passage and for use in the experiments. RPMI and FCS
were obtained from Gibco (Invitrogen, Paisley, Scotland).
SRB, DMSO, Penicillin and streptomycin were obtained
from Sigma (St. Louis MO, USA), TCA and acetic acid
from Baker BV (Deventer, NL) and PBS from NPBI BV
(Emmer-Compascuum, NL).
Physical measurements
The test compounds 1–3 and reference compounds were
dissolved in a concentration of 5 mg/ml in DMSO.
Compound 1 did not dissolve immediately in DMSO but
it required long vortexing. Compound 2 also had dissolu-
tion problem in DMSO. It was first stirred at 37°C and then
at 50°C but the compound did not dissolve completely, so
the suspension of 2 in DMSO was used to make the
dilutions in medium. On the other hand, compound 3 was
freely soluble in DMSO. The compounds were subsequently
diluted to a final concentration of 250,000 ng/ml in full
medium. Cytotoxicity was estimated by the microculture
sulforhodamine B (SRB) test [32].
The experiment was started on day 0. On day 0, 150 μl
of trypsinized tumor cells (1,500–2,000 cells/well) were
plated in 96-wells flat-bottomed micro-titer plates (Cellstar,
Greiner Bio-one). The plates were pre-incubated for 48 h at
37°C, 5 % CO2 to allow the cells to adhere to the bottom.
On day 2, a three-fold dilution sequence of ten steps was
made in full medium, starting with the 250,000 ng/ml stock
solution. Every dilution was used in quadruplicate by
adding 50μl to a column of four wells. This procedure
results in the highest concentration of 625,000 ng/ml being
present in column 12. Column 2 was used for the blank. To
column 1, medium was added to diminish interfering
evaporation.
On day 7, the incubation was terminated. Subsequent-
ly, the cells were fixed with 10% trichloroacetic acid in
PBS and placed at 4°C for an hour. After three washings
with tap water, the cells were stained for at least 15 min
with 0.4% SRB dissolved in 1% acetic acid. After
staining, the cells were washed with 1% acetic acid to
remove the unbound stain. The plates were air-dried and
the bound stain was dissolved in 150μl 10 mM tris-base.
The absorbance was measured at 540 nm using an
automated microplate reader (Labsystems Multiskan MS).
The data were used for construction of concentration-response
curves and determination of the ID50 values by use of
Deltasoft 3 software.
The variability of the in vitro cytotoxicity test depends
on the cell lines used and the serum applied. With the same
batch of cell lines and the same batch of serum the inter-
experimental CV (coefficient of variation) is 1–11%
depending on the cell line and the intra-experimental CV
is 2–4%. These values may be higher with other batches of
cell lines and/or serum.
Carbon, hydrogen and nitrogen analyses were performed
with a Perkin Elmer 2400 series II instrument. IR spectra in
the range 4,000–400 cm−1 were obtained on a Perkin Elmer
Spectrum BX series FT-IR spectrophotometer with samples
1
investigated as KBr discs. The H and 13C spectra were
recorded on a Bruker AMX 400 spectrometer and measured
at 400.13 and 100.62 MHz, respectively, while 119Sn NMR
spectra were recorded either on a Bruker AMX 400 or a
Jeol GX 270 spectrometer and measured at 149.18 and
1
100.75 MHz, respectively. The H, 13C and 119Sn chemical
shifts were referenced to Me4Si set at 0.00 ppm, CDCl3 set
at 77.0 ppm and Me4Sn set at 0.00 ppm respectively. The
Mössbauer spectra were recorded with a conventional
spectrometer operating in the transmission mode. The
source was Ca119SnO3 (Ritverc GmbH, St. Petersburg,
Russia; 10 mCi), moving at room temperature with constant
acceleration in a triangular waveform. The driving system,
multi-channel analyser, proportional counter gas detector
and the related electronics were purchased from Takes
(Ponteranica, Bergamo, Italy). The solid absorber samples,
containing ca. 0.5 mg cm−2 of 119Sn, were held between
aluminum foil windows at 77.3 K in a NRD-1258-DMB
liquid-nitrogen cryostat (Cryo Industries, USA). The velocity
was calibrated using a 10 mCi 57Co Mössbauer source and an
α57Fe foil, 4 μm thick, as absorber (both Ritverc GmbH, St.
Petersburg, Russia). The isomer shifts are relative to room
temperature Ca119SnO3.
Experimental protocol and cytotoxicity tests
The in vitro cytotoxicity test of dibutyltin(IV) compounds
1–3 was performed using the SRB test for the estimation of
cell viability. The human cancer cell lines examined in the
present study were WIDR (colon cancer), M19 MEL
(melanoma), A498 (renal cancer), IGROV (ovarian cancer)
and H226 (non-small cell lung cancer) belong to the
currently used anticancer screening panel of the NCI,
USA [31]. The MCF7 cell line is estrogen receptor
(ER)+/ progesterone receptor (PgR)+(breast cancer) and
the cell line EVSA-T is (ER)-/(Pgr)- (breast cancer). Prior
to the experiments, a mycoplasma test was carried out on
all cell lines and found to be negative. All cell lines were
maintained in a continuous logarithmic culture in RPMI
1640 medium with Hepes and phenol red. The medium was

Invest New Drugs (2011) 29:285–299
289
X-ray crystallography
336408; Fax: +44-1223-336003; e-mail: deposit@ccdc.
cam.ac.uk; Web site: http://www.ccdc.cam.ac.uk).
Single crystals suitable for an X-ray crystal-structure
determination were obtained from dichloromethane/n-heptane
(v/v 1:1) by slow evaporation of the solution of the
compound Bu2Sn(L1H)2.H2O (1). Crystal data for 1 i.e.
Computational methods
The molecular structures and geometries of the dibutyltin
(IV) compounds (1a i.e. 1 without water molecule and 3)
were fully optimized using the semiempirical quantum
chemistry method (PM6) [38–42] using the Program CP2K
2.1.7 (Development Version) working with quickstep
method [43]. Dockings of compounds 1a and 3 in the
active sites of various enzymes are performed using
ArgusLab 4.0.1. [38, 44–46]. This program was also
applied for visualization and molecular modeling of the
compounds. The three dimensional coordinates of the key
enzymes such as ribonucleotide reductase (pdb ID: 4R1R),
thymidylate synthase (pdb ID: 2G8D), thymidylate
phosphorylase (pdb ID: 1BRW) and topoisomerase II (pdb
ID: 1QZR) were obtained from the Internet at the Research
Collaboratory for Structural Bioinformatics (RCSB) Protein
Data Bank. The docking program implements an efficient
grid-based docking algorithm, which approximates an
exhaustive search within the free volume of the binding
site cavity. The conformational space was explored by the
geometry optimization of the flexible ligand (rings are
treated as rigid) in combination with the incremental
construction of the ligand torsions. Thus, docking occurs
between the flexible ligand part of the compounds and
enzymes. The ligand orientation is determined by a shape
scoring function based on Ascore and the final positions are
ranked by lowest interaction energy values. Prior to
docking, the ground state was optimized using the PM6QM
implemented in the geometry optimization module of pro-
C
42H54N4O7Sn: M=845.58, orthorhombic, P21212,
a=14.6214(7), b=22.6226(10), c=6.0367(3) Å, V=1996.78
(16) Å3, Z=2, Dx=1.406, F(000)=880, μ=0.694 mm−1.
Intensity data were collected at 98(1) K for a red-brown
prism (0.08×0.10×0.40 mm) on a Rigaku AFC12κ/SAT-
URN724 CCD using Mo Kα radiation so that θmax=27.5°.
There were 4379 unique absorption corrected [33] data
(Rint=0.023) and 4338 data with I ≥ 2σ(I). The structure was
solved by heavy-atom methods (PATTY in DIRDIF92 [34])
and refined (anisotropic displacement parameters, carbon-
bound H atoms in the riding model approximation, and a
weighting scheme w=1/[σ2(Fo )+(0.019P)2+1.861P] where
2
2
P=(Fo2+2Fc )/3) with SHELXL-97 on F2 [35]. The
hydroxyl-H atom was located in a difference Fourier map
but included in an idealised position. The final R was 0.023
(obs. data) and the wR (all data) was 0.056. The absolute
structure was determined on the basis of difference in Friedel
pairs included in the data set. The view of the molecule of 1
and supramolecular aggregation are shown in Figs. 1 and 2,
which were drawn with 50% probability ellipsoids [36] and
DIAMOND [37], respectively.
Crystallographic data (without structure factors) for the
structure(s) reported in this paper have been deposited with
the Cambridge Crystallographic Data Centre (CCDC) as
supplementary publication no. CCDC-744049. Copies of
the data can be obtained free of charge from the CCDC (12
Union Road, Cambridge CB2 1EZ, UK; Tel.: +44-1223-
Fig. 1 Molecular structure
and crystallographic numbering
scheme for 1. The diagram was
drawn with 50% displacement
ellipsoids and the atoms
indicated with a superscripted
i are related by the symmetry
operation: 1-x, 1-y, z. Selected
geometric parameters: Sn–O1
2.3280(16), Sn–O2 2.3334(15),
Sn–O4 2.2096(19), N1–N2
1.263(3) Å; O1–Sn–O2 55.85
(5), O1–Sn–O4 136.88(4),
O2–Sn–O4 81.07(4), O1–Sn–O1i
86.23(8), O2–Sn–O2i 162.14(8),
C18–Sn–C18i 179.53(14)°

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Invest New Drugs (2011) 29:285–299
L1HH’ was characterized by NMR and single crystal X-ray
diffraction [30]. The reactions between Bu2SnO and each
ligand in a 1:2 ratio, with azeotropic removal of generated
water from benzene, produces the new diorganotin(IV)
complexes of formulation Bu2Sn(LH)2 (Complexes 1–3;
Scheme 1). The complexes 1–3 are colored crystalline solids
with well defined melting points, stable in air and soluble in
common organic solvents. Analytical purities of the com-
plexes were established by elemental analyses and multinu-
clear NMR (1H, 13C and 119Sn) spectroscopic data.
The IR spectra of complexes 1–3 displayed a strong
sharp band at around 1,600 cm−1 which has been assigned
to the carboxylate antisymmetric [νasym(OCO)] stretching
vibration, in accord with our earlier reports [47–50]. The
assignment of the symmetric [νsym(OCO)] stretching
vibration band could not be made owing to the complex
1
pattern of the spectra. The H and 13C NMR data of the
Fig. 2 Supramolecular aggregation leading to a one-dimensional
chain in the crystal structure of 1. Hydrogen bonding is shown as
orange (O–H…O) and blue (O–H…N) dashed lines
ligands and their complexes 1–3 are given in the “Material
and Methods” section. The 1H NMR integration values
were completely consistent with the formulation of the
products. The 13C NMR spectra of the ligands and
complexes 1–3 displayed the expected carbon signals. In
gram package to confirm that no significant divergence in the
conformations of the complexes due to crystal packing effects.
1
the H NMR spectra, the signals due to CO2H of ligands
appear in the range 12–13 ppm and were found to be absent
in the complexes 1–3, confirming the bonding of the
carboxylato group to the Sn atom. The 119Sn NMR spectra
of 1–3 in CDCl3 solution reveal a sharp signal in the range
−140 to −146 ppm, consistent with those reported for other
R2Sn(O2CR’)2 systems [28, 49–53]. The similar range of
chemical shifts imply the same coordination arrangement, i.
e. a trapezoidal bipyramid, is preserved in all the three
complexes. This means that the aqua complex Bu2Sn
(L1H)2.H2O (1) dissociates in solution to give Bu2Sn
(L1H)2 (1a), similar to other two complexes Bu2Sn(L2H)2
(2) and Bu2Sn(L3H)2 (3).
Results and discussion
Synthesis and spectroscopy
Ligands L1HH’-L3HH’ were prepared using diazo-coupling
reactions. L1HH’ was synthesized by reacting o-carboxy-
benzenediazonium chloride with 4-tert-butyl-phenol [30]
while L2HH’ and L3HH’ were prepared by reacting
p-carboxybenzenediazonium chloride with 4-tert-butyl-phenol
and with 2-methylphenol, respectively, in alkaline solution
under cold conditions. Recently, one of the ligands,
Scheme 1 Syntheses of
dibutyltin(IV) complexes
(1–3) and their structures

Invest New Drugs (2011) 29:285–299
291
The Mössbauer spectra of the dibutyltin(IV) complexes
(1–3) have been recorded in order to obtain further insight
into the structure in the solid state. Typically, one doublet
spectra with narrow average full width at half maximum of
the resonant peaks (Γav, 0.82–0.99 mm s−1), characteristic
of occurrence of unique tin coordination site, were obtained
in all the compounds. From the deconvolution of the
obtained spectra, isomer shifts (δ) of 1.54 for 1, 1.44 for 2,
and 1.41 for 3mm s−1, typical of organotin(IV) derivatives
[54] were extracted. Furthermore, these δ values are
higher than the δ of the parent dibutyltin(IV) compound
(δ=1.08 mm s−1 for Bu2SnO) [55]. This would suggest that
the tin–ligand bond character, in all the complexes, is more
covalent than in the parent dibutyltin(IV)oxide [56]. As
previous reports where [R] were found to be −1.08 [60]
and −1.09 mm s−1 [61].
Crystal structure
The molecular structure of 1 is shown in Fig. 1 and selected
geometric parameters are collected in the figure caption.
The central tin atom is chelated by two carboxylate anions,
a water molecule, and the carbon atoms derived from two
butyl groups. The molecule conforms to crystallographic
two-fold symmetry with the tin atom and water-oxygen
atom lying on the two-fold axis. The carboxylate chelates
the tin centre by forming almost equidistant Sn–O bond
distances of 2.3280(16) and 2.3334(15) Å; the equivalence
of the Sn–O bonds is reflected in the C-O bond distances
which are equal within experimental error, i.e. 1.268(3) and
1.254(3) Å. The Sn–O bond formed by the water molecule,
at 2.2096(19) Å, is slightly shorter than the Sn–Ocarboxylate
bonds. Deprotonation at the carboxylate residue is con-
firmed by the short N1–N2 bond distance of 1.263(3) Å
and the formation of an intramolecular Ohydroxyl–H…N1azo
hydrogen bond (O3–H3o…N1=1.88Å, O3…N1=2.597(3)
Å with angle subtended at H3o of 142°) along with a
weaker Ohydroxyl–H…O2carboxylate interaction (O3–H3o…
O2=2.33Å, O3…O2 = 2.974(2) Å, with angle at H3o of
134°). A consequence of these intramolecular hydrogen
bonds is the near planarity of the carboxylate ligand as
evidenced by the magnitudes of the N1/N2/C8/C9 and
C1/C2/C7/N1 torsion angles of 2.6(3) and –8.5(3)°,
respectively. The coordination geometry at the tin atom is
based on a pentagonal bipyramidal arrangement with the
pentagonal plane defined by five oxygen atoms, and with the
axial positions occupied by the n-butyl groups; the C–Sn–C
axial angle is very nearly linear at 179.53(14)°.
Despite the rather large number of organotin carboxylate
structures available in the literature [16], there is only a
limited number conforming to the general formula R2Sn
(O2CR´)2(OH2) where the carboxylate ligand does not carry
functionality capable of forming additional interactions to
tin, such as a pyridine-nitrogen atom. There are only four
R2Sn(O2CR´)2(OH2) structures and three of these, i.e. with
R=butyl [63, 64] and cyclohexyl [65], having the same
structural motif, including the crystallographically imposed
two-fold symmetry, as reported herein. The exceptional
structure is that of Me2Sn(O2CCF3)(OH2), i.e. featuring the
somewhat electron-donating tin-bound methyl groups [66].
Here, the carboxylate ligands form quite asymmetric bonds
to tin with two Sn–O bonds, i.e. 2.081(3) and 2.205(3) Å,
being significantly shorter than the two Sn…O interactions,
i.e. 3.146(4) and 2.985(3) Å; the Sn–Owater distance is
2.238(3) Å. If the weaker Sn…O interactions are ignored,
the coordination geometry is based on a trigonal bipyrami-
dal arrangement with the equatorial positions defined by the
ꢀ
ꢀ
ꢀ
ꢀ
far as the quadrupole splitting ( Δexp ) values for the
complexes are concerned, the values for 2 (3.64 mm s−1)
and 3 (3.34 mm s−1) are indicative of a skew octahedral
configuration of the tin atom with trans- alkyl groups [54].
The ratio of the quadrupole splitting to isomer shift value
ꢀ
ꢀ
ꢀ
ꢀ
(r ¼ Δexp =d) indicates coordination number greater than
four [54].
According to Sham and Brancroft [57], for Sn(IV)
complexes containing a R2Sn(IV) moiety, quadrupole
splitting (|Δ|) values depend mainly on highly covalent
Sn–C bonds, so that the contributions of the other less
covalent bonds can be safely ignored. As a consequence,
the relationship between Δ and C–Sn–C bond angles can
be expressed according to
Â
Ã
1=2
jΔj ¼ ꢀ4½Rꢁ 1 ꢀ 3sin2qcos2q
ð1Þ
where [R] is the partial quadrupole splitting value (pqs
value) of organic groups bonded to the tin atom, and θ is
the C–Sn–C angle. Depending on the coordination number,
appropriate [R] values have been successfully used in Eq. 1
to calculate C–Sn–C angles satisfactorily in penta- and
hexa-coordinated Sn(IV) compounds [58]. For Bu2Sn
(L2H)2 (2) and Bu2Sn(L3H)2 (3), C–Sn–C angles have
been calculated as 147° and 138°, respectively, (in
agreement with the value 129.3° for 3 found by quantum
mechanical method (PM6) (Table 1)), clearly indicating
distortion from an ideal trans-R2Sn octahedral structure.
ꢀ
ꢀ
ꢀ
ꢀ
Furthermore, the 4.39 mm s−1 Δexp value for the
complex Bu2Sn(L1H)2.H2O (1) was found to be higher for
a hexa-coordinated organotin(IV) derivative, but is in good
agreement with the values found for several previously
reported hepta-coordinated organotin(IV) derivatives [59–
62], owing to a larger distortion which generally produce
greater |Δ | values than hexa- and penta- coordinated ones.
Applying Eq. 1 to the experimental |Δ | value and to the
crystallographically determined C–Sn–C angle (~180°), it
has been possible to measure a partial quadrupole splitting
value [R]=−1.10 mm s−1 which is in full agreement with

292
Invest New Drugs (2011) 29:285–299
Table 1 Selected bond lengths
(Å) and angles (º)a for energy
minimized structures of the
dibutyltin(IV) complexes 1a
and 3
Bond lengths (Å) and angles (°) 1a
3
Bond lengths (Å) and angles (°) 1a
3
Sn-O1
2.18
2.69
2.18
2.69
1.32
1.25
1.48
1.41
1.39
1.40
1.39
1.41
1.42
1.44
–
2.16 O2-C1-C2
2.71 C1-C2-C3
2.16 C1-C2-C7
2.71 C3- C2-C7
1.32 C2-C7-N1
1.24 C4-C5-N1
1.48 C6-C7-N1
1.40 C6-C5-N1
1.39 C7-N1-N2
1.41 C5-N1-N2
1.41 N1-N2-C8
1.39 N2-C8-C9
1.40 N2-C8-C13
129.1 126.2
118.1 119.0
122.9 120.3
118.9 120.6
Sn-O2
Sn-O1i
Sn-O2i
O1-C1
O2-C1
C1-C2
115.6
–
–
125.3
–
124.5
–
C2-C3
114.0
–
C3-C4
119.5
–
C4-C5
117.9
C5-C6
116.5 117.8
127.2 125.0
114.3 114.5
118.5 120.4
C6-C7
C7-C2
C7-N1
C5-N1
N1-N2
N2-C8
C8-C9
–
C13-C8-C9
1.45 C8-C9-O3
126.2
114.2
122.3
119.3
–
–
–
1.26
1.42
1.42
1.42
1.38
1.42
1.39
1.42
1.53
–
1.26 C10-C9-O3
1.44 C11-C12-C14
1.41 C13-C12-C14
1.39 C9-C10-C14
1.41 C11-C10-C14
1.41 C10-C11-O3
1.39 C12-C11-O3
1.41 O1-Sn-C18
–
–
C9-C10
C10-C11
C11-C12
C12-C13
C13-C8
C12-C14
C10-C14
C9-O3
C11-O3
Sn-C18
Sn-C15
Sn-C18i
Sn-C15i
120.9
121.4
123.3
114.3
–
–
–
–
106.4
–
–
O1-Sn-C15
1.49 O2-Sn-C18
O2-Sn-C15
2.13 O1-Sn-C18i
O1-Sn-C15i
2.13 O2-Sn-C18i
O2-Sn-C15i
107.4
–
86.9
–
1.34
–
–
87.1
–
106.5
–
2.13
–
–
107.4
86.8
–
2.13
–
–
86.7
–
2.13 C18-Sn-C18i
131.9
–
C15-Sn-C15i
129.3
–
O1-Sn-O2
O1i-Sn-O2i
Sn-O1-C1
Sn-O2-C1
O1-C1-C2
51.6
51.6
51.6
51.6
O1i -Sn-C18i
O1i-Sn-C15i
O2i-Sn-C18i
O2i-Sn-C15i
106.5
–
107.5
–
107.9
85.6
107.8
84.0
87.1
–
86.8
a Refer to Figs. 3 and 4 for the
numbering schemes
116.0
117.3
methyl groups and an oxygen atom of a carboxylate
residue.
angle subtended at H4o of 147° for symmetry operation
ii: x, y, 1+z.
The major feature of the note in the crystal packing is the
formation of O–H…O hydrogen bonding interactions. As
illustrated in Fig. 2, each of the water-bound H4o atoms
forms a hydrogen bond with a translationally related
carboxylate-O1 atom leading to the formation of supramo-
lecular chains along the c-direction, see Fig. S1 (refer to the
Supplementary materials) for a packing diagram. The
parameters associated with this hydrogen bonding are
O4–H4o…O1ii=1.91Å, O4…O1ii=2.657(2) Å, with an
Quantum chemical calculations
The geometries of the dibutyltin(IV) complexes (1a and 3)
were fully optimized using the quantum mechanical method
(PM6). Harmonic frequency calculations were performed
for all the stationary points to characterize their nature and
to ensure that the optimized structures correspond to global
minima. The molecular structures of 1a and 3 were

Invest New Drugs (2011) 29:285–299
293
obtained after full geometry optimization at PM6 levels are
shown in Figs. 3 and 4, respectively, while the optimized
geometric parameters are collected in Table 1. The ex-
perimental geometrical parameters for 1 generally do not
match closely with that of calculated values for 1a, as
expected, owing to the different coordination geometry
(pentagonal bipyramid) around the tin atom, caused by
coordination of the water molecule. Compounds 1a and 3
have a six-coordinate structure (Figs. 3 and 4). The
carboxylate groups on the ligands act as bidentate chelating
agents, giving a basal plane around the tin of four
asymmetrically coordinated oxygen atoms, whereas the
butyl groups are in the axial positions, but pinned back
somewhat to produce a skew-trapezoidal bipyramidal
structure. Such a configuration is commonly encountered
with diorganotin(IV) carboxylates [16]. Diorganotin(IV)
compounds of this type exhibiting this structural motif have
Sn–O1 values ≤2.2Å and Sn–O2 values ≥2.5Å. The
corresponding bond lengths observed for 1a are 2.18 (Sn–
O1) and 2.69 (Sn–O2), and for 3 are 2.16 (Sn–O1) and 2.71
(Sn–O2) (Table 1). In addition, 1a and 3 have the two butyl
substituents disposed over the longer Sn–O vectors with C–
Sn–C angles of around 130° (1a: 131.9° (C18-Sn-C18i) and
3: 129.3° (C15-Sn-C15i)). Since the basic structures and
coordination geometry of the dibutyltin(IV) complexes (1a
and 3) are similar for the differently substituted ligands, it
can be expected that the ligand properties may have direct
influences on the stability of the corresponding dibutyltin
(IV) complexes, as well as on their cytotoxic activity (see
below).
Cytotoxicity studies
The in vitro cytotoxic properties of diorganotin(IV) com-
pounds of formulations such as (a) Bu2Sn(L4H)2 (4) [28], (b)
{[Bu2Sn(L5H)2]2O}2 (5) [29], {[Bu2Sn(L6H)2]2O}2 (6) [29]
and triorganotin(IV) compounds, e.g., (c) [Ph3SnL7H]n (7)
[26] and [Ph3SnL8H]n (8) [26] are reported across a panel of
human tumor cell lines viz., A498, EVSA-T, H226, IGROV,
M19 MEL, MCF-7 and WIDR as per NCI protocol (Table 2).
It should be mentioned herein that the ligand skeleton in all
the compounds, i.e. LH is a carboxylate residue where L4H,
5-[(E)-2-(2-methylphenyl)-1-diazenyl]-2-hydroxybenzoate;
L5H, 5-[(E)-2-phenyl-1-diazenyl]-2-hydroxybenzoate; L6H,
5-[(E)-2-(4-methylphenyl)-1-diazenyl]-2-hydroxybenzoate,
L7H, 2-[(E)-2-(3-formyl-4-hydroxyphenyl)-1-diazenyl]ben-
zoate and L8H, 2-[(E)-2-(4-hydroxy-5-methylphenyl)-1-dia-
zenyl]benzoate all contain an azo group. On the whole,
appreciable cytotoxic effects were noted for these set of
compounds. However, owing to the limited numbers of
compounds studied, it is difficult to define the overall
spectrum of the activity of the compounds or their
possible selectivity. In the solid state, compounds of
formulation (a) have a skew trapezoidal geometry while
formulation (b) possesses a centrosymmetric tetranuclear
bis(dicarboxylatotetrabutyldistannoxane) frame-work
which contain a planar Sn4O2 core in which two μ3-oxo
O-atoms connect an Sn2O2 ring to two exocyclic Sn-atoms
[29] and each tin center has either trigonal bipyramidal or
octahedral geometry, depending on the donor atoms. On
the other hand, formulation (c) is polymeric, where the Sn
atom is trigonal bipyramidal in solid state, but collapses
(as revealed from 119Sn NMR spectroscopy) in the
solution state to give a monomeric tetrahedral tin species
[26].
Among the organotin(IV) compounds of formulations
(a–c) described above, the triphenyltin compounds 7 and
8 (formulation (c)) possess highest cytotoxicity and are
tetrahedral in solution [26]. It is also reported that the four
coordinated tin species have a stronger tendency to increase
the coordination numbers by O, S or N donor groups, while
five-coordinated species do not undergo further coordina-
tion and play no long term role in vivo chemistry of
organotin(IV) ester [67–70]. Further, the higher activity of
7 and 8 is attributed to the greater partition coefficient value
for triorganotins compared to diorganotins [67, 68]. Recent
docking studies indicated that the triphenyltin(IV)-2-[(E)-2-
(3-formyl-4-hydroxyphenyl)-1-diazenyl]benzoate (7) and 2-
[(E)-2-(4-hydroxy-5-methylphenyl)-1-diazenyl]benzoate (8)
exhibit hydrogen bonding interactions through azo nitrogen
atoms, formyl, carbonyl and ester oxygen atoms of the
molecule of triphenyltin compounds with the amino acid
residue of the enzymes and thus the presence of an azo
functionality in the ligand could be a possible reason for
Fig. 3 The structure of Bu2Sn(L1H)2 (1a) obtained after full geometry
optimization

294
Invest New Drugs (2011) 29:285–299
Fig. 4 The structure of Bu2Sn
(L3H)2 (3) obtained after full
geometry optimization
enhanced activity [26]. Such observations were also
concluded for the ruthenium complexes containing azo-
ligands which have shown higher activity in comparison to
the complexes with non-azo ligands [26]. Thus, it may be
inferred that the activity may depend on the coordination
geometry in solution, nature of R groups attached to Sn
atom and the skeletal framework of coordinating benzoato
ligand.
potential and the results are summarized in Table 2 and the
screening results are compared with the results from other
organotin(IV) compounds containing arylazobenzoate
framework. Under identical conditions, compound 3 dis-
played better activity than compounds 1 and 2, across a
panel of cell lines. The test compound 3 is found to be far
superior to CCDP (cisplatin), and ETO (etoposide), and the
activity is four times superior for the A498, EVSA-T and
MCF-7 cell lines than CCDP, and four, eight and sixteen
times superior for the A498, H226 and MCF-7 cell lines,
respectively, compared to ETO. This encouraging cytotoxic
In view of these information and to rationalize the
structure-activity, the three new dibutyltin(IV) complexes
1–3 of formulation (a), were investigated for their cytotoxic
Table 2 In vitro ID50 values (ng/ml) of test compounds 1–3 along with some reported organotin(IV) azo-compounds against some standard drugs
using as cell viability test in seven human tumor cell linesa
Test compoundb
Cell lines
A498
EVSA-T
H226
IGROV
M19 MEL
MCF-7
WIDR
Bu2Sn(L1H)2 (1)
Bu2Sn(L2H)2 (2)
Bu2Sn(L3H)2 (3)
Bu2Sn(L4H)2 (4) [28]
{[Bu2Sn(L5H)2]2O}2 (5) [29]
{[Bu2Sn(L6H)2]2O}2 (6) [29]
Ph3SnL7H (7)d [26]
Ph3SnL8H (8)d [26]
429
1169
382
387
135
140
103
101
90
134
371
134
63
816
1140
497
617
1489
449
179
149
155
101
109
60
374
901
294
122
52
236
416
162
124
27
896
2125
551
359
116
115
95
c
337
c
14
205
c
14
217
54
29
49
101
104
103
16
78
41
104
92
104
11
DOX
CDDP
5-FU
MTX
ETO
8
199
10
2253
143
37
422
475
5
3269
340
169
297
7
558
442
23
699
750
18
967
225
<3.2
150
<3.2
2287
3934
1314
<3.2
317
<3.2
580
<3.2
505
<3.2
2594
<3.2
TAX
<3.2
a DOX doxorubicin, CDDP cisplatin, 5-FU 5-fluorouracil, MTX methotrexate, ETO etoposide and TAX paclitaxel
b Standard drug reference values are cited immediately after the test compounds under identical conditions. For compounds 1–3, the reference
values for CCDP were 1503, 493, 645, 229, 711, 653 and 576 for A498, EVSA-T, H226, IGROV, M19 MEL, MCF-7 and WIDR cell lines,
respectively
c Reported dibutyltin(IV) compounds (4–6) have been included for comparison; see refs. 28, 29: LH is a carboxylate residue where L4 H, 5-[(E)-2-
(2-methylphenyl)-1-diazenyl]-2-hydroxybenzoate; L5 H, 5-[(E)-2-phenyl-1-diazenyl]-2-hydroxybenzoate; L6 H, 5-[(E)-2-(4-methylphenyl)-1-
diazenyl]-2-hydroxybenzoate
d Reported triphenyltin(IV) compounds (7–8) have been included for comparison; see refs. 26: LH is a carboxylate residue where L7 H, 2-[(E)-2-
(3-formyl-4-hydroxyphenyl)-1-diazenyl]benzoate; L8 H, 2-[(E)-2-(4-hydroxy-5-methylphenyl)-1-diazenyl]benzoate

Invest New Drugs (2011) 29:285–299
295
effect may be predictive of in vivo antitumor activity. The
lower activities of 1 and 2 compared to 3 may be attributed
to the steric crowding of bulky t-Bu of the ligand skeleton
of 1 and 2 which possibly hinders the binding ability.
Further, the cytotoxicity data of 3 is comparable to that
reported for compound 4 against A498 cell line. However,
much better activities were noted for the compounds 5 and
6 with formulation (b) and could be realized due to the
presence of two tin centres in the molecule. Notably, the
triphenyltin(IV) compounds 7 and 8 with formulation (c)
were found to be better performers than compounds of
other formulations ((a) and (b)), showing relevant cytotoxic
properties against all the cell lines used. However, the
variations in in vitro cytotoxicity among various formula-
tions (a–c) across tumor cell lines may also be due to
different kinetic and mechanistic behavior. Overall, the
results reported strongly support the view that the organotin
(IV) compounds are potent cytotoxins and deserve greater
attention as potential anticancer agents.
Fig. 5 Bu2Sn(L1H)2 (1a) docked into the binding site of the enzyme
ribonucleotide reductase (4R1R). Hydrogen bonding interactions
between the various groups of 1a and amino acid residues are shown.
Hydrogen atoms are omitted for clarity
Docking study
The encouraging cytotoxic activity for the test compounds
1–3 across a panel of cell lines viz., A498, EVSA-T, H226
IGROV, M19 MEL, MCF-7 and WIDR and current
information acquired from the docking studies of com-
pounds 7 and 8 performed with some enzymes [26],
prompted us to carry out molecular docking studies of 1a
and 3 (formulation (a)) to understand the complex-protein
interactions. The docking studies were performed with
some enzymes such as ribonucleotide reductase, thymidy-
late synthase, thymidylate phosphorylase and topoisomer-
ase II and their interactions with 1a and 3 are shown in
Figs. 5, 6, 7, 8, 9 and 10. The docking programme is
validated by docking ADP in the active site of enzyme
ribonucleotide reductase and a close overlapping between the
docked ligand and the native ligand was observed [26]. The
docking studies reveal that dibutyltin(IV) compounds 1a and
3 interact with enzymes at various sites and their hydrogen
bonding interactions are given in Table 3. They show
appreciable interactions with ribonucleotide reductase, thy-
midylate synthase and thymidylate phosphorylase, however,
they do not enter the active site of topoisomerase II.
Docking study shows that the compounds 1a and 3 are
masked inside the active site of ribonucleotide reductase
(Figs. 5 and 6) and both exhibit hydrogen bonding
interactions through azo nitrogen atoms, hydroxyl and
carbonyl groups with amino acid residues of the enzyme.
The interactions of 1a with ribonucleotide reductase are
better than those exhibited by 3.
Similarly, compound 1a when docked in the active site
of thymidylate synthase also shows hydrogen bonding
interactions with R23, L195 and S219 through carbonyl
oxygen atoms while azo nitrogen and hydroxyl oxygen
atoms interact with R218 amino acid residues of the
enzyme (Fig. 7). On the other hand, compound 3 shows
more hydrogen bonding interactions compared to 1a with
active site of thymidylate synthase. Compound 3 exhibits
hydrogen bonding interactions through azo nitrogen atoms,
OH and carbonyl groups with S219, C198, S219, R23 and
W85 amino acid residues of the enzyme (Fig. 8).
Likewise, the dockings of compounds 1a and 3 in the
active sites of thymidylate phosphorylase show the H-
bonding interactions between the various groups of this
compound and amino acid residues as shown in Figs. 9 and
10. Compound 3 interacts through the nitrogen atoms of the
azo groups with the amino acids G1113, R1168 and I1180,
and using carbonyl oxygen atoms, it forms hydrogen bonds
with the amino acids Y1165 and T1117. The hydroxyl
groups form hydrogen bonds with the amino acids G1119
and R1168 and besides this, it also forms hydrogen bonds
with D1122 and R1168 which are not visible in the present
view of Fig. 10. On the other hand, both the azo group
nitrogen atoms as well as two carbonyl oxygen atoms of
compound 1a show hydrogen bond interactions with the
amino acid Y1165. Nitrogen atoms of an azo group form
hydrogen bonds with L1114 and G1113 while a carbonyl
oxygen atom interacts with T1117. Unlike compound 3,
only one hydroxyl oxygen atom is involved in the hydrogen
bond formation with the amino acids G1113 and R1112.
Therefore, on the basis of docking studies, it is inferred
that the anticancer activities of compounds 1a and 3 might

296
Invest New Drugs (2011) 29:285–299
Fig. 6 Bu2Sn(L3H)2 (3) docked into the binding site of the enzyme
ribonucleotide reductase (4R1R). A number of hydrogen bonding
interactions between compound 3 and amino acid residues are shown.
Hydrogen atoms are omitted for clarity
Fig. 8 Bu2Sn(L3H)2 (3) docked into the binding site of the enzyme
thymidylate synthase (2G8D). Hydrogen bonding interactions be-
tween the ligand part of the molecule of 3 and amino acid residues are
shown. Hydrogen atoms are omitted for clarity
be emanating from their interactions with enzymes ribonu-
cleotide reductase, thymidylate synthase and thymidylate
phosphorylase. The docking studies also indicate that the
azo group nitrogen atoms and hydroxyl and carbonyl
oxygen atoms in the ligand moiety play an important role
during the dockings of the dibutyltin(IV) compounds into
the active sites of various enzymes as discussed above.
Nevertheless, the possibility of coordination through tin
beyond the active site of the enzymes cannot be ruled out
completely. It is very difficult to envisage the role of such
atoms in binding proteins in relation to improved cytotoxic
activity. However, the exact reasons for such activity cannot
be deduced at this stage from the available data.
Fig. 9 Bu2Sn(L1H)2 (1a) docked into the binding site of the enzyme
thymidylate phosphorylase (1BRW) showing hydrogen bonding
interactions through azo nitrogen, hydroxyl and carbonyl oxygen
atoms with various amino acid residues. Hydrogen atoms are omitted
for clarity
Fig. 7 Bu2Sn(L1H)2 (1a) docked into the binding site of the enzyme
thymidylate synthase (2G8D). Hydrogen bonding interactions be-
tween various groups of 1a and amino acid residues are shown.
Hydrogen atoms are omitted for clarity

Invest New Drugs (2011) 29:285–299
297
investigate the role of ligands on the cytotoxic properties of
some human tumor cell lines, viz., A498, EVSA-T, H226,
IGROV, M19 MEL, MCF-7 and WIDR and to realize the
binding capabilities toward some enzymes such as ribonu-
cleotide reductase, thymidylate synthase, thymidylate phos-
phorylase and topoisomerase II. The molecular structures
and geometries of compounds 1a and 3 were fully
optimized using the quantum mechanical method (PM6).
Both dibutyltin(IV) compounds 1 and 3 show very high
cytotoxicity when tested in vitro across seven human tumor
cell lines indicating their high potential as an anti-cancer
drug. Under identical conditions, compound 3 displayed
better activity than compounds 1 and 2 across a panel of
cell lines. The lower activities of 1 and 2 compared to 3
may be attributed to the steric crowding of bulky t-Bu of
the ligand skeleton of 1 and 2 which possibly hinders the
binding ability. Thus, the cytotoxicity of the compounds
depends on the nature of ligand constitution and shows
different activity. Nevertheless, the presence of the azo
group is required for anticancer activity. The docking
studies indicated that the azo group nitrogen atoms and
hydroxyl and carbonyl oxygen atoms of the dibutyltin(IV)
compounds are responsible for interacting with various key
enzymes that take part in the synthesis of raw materials for
DNA and its replication. The geometrical feature and size
of the molecule of organotin(IV) complexes seem to play
Fig. 10 Bu2Sn(L3H)2 (3) docked into the binding site of the enzyme
thymidylate phosphorylase (1BRW) showing hydrogen bonding
interactions through azo nitrogen, hydroxyl and carbonyl oxygen
atoms with various amino acid residues. Hydrogen atoms are omitted
for clarity
Conclusions and outlook
Three dibutyltin(IV) complexes of formulation Bu2Sn(LH)2
where LH is arylazobenzoate ligand were synthesized to
Table 3 Hydrogen bonding interactions of dibutyltin(IV) compounds 1a and 3 with ribonucleotide reductase (4R1R), thymidylate synthase
(2G8D) and thymidylate phosphorylase (1BRW)
Compound
Amino acids involved in hydrogen bonding
Compd.
No.
Groups
4R1R
2G8D
1BRW
Amino acid
residue
Bond
distances (Å)
Amino acid
residue
Bond
distances (Å)
Amino acid
residue
Bond
distances (Å)
1a
Azo group nitrogen
atom(s)
G253
2.74, 2.87
R218
2.86
Y1165
L1114
G1113
2.46, 2.75
1.14, 2.17
2.59, 2.99
2.95, 2.19
2.74, 1.14
2.16
Carbonyl oxygen atom(s) C439
Hydroxyl group oxygen R251
2.44
S219
R23
2.78
Y1165
T1117
G1113
R1112
G1113
R1168
I1180
2.71, 2.84
2.15
2.60
2.74
2.98
R218
2.69
atom(s)
C225
2.81
3
Azo group nitrogen
atom(s)
S224
W85
2.14, 2.51
2.32, 2.87
2.78
2.22, 1.74
2.90
Carbonyl oxygen atom(s) C439
2.19
2.99
C198
S219
R23
2.94
Y1165
T1117
2.89
2.53
2.97, 2.86
2.14, 2.51
Hydroxyl group oxygen N696
atom(s)
R218
G1119
R1168
D1122
2.97
2.54, 1.57
0.96, 2.76

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Invest New Drugs (2011) 29:285–299
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an important role in binding with enzymes. The cytotoxic
properties of 1 and 3 were compared with triphenyltin(IV)
compounds (7 and 8) of cognate systems. The cytotoxicity
results demonstrated that the structural modification in
coordination geometry and Sn-R groups in 1–3 did not
improve the cytotoxicity toward cell lines compared to 7
and 8. Data from the present study suggests complex 3
merits further investigation as a new drug and may be
suitable candidate for modification in order to improve
cytotoxic and dissolution properties. Further work in this
area is underway.
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of crystal packing on molecular geometry: a crystallographic and
theoretical investigation of selected diorganotin systems. J Phy
Chem A 102:2472–2482
20. Willem R, Verbruggen I, Gielen M, Biesemans M, Mahieu B,
Basu Baul TS, Tiekink ERT (1998) Correlating Mossbauer and
solution- and solid-state 117Sn NMR data with X-ray diffraction
structural data of triorganotin 2-[(E)-2-(2-Hydroxy-5-methyl-
phenyl)-1-diazenyl]benzoates. Organometallics 17:5758–5766
21. Dakternieks D, Duthie A, Smyth DR, Stapleton CPD, Tiekink
ERT (2003) Steric control over molecular structure and supramo-
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Acknowledgements The financial support of the Department of
Science & Technology, New Delhi, India (Grant No.SR/S1/IC-03/
2005,TSBB and SR/S1/OC-11A/2006, PS), of the Università degli
Studi di Palermo, Italy (Grants ORPA079E5M and ORPA0737W2)
and the University Grants Commission, New Delhi, India through
SAP-DSA, Phase-III, are gratefully acknowledged. The in vitro
cytotoxicity experiments were carried out by Ms. P. F. van Cuijk in
the Laboratory of Translational Pharmacology, Department of Medical
Oncology, Erasmus Medical Center, Rotterdam, The Netherlands,
under the supervision of Dr. E. A. C. Wiemer and Prof. Dr. G. Stoter.
23. Gielen M, Tiekink ERT (eds) (2005) Metallotherapeutic drug and
metal-based diagnostic agents: 50Sn Tin compounds and their
therapeutic potential. Wiley, Chichester, pp 421–439 (and refer-
ences therein)
24. Basu Baul TS, Basu S, de Vos D, Linden A (2009) Amino acetate
functionalized Schiff base organotin(IV) complexes as anticancer
drugs: synthesis, structural characterization, and in vitro cytotox-
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25. Basu Baul TS, Masharing C, Ruisi G, Jirásko R, Holčapek M,
de Vos D, Wolstenholme D, Linden A (2007) Self-assembly of
extended Schiff base amino acetate skeletons, 2-{[(2Z)-(3-
hydroxy-1-methyl-2-butenylidene)]amino}phenylpropionate and
2-{[(E)-1-(2-hydroxyaryl)alkylidene]amino}phenylpropionate
skeletons incorporating organotin(IV) moieties: synthesis, spec-
troscopic characterization, crystal structures, and in vitro cyto-
toxic activity. J Organomet Chem 692:4849–4862
26. Basu Baul TS, Paul A, Pellerito L, Scopelliti M, Singh P, Verma P,
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