O (4)-benzylfolic acid inactivates O (6)-alkylguanine-DNA alkyltransferase in brain tumor cell lines.
OBJECTIVE
The DNA repair protein, O (6)-alkylguanine-DNA alkyltransferase (AGT), is a primary source of tumor resistance to agents such as temozolomide and chloroethylnitrosoureas that form DNA lesions at the O (6)-position of guanines. To increase the efficacy of these drugs, pseudosubstrate inactivators of AGT such as O (6)-benzylguanine have been developed. A novel inactivator of AGT, O (4)-benzylfolic acid (O(4)-BFA), has been reported which is more potent and water soluble than O (6)-benzylguanine. Previous studies have suggested that uptake of O(4)-BFA is mediated by the folate receptor (FR), and, thus, its use may be limited to cells expressing FR.


METHODS
We measured AGT activity in cell extracts from a panel of brain tumor cells exposed to O(4)-BFA. Inactivation of AGT by O(4)-BFA was measured in cells grown without folic acid as well as in cells grown in folic acid-containing media. Competitive binding studies were performed using purified FR to determine its affinity for O(4)-BFA.


RESULTS
The observed IC(50) for O(4)-BFA in brain tumor cell lines ranged from 0.2 to 1.3 microM for cells grown in media containing 2.3 microM folic acid. At this concentration, folic acid would saturate the FR and the FR would be unable to take up O(4)-BFA. When cells were grown in folic acid free media, there was at most a 50% decrease in the observed IC(50)s, indicating that the FR was not essential for O(4)-BFA uptake. Competitive binding studies using purified FR confirmed that the IC(50) for O(4)-BFA is approximately 180 times greater than folic acid, i.e., it has a very weak affinity for FR.


CONCLUSIONS
These results indicate that O(4)-BFA has potentially broad use as an inactivator of AGT as its use is not limited to tumors expressing high levels of FR.
Johnson
Stewart P
SP
Department of Surgery, Duke University Medical Center, Box 2616, Durham, NC 27710, USA. stewart.johnson@duke.edu
Kamen
Barton A
BA
Bigner
Darell D
DD
Friedman
Henry S
HS
D018906:Antineoplastic Agents, Alkylating / Q000008:administration & dosage / Q000494:pharmacology*; D001667:Binding, Competitive; D001932:Brain Neoplasms / Q000188:drug therapy*; D002352:Carrier Proteins / Q000378:metabolism*; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects; D003606:Dacarbazine / Q000031:analogs & derivatives / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D019008:Drug Resistance, Neoplasm / Q000187:drug effects; D004354:Drug Screening Assays, Antitumor; D058973:Folate Receptors, GPI-Anchored; D005492:Folic Acid / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D006801:Humans; D020128:Inhibitory Concentration 50; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000187:drug effects* / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000378:metabolism*; D000077204:Temozolomide
D018906:Antineoplastic Agents, Alkylating; D002352:Carrier Proteins; D058973:Folate Receptors, GPI-Anchored; C524118:O(4)-benzylfolic acid; D011956:Receptors, Cell Surface; D003606:Dacarbazine; D005492:Folic Acid; D019853:O(6)-Methylguanine-DNA Methyltransferase; D000077204:Temozolomide