Fragmentation of Arylmethyl Quaternary Nitrogen Mustards
J. Phys. Chem. A, Vol. 101, No. 50, 1997 9709
radicals. Spectral data for the formation of the presumed
peroxyl radicals of 6 and 7 are given in Figures 4 and 5.
destroying hypoxic cells in tumors. It indicates that particular
attention should be directed to nitroheterocyclic analogues.
Fragmentation of these compounds at the one-electron reduction
level has the advantage of providing a more favorable stoichi-
ometry for the desired release of a cytotoxin from the com-
pounds and also avoids interception of subsequent reduction
products through other reactions. Although rapid unimolecular
fragmentation prevents reoxidation of the intermediate nitro
radicals by oxygen, this does not preclude selectivity for hypoxic
cells as there are possibly other mechanisms by which oxygen
inhibits the release of the cytotoxin from the compounds.21
Although benzyl radicals are formed promptly on one-electron
reduction of the non-nitro benzyl quaternary compounds 1 and
2, these low-potential compounds will be difficult to reduce
enzymatically. Their activation in tumors might be achievable
using ionizing radiation, although the efficiency would be
expected to be low because competing electron acceptors would
be unlikely to subsequently reduce these compounds. In light
of these findings we are currently investigating further nitro-
heterocyclic arylmethyl quaternary salts, incorporating much
more potent tertiary amine leaving groups such as amino seco-
-•
•
2
-
ArCH X f ArCH + X
(2)
2
•
2
•
ArCH + O f ArCH OO
2
2
Limits on the second-order rate constants for the electron-
transfer reaction of O2 with the radical anions of 6 and 7 can
6
6
-1 -1
be set as <6.0 × 10 and <8.0 × 10 L mol s , respectively.
The failure of oxygen to reoxidize the initial radical inter-
mediate, in competition with its first-order fragmentation, does
not preclude hypoxia-selective toxicity since 6 is 100-fold more
potent to EMT6 cells under anoxic than aerobic conditions
(unpublished data). This suggests that oxygen can inhibit
metabolic activation of the nitroarylmethyl quaternary com-
pounds by mechanisms other than redox cycling. Another
bioreductive drug which undergoes pseudo-first-order fragmen-
tation to release a cytotoxin on one-electron reduction, the
cobalt(III)-nitrogen mustard complex SN 24771, also shows
significant selective toxicity toward hypoxic cells despite the
fact that its initial one-electron reduction product (the Co(II)
intermediate) is not significantly reoxidized by O2.21 The latter
study suggested that competition between O2 and the bioreduc-
tive drug for metabolic reductants might be an alternative
mechanism by which O2 suppresses metabolic activation. This
mechanism may also contribute to the hypoxic selectivity of
nitroarylmethyl quaternary ammonium salts as bioreductive
drugs.
2
2
cyclopropylbenz[e]indolines, as drugs for enzymatic or radi-
olytic activation in hypoxic regions of tumors.
Acknowledgment. This study was supported by the National
Cancer Institute (contract no. NO1-CM47019) and The Health
Research Council of New Zealand.
References and Notes
(
1) Neta, P.; Behar, D. J. Am. Chem. Soc., 1980, 102, 4798.
Conclusions
(2) Bays, J. P.; Blumer, S. T.; Baral-Tosh, S.; Behar, D.; Neta, P. J.
Am. Chem. Soc. 1983, 105, 320.
This study has shown that there are multiple mechanisms by
which arylmethyl quaternary ammonium compounds fragment
on reduction. Unlike the nitrobenzyl halides, the nitrobenzyl
quaternary ammonium mustards show no evidence of intramo-
(3) Norris, R. K.; Barker, S. D.; Neta, P. J. Am. Chem. Soc. 1984,
06, 3140.
1
(4) Andrieux, C. P.; Le Gorande, A.; Saveant, J.-M. J. Am. Chem. Soc.
1
992, 114, 6892.
(
5) Tercel, M.; Wilson, W. R.; Denny, W. A. J. Med. Chem. 1993, 36,
2578.
(6) Denny, W. A.; Wilson, W. R.; Tercel, M.; Van Zijl, P.; Pullen, S.
M. Int. J. Radiat. Oncology Biol. Phys. 1994, 29, 317.
7) Tercel, M.; Wilson, W. R.; Anderson, R. F.; Denny, W. A. J. Med.
Chem. 1996, 39, 1084.
lecular electron transfer from their radical anions to form benzyl
radicals. Steady-state radiolysis studies14 show reductive release
of the tertiary amine (mechlorethamine) from such compounds
occurs from a product in which the nitro group is reduced further
by more than one electron. Fragmentation of the nonnitro
benzyl quaternary compounds does occur on one-electron
reduction, as demonstrated by the prompt formation of the
corresponding benzyl radicals in the present study. With the
low-potential benzene derivative 1, this appears to be predomi-
nantly due to direct electron attachment to the benzyl quaternary
moiety, while for the more electron-deficient 4-SO2CH3 deriva-
tive there is indirect evidence for initial electron attachment to
the aromatic system followed by very rapid transfer to the
benzylic position. In the case of the nitroheterocyclic analogues,
(
(
8) Bobrowski, K. J. Phys. Chem. 1981, 85, 382.
(9) Schuler, R. H.; Patterson, L. K.; Janata, E. J. Phys. Chem. 1980,
8
4, 2088.
(10) Simic, M.; Neta, P.; Hayon, E. J. Phys. Chem. 1969, 73, 3704.
(11) Patel, K. B.; Wilson, R. L. J. Chem. Soc., Faraday Trans. 1973,
9, 814.
12) Eaton, A. D., Clesceri, L. S., Greenberg, A. E., Eds. Standard
Methods for the Examination of Water and Wastewater, 19th ed.; United
6
(
Book Press: Inc.: Baltimore, MD, 1995; p 4-52.
(
13) Cummings, J.; MacLellan, A.; Smyth, J. F.; Farmer, P. B. Anal.
Chem. 1991, 63, 1514.
14) Wilson. W. R.; Ferry, D. M.; Tercel, M.; Anderson, R. F.; Denny,
W. A. Radiat. Res., in press.
15) Gordon, S.; Schmidt, K. H.; Hart, E. J. J. Phys. Chem. 1977, 81,
04.
(
(
6
and 7, dissociative intramolecular electron-transfer kinetics
1
are observed. The faster electron-transfer kinetics for the
nitroheterocycles than the nitrobenzene or nitronaphthalene
derivatives might well be related to their electron-rich π-systems
where six electrons are spread over five atoms and the addition
of a further electron aids the expulsion of the leaving group.
On the basis of results for the nitrobenzyl and 4-nitroimidazole
ring systems, the kinetics of intramolecular electron transfer are
slower for a tertiary amine rather for a halide leaving group,
but the 5-nitropyrrole system is an exception to this with the
nitro radical anion of the chloromethyl compound being
surprisingly stable.
(
16) Christensen, H. C.; Sehested, K.; Hart, E. J. J. Phys. Chem. 1973,
7
7, 983.
(17) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J.
Phys. Chem. Ref. Data 1988, 17, 513.
18) Anderson, R. F.; Patel, K. B. J. Chem. Soc., Faraday Trans. 1 1984,
(
8
2
6
0, 2693.
(19) Moreno, S. N. J.; Schreiber, J.; Mason, R. P. J. Biol. Chem. 1986,
61, 7811.
(20) Wardman, P.; Clarke, E. D. Biochem. Biophys. Res. Commun. 1976,
9, 942.
(21) Anderson, R. F.; Denny, W. A.; Ware, D. C.; Wilson, W. A. Br. J.
Cancer 1996, 74, Suppl. XXVII, S48.
22) Atwell, G. J.; Wilson, W. R.; Denny, W. A. Biorg. Med. Chem.
Lett. 1997, 7, 1493.
23) Stekhan, E.; Kuwana, T. Ber. Bunsen-Ges. Phys. Chem. 1974, 78,
253.
(
This investigation has several implications for the use of
arylmethyl quaternary salts as reductively-activated drugs for
(