Plasma and cerebrospinal fluid pharmacokinetics of O6-benzylguanine and time course of peripheral blood mononuclear cell O6-methylguanine-DNA methyltransferase inhibition in the nonhuman primate.
O6-Benzylguanine (O6BG) enhances the cytotoxicity of the nitrosoureas by irreversibly binding and inhibiting the DNA repair enzyme O6-methyl-guanine-DNA methyltransferase (MGMT). The plasma and cerebrospinal fluid (CSF) pharmacokinetics of O6BG and its active metabolite, O6-benzyl-8-oxoguanine, were studied in a nonhuman primate model after 200 mg/m2 had been injected i.v. The parent drug and the metabolite were measured with a reverse-phase HPLC assay. A pharmacokinetic model incorporating separate compartments for O6BG and the O6-benzyl-8-oxoguanine metabolite, first-order conversion of O6BG to the metabolite, and additional first-order elimination rate constants for each compound, was simultaneously fitted to the parent drug and metabolite plasma concentration time data. Elimination of O6BG from plasma was rapid; it had a half-life of 1.6 h and a clearance of 68 ml/min/m2. On the basis of the pharmacokinetic model, essentially all of the O6BG was converted to O6-benzyl-8-oxoguanine. The plasma pharmacokinetic profile of the metabolite differed considerably from that the parent drug. The half-life (14 h) was 10-fold longer and the area under the curve (2420 microM/h) was 11-fold higher than that of O6BG (212 microM/h). The clearance rate of O6-benzyl-8-oxoguanine was 6.4 ml/min/m2. The CSF:plasma ratio was 4.3% for O6BG and 36% for O6-benzyl-8-oxoguanine, and the metabolite area under the curve was 90-fold higher than that of O6BG in CSF. The excellent CSF penetration of the active metabolite provides a rationale for the use of O6BG as a chemosensitizing agent for brain tumors. We also studied the duration of MGMT inhibition in peripheral blood mononuclear cells. By 2 h after a 200 mg/m2 dose of O6BG, > 98% of MGMT activity was suppressed, and > 95% suppression of enzyme activity persisted at 18 and 48 h after the dose. By 2 weeks after the treatment, MGMT levels had returned to baseline. Persistent high concentrations of the active metabolite appear to provide a pharmacological explanation for the prolonged suppression of MGMT activity.
Berg
S L
SL
Pediatric Branch, National Cancer Institute, Bethesda, Maryland 20892, USA.
Gerson
S L
SL
Godwin
K
K
Cole
D E
DE
Liu
L
L
Balis
F M
FM
D000818:Animals; D004260:DNA Repair / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000097:blood / Q000134:cerebrospinal fluid / Q000493:pharmacokinetics*; D006147:Guanine / Q000031:analogs & derivatives* / Q000097:blood / Q000134:cerebrospinal fluid / Q000493:pharmacokinetics; D007963:Leukocytes, Mononuclear / Q000201:enzymology; D008253:Macaca mulatta; D008297:Male; D008780:Methyltransferases / Q000037:antagonists & inhibitors*; D019853:O(6)-Methylguanine-DNA Methyltransferase; D013997:Time Factors
D004791:Enzyme Inhibitors; C064976:O(6)-benzylguanine; C024829:8-hydroxyguanine; D006147:Guanine; D008780:Methyltransferases; D019853:O(6)-Methylguanine-DNA Methyltransferase