O6-Benzylguanine

O6-Benzylguanine (BG) is a potent inhibitor of MGMT and an antineoplastic agent [1].

O6-methylguanine DNA methyltransferase (MGMT, also known as O6-Alkylguanine-DNA alkyltransferase, or AGT) is a DNA repair protein that removes the alkyl group located on the O6-position of guanine from DNA and restores DNA integrity. MGMT is alkylated and irreversibly inactivated.

O6-Benzylguanine is a potent MGMT inhibitor. In HT29 cells, O6-Benzylguanine significantly reduced MGMT stability and the affinity of MGMT for DNA, increased its sensitivity to proteases. O6-Benzylguanine also increased the cytotoxic effect of alkylating agent BCNU [1]. In SF767 cells, BG (25 μM for 1 h) reduced >95% of MGMT activity, while 33% of the activity recovered within 24 h. When cells pretreated with BG (2.5 μM for 24 h) followed by the same low-dose treatment for 24 h completely depleted MGMT activity [2]. In HCT116 and HCT15 cells, treatment with BG and BCNU inactivated MGMT and arrested 70-80% of cells in G2/M phase in response to the DNA damages induced by BCNU [3].

In nude mice bearing SF767 human brain tumor xenografts, treatment with O6-Benzylguanine prior to BCNU significantly inhibited tumor growth as compared to BCNU alone [1]. In xenograft SF767 tumors, pre- and post-treatments (8 mg/kg over 24 h) combined with an i.p. bolus dose (80 mg/kg) of BG inhibited >95% of MGMT activity [2].

References:
[1].  Dolan ME, Pegg AE. O6-benzylguanine and its role in chemotherapy. Clin Cancer Res, 1997, 3(6): 837-847.
[2].  Kreklau EL, Kurpad C, Williams DA, et al. Prolonged inhibition of O(6)-methylguanine DNA methyltransferase in human tumor cells by O(6)-benzylguanine in vitro and in vivo. J Pharmacol Exp Ther, 1999, 291(3): 1269-1275.
[3].  Yan L, Donze JR, Liu L. Inactivated MGMT by O6-benzylguanine is associated with prolonged G2/M arrest in cancer cells treated with BCNU. Oncogene, 2005, 24(13): 2175-2183.