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Plasma and Cerebrospinal Fluid Pharmacokinetics of O⁶-Benzylguanine and Analogues in Nonhuman Primates¹

Department of Medicine, Cancer Research Center and Committee on Clinical Pharmacology, University of Chicago, Chicago, Illinois 60637 [L. L., S. K. R., M. E. D.]; Texas Children’s Cancer Center, Texas Children’s Hospital, Baylor College of Medicine, Houston, Texas 77030 [S. L. B.]; Pediatric Oncology Branch, National Cancer Institute, Bethesda, Maryland 20892 [C. L. M., F. M. B.]; and Chemistry of Carcinogenesis Laboratory, National Cancer Institute-Frederick Cancer Research and Development Center, Frederick, Maryland 21702-1201 [H-W. S-Y., R. C. M.]

O⁶-Benzylguanine (BG) is a potent, specific inactivator of the DNA repair protein, O⁶-alkylguanine-DNA alkyltransferase, that enhances the sensitivity of tumor cell lines and tumor xenografts to chloroethylnitrosoureas. To search for BG analogues with greater penetration into the cerebrospinal fluid (CSF), we evaluated plasma and CSF pharmacokinetics of BG, 8-aza-O⁶-benzylguanine (8-azaBG), O⁶-benzyl-8-bromoguanine (8-BrBG), O⁶-benzyl-8-oxoguanine (8-oxoBG), O⁶-benzyl-8-trifluoromethylguanine (8-tfmBG), and O⁶-benzyl-2'-deoxyguanosine (B2dG) after i.v. administration of 200 mg/m² of drug through an indwelling Ommaya reservoir in a nonhuman primate model. BG and its analogues were quantified in plasma and CSF using reverse-phase high-performance liquid chromatography assays. The plasma clearances of the four 8-substituted BG analogues were similar (0.04–0.06 l/h/kg), but half-lives ranged from <2 to >24 h. BG was converted to 8-oxoBG, an equally potent O⁶-alkylguanine-DNA alkyltransferase inactivator, and the elimination of 8-oxoBG was much slower than that of BG. As a result, the plasma area under the curve of 8-oxoBG was 3.5-fold greater than that of BG. B2dG was metabolized to BG and 8-oxoBG, but this pathway accounted for only 20% of B2dG elimination. The CSF penetration percentages (based on the ratio of AUC_CSF:AUC_plasma) for BG, 8-azaBG, 8-oxoBG, 8-tfmBG, 8-BrBG, and B2dG were 3.2, 0.18, 4.1, 1.4, <0.3, and 2.0%, respectively. The CSF penetration of BG and its active metabolite 8-oxoBG is greater than the penetration of 8-azaBG, 8-BrBG, 8-tfmBG, and B2dG.

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