Design and synthesis of C-8 linked pyrrolobenzodiazepine-naphthalimide hybrids as anti-tumour agents

Article abstract of DOI:10.1016/S0960-894X(02)00326-8

The facile synthesis of C-8 linked pyrrolobenzodiazepine-naphthalimide hybrid analogues is described. The compounds are prepared with varying degrees of linker length in order to probe the structural requirements for optimal in vitro anti-tumour activity. Some of these new hybrid compounds showed higher cytotoxic activity than the existing natural and synthetic pyrrolo[2,1-c][1,4]benzodiazepines.

Full text of DOI:10.1016/S0960-894X(02)00326-8

Bioorganic & Medicinal Chemistry Letters 12 (2002) 1933–1935
Design and Synthesis of C-8 Linked Pyrrolobenzodiazepine–
Naphthalimide Hybrids as Anti-Tumour Agents
Ahmed Kamal,* B. S. Narayan Reddy, G. Suresh Kumar Reddy and G. Ramesh
Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 15 March 2002; accepted 6 May 2002
Abstract—The facile synthesis of C-8 linked pyrrolobenzodiazepine–naphthalimide hybrid analogues is described. The compounds
are prepared with varying degrees of linker length in order to probe the structural requirements for optimal in vitro anti-tumour
activity. Some of these new hybrid compounds showed higher cytotoxic activity than the existing natural and synthetic pyrrolo[2,1-c]-
[1,4]benzodiazepines. # 2002 Elsevier Science Ltd. All rights reserved.
2
In the last few years, a growing interest has been shown
in the development of DNA-minor groove binders act-
ing as vehicles for the delivery of alkylating agents. It
has been observed that small molecules may recognize
an increasing range of DNA sequences,1 which have
DNA binding properties, including pyrrolo[2,1-c]-
[1,4]benzodiazepines (PBDs) and lexitropsins. PBDs are
a group of potent, naturally occurring anti-tumour
antibiotics produced by various Streptomyces species
and examples of which include anthramycin, tomay-
mycin, sibiromycin and the neothramycins A and B. A
key feature of these molecules with respect to their
mechanism of action is the N-10–C-11 carbinolamine (or
imine equalent). Nucleophilic attack by 2-NH2 of a gua-
nine base at the C-11 position of PBD forms a covalent
adductin hte minor groove of DNA.
Moreover, the
PBDs bind to DNA sequence selectively and have
potential not only as anti-tumour agents but also as
gene regulators and probes of DNA structure. Further,
a large number of methodologies have been developed
for the synthesis of PBD ring system.3
Confalone and coworkers4 have synthesized PBD ana-
logues with an epoxide group attached at C-11a position
with an objective of producing PBD system with DNA
cross-linking activity. Recently, Thurston and co-
workers, in order to increase cytotoxicity through the
production of DNA cross-links, have designed and
synthesized PBD dimers5 comprising of two PBD units
joined through their A-rings. Similarly, Baraldi and
coworkers6 as well as Lown and coworkers7 have pre-
pared new distamycin–PBD and lexitropin–PBD
hybrids to explore their biological properties. We have
also recently designed and synthesized non-cross-linking
mixed imine–amide PBD dimers that have significant
DNA-binding affinity and potent anti-tumour activity.8
Naphthalimides are DNA intercalating agents with high
anti-tumour activity9 and two members of this class
amonafide and mitonafide are in clinical trials. There-
fore, it has been considered of interest to design and
synthesize C-8 linked naphthalimide–PBD hybrids that
could be attractive targets with a view to have a combi-
nation of both the DNA-binding and intercalating
properties in the same molecule and may exhibit pro-
found anti-tumour activity. We have been interested in
the structural modifications of the PBD ring system and
the development of new synthetic strategies.10 In con-
tinuation of these efforts, we herein report the synthesis
*Corresponding author. Tel.: +91-40-719-3157; fax: +91-40-716-
0512; e-mail: ahmedkamal@iict.ap.nic.in
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00326-8

1934
A. Kamal et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1933–1935
and in vitro anti-tumour activity of novel C-8 linked
naphthalimide–PBD hybrids with different linker chain
lengths.
Further, compound 6 has been debenzylated with
.
BF3 OEt3/EtSH, and the resulting hydroxy compound
is coupled with corresponding naphthalimide (3) to give
the C-8 linked 1,8-naphthalimide nitroester (12). The
Synthesis of the naphthalimide–PBD hybrids11 has been
carried outemploying the commercially available vanil-
lin. Oxidation of vanillin followed by benzylation and
nitration by literature methods provides the starting
material (5). l-Proline methylester has been coupled to
5 to give the nitroester 6. This nitroester 6 on treatment
with DIBAL-H followed by protection of aldehyde with
ethanethiol gives diethyl thioacetal (7). This upon
nitroester (12) is reduced with SnCl2 2H2O to afford the
desired naphthalimide linked PBD dilactam (13) as
depicted in Scheme 3.
.
Interestingly, the data presented in Table 1 show that
the size of the linker spacer plays an important role in
this series of molecules for the anti-cancer activity.
.
debenzylation with BF3 OEt3/EtSH provides the com-
pound 8. This nitro-diethyl thioacetal 8 has been
coupled with appropriate bromo-N-alkyl-1,8-naphthal-
imide, which was prepared from commercially available
naphthalic anhydride (Scheme 1) to give C-8 linked 1,8-
naphthalimide nitro-diethyl thioacetal (9). The com-
.
pound 9 is efficiently reduced employing SnCl2 2H2O t o
afford the corresponding amino thioacetal (10). Depro-
tection of amino diethyl thioacetal (10) with HgCl2 and
HgO affords the desired C-8 linked 1,8-naphthalimide-
PBD imine12 (1) as shown in Scheme 2.
.
Scheme 3. (i) BF3 OEt2, EtSH, CH2Cl2; (ii) K2CO3, acetone, reflux;
.
(iii) SnCl2 2H2O, MeOH.
Scheme 1. (i) Liquid ammonia, reflux; (ii) dibromoalkane, K2CO3,
acetone, reflux.
Scheme 2. (i) Sulphamic acid, NaClO2; (ii) BnCl, NaOH; (iii) HNO3/H2SO4, CH2Cl2, À25 ꢀC; (iv) SOCl2, C6H6, l-proline, TFA, THF; (v) H2SO4,
ꢀ
.
.
MeOH, reflux; (vi) DIBAL-H, CH2Cl2, À78 C; (vii) EtSH, TMSCl; (viii) BF3 OEt3, EtSH, CH2Cl2; (ix) K2CO3, acetone, reflux; (x) SnCl2 2H2O,
MeOH; (xi) HgCl2, HgO, CH3CN/H2O.

A. Kamal et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1933–1935
1935
Table 1. Log LC50 (concentration in mol/L causing 50% lethality)
values for compounds 1a–ea
References and Notes
1. (a) Dervan, P. B. Science 1986, 232, 464. (b) Thurston,
D. E.; Thompson, A. S. Chem. Br. 1990, 26, 767.
2. (a) Thurston, D. E. In Molecular Aspects of Anticancer
Drug DNA Interactions; Neidle, D., Waring, M. J., Eds.;
Macmillan: London, 1993; p 54. (b) Tender, M. D.; Korman,
S. Nature 1963, 199, 501. (c) Hurley, L. H.; Petrusek, R. L.
Nature 1979, 282, 529.
3. (a) Thurston, D. E.; Bose, D. S. Chem. Rev. 1994, 94, 433.
(b) Kamal, A.; Rao, M. V.; Laxman, N.; Ramesh, G.; Reddy,
G. S. K. Curr. Med. Chem. Anti-cancer Agents 2002, 2, 215. (c)
Hu, W.-P.; Wang, J.-J.; Lin, F.-L.; Lin, Y.-C.; Lin, S.-R.; Hsu,
M.-H. J. Org. Chem. 2001, 66, 2881 and references cited
therein.
Cancer
1a
1b
1c
1d
1e
Leukaemia
Non-small-cell lung
Colon
À4.11
À4.05
À4.01
À4.00
À4.01
À4.01
À4.04
À4.00
À4.00
À4.14
À4.16
À4.34
À4.14
À4.23
À4.09
À4.57
À4.21
À4.22
À4.00
À4.34
À4.41
À4.00
À4.43
À4.06
À4.03
À4.00
À4.36
À4.11
À4.00
À4.08
À4.11
À4.09
À4.07
À4.02
À4.03
À4.02
À4.21
À4.08
À4.12
À4.29
À4.19
À4.11
À4.09
À4.18
À4.07
CNS
Melanoma
Ovarian
Renal
Prostate
Breast
aEach cancer type represents the average of six to eight different cancer
cell lines. For each histolasic cancer type, the average Àlog LC50
value was determined from an NCI panel consisting of six to eight
human cancer cell lines. The lower log LC50 values show the increase
of cytotoxicity.
4. Confalone, P. N.; Huie, E. M.; Ko, S. S.; Cole, G. M. J.
Org. Chem. 1988, 53, 482.
5. (a) Bose, D. S.; Thompson, A. S.; Ching, J. A.; Berardini,
M. D.; Jenkins, T. C.; Neidle, S.; Hurley, L. H.; Thurston,
D. E. J. Am. Chem. Soc. 1992, 114, 4939. (b) Thurston, D. E.;
Bose, D. S.; Thompson, A. S.; Howard, P. W.; Leoni, A.;
Croker, S. J.; Jenkins, T. C.; Neidle, S.; Hartley, J. A.; Hurley,
L. H. J. Org. Chem. 1996, 61, 8141. (c) Gregson, S. J.;
Howard, P. W.; Hartley, J. A.; Brooks, A. A.; Adams, L. J.;
Jenkins, T. C.; Kelland, L. R.; Thurston, D. E. J. Med. Chem.
2001, 44, 737.
6. Baraldi, P. G.; Balboni, G.; Cacciari, B.; Guiotto, A.;
Manfridini, S.; Romagnoli, R.; Spalluto, G.; Thurston, D. E.;
Howard, P. W.; Bianchi, N.; Rutigliano, C.; Mischiati, C.;
Gambari, R. J. Med. Chem. 1999, 42, 5131.
Compounds 1b and 1c have significant cytotoxic activity
in the various types of cancer cell lines. It appears that
cytotoxic activity is related to the length of the alkane
chain spacer, thus allowing 1b and 1c with four- and
five-carbon chain length for the proper snug fit in the
minor groove of double helix DNA. Further, compound
1b is more potent for colon and renal cancers, with
compound 1c likewise for colon and melanoma cancers.
However, compounds 13a–c did notexhibitany sig-
nificant anti-cancer activity.
7. Damayanthi, Y.; Reddy, B. S. P.; Lown, J. W. J. Org.
Chem. 1999, 64, 290.
In summary, the new hybrid compounds synthesized by
the combination of DNA-binding pyrrolobenzodiaze-
pines and DNA-intercalating naphthalimides have
exhibited promising in vitro anti-tumour activity and
have the potential to be developed as novel anti-cancer
agents. The detailed anti-cancer activity, molecular
modelling studies and DNA binding affinity will be
published in due course.
8. Kamal, A.; Laxman, N.; Ramesh, G.; Neelima, K.; Kon-
dapi, K. A. Chem. Commun. 2001, 437.
9. Brana, M. F.; Ramos, A. Curr. Med. Chem. Anti-cancer
Agents 2001, 1, 237.
10. (a) Kamal, A.; Rao, M. V.; Reddy, B. S. N. Khim. Geter-
osilil. Seodin. Chem. (Chem. Heterocycl. Compd. Engl.
Transl.) 1998, 12, 1588. (b) Kamal, A.; Reddy, G. S. K.;
Reddy, K. L. Tetrahedron Lett. 2001, 42, 6969. (c) Kamal, A.;
Reddy, G. S. K.; Raghavan, S. Bioorg. Med. Chem. Lett. 2001,
13, 387.
11. Kamal, A.; Reddy, B. S. N. Indian Patent Appl. No. 209/
DEL/2000.
Acknowledgements
12. Selected spectral data for 1c: 1H NMR (200 MHz, CDCl3)
d 1.2–2.4 (m, 10H), 3.5–3.8 (m, 4H), 3.9 (s, 3H), 4.0–4.3 (m,
3H), 6.8 (s, 1H), 7.5 (s, 1H), 7.65 (d, 1H, J=4.6 Hz), 7.7 (t,
2H, J=8.5 Hz), 8.2 (d, 2H, J=8.2 Hz), 8.6 (d, 2H, J=8 Hz).
FABMS: m/z=512 M+1.
We thank the National Cancer Institute, Maryland for
the anti-cancer assay in human cell lines. We are also
grateful to CSIR, New Delhi for the award of Senior
Research Fellowships to G.S.K.R. and G.R.