Targeting O⁶-methylguanine-DNA methyltransferase with specific inhibitors as a strategy in cancer therapy

Abstract

O (6)-methylguanine-DNA methyltransferase (MGMT) repairs the cancer chemotherapy-relevant DNA adducts, O (6)-methylguanine and O (6)-chloroethylguanine, induced by methylating and chloroethylating anticancer drugs, respectively. These adducts are cytotoxic, and given the overwhelming evidence that MGMT is a key factor in resistance, strategies for inactivating MGMT have been pursued. A number of drugs have been shown to inactivate MGMT in cells, human tumour models and cancer patients, and O (6)-benzylguanine and O (6)-[4-bromothenyl]guanine have been used in clinical trials. While these agents show no side effects per se, they also inactivate MGMT in normal tissues and hence exacerbate the toxic side effects of the alkylating drugs, requiring dose reduction. This might explain why, in any of the reported trials, the outcome has not been improved by their inclusion. It is, however, anticipated that, with the availability of tumour targeting strategies and hematopoetic stem cell protection, MGMT inactivators hold promise for enhancing the effectiveness of alkylating agent chemotherapy.

Fig. 1

O⁶-MeG and O⁶-chloroethylguanine driven cell death pathways, and protection by MGMT. During DNA replication, O⁶-MeG mispairs with thymine forming O⁶-MeG-thymine [142]. Mismatch repair removes thymine from O⁶-MeG-T mispairs. Due to the mispairing properties of O⁶-MeG, thymine is again inserted, which results in a futile repair cycle. This may result in single-strand DNA repair patches that block replication. In a subsequent round of replication this eventually results in DNA double-strand breaks [143] that are potent activators of the apoptotic pathway [144]. O⁶-chloroethylguanine in DNA is an unstable adduct undergoing a slow intramolecular rearrangement, forming the cyclic etheno adduct and subsequently a N1-guanine-N3-cytosine interstrand crosslink. If not repaired by the crosslink repair system, which involves the p53 regulated proteins DDB2 and XPC [145], these crosslinks are highly toxic, activating the apoptotic pathway [4]. MGMT repairs the initially formed O⁶-MeG as well as O⁶-MeG mispaired with thymine [20]. Therefore, its resynthesis exerts protection even some time after the primary lesion O⁶-MeG was induced. MGMT also repairs the O⁶-chloroethylguanine adduct by transferring the chloroethyl group to its own cysteine

Fig. 2

Dealkylation of O ⁶-methylguanosine and the pseudosubstrate O ⁶-benzylguanine by MGMT. a Following DNA methylation, the methyl group is transferred from O ⁶-methylguanosine to the active centre of MGMT. The same happens if the free base O ⁶-MeG is used as a pseudo-substrate. b In case of O ⁶-BG, the benzyl group is transferred to Cys 145 of MGMT. Alkyl group transfer to MGMT inactivates the protein and subjects it to degradation via ubiquitination

Fig. 3

Chemical structure of O ⁶-BG and O ⁶-BTG and the corresponding folate and glucose derivatives