Differences in cerebrospinal fluid penetration of corticosteroids: possible relationship to the prevention of meningeal leukemia.

The disposition of the synthetic corticosteroids, dexamethasone and prednisolone, in CSF was evaluated following bolus intravenous (IV) and intrathecal (IT) injection in a nonhuman primate model. Steroid concentration in plasma and CSF was measured with a radioimmunoassay following celite column chromatography. The CSF to plasma ratios of dexamethasone and prednisolone following IV bolus administration were 0.15 +/- 0.02 and 0.08 +/- 0.03, respectively. Although peak levels of the two steroids in the CSF reached equally potent levels when administered systemically in equipotent doses, the half-life of prednisolone in the CSF was shorter. In addition, there was a significant difference in the plasma protein binding of the two steroids, which may account for the differences in their CSF pharmacokinetics. Dexamethasone was 70% protein bound over a wide concentration range, while the protein binding of prednisolone was concentration dependent, ranging from 60% at 10 mumol/L to 95% at 0.5 mumol/L and below. After the initial distribution phase in plasma, CSF concentrations of dexamethasone and prednisolone approximated free plasma concentrations, indicating that penetration into the CSF was limited primarily by protein binding. At the plasma concentrations achieved following oral administration of standard doses of prednisone in children, the prednisolone (the active metabolite) is greater than 90% protein bound. The proportionally higher free plasma levels of dexamethasone result in greater penetration into the CSF. These findings may explain the lower rates of meningeal leukemia observed in children receiving dexamethasone instead of prednisone for the treatment of acute lymphoblastic leukemia (ALL).