Triazene compounds: mechanism of action and related DNA repair systems

Triazene compounds of clinical interest (i.e. dacarbazine and temozolomide) are a group of alkylating agents with similar chemical, physical, antitumour and mutagenic properties. Their mechanism of action is mainly related to methylation of O(6)-guanine, mediated by methyldiazonium ion, a highly reactive derivative of the two compounds. The cytotoxic/mutagenic effects of these drugs are based on the presence of DNA O(6)-methylguanine adducts that generate base/base mismatches with cytosine and with thymine. These adducts lead to cell death, or if the cell survives, provoke somatic point mutations represented by C:G-->T:A transition in DNA helix. Triazene compounds have excellent pharmacokinetic properties and limited toxicity. Dacarbazine requires hepatic activation whereas temozolomide is spontaneously converted into active metabolite in aqueous solution at physiological pH. Moreover, temozolomide is fully active when administrated orally (100% bioavailability). The biological effects of triazene compounds and cell resistance to them depend on at least three DNA repair systems, (a) O(6)-alkylguanine-DNA-alkyltransferase, called also methyl-guanine methyl-transferase (MGMT); (b) mismatch repair (MMR), and (c) base excision repair (BER). MGMT is a small enzyme-like protein that removes small alkyl adducts from the O(6) position of DNA guanine through a stoichiometric and auto-inactivating reaction. This reaction consists in a covalent transfer of the alkyl group from the alkylated site in DNA to an internal cysteine residue of MGMT protein. High levels of MGMT are responsible for normal and tumour cell resistance to triazenes. Therefore, pre-treatment with MGMT inhibitors - i.e. O(6)-benzylguanine or O(6)-(4-bromotenyl)guanine (Lomeguatrib) - is followed by a great increase in the activity of triazenes against target cells expressing high MGMT levels. MMR is represented by a protein complex dedicated to the repair of biosynthetic errors generated during DNA replication. The MMR system recognizes base mismatches and insertion-deletion loops, cuts the nucleotide sequence containing the lesion, and restores the correct base sequence. Therefore, not only MGMT but also MMR is involved in target cell susceptibility to triazenes. However, the system does not suppress, but instead promotes the cytotoxic effects of triazenes. In fact, MMR is not able to repair the incorrect base pairing determined by treatment with triazenes and, according to a predominant hypothesis, it causes reiterated "futile" attempts of damage repair leading to the activation of cell cycle arrest and apoptosis. BER removes lesions due to cellular metabolism, or to physical or chemical agents. BER is able to repair N(7)-methylguanine and N(3)-methyladenine determined by treatment with triazenes. Therefore, triazene compounds can also kill tumour cells by a N(3)-methyladenine-mediated mechanism if BER activity is inhibited by chemical agents (i.e. PARP inhibitors). In conclusion, in selected cases, triazenes can represent a therapeutic alternative to treatment of neoplastic diseases including haematological malignancies. Moreover, the susceptibility of neoplastic cells to these compounds can be substantially increased through pharmacological modulation of the expression level and functional activity of DNA repair enzymes.