Abstract
The DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) is an important suicide enzyme involved in the defense against O6-alkylating mutagens. It also plays a role in the resistance of tumors to anticancer drugs targeting the O6-position of guanine, such as temozolomide and fotemustine. Several potent MGMT inhibitors have been developed sensitizing cells to O6-alkylating agents. Aimed at targeting MGMT inhibitors to tumor cells, we synthesized MGMT inhibitory compounds conjugated with glucose to improve uptake in tumor cells. Here, we compared O6-benzylguanine, O6-2-fluoropyridinylmethylguanine (O6FPG), O6-3-iodobenzylguanine, O6-4-bromothenylguanine, and O6-5-iodothenylguanine with the corresponding C8-linker β-d-glucose derivatives. All glucose conjugated inhibitors were 3- to 5-fold less effective than the corresponding nonconjugated drugs as to MGMT inhibition that was measured in cell extracts (in vitro) and cultivated HeLaS3 cells (in vivo). Except for O6FPG, IC50 values of the guanine derivatives applied in vitro and in vivo were correlated. A similar correlation was not obvious for the corresponding glucosides, indicating differences in cellular uptake. C8-α-d-glucosides were less effective than β-glucosides. From the newly developed glucose-conjugated inhibitors tested, O6-4-bromothenylguanine-C8-β-d-glucoside (O6BTG-C8-βGlu) was most potent in inhibiting MGMT both in vitro and in vivo. At a concentration of 0.1 μM, it inhibited cellular MGMT to completion. It was not toxic, even when applied chronically to cells at high dose (up to 20 μM). O6BTG-C8-βGlu strongly potentiated the killing effect of fotemustine and temozolomide, causing reversal from MGMT+ to MGMT– phenotype. Therefore, O6BTG-C8-βGlu seems to be especially suitable for approaching MGMT inhibitor targeting in tumor therapy.
Footnotes
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This work was supported by the Deutsche Forschungsgemeinschaft Grants DFG-Ka724/12-1 and Ka724/13-1.
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doi:10.1124/jpet.104.071316.
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ABBREVIATIONS: MGMT, O6-methylguanine-DNA methyltransferase; O6BTG-C8-βGlu, O6-4-bromothenylguanine-C8-β-d-glucoside; O6FPG, O6-2-fluoropyridinylmethylguanine; O6IBG, O6-3-iodobenzylguanine; O6BTG, O6-4-bromothenylguanine; O6ITG, O6-5-iodothenylguanine.
- Received May 12, 2004.
- Accepted July 9, 2004.
- The American Society for Pharmacology and Experimental Therapeutics
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