title: O6-alkylguanine-DNA alkyltransferase inactivation by ester prodrugs of O6-benzylguanine derivatives and their rate of hydrolysis by cellular esterases.
abstract: To modulate the bioavailability and perhaps improve the tumor cell selectivity of O6-alkylguanine-DNA alkyltransferase (AGT) inactivators, pivaloyloxymethyl ester derivatives of O6-benzylguanine (BG) were synthesized and tested as AGT inactivators and as substrates for cellular esterases. The potential prodrugs examined were the 7- and 9-pivaloyloxymethyl derivatives of O6-benzylguanine (7- and 9-esterBG), and of 8-aza-O6-benzylguanine (8-aza-7-esterBG and 8-aza-9-esterBG) and the 9-pivaloyloxymethyl derivative of 8-bromo-O6-benzylguanine (8-bromo-9-esterBG). The benzylated purines were all potent inactivators of the pure AGT and of the AGT activity in HT29 cells and cell extracts. Each ester was at least 75 times less potent than the corresponding benzylated purine against the pure human AGT. In contrast, the activities of esters and their respective benzylated purine were similar in crude cell extracts and in intact cells. The increase in potency of esters in cellular extracts could be explained by a conversion of the respective prodrug to the more potent benzylated purine in the presence of cellular esterases. The apparent catalytic activity (Vmax/Km) of liver microsomal esterase for 8-azaBG ester prodrugs was 70-130 times greater than for BG prodrugs and 10-20 times greater than for 8-bromo-9-esterBG. Tumor cell hydrolysis of the esters varied considerably as a function of cell type and prodrug structure. These data suggest that these or related prodrugs may be advantageous for selective AGT inactivation in certain tumor types.
authors.affiliation: Section of Hematology-Oncology, University of Chicago, IL 60637, USA.
authors.forename: M E
authors.initials: ME
authors.lastname: Dolan
authors.forename: S K
authors.initials: SK
authors.lastname: Roy
authors.forename: B J
authors.initials: BJ
authors.lastname: Garbiras
authors.forename: P
authors.initials: P
authors.lastname: Helft
authors.forename: P
authors.initials: P
authors.lastname: Paras
authors.forename: M Y
authors.initials: MY
authors.lastname: Chae
authors.forename: R C
authors.initials: RC
authors.lastname: Moschel
authors.forename: A E
authors.initials: AE
authors.lastname: Pegg
mesh_terms: D000818:Animals; D002265:Carboxylic Ester Hydrolases / Q000378:metabolism*; D002478:Cells, Cultured; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D004952:Esters; D006147:Guanine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics; D006801:Humans; D006868:Hydrolysis; D015394:Molecular Structure; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000037:antagonists & inhibitors*; D011355:Prodrugs / Q000378:metabolism / Q000493:pharmacokinetics*; D013552:Swine; D014407:Tumor Cells, Cultured
chemical_list: D004791:Enzyme Inhibitors; D004952:Esters; D011355:Prodrugs; C064976:O(6)-benzylguanine; D006147:Guanine; D019853:O(6)-Methylguanine-DNA Methyltransferase; D002265:Carboxylic Ester Hydrolases