Establishing the Parabolic Relationship between Reactivity and Activity for Derivatives and Analogues of the Duocarmycin and CC-1065 Alkylation Subunits

Article abstract of DOI:10.1021/ja038162t

The preparation of a novel series of N-aryl CBI derivatives is detailed in which an aryl para substituent could be used to predictably modulate the reactivity of the resulting CC-1065/duocarmycin alkylation subunit analogue (ρ = 0.17). The derivatives were found to be exceptionally stable and to exhibit a well-defined relationship between reactivity and cytotoxic potency. When combined with the results of an extensive series of N-acyl CBI analogues and derivatives assembled over the past 15 years, the studies define a fundamental parabolic relationship between reactivity and cytotoxic potency. Copyright

Full text of DOI:10.1021/ja038162t

Published on Web 12/13/2003
Establishing the Parabolic Relationship between Reactivity and Activity for
Derivatives and Analogues of the Duocarmycin and CC-1065 Alkylation
Subunits
Jay P. Parrish, Terry V. Hughes, Inkyu Hwang, and Dale L. Boger*
Department of Chemistry and the Skaggs Institute for Chemical Biology, The Scripps Research Institute,
10550 North Torrey Pines Road, La Jolla, California 92037
Received August 27, 2003; E-mail: boger@scripps.edu
CC-1065 (1),1 the duocarmycins (2-3),2,3 and yatakemycin (4)4
are the parent members of a class of potent antitumor antibiotics
that derive their properties through a sequence-selective DNA
alkylation (Figure 1).5-7 An extensive series of studies have
characterized their structural features responsible for the DNA
alkylation reaction and have established fundamental relationships
between structure and activity.5-10 Among the most important of
these relationships is a direct correlation between chemical stability
and biological potency (cytotoxic activity).5,11 To date, this relation-
ship covered a wide range of modified alkylation subunits with
properties extending over a 106-fold range in both reactivities and
activities.5,12 Restricted to derivatives that possess sufficient reactiv-
ity to alkylate DNA, this relationship has been interpreted to reflect
the ability of the chemically more stable derivatives to more
effectively reach their biological target (DNA).5,11,13
Herein, we report an effective preparation and the examination
of a novel series of N2-aryl derivatives of 1,2,9,9a-tetrahydrocy-
clopropa[c]benz[e]indol-4-one (CBI)14,15 in which the electronic
properties of the aryl p-substituent could be systematically varied
Figure 1.
to predictably alter the reactivity. As detailed below, the derivatives
proved to be remarkably stable relative to the typical N-acyl
Scheme 1
derivatives, and they were found to exhibit a well-defined correla-
tion between reactivity and biological potency. When combined
with the results of preceding studies, this series served to complete
a well-defined parabolic relationship between reactivity and activity.
The N-aryl derivatives were prepared using either a Buchwald-
Hartwig16 Pd(0)-catalyzed or a Barton17 Cu(II)-catalyzed N-arylation
of either enantiomer of CBI (5) (Scheme 1), enlisting precursor
aryl chlorides or triarylbismuthines, respectively. The former pro-
vided effective couplings with electron-deficient aryl chlorides,
affording 12-15 (0.05 equiv Pd2(dba)3, 0.1 equiv (Cy)2P-
(DMAbp),18 1.6 equiv Cs2CO3, THF, reflux, 3-6 h, 53-96%), but
failed to provide products with electron-rich aryl halides under the
conditions examined. In these instances, the Cu(II)-catalyzed
coupling of 5 with the triarylbismuthines19 (1.1 equiv Cu(OAc)2,
1.0 equiv Et3N, CH2Cl2, 25 °C, 24-36 h, 39-64%) provided 6-11
in good conversions.
The derivatives 6-15 proved to be remarkably stable, displaying
readily measurable solvolysis reactivity only at pH 2. This reactivity
followed a well-defined correlation with σp (F ) 0.17) in which
electron-withdrawing substituents enhance and electron-donating
substituents decrease the solvolysis rate (Figure 2). Thus, the
solvolysis stability correlates with the expected extent of cross-
conjugated vinylogous amide stabilization of the cyclohexadienone
structure. In turn, this extent of the vinylogous amide conjugation
can be observed with the diagnostic N2-C3a bond length trends
found in the X-rays of 7 (1.380(3) Å, R ) OMe), 9 (1.395(4) Å,
R ) H), and 13 (1.399(6) Å, R ) CN).20
The biological properties (cytotoxic activity, L1210) of 6-14
also exhibited a well-defined trend correlating with their reactivity
(Table S1 and Figure S1). Thus, electron-withdrawing substituents
smoothly increase, whereas electron-donating substituents decrease,
the cytotoxic activity of the derivatives. Notably, 14, 13, and 12
bearing electron-withdrawing substituents displayed cytotoxic
potency (IC50 ) 40, 140, and 330 nM, respectively) comparable to
that of simple N-acyl-CBI derivatives (e.g., N-Boc-CBI, IC50 ) 80
nM), that of the naturally occurring alkylation subunits (e.g., N-Boc-
MeCPI, IC50 ) 300 nM), and analogous to that commonly
associated with efficacious antitumor drugs (e.g., mitomycin, IC50
9
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J. AM. CHEM. SOC. 2004, 126, 80-81
10.1021/ja038162t CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
the adherence to the parabolic relationship is remarkable, consistent
with a correlation of fundamental importance.
Acknowledgment. We gratefully acknowledge the support of
NIH (CA41986) and the Skaggs Institute for Chemical Biology.
We thank Dr. R. Chadha for the X-ray structures.
Supporting Information Available: Full experimental details for
the preparation of 6-15; solvolysis experimental details; tables of first-
order rate constants for 6-15, N-Boc-CBI analogues, and N-aryl-CBI
data; plot of log k vs log IC50 for 6-14; and structure key (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
References
(1) Chidester, C. G.; Krueger, W. C.; Mizsak, S. A.; Duchamp, D. J.; Martin,
D. G. J. Am. Chem. Soc. 1981, 103, 7629.
(2) Takahashi, I.; Takahashi, K.; Ichimura, M.; Morimoto, M.; Asano, K.;
Kawamoto, I.; Tomita, F.; Nakano, H. J. Antibiot. 1988, 41, 1915.
(3) (a) Ichimura, M.; Ogawa, T.; Takahashi, K.; Kobayashi, E.; Kawamoto,
I.; Yasuzawa, T.; Takahashi, I.; Nakano, H. J. Antibiot. 1990, 43, 1037.
(b) Yasuzawa, T.; Muroi, K.; Ichimura, M.; Takahashi, I.; Ogawa, T.;
Takahashi, K.; Sano, H.; Saitoh, Y. Chem. Pharm. Bull. 1995, 43, 378.
(4) Igarashi, Y.; Futamata, K.; Fujita, T.; Sekine, A.; Senda, H.; Naoki, H.;
Furumai, T. J. Antibiot. 2003, 56, 107.
Figure 2. Log k/kH (solvolysis, pH 2) vs σp.
(5) (a) Boger, D. L.; Johnson, D. S. Angew. Chem., Int. Ed. Engl. 1996, 35,
1438. For earlier reviews, see: (b) Boger, D. L. Acc. Chem. Res. 1995,
28, 20. (c) Boger, D. L.; Johnson, D. S. Proc. Natl. Acad. Sci., U.S.A.
1995, 92, 3642. (d) Boger, D. L. Chemtracts: Org. Chem. 1991, 4, 329.
(6) Warpehoski, M. A.; Hurley, L. H. Chem. Res. Toxicol. 1988, 1, 315.
(7) Parrish, J. P.; Kastrinsky, D. B.; Wolkenberg, S. E.; Tagarashi, Y.; Boger,
D. L. J. Am. Chem. Soc. 2003, 125, 10971.
(8) Boger, D. L.; Garbaccio, R. M. Bioorg. Med. Chem. 1997, 5, 263.
(9) Boger, D. L.; Garbaccio, R. M. Acc. Chem. Res. 1999, 32, 1043.
(10) Wolkenberg, S. W.; Boger, D. L. Chem. ReV. 2002, 102, 2477.
(11) (a) Boger, D. L.; Ishizaki, T. Tetrahedron Lett. 1990, 31, 793. (b) Boger,
D. L.; Yun, W. J. Am. Chem. Soc. 1994, 116, 5523. (c) Boger, D. L.;
Yun, W. J. Am. Chem. Soc. 1994, 116, 7996.
(12) Exceptions have been noted and typically constitute unreactive derivatives
(too stable) that fail to alkylate DNA (not active). These now may be fit
onto the parabolic relationships described herein. See: (a) Boger, D. L.;
Santilla´n, A., Jr.; Searcey, M.; Jin, Q. J. Am. Chem. Soc. 1998, 120, 11554.
(b) Castedo, L.; Delamano, J.; Enjo, J.; Fernandez, J.; Gravalos, D. G.;
Leis, R.; Lopez, C.; Marcos, C. F.; Rios, A.; Tojo, G. J. Am. Chem. Soc.
2001, 123, 5102 and ref 22.
(13) Boger, D. L.; Munk, S. A.; Ishizaki, T. J. Am. Chem. Soc. 1991, 113,
2779.
(14) Review: Boger, D. L.; Boyce, C. W.; Garbaccio, R. M.; Goldberg, J.
Chem. ReV. 1997, 97, 787.
Figure 3. Relationship between reactivity (solvolysis k, pH 3) and cytotoxic
potency (L1210).
(15) (a) Boger, D. L.; Ishizaki, T.; Kitos, P. A.; Suntornwat, O. J. Org. Chem.
1990, 55, 5823. (b) Parrish, J. P.; Kastrinsky, D. B.; Stauffer, F.; Hedrick,
M. P.; Hwang, I.; Boger, D. L. Bioorg. Med. Chem. 2003, 11, 3815 and
refs cited therein. (c) Drost, K. J.; Cava, M. P. J. Org. Chem. 1991, 56,
2240. (d) Aristoff, P. A.; Johnson, P. D. J. Org. Chem. 1992, 57, 6234.
(16) (a) Louie, J.; Hartwig, J. F. Tetrahedron Lett. 1995, 36, 3609. (b) Guram,
A. S.; Rennels, R. A.; Buchwald, S. L. Angew. Chem., Int. Ed. 1995, 34,
1348.
) 90 nM). Moreover, when this correlation between reactivity
(-log k, pH 3) and cytotoxic activity (-log IC50, L1210) is plotted
along with prior data for the more reactive N-acyl-CBI derivatives
and its analogues, it established a well-defined parabolic relationship
between reactivity and biological potency (Figure 3). This plot
incorporates not only the N-aryl derivatives disclosed herein, but
all N-Boc derivatives of the alkylation subunits that we have
examined to date,21 including a class of unusually stable C3 halogen
CBI derivatives 16-1921,22 and a series of simple N-acyl CBI
derivatives 20-23.21,11 The parabolic relationship establishes that
the compounds should possess sufficient stability to reach their
biological target (DNA), yet maintain sufficient reactivity to alkylate
DNA upon reaching the biological target and, importantly, defines
this optimal balance of stability and reactivity.23
The variations that appear in this correlation may be attributed
not only to the inherent error in the cytotoxic assay especially with
data that has been collected intermittently over a 15-year period
but also to structural differences in the derivatives that impact
features beyond reactivity (e.g., cell penetration and distribution,
DNA binding affinity). Even without accounting for such variables,
(17) (a) Barton, D. H. R.; Finet, J.-P.; Khamsi, J. Tetrahedron Lett. 1987, 28,
887. (b) Review: Elliott, G. I.; Konopelski, J. P. Tetrahedron 2001, 57,
5683.
(18) (Cy)2P(DMAbp) ) 2-dicyclohexylphosphino-2′-(N, N-dimethylamino)-
biphenyl. Tomori, H.; Fox, J. M.; Buchwald, S. L. J. Org. Chem. 2000,
65, 5334.
(19) Prepared by treatment of corresponding aryl bromide (1.0 equiv) with
n-BuLi (1.1 equiv) in THF at -78 °C for 1 h, followed by addition of a
THF solution of BiCl3 (0.33 equiv). Solvent removal and recrystallization
from toluene afforded pure Ar3Bi. For a representative example, see:
Hassan, A.; Breeze, S. R.; Courtenay, S.; Deslippe, C.; Wang, S.
Organometallics 1996, 15, 5614.
(20) The X-ray structures have been deposited with the Cambridge Crystal-
lographic Data Centre: 7 (CCDC 216387), 9 (CCDC 216392), and 13
(CCDC 216386).
(21) Structures, references, and data for these compounds are provided in the
Supporting Information.
(22) Boger, D. L.; Brunette, S. R.; Garbaccio, R. M. J. Org. Chem. 2001, 66,
5163.
(23) The unnatural enantiomers follow an analogous parabolic relationship,
albeit with 5-10-fold less potent cytotoxic activity. See Supporting
Information Figure S2.
JA038162T
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