Chemistry Reference
In-Depth Information
The alternative, the elimination of
•
OH [reaction (69)] has been considered to
be much less likely. As has been discussed in Chapter 6.9, water elimination re-
actions may considerably change the properties of a radical. This also applies
to the tirapazamine system. The primary 'reducing' tirapazamine radical turns
into an oxidizing one upon water elimination. Its rate constant with dGMP has
been determined at 1.4
10
8
dm
3
mol
−1
s
−1
. It has been reported to react also
with 2-deoxyribose by H-abstraction at 3.7
×
10
6
dm
3
mol
−1
s
−1
(see also Shinde
et al. 2004). At first sight, this is very surprising for a radical for which many
mesomeric forms can be written. However according to DFT calculations, H-
abstraction from the sugar moiety is exoenergetic and H-transfer would occur
to the exocyclic NH function, the site of highest spin density (Naumov and von
Sonntag 2005, unpublished results).
It is commonly agreed that the one-electron reduced drug is capable of induc-
ing DNA strand breaks, even DSBs (Brown 1993). With DNA in aqueous solution,
SSBs dominate considerably over DSBs (Jones and Weinfeld 1996). The major
detectable lesion (32% of the 3
×
ends) has been identified as the phosphoglyco-
late. Its yield dramatically increases with increasing tirapazamine concentra-
tion, and it has been suggested that this reaction is caused by an abstraction of
H4
′
by the activated drug and donation of an oxygen atom (possibly the
N
-oxid
oxygen) to the
C
(4
′
) radical by a second tirapazamine. This reaction would be
reminiscent of the action of nitroaromatic sensitizers (Chap. 6.3), but quite dif-
ferent from a mere one-electron oxidation of a reducing radical by tirapazamine.
For example, when the
C
(1
′
) radical is generated photolytically in an adequately
substituted ss/dsODNs, it is rapidly oxidized by the drug to the 2-dRL lesion
(Daniels et al. 1998; Hwang et al. 1999b), thereby another drug molecule is acti-
vated (Wardman et al. 2003). The rate of reaction with the ssDNA model is 2.5
′
×
10
8
dm
3
mol
−1
s
−1
, and the rate of reaction drops to 4.6
10
6
dm
3
mol
−1
s
−1
with
the dsDNA model (Hwang et al. 1999b). This dramatic drop in rate may be due
to the fact that the
C
(1
×
) radical is deeply hidden in the minor groove (Sect. 12.2),
and thus difficult to approach.
SSB formation is suppressed by typical
•
OH-scavengers such as MeOH, DMSO
and
t
BuOH at high concentrations (0.1 mol dm
−3
); but the suppression is only
partial (Daniels and Gates 1996). The involvement of (free)
•
OH, as suggested,
cannot be the major cause of the DNA damage, since in the reaction with DMSO
(in the absence of DNA) the effectiveness of xanthine/xanthine oxidase activated
tirapazamine in forming methanesulfinic acid is only in the 1% range (based on
added tirapazamine; 92% is the theoretical value if full conversion is achieved;
Chap. 3.2; for a caveat that the xanthine/xanthine oxidase system as such may
nick DNA under certain conditions via
•
OH see Jones and Weinfeld 1996). The
formation of some free
•
OH is supported by spin-trapping experiments (Patter-
son and Taiwo 2000), but strongly oxidizing species other than
•
OH give also
rise to the this spin-adduct (Sect. 12.9.5). Tirapazamine is much more effective
in creating base damage (Birincioglu et al. 2003) then can be accounted for by
free
•
OH that reacts with DNA (for the competition kinetics as a function of the
scavenger capacity of the solution see Udovicic et al. 1991a).
Little if any base damage was detected using the
32
P-postlabeling assay (Jones
and Weinfeld 1996). Yet, this assay only records a limited number of such lesions
′
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