DNA alkylators

Fig. 3. Structures of DNA alkylating/cross-linking agents given in Table 2.

Benzyl chloride also induced in vitro cellular transformation in Syrian hamster embryo cultures and DNA alkylation in several organs of the male mouse following iv adrninistration. In summary, lARC states there is limited evidence that benzyl chloride is carcinogenic in experimental animals epidemiological data was inadequate to evaluate carcinogenicity to humans (67). 

Shape-dependent catalysis for the CC-1065 and duocarmycin DNA alkylation reaction 99ACR1043. 

The requirement for reduction prior to DNA alkylation and crosslinking was first demonstrated by Iyer and Szybalski in 1964 [29], and can be induced both by chemical reducing agents such as sodium dithionite and thiols in vitro and by various reductive enzymes such as DT-diaphorase (NAD(P)H-quinone oxidoreduc-tase) in vitro and in vivo [47]. Much work to characterize the mechanism of reductive activation and alkylation has been carried out, principally by the Tomasz and Kohn groups, and Figure 11.1 illustrates a generally accepted pathway for mitomycin C [16, 48-50] based on these experiments, which is very similar to the mechanism originally proposed by Iyer and Szybalski [29]. 

Figure 11.1 Proposed nrechanisur for reductive activation of and DNA alkylation/crosslmkmg by mitomyci... 

It was concluded that the crosslinking reaction proceeded through initial alkylation of N-7 of adenine by the aziridine ring, followed by guanine N-7 alkylation by the epoxide (Figure 11.12). This seems inconsistent with Armstrong s earlier conclusions, but the very short oligonucleotides used by Saito et al. may influence the DNA alkylation chemistry. 

Coleman et al. have performed a preliminary structure/activity study for a series of analogues of 77 [149]. They found that removal of the naphthoate moiety (88 Scheme 11.11) dramatically reduced the yield of DNA alkylation, while replacement of the NH2 group with O-benzyl (89 and 90) abolished DNA alkylation completely. Compound 91 alkylated DNA with reduced efficiency, so this effect is not simply due to a requirement for a hydrogen bond donor at this position. Perhaps the amide is required at this position to increase the ability of the C=0 to act as a hydrogen bond acceptor. Importantly, they found a strong correlation between the extent of in vitro DNA alkylation and cell culture cytotoxicity. 

Reactions Where the DNA Base Serves as a Leaving Group Thiolysis, Hydrolysis, Elimination, and Reversible DNA Alkylation... 

In a few interesting cases, adducts decompose via reactions in which DNA serves as a leaving group and the original DNA-alkylating electrophile is regenerated. Reversible... 

DNA alkylation has the potential to yield a time-dependent spectrum of adducts, in which initially formed kinetically favored lesions give way to the thermodynamically favored adducts over time. Reversible alkylation has been observed at several of the nucleophilic sites in DNA, including N3A (CC-1065,7, Scheme 8.10, duocarmycin, 8)," " NIA (qui-none methide, 9)," N7G (leinamycin, Schane 8.11, aflatoxin Bj epoxide, 10 and quinone methide, 9),57.ii4.ii8 (quinone methide, 9)," and bPG (ecteinascidin 743,11)." The bidentate Nl/ISPG adduct of malondialdehyde also forms reversibly. ... 

A large number of DNA-alkylating agents are known and we will not attempt a comprehensive survey here. A number of excellent reviews provide an overview of this area. " Here we will review DNA alkylation by three types of reactive intermediates that are important in medicinal chemistry and toxicology— episulfonium ions, aziridinium ions, and carbocations. 

The first of the few low-temperature methods for the formation of an o-QM was a method developed by Rokita.5 It is principally used for reversible DNA alkylation. However, it has recently begun to find its way into some synthetic applications. It utilizes a silylated phenol, which proves vastly more manageable as an o-QM precursor than the corresponding o-hydroxyl benzyl halide (Fig. 4.6). In this kinetically controlled process, expulsion of a benzylic leaving group is triggered at low temperature by treatment with a fluoride ion, which causes a (3-elimination. 

Dithionite-mediated reductive activation of mitomycin C has been employed in the study of its DNA alkylation chemistry.6,63 However, dithionite activated mitomycin C possesses different DNA alkylation properties than that activated by catalytic hydrogenation and enzymatic reduction. We postulated that a new alkylating species is produced by dithionite reductive activation resulting in different reactivity than the iminium methide species. To investigate dithionite-mediated reductive activation further, we treated 13 C-labeled analogues of WV-15 with dithionite and carried out spectral and product studies. 

In vivo studies were carried on the aziridinated cyclopent[Z ]indole quinone out before it was discovered that the aziridinyl ring did not participate in DNA alkylation. The results in Fig. 7.22 for the B16 melanoma syngraft model reveal that there was substantial reduction of tumor mass at 3 mg/kg. However, toxicity (animal deaths) became apparent at 5 mg/kg. On the other hand, human lung cancer xenografts in SCID (severe combined immunodeficient) mice were reduced to 50% mass with 3x1 mg/kg doses without any animal deaths. 

Ping, W. Yang, S. Lixia, Z. Hanping, H. Xiang, Z. Quinone methide derivatives important intermediates to DNA alkylating and DNA cross-linking actions. Curr. Med. Chem. 2005, 12, 2893-2913. 

Ouyang, A. Skibo, E. B. The iminium ion chemistry of mitosene DNA alkylating agents, enriched 13C-NMR studies. Biochemistry 2000, 39, 5817-5830. 

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