Introduction: Topotecan (TPT) has excellent CNS penetration, suggesting its promising role in treating CNS malignancies. To understand the potential mechanism of high TPT CNS penetration, we studied the TPT distribution into either brain extracellular fluid (ECF) or ventricular CSF (vCSF) in a mouse model. Since TPT is a substrate for many ABC transporters (e.g., ABCG2, ABCC1) expressed at the blood-brain barrier (BBB) as well as the blood-CSF barrier (BCB), the role of active transporters in TPT CNS penetration was explored alone and in combination with a putative ABC transport modulator gefitinib. Methods: TPT distribution into brain ECF and vCSF was studied in FVB mice using microdialysis coupled with an on-line microbore HPLC system after 4 and 8 mg/kg i.v. injection. In vivo recovery was estimated for each individual mouse. A limited sampling model for unbound TPT lactone plasma pharmacokinetics (PK) was developed based on unbound TPT lactone population PK after 4 and 8 mg/kg i.v. injection. Unbound TPT lactone in plasma was obtained after correcting for protein binding. To study the effect of gefitinib on brain ECF and vCSF penetration of TPT lactone, 200mg/kg gefitinib was given orally one hour before 4mg/kg TPT i.v. injection as suggested by the gefitinib PK profile. The area under the curve (AUC) for TPT lactone in brain was estimated by the linear trapezoidal rule and addition of a residual area. Unbound TPT AUC in plasma was estimated using PK parameters obtained using a MAP-Bayesian approach. The vCSF: plasma and brain ECF: plasma AUC ratios were calculated and used as a measurement of TPT CNS penetration. Abcg2 expression at the BCB was studied by immunohistochemistry using the monoclonal antibody BXP-53. Results: After 4 and 8 mg/kg TPT i.v. injection, the unbound TPT lactone brain ECF: plasma AUC ratio was 20.8±3.7% and 61.1±16.2%, respectively; the vCSF: plasma AUC ratio was 118.0±10.0% and 130.4±12.6%, respectively. After 200 mg/kg gefitinib oral administration followed by 4 mg/kg TPT i.v. injection, the unbound TPT lactone brain ECF: plasma AUC ratio increased to 35.2±3.5%, which is significantly different from that without gefitinib (P<0.05). The vCSF: plasma AUC ratio significantly decreased to 97.9±5.2 (P<0.05). Abcg2 was detected at the apical side of the choroid plexus. Conclusions: TPT brain ECF penetration was much lower compared to vCSF penetration. Gefitinib increased TPT brain ECF penetration but decreased the vCSF penetration. These results are consistent with the possibility that expression of Abcg2 at the apical side of the choroid plexus facilitates an influx transport mechanism across the BCB resulting in high TPT CSF penetration, which could be inhibited by ABC transport modulators such as gefitinib. (Supported by NIH Grant CA23099 & GM071321)

[Proc Amer Assoc Cancer Res, Volume 47, 2006]