2-Amino-O⁴-benzylpteridine derivatives: Potent inactivators of O⁶-alkylguanine-DNA alkyltransferase

2-Amino-O⁴-benzylpteridine (1), 2-amino-O⁴-benzyl-6, 7-dimethylpteridine (2), 2-amino-O⁴-benzyl-6-hydroxymethylpteridine (4), 2-amino-O⁴-benzylpteridine-6-carboxylic acid (5), 2-amino-O ⁴-benzyl-6-formylpteridine (6), and O⁴-benzylfolic acid (7) are shown to be as potent or more potent inactivators of the human DNA repair protein O⁶-alkylguanine-DNA alkyltransferase (alkyltransferase) in vitro than O⁶-benzylguanine, the prototype alkyltransferase inactivator currently in clinical trials. Additionally, the negatively charged (at physiological pH) inactivators 2-amino-O ⁴-benzylpteridine-6-carboxylic acid (5) and O⁴- benzylfolate (7) are far more water soluble than O⁶-benzylguanine. The activity of O⁴-benzylfolic acid (7) is particularly noteworthy because it is roughly 30 times more active than O⁶-benzylguanine against the wild-type alkyltransferase and is even capable of inactivating the P140K mutant alkyltransferase that is resistant to inactivation by O ⁶-benzylguanine. All the pteridine derivatives except 2-amino-O ⁴-benzylpteridine-6-carboxylic acid are effective in enhancing cell killing by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). However, the effectiveness of O⁴-benzylfolate as an adjuvant for cell killing by BCNU appears to be a function of a cell's α-folate receptor expression. Thus, O⁴-benzylfolate is least effective as an adjuvant in A549 cells (which express little if any receptor), is moderately effective in HT29 cells (which express low levels of the receptor), but is very effective in KB cells (which are known to express high levels of the α-folate receptor). Therefore, O⁴-benzylfolic acid shows promise as an agent for possible tumor-selective alkyltransferase inactivation, which suggests it may prove to be superior to O⁶-benzylguanine as a chemotherapy adjuvant.