Cytotoxicity strand breaks

The abasic sites (3, Scheme 8.2) resulting from the loss of alkylated bases from DNA are both cytotoxic and mutagenic. " The cyclic acetal (3) exists in equilibrium with small amounts (—1%) of the open chain aldehyde (4). The acidic nature of a-proton in the aldehyde form of the abasic lesion facilitates 3-elimination of the 3 -phosphate residue to yield a strand break. " This reaction occurs with a half-life of about 200 h under physiological conditions (pH 7.4, 37°C), but can be accelerated by heat, basic conditions, or the presence of various amines. " ... 

Sze C-C, Shi C-Y, Ong C-N. 1996. Cytotoxicity and DNA strand breaks induced by benzene and its metabolites in Chinese Hamster ovary cells. J Appl Toxicol 16 259-264. 

The covalent trapping of the enzyme leads to a depletion of the cellular pool of DNMTs and subsequent DNA hypomethylation. This in turn results in activation with respect to the reactivation of silenced genes. Additionally, the covalently trapped DNMT may inhibit RNA and DNA polymerases, which leads to an inhibition of protein biosynthesis and DNA strand breaks. This may lead to apoptosis and hence cytotoxicity. Thus, it is not easy to dissect the reasons for the clinical efficacy of these inhibitors in terms of real epigenetic and plain cytotoxic effects [81]. 

CPTs kill cells in S-phase by the mechanism involving the replication fork collision model. It is known that reversible TOPO-1 CPT DNA cleavable complexes are in themselves nonlethal and that collisions with an advancing replication fork cause cell death (8). Several biochemical events follow collision with the replication fork the formation of double-strand breaks, irreversible arrest of the replication fork, and the formation of TOPO-1-linked DNA break at the site of collision. These mechanisms account for the S-phase-specific cytotoxicity seen at relatively low doses of camptothecin. 

Novel mechanisms of interest include sensitizing hypoxic tumor cell lines to enhance radiotoxicity. Tirapazamine is a hypoxia-selective compound 1-2-fold greater in magnitude in comparison to mitomycin C or porfiromycin (84). Its mechanism of action results in a one-electron reduction inducing DNA double-strand breaks and cell death under hypoxic conditions. The free radical is oxidized back to the parent compound under aerobic conditions. When combined with the platinum compounds, the cytotoxic effects may be equivalent to that seen with five times the dose of cisplatin without the toxicities that would be encountered if actually administered (85). 

Doxorubicin binds tightly to DNA by its ability to intercalate between base pairs and therefore is preferentially concentrated in nuclear structures. Intercalation results in steric hindrance, hence production of single-strand breaks in DNA and inhibition of DNA synthesis and DNA-dependent RNA synthesis. The enzyme topoisomerase II is thought to be involved in the generation of DNA strand breaks by the anthracydines. Cells in S-phase are most sensitive to doxorubicin, although cytotoxicity also occurs in other phases of the cell cycle. 

The role of CYP2E1 in a-oxidation of A-nitrosodiethanolamine was probed by using the deuterated isotopomers A-nitroso[a-D4]diethanolamine and 7V-nitroso-[P-D4]diethanolamine. 7V-Nitrosodiethanolamine and 7V-nitroso[P-D4]diethanolamine were equally cytotoxic to human CYP2E1-transfected V79 cells, while 7V-nitroso-[a-D4]diethanolamine was not. Significant DNA single-strand break levels were... 

Bradley, M.O., Taylor, V.I., Armstrong, M.J. Galloway, S.M. (1987) Relationships among cytotoxicity, lysosomal breakdown, chromosome aberrations, and DNA double-strand breaks. Mutat. Res., 189, 69-79... 

Susa N, Ueno S, Furukawa Y, et al. 1997a. Potent protective effect of melatonin on chromium(VI)-induced DNA single-strand breaks, cytotoxicity, and lipid peroxidation in primary cultures of rat hepatocytes. Toxicol Appl Pharmacol 144 377-384. 

Fig. 4. Possible role of mismatch repair in the cytotoxicity of cisplatin. A) During replicative bypass, a mismatch is incorporated across from the cisplatin-DNA adduct. This compound lesion is bound by the mismatch repair proteins, which cut the DNA on the strand opposite the platinum. Repair synthesis would reproduce the same mismatch, resulting in a futile cycle and possibly the accumulation of DNA strand breaks which would activate apoptosis. B) Alternatively, the mismatch repair complex can recognize the cisplatin-DNA adduct alone and generate a signal that triggers apoptosis.

Crook TR, Souhami RL, McLean AE. 1986. Cytotoxicity, DNA cross-linking, and single strand breaks induced by activated cyclophosphamide and acrolein in human leukemia cells. Cancer Res 46 5029- 5034. 

Poly (ADP-ribose) polymerase-1 (PARP-1) facilitates the repair of DNA strand breaks (Calabrese et al, 2004). Inhibiting PARP-1 increases the cytotoxicity of DNA-damaging chemotherapy and radiation therapy in vitro. But classical PARP-1 inhibitors have limited clinical utility. AG14361 is, to our knowledge, the first high-potency... 

---