Mitomycins reductive activation

The mitomycins do not react directly with DNA, but require prior activation by reduction of the quinone. This property of bioreductive activation has inspired the design and development of synthetic anticancer drugs that are also activated by reduction, as this is expected to confer a degree of tumor selectivity [45, 46]. Many solid tumors are short of oxygen relative to normal tissue, so reductive activation of the mitomycins and other bioreductive drugs can proceed in tumors, while it is inhibited by the oxidizing environments in normal tissues. 

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]. 

Like the mitomycins, FR900482 (6), FR66979 (7), FK973 (8), and FK317 (9) have also been shown to crosslink DNA both in vivo [68-70], and in vitro after reductive activation [71-76] with selectivity for the 5 -CpG-3 sequence [77]. The mechanism outlined in Figure 11.2 was originally proposed by Goto and Fukuyama [78] and has been verified by the experimental work of Williams and Hopkins [71-77, 79]. Reduction of the N-O bond produces intermediate 27, which can lose a molecule of water to form 28, which reacts with DNA by a mechanism similar to that found... 

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. 

Schiltz, P. Kohn, H. Sodium dithionite-mediate mitomycin C reductive activation processes. Tetrahedron Lett. 1992, 33, 4709 1712. 

Suresh Kumar G, Lipman R, Cummings J, et al. Mitomycin C-DNA adducts generated by DT-diaphorase. Revised mechanism of the enzymatic reductive activation of mitomycin C. Biochemistry 1997 36(46) 14128—14136. 

Antitumor agents mitomycin A (61 A) and mitomycin C (61C) contain a latent quinone functionality, which is exposed by reductive activation and elimination of a glycoside or an alcohol followed by opening of the aziridine ring. These quinone methides then react with nucleic acids to form bis-adducts.103 The reductive activation of mitomycins provides selectivity in targeting solid tumors, because this is favored in the oxygen-deprived environment of tumor cells, and inhibited by the oxygen-rich environment of healthy tissues.107... 

The reductive activation of mitomycins in the cell is thought to be an enzymatic process.108 Reduction of mitomycins A and C in vitro by H2/Pt02 or by Na2S204 gives 62, which then breaks down to the quinone methide 63A.109 This quinone methide reacts with DNA to give a complex mixture of alkylated DNA and cross-linked oligonucleotides. Mitomycin A is both more easily reduced and more toxic than mitomycin C, and there is some evidence that the toxicity of... 

Pan, S., Andrews, P.A., Glover, C.J., and Bachur, N.R., 1984, Reductive activation of mitomycin C and mitomycin C metabolites catalysed by NADPH-cytochorome P-450 reductase and xantine oxidase. J. Biol. Chem. 259 959-962 Pollakis, G., Goormaghtigh, E., and Ruysschaert, J.-M., 1983, Role of quinone structure in the mitochondrial damage induced by antitumor anthracyclines. FEBS Lett. 155 267-272 Rappaport, S.M., McDonald, T.A., and Yeowell-O Connell, K., 1996, The use ofprotein adducts to investigate the disposition of reactive metabolites of benzene. Environ. Health Perspect. 104Suppl6 1235-1237... 

The mechanism of action of the anti-neoplastic agent mitomycin C is believed to proceed by its initial reductive activation followed by covalent binding of the activated species to DNA. Drug attachment occurs sequentially at C(l) and C(10) leading to the formation of cross-linked DNA products. A new covalent mitomycin C-DNA adduct (26S), in which tho-e is an intra-strand cross-link, has been isolated from DNA exposed to reductively activated mitomycin C. 

Paz, M.M., Das, A., and Tomasz, M., Mitomycin C hnked to DNA minor groove binding agents synthesis, reductive activation, DNA binding and cross-hnking properties and in vitro antitumor activity, Bioorg. Med. Chem., 7,2713,1999. 

Another class of DNA alkylating agents, the Mitomycins, proved to be most promising in clinical trials. Among these, mitomycin C, shown in Fig. 6.1, exhibits significant anti-tumor activity. Its mechanism of activation consists of a complex bioreductive process. The first step is the reduction to hydroquinone, followed by a loss of methanol. This reaction fa-... 

Although reduction of quinones is usually a detoxication pathway, there are examples such as mitomycin C in which the hydroquinone is more toxic than the quinone as shown in Figure 5.12 and this may increase the susceptibility of cancers that express high levels of NQO. In this case, the reduction of the quinone leads to the loss of methanol, which is the first step in the activation of this anticancer agent (20). 

Short term treatment with TPA sensitized human 2008 ovarian carcinoma cells to cis-platin. This sensitization disappeared completely by seven hours after treatment, indicating that not inhibition, but activation of PKC sensitizes 2008 cells to the antiproliferative activity of cis-platin (Isonishi et al., 1990). Pretreatment of HeLa cells with TPA or PdBu caused a 9-fold increase in cellular sensitivity to cis-platin and 2.5-fold to melphalan, but had now effect on the antiproliferative activity of bleomycin, adriamycin, vincristine, or mitomycin C. The sensitization of HeLa cells by TPA was associated with a 6-fold stimulation of PKC activation and a concentration- and time-dependent increase in cellular platinum content. (Basu et al. 1990). PKC activity was found to be decreased significantly in cis-platin-resistant human small cell lung H69/CP cancer cells compared to the drug-sensitive variant. A similar reduction in PKC activity was noted in ovarian carcinoma 2008 cells that were resistant to cis-platin. A modest decrease in PKC activity was also observed in etoposide-resistant H69 cells but not in taxol-resistant H69 cells or bleomycin-resistant human head and neck carcinoma A-253 cells (Basu et al., 1996), indicating that reduced PKC activity leads to decreased sensitivity in this system. 

Mitomycin (mitomycin C, Mitocin-C, Mutamycin) is an antibiotic that is derived from a species of Streptomyces. It is sometimes classified as an alkylating agent because it can covalently bind to and cross-link DNA. Mitomycin is thought to inhibit DNA synthesis through its abihty to alkylate double-strand DNA and bring about interstrand cross-hnking. There is evidence that enzymatic reduction by a reduced nicotinamide-adenine dinucleotide phosphate (NADPH) dependent reductase is necessary to activate the drug. 

These compounds exert their biological activity as DNA cross-linking agents. The anticancer activity of this series of compounds is almost identical (albeit more potent and less toxic) to the mitomycins in that the natural product itself must be activated in order to cross-link DNA. A two-electron reduction of FR900482 converts it to a mixture of 307 and 308. Loss of water provides a leucoaziridinomitosene 309. It is this metabolite that undergoes DNA cross-linking to form the DNA adduct 310 (Scheme 59). 

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