Plasma and CNS pharmacokinetics of O 4 -benzylfolic acid (O 4 BF) and metabolite in a non-human primate model

Purpose O 6 -alkylguanine-DNA alkyltransferase (AGT) repairs DNA damage from alkylating agents by transferring the alkyl adducts from the O 6 -position of guanine in DNA to AGT. The folate analog O 4 -benzylfolic acid (O 4 BF) is an inhibitor of AGT with reported selectivity of the alpha-folate receptor in tumors. We studied plasma and cerebrospinal fluid (CSF) pharmacokinetics and CSF penetration of O 4 BF in a non-human primate model. Methods Rhesus monkeys ( Macaca mulatta) received O 4 BF (10–50 mg/kg) intravenously, and serial blood and CSF samples were obtained. Analyte concentrations in plasma were measured by HPLC/photo diode array, and an HPLC/MS/MS assay was used for CSF samples. Results A putative metabolite of O 4 BF was detected in plasma and CSF. O 4 BF and the metabolite inactivated purified AGT with ED 50 of 0.04 mcM. The median clearance of O 4 BF was 8 ml/min/kg and half-life was 1.1 h. The metabolite had a substantially longer half-life (>20 h) and greater AUC than O 4 BF. The AUC of the metabolite increased disproportionately to the dose of O 4 BF, suggesting saturable elimination. CSF penetration of O 4 BF and its metabolite was < 1%. At the 50 mg/kg dose level, the C max in CSF for O 4 BF was less than 0.09 mcM and for the metabolite the C max ranged from 0.02 to 0.04 mcM (O 4 BF equivalents). Conclusions Concentrations of O 4 BF and the metabolite in CSF exceeded the ED 50 of AGT; however, recently reported lack of receptor specificity and pharmacokinetic data suggesting saturable elimination of both O 4 BF and its metabolite may limit dose-escalation and future clinical development of this agent.