Abstract 4645: Clofarabine, a potent anticancer compound with limited penetration in an orthotopic murine model of ependymoma

Clofarabine, a deoxyadenosine analog, was a potent hit in our in vitro high-throughput screening against murine ependymoma neurospheres. To prioritize clofarabine for further preclinical efficacy studies, we evaluated the plasma pharmacokinetic (PK) disposition and central nervous system penetration in a murine model of ependymoma. A plasma PK study of clofarabine (45 mg/kg IP) was performed using CD1 nude mice bearing ependymoma cortical allographs (Ink4a/Arf-null + RTBDN) to obtain initial plasma PK parameters. These estimates were used to derive D-optimal plasma sampling time-points (e.g., 0.25, 2.5, and 5 hr) for cerebral microdialysis studies. Comparison of the clofarabine systemic exposure obtained from the plasma PK study and that simulated from pediatric patients using a published population PK model (Bonate, Cancer Chemotherap Pharmacol, 2011) suggested a dosage of 30 mg/kg in mice would be equivalent to a pediatric dosage of 180 mg/m2 given as a 2 hr infusion. Cerebral microdialysis was applied in CD1 nude mice bearing ependymoma cortical allographs (Ink4a/Arf-null + RTBDN), which permitted repeated in situ sampling of clofarabine tumor extracellular fluid (tECF). A microdialysis probe (BASi; 1 mm membrane) was introduced into the tumor through a cannula inserted during tumor cell implantation. After microdialysis probe equilibration, 7 mice were dosed with 30 mg/kg of clofarabine IP. In each mouse, serial plasma samples were collected at 0.25, 2.5, and 5 h post-dose, and tECF dialysate fractions were collected over 60 min intervals for up to 5 h post-dose. To measure clofarabine in both plasma and tECF, a robust, sensitive LC-MS/MS method was developed and validated. Both within-day and between-day precision (%CV) were ≤ 5.1% and accuracy ranged from 86% to 109%. A two-compartment model with absorption and tumor compartments linked to a central compartment was fitted to plasma and tECF concentration-time data using a nonlinear mixed effects modeling approach (NONMEM 7.2.0). For modeling purposes, the volume of the tECF compartment was fixed to published values. Unbound fraction of clofarabine in murine plasma was 0.82 ± 0.14. The model derived area under unbound concentration-time curve (AUCu,0-8) values for 30 and 45 mg/kg dosages were 5185 ± 550 µg/L*hr and 7677 ± 699 µg/L*hr, respectively. Clofarabine was absorbed rapidly from the peritoneal cavity with Tmax (time to reach maximum concentration) value of 0.33 ± 0.17 hr. Tumor to plasma partit